rpms
/
perl-Pod-Perldoc
2
0
Fork 0
Code Issues Pull Requests Packages Projects Releases Wiki Activity
perl-Pod-Perldoc/Pod-Perldoc-3.28-Add-a-test...

3737 lines
146 KiB
Diff
Raw Permalink Blame History

From 0729324bcb3f8186541fcc7fabadf474b86a7411 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Petr=20P=C3=ADsa=C5=99?= <ppisar@redhat.com>
Date: Wed, 14 Aug 2019 17:35:22 +0200
Subject: [PATCH 1/2] Add a test for a truncated perldoc -f "tr" output
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
Please note that it bundles perlop POD from Perl 5.30.0 to have
a non-moving test target.
CPAN RT#86506
Signed-off-by: Petr Písař <ppisar@redhat.com>
---
MANIFEST | 2 +
Makefile.PL | 1 +
corpus/perlop.pod | 3610 +++++++++++++++++++++++++++++++++++++
t/03_builtin_pod_output.t | 59 +
4 files changed, 3672 insertions(+)
create mode 100644 corpus/perlop.pod
create mode 100644 t/03_builtin_pod_output.t
diff --git a/MANIFEST b/MANIFEST
index 8350f18..8e1dbae 100644
--- a/MANIFEST
+++ b/MANIFEST
@@ -1,6 +1,7 @@
Changes
corpus/no-head.pod
corpus/perlfunc.pod
+corpus/perlop.pod
corpus/utf8.pod
lib/Pod/Perldoc.pm
lib/Pod/Perldoc/BaseTo.pm
@@ -28,3 +29,4 @@ t/man/_get_columns.t
t/pod.t
t/01_about_verbose.t
t/02_module_pod_output.t
+t/03_builtin_pod_output.t
diff --git a/Makefile.PL b/Makefile.PL
index f76d364..8f5b159 100644
--- a/Makefile.PL
+++ b/Makefile.PL
@@ -15,6 +15,7 @@ WriteMakefile(
'PREREQ_PM' => {
# Are there any hard dependencies not covered here?
+ 'blib' => '0',
'Config' => '0',
'Encode' => '0',
'Fcntl' => '0',
diff --git a/corpus/perlop.pod b/corpus/perlop.pod
new file mode 100644
index 0000000..dd658bf
--- /dev/null
+++ b/corpus/perlop.pod
@@ -0,0 +1,3610 @@
+=head1 NAME
+X<operator>
+
+perlop - Perl operators and precedence
+
+=head1 DESCRIPTION
+
+In Perl, the operator determines what operation is performed,
+independent of the type of the operands. For example S<C<$x + $y>>
+is always a numeric addition, and if C<$x> or C<$y> do not contain
+numbers, an attempt is made to convert them to numbers first.
+
+This is in contrast to many other dynamic languages, where the
+operation is determined by the type of the first argument. It also
+means that Perl has two versions of some operators, one for numeric
+and one for string comparison. For example S<C<$x == $y>> compares
+two numbers for equality, and S<C<$x eq $y>> compares two strings.
+
+There are a few exceptions though: C<x> can be either string
+repetition or list repetition, depending on the type of the left
+operand, and C<&>, C<|>, C<^> and C<~> can be either string or numeric bit
+operations.
+
+=head2 Operator Precedence and Associativity
+X<operator, precedence> X<precedence> X<associativity>
+
+Operator precedence and associativity work in Perl more or less like
+they do in mathematics.
+
+I<Operator precedence> means some operators group more tightly than others.
+For example, in C<2 + 4 * 5>, the multiplication has higher precedence, so C<4
+* 5> is grouped together as the right-hand operand of the addition, rather
+than C<2 + 4> being grouped together as the left-hand operand of the
+multiplication. It is as if the expression were written C<2 + (4 * 5)>, not
+C<(2 + 4) * 5>. So the expression yields C<2 + 20 == 22>, rather than
+C<6 * 5 == 30>.
+
+I<Operator associativity> defines what happens if a sequence of the same
+operators is used one after another: whether they will be grouped at the left
+or the right. For example, in C<9 - 3 - 2>, subtraction is left associative,
+so C<9 - 3> is grouped together as the left-hand operand of the second
+subtraction, rather than C<3 - 2> being grouped together as the right-hand
+operand of the first subtraction. It is as if the expression were written
+C<(9 - 3) - 2>, not C<9 - (3 - 2)>. So the expression yields C<6 - 2 == 4>,
+rather than C<9 - 1 == 8>.
+
+For simple operators that evaluate all their operands and then combine the
+values in some way, precedence and associativity (and parentheses) imply some
+ordering requirements on those combining operations. For example, in C<2 + 4 *
+5>, the grouping implied by precedence means that the multiplication of 4 and
+5 must be performed before the addition of 2 and 20, simply because the result
+of that multiplication is required as one of the operands of the addition. But
+the order of operations is not fully determined by this: in C<2 * 2 + 4 * 5>
+both multiplications must be performed before the addition, but the grouping
+does not say anything about the order in which the two multiplications are
+performed. In fact Perl has a general rule that the operands of an operator
+are evaluated in left-to-right order. A few operators such as C<&&=> have
+special evaluation rules that can result in an operand not being evaluated at
+all; in general, the top-level operator in an expression has control of
+operand evaluation.
+
+Perl operators have the following associativity and precedence,
+listed from highest precedence to lowest. Operators borrowed from
+C keep the same precedence relationship with each other, even where
+C's precedence is slightly screwy. (This makes learning Perl easier
+for C folks.) With very few exceptions, these all operate on scalar
+values only, not array values.
+
+ left terms and list operators (leftward)
+ left ->
+ nonassoc ++ --
+ right **
+ right ! ~ \ and unary + and -
+ left =~ !~
+ left * / % x
+ left + - .
+ left << >>
+ nonassoc named unary operators
+ nonassoc < > <= >= lt gt le ge
+ nonassoc == != <=> eq ne cmp ~~
+ left &
+ left | ^
+ left &&
+ left || //
+ nonassoc .. ...
+ right ?:
+ right = += -= *= etc. goto last next redo dump
+ left , =>
+ nonassoc list operators (rightward)
+ right not
+ left and
+ left or xor
+
+In the following sections, these operators are covered in detail, in the
+same order in which they appear in the table above.
+
+Many operators can be overloaded for objects. See L<overload>.
+
+=head2 Terms and List Operators (Leftward)
+X<list operator> X<operator, list> X<term>
+
+A TERM has the highest precedence in Perl. They include variables,
+quote and quote-like operators, any expression in parentheses,
+and any function whose arguments are parenthesized. Actually, there
+aren't really functions in this sense, just list operators and unary
+operators behaving as functions because you put parentheses around
+the arguments. These are all documented in L<perlfunc>.
+
+If any list operator (C<print()>, etc.) or any unary operator (C<chdir()>, etc.)
+is followed by a left parenthesis as the next token, the operator and
+arguments within parentheses are taken to be of highest precedence,
+just like a normal function call.
+
+In the absence of parentheses, the precedence of list operators such as
+C<print>, C<sort>, or C<chmod> is either very high or very low depending on
+whether you are looking at the left side or the right side of the operator.
+For example, in
+
+ @ary = (1, 3, sort 4, 2);
+ print @ary; # prints 1324
+
+the commas on the right of the C<sort> are evaluated before the C<sort>,
+but the commas on the left are evaluated after. In other words,
+list operators tend to gobble up all arguments that follow, and
+then act like a simple TERM with regard to the preceding expression.
+Be careful with parentheses:
+
+ # These evaluate exit before doing the print:
+ print($foo, exit); # Obviously not what you want.
+ print $foo, exit; # Nor is this.
+
+ # These do the print before evaluating exit:
+ (print $foo), exit; # This is what you want.
+ print($foo), exit; # Or this.
+ print ($foo), exit; # Or even this.
+
+Also note that
+
+ print ($foo & 255) + 1, "\n";
+
+probably doesn't do what you expect at first glance. The parentheses
+enclose the argument list for C<print> which is evaluated (printing
+the result of S<C<$foo & 255>>). Then one is added to the return value
+of C<print> (usually 1). The result is something like this:
+
+ 1 + 1, "\n"; # Obviously not what you meant.
+
+To do what you meant properly, you must write:
+
+ print(($foo & 255) + 1, "\n");
+
+See L</Named Unary Operators> for more discussion of this.
+
+Also parsed as terms are the S<C<do {}>> and S<C<eval {}>> constructs, as
+well as subroutine and method calls, and the anonymous
+constructors C<[]> and C<{}>.
+
+See also L</Quote and Quote-like Operators> toward the end of this section,
+as well as L</"I/O Operators">.
+
+=head2 The Arrow Operator
+X<arrow> X<dereference> X<< -> >>
+
+"C<< -> >>" is an infix dereference operator, just as it is in C
+and C++. If the right side is either a C<[...]>, C<{...}>, or a
+C<(...)> subscript, then the left side must be either a hard or
+symbolic reference to an array, a hash, or a subroutine respectively.
+(Or technically speaking, a location capable of holding a hard
+reference, if it's an array or hash reference being used for
+assignment.) See L<perlreftut> and L<perlref>.
+
+Otherwise, the right side is a method name or a simple scalar
+variable containing either the method name or a subroutine reference,
+and the left side must be either an object (a blessed reference)
+or a class name (that is, a package name). See L<perlobj>.
+
+The dereferencing cases (as opposed to method-calling cases) are
+somewhat extended by the C<postderef> feature. For the
+details of that feature, consult L<perlref/Postfix Dereference Syntax>.
+
+=head2 Auto-increment and Auto-decrement
+X<increment> X<auto-increment> X<++> X<decrement> X<auto-decrement> X<-->
+
+C<"++"> and C<"--"> work as in C. That is, if placed before a variable,
+they increment or decrement the variable by one before returning the
+value, and if placed after, increment or decrement after returning the
+value.
+
+ $i = 0; $j = 0;
+ print $i++; # prints 0
+ print ++$j; # prints 1
+
+Note that just as in C, Perl doesn't define B<when> the variable is
+incremented or decremented. You just know it will be done sometime
+before or after the value is returned. This also means that modifying
+a variable twice in the same statement will lead to undefined behavior.
+Avoid statements like:
+
+ $i = $i ++;
+ print ++ $i + $i ++;
+
+Perl will not guarantee what the result of the above statements is.
+
+The auto-increment operator has a little extra builtin magic to it. If
+you increment a variable that is numeric, or that has ever been used in
+a numeric context, you get a normal increment. If, however, the
+variable has been used in only string contexts since it was set, and
+has a value that is not the empty string and matches the pattern
+C</^[a-zA-Z]*[0-9]*\z/>, the increment is done as a string, preserving each
+character within its range, with carry:
+
+ print ++($foo = "99"); # prints "100"
+ print ++($foo = "a0"); # prints "a1"
+ print ++($foo = "Az"); # prints "Ba"
+ print ++($foo = "zz"); # prints "aaa"
+
+C<undef> is always treated as numeric, and in particular is changed
+to C<0> before incrementing (so that a post-increment of an undef value
+will return C<0> rather than C<undef>).
+
+The auto-decrement operator is not magical.
+
+=head2 Exponentiation
+X<**> X<exponentiation> X<power>
+
+Binary C<"**"> is the exponentiation operator. It binds even more
+tightly than unary minus, so C<-2**4> is C<-(2**4)>, not C<(-2)**4>.
+(This is
+implemented using C's C<pow(3)> function, which actually works on doubles
+internally.)
+
+Note that certain exponentiation expressions are ill-defined:
+these include C<0**0>, C<1**Inf>, and C<Inf**0>. Do not expect
+any particular results from these special cases, the results
+are platform-dependent.
+
+=head2 Symbolic Unary Operators
+X<unary operator> X<operator, unary>
+
+Unary C<"!"> performs logical negation, that is, "not". See also
+L<C<not>|/Logical Not> for a lower precedence version of this.
+X<!>
+
+Unary C<"-"> performs arithmetic negation if the operand is numeric,
+including any string that looks like a number. If the operand is
+an identifier, a string consisting of a minus sign concatenated
+with the identifier is returned. Otherwise, if the string starts
+with a plus or minus, a string starting with the opposite sign is
+returned. One effect of these rules is that C<-bareword> is equivalent
+to the string C<"-bareword">. If, however, the string begins with a
+non-alphabetic character (excluding C<"+"> or C<"-">), Perl will attempt
+to convert
+the string to a numeric, and the arithmetic negation is performed. If the
+string cannot be cleanly converted to a numeric, Perl will give the warning
+B<Argument "the string" isn't numeric in negation (-) at ...>.
+X<-> X<negation, arithmetic>
+
+Unary C<"~"> performs bitwise negation, that is, 1's complement. For
+example, S<C<0666 & ~027>> is 0640. (See also L</Integer Arithmetic> and
+L</Bitwise String Operators>.) Note that the width of the result is
+platform-dependent: C<~0> is 32 bits wide on a 32-bit platform, but 64
+bits wide on a 64-bit platform, so if you are expecting a certain bit
+width, remember to use the C<"&"> operator to mask off the excess bits.
+X<~> X<negation, binary>
+
+Starting in Perl 5.28, it is a fatal error to try to complement a string
+containing a character with an ordinal value above 255.
+
+If the "bitwise" feature is enabled via S<C<use
+feature 'bitwise'>> or C<use v5.28>, then unary
+C<"~"> always treats its argument as a number, and an
+alternate form of the operator, C<"~.">, always treats its argument as a
+string. So C<~0> and C<~"0"> will both give 2**32-1 on 32-bit platforms,
+whereas C<~.0> and C<~."0"> will both yield C<"\xff">. Until Perl 5.28,
+this feature produced a warning in the C<"experimental::bitwise"> category.
+
+Unary C<"+"> has no effect whatsoever, even on strings. It is useful
+syntactically for separating a function name from a parenthesized expression
+that would otherwise be interpreted as the complete list of function
+arguments. (See examples above under L</Terms and List Operators (Leftward)>.)
+X<+>
+
+Unary C<"\"> creates references. If its operand is a single sigilled
+thing, it creates a reference to that object. If its operand is a
+parenthesised list, then it creates references to the things mentioned
+in the list. Otherwise it puts its operand in list context, and creates
+a list of references to the scalars in the list provided by the operand.
+See L<perlreftut>
+and L<perlref>. Do not confuse this behavior with the behavior of
+backslash within a string, although both forms do convey the notion
+of protecting the next thing from interpolation.
+X<\> X<reference> X<backslash>
+
+=head2 Binding Operators
+X<binding> X<operator, binding> X<=~> X<!~>
+
+Binary C<"=~"> binds a scalar expression to a pattern match. Certain operations
+search or modify the string C<$_> by default. This operator makes that kind
+of operation work on some other string. The right argument is a search
+pattern, substitution, or transliteration. The left argument is what is
+supposed to be searched, substituted, or transliterated instead of the default
+C<$_>. When used in scalar context, the return value generally indicates the
+success of the operation. The exceptions are substitution (C<s///>)
+and transliteration (C<y///>) with the C</r> (non-destructive) option,
+which cause the B<r>eturn value to be the result of the substitution.
+Behavior in list context depends on the particular operator.
+See L</"Regexp Quote-Like Operators"> for details and L<perlretut> for
+examples using these operators.
+
+If the right argument is an expression rather than a search pattern,
+substitution, or transliteration, it is interpreted as a search pattern at run
+time. Note that this means that its
+contents will be interpolated twice, so
+
+ '\\' =~ q'\\';
+
+is not ok, as the regex engine will end up trying to compile the
+pattern C<\>, which it will consider a syntax error.
+
+Binary C<"!~"> is just like C<"=~"> except the return value is negated in
+the logical sense.
+
+Binary C<"!~"> with a non-destructive substitution (C<s///r>) or transliteration
+(C<y///r>) is a syntax error.
+
+=head2 Multiplicative Operators
+X<operator, multiplicative>
+
+Binary C<"*"> multiplies two numbers.
+X<*>
+
+Binary C<"/"> divides two numbers.
+X</> X<slash>
+
+Binary C<"%"> is the modulo operator, which computes the division
+remainder of its first argument with respect to its second argument.
+Given integer
+operands C<$m> and C<$n>: If C<$n> is positive, then S<C<$m % $n>> is
+C<$m> minus the largest multiple of C<$n> less than or equal to
+C<$m>. If C<$n> is negative, then S<C<$m % $n>> is C<$m> minus the
+smallest multiple of C<$n> that is not less than C<$m> (that is, the
+result will be less than or equal to zero). If the operands
+C<$m> and C<$n> are floating point values and the absolute value of
+C<$n> (that is C<abs($n)>) is less than S<C<(UV_MAX + 1)>>, only
+the integer portion of C<$m> and C<$n> will be used in the operation
+(Note: here C<UV_MAX> means the maximum of the unsigned integer type).
+If the absolute value of the right operand (C<abs($n)>) is greater than
+or equal to S<C<(UV_MAX + 1)>>, C<"%"> computes the floating-point remainder
+C<$r> in the equation S<C<($r = $m - $i*$n)>> where C<$i> is a certain
+integer that makes C<$r> have the same sign as the right operand
+C<$n> (B<not> as the left operand C<$m> like C function C<fmod()>)
+and the absolute value less than that of C<$n>.
+Note that when S<C<use integer>> is in scope, C<"%"> gives you direct access
+to the modulo operator as implemented by your C compiler. This
+operator is not as well defined for negative operands, but it will
+execute faster.
+X<%> X<remainder> X<modulo> X<mod>
+
+Binary C<x> is the repetition operator. In scalar context, or if the
+left operand is neither enclosed in parentheses nor a C<qw//> list,
+it performs a string repetition. In that case it supplies scalar
+context to the left operand, and returns a string consisting of the
+left operand string repeated the number of times specified by the right
+operand. If the C<x> is in list context, and the left operand is either
+enclosed in parentheses or a C<qw//> list, it performs a list repetition.
+In that case it supplies list context to the left operand, and returns
+a list consisting of the left operand list repeated the number of times
+specified by the right operand.
+If the right operand is zero or negative (raising a warning on
+negative), it returns an empty string
+or an empty list, depending on the context.
+X<x>
+
+ print '-' x 80; # print row of dashes
+
+ print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
+
+ @ones = (1) x 80; # a list of 80 1's
+ @ones = (5) x @ones; # set all elements to 5
+
+
+=head2 Additive Operators
+X<operator, additive>
+
+Binary C<"+"> returns the sum of two numbers.
+X<+>
+
+Binary C<"-"> returns the difference of two numbers.
+X<->
+
+Binary C<"."> concatenates two strings.
+X<string, concatenation> X<concatenation>
+X<cat> X<concat> X<concatenate> X<.>
+
+=head2 Shift Operators
+X<shift operator> X<operator, shift> X<<< << >>>
+X<<< >> >>> X<right shift> X<left shift> X<bitwise shift>
+X<shl> X<shr> X<shift, right> X<shift, left>
+
+Binary C<<< "<<" >>> returns the value of its left argument shifted left by the
+number of bits specified by the right argument. Arguments should be
+integers. (See also L</Integer Arithmetic>.)
+
+Binary C<<< ">>" >>> returns the value of its left argument shifted right by
+the number of bits specified by the right argument. Arguments should
+be integers. (See also L</Integer Arithmetic>.)
+
+If S<C<use integer>> (see L</Integer Arithmetic>) is in force then
+signed C integers are used (I<arithmetic shift>), otherwise unsigned C
+integers are used (I<logical shift>), even for negative shiftees.
+In arithmetic right shift the sign bit is replicated on the left,
+in logical shift zero bits come in from the left.
+
+Either way, the implementation isn't going to generate results larger
+than the size of the integer type Perl was built with (32 bits or 64 bits).
+
+Shifting by negative number of bits means the reverse shift: left
+shift becomes right shift, right shift becomes left shift. This is
+unlike in C, where negative shift is undefined.
+
+Shifting by more bits than the size of the integers means most of the
+time zero (all bits fall off), except that under S<C<use integer>>
+right overshifting a negative shiftee results in -1. This is unlike
+in C, where shifting by too many bits is undefined. A common C
+behavior is "shift by modulo wordbits", so that for example
+
+ 1 >> 64 == 1 >> (64 % 64) == 1 >> 0 == 1 # Common C behavior.
+
+but that is completely accidental.
+
+If you get tired of being subject to your platform's native integers,
+the S<C<use bigint>> pragma neatly sidesteps the issue altogether:
+
+ print 20 << 20; # 20971520
+ print 20 << 40; # 5120 on 32-bit machines,
+ # 21990232555520 on 64-bit machines
+ use bigint;
+ print 20 << 100; # 25353012004564588029934064107520
+
+=head2 Named Unary Operators
+X<operator, named unary>
+
+The various named unary operators are treated as functions with one
+argument, with optional parentheses.
+
+If any list operator (C<print()>, etc.) or any unary operator (C<chdir()>, etc.)
+is followed by a left parenthesis as the next token, the operator and
+arguments within parentheses are taken to be of highest precedence,
+just like a normal function call. For example,
+because named unary operators are higher precedence than C<||>:
+
+ chdir $foo || die; # (chdir $foo) || die
+ chdir($foo) || die; # (chdir $foo) || die
+ chdir ($foo) || die; # (chdir $foo) || die
+ chdir +($foo) || die; # (chdir $foo) || die
+
+but, because C<"*"> is higher precedence than named operators:
+
+ chdir $foo * 20; # chdir ($foo * 20)
+ chdir($foo) * 20; # (chdir $foo) * 20
+ chdir ($foo) * 20; # (chdir $foo) * 20
+ chdir +($foo) * 20; # chdir ($foo * 20)
+
+ rand 10 * 20; # rand (10 * 20)
+ rand(10) * 20; # (rand 10) * 20
+ rand (10) * 20; # (rand 10) * 20
+ rand +(10) * 20; # rand (10 * 20)
+
+Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are
+treated like named unary operators, but they don't follow this functional
+parenthesis rule. That means, for example, that C<-f($file).".bak"> is
+equivalent to S<C<-f "$file.bak">>.
+X<-X> X<filetest> X<operator, filetest>
+
+See also L</"Terms and List Operators (Leftward)">.
+
+=head2 Relational Operators
+X<relational operator> X<operator, relational>
+
+Perl operators that return true or false generally return values
+that can be safely used as numbers. For example, the relational
+operators in this section and the equality operators in the next
+one return C<1> for true and a special version of the defined empty
+string, C<"">, which counts as a zero but is exempt from warnings
+about improper numeric conversions, just as S<C<"0 but true">> is.
+
+Binary C<< "<" >> returns true if the left argument is numerically less than
+the right argument.
+X<< < >>
+
+Binary C<< ">" >> returns true if the left argument is numerically greater
+than the right argument.
+X<< > >>
+
+Binary C<< "<=" >> returns true if the left argument is numerically less than
+or equal to the right argument.
+X<< <= >>
+
+Binary C<< ">=" >> returns true if the left argument is numerically greater
+than or equal to the right argument.
+X<< >= >>
+
+Binary C<"lt"> returns true if the left argument is stringwise less than
+the right argument.
+X<< lt >>
+
+Binary C<"gt"> returns true if the left argument is stringwise greater
+than the right argument.
+X<< gt >>
+
+Binary C<"le"> returns true if the left argument is stringwise less than
+or equal to the right argument.
+X<< le >>
+
+Binary C<"ge"> returns true if the left argument is stringwise greater
+than or equal to the right argument.
+X<< ge >>
+
+=head2 Equality Operators
+X<equality> X<equal> X<equals> X<operator, equality>
+
+Binary C<< "==" >> returns true if the left argument is numerically equal to
+the right argument.
+X<==>
+
+Binary C<< "!=" >> returns true if the left argument is numerically not equal
+to the right argument.
+X<!=>
+
+Binary C<< "<=>" >> returns -1, 0, or 1 depending on whether the left
+argument is numerically less than, equal to, or greater than the right
+argument. If your platform supports C<NaN>'s (not-a-numbers) as numeric
+values, using them with C<< "<=>" >> returns undef. C<NaN> is not
+C<< "<" >>, C<< "==" >>, C<< ">" >>, C<< "<=" >> or C<< ">=" >> anything
+(even C<NaN>), so those 5 return false. S<C<< NaN != NaN >>> returns
+true, as does S<C<NaN !=> I<anything else>>. If your platform doesn't
+support C<NaN>'s then C<NaN> is just a string with numeric value 0.
+X<< <=> >>
+X<spaceship>
+
+ $ perl -le '$x = "NaN"; print "No NaN support here" if $x == $x'
+ $ perl -le '$x = "NaN"; print "NaN support here" if $x != $x'
+
+(Note that the L<bigint>, L<bigrat>, and L<bignum> pragmas all
+support C<"NaN">.)
+
+Binary C<"eq"> returns true if the left argument is stringwise equal to
+the right argument.
+X<eq>
+
+Binary C<"ne"> returns true if the left argument is stringwise not equal
+to the right argument.
+X<ne>
+
+Binary C<"cmp"> returns -1, 0, or 1 depending on whether the left
+argument is stringwise less than, equal to, or greater than the right
+argument.
+X<cmp>
+
+Binary C<"~~"> does a smartmatch between its arguments. Smart matching
+is described in the next section.
+X<~~>
+
+C<"lt">, C<"le">, C<"ge">, C<"gt"> and C<"cmp"> use the collation (sort)
+order specified by the current C<LC_COLLATE> locale if a S<C<use
+locale>> form that includes collation is in effect. See L<perllocale>.
+Do not mix these with Unicode,
+only use them with legacy 8-bit locale encodings.
+The standard C<L<Unicode::Collate>> and
+C<L<Unicode::Collate::Locale>> modules offer much more powerful
+solutions to collation issues.
+
+For case-insensitive comparisons, look at the L<perlfunc/fc> case-folding
+function, available in Perl v5.16 or later:
+
+ if ( fc($x) eq fc($y) ) { ... }
+
+=head2 Smartmatch Operator
+
+First available in Perl 5.10.1 (the 5.10.0 version behaved differently),
+binary C<~~> does a "smartmatch" between its arguments. This is mostly
+used implicitly in the C<when> construct described in L<perlsyn>, although
+not all C<when> clauses call the smartmatch operator. Unique among all of
+Perl's operators, the smartmatch operator can recurse. The smartmatch
+operator is L<experimental|perlpolicy/experimental> and its behavior is
+subject to change.
+
+It is also unique in that all other Perl operators impose a context
+(usually string or numeric context) on their operands, autoconverting
+those operands to those imposed contexts. In contrast, smartmatch
+I<infers> contexts from the actual types of its operands and uses that
+type information to select a suitable comparison mechanism.
+
+The C<~~> operator compares its operands "polymorphically", determining how
+to compare them according to their actual types (numeric, string, array,
+hash, etc.). Like the equality operators with which it shares the same
+precedence, C<~~> returns 1 for true and C<""> for false. It is often best
+read aloud as "in", "inside of", or "is contained in", because the left
+operand is often looked for I<inside> the right operand. That makes the
+order of the operands to the smartmatch operand often opposite that of
+the regular match operator. In other words, the "smaller" thing is usually
+placed in the left operand and the larger one in the right.
+
+The behavior of a smartmatch depends on what type of things its arguments
+are, as determined by the following table. The first row of the table
+whose types apply determines the smartmatch behavior. Because what
+actually happens is mostly determined by the type of the second operand,
+the table is sorted on the right operand instead of on the left.
+
+ Left Right Description and pseudocode
+ ===============================================================
+ Any undef check whether Any is undefined
+ like: !defined Any
+
+ Any Object invoke ~~ overloading on Object, or die
+
+ Right operand is an ARRAY:
+
+ Left Right Description and pseudocode
+ ===============================================================
+ ARRAY1 ARRAY2 recurse on paired elements of ARRAY1 and ARRAY2[2]
+ like: (ARRAY1[0] ~~ ARRAY2[0])
+ && (ARRAY1[1] ~~ ARRAY2[1]) && ...
+ HASH ARRAY any ARRAY elements exist as HASH keys
+ like: grep { exists HASH->{$_} } ARRAY
+ Regexp ARRAY any ARRAY elements pattern match Regexp
+ like: grep { /Regexp/ } ARRAY
+ undef ARRAY undef in ARRAY
+ like: grep { !defined } ARRAY
+ Any ARRAY smartmatch each ARRAY element[3]
+ like: grep { Any ~~ $_ } ARRAY
+
+ Right operand is a HASH:
+
+ Left Right Description and pseudocode
+ ===============================================================
+ HASH1 HASH2 all same keys in both HASHes
+ like: keys HASH1 ==
+ grep { exists HASH2->{$_} } keys HASH1
+ ARRAY HASH any ARRAY elements exist as HASH keys
+ like: grep { exists HASH->{$_} } ARRAY
+ Regexp HASH any HASH keys pattern match Regexp
+ like: grep { /Regexp/ } keys HASH
+ undef HASH always false (undef can't be a key)
+ like: 0 == 1
+ Any HASH HASH key existence
+ like: exists HASH->{Any}
+
+ Right operand is CODE:
+
+ Left Right Description and pseudocode
+ ===============================================================
+ ARRAY CODE sub returns true on all ARRAY elements[1]
+ like: !grep { !CODE->($_) } ARRAY
+ HASH CODE sub returns true on all HASH keys[1]
+ like: !grep { !CODE->($_) } keys HASH
+ Any CODE sub passed Any returns true
+ like: CODE->(Any)
+
+Right operand is a Regexp:
+
+ Left Right Description and pseudocode
+ ===============================================================
+ ARRAY Regexp any ARRAY elements match Regexp
+ like: grep { /Regexp/ } ARRAY
+ HASH Regexp any HASH keys match Regexp
+ like: grep { /Regexp/ } keys HASH
+ Any Regexp pattern match
+ like: Any =~ /Regexp/
+
+ Other:
+
+ Left Right Description and pseudocode
+ ===============================================================
+ Object Any invoke ~~ overloading on Object,
+ or fall back to...
+
+ Any Num numeric equality
+ like: Any == Num
+ Num nummy[4] numeric equality
+ like: Num == nummy
+ undef Any check whether undefined
+ like: !defined(Any)
+ Any Any string equality
+ like: Any eq Any
+
+
+Notes:
+
+=over
+
+=item 1.
+Empty hashes or arrays match.
+
+=item 2.
+That is, each element smartmatches the element of the same index in the other array.[3]
+
+=item 3.
+If a circular reference is found, fall back to referential equality.
+
+=item 4.
+Either an actual number, or a string that looks like one.
+
+=back
+
+The smartmatch implicitly dereferences any non-blessed hash or array
+reference, so the C<I<HASH>> and C<I<ARRAY>> entries apply in those cases.
+For blessed references, the C<I<Object>> entries apply. Smartmatches
+involving hashes only consider hash keys, never hash values.
+
+The "like" code entry is not always an exact rendition. For example, the
+smartmatch operator short-circuits whenever possible, but C<grep> does
+not. Also, C<grep> in scalar context returns the number of matches, but
+C<~~> returns only true or false.
+
+Unlike most operators, the smartmatch operator knows to treat C<undef>
+specially:
+
+ use v5.10.1;
+ @array = (1, 2, 3, undef, 4, 5);
+ say "some elements undefined" if undef ~~ @array;
+
+Each operand is considered in a modified scalar context, the modification
+being that array and hash variables are passed by reference to the
+operator, which implicitly dereferences them. Both elements
+of each pair are the same:
+
+ use v5.10.1;
+
+ my %hash = (red => 1, blue => 2, green => 3,
+ orange => 4, yellow => 5, purple => 6,
+ black => 7, grey => 8, white => 9);
+
+ my @array = qw(red blue green);
+
+ say "some array elements in hash keys" if @array ~~ %hash;
+ say "some array elements in hash keys" if \@array ~~ \%hash;
+
+ say "red in array" if "red" ~~ @array;
+ say "red in array" if "red" ~~ \@array;
+
+ say "some keys end in e" if /e$/ ~~ %hash;
+ say "some keys end in e" if /e$/ ~~ \%hash;
+
+Two arrays smartmatch if each element in the first array smartmatches
+(that is, is "in") the corresponding element in the second array,
+recursively.
+
+ use v5.10.1;
+ my @little = qw(red blue green);
+ my @bigger = ("red", "blue", [ "orange", "green" ] );
+ if (@little ~~ @bigger) { # true!
+ say "little is contained in bigger";
+ }
+
+Because the smartmatch operator recurses on nested arrays, this
+will still report that "red" is in the array.
+
+ use v5.10.1;
+ my @array = qw(red blue green);
+ my $nested_array = [[[[[[[ @array ]]]]]]];
+ say "red in array" if "red" ~~ $nested_array;
+
+If two arrays smartmatch each other, then they are deep
+copies of each others' values, as this example reports:
+
+ use v5.12.0;
+ my @a = (0, 1, 2, [3, [4, 5], 6], 7);
+ my @b = (0, 1, 2, [3, [4, 5], 6], 7);
+
+ if (@a ~~ @b && @b ~~ @a) {
+ say "a and b are deep copies of each other";
+ }
+ elsif (@a ~~ @b) {
+ say "a smartmatches in b";
+ }
+ elsif (@b ~~ @a) {
+ say "b smartmatches in a";
+ }
+ else {
+ say "a and b don't smartmatch each other at all";
+ }
+
+
+If you were to set S<C<$b[3] = 4>>, then instead of reporting that "a and b
+are deep copies of each other", it now reports that C<"b smartmatches in a">.
+That's because the corresponding position in C<@a> contains an array that
+(eventually) has a 4 in it.
+
+Smartmatching one hash against another reports whether both contain the
+same keys, no more and no less. This could be used to see whether two
+records have the same field names, without caring what values those fields
+might have. For example:
+
+ use v5.10.1;
+ sub make_dogtag {
+ state $REQUIRED_FIELDS = { name=>1, rank=>1, serial_num=>1 };
+
+ my ($class, $init_fields) = @_;
+
+ die "Must supply (only) name, rank, and serial number"
+ unless $init_fields ~~ $REQUIRED_FIELDS;
+
+ ...
+ }
+
+However, this only does what you mean if C<$init_fields> is indeed a hash
+reference. The condition C<$init_fields ~~ $REQUIRED_FIELDS> also allows the
+strings C<"name">, C<"rank">, C<"serial_num"> as well as any array reference
+that contains C<"name"> or C<"rank"> or C<"serial_num"> anywhere to pass
+through.
+
+The smartmatch operator is most often used as the implicit operator of a
+C<when> clause. See the section on "Switch Statements" in L<perlsyn>.
+
+=head3 Smartmatching of Objects
+
+To avoid relying on an object's underlying representation, if the
+smartmatch's right operand is an object that doesn't overload C<~~>,
+it raises the exception "C<Smartmatching a non-overloaded object
+breaks encapsulation>". That's because one has no business digging
+around to see whether something is "in" an object. These are all
+illegal on objects without a C<~~> overload:
+
+ %hash ~~ $object
+ 42 ~~ $object
+ "fred" ~~ $object
+
+However, you can change the way an object is smartmatched by overloading
+the C<~~> operator. This is allowed to
+extend the usual smartmatch semantics.
+For objects that do have an C<~~> overload, see L<overload>.
+
+Using an object as the left operand is allowed, although not very useful.
+Smartmatching rules take precedence over overloading, so even if the
+object in the left operand has smartmatch overloading, this will be
+ignored. A left operand that is a non-overloaded object falls back on a
+string or numeric comparison of whatever the C<ref> operator returns. That
+means that
+
+ $object ~~ X
+
+does I<not> invoke the overload method with C<I<X>> as an argument.
+Instead the above table is consulted as normal, and based on the type of
+C<I<X>>, overloading may or may not be invoked. For simple strings or
+numbers, "in" becomes equivalent to this:
+
+ $object ~~ $number ref($object) == $number
+ $object ~~ $string ref($object) eq $string
+
+For example, this reports that the handle smells IOish
+(but please don't really do this!):
+
+ use IO::Handle;
+ my $fh = IO::Handle->new();
+ if ($fh ~~ /\bIO\b/) {
+ say "handle smells IOish";
+ }
+
+That's because it treats C<$fh> as a string like
+C<"IO::Handle=GLOB(0x8039e0)">, then pattern matches against that.
+
+=head2 Bitwise And
+X<operator, bitwise, and> X<bitwise and> X<&>
+
+Binary C<"&"> returns its operands ANDed together bit by bit. Although no
+warning is currently raised, the result is not well defined when this operation
+is performed on operands that aren't either numbers (see
+L</Integer Arithmetic>) nor bitstrings (see L</Bitwise String Operators>).
+
+Note that C<"&"> has lower priority than relational operators, so for example
+the parentheses are essential in a test like
+
+ print "Even\n" if ($x & 1) == 0;
+
+If the "bitwise" feature is enabled via S<C<use feature 'bitwise'>> or
+C<use v5.28>, then this operator always treats its operands as numbers.
+Before Perl 5.28 this feature produced a warning in the
+C<"experimental::bitwise"> category.
+
+=head2 Bitwise Or and Exclusive Or
+X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor>
+X<bitwise xor> X<^>
+
+Binary C<"|"> returns its operands ORed together bit by bit.
+
+Binary C<"^"> returns its operands XORed together bit by bit.
+
+Although no warning is currently raised, the results are not well
+defined when these operations are performed on operands that aren't either
+numbers (see L</Integer Arithmetic>) nor bitstrings (see L</Bitwise String
+Operators>).
+
+Note that C<"|"> and C<"^"> have lower priority than relational operators, so
+for example the parentheses are essential in a test like
+
+ print "false\n" if (8 | 2) != 10;
+
+If the "bitwise" feature is enabled via S<C<use feature 'bitwise'>> or
+C<use v5.28>, then this operator always treats its operands as numbers.
+Before Perl 5.28. this feature produced a warning in the
+C<"experimental::bitwise"> category.
+
+=head2 C-style Logical And
+X<&&> X<logical and> X<operator, logical, and>
+
+Binary C<"&&"> performs a short-circuit logical AND operation. That is,
+if the left operand is false, the right operand is not even evaluated.
+Scalar or list context propagates down to the right operand if it
+is evaluated.
+
+=head2 C-style Logical Or
+X<||> X<operator, logical, or>
+
+Binary C<"||"> performs a short-circuit logical OR operation. That is,
+if the left operand is true, the right operand is not even evaluated.
+Scalar or list context propagates down to the right operand if it
+is evaluated.
+
+=head2 Logical Defined-Or
+X<//> X<operator, logical, defined-or>
+
+Although it has no direct equivalent in C, Perl's C<//> operator is related
+to its C-style "or". In fact, it's exactly the same as C<||>, except that it
+tests the left hand side's definedness instead of its truth. Thus,
+S<C<< EXPR1 // EXPR2 >>> returns the value of C<< EXPR1 >> if it's defined,
+otherwise, the value of C<< EXPR2 >> is returned.
+(C<< EXPR1 >> is evaluated in scalar context, C<< EXPR2 >>
+in the context of C<< // >> itself). Usually,
+this is the same result as S<C<< defined(EXPR1) ? EXPR1 : EXPR2 >>> (except that
+the ternary-operator form can be used as a lvalue, while S<C<< EXPR1 // EXPR2 >>>
+cannot). This is very useful for
+providing default values for variables. If you actually want to test if
+at least one of C<$x> and C<$y> is defined, use S<C<defined($x // $y)>>.
+
+The C<||>, C<//> and C<&&> operators return the last value evaluated
+(unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably
+portable way to find out the home directory might be:
+
+ $home = $ENV{HOME}
+ // $ENV{LOGDIR}
+ // (getpwuid($<))[7]
+ // die "You're homeless!\n";
+
+In particular, this means that you shouldn't use this
+for selecting between two aggregates for assignment:
+
+ @a = @b || @c; # This doesn't do the right thing
+ @a = scalar(@b) || @c; # because it really means this.
+ @a = @b ? @b : @c; # This works fine, though.
+
+As alternatives to C<&&> and C<||> when used for
+control flow, Perl provides the C<and> and C<or> operators (see below).
+The short-circuit behavior is identical. The precedence of C<"and">
+and C<"or"> is much lower, however, so that you can safely use them after a
+list operator without the need for parentheses:
+
+ unlink "alpha", "beta", "gamma"
+ or gripe(), next LINE;
+
+With the C-style operators that would have been written like this:
+
+ unlink("alpha", "beta", "gamma")
+ || (gripe(), next LINE);
+
+It would be even more readable to write that this way:
+
+ unless(unlink("alpha", "beta", "gamma")) {
+ gripe();
+ next LINE;
+ }
+
+Using C<"or"> for assignment is unlikely to do what you want; see below.
+
+=head2 Range Operators
+X<operator, range> X<range> X<..> X<...>
+
+Binary C<".."> is the range operator, which is really two different
+operators depending on the context. In list context, it returns a
+list of values counting (up by ones) from the left value to the right
+value. If the left value is greater than the right value then it
+returns the empty list. The range operator is useful for writing
+S<C<foreach (1..10)>> loops and for doing slice operations on arrays. In
+the current implementation, no temporary array is created when the
+range operator is used as the expression in C<foreach> loops, but older
+versions of Perl might burn a lot of memory when you write something
+like this:
+
+ for (1 .. 1_000_000) {
+ # code
+ }
+
+The range operator also works on strings, using the magical
+auto-increment, see below.
+
+In scalar context, C<".."> returns a boolean value. The operator is
+bistable, like a flip-flop, and emulates the line-range (comma)
+operator of B<sed>, B<awk>, and various editors. Each C<".."> operator
+maintains its own boolean state, even across calls to a subroutine
+that contains it. It is false as long as its left operand is false.
+Once the left operand is true, the range operator stays true until the
+right operand is true, I<AFTER> which the range operator becomes false
+again. It doesn't become false till the next time the range operator
+is evaluated. It can test the right operand and become false on the
+same evaluation it became true (as in B<awk>), but it still returns
+true once. If you don't want it to test the right operand until the
+next evaluation, as in B<sed>, just use three dots (C<"...">) instead of
+two. In all other regards, C<"..."> behaves just like C<".."> does.
+
+The right operand is not evaluated while the operator is in the
+"false" state, and the left operand is not evaluated while the
+operator is in the "true" state. The precedence is a little lower
+than || and &&. The value returned is either the empty string for
+false, or a sequence number (beginning with 1) for true. The sequence
+number is reset for each range encountered. The final sequence number
+in a range has the string C<"E0"> appended to it, which doesn't affect
+its numeric value, but gives you something to search for if you want
+to exclude the endpoint. You can exclude the beginning point by
+waiting for the sequence number to be greater than 1.
+
+If either operand of scalar C<".."> is a constant expression,
+that operand is considered true if it is equal (C<==>) to the current
+input line number (the C<$.> variable).
+
+To be pedantic, the comparison is actually S<C<int(EXPR) == int(EXPR)>>,
+but that is only an issue if you use a floating point expression; when
+implicitly using C<$.> as described in the previous paragraph, the
+comparison is S<C<int(EXPR) == int($.)>> which is only an issue when C<$.>
+is set to a floating point value and you are not reading from a file.
+Furthermore, S<C<"span" .. "spat">> or S<C<2.18 .. 3.14>> will not do what
+you want in scalar context because each of the operands are evaluated
+using their integer representation.
+
+Examples:
+
+As a scalar operator:
+
+ if (101 .. 200) { print; } # print 2nd hundred lines, short for
+ # if ($. == 101 .. $. == 200) { print; }
+
+ next LINE if (1 .. /^$/); # skip header lines, short for
+ # next LINE if ($. == 1 .. /^$/);
+ # (typically in a loop labeled LINE)
+
+ s/^/> / if (/^$/ .. eof()); # quote body
+
+ # parse mail messages
+ while (<>) {
+ $in_header = 1 .. /^$/;
+ $in_body = /^$/ .. eof;
+ if ($in_header) {
+ # do something
+ } else { # in body
+ # do something else
+ }
+ } continue {
+ close ARGV if eof; # reset $. each file
+ }
+
+Here's a simple example to illustrate the difference between
+the two range operators:
+
+ @lines = (" - Foo",
+ "01 - Bar",
+ "1 - Baz",
+ " - Quux");
+
+ foreach (@lines) {
+ if (/0/ .. /1/) {
+ print "$_\n";
+ }
+ }
+
+This program will print only the line containing "Bar". If
+the range operator is changed to C<...>, it will also print the
+"Baz" line.
+
+And now some examples as a list operator:
+
+ for (101 .. 200) { print } # print $_ 100 times
+ @foo = @foo[0 .. $#foo]; # an expensive no-op
+ @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
+
+The range operator (in list context) makes use of the magical
+auto-increment algorithm if the operands are strings. You
+can say
+
+ @alphabet = ("A" .. "Z");
+
+to get all normal letters of the English alphabet, or
+
+ $hexdigit = (0 .. 9, "a" .. "f")[$num & 15];
+
+to get a hexadecimal digit, or
+
+ @z2 = ("01" .. "31");
+ print $z2[$mday];
+
+to get dates with leading zeros.
+
+If the final value specified is not in the sequence that the magical
+increment would produce, the sequence goes until the next value would
+be longer than the final value specified.
+
+As of Perl 5.26, the list-context range operator on strings works as expected
+in the scope of L<< S<C<"use feature 'unicode_strings">>|feature/The
+'unicode_strings' feature >>. In previous versions, and outside the scope of
+that feature, it exhibits L<perlunicode/The "Unicode Bug">: its behavior
+depends on the internal encoding of the range endpoint.
+
+If the initial value specified isn't part of a magical increment
+sequence (that is, a non-empty string matching C</^[a-zA-Z]*[0-9]*\z/>),
+only the initial value will be returned. So the following will only
+return an alpha:
+
+ use charnames "greek";
+ my @greek_small = ("\N{alpha}" .. "\N{omega}");
+
+To get the 25 traditional lowercase Greek letters, including both sigmas,
+you could use this instead:
+
+ use charnames "greek";
+ my @greek_small = map { chr } ( ord("\N{alpha}")
+ ..
+ ord("\N{omega}")
+ );
+
+However, because there are I<many> other lowercase Greek characters than
+just those, to match lowercase Greek characters in a regular expression,
+you could use the pattern C</(?:(?=\p{Greek})\p{Lower})+/> (or the
+L<experimental feature|perlrecharclass/Extended Bracketed Character
+Classes> C<S</(?[ \p{Greek} & \p{Lower} ])+/>>).
+
+Because each operand is evaluated in integer form, S<C<2.18 .. 3.14>> will
+return two elements in list context.
+
+ @list = (2.18 .. 3.14); # same as @list = (2 .. 3);
+
+=head2 Conditional Operator
+X<operator, conditional> X<operator, ternary> X<ternary> X<?:>
+
+Ternary C<"?:"> is the conditional operator, just as in C. It works much
+like an if-then-else. If the argument before the C<?> is true, the
+argument before the C<:> is returned, otherwise the argument after the
+C<:> is returned. For example:
+
+ printf "I have %d dog%s.\n", $n,
+ ($n == 1) ? "" : "s";
+
+Scalar or list context propagates downward into the 2nd
+or 3rd argument, whichever is selected.
+
+ $x = $ok ? $y : $z; # get a scalar
+ @x = $ok ? @y : @z; # get an array
+ $x = $ok ? @y : @z; # oops, that's just a count!
+
+The operator may be assigned to if both the 2nd and 3rd arguments are
+legal lvalues (meaning that you can assign to them):
+
+ ($x_or_y ? $x : $y) = $z;
+
+Because this operator produces an assignable result, using assignments
+without parentheses will get you in trouble. For example, this:
+
+ $x % 2 ? $x += 10 : $x += 2
+
+Really means this:
+
+ (($x % 2) ? ($x += 10) : $x) += 2
+
+Rather than this:
+
+ ($x % 2) ? ($x += 10) : ($x += 2)
+
+That should probably be written more simply as:
+
+ $x += ($x % 2) ? 10 : 2;
+
+=head2 Assignment Operators
+X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=>
+X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=>
+X<%=> X<^=> X<x=> X<&.=> X<|.=> X<^.=>
+
+C<"="> is the ordinary assignment operator.
+
+Assignment operators work as in C. That is,
+
+ $x += 2;
+
+is equivalent to
+
+ $x = $x + 2;
+
+although without duplicating any side effects that dereferencing the lvalue
+might trigger, such as from C<tie()>. Other assignment operators work similarly.
+The following are recognized:
+
+ **= += *= &= &.= <<= &&=
+ -= /= |= |.= >>= ||=
+ .= %= ^= ^.= //=
+ x=
+
+Although these are grouped by family, they all have the precedence
+of assignment. These combined assignment operators can only operate on
+scalars, whereas the ordinary assignment operator can assign to arrays,
+hashes, lists and even references. (See L<"Context"|perldata/Context>
+and L<perldata/List value constructors>, and L<perlref/Assigning to
+References>.)
+
+Unlike in C, the scalar assignment operator produces a valid lvalue.
+Modifying an assignment is equivalent to doing the assignment and
+then modifying the variable that was assigned to. This is useful
+for modifying a copy of something, like this:
+
+ ($tmp = $global) =~ tr/13579/24680/;
+
+Although as of 5.14, that can be also be accomplished this way:
+
+ use v5.14;
+ $tmp = ($global =~ tr/13579/24680/r);
+
+Likewise,
+
+ ($x += 2) *= 3;
+
+is equivalent to
+
+ $x += 2;
+ $x *= 3;
+
+Similarly, a list assignment in list context produces the list of
+lvalues assigned to, and a list assignment in scalar context returns
+the number of elements produced by the expression on the right hand
+side of the assignment.
+
+The three dotted bitwise assignment operators (C<&.=> C<|.=> C<^.=>) are new in
+Perl 5.22. See L</Bitwise String Operators>.
+
+=head2 Comma Operator
+X<comma> X<operator, comma> X<,>
+
+Binary C<","> is the comma operator. In scalar context it evaluates
+its left argument, throws that value away, then evaluates its right
+argument and returns that value. This is just like C's comma operator.
+
+In list context, it's just the list argument separator, and inserts
+both its arguments into the list. These arguments are also evaluated
+from left to right.
+
+The C<< => >> operator (sometimes pronounced "fat comma") is a synonym
+for the comma except that it causes a
+word on its left to be interpreted as a string if it begins with a letter
+or underscore and is composed only of letters, digits and underscores.
+This includes operands that might otherwise be interpreted as operators,
+constants, single number v-strings or function calls. If in doubt about
+this behavior, the left operand can be quoted explicitly.
+
+Otherwise, the C<< => >> operator behaves exactly as the comma operator
+or list argument separator, according to context.
+
+For example:
+
+ use constant FOO => "something";
+
+ my %h = ( FOO => 23 );
+
+is equivalent to:
+
+ my %h = ("FOO", 23);
+
+It is I<NOT>:
+
+ my %h = ("something", 23);
+
+The C<< => >> operator is helpful in documenting the correspondence
+between keys and values in hashes, and other paired elements in lists.
+
+ %hash = ( $key => $value );
+ login( $username => $password );
+
+The special quoting behavior ignores precedence, and hence may apply to
+I<part> of the left operand:
+
+ print time.shift => "bbb";
+
+That example prints something like C<"1314363215shiftbbb">, because the
+C<< => >> implicitly quotes the C<shift> immediately on its left, ignoring
+the fact that C<time.shift> is the entire left operand.
+
+=head2 List Operators (Rightward)
+X<operator, list, rightward> X<list operator>
+
+On the right side of a list operator, the comma has very low precedence,
+such that it controls all comma-separated expressions found there.
+The only operators with lower precedence are the logical operators
+C<"and">, C<"or">, and C<"not">, which may be used to evaluate calls to list
+operators without the need for parentheses:
+
+ open HANDLE, "< :encoding(UTF-8)", "filename"
+ or die "Can't open: $!\n";
+
+However, some people find that code harder to read than writing
+it with parentheses:
+
+ open(HANDLE, "< :encoding(UTF-8)", "filename")
+ or die "Can't open: $!\n";
+
+in which case you might as well just use the more customary C<"||"> operator:
+
+ open(HANDLE, "< :encoding(UTF-8)", "filename")
+ || die "Can't open: $!\n";
+
+See also discussion of list operators in L</Terms and List Operators (Leftward)>.
+
+=head2 Logical Not
+X<operator, logical, not> X<not>
+
+Unary C<"not"> returns the logical negation of the expression to its right.
+It's the equivalent of C<"!"> except for the very low precedence.
+
+=head2 Logical And
+X<operator, logical, and> X<and>
+
+Binary C<"and"> returns the logical conjunction of the two surrounding
+expressions. It's equivalent to C<&&> except for the very low
+precedence. This means that it short-circuits: the right
+expression is evaluated only if the left expression is true.
+
+=head2 Logical or and Exclusive Or
+X<operator, logical, or> X<operator, logical, xor>
+X<operator, logical, exclusive or>
+X<or> X<xor>
+
+Binary C<"or"> returns the logical disjunction of the two surrounding
+expressions. It's equivalent to C<||> except for the very low precedence.
+This makes it useful for control flow:
+
+ print FH $data or die "Can't write to FH: $!";
+
+This means that it short-circuits: the right expression is evaluated
+only if the left expression is false. Due to its precedence, you must
+be careful to avoid using it as replacement for the C<||> operator.
+It usually works out better for flow control than in assignments:
+
+ $x = $y or $z; # bug: this is wrong
+ ($x = $y) or $z; # really means this
+ $x = $y || $z; # better written this way
+
+However, when it's a list-context assignment and you're trying to use
+C<||> for control flow, you probably need C<"or"> so that the assignment
+takes higher precedence.
+
+ @info = stat($file) || die; # oops, scalar sense of stat!
+ @info = stat($file) or die; # better, now @info gets its due
+
+Then again, you could always use parentheses.
+
+Binary C<"xor"> returns the exclusive-OR of the two surrounding expressions.
+It cannot short-circuit (of course).
+
+There is no low precedence operator for defined-OR.
+
+=head2 C Operators Missing From Perl
+X<operator, missing from perl> X<&> X<*>
+X<typecasting> X<(TYPE)>
+
+Here is what C has that Perl doesn't:
+
+=over 8
+
+=item unary &
+
+Address-of operator. (But see the C<"\"> operator for taking a reference.)
+
+=item unary *
+
+Dereference-address operator. (Perl's prefix dereferencing
+operators are typed: C<$>, C<@>, C<%>, and C<&>.)
+
+=item (TYPE)
+
+Type-casting operator.
+
+=back
+
+=head2 Quote and Quote-like Operators
+X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m>
+X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>>
+X<escape sequence> X<escape>
+
+While we usually think of quotes as literal values, in Perl they
+function as operators, providing various kinds of interpolating and
+pattern matching capabilities. Perl provides customary quote characters
+for these behaviors, but also provides a way for you to choose your
+quote character for any of them. In the following table, a C<{}> represents
+any pair of delimiters you choose.
+
+ Customary Generic Meaning Interpolates
+ '' q{} Literal no
+ "" qq{} Literal yes
+ `` qx{} Command yes*
+ qw{} Word list no
+ // m{} Pattern match yes*
+ qr{} Pattern yes*
+ s{}{} Substitution yes*
+ tr{}{} Transliteration no (but see below)
+ y{}{} Transliteration no (but see below)
+ <<EOF here-doc yes*
+
+ * unless the delimiter is ''.
+
+Non-bracketing delimiters use the same character fore and aft, but the four
+sorts of ASCII brackets (round, angle, square, curly) all nest, which means
+that
+
+ q{foo{bar}baz}
+
+is the same as
+
+ 'foo{bar}baz'
+
+Note, however, that this does not always work for quoting Perl code:
+
+ $s = q{ if($x eq "}") ... }; # WRONG
+
+is a syntax error. The C<L<Text::Balanced>> module (standard as of v5.8,
+and from CPAN before then) is able to do this properly.
+
+There can (and in some cases, must) be whitespace between the operator
+and the quoting
+characters, except when C<#> is being used as the quoting character.
+C<q#foo#> is parsed as the string C<foo>, while S<C<q #foo#>> is the
+operator C<q> followed by a comment. Its argument will be taken
+from the next line. This allows you to write:
+
+ s {foo} # Replace foo
+ {bar} # with bar.
+
+The cases where whitespace must be used are when the quoting character
+is a word character (meaning it matches C</\w/>):
+
+ q XfooX # Works: means the string 'foo'
+ qXfooX # WRONG!
+
+The following escape sequences are available in constructs that interpolate,
+and in transliterations whose delimiters aren't single quotes (C<"'">).
+X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N> X<\N{}>
+X<\o{}>
+
+ Sequence Note Description
+ \t tab (HT, TAB)
+ \n newline (NL)
+ \r return (CR)
+ \f form feed (FF)
+ \b backspace (BS)
+ \a alarm (bell) (BEL)
+ \e escape (ESC)
+ \x{263A} [1,8] hex char (example: SMILEY)
+ \x1b [2,8] restricted range hex char (example: ESC)
+ \N{name} [3] named Unicode character or character sequence
+ \N{U+263D} [4,8] Unicode character (example: FIRST QUARTER MOON)
+ \c[ [5] control char (example: chr(27))
+ \o{23072} [6,8] octal char (example: SMILEY)
+ \033 [7,8] restricted range octal char (example: ESC)
+
+=over 4
+
+=item [1]
+
+The result is the character specified by the hexadecimal number between
+the braces. See L</[8]> below for details on which character.
+
+Only hexadecimal digits are valid between the braces. If an invalid
+character is encountered, a warning will be issued and the invalid
+character and all subsequent characters (valid or invalid) within the
+braces will be discarded.
+
+If there are no valid digits between the braces, the generated character is
+the NULL character (C<\x{00}>). However, an explicit empty brace (C<\x{}>)
+will not cause a warning (currently).
+
+=item [2]
+
+The result is the character specified by the hexadecimal number in the range
+0x00 to 0xFF. See L</[8]> below for details on which character.
+
+Only hexadecimal digits are valid following C<\x>. When C<\x> is followed
+by fewer than two valid digits, any valid digits will be zero-padded. This
+means that C<\x7> will be interpreted as C<\x07>, and a lone C<"\x"> will be
+interpreted as C<\x00>. Except at the end of a string, having fewer than
+two valid digits will result in a warning. Note that although the warning
+says the illegal character is ignored, it is only ignored as part of the
+escape and will still be used as the subsequent character in the string.
+For example:
+
+ Original Result Warns?
+ "\x7" "\x07" no
+ "\x" "\x00" no
+ "\x7q" "\x07q" yes
+ "\xq" "\x00q" yes
+
+=item [3]
+
+The result is the Unicode character or character sequence given by I<name>.
+See L<charnames>.
+
+=item [4]
+
+S<C<\N{U+I<hexadecimal number>}>> means the Unicode character whose Unicode code
+point is I<hexadecimal number>.
+
+=item [5]
+
+The character following C<\c> is mapped to some other character as shown in the
+table:
+
+ Sequence Value
+ \c@ chr(0)
+ \cA chr(1)
+ \ca chr(1)
+ \cB chr(2)
+ \cb chr(2)
+ ...
+ \cZ chr(26)
+ \cz chr(26)
+ \c[ chr(27)
+ # See below for chr(28)
+ \c] chr(29)
+ \c^ chr(30)
+ \c_ chr(31)
+ \c? chr(127) # (on ASCII platforms; see below for link to
+ # EBCDIC discussion)
+
+In other words, it's the character whose code point has had 64 xor'd with
+its uppercase. C<\c?> is DELETE on ASCII platforms because
+S<C<ord("?") ^ 64>> is 127, and
+C<\c@> is NULL because the ord of C<"@"> is 64, so xor'ing 64 itself produces 0.
+
+Also, C<\c\I<X>> yields S<C< chr(28) . "I<X>">> for any I<X>, but cannot come at the
+end of a string, because the backslash would be parsed as escaping the end
+quote.
+
+On ASCII platforms, the resulting characters from the list above are the
+complete set of ASCII controls. This isn't the case on EBCDIC platforms; see
+L<perlebcdic/OPERATOR DIFFERENCES> for a full discussion of the
+differences between these for ASCII versus EBCDIC platforms.
+
+Use of any other character following the C<"c"> besides those listed above is
+discouraged, and as of Perl v5.20, the only characters actually allowed
+are the printable ASCII ones, minus the left brace C<"{">. What happens
+for any of the allowed other characters is that the value is derived by
+xor'ing with the seventh bit, which is 64, and a warning raised if
+enabled. Using the non-allowed characters generates a fatal error.
+
+To get platform independent controls, you can use C<\N{...}>.
+
+=item [6]
+
+The result is the character specified by the octal number between the braces.
+See L</[8]> below for details on which character.
+
+If a character that isn't an octal digit is encountered, a warning is raised,
+and the value is based on the octal digits before it, discarding it and all
+following characters up to the closing brace. It is a fatal error if there are
+no octal digits at all.
+
+=item [7]
+
+The result is the character specified by the three-digit octal number in the
+range 000 to 777 (but best to not use above 077, see next paragraph). See
+L</[8]> below for details on which character.
+
+Some contexts allow 2 or even 1 digit, but any usage without exactly
+three digits, the first being a zero, may give unintended results. (For
+example, in a regular expression it may be confused with a backreference;
+see L<perlrebackslash/Octal escapes>.) Starting in Perl 5.14, you may
+use C<\o{}> instead, which avoids all these problems. Otherwise, it is best to
+use this construct only for ordinals C<\077> and below, remembering to pad to
+the left with zeros to make three digits. For larger ordinals, either use
+C<\o{}>, or convert to something else, such as to hex and use C<\N{U+}>
+(which is portable between platforms with different character sets) or
+C<\x{}> instead.
+
+=item [8]
+
+Several constructs above specify a character by a number. That number
+gives the character's position in the character set encoding (indexed from 0).
+This is called synonymously its ordinal, code position, or code point. Perl
+works on platforms that have a native encoding currently of either ASCII/Latin1
+or EBCDIC, each of which allow specification of 256 characters. In general, if
+the number is 255 (0xFF, 0377) or below, Perl interprets this in the platform's
+native encoding. If the number is 256 (0x100, 0400) or above, Perl interprets
+it as a Unicode code point and the result is the corresponding Unicode
+character. For example C<\x{50}> and C<\o{120}> both are the number 80 in
+decimal, which is less than 256, so the number is interpreted in the native
+character set encoding. In ASCII the character in the 80th position (indexed
+from 0) is the letter C<"P">, and in EBCDIC it is the ampersand symbol C<"&">.
+C<\x{100}> and C<\o{400}> are both 256 in decimal, so the number is interpreted
+as a Unicode code point no matter what the native encoding is. The name of the
+character in the 256th position (indexed by 0) in Unicode is
+C<LATIN CAPITAL LETTER A WITH MACRON>.
+
+An exception to the above rule is that S<C<\N{U+I<hex number>}>> is
+always interpreted as a Unicode code point, so that C<\N{U+0050}> is C<"P"> even
+on EBCDIC platforms.
+
+=back
+
+B<NOTE>: Unlike C and other languages, Perl has no C<\v> escape sequence for
+the vertical tab (VT, which is 11 in both ASCII and EBCDIC), but you may
+use C<\N{VT}>, C<\ck>, C<\N{U+0b}>, or C<\x0b>. (C<\v>
+does have meaning in regular expression patterns in Perl, see L<perlre>.)
+
+The following escape sequences are available in constructs that interpolate,
+but not in transliterations.
+X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q> X<\F>
+
+ \l lowercase next character only
+ \u titlecase (not uppercase!) next character only
+ \L lowercase all characters till \E or end of string
+ \U uppercase all characters till \E or end of string
+ \F foldcase all characters till \E or end of string
+ \Q quote (disable) pattern metacharacters till \E or
+ end of string
+ \E end either case modification or quoted section
+ (whichever was last seen)
+
+See L<perlfunc/quotemeta> for the exact definition of characters that
+are quoted by C<\Q>.
+
+C<\L>, C<\U>, C<\F>, and C<\Q> can stack, in which case you need one
+C<\E> for each. For example:
+
+ say"This \Qquoting \ubusiness \Uhere isn't quite\E done yet,\E is it?";
+ This quoting\ Business\ HERE\ ISN\'T\ QUITE\ done\ yet\, is it?
+
+If a S<C<use locale>> form that includes C<LC_CTYPE> is in effect (see
+L<perllocale>), the case map used by C<\l>, C<\L>, C<\u>, and C<\U> is
+taken from the current locale. If Unicode (for example, C<\N{}> or code
+points of 0x100 or beyond) is being used, the case map used by C<\l>,
+C<\L>, C<\u>, and C<\U> is as defined by Unicode. That means that
+case-mapping a single character can sometimes produce a sequence of
+several characters.
+Under S<C<use locale>>, C<\F> produces the same results as C<\L>
+for all locales but a UTF-8 one, where it instead uses the Unicode
+definition.
+
+All systems use the virtual C<"\n"> to represent a line terminator,
+called a "newline". There is no such thing as an unvarying, physical
+newline character. It is only an illusion that the operating system,
+device drivers, C libraries, and Perl all conspire to preserve. Not all
+systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
+on the ancient Macs (pre-MacOS X) of yesteryear, these used to be reversed,
+and on systems without a line terminator,
+printing C<"\n"> might emit no actual data. In general, use C<"\n"> when
+you mean a "newline" for your system, but use the literal ASCII when you
+need an exact character. For example, most networking protocols expect
+and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
+and although they often accept just C<"\012">, they seldom tolerate just
+C<"\015">. If you get in the habit of using C<"\n"> for networking,
+you may be burned some day.
+X<newline> X<line terminator> X<eol> X<end of line>
+X<\n> X<\r> X<\r\n>
+
+For constructs that do interpolate, variables beginning with "C<$>"
+or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
+C<< $href->{key}[0] >> are also interpolated, as are array and hash slices.
+But method calls such as C<< $obj->meth >> are not.
+
+Interpolating an array or slice interpolates the elements in order,
+separated by the value of C<$">, so is equivalent to interpolating
+S<C<join $", @array>>. "Punctuation" arrays such as C<@*> are usually
+interpolated only if the name is enclosed in braces C<@{*}>, but the
+arrays C<@_>, C<@+>, and C<@-> are interpolated even without braces.
+
+For double-quoted strings, the quoting from C<\Q> is applied after
+interpolation and escapes are processed.
+
+ "abc\Qfoo\tbar$s\Exyz"
+
+is equivalent to
+
+ "abc" . quotemeta("foo\tbar$s") . "xyz"
+
+For the pattern of regex operators (C<qr//>, C<m//> and C<s///>),
+the quoting from C<\Q> is applied after interpolation is processed,
+but before escapes are processed. This allows the pattern to match
+literally (except for C<$> and C<@>). For example, the following matches:
+
+ '\s\t' =~ /\Q\s\t/
+
+Because C<$> or C<@> trigger interpolation, you'll need to use something
+like C</\Quser\E\@\Qhost/> to match them literally.
+
+Patterns are subject to an additional level of interpretation as a
+regular expression. This is done as a second pass, after variables are
+interpolated, so that regular expressions may be incorporated into the
+pattern from the variables. If this is not what you want, use C<\Q> to
+interpolate a variable literally.
+
+Apart from the behavior described above, Perl does not expand
+multiple levels of interpolation. In particular, contrary to the
+expectations of shell programmers, back-quotes do I<NOT> interpolate
+within double quotes, nor do single quotes impede evaluation of
+variables when used within double quotes.
+
+=head2 Regexp Quote-Like Operators
+X<operator, regexp>
+
+Here are the quote-like operators that apply to pattern
+matching and related activities.
+
+=over 8
+
+=item C<qr/I<STRING>/msixpodualn>
+X<qr> X</i> X</m> X</o> X</s> X</x> X</p>
+
+This operator quotes (and possibly compiles) its I<STRING> as a regular
+expression. I<STRING> is interpolated the same way as I<PATTERN>
+in C<m/I<PATTERN>/>. If C<"'"> is used as the delimiter, no variable
+interpolation is done. Returns a Perl value which may be used instead of the
+corresponding C</I<STRING>/msixpodualn> expression. The returned value is a
+normalized version of the original pattern. It magically differs from
+a string containing the same characters: C<ref(qr/x/)> returns "Regexp";
+however, dereferencing it is not well defined (you currently get the
+normalized version of the original pattern, but this may change).
+
+
+For example,
+
+ $rex = qr/my.STRING/is;
+ print $rex; # prints (?si-xm:my.STRING)
+ s/$rex/foo/;
+
+is equivalent to
+
+ s/my.STRING/foo/is;
+
+The result may be used as a subpattern in a match:
+
+ $re = qr/$pattern/;
+ $string =~ /foo${re}bar/; # can be interpolated in other
+ # patterns
+ $string =~ $re; # or used standalone
+ $string =~ /$re/; # or this way
+
+Since Perl may compile the pattern at the moment of execution of the C<qr()>
+operator, using C<qr()> may have speed advantages in some situations,
+notably if the result of C<qr()> is used standalone:
+
+ sub match {
+ my $patterns = shift;
+ my @compiled = map qr/$_/i, @$patterns;
+ grep {
+ my $success = 0;
+ foreach my $pat (@compiled) {
+ $success = 1, last if /$pat/;
+ }
+ $success;
+ } @_;
+ }
+
+Precompilation of the pattern into an internal representation at
+the moment of C<qr()> avoids the need to recompile the pattern every
+time a match C</$pat/> is attempted. (Perl has many other internal
+optimizations, but none would be triggered in the above example if
+we did not use C<qr()> operator.)
+
+Options (specified by the following modifiers) are:
+
+ m Treat string as multiple lines.
+ s Treat string as single line. (Make . match a newline)
+ i Do case-insensitive pattern matching.
+ x Use extended regular expressions; specifying two
+ x's means \t and the SPACE character are ignored within
+ square-bracketed character classes
+ p When matching preserve a copy of the matched string so
+ that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be
+ defined (ignored starting in v5.20) as these are always
+ defined starting in that release
+ o Compile pattern only once.
+ a ASCII-restrict: Use ASCII for \d, \s, \w and [[:posix:]]
+ character classes; specifying two a's adds the further
+ restriction that no ASCII character will match a
+ non-ASCII one under /i.
+ l Use the current run-time locale's rules.
+ u Use Unicode rules.
+ d Use Unicode or native charset, as in 5.12 and earlier.
+ n Non-capture mode. Don't let () fill in $1, $2, etc...
+
+If a precompiled pattern is embedded in a larger pattern then the effect
+of C<"msixpluadn"> will be propagated appropriately. The effect that the
+C</o> modifier has is not propagated, being restricted to those patterns
+explicitly using it.
+
+The C</a>, C</d>, C</l>, and C</u> modifiers (added in Perl 5.14)
+control the character set rules, but C</a> is the only one you are likely
+to want to specify explicitly; the other three are selected
+automatically by various pragmas.
+
+See L<perlre> for additional information on valid syntax for I<STRING>, and
+for a detailed look at the semantics of regular expressions. In
+particular, all modifiers except the largely obsolete C</o> are further
+explained in L<perlre/Modifiers>. C</o> is described in the next section.
+
+=item C<m/I<PATTERN>/msixpodualngc>
+X<m> X<operator, match>
+X<regexp, options> X<regexp> X<regex, options> X<regex>
+X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c>
+
+=item C</I<PATTERN>/msixpodualngc>
+
+Searches a string for a pattern match, and in scalar context returns
+true if it succeeds, false if it fails. If no string is specified
+via the C<=~> or C<!~> operator, the C<$_> string is searched. (The
+string specified with C<=~> need not be an lvalue--it may be the
+result of an expression evaluation, but remember the C<=~> binds
+rather tightly.) See also L<perlre>.
+
+Options are as described in C<qr//> above; in addition, the following match
+process modifiers are available:
+
+ g Match globally, i.e., find all occurrences.
+ c Do not reset search position on a failed match when /g is
+ in effect.
+
+If C<"/"> is the delimiter then the initial C<m> is optional. With the C<m>
+you can use any pair of non-whitespace (ASCII) characters
+as delimiters. This is particularly useful for matching path names
+that contain C<"/">, to avoid LTS (leaning toothpick syndrome). If C<"?"> is
+the delimiter, then a match-only-once rule applies,
+described in C<m?I<PATTERN>?> below. If C<"'"> (single quote) is the delimiter,
+no variable interpolation is performed on the I<PATTERN>.
+When using a delimiter character valid in an identifier, whitespace is required
+after the C<m>.
+
+I<PATTERN> may contain variables, which will be interpolated
+every time the pattern search is evaluated, except
+for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
+C<$|> are not interpolated because they look like end-of-string tests.)
+Perl will not recompile the pattern unless an interpolated
+variable that it contains changes. You can force Perl to skip the
+test and never recompile by adding a C</o> (which stands for "once")
+after the trailing delimiter.
+Once upon a time, Perl would recompile regular expressions
+unnecessarily, and this modifier was useful to tell it not to do so, in the
+interests of speed. But now, the only reasons to use C</o> are one of:
+
+=over
+
+=item 1
+
+The variables are thousands of characters long and you know that they
+don't change, and you need to wring out the last little bit of speed by
+having Perl skip testing for that. (There is a maintenance penalty for
+doing this, as mentioning C</o> constitutes a promise that you won't
+change the variables in the pattern. If you do change them, Perl won't
+even notice.)
+
+=item 2
+
+you want the pattern to use the initial values of the variables
+regardless of whether they change or not. (But there are saner ways
+of accomplishing this than using C</o>.)
+
+=item 3
+
+If the pattern contains embedded code, such as
+
+ use re 'eval';
+ $code = 'foo(?{ $x })';
+ /$code/
+
+then perl will recompile each time, even though the pattern string hasn't
+changed, to ensure that the current value of C<$x> is seen each time.
+Use C</o> if you want to avoid this.
+
+=back
+
+The bottom line is that using C</o> is almost never a good idea.
+
+=item The empty pattern C<//>
+
+If the I<PATTERN> evaluates to the empty string, the last
+I<successfully> matched regular expression is used instead. In this
+case, only the C<g> and C<c> flags on the empty pattern are honored;
+the other flags are taken from the original pattern. If no match has
+previously succeeded, this will (silently) act instead as a genuine
+empty pattern (which will always match).
+
+Note that it's possible to confuse Perl into thinking C<//> (the empty
+regex) is really C<//> (the defined-or operator). Perl is usually pretty
+good about this, but some pathological cases might trigger this, such as
+C<$x///> (is that S<C<($x) / (//)>> or S<C<$x // />>?) and S<C<print $fh //>>
+(S<C<print $fh(//>> or S<C<print($fh //>>?). In all of these examples, Perl
+will assume you meant defined-or. If you meant the empty regex, just
+use parentheses or spaces to disambiguate, or even prefix the empty
+regex with an C<m> (so C<//> becomes C<m//>).
+
+=item Matching in list context
+
+If the C</g> option is not used, C<m//> in list context returns a
+list consisting of the subexpressions matched by the parentheses in the
+pattern, that is, (C<$1>, C<$2>, C<$3>...) (Note that here C<$1> etc. are
+also set). When there are no parentheses in the pattern, the return
+value is the list C<(1)> for success.
+With or without parentheses, an empty list is returned upon failure.
+
+Examples:
+
+ open(TTY, "+</dev/tty")
+ || die "can't access /dev/tty: $!";
+
+ <TTY> =~ /^y/i && foo(); # do foo if desired
+
+ if (/Version: *([0-9.]*)/) { $version = $1; }
+
+ next if m#^/usr/spool/uucp#;
+
+ # poor man's grep
+ $arg = shift;
+ while (<>) {
+ print if /$arg/o; # compile only once (no longer needed!)
+ }
+
+ if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
+
+This last example splits C<$foo> into the first two words and the
+remainder of the line, and assigns those three fields to C<$F1>, C<$F2>, and
+C<$Etc>. The conditional is true if any variables were assigned; that is,
+if the pattern matched.
+
+The C</g> modifier specifies global pattern matching--that is,
+matching as many times as possible within the string. How it behaves
+depends on the context. In list context, it returns a list of the
+substrings matched by any capturing parentheses in the regular
+expression. If there are no parentheses, it returns a list of all
+the matched strings, as if there were parentheses around the whole
+pattern.
+
+In scalar context, each execution of C<m//g> finds the next match,
+returning true if it matches, and false if there is no further match.
+The position after the last match can be read or set using the C<pos()>
+function; see L<perlfunc/pos>. A failed match normally resets the
+search position to the beginning of the string, but you can avoid that
+by adding the C</c> modifier (for example, C<m//gc>). Modifying the target
+string also resets the search position.
+
+=item C<\G I<assertion>>
+
+You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
+zero-width assertion that matches the exact position where the
+previous C<m//g>, if any, left off. Without the C</g> modifier, the
+C<\G> assertion still anchors at C<pos()> as it was at the start of
+the operation (see L<perlfunc/pos>), but the match is of course only
+attempted once. Using C<\G> without C</g> on a target string that has
+not previously had a C</g> match applied to it is the same as using
+the C<\A> assertion to match the beginning of the string. Note also
+that, currently, C<\G> is only properly supported when anchored at the
+very beginning of the pattern.
+
+Examples:
+
+ # list context
+ ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
+
+ # scalar context
+ local $/ = "";
+ while ($paragraph = <>) {
+ while ($paragraph =~ /\p{Ll}['")]*[.!?]+['")]*\s/g) {
+ $sentences++;
+ }
+ }
+ say $sentences;
+
+Here's another way to check for sentences in a paragraph:
+
+ my $sentence_rx = qr{
+ (?: (?<= ^ ) | (?<= \s ) ) # after start-of-string or
+ # whitespace
+ \p{Lu} # capital letter
+ .*? # a bunch of anything
+ (?<= \S ) # that ends in non-
+ # whitespace
+ (?<! \b [DMS]r ) # but isn't a common abbr.
+ (?<! \b Mrs )
+ (?<! \b Sra )
+ (?<! \b St )
+ [.?!] # followed by a sentence
+ # ender
+ (?= $ | \s ) # in front of end-of-string
+ # or whitespace
+ }sx;
+ local $/ = "";
+ while (my $paragraph = <>) {
+ say "NEW PARAGRAPH";
+ my $count = 0;
+ while ($paragraph =~ /($sentence_rx)/g) {
+ printf "\tgot sentence %d: <%s>\n", ++$count, $1;
+ }
+ }
+
+Here's how to use C<m//gc> with C<\G>:
+
+ $_ = "ppooqppqq";
+ while ($i++ < 2) {
+ print "1: '";
+ print $1 while /(o)/gc; print "', pos=", pos, "\n";
+ print "2: '";
+ print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
+ print "3: '";
+ print $1 while /(p)/gc; print "', pos=", pos, "\n";
+ }
+ print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
+
+The last example should print:
+
+ 1: 'oo', pos=4
+ 2: 'q', pos=5
+ 3: 'pp', pos=7
+ 1: '', pos=7
+ 2: 'q', pos=8
+ 3: '', pos=8
+ Final: 'q', pos=8
+
+Notice that the final match matched C<q> instead of C<p>, which a match
+without the C<\G> anchor would have done. Also note that the final match
+did not update C<pos>. C<pos> is only updated on a C</g> match. If the
+final match did indeed match C<p>, it's a good bet that you're running an
+ancient (pre-5.6.0) version of Perl.
+
+A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
+combine several regexps like this to process a string part-by-part,
+doing different actions depending on which regexp matched. Each
+regexp tries to match where the previous one leaves off.
+
+ $_ = <<'EOL';
+ $url = URI::URL->new( "http://example.com/" );
+ die if $url eq "xXx";
+ EOL
+
+ LOOP: {
+ print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
+ print(" lowercase"), redo LOOP
+ if /\G\p{Ll}+\b[,.;]?\s*/gc;
+ print(" UPPERCASE"), redo LOOP
+ if /\G\p{Lu}+\b[,.;]?\s*/gc;
+ print(" Capitalized"), redo LOOP
+ if /\G\p{Lu}\p{Ll}+\b[,.;]?\s*/gc;
+ print(" MiXeD"), redo LOOP if /\G\pL+\b[,.;]?\s*/gc;
+ print(" alphanumeric"), redo LOOP
+ if /\G[\p{Alpha}\pN]+\b[,.;]?\s*/gc;
+ print(" line-noise"), redo LOOP if /\G\W+/gc;
+ print ". That's all!\n";
+ }
+
+Here is the output (split into several lines):
+
+ line-noise lowercase line-noise UPPERCASE line-noise UPPERCASE
+ line-noise lowercase line-noise lowercase line-noise lowercase
+ lowercase line-noise lowercase lowercase line-noise lowercase
+ lowercase line-noise MiXeD line-noise. That's all!
+
+=item C<m?I<PATTERN>?msixpodualngc>
+X<?> X<operator, match-once>
+
+This is just like the C<m/I<PATTERN>/> search, except that it matches
+only once between calls to the C<reset()> operator. This is a useful
+optimization when you want to see only the first occurrence of
+something in each file of a set of files, for instance. Only C<m??>
+patterns local to the current package are reset.
+
+ while (<>) {
+ if (m?^$?) {
+ # blank line between header and body
+ }
+ } continue {
+ reset if eof; # clear m?? status for next file
+ }
+
+Another example switched the first "latin1" encoding it finds
+to "utf8" in a pod file:
+
+ s//utf8/ if m? ^ =encoding \h+ \K latin1 ?x;
+
+The match-once behavior is controlled by the match delimiter being
+C<?>; with any other delimiter this is the normal C<m//> operator.
+
+In the past, the leading C<m> in C<m?I<PATTERN>?> was optional, but omitting it
+would produce a deprecation warning. As of v5.22.0, omitting it produces a
+syntax error. If you encounter this construct in older code, you can just add
+C<m>.
+
+=item C<s/I<PATTERN>/I<REPLACEMENT>/msixpodualngcer>
+X<s> X<substitute> X<substitution> X<replace> X<regexp, replace>
+X<regexp, substitute> X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> X</e> X</r>
+
+Searches a string for a pattern, and if found, replaces that pattern
+with the replacement text and returns the number of substitutions
+made. Otherwise it returns false (a value that is both an empty string (C<"">)
+and numeric zero (C<0>) as described in L</Relational Operators>).
+
+If the C</r> (non-destructive) option is used then it runs the
+substitution on a copy of the string and instead of returning the
+number of substitutions, it returns the copy whether or not a
+substitution occurred. The original string is never changed when
+C</r> is used. The copy will always be a plain string, even if the
+input is an object or a tied variable.
+
+If no string is specified via the C<=~> or C<!~> operator, the C<$_>
+variable is searched and modified. Unless the C</r> option is used,
+the string specified must be a scalar variable, an array element, a
+hash element, or an assignment to one of those; that is, some sort of
+scalar lvalue.
+
+If the delimiter chosen is a single quote, no variable interpolation is
+done on either the I<PATTERN> or the I<REPLACEMENT>. Otherwise, if the
+I<PATTERN> contains a C<$> that looks like a variable rather than an
+end-of-string test, the variable will be interpolated into the pattern
+at run-time. If you want the pattern compiled only once the first time
+the variable is interpolated, use the C</o> option. If the pattern
+evaluates to the empty string, the last successfully executed regular
+expression is used instead. See L<perlre> for further explanation on these.
+
+Options are as with C<m//> with the addition of the following replacement
+specific options:
+
+ e Evaluate the right side as an expression.
+ ee Evaluate the right side as a string then eval the
+ result.
+ r Return substitution and leave the original string
+ untouched.
+
+Any non-whitespace delimiter may replace the slashes. Add space after
+the C<s> when using a character allowed in identifiers. If single quotes
+are used, no interpretation is done on the replacement string (the C</e>
+modifier overrides this, however). Note that Perl treats backticks
+as normal delimiters; the replacement text is not evaluated as a command.
+If the I<PATTERN> is delimited by bracketing quotes, the I<REPLACEMENT> has
+its own pair of quotes, which may or may not be bracketing quotes, for example,
+C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
+replacement portion to be treated as a full-fledged Perl expression
+and evaluated right then and there. It is, however, syntax checked at
+compile-time. A second C<e> modifier will cause the replacement portion
+to be C<eval>ed before being run as a Perl expression.
+
+Examples:
+
+ s/\bgreen\b/mauve/g; # don't change wintergreen
+
+ $path =~ s|/usr/bin|/usr/local/bin|;
+
+ s/Login: $foo/Login: $bar/; # run-time pattern
+
+ ($foo = $bar) =~ s/this/that/; # copy first, then
+ # change
+ ($foo = "$bar") =~ s/this/that/; # convert to string,
+ # copy, then change
+ $foo = $bar =~ s/this/that/r; # Same as above using /r
+ $foo = $bar =~ s/this/that/r
+ =~ s/that/the other/r; # Chained substitutes
+ # using /r
+ @foo = map { s/this/that/r } @bar # /r is very useful in
+ # maps
+
+ $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-cnt
+
+ $_ = 'abc123xyz';
+ s/\d+/$&*2/e; # yields 'abc246xyz'
+ s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
+ s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
+
+ s/%(.)/$percent{$1}/g; # change percent escapes; no /e
+ s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
+ s/^=(\w+)/pod($1)/ge; # use function call
+
+ $_ = 'abc123xyz';
+ $x = s/abc/def/r; # $x is 'def123xyz' and
+ # $_ remains 'abc123xyz'.
+
+ # expand variables in $_, but dynamics only, using
+ # symbolic dereferencing
+ s/\$(\w+)/${$1}/g;
+
+ # Add one to the value of any numbers in the string
+ s/(\d+)/1 + $1/eg;
+
+ # Titlecase words in the last 30 characters only
+ substr($str, -30) =~ s/\b(\p{Alpha}+)\b/\u\L$1/g;
+
+ # This will expand any embedded scalar variable
+ # (including lexicals) in $_ : First $1 is interpolated
+ # to the variable name, and then evaluated
+ s/(\$\w+)/$1/eeg;
+
+ # Delete (most) C comments.
+ $program =~ s {
+ /\* # Match the opening delimiter.
+ .*? # Match a minimal number of characters.
+ \*/ # Match the closing delimiter.
+ } []gsx;
+
+ s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_,
+ # expensively
+
+ for ($variable) { # trim whitespace in $variable,
+ # cheap
+ s/^\s+//;
+ s/\s+$//;
+ }
+
+ s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
+
+ $foo !~ s/A/a/g; # Lowercase all A's in $foo; return
+ # 0 if any were found and changed;
+ # otherwise return 1
+
+Note the use of C<$> instead of C<\> in the last example. Unlike
+B<sed>, we use the \<I<digit>> form only in the left hand side.
+Anywhere else it's $<I<digit>>.
+
+Occasionally, you can't use just a C</g> to get all the changes
+to occur that you might want. Here are two common cases:
+
+ # put commas in the right places in an integer
+ 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
+
+ # expand tabs to 8-column spacing
+ 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
+
+=back
+
+=head2 Quote-Like Operators
+X<operator, quote-like>
+
+=over 4
+
+=item C<q/I<STRING>/>
+X<q> X<quote, single> X<'> X<''>
+
+=item C<'I<STRING>'>
+
+A single-quoted, literal string. A backslash represents a backslash
+unless followed by the delimiter or another backslash, in which case
+the delimiter or backslash is interpolated.
+
+ $foo = q!I said, "You said, 'She said it.'"!;
+ $bar = q('This is it.');
+ $baz = '\n'; # a two-character string
+
+=item C<qq/I<STRING>/>
+X<qq> X<quote, double> X<"> X<"">
+
+=item "I<STRING>"
+
+A double-quoted, interpolated string.
+
+ $_ .= qq
+ (*** The previous line contains the naughty word "$1".\n)
+ if /\b(tcl|java|python)\b/i; # :-)
+ $baz = "\n"; # a one-character string
+
+=item C<qx/I<STRING>/>
+X<qx> X<`> X<``> X<backtick>
+
+=item C<`I<STRING>`>
+
+A string which is (possibly) interpolated and then executed as a
+system command with F</bin/sh> or its equivalent. Shell wildcards,
+pipes, and redirections will be honored. The collected standard
+output of the command is returned; standard error is unaffected. In
+scalar context, it comes back as a single (potentially multi-line)
+string, or C<undef> if the command failed. In list context, returns a
+list of lines (however you've defined lines with C<$/> or
+C<$INPUT_RECORD_SEPARATOR>), or an empty list if the command failed.
+
+Because backticks do not affect standard error, use shell file descriptor
+syntax (assuming the shell supports this) if you care to address this.
+To capture a command's STDERR and STDOUT together:
+
+ $output = `cmd 2>&1`;
+
+To capture a command's STDOUT but discard its STDERR:
+
+ $output = `cmd 2>/dev/null`;
+
+To capture a command's STDERR but discard its STDOUT (ordering is
+important here):
+
+ $output = `cmd 2>&1 1>/dev/null`;
+
+To exchange a command's STDOUT and STDERR in order to capture the STDERR
+but leave its STDOUT to come out the old STDERR:
+
+ $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
+
+To read both a command's STDOUT and its STDERR separately, it's easiest
+to redirect them separately to files, and then read from those files
+when the program is done:
+
+ system("program args 1>program.stdout 2>program.stderr");
+
+The STDIN filehandle used by the command is inherited from Perl's STDIN.
+For example:
+
+ open(SPLAT, "stuff") || die "can't open stuff: $!";
+ open(STDIN, "<&SPLAT") || die "can't dupe SPLAT: $!";
+ print STDOUT `sort`;
+
+will print the sorted contents of the file named F<"stuff">.
+
+Using single-quote as a delimiter protects the command from Perl's
+double-quote interpolation, passing it on to the shell instead:
+
+ $perl_info = qx(ps $$); # that's Perl's $$
+ $shell_info = qx'ps $$'; # that's the new shell's $$
+
+How that string gets evaluated is entirely subject to the command
+interpreter on your system. On most platforms, you will have to protect
+shell metacharacters if you want them treated literally. This is in
+practice difficult to do, as it's unclear how to escape which characters.
+See L<perlsec> for a clean and safe example of a manual C<fork()> and C<exec()>
+to emulate backticks safely.
+
+On some platforms (notably DOS-like ones), the shell may not be
+capable of dealing with multiline commands, so putting newlines in
+the string may not get you what you want. You may be able to evaluate
+multiple commands in a single line by separating them with the command
+separator character, if your shell supports that (for example, C<;> on
+many Unix shells and C<&> on the Windows NT C<cmd> shell).
+
+Perl will attempt to flush all files opened for
+output before starting the child process, but this may not be supported
+on some platforms (see L<perlport>). To be safe, you may need to set
+C<$|> (C<$AUTOFLUSH> in C<L<English>>) or call the C<autoflush()> method of
+C<L<IO::Handle>> on any open handles.
+
+Beware that some command shells may place restrictions on the length
+of the command line. You must ensure your strings don't exceed this
+limit after any necessary interpolations. See the platform-specific
+release notes for more details about your particular environment.
+
+Using this operator can lead to programs that are difficult to port,
+because the shell commands called vary between systems, and may in
+fact not be present at all. As one example, the C<type> command under
+the POSIX shell is very different from the C<type> command under DOS.
+That doesn't mean you should go out of your way to avoid backticks
+when they're the right way to get something done. Perl was made to be
+a glue language, and one of the things it glues together is commands.
+Just understand what you're getting yourself into.
+
+Like C<system>, backticks put the child process exit code in C<$?>.
+If you'd like to manually inspect failure, you can check all possible
+failure modes by inspecting C<$?> like this:
+
+ if ($? == -1) {
+ print "failed to execute: $!\n";
+ }
+ elsif ($? & 127) {
+ printf "child died with signal %d, %s coredump\n",
+ ($? & 127), ($? & 128) ? 'with' : 'without';
+ }
+ else {
+ printf "child exited with value %d\n", $? >> 8;
+ }
+
+Use the L<open> pragma to control the I/O layers used when reading the
+output of the command, for example:
+
+ use open IN => ":encoding(UTF-8)";
+ my $x = `cmd-producing-utf-8`;
+
+C<qx//> can also be called like a function with L<perlfunc/readpipe>.
+
+See L</"I/O Operators"> for more discussion.
+
+=item C<qw/I<STRING>/>
+X<qw> X<quote, list> X<quote, words>
+
+Evaluates to a list of the words extracted out of I<STRING>, using embedded
+whitespace as the word delimiters. It can be understood as being roughly
+equivalent to:
+
+ split(" ", q/STRING/);
+
+the differences being that it only splits on ASCII whitespace,
+generates a real list at compile time, and
+in scalar context it returns the last element in the list. So
+this expression:
+
+ qw(foo bar baz)
+
+is semantically equivalent to the list:
+
+ "foo", "bar", "baz"
+
+Some frequently seen examples:
+
+ use POSIX qw( setlocale localeconv )
+ @EXPORT = qw( foo bar baz );
+
+A common mistake is to try to separate the words with commas or to
+put comments into a multi-line C<qw>-string. For this reason, the
+S<C<use warnings>> pragma and the B<-w> switch (that is, the C<$^W> variable)
+produces warnings if the I<STRING> contains the C<","> or the C<"#"> character.
+
+=item C<tr/I<SEARCHLIST>/I<REPLACEMENTLIST>/cdsr>
+X<tr> X<y> X<transliterate> X</c> X</d> X</s>
+
+=item C<y/I<SEARCHLIST>/I<REPLACEMENTLIST>/cdsr>
+
+Transliterates all occurrences of the characters found (or not found
+if the C</c> modifier is specified) in the search list with the
+positionally corresponding character in the replacement list, possibly
+deleting some, depending on the modifiers specified. It returns the
+number of characters replaced or deleted. If no string is specified via
+the C<=~> or C<!~> operator, the C<$_> string is transliterated.
+
+For B<sed> devotees, C<y> is provided as a synonym for C<tr>.
+
+If the C</r> (non-destructive) option is present, a new copy of the string
+is made and its characters transliterated, and this copy is returned no
+matter whether it was modified or not: the original string is always
+left unchanged. The new copy is always a plain string, even if the input
+string is an object or a tied variable.
+
+Unless the C</r> option is used, the string specified with C<=~> must be a
+scalar variable, an array element, a hash element, or an assignment to one
+of those; in other words, an lvalue.
+
+If the characters delimiting I<SEARCHLIST> and I<REPLACEMENTLIST>
+are single quotes (C<tr'I<SEARCHLIST>'I<REPLACEMENTLIST>'>), the only
+interpolation is removal of C<\> from pairs of C<\\>.
+
+Otherwise, a character range may be specified with a hyphen, so
+C<tr/A-J/0-9/> does the same replacement as
+C<tr/ACEGIBDFHJ/0246813579/>.
+
+If the I<SEARCHLIST> is delimited by bracketing quotes, the
+I<REPLACEMENTLIST> must have its own pair of quotes, which may or may
+not be bracketing quotes; for example, C<tr[aeiouy][yuoiea]> or
+C<tr(+\-*/)/ABCD/>.
+
+Characters may be literals, or (if the delimiters aren't single quotes)
+any of the escape sequences accepted in double-quoted strings. But
+there is never any variable interpolation, so C<"$"> and C<"@"> are
+always treated as literals. A hyphen at the beginning or end, or
+preceded by a backslash is also always considered a literal. Escape
+sequence details are in L<the table near the beginning of this
+section|/Quote and Quote-like Operators>.
+
+Note that C<tr> does B<not> do regular expression character classes such as
+C<\d> or C<\pL>. The C<tr> operator is not equivalent to the C<L<tr(1)>>
+utility. C<tr[a-z][A-Z]> will uppercase the 26 letters "a" through "z",
+but for case changing not confined to ASCII, use
+L<C<lc>|perlfunc/lc>, L<C<uc>|perlfunc/uc>,
+L<C<lcfirst>|perlfunc/lcfirst>, L<C<ucfirst>|perlfunc/ucfirst>
+(all documented in L<perlfunc>), or the
+L<substitution operator C<sE<sol>I<PATTERN>E<sol>I<REPLACEMENT>E<sol>>|/sE<sol>PATTERNE<sol>REPLACEMENTE<sol>msixpodualngcer>
+(with C<\U>, C<\u>, C<\L>, and C<\l> string-interpolation escapes in the
+I<REPLACEMENT> portion).
+
+Most ranges are unportable between character sets, but certain ones
+signal Perl to do special handling to make them portable. There are two
+classes of portable ranges. The first are any subsets of the ranges
+C<A-Z>, C<a-z>, and C<0-9>, when expressed as literal characters.
+
+ tr/h-k/H-K/
+
+capitalizes the letters C<"h">, C<"i">, C<"j">, and C<"k"> and nothing
+else, no matter what the platform's character set is. In contrast, all
+of
+
+ tr/\x68-\x6B/\x48-\x4B/
+ tr/h-\x6B/H-\x4B/
+ tr/\x68-k/\x48-K/
+
+do the same capitalizations as the previous example when run on ASCII
+platforms, but something completely different on EBCDIC ones.
+
+The second class of portable ranges is invoked when one or both of the
+range's end points are expressed as C<\N{...}>
+
+ $string =~ tr/\N{U+20}-\N{U+7E}//d;
+
+removes from C<$string> all the platform's characters which are
+equivalent to any of Unicode U+0020, U+0021, ... U+007D, U+007E. This
+is a portable range, and has the same effect on every platform it is
+run on. In this example, these are the ASCII
+printable characters. So after this is run, C<$string> has only
+controls and characters which have no ASCII equivalents.
+
+But, even for portable ranges, it is not generally obvious what is
+included without having to look things up in the manual. A sound
+principle is to use only ranges that both begin from, and end at, either
+ASCII alphabetics of equal case (C<b-e>, C<B-E>), or digits (C<1-4>).
+Anything else is unclear (and unportable unless C<\N{...}> is used). If
+in doubt, spell out the character sets in full.
+
+Options:
+
+ c Complement the SEARCHLIST.
+ d Delete found but unreplaced characters.
+ r Return the modified string and leave the original string
+ untouched.
+ s Squash duplicate replaced characters.
+
+If the C</d> modifier is specified, any characters specified by
+I<SEARCHLIST> not found in I<REPLACEMENTLIST> are deleted. (Note that
+this is slightly more flexible than the behavior of some B<tr> programs,
+which delete anything they find in the I<SEARCHLIST>, period.)
+
+If the C</s> modifier is specified, sequences of characters, all in a
+row, that were transliterated to the same character are squashed down to
+a single instance of that character.
+
+ my $a = "aaaba"
+ $a =~ tr/a/a/s # $a now is "aba"
+
+If the C</d> modifier is used, the I<REPLACEMENTLIST> is always interpreted
+exactly as specified. Otherwise, if the I<REPLACEMENTLIST> is shorter
+than the I<SEARCHLIST>, the final character, if any, is replicated until
+it is long enough. There won't be a final character if and only if the
+I<REPLACEMENTLIST> is empty, in which case I<REPLACEMENTLIST> is
+copied from I<SEARCHLIST>. An empty I<REPLACEMENTLIST> is useful
+for counting characters in a class, or for squashing character sequences
+in a class.
+
+ tr/abcd// tr/abcd/abcd/
+ tr/abcd/AB/ tr/abcd/ABBB/
+ tr/abcd//d s/[abcd]//g
+ tr/abcd/AB/d (tr/ab/AB/ + s/[cd]//g) - but run together
+
+If the C</c> modifier is specified, the characters to be transliterated
+are the ones NOT in I<SEARCHLIST>, that is, it is complemented. If
+C</d> and/or C</s> are also specified, they apply to the complemented
+I<SEARCHLIST>. Recall, that if I<REPLACEMENTLIST> is empty (except
+under C</d>) a copy of I<SEARCHLIST> is used instead. That copy is made
+after complementing under C</c>. I<SEARCHLIST> is sorted by code point
+order after complementing, and any I<REPLACEMENTLIST> is applied to
+that sorted result. This means that under C</c>, the order of the
+characters specified in I<SEARCHLIST> is irrelevant. This can
+lead to different results on EBCDIC systems if I<REPLACEMENTLIST>
+contains more than one character, hence it is generally non-portable to
+use C</c> with such a I<REPLACEMENTLIST>.
+
+Another way of describing the operation is this:
+If C</c> is specified, the I<SEARCHLIST> is sorted by code point order,
+then complemented. If I<REPLACEMENTLIST> is empty and C</d> is not
+specified, I<REPLACEMENTLIST> is replaced by a copy of I<SEARCHLIST> (as
+modified under C</c>), and these potentially modified lists are used as
+the basis for what follows. Any character in the target string that
+isn't in I<SEARCHLIST> is passed through unchanged. Every other
+character in the target string is replaced by the character in
+I<REPLACEMENTLIST> that positionally corresponds to its mate in
+I<SEARCHLIST>, except that under C</s>, the 2nd and following characters
+are squeezed out in a sequence of characters in a row that all translate
+to the same character. If I<SEARCHLIST> is longer than
+I<REPLACEMENTLIST>, characters in the target string that match a
+character in I<SEARCHLIST> that doesn't have a correspondence in
+I<REPLACEMENTLIST> are either deleted from the target string if C</d> is
+specified; or replaced by the final character in I<REPLACEMENTLIST> if
+C</d> isn't specified.
+
+Some examples:
+
+ $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case ASCII
+
+ $cnt = tr/*/*/; # count the stars in $_
+ $cnt = tr/*//; # same thing
+
+ $cnt = $sky =~ tr/*/*/; # count the stars in $sky
+ $cnt = $sky =~ tr/*//; # same thing
+
+ $cnt = $sky =~ tr/*//c; # count all the non-stars in $sky
+ $cnt = $sky =~ tr/*/*/c; # same, but transliterate each non-star
+ # into a star, leaving the already-stars
+ # alone. Afterwards, everything in $sky
+ # is a star.
+
+ $cnt = tr/0-9//; # count the ASCII digits in $_
+
+ tr/a-zA-Z//s; # bookkeeper -> bokeper
+ tr/o/o/s; # bookkeeper -> bokkeeper
+ tr/oe/oe/s; # bookkeeper -> bokkeper
+ tr/oe//s; # bookkeeper -> bokkeper
+ tr/oe/o/s; # bookkeeper -> bokkopor
+
+ ($HOST = $host) =~ tr/a-z/A-Z/;
+ $HOST = $host =~ tr/a-z/A-Z/r; # same thing
+
+ $HOST = $host =~ tr/a-z/A-Z/r # chained with s///r
+ =~ s/:/ -p/r;
+
+ tr/a-zA-Z/ /cs; # change non-alphas to single space
+
+ @stripped = map tr/a-zA-Z/ /csr, @original;
+ # /r with map
+
+ tr [\200-\377]
+ [\000-\177]; # wickedly delete 8th bit
+
+ $foo !~ tr/A/a/ # transliterate all the A's in $foo to 'a',
+ # return 0 if any were found and changed.
+ # Otherwise return 1
+
+If multiple transliterations are given for a character, only the
+first one is used:
+
+ tr/AAA/XYZ/
+
+will transliterate any A to X.
+
+Because the transliteration table is built at compile time, neither
+the I<SEARCHLIST> nor the I<REPLACEMENTLIST> are subjected to double quote
+interpolation. That means that if you want to use variables, you
+must use an C<eval()>:
+
+ eval "tr/$oldlist/$newlist/";
+ die $@ if $@;
+
+ eval "tr/$oldlist/$newlist/, 1" or die $@;
+
+=item C<< <<I<EOF> >>
+X<here-doc> X<heredoc> X<here-document> X<<< << >>>
+
+A line-oriented form of quoting is based on the shell "here-document"
+syntax. Following a C<< << >> you specify a string to terminate
+the quoted material, and all lines following the current line down to
+the terminating string are the value of the item.
+
+Prefixing the terminating string with a C<~> specifies that you
+want to use L</Indented Here-docs> (see below).
+
+The terminating string may be either an identifier (a word), or some
+quoted text. An unquoted identifier works like double quotes.
+There may not be a space between the C<< << >> and the identifier,
+unless the identifier is explicitly quoted. The terminating string
+must appear by itself (unquoted and with no surrounding whitespace)
+on the terminating line.
+
+If the terminating string is quoted, the type of quotes used determine
+the treatment of the text.
+
+=over 4
+
+=item Double Quotes
+
+Double quotes indicate that the text will be interpolated using exactly
+the same rules as normal double quoted strings.
+
+ print <<EOF;
+ The price is $Price.
+ EOF
+
+ print << "EOF"; # same as above
+ The price is $Price.
+ EOF
+
+
+=item Single Quotes
+
+Single quotes indicate the text is to be treated literally with no
+interpolation of its content. This is similar to single quoted
+strings except that backslashes have no special meaning, with C<\\>
+being treated as two backslashes and not one as they would in every
+other quoting construct.
+
+Just as in the shell, a backslashed bareword following the C<<< << >>>
+means the same thing as a single-quoted string does:
+
+ $cost = <<'VISTA'; # hasta la ...
+ That'll be $10 please, ma'am.
+ VISTA
+
+ $cost = <<\VISTA; # Same thing!
+ That'll be $10 please, ma'am.
+ VISTA
+
+This is the only form of quoting in perl where there is no need
+to worry about escaping content, something that code generators
+can and do make good use of.
+
+=item Backticks
+
+The content of the here doc is treated just as it would be if the
+string were embedded in backticks. Thus the content is interpolated
+as though it were double quoted and then executed via the shell, with
+the results of the execution returned.
+
+ print << `EOC`; # execute command and get results
+ echo hi there
+ EOC
+
+=back
+
+=over 4
+
+=item Indented Here-docs
+
+The here-doc modifier C<~> allows you to indent your here-docs to make
+the code more readable:
+
+ if ($some_var) {
+ print <<~EOF;
+ This is a here-doc
+ EOF
+ }
+
+This will print...
+
+ This is a here-doc
+
+...with no leading whitespace.
+
+The delimiter is used to determine the B<exact> whitespace to
+remove from the beginning of each line. All lines B<must> have
+at least the same starting whitespace (except lines only
+containing a newline) or perl will croak. Tabs and spaces can
+be mixed, but are matched exactly. One tab will not be equal to
+8 spaces!
+
+Additional beginning whitespace (beyond what preceded the
+delimiter) will be preserved:
+
+ print <<~EOF;
+ This text is not indented
+ This text is indented with two spaces
+ This text is indented with two tabs
+ EOF
+
+Finally, the modifier may be used with all of the forms
+mentioned above:
+
+ <<~\EOF;
+ <<~'EOF'
+ <<~"EOF"
+ <<~`EOF`
+
+And whitespace may be used between the C<~> and quoted delimiters:
+
+ <<~ 'EOF'; # ... "EOF", `EOF`
+
+=back
+
+It is possible to stack multiple here-docs in a row:
+
+ print <<"foo", <<"bar"; # you can stack them
+ I said foo.
+ foo
+ I said bar.
+ bar
+
+ myfunc(<< "THIS", 23, <<'THAT');
+ Here's a line
+ or two.
+ THIS
+ and here's another.
+ THAT
+
+Just don't forget that you have to put a semicolon on the end
+to finish the statement, as Perl doesn't know you're not going to
+try to do this:
+
+ print <<ABC
+ 179231
+ ABC
+ + 20;
+
+If you want to remove the line terminator from your here-docs,
+use C<chomp()>.
+
+ chomp($string = <<'END');
+ This is a string.
+ END
+
+If you want your here-docs to be indented with the rest of the code,
+use the C<<< <<~FOO >>> construct described under L</Indented Here-docs>:
+
+ $quote = <<~'FINIS';
+ The Road goes ever on and on,
+ down from the door where it began.
+ FINIS
+
+If you use a here-doc within a delimited construct, such as in C<s///eg>,
+the quoted material must still come on the line following the
+C<<< <<FOO >>> marker, which means it may be inside the delimited
+construct:
+
+ s/this/<<E . 'that'
+ the other
+ E
+ . 'more '/eg;
+
+It works this way as of Perl 5.18. Historically, it was inconsistent, and
+you would have to write
+
+ s/this/<<E . 'that'
+ . 'more '/eg;
+ the other
+ E
+
+outside of string evals.
+
+Additionally, quoting rules for the end-of-string identifier are
+unrelated to Perl's quoting rules. C<q()>, C<qq()>, and the like are not
+supported in place of C<''> and C<"">, and the only interpolation is for
+backslashing the quoting character:
+
+ print << "abc\"def";
+ testing...
+ abc"def
+
+Finally, quoted strings cannot span multiple lines. The general rule is
+that the identifier must be a string literal. Stick with that, and you
+should be safe.
+
+=back
+
+=head2 Gory details of parsing quoted constructs
+X<quote, gory details>
+
+When presented with something that might have several different
+interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
+principle to pick the most probable interpretation. This strategy
+is so successful that Perl programmers often do not suspect the
+ambivalence of what they write. But from time to time, Perl's
+notions differ substantially from what the author honestly meant.
+
+This section hopes to clarify how Perl handles quoted constructs.
+Although the most common reason to learn this is to unravel labyrinthine
+regular expressions, because the initial steps of parsing are the
+same for all quoting operators, they are all discussed together.
+
+The most important Perl parsing rule is the first one discussed
+below: when processing a quoted construct, Perl first finds the end
+of that construct, then interprets its contents. If you understand
+this rule, you may skip the rest of this section on the first
+reading. The other rules are likely to contradict the user's
+expectations much less frequently than this first one.
+
+Some passes discussed below are performed concurrently, but because
+their results are the same, we consider them individually. For different
+quoting constructs, Perl performs different numbers of passes, from
+one to four, but these passes are always performed in the same order.
+
+=over 4
+
+=item Finding the end
+
+The first pass is finding the end of the quoted construct. This results
+in saving to a safe location a copy of the text (between the starting
+and ending delimiters), normalized as necessary to avoid needing to know
+what the original delimiters were.
+
+If the construct is a here-doc, the ending delimiter is a line
+that has a terminating string as the content. Therefore C<<<EOF> is
+terminated by C<EOF> immediately followed by C<"\n"> and starting
+from the first column of the terminating line.
+When searching for the terminating line of a here-doc, nothing
+is skipped. In other words, lines after the here-doc syntax
+are compared with the terminating string line by line.
+
+For the constructs except here-docs, single characters are used as starting
+and ending delimiters. If the starting delimiter is an opening punctuation
+(that is C<(>, C<[>, C<{>, or C<< < >>), the ending delimiter is the
+corresponding closing punctuation (that is C<)>, C<]>, C<}>, or C<< > >>).
+If the starting delimiter is an unpaired character like C</> or a closing
+punctuation, the ending delimiter is the same as the starting delimiter.
+Therefore a C</> terminates a C<qq//> construct, while a C<]> terminates
+both C<qq[]> and C<qq]]> constructs.
+
+When searching for single-character delimiters, escaped delimiters
+and C<\\> are skipped. For example, while searching for terminating C</>,
+combinations of C<\\> and C<\/> are skipped. If the delimiters are
+bracketing, nested pairs are also skipped. For example, while searching
+for a closing C<]> paired with the opening C<[>, combinations of C<\\>, C<\]>,
+and C<\[> are all skipped, and nested C<[> and C<]> are skipped as well.
+However, when backslashes are used as the delimiters (like C<qq\\> and
+C<tr\\\>), nothing is skipped.
+During the search for the end, backslashes that escape delimiters or
+other backslashes are removed (exactly speaking, they are not copied to the
+safe location).
+
+For constructs with three-part delimiters (C<s///>, C<y///>, and
+C<tr///>), the search is repeated once more.
+If the first delimiter is not an opening punctuation, the three delimiters must
+be the same, such as C<s!!!> and C<tr)))>,
+in which case the second delimiter
+terminates the left part and starts the right part at once.
+If the left part is delimited by bracketing punctuation (that is C<()>,
+C<[]>, C<{}>, or C<< <> >>), the right part needs another pair of
+delimiters such as C<s(){}> and C<tr[]//>. In these cases, whitespace
+and comments are allowed between the two parts, although the comment must follow
+at least one whitespace character; otherwise a character expected as the
+start of the comment may be regarded as the starting delimiter of the right part.
+
+During this search no attention is paid to the semantics of the construct.
+Thus:
+
+ "$hash{"$foo/$bar"}"
+
+or:
+
+ m/
+ bar # NOT a comment, this slash / terminated m//!
+ /x
+
+do not form legal quoted expressions. The quoted part ends on the
+first C<"> and C</>, and the rest happens to be a syntax error.
+Because the slash that terminated C<m//> was followed by a C<SPACE>,
+the example above is not C<m//x>, but rather C<m//> with no C</x>
+modifier. So the embedded C<#> is interpreted as a literal C<#>.
+
+Also no attention is paid to C<\c\> (multichar control char syntax) during
+this search. Thus the second C<\> in C<qq/\c\/> is interpreted as a part
+of C<\/>, and the following C</> is not recognized as a delimiter.
+Instead, use C<\034> or C<\x1c> at the end of quoted constructs.
+
+=item Interpolation
+X<interpolation>
+
+The next step is interpolation in the text obtained, which is now
+delimiter-independent. There are multiple cases.
+
+=over 4
+
+=item C<<<'EOF'>
+
+No interpolation is performed.
+Note that the combination C<\\> is left intact, since escaped delimiters
+are not available for here-docs.
+
+=item C<m''>, the pattern of C<s'''>
+
+No interpolation is performed at this stage.
+Any backslashed sequences including C<\\> are treated at the stage
+to L</"parsing regular expressions">.
+
+=item C<''>, C<q//>, C<tr'''>, C<y'''>, the replacement of C<s'''>
+
+The only interpolation is removal of C<\> from pairs of C<\\>.
+Therefore C<"-"> in C<tr'''> and C<y'''> is treated literally
+as a hyphen and no character range is available.
+C<\1> in the replacement of C<s'''> does not work as C<$1>.
+
+=item C<tr///>, C<y///>
+
+No variable interpolation occurs. String modifying combinations for
+case and quoting such as C<\Q>, C<\U>, and C<\E> are not recognized.
+The other escape sequences such as C<\200> and C<\t> and backslashed
+characters such as C<\\> and C<\-> are converted to appropriate literals.
+The character C<"-"> is treated specially and therefore C<\-> is treated
+as a literal C<"-">.
+
+=item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF">
+
+C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F> (possibly paired with C<\E>) are
+converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
+is converted to S<C<$foo . (quotemeta("baz" . $bar))>> internally.
+The other escape sequences such as C<\200> and C<\t> and backslashed
+characters such as C<\\> and C<\-> are replaced with appropriate
+expansions.
+
+Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
+is interpolated in the usual way. Something like C<"\Q\\E"> has
+no C<\E> inside. Instead, it has C<\Q>, C<\\>, and C<E>, so the
+result is the same as for C<"\\\\E">. As a general rule, backslashes
+between C<\Q> and C<\E> may lead to counterintuitive results. So,
+C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
+as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
+
+ $str = '\t';
+ return "\Q$str";
+
+may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
+
+Interpolated scalars and arrays are converted internally to the C<join> and
+C<"."> catenation operations. Thus, S<C<"$foo XXX '@arr'">> becomes:
+
+ $foo . " XXX '" . (join $", @arr) . "'";
+
+All operations above are performed simultaneously, left to right.
+
+Because the result of S<C<"\Q I<STRING> \E">> has all metacharacters
+quoted, there is no way to insert a literal C<$> or C<@> inside a
+C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to become
+C<"\\\$">; if not, it is interpreted as the start of an interpolated
+scalar.
+
+Note also that the interpolation code needs to make a decision on
+where the interpolated scalar ends. For instance, whether
+S<C<< "a $x -> {c}" >>> really means:
+
+ "a " . $x . " -> {c}";
+
+or:
+
+ "a " . $x -> {c};
+
+Most of the time, the longest possible text that does not include
+spaces between components and which contains matching braces or
+brackets. because the outcome may be determined by voting based
+on heuristic estimators, the result is not strictly predictable.
+Fortunately, it's usually correct for ambiguous cases.
+
+=item the replacement of C<s///>
+
+Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F> and interpolation
+happens as with C<qq//> constructs.
+
+It is at this step that C<\1> is begrudgingly converted to C<$1> in
+the replacement text of C<s///>, in order to correct the incorrigible
+I<sed> hackers who haven't picked up the saner idiom yet. A warning
+is emitted if the S<C<use warnings>> pragma or the B<-w> command-line flag
+(that is, the C<$^W> variable) was set.
+
+=item C<RE> in C<m?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
+
+Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F>, C<\E>,
+and interpolation happens (almost) as with C<qq//> constructs.
+
+Processing of C<\N{...}> is also done here, and compiled into an intermediate
+form for the regex compiler. (This is because, as mentioned below, the regex
+compilation may be done at execution time, and C<\N{...}> is a compile-time
+construct.)
+
+However any other combinations of C<\> followed by a character
+are not substituted but only skipped, in order to parse them
+as regular expressions at the following step.
+As C<\c> is skipped at this step, C<@> of C<\c@> in RE is possibly
+treated as an array symbol (for example C<@foo>),
+even though the same text in C<qq//> gives interpolation of C<\c@>.
+
+Code blocks such as C<(?{BLOCK})> are handled by temporarily passing control
+back to the perl parser, in a similar way that an interpolated array
+subscript expression such as C<"foo$array[1+f("[xyz")]bar"> would be.
+
+Moreover, inside C<(?{BLOCK})>, S<C<(?# comment )>>, and
+a C<#>-comment in a C</x>-regular expression, no processing is
+performed whatsoever. This is the first step at which the presence
+of the C</x> modifier is relevant.
+
+Interpolation in patterns has several quirks: C<$|>, C<$(>, C<$)>, C<@+>
+and C<@-> are not interpolated, and constructs C<$var[SOMETHING]> are
+voted (by several different estimators) to be either an array element
+or C<$var> followed by an RE alternative. This is where the notation
+C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
+array element C<-9>, not as a regular expression from the variable
+C<$arr> followed by a digit, which would be the interpretation of
+C</$arr[0-9]/>. Since voting among different estimators may occur,
+the result is not predictable.
+
+The lack of processing of C<\\> creates specific restrictions on
+the post-processed text. If the delimiter is C</>, one cannot get
+the combination C<\/> into the result of this step. C</> will
+finish the regular expression, C<\/> will be stripped to C</> on
+the previous step, and C<\\/> will be left as is. Because C</> is
+equivalent to C<\/> inside a regular expression, this does not
+matter unless the delimiter happens to be character special to the
+RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<m?foo?>; or an
+alphanumeric char, as in:
+
+ m m ^ a \s* b mmx;
+
+In the RE above, which is intentionally obfuscated for illustration, the
+delimiter is C<m>, the modifier is C<mx>, and after delimiter-removal the
+RE is the same as for S<C<m/ ^ a \s* b /mx>>. There's more than one
+reason you're encouraged to restrict your delimiters to non-alphanumeric,
+non-whitespace choices.
+
+=back
+
+This step is the last one for all constructs except regular expressions,
+which are processed further.
+
+=item parsing regular expressions
+X<regexp, parse>
+
+Previous steps were performed during the compilation of Perl code,
+but this one happens at run time, although it may be optimized to
+be calculated at compile time if appropriate. After preprocessing
+described above, and possibly after evaluation if concatenation,
+joining, casing translation, or metaquoting are involved, the
+resulting I<string> is passed to the RE engine for compilation.
+
+Whatever happens in the RE engine might be better discussed in L<perlre>,
+but for the sake of continuity, we shall do so here.
+
+This is another step where the presence of the C</x> modifier is
+relevant. The RE engine scans the string from left to right and
+converts it into a finite automaton.
+
+Backslashed characters are either replaced with corresponding
+literal strings (as with C<\{>), or else they generate special nodes
+in the finite automaton (as with C<\b>). Characters special to the
+RE engine (such as C<|>) generate corresponding nodes or groups of
+nodes. C<(?#...)> comments are ignored. All the rest is either
+converted to literal strings to match, or else is ignored (as is
+whitespace and C<#>-style comments if C</x> is present).
+
+Parsing of the bracketed character class construct, C<[...]>, is
+rather different than the rule used for the rest of the pattern.
+The terminator of this construct is found using the same rules as
+for finding the terminator of a C<{}>-delimited construct, the only
+exception being that C<]> immediately following C<[> is treated as
+though preceded by a backslash.
+
+The terminator of runtime C<(?{...})> is found by temporarily switching
+control to the perl parser, which should stop at the point where the
+logically balancing terminating C<}> is found.
+
+It is possible to inspect both the string given to RE engine and the
+resulting finite automaton. See the arguments C<debug>/C<debugcolor>
+in the S<C<use L<re>>> pragma, as well as Perl's B<-Dr> command-line
+switch documented in L<perlrun/"Command Switches">.
+
+=item Optimization of regular expressions
+X<regexp, optimization>
+
+This step is listed for completeness only. Since it does not change
+semantics, details of this step are not documented and are subject
+to change without notice. This step is performed over the finite
+automaton that was generated during the previous pass.
+
+It is at this stage that C<split()> silently optimizes C</^/> to
+mean C</^/m>.
+
+=back
+
+=head2 I/O Operators
+X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle>
+X<< <> >> X<< <<>> >> X<@ARGV>
+
+There are several I/O operators you should know about.
+
+A string enclosed by backticks (grave accents) first undergoes
+double-quote interpolation. It is then interpreted as an external
+command, and the output of that command is the value of the
+backtick string, like in a shell. In scalar context, a single string
+consisting of all output is returned. In list context, a list of
+values is returned, one per line of output. (You can set C<$/> to use
+a different line terminator.) The command is executed each time the
+pseudo-literal is evaluated. The status value of the command is
+returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
+Unlike in B<csh>, no translation is done on the return data--newlines
+remain newlines. Unlike in any of the shells, single quotes do not
+hide variable names in the command from interpretation. To pass a
+literal dollar-sign through to the shell you need to hide it with a
+backslash. The generalized form of backticks is C<qx//>, or you can
+call the L<perlfunc/readpipe> function. (Because
+backticks always undergo shell expansion as well, see L<perlsec> for
+security concerns.)
+X<qx> X<`> X<``> X<backtick> X<glob>
+
+In scalar context, evaluating a filehandle in angle brackets yields
+the next line from that file (the newline, if any, included), or
+C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
+(sometimes known as file-slurp mode) and the file is empty, it
+returns C<''> the first time, followed by C<undef> subsequently.
+
+Ordinarily you must assign the returned value to a variable, but
+there is one situation where an automatic assignment happens. If
+and only if the input symbol is the only thing inside the conditional
+of a C<while> statement (even if disguised as a C<for(;;)> loop),
+the value is automatically assigned to the global variable C<$_>,
+destroying whatever was there previously. (This may seem like an
+odd thing to you, but you'll use the construct in almost every Perl
+script you write.) The C<$_> variable is not implicitly localized.
+You'll have to put a S<C<local $_;>> before the loop if you want that
+to happen. Furthermore, if the input symbol or an explicit assignment
+of the input symbol to a scalar is used as a C<while>/C<for> condition,
+then the condition actually tests for definedness of the expression's
+value, not for its regular truth value.
+
+Thus the following lines are equivalent:
+
+ while (defined($_ = <STDIN>)) { print; }
+ while ($_ = <STDIN>) { print; }
+ while (<STDIN>) { print; }
+ for (;<STDIN>;) { print; }
+ print while defined($_ = <STDIN>);
+ print while ($_ = <STDIN>);
+ print while <STDIN>;
+
+This also behaves similarly, but assigns to a lexical variable
+instead of to C<$_>:
+
+ while (my $line = <STDIN>) { print $line }
+
+In these loop constructs, the assigned value (whether assignment
+is automatic or explicit) is then tested to see whether it is
+defined. The defined test avoids problems where the line has a string
+value that would be treated as false by Perl; for example a "" or
+a C<"0"> with no trailing newline. If you really mean for such values
+to terminate the loop, they should be tested for explicitly:
+
+ while (($_ = <STDIN>) ne '0') { ... }
+ while (<STDIN>) { last unless $_; ... }
+
+In other boolean contexts, C<< <I<FILEHANDLE>> >> without an
+explicit C<defined> test or comparison elicits a warning if the
+S<C<use warnings>> pragma or the B<-w>
+command-line switch (the C<$^W> variable) is in effect.
+
+The filehandles STDIN, STDOUT, and STDERR are predefined. (The
+filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
+in packages, where they would be interpreted as local identifiers
+rather than global.) Additional filehandles may be created with
+the C<open()> function, amongst others. See L<perlopentut> and
+L<perlfunc/open> for details on this.
+X<stdin> X<stdout> X<sterr>
+
+If a C<< <I<FILEHANDLE>> >> is used in a context that is looking for
+a list, a list comprising all input lines is returned, one line per
+list element. It's easy to grow to a rather large data space this
+way, so use with care.
+
+C<< <I<FILEHANDLE>> >> may also be spelled C<readline(*I<FILEHANDLE>)>.
+See L<perlfunc/readline>.
+
+The null filehandle C<< <> >> is special: it can be used to emulate the
+behavior of B<sed> and B<awk>, and any other Unix filter program
+that takes a list of filenames, doing the same to each line
+of input from all of them. Input from C<< <> >> comes either from
+standard input, or from each file listed on the command line. Here's
+how it works: the first time C<< <> >> is evaluated, the C<@ARGV> array is
+checked, and if it is empty, C<$ARGV[0]> is set to C<"-">, which when opened
+gives you standard input. The C<@ARGV> array is then processed as a list
+of filenames. The loop
+
+ while (<>) {
+ ... # code for each line
+ }
+
+is equivalent to the following Perl-like pseudo code:
+
+ unshift(@ARGV, '-') unless @ARGV;
+ while ($ARGV = shift) {
+ open(ARGV, $ARGV);
+ while (<ARGV>) {
+ ... # code for each line
+ }
+ }
+
+except that it isn't so cumbersome to say, and will actually work.
+It really does shift the C<@ARGV> array and put the current filename
+into the C<$ARGV> variable. It also uses filehandle I<ARGV>
+internally. C<< <> >> is just a synonym for C<< <ARGV> >>, which
+is magical. (The pseudo code above doesn't work because it treats
+C<< <ARGV> >> as non-magical.)
+
+Since the null filehandle uses the two argument form of L<perlfunc/open>
+it interprets special characters, so if you have a script like this:
+
+ while (<>) {
+ print;
+ }
+
+and call it with S<C<perl dangerous.pl 'rm -rfv *|'>>, it actually opens a
+pipe, executes the C<rm> command and reads C<rm>'s output from that pipe.
+If you want all items in C<@ARGV> to be interpreted as file names, you
+can use the module C<ARGV::readonly> from CPAN, or use the double bracket:
+
+ while (<<>>) {
+ print;
+ }
+
+Using double angle brackets inside of a while causes the open to use the
+three argument form (with the second argument being C<< < >>), so all
+arguments in C<ARGV> are treated as literal filenames (including C<"-">).
+(Note that for convenience, if you use C<< <<>> >> and if C<@ARGV> is
+empty, it will still read from the standard input.)
+
+You can modify C<@ARGV> before the first C<< <> >> as long as the array ends up
+containing the list of filenames you really want. Line numbers (C<$.>)
+continue as though the input were one big happy file. See the example
+in L<perlfunc/eof> for how to reset line numbers on each file.
+
+If you want to set C<@ARGV> to your own list of files, go right ahead.
+This sets C<@ARGV> to all plain text files if no C<@ARGV> was given:
+
+ @ARGV = grep { -f && -T } glob('*') unless @ARGV;
+
+You can even set them to pipe commands. For example, this automatically
+filters compressed arguments through B<gzip>:
+
+ @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
+
+If you want to pass switches into your script, you can use one of the
+C<Getopts> modules or put a loop on the front like this:
+
+ while ($_ = $ARGV[0], /^-/) {
+ shift;
+ last if /^--$/;
+ if (/^-D(.*)/) { $debug = $1 }
+ if (/^-v/) { $verbose++ }
+ # ... # other switches
+ }
+
+ while (<>) {
+ # ... # code for each line
+ }
+
+The C<< <> >> symbol will return C<undef> for end-of-file only once.
+If you call it again after this, it will assume you are processing another
+C<@ARGV> list, and if you haven't set C<@ARGV>, will read input from STDIN.
+
+If what the angle brackets contain is a simple scalar variable (for example,
+C<$foo>), then that variable contains the name of the
+filehandle to input from, or its typeglob, or a reference to the
+same. For example:
+
+ $fh = \*STDIN;
+ $line = <$fh>;
+
+If what's within the angle brackets is neither a filehandle nor a simple
+scalar variable containing a filehandle name, typeglob, or typeglob
+reference, it is interpreted as a filename pattern to be globbed, and
+either a list of filenames or the next filename in the list is returned,
+depending on context. This distinction is determined on syntactic
+grounds alone. That means C<< <$x> >> is always a C<readline()> from
+an indirect handle, but C<< <$hash{key}> >> is always a C<glob()>.
+That's because C<$x> is a simple scalar variable, but C<$hash{key}> is
+not--it's a hash element. Even C<< <$x > >> (note the extra space)
+is treated as C<glob("$x ")>, not C<readline($x)>.
+
+One level of double-quote interpretation is done first, but you can't
+say C<< <$foo> >> because that's an indirect filehandle as explained
+in the previous paragraph. (In older versions of Perl, programmers
+would insert curly brackets to force interpretation as a filename glob:
+C<< <${foo}> >>. These days, it's considered cleaner to call the
+internal function directly as C<glob($foo)>, which is probably the right
+way to have done it in the first place.) For example:
+
+ while (<*.c>) {
+ chmod 0644, $_;
+ }
+
+is roughly equivalent to:
+
+ open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
+ while (<FOO>) {
+ chomp;
+ chmod 0644, $_;
+ }
+
+except that the globbing is actually done internally using the standard
+C<L<File::Glob>> extension. Of course, the shortest way to do the above is:
+
+ chmod 0644, <*.c>;
+
+A (file)glob evaluates its (embedded) argument only when it is
+starting a new list. All values must be read before it will start
+over. In list context, this isn't important because you automatically
+get them all anyway. However, in scalar context the operator returns
+the next value each time it's called, or C<undef> when the list has
+run out. As with filehandle reads, an automatic C<defined> is
+generated when the glob occurs in the test part of a C<while>,
+because legal glob returns (for example,
+a file called F<0>) would otherwise
+terminate the loop. Again, C<undef> is returned only once. So if
+you're expecting a single value from a glob, it is much better to
+say
+
+ ($file) = <blurch*>;
+
+than
+
+ $file = <blurch*>;
+
+because the latter will alternate between returning a filename and
+returning false.
+
+If you're trying to do variable interpolation, it's definitely better
+to use the C<glob()> function, because the older notation can cause people
+to become confused with the indirect filehandle notation.
+
+ @files = glob("$dir/*.[ch]");
+ @files = glob($files[$i]);
+
+If an angle-bracket-based globbing expression is used as the condition of
+a C<while> or C<for> loop, then it will be implicitly assigned to C<$_>.
+If either a globbing expression or an explicit assignment of a globbing
+expression to a scalar is used as a C<while>/C<for> condition, then
+the condition actually tests for definedness of the expression's value,
+not for its regular truth value.
+
+=head2 Constant Folding
+X<constant folding> X<folding>
+
+Like C, Perl does a certain amount of expression evaluation at
+compile time whenever it determines that all arguments to an
+operator are static and have no side effects. In particular, string
+concatenation happens at compile time between literals that don't do
+variable substitution. Backslash interpolation also happens at
+compile time. You can say
+
+ 'Now is the time for all'
+ . "\n"
+ . 'good men to come to.'
+
+and this all reduces to one string internally. Likewise, if
+you say
+
+ foreach $file (@filenames) {
+ if (-s $file > 5 + 100 * 2**16) { }
+ }
+
+the compiler precomputes the number which that expression
+represents so that the interpreter won't have to.
+
+=head2 No-ops
+X<no-op> X<nop>
+
+Perl doesn't officially have a no-op operator, but the bare constants
+C<0> and C<1> are special-cased not to produce a warning in void
+context, so you can for example safely do
+
+ 1 while foo();
+
+=head2 Bitwise String Operators
+X<operator, bitwise, string> X<&.> X<|.> X<^.> X<~.>
+
+Bitstrings of any size may be manipulated by the bitwise operators
+(C<~ | & ^>).
+
+If the operands to a binary bitwise op are strings of different
+sizes, B<|> and B<^> ops act as though the shorter operand had
+additional zero bits on the right, while the B<&> op acts as though
+the longer operand were truncated to the length of the shorter.
+The granularity for such extension or truncation is one or more
+bytes.
+
+ # ASCII-based examples
+ print "j p \n" ^ " a h"; # prints "JAPH\n"
+ print "JA" | " ph\n"; # prints "japh\n"
+ print "japh\nJunk" & '_____'; # prints "JAPH\n";
+ print 'p N$' ^ " E<H\n"; # prints "Perl\n";
+
+If you are intending to manipulate bitstrings, be certain that
+you're supplying bitstrings: If an operand is a number, that will imply
+a B<numeric> bitwise operation. You may explicitly show which type of
+operation you intend by using C<""> or C<0+>, as in the examples below.
+
+ $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
+ $foo = '150' | 105; # yields 255
+ $foo = 150 | '105'; # yields 255
+ $foo = '150' | '105'; # yields string '155' (under ASCII)
+
+ $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
+ $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
+
+This somewhat unpredictable behavior can be avoided with the "bitwise"
+feature, new in Perl 5.22. You can enable it via S<C<use feature
+'bitwise'>> or C<use v5.28>. Before Perl 5.28, it used to emit a warning
+in the C<"experimental::bitwise"> category. Under this feature, the four
+standard bitwise operators (C<~ | & ^>) are always numeric. Adding a dot
+after each operator (C<~. |. &. ^.>) forces it to treat its operands as
+strings:
+
+ use feature "bitwise";
+ $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
+ $foo = '150' | 105; # yields 255
+ $foo = 150 | '105'; # yields 255
+ $foo = '150' | '105'; # yields 255
+ $foo = 150 |. 105; # yields string '155'
+ $foo = '150' |. 105; # yields string '155'
+ $foo = 150 |.'105'; # yields string '155'
+ $foo = '150' |.'105'; # yields string '155'
+
+ $baz = $foo & $bar; # both operands numeric
+ $biz = $foo ^. $bar; # both operands stringy
+
+The assignment variants of these operators (C<&= |= ^= &.= |.= ^.=>)
+behave likewise under the feature.
+
+It is a fatal error if an operand contains a character whose ordinal
+value is above 0xFF, and hence not expressible except in UTF-8. The
+operation is performed on a non-UTF-8 copy for other operands encoded in
+UTF-8. See L<perlunicode/Byte and Character Semantics>.
+
+See L<perlfunc/vec> for information on how to manipulate individual bits
+in a bit vector.
+
+=head2 Integer Arithmetic
+X<integer>
+
+By default, Perl assumes that it must do most of its arithmetic in
+floating point. But by saying
+
+ use integer;
+
+you may tell the compiler to use integer operations
+(see L<integer> for a detailed explanation) from here to the end of
+the enclosing BLOCK. An inner BLOCK may countermand this by saying
+
+ no integer;
+
+which lasts until the end of that BLOCK. Note that this doesn't
+mean everything is an integer, merely that Perl will use integer
+operations for arithmetic, comparison, and bitwise operators. For
+example, even under S<C<use integer>>, if you take the C<sqrt(2)>, you'll
+still get C<1.4142135623731> or so.
+
+Used on numbers, the bitwise operators (C<&> C<|> C<^> C<~> C<< << >>
+C<< >> >>) always produce integral results. (But see also
+L</Bitwise String Operators>.) However, S<C<use integer>> still has meaning for
+them. By default, their results are interpreted as unsigned integers, but
+if S<C<use integer>> is in effect, their results are interpreted
+as signed integers. For example, C<~0> usually evaluates to a large
+integral value. However, S<C<use integer; ~0>> is C<-1> on two's-complement
+machines.
+
+=head2 Floating-point Arithmetic
+
+X<floating-point> X<floating point> X<float> X<real>
+
+While S<C<use integer>> provides integer-only arithmetic, there is no
+analogous mechanism to provide automatic rounding or truncation to a
+certain number of decimal places. For rounding to a certain number
+of digits, C<sprintf()> or C<printf()> is usually the easiest route.
+See L<perlfaq4>.
+
+Floating-point numbers are only approximations to what a mathematician
+would call real numbers. There are infinitely more reals than floats,
+so some corners must be cut. For example:
+
+ printf "%.20g\n", 123456789123456789;
+ # produces 123456789123456784
+
+Testing for exact floating-point equality or inequality is not a
+good idea. Here's a (relatively expensive) work-around to compare
+whether two floating-point numbers are equal to a particular number of
+decimal places. See Knuth, volume II, for a more robust treatment of
+this topic.
+
+ sub fp_equal {
+ my ($X, $Y, $POINTS) = @_;
+ my ($tX, $tY);
+ $tX = sprintf("%.${POINTS}g", $X);
+ $tY = sprintf("%.${POINTS}g", $Y);
+ return $tX eq $tY;
+ }
+
+The POSIX module (part of the standard perl distribution) implements
+C<ceil()>, C<floor()>, and other mathematical and trigonometric functions.
+The C<L<Math::Complex>> module (part of the standard perl distribution)
+defines mathematical functions that work on both the reals and the
+imaginary numbers. C<Math::Complex> is not as efficient as POSIX, but
+POSIX can't work with complex numbers.
+
+Rounding in financial applications can have serious implications, and
+the rounding method used should be specified precisely. In these
+cases, it probably pays not to trust whichever system rounding is
+being used by Perl, but to instead implement the rounding function you
+need yourself.
+
+=head2 Bigger Numbers
+X<number, arbitrary precision>
+
+The standard C<L<Math::BigInt>>, C<L<Math::BigRat>>, and
+C<L<Math::BigFloat>> modules,
+along with the C<bignum>, C<bigint>, and C<bigrat> pragmas, provide
+variable-precision arithmetic and overloaded operators, although
+they're currently pretty slow. At the cost of some space and
+considerable speed, they avoid the normal pitfalls associated with
+limited-precision representations.
+
+ use 5.010;
+ use bigint; # easy interface to Math::BigInt
+ $x = 123456789123456789;
+ say $x * $x;
+ +15241578780673678515622620750190521
+
+Or with rationals:
+
+ use 5.010;
+ use bigrat;
+ $x = 3/22;
+ $y = 4/6;
+ say "x/y is ", $x/$y;
+ say "x*y is ", $x*$y;
+ x/y is 9/44
+ x*y is 1/11
+
+Several modules let you calculate with unlimited or fixed precision
+(bound only by memory and CPU time). There
+are also some non-standard modules that
+provide faster implementations via external C libraries.
+
+Here is a short, but incomplete summary:
+
+ Math::String treat string sequences like numbers
+ Math::FixedPrecision calculate with a fixed precision
+ Math::Currency for currency calculations
+ Bit::Vector manipulate bit vectors fast (uses C)
+ Math::BigIntFast Bit::Vector wrapper for big numbers
+ Math::Pari provides access to the Pari C library
+ Math::Cephes uses the external Cephes C library (no
+ big numbers)
+ Math::Cephes::Fraction fractions via the Cephes library
+ Math::GMP another one using an external C library
+ Math::GMPz an alternative interface to libgmp's big ints
+ Math::GMPq an interface to libgmp's fraction numbers
+ Math::GMPf an interface to libgmp's floating point numbers
+
+Choose wisely.
+
+=cut
diff --git a/t/03_builtin_pod_output.t b/t/03_builtin_pod_output.t
new file mode 100644
index 0000000..70f8549
--- /dev/null
+++ b/t/03_builtin_pod_output.t
@@ -0,0 +1,59 @@
+
+use File::Spec;
+use FindBin qw($Bin);
+
+use IPC::Open3;
+use Test::More;
+use Config;
+
+my $pid = undef;
+my $stdout = undef;
+my $stderr = undef;
+
+# get path to perldoc exec in a hopefully platform neutral way..
+my ($volume, $bindir, undef) = File::Spec->splitpath($Bin);
+my $perldoc = File::Spec->catpath($volume,$bindir, File::Spec->catfile(qw(blib script perldoc)));
+if ($ENV{PERL_CORE}) {
+ $perldoc = File::Spec->catfile('..','..','utils',
+ ($Config{usecperl}?'c':'').'perldoc');
+}
+
+# Hash of builtin => [output_start_regexp, output_end_regexp]
+my %builtins = (
+ 'tr' => [ # CPAN RT#86506
+ qr/\A\s+"tr\/\*SEARCHLIST\*\/\*REPLACEMENTLIST\*\/cdsr"\n/,
+ qr/\n\s+eval "tr\/\$oldlist\/\$newlist\/, 1" or die \$\@;\n\n\z/
+ ],
+);
+
+plan tests => 5 * scalar keys %builtins;
+
+for my $builtin (sort keys %builtins) {
+ my ($pid, $stdout, $stderr);
+
+ eval {
+ $pid = open3(\*CHLD_IN, \*CHLD_OUT1, \*CHLD_ERR1,
+ $^X, '-Mblib', '-Icorpus', $perldoc, '-T', '-t', '-f', $builtin);
+ };
+
+ is(length($@), 0, "open for $builtin succeeded"); # returns '' not undef
+ ok(defined($pid), "got process ID for $builtin");
+
+ # gather STDERR
+ while(<CHLD_ERR1>){
+ $stderr .= $_;
+ }
+
+ # check STDERR
+ is($stderr, undef, "no STDERR for $builtin");
+
+ # gather STDOUT
+ while(<CHLD_OUT1>){
+ $stdout .= $_;
+ }
+
+ # check STDOUT
+ like($stdout, $builtins{$builtin}->[0], "output for $builtin starts as expected");
+ like($stdout, $builtins{$builtin}->[1], "output for $builtin ends as expected");
+}
+
--
2.21.0
Reference in New Issue View Git Blame Copy Permalink