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a0d0e21e 1=head1 NAME
d74e8afc 2X<operator>
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4perlop - Perl operators and precedence
5
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6=head1 DESCRIPTION
7
ae3f7391 8In Perl, the operator determines what operation is performed,
ba7f043c 9independent of the type of the operands. For example S<C<$x + $y>>
db691027 10is always a numeric addition, and if C<$x> or C<$y> do not contain
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11numbers, an attempt is made to convert them to numbers first.
12
13This is in contrast to many other dynamic languages, where the
46f8a5ea 14operation is determined by the type of the first argument. It also
ae3f7391 15means that Perl has two versions of some operators, one for numeric
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16and one for string comparison. For example S<C<$x == $y>> compares
17two numbers for equality, and S<C<$x eq $y>> compares two strings.
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18
19There are a few exceptions though: C<x> can be either string
20repetition or list repetition, depending on the type of the left
0b55efd7 21operand, and C<&>, C<|>, C<^> and C<~> can be either string or numeric bit
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22operations.
23
89d205f2 24=head2 Operator Precedence and Associativity
d74e8afc 25X<operator, precedence> X<precedence> X<associativity>
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26
27Operator precedence and associativity work in Perl more or less like
28they do in mathematics.
29
30I<Operator precedence> means some operators are evaluated before
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31others. For example, in S<C<2 + 4 * 5>>, the multiplication has higher
32precedence so S<C<4 * 5>> is evaluated first yielding S<C<2 + 20 ==
3322>> and not S<C<6 * 5 == 30>>.
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34
35I<Operator associativity> defines what happens if a sequence of the
36same operators is used one after another: whether the evaluator will
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37evaluate the left operations first, or the right first. For example, in
38S<C<8 - 4 - 2>>, subtraction is left associative so Perl evaluates the
39expression left to right. S<C<8 - 4>> is evaluated first making the
40expression S<C<4 - 2 == 2>> and not S<C<8 - 2 == 6>>.
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41
42Perl operators have the following associativity and precedence,
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43listed from highest precedence to lowest. Operators borrowed from
44C keep the same precedence relationship with each other, even where
45C's precedence is slightly screwy. (This makes learning Perl easier
46for C folks.) With very few exceptions, these all operate on scalar
47values only, not array values.
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48
49 left terms and list operators (leftward)
50 left ->
51 nonassoc ++ --
52 right **
53 right ! ~ \ and unary + and -
54310121 54 left =~ !~
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55 left * / % x
56 left + - .
57 left << >>
58 nonassoc named unary operators
59 nonassoc < > <= >= lt gt le ge
0d863452 60 nonassoc == != <=> eq ne cmp ~~
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61 left &
62 left | ^
63 left &&
c963b151 64 left || //
137443ea 65 nonassoc .. ...
a0d0e21e 66 right ?:
2ba1f20a 67 right = += -= *= etc. goto last next redo dump
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68 left , =>
69 nonassoc list operators (rightward)
a5f75d66 70 right not
a0d0e21e 71 left and
f23102e2 72 left or xor
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73
74In the following sections, these operators are covered in precedence order.
75
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76Many operators can be overloaded for objects. See L<overload>.
77
a0d0e21e 78=head2 Terms and List Operators (Leftward)
d74e8afc 79X<list operator> X<operator, list> X<term>
a0d0e21e 80
62c18ce2 81A TERM has the highest precedence in Perl. They include variables,
5f05dabc 82quote and quote-like operators, any expression in parentheses,
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83and any function whose arguments are parenthesized. Actually, there
84aren't really functions in this sense, just list operators and unary
85operators behaving as functions because you put parentheses around
86the arguments. These are all documented in L<perlfunc>.
87
ba7f043c 88If any list operator (C<print()>, etc.) or any unary operator (C<chdir()>, etc.)
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89is followed by a left parenthesis as the next token, the operator and
90arguments within parentheses are taken to be of highest precedence,
91just like a normal function call.
92
93In the absence of parentheses, the precedence of list operators such as
94C<print>, C<sort>, or C<chmod> is either very high or very low depending on
54310121 95whether you are looking at the left side or the right side of the operator.
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96For example, in
97
98 @ary = (1, 3, sort 4, 2);
99 print @ary; # prints 1324
100
ba7f043c 101the commas on the right of the C<sort> are evaluated before the C<sort>,
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102but the commas on the left are evaluated after. In other words,
103list operators tend to gobble up all arguments that follow, and
a0d0e21e 104then act like a simple TERM with regard to the preceding expression.
19799a22 105Be careful with parentheses:
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106
107 # These evaluate exit before doing the print:
108 print($foo, exit); # Obviously not what you want.
109 print $foo, exit; # Nor is this.
110
111 # These do the print before evaluating exit:
112 (print $foo), exit; # This is what you want.
113 print($foo), exit; # Or this.
114 print ($foo), exit; # Or even this.
115
116Also note that
117
118 print ($foo & 255) + 1, "\n";
119
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120probably doesn't do what you expect at first glance. The parentheses
121enclose the argument list for C<print> which is evaluated (printing
ba7f043c 122the result of S<C<$foo & 255>>). Then one is added to the return value
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123of C<print> (usually 1). The result is something like this:
124
125 1 + 1, "\n"; # Obviously not what you meant.
126
127To do what you meant properly, you must write:
128
129 print(($foo & 255) + 1, "\n");
130
131See L<Named Unary Operators> for more discussion of this.
a0d0e21e 132
ba7f043c 133Also parsed as terms are the S<C<do {}>> and S<C<eval {}>> constructs, as
54310121 134well as subroutine and method calls, and the anonymous
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135constructors C<[]> and C<{}>.
136
2ae324a7 137See also L<Quote and Quote-like Operators> toward the end of this section,
da87341d 138as well as L</"I/O Operators">.
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139
140=head2 The Arrow Operator
d74e8afc 141X<arrow> X<dereference> X<< -> >>
a0d0e21e 142
35f2feb0 143"C<< -> >>" is an infix dereference operator, just as it is in C
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144and C++. If the right side is either a C<[...]>, C<{...}>, or a
145C<(...)> subscript, then the left side must be either a hard or
146symbolic reference to an array, a hash, or a subroutine respectively.
147(Or technically speaking, a location capable of holding a hard
148reference, if it's an array or hash reference being used for
149assignment.) See L<perlreftut> and L<perlref>.
a0d0e21e 150
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151Otherwise, the right side is a method name or a simple scalar
152variable containing either the method name or a subroutine reference,
153and the left side must be either an object (a blessed reference)
154or a class name (that is, a package name). See L<perlobj>.
a0d0e21e 155
821361b6 156The dereferencing cases (as opposed to method-calling cases) are
2ad792cd 157somewhat extended by the C<postderef> feature. For the
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158details of that feature, consult L<perlref/Postfix Dereference Syntax>.
159
5f05dabc 160=head2 Auto-increment and Auto-decrement
d74e8afc 161X<increment> X<auto-increment> X<++> X<decrement> X<auto-decrement> X<-->
a0d0e21e 162
ba7f043c 163C<"++"> and C<"--"> work as in C. That is, if placed before a variable,
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164they increment or decrement the variable by one before returning the
165value, and if placed after, increment or decrement after returning the
166value.
167
168 $i = 0; $j = 0;
169 print $i++; # prints 0
170 print ++$j; # prints 1
a0d0e21e 171
b033823e 172Note that just as in C, Perl doesn't define B<when> the variable is
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173incremented or decremented. You just know it will be done sometime
174before or after the value is returned. This also means that modifying
c543c01b 175a variable twice in the same statement will lead to undefined behavior.
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176Avoid statements like:
177
178 $i = $i ++;
179 print ++ $i + $i ++;
180
181Perl will not guarantee what the result of the above statements is.
182
54310121 183The auto-increment operator has a little extra builtin magic to it. If
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184you increment a variable that is numeric, or that has ever been used in
185a numeric context, you get a normal increment. If, however, the
5f05dabc 186variable has been used in only string contexts since it was set, and
5a964f20 187has a value that is not the empty string and matches the pattern
9c0670e1 188C</^[a-zA-Z]*[0-9]*\z/>, the increment is done as a string, preserving each
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189character within its range, with carry:
190
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191 print ++($foo = "99"); # prints "100"
192 print ++($foo = "a0"); # prints "a1"
193 print ++($foo = "Az"); # prints "Ba"
194 print ++($foo = "zz"); # prints "aaa"
a0d0e21e 195
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196C<undef> is always treated as numeric, and in particular is changed
197to C<0> before incrementing (so that a post-increment of an undef value
198will return C<0> rather than C<undef>).
199
5f05dabc 200The auto-decrement operator is not magical.
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201
202=head2 Exponentiation
d74e8afc 203X<**> X<exponentiation> X<power>
a0d0e21e 204
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205Binary C<"**"> is the exponentiation operator. It binds even more
206tightly than unary minus, so C<-2**4> is C<-(2**4)>, not C<(-2)**4>.
207(This is
208implemented using C's C<pow(3)> function, which actually works on doubles
cb1a09d0 209internally.)
a0d0e21e 210
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211Note that certain exponentiation expressions are ill-defined:
212these include C<0**0>, C<1**Inf>, and C<Inf**0>. Do not expect
213any particular results from these special cases, the results
214are platform-dependent.
215
a0d0e21e 216=head2 Symbolic Unary Operators
d74e8afc 217X<unary operator> X<operator, unary>
a0d0e21e 218
ba7f043c 219Unary C<"!"> performs logical negation, that is, "not". See also C<not> for a lower
a0d0e21e 220precedence version of this.
d74e8afc 221X<!>
a0d0e21e 222
ba7f043c 223Unary C<"-"> performs arithmetic negation if the operand is numeric,
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224including any string that looks like a number. If the operand is
225an identifier, a string consisting of a minus sign concatenated
226with the identifier is returned. Otherwise, if the string starts
227with a plus or minus, a string starting with the opposite sign is
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228returned. One effect of these rules is that C<-bareword> is equivalent
229to the string C<"-bareword">. If, however, the string begins with a
230non-alphabetic character (excluding C<"+"> or C<"-">), Perl will attempt
231to convert
232the string to a numeric, and the arithmetic negation is performed. If the
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233string cannot be cleanly converted to a numeric, Perl will give the warning
234B<Argument "the string" isn't numeric in negation (-) at ...>.
d74e8afc 235X<-> X<negation, arithmetic>
a0d0e21e 236
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237Unary C<"~"> performs bitwise negation, that is, 1's complement. For
238example, S<C<0666 & ~027>> is 0640. (See also L<Integer Arithmetic> and
972b05a9 239L<Bitwise String Operators>.) Note that the width of the result is
ba7f043c 240platform-dependent: C<~0> is 32 bits wide on a 32-bit platform, but 64
972b05a9 241bits wide on a 64-bit platform, so if you are expecting a certain bit
ba7f043c 242width, remember to use the C<"&"> operator to mask off the excess bits.
d74e8afc 243X<~> X<negation, binary>
a0d0e21e 244
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245When complementing strings, if all characters have ordinal values under
246256, then their complements will, also. But if they do not, all
247characters will be in either 32- or 64-bit complements, depending on your
248architecture. So for example, C<~"\x{3B1}"> is C<"\x{FFFF_FC4E}"> on
24932-bit machines and C<"\x{FFFF_FFFF_FFFF_FC4E}"> on 64-bit machines.
250
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251If the experimental "bitwise" feature is enabled via S<C<use feature
252'bitwise'>>, then unary C<"~"> always treats its argument as a number, and an
253alternate form of the operator, C<"~.">, always treats its argument as a
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254string. So C<~0> and C<~"0"> will both give 2**32-1 on 32-bit platforms,
255whereas C<~.0> and C<~."0"> will both yield C<"\xff">. This feature
ba7f043c 256produces a warning unless you use S<C<no warnings 'experimental::bitwise'>>.
fb7054ba 257
ba7f043c 258Unary C<"+"> has no effect whatsoever, even on strings. It is useful
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259syntactically for separating a function name from a parenthesized expression
260that would otherwise be interpreted as the complete list of function
5ba421f6 261arguments. (See examples above under L<Terms and List Operators (Leftward)>.)
d74e8afc 262X<+>
a0d0e21e 263
ba7f043c 264Unary C<"\"> creates a reference to whatever follows it. See L<perlreftut>
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265and L<perlref>. Do not confuse this behavior with the behavior of
266backslash within a string, although both forms do convey the notion
267of protecting the next thing from interpolation.
d74e8afc 268X<\> X<reference> X<backslash>
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269
270=head2 Binding Operators
d74e8afc 271X<binding> X<operator, binding> X<=~> X<!~>
a0d0e21e 272
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273Binary C<"=~"> binds a scalar expression to a pattern match. Certain operations
274search or modify the string C<$_> by default. This operator makes that kind
cb1a09d0 275of operation work on some other string. The right argument is a search
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276pattern, substitution, or transliteration. The left argument is what is
277supposed to be searched, substituted, or transliterated instead of the default
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278C<$_>. When used in scalar context, the return value generally indicates the
279success of the operation. The exceptions are substitution (C<s///>)
280and transliteration (C<y///>) with the C</r> (non-destructive) option,
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281which cause the B<r>eturn value to be the result of the substitution.
282Behavior in list context depends on the particular operator.
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283See L</"Regexp Quote-Like Operators"> for details and L<perlretut> for
284examples using these operators.
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285
286If the right argument is an expression rather than a search pattern,
2c268ad5 287substitution, or transliteration, it is interpreted as a search pattern at run
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288time. Note that this means that its
289contents will be interpolated twice, so
89d205f2 290
1ca345ed 291 '\\' =~ q'\\';
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292
293is not ok, as the regex engine will end up trying to compile the
294pattern C<\>, which it will consider a syntax error.
a0d0e21e 295
ba7f043c 296Binary C<"!~"> is just like C<"=~"> except the return value is negated in
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297the logical sense.
298
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299Binary C<"!~"> with a non-destructive substitution (C<s///r>) or transliteration
300(C<y///r>) is a syntax error.
4f4d7508 301
a0d0e21e 302=head2 Multiplicative Operators
d74e8afc 303X<operator, multiplicative>
a0d0e21e 304
ba7f043c 305Binary C<"*"> multiplies two numbers.
d74e8afc 306X<*>
a0d0e21e 307
ba7f043c 308Binary C<"/"> divides two numbers.
d74e8afc 309X</> X<slash>
a0d0e21e 310
ba7f043c 311Binary C<"%"> is the modulo operator, which computes the division
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312remainder of its first argument with respect to its second argument.
313Given integer
ba7f043c 314operands C<$m> and C<$n>: If C<$n> is positive, then S<C<$m % $n>> is
db691027 315C<$m> minus the largest multiple of C<$n> less than or equal to
ba7f043c 316C<$m>. If C<$n> is negative, then S<C<$m % $n>> is C<$m> minus the
db691027 317smallest multiple of C<$n> that is not less than C<$m> (that is, the
89b4f0ad 318result will be less than or equal to zero). If the operands
db691027 319C<$m> and C<$n> are floating point values and the absolute value of
ba7f043c 320C<$n> (that is C<abs($n)>) is less than S<C<(UV_MAX + 1)>>, only
db691027 321the integer portion of C<$m> and C<$n> will be used in the operation
4848a83b 322(Note: here C<UV_MAX> means the maximum of the unsigned integer type).
db691027 323If the absolute value of the right operand (C<abs($n)>) is greater than
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324or equal to S<C<(UV_MAX + 1)>>, C<"%"> computes the floating-point remainder
325C<$r> in the equation S<C<($r = $m - $i*$n)>> where C<$i> is a certain
f7918450 326integer that makes C<$r> have the same sign as the right operand
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327C<$n> (B<not> as the left operand C<$m> like C function C<fmod()>)
328and the absolute value less than that of C<$n>.
ba7f043c 329Note that when S<C<use integer>> is in scope, C<"%"> gives you direct access
f7918450 330to the modulo operator as implemented by your C compiler. This
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331operator is not as well defined for negative operands, but it will
332execute faster.
f7918450 333X<%> X<remainder> X<modulo> X<mod>
55d729e4 334
ba7f043c 335Binary C<"x"> is the repetition operator. In scalar context or if the left
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336operand is not enclosed in parentheses, it returns a string consisting
337of the left operand repeated the number of times specified by the right
338operand. In list context, if the left operand is enclosed in
ba7f043c 339parentheses or is a list formed by C<qw/I<STRING>/>, it repeats the list.
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340If the right operand is zero or negative (raising a warning on
341negative), it returns an empty string
3585017f 342or an empty list, depending on the context.
d74e8afc 343X<x>
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344
345 print '-' x 80; # print row of dashes
346
347 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
348
349 @ones = (1) x 80; # a list of 80 1's
350 @ones = (5) x @ones; # set all elements to 5
351
352
353=head2 Additive Operators
d74e8afc 354X<operator, additive>
a0d0e21e 355
ba7f043c 356Binary C<"+"> returns the sum of two numbers.
d74e8afc 357X<+>
a0d0e21e 358
ba7f043c 359Binary C<"-"> returns the difference of two numbers.
d74e8afc 360X<->
a0d0e21e 361
ba7f043c 362Binary C<"."> concatenates two strings.
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363X<string, concatenation> X<concatenation>
364X<cat> X<concat> X<concatenate> X<.>
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365
366=head2 Shift Operators
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367X<shift operator> X<operator, shift> X<<< << >>>
368X<<< >> >>> X<right shift> X<left shift> X<bitwise shift>
369X<shl> X<shr> X<shift, right> X<shift, left>
a0d0e21e 370
ba7f043c 371Binary C<<< "<<" >>> returns the value of its left argument shifted left by the
55497cff 372number of bits specified by the right argument. Arguments should be
982ce180 373integers. (See also L<Integer Arithmetic>.)
a0d0e21e 374
ba7f043c 375Binary C<<< ">>" >>> returns the value of its left argument shifted right by
55497cff 376the number of bits specified by the right argument. Arguments should
982ce180 377be integers. (See also L<Integer Arithmetic>.)
a0d0e21e 378
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379If S<C<use integer>> (see L<Integer Arithmetic>) is in force then
380signed C integers are used (I<arithmetic shift>), otherwise unsigned C
381integers are used (I<logical shift>), even for negative shiftees.
382In arithmetic right shift the sign bit is replicated on the left,
383in logical shift zero bits come in from the left.
384
385Either way, the implementation isn't going to generate results larger
386than the size of the integer type Perl was built with (32 bits or 64 bits).
387
388Shifting by negative number of bits means the reverse shift: left
389shift becomes right shift, right shift becomes left shift. This is
390unlike in C, where negative shift is undefined.
391
392Shifting by more bits than the size of the integers means most of the
393time zero (all bits fall off), except that under S<C<use integer>>
394right overshifting a negative shiftee results in -1. This is unlike
395in C, where shifting by too many bits is undefined. A common C
396behavior is "shift by modulo wordbits", so that for example
397
398 1 >> 64 == 1 >> (64 % 64) == 1 >> 0 == 1 # Common C behavior.
399
400but that is completely accidental.
b16cf6df 401
1ca345ed 402If you get tired of being subject to your platform's native integers,
ba7f043c 403the S<C<use bigint>> pragma neatly sidesteps the issue altogether:
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404
405 print 20 << 20; # 20971520
406 print 20 << 40; # 5120 on 32-bit machines,
407 # 21990232555520 on 64-bit machines
408 use bigint;
409 print 20 << 100; # 25353012004564588029934064107520
410
a0d0e21e 411=head2 Named Unary Operators
d74e8afc 412X<operator, named unary>
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413
414The various named unary operators are treated as functions with one
568e6d8b 415argument, with optional parentheses.
a0d0e21e 416
ba7f043c 417If any list operator (C<print()>, etc.) or any unary operator (C<chdir()>, etc.)
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418is followed by a left parenthesis as the next token, the operator and
419arguments within parentheses are taken to be of highest precedence,
3981b0eb 420just like a normal function call. For example,
1ca345ed 421because named unary operators are higher precedence than C<||>:
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422
423 chdir $foo || die; # (chdir $foo) || die
424 chdir($foo) || die; # (chdir $foo) || die
425 chdir ($foo) || die; # (chdir $foo) || die
426 chdir +($foo) || die; # (chdir $foo) || die
427
ba7f043c 428but, because C<"*"> is higher precedence than named operators:
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429
430 chdir $foo * 20; # chdir ($foo * 20)
431 chdir($foo) * 20; # (chdir $foo) * 20
432 chdir ($foo) * 20; # (chdir $foo) * 20
433 chdir +($foo) * 20; # chdir ($foo * 20)
434
435 rand 10 * 20; # rand (10 * 20)
436 rand(10) * 20; # (rand 10) * 20
437 rand (10) * 20; # (rand 10) * 20
438 rand +(10) * 20; # rand (10 * 20)
439
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440Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are
441treated like named unary operators, but they don't follow this functional
442parenthesis rule. That means, for example, that C<-f($file).".bak"> is
ba7f043c 443equivalent to S<C<-f "$file.bak">>.
d74e8afc 444X<-X> X<filetest> X<operator, filetest>
568e6d8b 445
5ba421f6 446See also L<"Terms and List Operators (Leftward)">.
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447
448=head2 Relational Operators
d74e8afc 449X<relational operator> X<operator, relational>
a0d0e21e 450
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451Perl operators that return true or false generally return values
452that can be safely used as numbers. For example, the relational
453operators in this section and the equality operators in the next
454one return C<1> for true and a special version of the defined empty
455string, C<"">, which counts as a zero but is exempt from warnings
ba7f043c 456about improper numeric conversions, just as S<C<"0 but true">> is.
1ca345ed 457
ba7f043c 458Binary C<< "<" >> returns true if the left argument is numerically less than
a0d0e21e 459the right argument.
d74e8afc 460X<< < >>
a0d0e21e 461
ba7f043c 462Binary C<< ">" >> returns true if the left argument is numerically greater
a0d0e21e 463than the right argument.
d74e8afc 464X<< > >>
a0d0e21e 465
ba7f043c 466Binary C<< "<=" >> returns true if the left argument is numerically less than
a0d0e21e 467or equal to the right argument.
d74e8afc 468X<< <= >>
a0d0e21e 469
ba7f043c 470Binary C<< ">=" >> returns true if the left argument is numerically greater
a0d0e21e 471than or equal to the right argument.
d74e8afc 472X<< >= >>
a0d0e21e 473
ba7f043c 474Binary C<"lt"> returns true if the left argument is stringwise less than
a0d0e21e 475the right argument.
d74e8afc 476X<< lt >>
a0d0e21e 477
ba7f043c 478Binary C<"gt"> returns true if the left argument is stringwise greater
a0d0e21e 479than the right argument.
d74e8afc 480X<< gt >>
a0d0e21e 481
ba7f043c 482Binary C<"le"> returns true if the left argument is stringwise less than
a0d0e21e 483or equal to the right argument.
d74e8afc 484X<< le >>
a0d0e21e 485
ba7f043c 486Binary C<"ge"> returns true if the left argument is stringwise greater
a0d0e21e 487than or equal to the right argument.
d74e8afc 488X<< ge >>
a0d0e21e
LW
489
490=head2 Equality Operators
d74e8afc 491X<equality> X<equal> X<equals> X<operator, equality>
a0d0e21e 492
ba7f043c 493Binary C<< "==" >> returns true if the left argument is numerically equal to
a0d0e21e 494the right argument.
d74e8afc 495X<==>
a0d0e21e 496
ba7f043c 497Binary C<< "!=" >> returns true if the left argument is numerically not equal
a0d0e21e 498to the right argument.
d74e8afc 499X<!=>
a0d0e21e 500
ba7f043c 501Binary C<< "<=>" >> returns -1, 0, or 1 depending on whether the left
6ee5d4e7 502argument is numerically less than, equal to, or greater than the right
ba7f043c
KW
503argument. If your platform supports C<NaN>'s (not-a-numbers) as numeric
504values, using them with C<< "<=>" >> returns undef. C<NaN> is not
505C<< "<" >>, C<< "==" >>, C<< ">" >>, C<< "<=" >> or C<< ">=" >> anything
506(even C<NaN>), so those 5 return false. S<C<< NaN != NaN >>> returns
507true, as does S<C<NaN !=> I<anything else>>. If your platform doesn't
508support C<NaN>'s then C<NaN> is just a string with numeric value 0.
509X<< <=> >>
510X<spaceship>
7d3a9d88 511
db691027
SF
512 $ perl -le '$x = "NaN"; print "No NaN support here" if $x == $x'
513 $ perl -le '$x = "NaN"; print "NaN support here" if $x != $x'
1ca345ed 514
db691027 515(Note that the L<bigint>, L<bigrat>, and L<bignum> pragmas all
ba7f043c 516support C<"NaN">.)
a0d0e21e 517
ba7f043c 518Binary C<"eq"> returns true if the left argument is stringwise equal to
a0d0e21e 519the right argument.
d74e8afc 520X<eq>
a0d0e21e 521
ba7f043c 522Binary C<"ne"> returns true if the left argument is stringwise not equal
a0d0e21e 523to the right argument.
d74e8afc 524X<ne>
a0d0e21e 525
ba7f043c 526Binary C<"cmp"> returns -1, 0, or 1 depending on whether the left
d4ad863d
JH
527argument is stringwise less than, equal to, or greater than the right
528argument.
d74e8afc 529X<cmp>
a0d0e21e 530
ba7f043c 531Binary C<"~~"> does a smartmatch between its arguments. Smart matching
1ca345ed 532is described in the next section.
0d863452
RH
533X<~~>
534
ba7f043c
KW
535C<"lt">, C<"le">, C<"ge">, C<"gt"> and C<"cmp"> use the collation (sort)
536order specified by the current C<LC_COLLATE> locale if a S<C<use
537locale>> form that includes collation is in effect. See L<perllocale>.
538Do not mix these with Unicode,
539only use them with legacy 8-bit locale encodings.
540The standard C<L<Unicode::Collate>> and
541C<L<Unicode::Collate::Locale>> modules offer much more powerful
542solutions to collation issues.
1ca345ed 543
82365311
DG
544For case-insensitive comparisions, look at the L<perlfunc/fc> case-folding
545function, available in Perl v5.16 or later:
546
547 if ( fc($x) eq fc($y) ) { ... }
548
1ca345ed
TC
549=head2 Smartmatch Operator
550
551First available in Perl 5.10.1 (the 5.10.0 version behaved differently),
552binary C<~~> does a "smartmatch" between its arguments. This is mostly
553used implicitly in the C<when> construct described in L<perlsyn>, although
554not all C<when> clauses call the smartmatch operator. Unique among all of
cc08d69f
RS
555Perl's operators, the smartmatch operator can recurse. The smartmatch
556operator is L<experimental|perlpolicy/experimental> and its behavior is
557subject to change.
1ca345ed
TC
558
559It is also unique in that all other Perl operators impose a context
560(usually string or numeric context) on their operands, autoconverting
561those operands to those imposed contexts. In contrast, smartmatch
562I<infers> contexts from the actual types of its operands and uses that
563type information to select a suitable comparison mechanism.
564
565The C<~~> operator compares its operands "polymorphically", determining how
566to compare them according to their actual types (numeric, string, array,
567hash, etc.) Like the equality operators with which it shares the same
568precedence, C<~~> returns 1 for true and C<""> for false. It is often best
569read aloud as "in", "inside of", or "is contained in", because the left
570operand is often looked for I<inside> the right operand. That makes the
40bec8a5 571order of the operands to the smartmatch operand often opposite that of
1ca345ed
TC
572the regular match operator. In other words, the "smaller" thing is usually
573placed in the left operand and the larger one in the right.
574
575The behavior of a smartmatch depends on what type of things its arguments
576are, as determined by the following table. The first row of the table
577whose types apply determines the smartmatch behavior. Because what
578actually happens is mostly determined by the type of the second operand,
579the table is sorted on the right operand instead of on the left.
580
581 Left Right Description and pseudocode
582 ===============================================================
583 Any undef check whether Any is undefined
584 like: !defined Any
585
586 Any Object invoke ~~ overloading on Object, or die
587
588 Right operand is an ARRAY:
589
590 Left Right Description and pseudocode
591 ===============================================================
592 ARRAY1 ARRAY2 recurse on paired elements of ARRAY1 and ARRAY2[2]
593 like: (ARRAY1[0] ~~ ARRAY2[0])
594 && (ARRAY1[1] ~~ ARRAY2[1]) && ...
595 HASH ARRAY any ARRAY elements exist as HASH keys
596 like: grep { exists HASH->{$_} } ARRAY
597 Regexp ARRAY any ARRAY elements pattern match Regexp
598 like: grep { /Regexp/ } ARRAY
599 undef ARRAY undef in ARRAY
600 like: grep { !defined } ARRAY
40bec8a5 601 Any ARRAY smartmatch each ARRAY element[3]
1ca345ed
TC
602 like: grep { Any ~~ $_ } ARRAY
603
604 Right operand is a HASH:
605
606 Left Right Description and pseudocode
607 ===============================================================
608 HASH1 HASH2 all same keys in both HASHes
609 like: keys HASH1 ==
610 grep { exists HASH2->{$_} } keys HASH1
611 ARRAY HASH any ARRAY elements exist as HASH keys
612 like: grep { exists HASH->{$_} } ARRAY
613 Regexp HASH any HASH keys pattern match Regexp
614 like: grep { /Regexp/ } keys HASH
615 undef HASH always false (undef can't be a key)
616 like: 0 == 1
617 Any HASH HASH key existence
618 like: exists HASH->{Any}
619
620 Right operand is CODE:
f703fc96 621
1ca345ed
TC
622 Left Right Description and pseudocode
623 ===============================================================
624 ARRAY CODE sub returns true on all ARRAY elements[1]
625 like: !grep { !CODE->($_) } ARRAY
626 HASH CODE sub returns true on all HASH keys[1]
627 like: !grep { !CODE->($_) } keys HASH
628 Any CODE sub passed Any returns true
629 like: CODE->(Any)
630
631Right operand is a Regexp:
632
633 Left Right Description and pseudocode
634 ===============================================================
635 ARRAY Regexp any ARRAY elements match Regexp
636 like: grep { /Regexp/ } ARRAY
637 HASH Regexp any HASH keys match Regexp
638 like: grep { /Regexp/ } keys HASH
639 Any Regexp pattern match
640 like: Any =~ /Regexp/
641
642 Other:
643
644 Left Right Description and pseudocode
645 ===============================================================
646 Object Any invoke ~~ overloading on Object,
647 or fall back to...
648
649 Any Num numeric equality
650 like: Any == Num
651 Num nummy[4] numeric equality
652 like: Num == nummy
653 undef Any check whether undefined
654 like: !defined(Any)
655 Any Any string equality
656 like: Any eq Any
657
658
659Notes:
660
661=over
662
663=item 1.
664Empty hashes or arrays match.
665
666=item 2.
40bec8a5 667That is, each element smartmatches the element of the same index in the other array.[3]
1ca345ed
TC
668
669=item 3.
670If a circular reference is found, fall back to referential equality.
671
672=item 4.
673Either an actual number, or a string that looks like one.
674
675=back
676
677The smartmatch implicitly dereferences any non-blessed hash or array
678reference, so the C<I<HASH>> and C<I<ARRAY>> entries apply in those cases.
679For blessed references, the C<I<Object>> entries apply. Smartmatches
680involving hashes only consider hash keys, never hash values.
681
682The "like" code entry is not always an exact rendition. For example, the
40bec8a5 683smartmatch operator short-circuits whenever possible, but C<grep> does
1ca345ed
TC
684not. Also, C<grep> in scalar context returns the number of matches, but
685C<~~> returns only true or false.
686
687Unlike most operators, the smartmatch operator knows to treat C<undef>
688specially:
689
690 use v5.10.1;
691 @array = (1, 2, 3, undef, 4, 5);
692 say "some elements undefined" if undef ~~ @array;
693
694Each operand is considered in a modified scalar context, the modification
695being that array and hash variables are passed by reference to the
696operator, which implicitly dereferences them. Both elements
697of each pair are the same:
698
699 use v5.10.1;
700
701 my %hash = (red => 1, blue => 2, green => 3,
702 orange => 4, yellow => 5, purple => 6,
703 black => 7, grey => 8, white => 9);
704
705 my @array = qw(red blue green);
706
707 say "some array elements in hash keys" if @array ~~ %hash;
708 say "some array elements in hash keys" if \@array ~~ \%hash;
709
710 say "red in array" if "red" ~~ @array;
711 say "red in array" if "red" ~~ \@array;
712
713 say "some keys end in e" if /e$/ ~~ %hash;
714 say "some keys end in e" if /e$/ ~~ \%hash;
715
40bec8a5
TC
716Two arrays smartmatch if each element in the first array smartmatches
717(that is, is "in") the corresponding element in the second array,
718recursively.
1ca345ed
TC
719
720 use v5.10.1;
721 my @little = qw(red blue green);
722 my @bigger = ("red", "blue", [ "orange", "green" ] );
723 if (@little ~~ @bigger) { # true!
724 say "little is contained in bigger";
725 }
726
727Because the smartmatch operator recurses on nested arrays, this
728will still report that "red" is in the array.
729
730 use v5.10.1;
731 my @array = qw(red blue green);
732 my $nested_array = [[[[[[[ @array ]]]]]]];
733 say "red in array" if "red" ~~ $nested_array;
734
735If two arrays smartmatch each other, then they are deep
736copies of each others' values, as this example reports:
737
738 use v5.12.0;
739 my @a = (0, 1, 2, [3, [4, 5], 6], 7);
740 my @b = (0, 1, 2, [3, [4, 5], 6], 7);
741
742 if (@a ~~ @b && @b ~~ @a) {
743 say "a and b are deep copies of each other";
744 }
745 elsif (@a ~~ @b) {
746 say "a smartmatches in b";
747 }
748 elsif (@b ~~ @a) {
749 say "b smartmatches in a";
750 }
751 else {
752 say "a and b don't smartmatch each other at all";
753 }
754
755
ba7f043c
KW
756If you were to set S<C<$b[3] = 4>>, then instead of reporting that "a and b
757are deep copies of each other", it now reports that C<"b smartmatches in a">.
758That's because the corresponding position in C<@a> contains an array that
1ca345ed
TC
759(eventually) has a 4 in it.
760
761Smartmatching one hash against another reports whether both contain the
46f8a5ea 762same keys, no more and no less. This could be used to see whether two
1ca345ed
TC
763records have the same field names, without caring what values those fields
764might have. For example:
765
766 use v5.10.1;
767 sub make_dogtag {
768 state $REQUIRED_FIELDS = { name=>1, rank=>1, serial_num=>1 };
769
770 my ($class, $init_fields) = @_;
771
772 die "Must supply (only) name, rank, and serial number"
773 unless $init_fields ~~ $REQUIRED_FIELDS;
774
775 ...
776 }
777
778or, if other non-required fields are allowed, use ARRAY ~~ HASH:
779
780 use v5.10.1;
781 sub make_dogtag {
782 state $REQUIRED_FIELDS = { name=>1, rank=>1, serial_num=>1 };
783
784 my ($class, $init_fields) = @_;
785
786 die "Must supply (at least) name, rank, and serial number"
787 unless [keys %{$init_fields}] ~~ $REQUIRED_FIELDS;
788
789 ...
790 }
791
792The smartmatch operator is most often used as the implicit operator of a
793C<when> clause. See the section on "Switch Statements" in L<perlsyn>.
794
795=head3 Smartmatching of Objects
796
40bec8a5
TC
797To avoid relying on an object's underlying representation, if the
798smartmatch's right operand is an object that doesn't overload C<~~>,
799it raises the exception "C<Smartmatching a non-overloaded object
46f8a5ea
FC
800breaks encapsulation>". That's because one has no business digging
801around to see whether something is "in" an object. These are all
40bec8a5 802illegal on objects without a C<~~> overload:
1ca345ed
TC
803
804 %hash ~~ $object
805 42 ~~ $object
806 "fred" ~~ $object
807
808However, you can change the way an object is smartmatched by overloading
46f8a5ea
FC
809the C<~~> operator. This is allowed to
810extend the usual smartmatch semantics.
1ca345ed
TC
811For objects that do have an C<~~> overload, see L<overload>.
812
813Using an object as the left operand is allowed, although not very useful.
814Smartmatching rules take precedence over overloading, so even if the
815object in the left operand has smartmatch overloading, this will be
816ignored. A left operand that is a non-overloaded object falls back on a
817string or numeric comparison of whatever the C<ref> operator returns. That
818means that
819
820 $object ~~ X
821
822does I<not> invoke the overload method with C<I<X>> as an argument.
823Instead the above table is consulted as normal, and based on the type of
824C<I<X>>, overloading may or may not be invoked. For simple strings or
ba7f043c 825numbers, "in" becomes equivalent to this:
1ca345ed
TC
826
827 $object ~~ $number ref($object) == $number
828 $object ~~ $string ref($object) eq $string
829
830For example, this reports that the handle smells IOish
831(but please don't really do this!):
832
833 use IO::Handle;
834 my $fh = IO::Handle->new();
835 if ($fh ~~ /\bIO\b/) {
836 say "handle smells IOish";
837 }
838
839That's because it treats C<$fh> as a string like
840C<"IO::Handle=GLOB(0x8039e0)">, then pattern matches against that.
a034a98d 841
a0d0e21e 842=head2 Bitwise And
d74e8afc 843X<operator, bitwise, and> X<bitwise and> X<&>
a0d0e21e 844
ba7f043c 845Binary C<"&"> returns its operands ANDed together bit by bit. Although no
c791a246
KW
846warning is currently raised, the result is not well defined when this operation
847is performed on operands that aren't either numbers (see
ba7f043c 848L<Integer Arithmetic>) nor bitstrings (see L<Bitwise String Operators>).
a0d0e21e 849
ba7f043c 850Note that C<"&"> has lower priority than relational operators, so for example
1ca345ed 851the parentheses are essential in a test like
2cdc098b 852
1ca345ed 853 print "Even\n" if ($x & 1) == 0;
2cdc098b 854
ba7f043c
KW
855If the experimental "bitwise" feature is enabled via S<C<use feature
856'bitwise'>>, then this operator always treats its operand as numbers. This
857feature produces a warning unless you also use C<S<no warnings
858'experimental::bitwise'>>.
fb7054ba 859
a0d0e21e 860=head2 Bitwise Or and Exclusive Or
d74e8afc
ITB
861X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor>
862X<bitwise xor> X<^>
a0d0e21e 863
ba7f043c 864Binary C<"|"> returns its operands ORed together bit by bit.
a0d0e21e 865
ba7f043c 866Binary C<"^"> returns its operands XORed together bit by bit.
c791a246
KW
867
868Although no warning is currently raised, the results are not well
869defined when these operations are performed on operands that aren't either
ba7f043c 870numbers (see L<Integer Arithmetic>) nor bitstrings (see L<Bitwise String
c791a246 871Operators>).
a0d0e21e 872
ba7f043c
KW
873Note that C<"|"> and C<"^"> have lower priority than relational operators, so
874for example the parentheses are essential in a test like
2cdc098b 875
1ca345ed 876 print "false\n" if (8 | 2) != 10;
2cdc098b 877
ba7f043c
KW
878If the experimental "bitwise" feature is enabled via S<C<use feature
879'bitwise'>>, then this operator always treats its operand as numbers. This
880feature produces a warning unless you also use S<C<no warnings
881'experimental::bitwise'>>.
fb7054ba 882
a0d0e21e 883=head2 C-style Logical And
d74e8afc 884X<&&> X<logical and> X<operator, logical, and>
a0d0e21e 885
ba7f043c 886Binary C<"&&"> performs a short-circuit logical AND operation. That is,
a0d0e21e
LW
887if the left operand is false, the right operand is not even evaluated.
888Scalar or list context propagates down to the right operand if it
889is evaluated.
890
891=head2 C-style Logical Or
d74e8afc 892X<||> X<operator, logical, or>
a0d0e21e 893
ba7f043c 894Binary C<"||"> performs a short-circuit logical OR operation. That is,
a0d0e21e
LW
895if the left operand is true, the right operand is not even evaluated.
896Scalar or list context propagates down to the right operand if it
897is evaluated.
898
26d9d83b 899=head2 Logical Defined-Or
d74e8afc 900X<//> X<operator, logical, defined-or>
c963b151
BD
901
902Although it has no direct equivalent in C, Perl's C<//> operator is related
ba7f043c 903to its C-style "or". In fact, it's exactly the same as C<||>, except that it
95bee9ba 904tests the left hand side's definedness instead of its truth. Thus,
ba7f043c 905S<C<< EXPR1 // EXPR2 >>> returns the value of C<< EXPR1 >> if it's defined,
46f8a5ea
FC
906otherwise, the value of C<< EXPR2 >> is returned.
907(C<< EXPR1 >> is evaluated in scalar context, C<< EXPR2 >>
908in the context of C<< // >> itself). Usually,
ba7f043c
KW
909this is the same result as S<C<< defined(EXPR1) ? EXPR1 : EXPR2 >>> (except that
910the ternary-operator form can be used as a lvalue, while S<C<< EXPR1 // EXPR2 >>>
46f8a5ea 911cannot). This is very useful for
bdc7923b 912providing default values for variables. If you actually want to test if
ba7f043c 913at least one of C<$x> and C<$y> is defined, use S<C<defined($x // $y)>>.
c963b151 914
d042e63d 915The C<||>, C<//> and C<&&> operators return the last value evaluated
46f8a5ea 916(unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably
d042e63d 917portable way to find out the home directory might be:
a0d0e21e 918
c543c01b
TC
919 $home = $ENV{HOME}
920 // $ENV{LOGDIR}
921 // (getpwuid($<))[7]
922 // die "You're homeless!\n";
a0d0e21e 923
5a964f20
TC
924In particular, this means that you shouldn't use this
925for selecting between two aggregates for assignment:
926
927 @a = @b || @c; # this is wrong
928 @a = scalar(@b) || @c; # really meant this
929 @a = @b ? @b : @c; # this works fine, though
930
1ca345ed 931As alternatives to C<&&> and C<||> when used for
f23102e2 932control flow, Perl provides the C<and> and C<or> operators (see below).
ba7f043c
KW
933The short-circuit behavior is identical. The precedence of C<"and">
934and C<"or"> is much lower, however, so that you can safely use them after a
5a964f20 935list operator without the need for parentheses:
a0d0e21e
LW
936
937 unlink "alpha", "beta", "gamma"
938 or gripe(), next LINE;
939
940With the C-style operators that would have been written like this:
941
942 unlink("alpha", "beta", "gamma")
943 || (gripe(), next LINE);
944
1ca345ed
TC
945It would be even more readable to write that this way:
946
947 unless(unlink("alpha", "beta", "gamma")) {
948 gripe();
949 next LINE;
950 }
951
ba7f043c 952Using C<"or"> for assignment is unlikely to do what you want; see below.
5a964f20
TC
953
954=head2 Range Operators
d74e8afc 955X<operator, range> X<range> X<..> X<...>
a0d0e21e 956
ba7f043c 957Binary C<".."> is the range operator, which is really two different
fb53bbb2 958operators depending on the context. In list context, it returns a
54ae734e 959list of values counting (up by ones) from the left value to the right
2cdbc966 960value. If the left value is greater than the right value then it
fb53bbb2 961returns the empty list. The range operator is useful for writing
ba7f043c 962S<C<foreach (1..10)>> loops and for doing slice operations on arrays. In
2cdbc966
JD
963the current implementation, no temporary array is created when the
964range operator is used as the expression in C<foreach> loops, but older
965versions of Perl might burn a lot of memory when you write something
966like this:
a0d0e21e
LW
967
968 for (1 .. 1_000_000) {
969 # code
54310121 970 }
a0d0e21e 971
8f0f46f8 972The range operator also works on strings, using the magical
973auto-increment, see below.
54ae734e 974
ba7f043c 975In scalar context, C<".."> returns a boolean value. The operator is
8f0f46f8 976bistable, like a flip-flop, and emulates the line-range (comma)
ba7f043c 977operator of B<sed>, B<awk>, and various editors. Each C<".."> operator
8f0f46f8 978maintains its own boolean state, even across calls to a subroutine
46f8a5ea 979that contains it. It is false as long as its left operand is false.
a0d0e21e
LW
980Once the left operand is true, the range operator stays true until the
981right operand is true, I<AFTER> which the range operator becomes false
8f0f46f8 982again. It doesn't become false till the next time the range operator
983is evaluated. It can test the right operand and become false on the
984same evaluation it became true (as in B<awk>), but it still returns
46f8a5ea 985true once. If you don't want it to test the right operand until the
ba7f043c
KW
986next evaluation, as in B<sed>, just use three dots (C<"...">) instead of
987two. In all other regards, C<"..."> behaves just like C<".."> does.
19799a22
GS
988
989The right operand is not evaluated while the operator is in the
990"false" state, and the left operand is not evaluated while the
991operator is in the "true" state. The precedence is a little lower
992than || and &&. The value returned is either the empty string for
8f0f46f8 993false, or a sequence number (beginning with 1) for true. The sequence
994number is reset for each range encountered. The final sequence number
ba7f043c 995in a range has the string C<"E0"> appended to it, which doesn't affect
8f0f46f8 996its numeric value, but gives you something to search for if you want
997to exclude the endpoint. You can exclude the beginning point by
998waiting for the sequence number to be greater than 1.
df5f8116 999
ba7f043c 1000If either operand of scalar C<".."> is a constant expression,
df5f8116
CW
1001that operand is considered true if it is equal (C<==>) to the current
1002input line number (the C<$.> variable).
1003
ba7f043c 1004To be pedantic, the comparison is actually S<C<int(EXPR) == int(EXPR)>>,
df5f8116
CW
1005but that is only an issue if you use a floating point expression; when
1006implicitly using C<$.> as described in the previous paragraph, the
ba7f043c 1007comparison is S<C<int(EXPR) == int($.)>> which is only an issue when C<$.>
df5f8116 1008is set to a floating point value and you are not reading from a file.
ba7f043c 1009Furthermore, S<C<"span" .. "spat">> or S<C<2.18 .. 3.14>> will not do what
df5f8116
CW
1010you want in scalar context because each of the operands are evaluated
1011using their integer representation.
1012
1013Examples:
a0d0e21e
LW
1014
1015As a scalar operator:
1016
df5f8116 1017 if (101 .. 200) { print; } # print 2nd hundred lines, short for
950b09ed 1018 # if ($. == 101 .. $. == 200) { print; }
9f10b797
RGS
1019
1020 next LINE if (1 .. /^$/); # skip header lines, short for
f343f960 1021 # next LINE if ($. == 1 .. /^$/);
9f10b797
RGS
1022 # (typically in a loop labeled LINE)
1023
1024 s/^/> / if (/^$/ .. eof()); # quote body
a0d0e21e 1025
5a964f20
TC
1026 # parse mail messages
1027 while (<>) {
1028 $in_header = 1 .. /^$/;
df5f8116
CW
1029 $in_body = /^$/ .. eof;
1030 if ($in_header) {
f343f960 1031 # do something
df5f8116 1032 } else { # in body
f343f960 1033 # do something else
df5f8116 1034 }
5a964f20 1035 } continue {
df5f8116 1036 close ARGV if eof; # reset $. each file
5a964f20
TC
1037 }
1038
acf31ca5
SF
1039Here's a simple example to illustrate the difference between
1040the two range operators:
1041
1042 @lines = (" - Foo",
1043 "01 - Bar",
1044 "1 - Baz",
1045 " - Quux");
1046
9f10b797
RGS
1047 foreach (@lines) {
1048 if (/0/ .. /1/) {
acf31ca5
SF
1049 print "$_\n";
1050 }
1051 }
1052
46f8a5ea 1053This program will print only the line containing "Bar". If
9f10b797 1054the range operator is changed to C<...>, it will also print the
acf31ca5
SF
1055"Baz" line.
1056
1057And now some examples as a list operator:
a0d0e21e 1058
1ca345ed
TC
1059 for (101 .. 200) { print } # print $_ 100 times
1060 @foo = @foo[0 .. $#foo]; # an expensive no-op
1061 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
a0d0e21e 1062
5a964f20 1063The range operator (in list context) makes use of the magical
5f05dabc 1064auto-increment algorithm if the operands are strings. You
a0d0e21e
LW
1065can say
1066
c543c01b 1067 @alphabet = ("A" .. "Z");
a0d0e21e 1068
54ae734e 1069to get all normal letters of the English alphabet, or
a0d0e21e 1070
c543c01b 1071 $hexdigit = (0 .. 9, "a" .. "f")[$num & 15];
a0d0e21e
LW
1072
1073to get a hexadecimal digit, or
1074
1ca345ed
TC
1075 @z2 = ("01" .. "31");
1076 print $z2[$mday];
a0d0e21e 1077
ea4f5703
YST
1078to get dates with leading zeros.
1079
1080If the final value specified is not in the sequence that the magical
1081increment would produce, the sequence goes until the next value would
1082be longer than the final value specified.
1083
1084If the initial value specified isn't part of a magical increment
c543c01b 1085sequence (that is, a non-empty string matching C</^[a-zA-Z]*[0-9]*\z/>),
ea4f5703
YST
1086only the initial value will be returned. So the following will only
1087return an alpha:
1088
c543c01b 1089 use charnames "greek";
ea4f5703
YST
1090 my @greek_small = ("\N{alpha}" .. "\N{omega}");
1091
c543c01b
TC
1092To get the 25 traditional lowercase Greek letters, including both sigmas,
1093you could use this instead:
ea4f5703 1094
c543c01b 1095 use charnames "greek";
1ca345ed
TC
1096 my @greek_small = map { chr } ( ord("\N{alpha}")
1097 ..
1098 ord("\N{omega}")
1099 );
c543c01b
TC
1100
1101However, because there are I<many> other lowercase Greek characters than
1102just those, to match lowercase Greek characters in a regular expression,
47c56cc8
KW
1103you could use the pattern C</(?:(?=\p{Greek})\p{Lower})+/> (or the
1104L<experimental feature|perlrecharclass/Extended Bracketed Character
1105Classes> C<S</(?[ \p{Greek} & \p{Lower} ])+/>>).
a0d0e21e 1106
ba7f043c 1107Because each operand is evaluated in integer form, S<C<2.18 .. 3.14>> will
df5f8116
CW
1108return two elements in list context.
1109
1110 @list = (2.18 .. 3.14); # same as @list = (2 .. 3);
1111
a0d0e21e 1112=head2 Conditional Operator
d74e8afc 1113X<operator, conditional> X<operator, ternary> X<ternary> X<?:>
a0d0e21e 1114
ba7f043c
KW
1115Ternary C<"?:"> is the conditional operator, just as in C. It works much
1116like an if-then-else. If the argument before the C<?> is true, the
1117argument before the C<:> is returned, otherwise the argument after the
1118C<:> is returned. For example:
cb1a09d0 1119
54310121 1120 printf "I have %d dog%s.\n", $n,
c543c01b 1121 ($n == 1) ? "" : "s";
cb1a09d0
AD
1122
1123Scalar or list context propagates downward into the 2nd
54310121 1124or 3rd argument, whichever is selected.
cb1a09d0 1125
db691027
SF
1126 $x = $ok ? $y : $z; # get a scalar
1127 @x = $ok ? @y : @z; # get an array
1128 $x = $ok ? @y : @z; # oops, that's just a count!
cb1a09d0
AD
1129
1130The operator may be assigned to if both the 2nd and 3rd arguments are
1131legal lvalues (meaning that you can assign to them):
a0d0e21e 1132
db691027 1133 ($x_or_y ? $x : $y) = $z;
a0d0e21e 1134
5a964f20
TC
1135Because this operator produces an assignable result, using assignments
1136without parentheses will get you in trouble. For example, this:
1137
db691027 1138 $x % 2 ? $x += 10 : $x += 2
5a964f20
TC
1139
1140Really means this:
1141
db691027 1142 (($x % 2) ? ($x += 10) : $x) += 2
5a964f20
TC
1143
1144Rather than this:
1145
db691027 1146 ($x % 2) ? ($x += 10) : ($x += 2)
5a964f20 1147
19799a22
GS
1148That should probably be written more simply as:
1149
db691027 1150 $x += ($x % 2) ? 10 : 2;
19799a22 1151
4633a7c4 1152=head2 Assignment Operators
d74e8afc 1153X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=>
5ac3b81c 1154X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=>
fb7054ba 1155X<%=> X<^=> X<x=> X<&.=> X<|.=> X<^.=>
a0d0e21e 1156
ba7f043c 1157C<"="> is the ordinary assignment operator.
a0d0e21e
LW
1158
1159Assignment operators work as in C. That is,
1160
db691027 1161 $x += 2;
a0d0e21e
LW
1162
1163is equivalent to
1164
db691027 1165 $x = $x + 2;
a0d0e21e
LW
1166
1167although without duplicating any side effects that dereferencing the lvalue
ba7f043c 1168might trigger, such as from C<tie()>. Other assignment operators work similarly.
54310121 1169The following are recognized:
a0d0e21e 1170
fb7054ba
FC
1171 **= += *= &= &.= <<= &&=
1172 -= /= |= |.= >>= ||=
1173 .= %= ^= ^.= //=
9f10b797 1174 x=
a0d0e21e 1175
19799a22 1176Although these are grouped by family, they all have the precedence
82848c10
FC
1177of assignment. These combined assignment operators can only operate on
1178scalars, whereas the ordinary assignment operator can assign to arrays,
1179hashes, lists and even references. (See L<"Context"|perldata/Context>
1180and L<perldata/List value constructors>, and L<perlref/Assigning to
1181References>.)
a0d0e21e 1182
b350dd2f
GS
1183Unlike in C, the scalar assignment operator produces a valid lvalue.
1184Modifying an assignment is equivalent to doing the assignment and
1185then modifying the variable that was assigned to. This is useful
1186for modifying a copy of something, like this:
a0d0e21e 1187
1ca345ed
TC
1188 ($tmp = $global) =~ tr/13579/24680/;
1189
1190Although as of 5.14, that can be also be accomplished this way:
1191
1192 use v5.14;
1193 $tmp = ($global =~ tr/13579/24680/r);
a0d0e21e
LW
1194
1195Likewise,
1196
db691027 1197 ($x += 2) *= 3;
a0d0e21e
LW
1198
1199is equivalent to
1200
db691027
SF
1201 $x += 2;
1202 $x *= 3;
a0d0e21e 1203
b350dd2f
GS
1204Similarly, a list assignment in list context produces the list of
1205lvalues assigned to, and a list assignment in scalar context returns
1206the number of elements produced by the expression on the right hand
1207side of the assignment.
1208
ba7f043c 1209The three dotted bitwise assignment operators (C<&.=> C<|.=> C<^.=>) are new in
fb7054ba
FC
1210Perl 5.22 and experimental. See L</Bitwise String Operators>.
1211
748a9306 1212=head2 Comma Operator
d74e8afc 1213X<comma> X<operator, comma> X<,>
a0d0e21e 1214
ba7f043c 1215Binary C<","> is the comma operator. In scalar context it evaluates
a0d0e21e
LW
1216its left argument, throws that value away, then evaluates its right
1217argument and returns that value. This is just like C's comma operator.
1218
5a964f20 1219In list context, it's just the list argument separator, and inserts
ed5c6d31
PJ
1220both its arguments into the list. These arguments are also evaluated
1221from left to right.
a0d0e21e 1222
ba7f043c
KW
1223The C<< => >> operator (sometimes pronounced "fat comma") is a synonym
1224for the comma except that it causes a
4e1988c6 1225word on its left to be interpreted as a string if it begins with a letter
344f2c40
IG
1226or underscore and is composed only of letters, digits and underscores.
1227This includes operands that might otherwise be interpreted as operators,
46f8a5ea 1228constants, single number v-strings or function calls. If in doubt about
c543c01b 1229this behavior, the left operand can be quoted explicitly.
344f2c40
IG
1230
1231Otherwise, the C<< => >> operator behaves exactly as the comma operator
1232or list argument separator, according to context.
1233
1234For example:
a44e5664
MS
1235
1236 use constant FOO => "something";
1237
1238 my %h = ( FOO => 23 );
1239
1240is equivalent to:
1241
1242 my %h = ("FOO", 23);
1243
1244It is I<NOT>:
1245
1246 my %h = ("something", 23);
1247
719b43e8
RGS
1248The C<< => >> operator is helpful in documenting the correspondence
1249between keys and values in hashes, and other paired elements in lists.
748a9306 1250
a12b8f3c
FC
1251 %hash = ( $key => $value );
1252 login( $username => $password );
a44e5664 1253
4e1988c6
FC
1254The special quoting behavior ignores precedence, and hence may apply to
1255I<part> of the left operand:
1256
1257 print time.shift => "bbb";
1258
ba7f043c 1259That example prints something like C<"1314363215shiftbbb">, because the
4e1988c6
FC
1260C<< => >> implicitly quotes the C<shift> immediately on its left, ignoring
1261the fact that C<time.shift> is the entire left operand.
1262
a0d0e21e 1263=head2 List Operators (Rightward)
d74e8afc 1264X<operator, list, rightward> X<list operator>
a0d0e21e 1265
c543c01b 1266On the right side of a list operator, the comma has very low precedence,
a0d0e21e
LW
1267such that it controls all comma-separated expressions found there.
1268The only operators with lower precedence are the logical operators
ba7f043c 1269C<"and">, C<"or">, and C<"not">, which may be used to evaluate calls to list
1ca345ed
TC
1270operators without the need for parentheses:
1271
1272 open HANDLE, "< :utf8", "filename" or die "Can't open: $!\n";
1273
1274However, some people find that code harder to read than writing
1275it with parentheses:
1276
1277 open(HANDLE, "< :utf8", "filename") or die "Can't open: $!\n";
1278
ba7f043c 1279in which case you might as well just use the more customary C<"||"> operator:
a0d0e21e 1280
1ca345ed 1281 open(HANDLE, "< :utf8", "filename") || die "Can't open: $!\n";
a0d0e21e 1282
5ba421f6 1283See also discussion of list operators in L<Terms and List Operators (Leftward)>.
a0d0e21e
LW
1284
1285=head2 Logical Not
d74e8afc 1286X<operator, logical, not> X<not>
a0d0e21e 1287
ba7f043c
KW
1288Unary C<"not"> returns the logical negation of the expression to its right.
1289It's the equivalent of C<"!"> except for the very low precedence.
a0d0e21e
LW
1290
1291=head2 Logical And
d74e8afc 1292X<operator, logical, and> X<and>
a0d0e21e 1293
ba7f043c 1294Binary C<"and"> returns the logical conjunction of the two surrounding
c543c01b
TC
1295expressions. It's equivalent to C<&&> except for the very low
1296precedence. This means that it short-circuits: the right
a0d0e21e
LW
1297expression is evaluated only if the left expression is true.
1298
59ab9d6e 1299=head2 Logical or and Exclusive Or
f23102e2 1300X<operator, logical, or> X<operator, logical, xor>
59ab9d6e 1301X<operator, logical, exclusive or>
f23102e2 1302X<or> X<xor>
a0d0e21e 1303
ba7f043c 1304Binary C<"or"> returns the logical disjunction of the two surrounding
c543c01b
TC
1305expressions. It's equivalent to C<||> except for the very low precedence.
1306This makes it useful for control flow:
5a964f20
TC
1307
1308 print FH $data or die "Can't write to FH: $!";
1309
c543c01b
TC
1310This means that it short-circuits: the right expression is evaluated
1311only if the left expression is false. Due to its precedence, you must
1312be careful to avoid using it as replacement for the C<||> operator.
1313It usually works out better for flow control than in assignments:
5a964f20 1314
db691027
SF
1315 $x = $y or $z; # bug: this is wrong
1316 ($x = $y) or $z; # really means this
1317 $x = $y || $z; # better written this way
5a964f20 1318
19799a22 1319However, when it's a list-context assignment and you're trying to use
ba7f043c 1320C<||> for control flow, you probably need C<"or"> so that the assignment
5a964f20
TC
1321takes higher precedence.
1322
1323 @info = stat($file) || die; # oops, scalar sense of stat!
1324 @info = stat($file) or die; # better, now @info gets its due
1325
c963b151
BD
1326Then again, you could always use parentheses.
1327
ba7f043c 1328Binary C<"xor"> returns the exclusive-OR of the two surrounding expressions.
c543c01b 1329It cannot short-circuit (of course).
a0d0e21e 1330
59ab9d6e
MB
1331There is no low precedence operator for defined-OR.
1332
a0d0e21e 1333=head2 C Operators Missing From Perl
d74e8afc
ITB
1334X<operator, missing from perl> X<&> X<*>
1335X<typecasting> X<(TYPE)>
a0d0e21e
LW
1336
1337Here is what C has that Perl doesn't:
1338
1339=over 8
1340
1341=item unary &
1342
ba7f043c 1343Address-of operator. (But see the C<"\"> operator for taking a reference.)
a0d0e21e
LW
1344
1345=item unary *
1346
46f8a5ea 1347Dereference-address operator. (Perl's prefix dereferencing
ba7f043c 1348operators are typed: C<$>, C<@>, C<%>, and C<&>.)
a0d0e21e
LW
1349
1350=item (TYPE)
1351
19799a22 1352Type-casting operator.
a0d0e21e
LW
1353
1354=back
1355
5f05dabc 1356=head2 Quote and Quote-like Operators
89d205f2 1357X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m>
d74e8afc
ITB
1358X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>>
1359X<escape sequence> X<escape>
1360
a0d0e21e
LW
1361While we usually think of quotes as literal values, in Perl they
1362function as operators, providing various kinds of interpolating and
1363pattern matching capabilities. Perl provides customary quote characters
1364for these behaviors, but also provides a way for you to choose your
1365quote character for any of them. In the following table, a C<{}> represents
9f10b797 1366any pair of delimiters you choose.
a0d0e21e 1367
2c268ad5
TP
1368 Customary Generic Meaning Interpolates
1369 '' q{} Literal no
1370 "" qq{} Literal yes
af9219ee 1371 `` qx{} Command yes*
2c268ad5 1372 qw{} Word list no
af9219ee
MG
1373 // m{} Pattern match yes*
1374 qr{} Pattern yes*
1375 s{}{} Substitution yes*
2c268ad5 1376 tr{}{} Transliteration no (but see below)
c543c01b 1377 y{}{} Transliteration no (but see below)
7e3b091d 1378 <<EOF here-doc yes*
a0d0e21e 1379
af9219ee
MG
1380 * unless the delimiter is ''.
1381
87275199 1382Non-bracketing delimiters use the same character fore and aft, but the four
c543c01b 1383sorts of ASCII brackets (round, angle, square, curly) all nest, which means
9f10b797 1384that
87275199 1385
c543c01b 1386 q{foo{bar}baz}
35f2feb0 1387
9f10b797 1388is the same as
87275199 1389
c543c01b 1390 'foo{bar}baz'
87275199
GS
1391
1392Note, however, that this does not always work for quoting Perl code:
1393
db691027 1394 $s = q{ if($x eq "}") ... }; # WRONG
87275199 1395
ba7f043c 1396is a syntax error. The C<L<Text::Balanced>> module (standard as of v5.8,
c543c01b 1397and from CPAN before then) is able to do this properly.
87275199 1398
19799a22 1399There can be whitespace between the operator and the quoting
fb73857a 1400characters, except when C<#> is being used as the quoting character.
ba7f043c 1401C<q#foo#> is parsed as the string C<foo>, while S<C<q #foo#>> is the
19799a22
GS
1402operator C<q> followed by a comment. Its argument will be taken
1403from the next line. This allows you to write:
fb73857a
PP
1404
1405 s {foo} # Replace foo
1406 {bar} # with bar.
1407
c543c01b
TC
1408The following escape sequences are available in constructs that interpolate,
1409and in transliterations:
5691ca5f 1410X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N> X<\N{}>
04341565 1411X<\o{}>
5691ca5f 1412
2c4c1ff2
KW
1413 Sequence Note Description
1414 \t tab (HT, TAB)
1415 \n newline (NL)
1416 \r return (CR)
1417 \f form feed (FF)
1418 \b backspace (BS)
1419 \a alarm (bell) (BEL)
1420 \e escape (ESC)
c543c01b 1421 \x{263A} [1,8] hex char (example: SMILEY)
2c4c1ff2 1422 \x1b [2,8] restricted range hex char (example: ESC)
fb121860 1423 \N{name} [3] named Unicode character or character sequence
2c4c1ff2
KW
1424 \N{U+263D} [4,8] Unicode character (example: FIRST QUARTER MOON)
1425 \c[ [5] control char (example: chr(27))
1426 \o{23072} [6,8] octal char (example: SMILEY)
1427 \033 [7,8] restricted range octal char (example: ESC)
5691ca5f
KW
1428
1429=over 4
1430
1431=item [1]
1432
2c4c1ff2
KW
1433The result is the character specified by the hexadecimal number between
1434the braces. See L</[8]> below for details on which character.
96448467 1435
46f8a5ea 1436Only hexadecimal digits are valid between the braces. If an invalid
96448467
DG
1437character is encountered, a warning will be issued and the invalid
1438character and all subsequent characters (valid or invalid) within the
1439braces will be discarded.
1440
1441If there are no valid digits between the braces, the generated character is
1442the NULL character (C<\x{00}>). However, an explicit empty brace (C<\x{}>)
c543c01b 1443will not cause a warning (currently).
40687185
KW
1444
1445=item [2]
1446
2c4c1ff2
KW
1447The result is the character specified by the hexadecimal number in the range
14480x00 to 0xFF. See L</[8]> below for details on which character.
96448467
DG
1449
1450Only hexadecimal digits are valid following C<\x>. When C<\x> is followed
2c4c1ff2 1451by fewer than two valid digits, any valid digits will be zero-padded. This
ba7f043c 1452means that C<\x7> will be interpreted as C<\x07>, and a lone C<"\x"> will be
2c4c1ff2 1453interpreted as C<\x00>. Except at the end of a string, having fewer than
c543c01b 1454two valid digits will result in a warning. Note that although the warning
96448467
DG
1455says the illegal character is ignored, it is only ignored as part of the
1456escape and will still be used as the subsequent character in the string.
1457For example:
1458
1459 Original Result Warns?
1460 "\x7" "\x07" no
1461 "\x" "\x00" no
1462 "\x7q" "\x07q" yes
1463 "\xq" "\x00q" yes
1464
40687185
KW
1465=item [3]
1466
fb121860 1467The result is the Unicode character or character sequence given by I<name>.
2c4c1ff2 1468See L<charnames>.
40687185
KW
1469
1470=item [4]
1471
ba7f043c 1472S<C<\N{U+I<hexadecimal number>}>> means the Unicode character whose Unicode code
2c4c1ff2 1473point is I<hexadecimal number>.
40687185
KW
1474
1475=item [5]
1476
5691ca5f
KW
1477The character following C<\c> is mapped to some other character as shown in the
1478table:
1479
1480 Sequence Value
1481 \c@ chr(0)
1482 \cA chr(1)
1483 \ca chr(1)
1484 \cB chr(2)
1485 \cb chr(2)
1486 ...
1487 \cZ chr(26)
1488 \cz chr(26)
1489 \c[ chr(27)
ba7f043c 1490 # See below for chr(28)
5691ca5f
KW
1491 \c] chr(29)
1492 \c^ chr(30)
c3e9d7a9 1493 \c_ chr(31)
ba7f043c
KW
1494 \c? chr(127) # (on ASCII platforms; see below for link to
1495 # EBCDIC discussion)
5691ca5f 1496
d813941f 1497In other words, it's the character whose code point has had 64 xor'd with
c3e9d7a9
KW
1498its uppercase. C<\c?> is DELETE on ASCII platforms because
1499S<C<ord("?") ^ 64>> is 127, and
ba7f043c 1500C<\c@> is NULL because the ord of C<"@"> is 64, so xor'ing 64 itself produces 0.
d813941f 1501
ba7f043c 1502Also, C<\c\I<X>> yields S<C< chr(28) . "I<X>">> for any I<X>, but cannot come at the
5691ca5f
KW
1503end of a string, because the backslash would be parsed as escaping the end
1504quote.
1505
1506On ASCII platforms, the resulting characters from the list above are the
1507complete set of ASCII controls. This isn't the case on EBCDIC platforms; see
c3e9d7a9
KW
1508L<perlebcdic/OPERATOR DIFFERENCES> for a full discussion of the
1509differences between these for ASCII versus EBCDIC platforms.
5691ca5f 1510
c3e9d7a9 1511Use of any other character following the C<"c"> besides those listed above is
63a63d81
KW
1512discouraged, and as of Perl v5.20, the only characters actually allowed
1513are the printable ASCII ones, minus the left brace C<"{">. What happens
1514for any of the allowed other characters is that the value is derived by
1515xor'ing with the seventh bit, which is 64, and a warning raised if
1516enabled. Using the non-allowed characters generates a fatal error.
5691ca5f
KW
1517
1518To get platform independent controls, you can use C<\N{...}>.
1519
40687185
KW
1520=item [6]
1521
2c4c1ff2
KW
1522The result is the character specified by the octal number between the braces.
1523See L</[8]> below for details on which character.
04341565
DG
1524
1525If a character that isn't an octal digit is encountered, a warning is raised,
1526and the value is based on the octal digits before it, discarding it and all
1527following characters up to the closing brace. It is a fatal error if there are
1528no octal digits at all.
1529
1530=item [7]
1531
c543c01b 1532The result is the character specified by the three-digit octal number in the
2c4c1ff2
KW
1533range 000 to 777 (but best to not use above 077, see next paragraph). See
1534L</[8]> below for details on which character.
1535
1536Some contexts allow 2 or even 1 digit, but any usage without exactly
40687185 1537three digits, the first being a zero, may give unintended results. (For
5db3e519
FC
1538example, in a regular expression it may be confused with a backreference;
1539see L<perlrebackslash/Octal escapes>.) Starting in Perl 5.14, you may
c543c01b 1540use C<\o{}> instead, which avoids all these problems. Otherwise, it is best to
04341565
DG
1541use this construct only for ordinals C<\077> and below, remembering to pad to
1542the left with zeros to make three digits. For larger ordinals, either use
ba7f043c
KW
1543C<\o{}>, or convert to something else, such as to hex and use C<\N{U+}>
1544(which is portable between platforms with different character sets) or
1545C<\x{}> instead.
40687185 1546
2c4c1ff2
KW
1547=item [8]
1548
c543c01b 1549Several constructs above specify a character by a number. That number
2c4c1ff2 1550gives the character's position in the character set encoding (indexed from 0).
c543c01b 1551This is called synonymously its ordinal, code position, or code point. Perl
2c4c1ff2
KW
1552works on platforms that have a native encoding currently of either ASCII/Latin1
1553or EBCDIC, each of which allow specification of 256 characters. In general, if
1554the number is 255 (0xFF, 0377) or below, Perl interprets this in the platform's
1555native encoding. If the number is 256 (0x100, 0400) or above, Perl interprets
c543c01b 1556it as a Unicode code point and the result is the corresponding Unicode
2c4c1ff2
KW
1557character. For example C<\x{50}> and C<\o{120}> both are the number 80 in
1558decimal, which is less than 256, so the number is interpreted in the native
1559character set encoding. In ASCII the character in the 80th position (indexed
ba7f043c 1560from 0) is the letter C<"P">, and in EBCDIC it is the ampersand symbol C<"&">.
2c4c1ff2
KW
1561C<\x{100}> and C<\o{400}> are both 256 in decimal, so the number is interpreted
1562as a Unicode code point no matter what the native encoding is. The name of the
9fef6a0d 1563character in the 256th position (indexed by 0) in Unicode is
2c4c1ff2
KW
1564C<LATIN CAPITAL LETTER A WITH MACRON>.
1565
9fef6a0d 1566There are a couple of exceptions to the above rule. S<C<\N{U+I<hex number>}>> is
ba7f043c
KW
1567always interpreted as a Unicode code point, so that C<\N{U+0050}> is C<"P"> even
1568on EBCDIC platforms. And if C<S<L<use encoding|encoding>>> is in effect, the
2c4c1ff2
KW
1569number is considered to be in that encoding, and is translated from that into
1570the platform's native encoding if there is a corresponding native character;
1571otherwise to Unicode.
1572
5691ca5f 1573=back
4c77eaa2 1574
e526e8bb 1575B<NOTE>: Unlike C and other languages, Perl has no C<\v> escape sequence for
8b312c40 1576the vertical tab (VT, which is 11 in both ASCII and EBCDIC), but you may
ba7f043c 1577use C<\N{VT}>, C<\ck>, C<\N{U+0b}>, or C<\x0b>. (C<\v>
e526e8bb
KW
1578does have meaning in regular expression patterns in Perl, see L<perlre>.)
1579
1580The following escape sequences are available in constructs that interpolate,
904501ec 1581but not in transliterations.
628253b8 1582X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q> X<\F>
904501ec 1583
c543c01b
TC
1584 \l lowercase next character only
1585 \u titlecase (not uppercase!) next character only
e4d34742
EB
1586 \L lowercase all characters till \E or end of string
1587 \U uppercase all characters till \E or end of string
628253b8 1588 \F foldcase all characters till \E or end of string
736fe711
KW
1589 \Q quote (disable) pattern metacharacters till \E or
1590 end of string
7e31b643 1591 \E end either case modification or quoted section
c543c01b
TC
1592 (whichever was last seen)
1593
736fe711
KW
1594See L<perlfunc/quotemeta> for the exact definition of characters that
1595are quoted by C<\Q>.
1596
628253b8 1597C<\L>, C<\U>, C<\F>, and C<\Q> can stack, in which case you need one
c543c01b
TC
1598C<\E> for each. For example:
1599
9fef6a0d
KW
1600 say"This \Qquoting \ubusiness \Uhere isn't quite\E done yet,\E is it?";
1601 This quoting\ Business\ HERE\ ISN\'T\ QUITE\ done\ yet\, is it?
a0d0e21e 1602
ba7f043c
KW
1603If a S<C<use locale>> form that includes C<LC_CTYPE> is in effect (see
1604L<perllocale>), the case map used by C<\l>, C<\L>, C<\u>, and C<\U> is
1605taken from the current locale. If Unicode (for example, C<\N{}> or code
1606points of 0x100 or beyond) is being used, the case map used by C<\l>,
1607C<\L>, C<\u>, and C<\U> is as defined by Unicode. That means that
1608case-mapping a single character can sometimes produce a sequence of
1609several characters.
1610Under S<C<use locale>>, C<\F> produces the same results as C<\L>
31f05a37
KW
1611for all locales but a UTF-8 one, where it instead uses the Unicode
1612definition.
a034a98d 1613
5a964f20
TC
1614All systems use the virtual C<"\n"> to represent a line terminator,
1615called a "newline". There is no such thing as an unvarying, physical
19799a22 1616newline character. It is only an illusion that the operating system,
5a964f20
TC
1617device drivers, C libraries, and Perl all conspire to preserve. Not all
1618systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
c543c01b 1619on the ancient Macs (pre-MacOS X) of yesteryear, these used to be reversed,
ba7f043c 1620and on systems without a line terminator,
c543c01b 1621printing C<"\n"> might emit no actual data. In general, use C<"\n"> when
5a964f20
TC
1622you mean a "newline" for your system, but use the literal ASCII when you
1623need an exact character. For example, most networking protocols expect
2a380090 1624and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
5a964f20
TC
1625and although they often accept just C<"\012">, they seldom tolerate just
1626C<"\015">. If you get in the habit of using C<"\n"> for networking,
1627you may be burned some day.
d74e8afc
ITB
1628X<newline> X<line terminator> X<eol> X<end of line>
1629X<\n> X<\r> X<\r\n>
5a964f20 1630
904501ec
MG
1631For constructs that do interpolate, variables beginning with "C<$>"
1632or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
ad0f383a
A
1633C<< $href->{key}[0] >> are also interpolated, as are array and hash slices.
1634But method calls such as C<< $obj->meth >> are not.
af9219ee
MG
1635
1636Interpolating an array or slice interpolates the elements in order,
1637separated by the value of C<$">, so is equivalent to interpolating
ba7f043c 1638S<C<join $", @array>>. "Punctuation" arrays such as C<@*> are usually
c543c01b
TC
1639interpolated only if the name is enclosed in braces C<@{*}>, but the
1640arrays C<@_>, C<@+>, and C<@-> are interpolated even without braces.
af9219ee 1641
bc7b91c6
EB
1642For double-quoted strings, the quoting from C<\Q> is applied after
1643interpolation and escapes are processed.
1644
1645 "abc\Qfoo\tbar$s\Exyz"
1646
1647is equivalent to
1648
1649 "abc" . quotemeta("foo\tbar$s") . "xyz"
1650
1651For the pattern of regex operators (C<qr//>, C<m//> and C<s///>),
1652the quoting from C<\Q> is applied after interpolation is processed,
46f8a5ea
FC
1653but before escapes are processed. This allows the pattern to match
1654literally (except for C<$> and C<@>). For example, the following matches:
bc7b91c6
EB
1655
1656 '\s\t' =~ /\Q\s\t/
1657
1658Because C<$> or C<@> trigger interpolation, you'll need to use something
1659like C</\Quser\E\@\Qhost/> to match them literally.
1d2dff63 1660
a0d0e21e
LW
1661Patterns are subject to an additional level of interpretation as a
1662regular expression. This is done as a second pass, after variables are
1663interpolated, so that regular expressions may be incorporated into the
1664pattern from the variables. If this is not what you want, use C<\Q> to
1665interpolate a variable literally.
1666
19799a22
GS
1667Apart from the behavior described above, Perl does not expand
1668multiple levels of interpolation. In particular, contrary to the
1669expectations of shell programmers, back-quotes do I<NOT> interpolate
1670within double quotes, nor do single quotes impede evaluation of
1671variables when used within double quotes.
a0d0e21e 1672
5f05dabc 1673=head2 Regexp Quote-Like Operators
d74e8afc 1674X<operator, regexp>
cb1a09d0 1675
5f05dabc 1676Here are the quote-like operators that apply to pattern
cb1a09d0
AD
1677matching and related activities.
1678
a0d0e21e
LW
1679=over 8
1680
ba7f043c 1681=item C<qr/I<STRING>/msixpodualn>
01c6f5f4 1682X<qr> X</i> X</m> X</o> X</s> X</x> X</p>
a0d0e21e 1683
87e95b7f
YO
1684This operator quotes (and possibly compiles) its I<STRING> as a regular
1685expression. I<STRING> is interpolated the same way as I<PATTERN>
ba7f043c 1686in C<m/I<PATTERN>/>. If C<"'"> is used as the delimiter, no interpolation
87e95b7f 1687is done. Returns a Perl value which may be used instead of the
ba7f043c 1688corresponding C</I<STRING>/msixpodualn> expression. The returned value is a
46f8a5ea 1689normalized version of the original pattern. It magically differs from
1c8ee595
CO
1690a string containing the same characters: C<ref(qr/x/)> returns "Regexp";
1691however, dereferencing it is not well defined (you currently get the
1692normalized version of the original pattern, but this may change).
1693
a0d0e21e 1694
87e95b7f
YO
1695For example,
1696
1697 $rex = qr/my.STRING/is;
85dd5c8b 1698 print $rex; # prints (?si-xm:my.STRING)
87e95b7f
YO
1699 s/$rex/foo/;
1700
1701is equivalent to
1702
1703 s/my.STRING/foo/is;
1704
1705The result may be used as a subpattern in a match:
1706
1707 $re = qr/$pattern/;
7188ca43
KW
1708 $string =~ /foo${re}bar/; # can be interpolated in other
1709 # patterns
87e95b7f
YO
1710 $string =~ $re; # or used standalone
1711 $string =~ /$re/; # or this way
1712
ba7f043c
KW
1713Since Perl may compile the pattern at the moment of execution of the C<qr()>
1714operator, using C<qr()> may have speed advantages in some situations,
1715notably if the result of C<qr()> is used standalone:
87e95b7f
YO
1716
1717 sub match {
1718 my $patterns = shift;
1719 my @compiled = map qr/$_/i, @$patterns;
1720 grep {
1721 my $success = 0;
1722 foreach my $pat (@compiled) {
1723 $success = 1, last if /$pat/;
1724 }
1725 $success;
1726 } @_;
5a964f20
TC
1727 }
1728
87e95b7f 1729Precompilation of the pattern into an internal representation at
ba7f043c 1730the moment of C<qr()> avoids the need to recompile the pattern every
87e95b7f
YO
1731time a match C</$pat/> is attempted. (Perl has many other internal
1732optimizations, but none would be triggered in the above example if
ba7f043c 1733we did not use C<qr()> operator.)
87e95b7f 1734
765fa144 1735Options (specified by the following modifiers) are:
87e95b7f
YO
1736
1737 m Treat string as multiple lines.
1738 s Treat string as single line. (Make . match a newline)
1739 i Do case-insensitive pattern matching.
1740 x Use extended regular expressions.
1741 p When matching preserve a copy of the matched string so
7188ca43 1742 that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be
ba7f043c
KW
1743 defined (ignored starting in v5.20) as these are always
1744 defined starting in that relese
87e95b7f 1745 o Compile pattern only once.
7188ca43 1746 a ASCII-restrict: Use ASCII for \d, \s, \w; specifying two
ba7f043c
KW
1747 a's further restricts things to that that no ASCII
1748 character will match a non-ASCII one under /i.
1749 l Use the current run-time locale's rules.
48cbae4f
SK
1750 u Use Unicode rules.
1751 d Use Unicode or native charset, as in 5.12 and earlier.
33be4c61 1752 n Non-capture mode. Don't let () fill in $1, $2, etc...
87e95b7f
YO
1753
1754If a precompiled pattern is embedded in a larger pattern then the effect
ba7f043c
KW
1755of C<"msixpluadn"> will be propagated appropriately. The effect that the
1756C</o> modifier has is not propagated, being restricted to those patterns
87e95b7f
YO
1757explicitly using it.
1758
b6fa137b 1759The last four modifiers listed above, added in Perl 5.14,
850b7ec9 1760control the character set rules, but C</a> is the only one you are likely
18509dec
KW
1761to want to specify explicitly; the other three are selected
1762automatically by various pragmas.
da392a17 1763
ba7f043c 1764See L<perlre> for additional information on valid syntax for I<STRING>, and
5e2aa8f5 1765for a detailed look at the semantics of regular expressions. In
1ca345ed
TC
1766particular, all modifiers except the largely obsolete C</o> are further
1767explained in L<perlre/Modifiers>. C</o> is described in the next section.
a0d0e21e 1768
ba7f043c 1769=item C<m/I<PATTERN>/msixpodualngc>
89d205f2
YO
1770X<m> X<operator, match>
1771X<regexp, options> X<regexp> X<regex, options> X<regex>
01c6f5f4 1772X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c>
a0d0e21e 1773
ba7f043c 1774=item C</I<PATTERN>/msixpodualngc>
a0d0e21e 1775
5a964f20 1776Searches a string for a pattern match, and in scalar context returns
19799a22 1777true if it succeeds, false if it fails. If no string is specified
ba7f043c 1778via the C<=~> or C<!~> operator, the C<$_> string is searched. (The
19799a22
GS
1779string specified with C<=~> need not be an lvalue--it may be the
1780result of an expression evaluation, but remember the C<=~> binds
006671a6 1781rather tightly.) See also L<perlre>.
a0d0e21e 1782
f6050459 1783Options are as described in C<qr//> above; in addition, the following match
01c6f5f4 1784process modifiers are available:
a0d0e21e 1785
950b09ed 1786 g Match globally, i.e., find all occurrences.
7188ca43
KW
1787 c Do not reset search position on a failed match when /g is
1788 in effect.
a0d0e21e 1789
ba7f043c 1790If C<"/"> is the delimiter then the initial C<m> is optional. With the C<m>
c543c01b 1791you can use any pair of non-whitespace (ASCII) characters
725a61d7 1792as delimiters. This is particularly useful for matching path names
ba7f043c 1793that contain C<"/">, to avoid LTS (leaning toothpick syndrome). If C<"?"> is
725a61d7 1794the delimiter, then a match-only-once rule applies,
ba7f043c
KW
1795described in C<m?I<PATTERN>?> below. If C<"'"> (single quote) is the delimiter,
1796no interpolation is performed on the I<PATTERN>.
1797When using a delimiter character valid in an identifier, whitespace is required
ed02a3bf 1798after the C<m>.
a0d0e21e 1799
ba7f043c 1800I<PATTERN> may contain variables, which will be interpolated
532c9e80 1801every time the pattern search is evaluated, except
1f247705
GS
1802for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
1803C<$|> are not interpolated because they look like end-of-string tests.)
532c9e80
KW
1804Perl will not recompile the pattern unless an interpolated
1805variable that it contains changes. You can force Perl to skip the
1806test and never recompile by adding a C</o> (which stands for "once")
1807after the trailing delimiter.
1808Once upon a time, Perl would recompile regular expressions
1809unnecessarily, and this modifier was useful to tell it not to do so, in the
5cc41653 1810interests of speed. But now, the only reasons to use C</o> are one of:
532c9e80
KW
1811
1812=over
1813
1814=item 1
1815
1816The variables are thousands of characters long and you know that they
1817don't change, and you need to wring out the last little bit of speed by
1818having Perl skip testing for that. (There is a maintenance penalty for
1819doing this, as mentioning C</o> constitutes a promise that you won't
18509dec 1820change the variables in the pattern. If you do change them, Perl won't
532c9e80
KW
1821even notice.)
1822
1823=item 2
1824
1825you want the pattern to use the initial values of the variables
1826regardless of whether they change or not. (But there are saner ways
1827of accomplishing this than using C</o>.)
1828
fa9b8686
DM
1829=item 3
1830
1831If the pattern contains embedded code, such as
1832
1833 use re 'eval';
1834 $code = 'foo(?{ $x })';
1835 /$code/
1836
1837then perl will recompile each time, even though the pattern string hasn't
1838changed, to ensure that the current value of C<$x> is seen each time.
1839Use C</o> if you want to avoid this.
1840
532c9e80 1841=back
a0d0e21e 1842
18509dec
KW
1843The bottom line is that using C</o> is almost never a good idea.
1844
ba7f043c 1845=item The empty pattern C<//>
e9d89077 1846
ba7f043c 1847If the I<PATTERN> evaluates to the empty string, the last
46f8a5ea 1848I<successfully> matched regular expression is used instead. In this
c543c01b 1849case, only the C<g> and C<c> flags on the empty pattern are honored;
46f8a5ea 1850the other flags are taken from the original pattern. If no match has
d65afb4b
HS
1851previously succeeded, this will (silently) act instead as a genuine
1852empty pattern (which will always match).
a0d0e21e 1853
89d205f2
YO
1854Note that it's possible to confuse Perl into thinking C<//> (the empty
1855regex) is really C<//> (the defined-or operator). Perl is usually pretty
1856good about this, but some pathological cases might trigger this, such as
ba7f043c
KW
1857C<$x///> (is that S<C<($x) / (//)>> or S<C<$x // />>?) and S<C<print $fh //>>
1858(S<C<print $fh(//>> or S<C<print($fh //>>?). In all of these examples, Perl
89d205f2
YO
1859will assume you meant defined-or. If you meant the empty regex, just
1860use parentheses or spaces to disambiguate, or even prefix the empty
c963b151
BD
1861regex with an C<m> (so C<//> becomes C<m//>).
1862
e9d89077
DN
1863=item Matching in list context
1864
19799a22 1865If the C</g> option is not used, C<m//> in list context returns a
a0d0e21e 1866list consisting of the subexpressions matched by the parentheses in the
3ff8ecf9
BF
1867pattern, that is, (C<$1>, C<$2>, C<$3>...) (Note that here C<$1> etc. are
1868also set). When there are no parentheses in the pattern, the return
1869value is the list C<(1)> for success.
1870With or without parentheses, an empty list is returned upon failure.
a0d0e21e
LW
1871
1872Examples:
1873
7188ca43
KW
1874 open(TTY, "+</dev/tty")
1875 || die "can't access /dev/tty: $!";
c543c01b 1876
7188ca43 1877 <TTY> =~ /^y/i && foo(); # do foo if desired
a0d0e21e 1878
7188ca43 1879 if (/Version: *([0-9.]*)/) { $version = $1; }
a0d0e21e 1880
7188ca43 1881 next if m#^/usr/spool/uucp#;
a0d0e21e 1882
7188ca43
KW
1883 # poor man's grep
1884 $arg = shift;
1885 while (<>) {
1886 print if /$arg/o; # compile only once (no longer needed!)
1887 }
a0d0e21e 1888
7188ca43 1889 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
a0d0e21e 1890
ba7f043c
KW
1891This last example splits C<$foo> into the first two words and the
1892remainder of the line, and assigns those three fields to C<$F1>, C<$F2>, and
1893C<$Etc>. The conditional is true if any variables were assigned; that is,
c543c01b 1894if the pattern matched.
a0d0e21e 1895
19799a22 1896The C</g> modifier specifies global pattern matching--that is,
46f8a5ea
FC
1897matching as many times as possible within the string. How it behaves
1898depends on the context. In list context, it returns a list of the
19799a22 1899substrings matched by any capturing parentheses in the regular
46f8a5ea 1900expression. If there are no parentheses, it returns a list of all
19799a22
GS
1901the matched strings, as if there were parentheses around the whole
1902pattern.
a0d0e21e 1903
7e86de3e 1904In scalar context, each execution of C<m//g> finds the next match,
19799a22 1905returning true if it matches, and false if there is no further match.
3dd93342 1906The position after the last match can be read or set using the C<pos()>
46f8a5ea 1907function; see L<perlfunc/pos>. A failed match normally resets the
7e86de3e 1908search position to the beginning of the string, but you can avoid that
46f8a5ea 1909by adding the C</c> modifier (for example, C<m//gc>). Modifying the target
7e86de3e 1910string also resets the search position.
c90c0ff4 1911
ba7f043c 1912=item C<\G I<assertion>>
e9d89077 1913
c90c0ff4 1914You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
3dd93342 1915zero-width assertion that matches the exact position where the
46f8a5ea 1916previous C<m//g>, if any, left off. Without the C</g> modifier, the
3dd93342 1917C<\G> assertion still anchors at C<pos()> as it was at the start of
1918the operation (see L<perlfunc/pos>), but the match is of course only
46f8a5ea 1919attempted once. Using C<\G> without C</g> on a target string that has
3dd93342 1920not previously had a C</g> match applied to it is the same as using
1921the C<\A> assertion to match the beginning of the string. Note also
1922that, currently, C<\G> is only properly supported when anchored at the
1923very beginning of the pattern.
c90c0ff4
PP
1924
1925Examples:
a0d0e21e
LW
1926
1927 # list context
1928 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
1929
1930 # scalar context
c543c01b
TC
1931 local $/ = "";
1932 while ($paragraph = <>) {
1933 while ($paragraph =~ /\p{Ll}['")]*[.!?]+['")]*\s/g) {
19799a22 1934 $sentences++;
a0d0e21e
LW
1935 }
1936 }
c543c01b
TC
1937 say $sentences;
1938
1939Here's another way to check for sentences in a paragraph:
1940
7188ca43
KW
1941 my $sentence_rx = qr{
1942 (?: (?<= ^ ) | (?<= \s ) ) # after start-of-string or
1943 # whitespace
1944 \p{Lu} # capital letter
1945 .*? # a bunch of anything
1946 (?<= \S ) # that ends in non-
1947 # whitespace
1948 (?<! \b [DMS]r ) # but isn't a common abbr.
1949 (?<! \b Mrs )
1950 (?<! \b Sra )
1951 (?<! \b St )
1952 [.?!] # followed by a sentence
1953 # ender
1954 (?= $ | \s ) # in front of end-of-string
1955 # or whitespace
1956 }sx;
1957 local $/ = "";
1958 while (my $paragraph = <>) {
1959 say "NEW PARAGRAPH";
1960 my $count = 0;
1961 while ($paragraph =~ /($sentence_rx)/g) {
1962 printf "\tgot sentence %d: <%s>\n", ++$count, $1;
c543c01b 1963 }
7188ca43 1964 }
c543c01b
TC
1965
1966Here's how to use C<m//gc> with C<\G>:
a0d0e21e 1967
137443ea 1968 $_ = "ppooqppqq";
44a8e56a
PP
1969 while ($i++ < 2) {
1970 print "1: '";
c90c0ff4 1971 print $1 while /(o)/gc; print "', pos=", pos, "\n";
44a8e56a 1972 print "2: '";
c90c0ff4 1973 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
44a8e56a 1974 print "3: '";
c90c0ff4 1975 print $1 while /(p)/gc; print "', pos=", pos, "\n";
44a8e56a 1976 }
5d43e42d 1977 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
44a8e56a
PP
1978
1979The last example should print:
1980
1981 1: 'oo', pos=4
137443ea 1982 2: 'q', pos=5
44a8e56a
PP
1983 3: 'pp', pos=7
1984 1: '', pos=7
137443ea
PP
1985 2: 'q', pos=8
1986 3: '', pos=8
5d43e42d
DC
1987 Final: 'q', pos=8
1988
1989Notice that the final match matched C<q> instead of C<p>, which a match
46f8a5ea
FC
1990without the C<\G> anchor would have done. Also note that the final match
1991did not update C<pos>. C<pos> is only updated on a C</g> match. If the
c543c01b
TC
1992final match did indeed match C<p>, it's a good bet that you're running a
1993very old (pre-5.6.0) version of Perl.
44a8e56a 1994
c90c0ff4 1995A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
e7ea3e70 1996combine several regexps like this to process a string part-by-part,
c90c0ff4
PP
1997doing different actions depending on which regexp matched. Each
1998regexp tries to match where the previous one leaves off.
e7ea3e70 1999
3fe9a6f1 2000 $_ = <<'EOL';
7188ca43
KW
2001 $url = URI::URL->new( "http://example.com/" );
2002 die if $url eq "xXx";
3fe9a6f1 2003 EOL
c543c01b
TC
2004
2005 LOOP: {
950b09ed 2006 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
7188ca43
KW
2007 print(" lowercase"), redo LOOP
2008 if /\G\p{Ll}+\b[,.;]?\s*/gc;
2009 print(" UPPERCASE"), redo LOOP
2010 if /\G\p{Lu}+\b[,.;]?\s*/gc;
2011 print(" Capitalized"), redo LOOP
2012 if /\G\p{Lu}\p{Ll}+\b[,.;]?\s*/gc;
c543c01b 2013 print(" MiXeD"), redo LOOP if /\G\pL+\b[,.;]?\s*/gc;
7188ca43
KW
2014 print(" alphanumeric"), redo LOOP
2015 if /\G[\p{Alpha}\pN]+\b[,.;]?\s*/gc;
c543c01b 2016 print(" line-noise"), redo LOOP if /\G\W+/gc;
950b09ed 2017 print ". That's all!\n";
c543c01b 2018 }
e7ea3e70
IZ
2019
2020Here is the output (split into several lines):
2021
7188ca43
KW
2022 line-noise lowercase line-noise UPPERCASE line-noise UPPERCASE
2023 line-noise lowercase line-noise lowercase line-noise lowercase
2024 lowercase line-noise lowercase lowercase line-noise lowercase
2025 lowercase line-noise MiXeD line-noise. That's all!
44a8e56a 2026
ba7f043c 2027=item C<m?I<PATTERN>?msixpodualngc>
725a61d7 2028X<?> X<operator, match-once>
87e95b7f 2029
ba7f043c 2030=item C<?I<PATTERN>?msixpodualngc>
55d389e7 2031
ba7f043c
KW
2032This is just like the C<m/I<PATTERN>/> search, except that it matches
2033only once between calls to the C<reset()> operator. This is a useful
87e95b7f 2034optimization when you want to see only the first occurrence of
ceb131e8 2035something in each file of a set of files, for instance. Only C<m??>
87e95b7f
YO
2036patterns local to the current package are reset.
2037
2038 while (<>) {
ceb131e8 2039 if (m?^$?) {
87e95b7f
YO
2040 # blank line between header and body
2041 }
2042 } continue {
725a61d7 2043 reset if eof; # clear m?? status for next file
87e95b7f
YO
2044 }
2045
c543c01b
TC
2046Another example switched the first "latin1" encoding it finds
2047to "utf8" in a pod file:
2048
2049 s//utf8/ if m? ^ =encoding \h+ \K latin1 ?x;
2050
2051The match-once behavior is controlled by the match delimiter being
4932eeca 2052C<?>; with any other delimiter this is the normal C<m//> operator.
725a61d7 2053
ba7f043c 2054In the past, the leading C<m> in C<m?I<PATTERN>?> was optional, but omitting it
0381ecf1
MH
2055would produce a deprecation warning. As of v5.22.0, omitting it produces a
2056syntax error. If you encounter this construct in older code, you can just add
2057C<m>.
87e95b7f 2058
ba7f043c 2059=item C<s/I<PATTERN>/I<REPLACEMENT>/msixpodualngcer>
87e95b7f 2060X<substitute> X<substitution> X<replace> X<regexp, replace>
4f4d7508 2061X<regexp, substitute> X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> X</e> X</r>
87e95b7f
YO
2062
2063Searches a string for a pattern, and if found, replaces that pattern
2064with the replacement text and returns the number of substitutions
2065made. Otherwise it returns false (specifically, the empty string).
2066
c543c01b 2067If the C</r> (non-destructive) option is used then it runs the
679563bb
KW
2068substitution on a copy of the string and instead of returning the
2069number of substitutions, it returns the copy whether or not a
c543c01b
TC
2070substitution occurred. The original string is never changed when
2071C</r> is used. The copy will always be a plain string, even if the
2072input is an object or a tied variable.
4f4d7508 2073
87e95b7f 2074If no string is specified via the C<=~> or C<!~> operator, the C<$_>
c543c01b
TC
2075variable is searched and modified. Unless the C</r> option is used,
2076the string specified must be a scalar variable, an array element, a
2077hash element, or an assignment to one of those; that is, some sort of
2078scalar lvalue.
87e95b7f
YO
2079
2080If the delimiter chosen is a single quote, no interpolation is
ba7f043c
KW
2081done on either the I<PATTERN> or the I<REPLACEMENT>. Otherwise, if the
2082I<PATTERN> contains a C<$> that looks like a variable rather than an
87e95b7f
YO
2083end-of-string test, the variable will be interpolated into the pattern
2084at run-time. If you want the pattern compiled only once the first time
2085the variable is interpolated, use the C</o> option. If the pattern
2086evaluates to the empty string, the last successfully executed regular
2087expression is used instead. See L<perlre> for further explanation on these.
87e95b7f 2088
ba7f043c 2089Options are as with C<m//> with the addition of the following replacement
87e95b7f
YO
2090specific options:
2091
2092 e Evaluate the right side as an expression.
7188ca43
KW
2093 ee Evaluate the right side as a string then eval the
2094 result.
2095 r Return substitution and leave the original string
2096 untouched.
87e95b7f 2097
ed02a3bf
DN
2098Any non-whitespace delimiter may replace the slashes. Add space after
2099the C<s> when using a character allowed in identifiers. If single quotes
2100are used, no interpretation is done on the replacement string (the C</e>
3ff8ecf9 2101modifier overrides this, however). Note that Perl treats backticks
ed02a3bf 2102as normal delimiters; the replacement text is not evaluated as a command.
ba7f043c 2103If the I<PATTERN> is delimited by bracketing quotes, the I<REPLACEMENT> has
1ca345ed 2104its own pair of quotes, which may or may not be bracketing quotes, for example,
87e95b7f
YO
2105C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
2106replacement portion to be treated as a full-fledged Perl expression
2107and evaluated right then and there. It is, however, syntax checked at
46f8a5ea 2108compile-time. A second C<e> modifier will cause the replacement portion
87e95b7f
YO
2109to be C<eval>ed before being run as a Perl expression.
2110
2111Examples:
2112
7188ca43 2113 s/\bgreen\b/mauve/g; # don't change wintergreen
87e95b7f
YO
2114
2115 $path =~ s|/usr/bin|/usr/local/bin|;
2116
2117 s/Login: $foo/Login: $bar/; # run-time pattern
2118
7188ca43
KW
2119 ($foo = $bar) =~ s/this/that/; # copy first, then
2120 # change
2121 ($foo = "$bar") =~ s/this/that/; # convert to string,
2122 # copy, then change
4f4d7508
DC
2123 $foo = $bar =~ s/this/that/r; # Same as above using /r
2124 $foo = $bar =~ s/this/that/r
7188ca43
KW
2125 =~ s/that/the other/r; # Chained substitutes
2126 # using /r
2127 @foo = map { s/this/that/r } @bar # /r is very useful in
2128 # maps
87e95b7f 2129
7188ca43 2130 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-cnt
87e95b7f
YO
2131
2132 $_ = 'abc123xyz';
2133 s/\d+/$&*2/e; # yields 'abc246xyz'
2134 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
2135 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
2136
2137 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
2138 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
2139 s/^=(\w+)/pod($1)/ge; # use function call
2140
4f4d7508 2141 $_ = 'abc123xyz';
db691027 2142 $x = s/abc/def/r; # $x is 'def123xyz' and
4f4d7508
DC
2143 # $_ remains 'abc123xyz'.
2144
87e95b7f
YO
2145 # expand variables in $_, but dynamics only, using
2146 # symbolic dereferencing
2147 s/\$(\w+)/${$1}/g;
2148
2149 # Add one to the value of any numbers in the string
2150 s/(\d+)/1 + $1/eg;
2151
c543c01b
TC
2152 # Titlecase words in the last 30 characters only
2153 substr($str, -30) =~ s/\b(\p{Alpha}+)\b/\u\L$1/g;
2154
87e95b7f
YO
2155 # This will expand any embedded scalar variable
2156 # (including lexicals) in $_ : First $1 is interpolated
2157 # to the variable name, and then evaluated
2158 s/(\$\w+)/$1/eeg;
2159
2160 # Delete (most) C comments.
2161 $program =~ s {
2162 /\* # Match the opening delimiter.
2163 .*? # Match a minimal number of characters.
2164 \*/ # Match the closing delimiter.
2165 } []gsx;
2166
7188ca43
KW
2167 s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_,
2168 # expensively
87e95b7f 2169
7188ca43
KW
2170 for ($variable) { # trim whitespace in $variable,
2171 # cheap
87e95b7f
YO
2172 s/^\s+//;
2173 s/\s+$//;
2174 }
2175
2176 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
2177
ba7f043c
KW
2178Note the use of C<$> instead of C<\> in the last example. Unlike
2179B<sed>, we use the \<I<digit>> form only in the left hand side.
87e95b7f
YO
2180Anywhere else it's $<I<digit>>.
2181
2182Occasionally, you can't use just a C</g> to get all the changes
2183to occur that you might want. Here are two common cases:
2184
2185 # put commas in the right places in an integer
2186 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
2187
2188 # expand tabs to 8-column spacing
2189 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
2190
2191=back
2192
2193=head2 Quote-Like Operators
2194X<operator, quote-like>
2195
01c6f5f4
RGS
2196=over 4
2197
ba7f043c 2198=item C<q/I<STRING>/>
5d44bfff 2199X<q> X<quote, single> X<'> X<''>
a0d0e21e 2200
ba7f043c 2201=item C<'I<STRING>'>
a0d0e21e 2202
19799a22 2203A single-quoted, literal string. A backslash represents a backslash
68dc0745
PP
2204unless followed by the delimiter or another backslash, in which case
2205the delimiter or backslash is interpolated.
a0d0e21e
LW
2206
2207 $foo = q!I said, "You said, 'She said it.'"!;
2208 $bar = q('This is it.');
68dc0745 2209 $baz = '\n'; # a two-character string
a0d0e21e 2210
ba7f043c 2211=item C<qq/I<STRING>/>
d74e8afc 2212X<qq> X<quote, double> X<"> X<"">
a0d0e21e 2213
ba7f043c 2214=item "I<STRING>"
a0d0e21e
LW
2215
2216A double-quoted, interpolated string.
2217
2218 $_ .= qq
2219 (*** The previous line contains the naughty word "$1".\n)
19799a22 2220 if /\b(tcl|java|python)\b/i; # :-)
68dc0745 2221 $baz = "\n"; # a one-character string
a0d0e21e 2222
ba7f043c 2223=item C<qx/I<STRING>/>
d74e8afc 2224X<qx> X<`> X<``> X<backtick>
a0d0e21e 2225
ba7f043c 2226=item C<`I<STRING>`>
a0d0e21e 2227
43dd4d21 2228A string which is (possibly) interpolated and then executed as a
f703fc96 2229system command with F</bin/sh> or its equivalent. Shell wildcards,
43dd4d21
JH
2230pipes, and redirections will be honored. The collected standard
2231output of the command is returned; standard error is unaffected. In
2232scalar context, it comes back as a single (potentially multi-line)
ba7f043c
KW
2233string, or C<undef> if the command failed. In list context, returns a
2234list of lines (however you've defined lines with C<$/> or
2235C<$INPUT_RECORD_SEPARATOR>), or an empty list if the command failed.
5a964f20
TC
2236
2237Because backticks do not affect standard error, use shell file descriptor
2238syntax (assuming the shell supports this) if you care to address this.
2239To capture a command's STDERR and STDOUT together:
a0d0e21e 2240
5a964f20
TC
2241 $output = `cmd 2>&1`;
2242
2243To capture a command's STDOUT but discard its STDERR:
2244
2245 $output = `cmd 2>/dev/null`;
2246
2247To capture a command's STDERR but discard its STDOUT (ordering is
2248important here):
2249
2250 $output = `cmd 2>&1 1>/dev/null`;
2251
2252To exchange a command's STDOUT and STDERR in order to capture the STDERR
2253but leave its STDOUT to come out the old STDERR:
2254
2255 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
2256
2257To read both a command's STDOUT and its STDERR separately, it's easiest
2359510d
SD
2258to redirect them separately to files, and then read from those files
2259when the program is done:
5a964f20 2260
2359510d 2261 system("program args 1>program.stdout 2>program.stderr");
5a964f20 2262
30398227
SP
2263The STDIN filehandle used by the command is inherited from Perl's STDIN.
2264For example:
2265
c543c01b
TC
2266 open(SPLAT, "stuff") || die "can't open stuff: $!";
2267 open(STDIN, "<&SPLAT") || die "can't dupe SPLAT: $!";
40bbb707 2268 print STDOUT `sort`;
30398227 2269
40bbb707 2270will print the sorted contents of the file named F<"stuff">.
30398227 2271
5a964f20
TC
2272Using single-quote as a delimiter protects the command from Perl's
2273double-quote interpolation, passing it on to the shell instead:
2274
2275 $perl_info = qx(ps $$); # that's Perl's $$
2276 $shell_info = qx'ps $$'; # that's the new shell's $$
2277
19799a22 2278How that string gets evaluated is entirely subject to the command
5a964f20
TC
2279interpreter on your system. On most platforms, you will have to protect
2280shell metacharacters if you want them treated literally. This is in
2281practice difficult to do, as it's unclear how to escape which characters.
ba7f043c 2282See L<perlsec> for a clean and safe example of a manual C<fork()> and C<exec()>
5a964f20 2283to emulate backticks safely.
a0d0e21e 2284
bb32b41a
GS
2285On some platforms (notably DOS-like ones), the shell may not be
2286capable of dealing with multiline commands, so putting newlines in
2287the string may not get you what you want. You may be able to evaluate
2288multiple commands in a single line by separating them with the command
1ca345ed
TC
2289separator character, if your shell supports that (for example, C<;> on
2290many Unix shells and C<&> on the Windows NT C<cmd> shell).
bb32b41a 2291
3ff8ecf9 2292Perl will attempt to flush all files opened for
0f897271
GS
2293output before starting the child process, but this may not be supported
2294on some platforms (see L<perlport>). To be safe, you may need to set
ba7f043c
KW
2295C<$|> (C<$AUTOFLUSH> in C<L<English>>) or call the C<autoflush()> method of
2296C<L<IO::Handle>> on any open handles.
0f897271 2297
bb32b41a
GS
2298Beware that some command shells may place restrictions on the length
2299of the command line. You must ensure your strings don't exceed this
2300limit after any necessary interpolations. See the platform-specific
2301release notes for more details about your particular environment.
2302
5a964f20
TC
2303Using this operator can lead to programs that are difficult to port,
2304because the shell commands called vary between systems, and may in
2305fact not be present at all. As one example, the C<type> command under
2306the POSIX shell is very different from the C<type> command under DOS.
2307That doesn't mean you should go out of your way to avoid backticks
2308when they're the right way to get something done. Perl was made to be
2309a glue language, and one of the things it glues together is commands.
2310Just understand what you're getting yourself into.
bb32b41a 2311
da87341d 2312See L</"I/O Operators"> for more discussion.
a0d0e21e 2313
ba7f043c 2314=item C<qw/I<STRING>/>
d74e8afc 2315X<qw> X<quote, list> X<quote, words>
945c54fd 2316
ba7f043c 2317Evaluates to a list of the words extracted out of I<STRING>, using embedded
945c54fd
JH
2318whitespace as the word delimiters. It can be understood as being roughly
2319equivalent to:
2320
c543c01b 2321 split(" ", q/STRING/);
945c54fd 2322
efb1e162
CW
2323the differences being that it generates a real list at compile time, and
2324in scalar context it returns the last element in the list. So
945c54fd
JH
2325this expression:
2326
2327 qw(foo bar baz)
2328
2329is semantically equivalent to the list:
2330
c543c01b 2331 "foo", "bar", "baz"
945c54fd
JH
2332
2333Some frequently seen examples:
2334
2335 use POSIX qw( setlocale localeconv )
2336 @EXPORT = qw( foo bar baz );
2337
ba7f043c 2338A common mistake is to try to separate the words with commas or to
945c54fd 2339put comments into a multi-line C<qw>-string. For this reason, the
ba7f043c
KW
2340S<C<use warnings>> pragma and the B<-w> switch (that is, the C<$^W> variable)
2341produces warnings if the I<STRING> contains the C<","> or the C<"#"> character.
945c54fd 2342
ba7f043c 2343=item C<tr/I<SEARCHLIST>/I<REPLACEMENTLIST>/cdsr>
d74e8afc 2344X<tr> X<y> X<transliterate> X</c> X</d> X</s>
a0d0e21e 2345
ba7f043c 2346=item C<y/I<SEARCHLIST>/I<REPLACEMENTLIST>/cdsr>
a0d0e21e 2347
2c268ad5 2348Transliterates all occurrences of the characters found in the search list
a0d0e21e
LW
2349with the corresponding character in the replacement list. It returns
2350the number of characters replaced or deleted. If no string is
ba7f043c 2351specified via the C<=~> or C<!~> operator, the C<$_> string is transliterated.
c543c01b
TC
2352
2353If the C</r> (non-destructive) option is present, a new copy of the string
2354is made and its characters transliterated, and this copy is returned no
2355matter whether it was modified or not: the original string is always
2356left unchanged. The new copy is always a plain string, even if the input
2357string is an object or a tied variable.
8ada0baa 2358
c543c01b
TC
2359Unless the C</r> option is used, the string specified with C<=~> must be a
2360scalar variable, an array element, a hash element, or an assignment to one
2361of those; in other words, an lvalue.
8ff32507 2362
89d205f2 2363A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
2c268ad5 2364does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
54310121 2365For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
ba7f043c 2366I<SEARCHLIST> is delimited by bracketing quotes, the I<REPLACEMENTLIST> has
c543c01b
TC
2367its own pair of quotes, which may or may not be bracketing quotes;
2368for example, C<tr[aeiouy][yuoiea]> or C<tr(+\-*/)/ABCD/>.
2369
ba7f043c
KW
2370Characters may be literals or any of the escape sequences accepted in
2371double-quoted strings. But there is no interpolation, so C<"$"> and
2372C<"@"> are treated as literals. A hyphen at the beginning or end, or
2373preceded by a backslash is considered a literal. Escape sequence
2374details are in L<the table near the beginning of this section|/Quote and
2375Quote-like Operators>. It is a bug in Perl v5.22 that something like
2376
2377 tr/\N{U+20}-\N{U+7E}foobar//
2378
2379does not treat that range as fully Unicode.
2380
c543c01b 2381Note that C<tr> does B<not> do regular expression character classes such as
ba7f043c 2382C<\d> or C<\pL>. The C<tr> operator is not equivalent to the C<L<tr(1)>>
c543c01b
TC
2383utility. If you want to map strings between lower/upper cases, see
2384L<perlfunc/lc> and L<perlfunc/uc>, and in general consider using the C<s>
2385operator if you need regular expressions. The C<\U>, C<\u>, C<\L>, and
2386C<\l> string-interpolation escapes on the right side of a substitution
2387operator will perform correct case-mappings, but C<tr[a-z][A-Z]> will not
2388(except sometimes on legacy 7-bit data).
cc255d5f 2389
8ada0baa
JH
2390Note also that the whole range idea is rather unportable between
2391character sets--and even within character sets they may cause results
2392you probably didn't expect. A sound principle is to use only ranges
2393that begin from and end at either alphabets of equal case (a-e, A-E),
2394or digits (0-4). Anything else is unsafe. If in doubt, spell out the
2395character sets in full.
2396
a0d0e21e
LW
2397Options:
2398
2399 c Complement the SEARCHLIST.
2400 d Delete found but unreplaced characters.
2401 s Squash duplicate replaced characters.
8ff32507
FC
2402 r Return the modified string and leave the original string
2403 untouched.
a0d0e21e 2404
ba7f043c 2405If the C</c> modifier is specified, the I<SEARCHLIST> character set
19799a22 2406is complemented. If the C</d> modifier is specified, any characters
ba7f043c 2407specified by I<SEARCHLIST> not found in I<REPLACEMENTLIST> are deleted.
19799a22 2408(Note that this is slightly more flexible than the behavior of some
ba7f043c 2409B<tr> programs, which delete anything they find in the I<SEARCHLIST>,
46f8a5ea 2410period.) If the C</s> modifier is specified, sequences of characters
19799a22
GS
2411that were transliterated to the same character are squashed down
2412to a single instance of the character.
a0d0e21e 2413
ba7f043c
KW
2414If the C</d> modifier is used, the I<REPLACEMENTLIST> is always interpreted
2415exactly as specified. Otherwise, if the I<REPLACEMENTLIST> is shorter
2416than the I<SEARCHLIST>, the final character is replicated till it is long
2417enough. If the I<REPLACEMENTLIST> is empty, the I<SEARCHLIST> is replicated.
a0d0e21e
LW
2418This latter is useful for counting characters in a class or for
2419squashing character sequences in a class.
2420
2421Examples:
2422
c543c01b 2423 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case ASCII
a0d0e21e
LW
2424
2425 $cnt = tr/*/*/; # count the stars in $_
2426
2427 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
2428
2429 $cnt = tr/0-9//; # count the digits in $_
2430
2431 tr/a-zA-Z//s; # bookkeeper -> bokeper
2432
2433 ($HOST = $host) =~ tr/a-z/A-Z/;
c543c01b 2434 $HOST = $host =~ tr/a-z/A-Z/r; # same thing
8ff32507 2435
c543c01b 2436 $HOST = $host =~ tr/a-z/A-Z/r # chained with s///r
8ff32507 2437 =~ s/:/ -p/r;
a0d0e21e
LW
2438
2439 tr/a-zA-Z/ /cs; # change non-alphas to single space
2440
8ff32507
FC
2441 @stripped = map tr/a-zA-Z/ /csr, @original;
2442 # /r with map
2443
a0d0e21e 2444 tr [\200-\377]
c543c01b 2445 [\000-\177]; # wickedly delete 8th bit
a0d0e21e 2446
19799a22
GS
2447If multiple transliterations are given for a character, only the
2448first one is used:
748a9306
LW
2449
2450 tr/AAA/XYZ/
2451
2c268ad5 2452will transliterate any A to X.
748a9306 2453
19799a22 2454Because the transliteration table is built at compile time, neither
ba7f043c 2455the I<SEARCHLIST> nor the I<REPLACEMENTLIST> are subjected to double quote
19799a22 2456interpolation. That means that if you want to use variables, you
ba7f043c 2457must use an C<eval()>:
a0d0e21e
LW
2458
2459 eval "tr/$oldlist/$newlist/";
2460 die $@ if $@;
2461
2462 eval "tr/$oldlist/$newlist/, 1" or die $@;
2463
ba7f043c 2464=item C<< <<I<EOF> >>
d74e8afc 2465X<here-doc> X<heredoc> X<here-document> X<<< << >>>
7e3b091d
DA
2466
2467A line-oriented form of quoting is based on the shell "here-document"
2468syntax. Following a C<< << >> you specify a string to terminate
2469the quoted material, and all lines following the current line down to
89d205f2
YO
2470the terminating string are the value of the item.
2471
2472The terminating string may be either an identifier (a word), or some
2473quoted text. An unquoted identifier works like double quotes.
2474There may not be a space between the C<< << >> and the identifier,
2475unless the identifier is explicitly quoted. (If you put a space it
2476will be treated as a null identifier, which is valid, and matches the
2477first empty line.) The terminating string must appear by itself
2478(unquoted and with no surrounding whitespace) on the terminating line.
2479
2480If the terminating string is quoted, the type of quotes used determine
2481the treatment of the text.
2482
2483=over 4
2484
2485=item Double Quotes
2486
2487Double quotes indicate that the text will be interpolated using exactly
2488the same rules as normal double quoted strings.
7e3b091d
DA
2489
2490 print <<EOF;
2491 The price is $Price.
2492 EOF
2493
2494 print << "EOF"; # same as above
2495 The price is $Price.
2496 EOF
2497
89d205f2
YO
2498
2499=item Single Quotes
2500
2501Single quotes indicate the text is to be treated literally with no
46f8a5ea 2502interpolation of its content. This is similar to single quoted
89d205f2
YO
2503strings except that backslashes have no special meaning, with C<\\>
2504being treated as two backslashes and not one as they would in every
2505other quoting construct.
2506
c543c01b
TC
2507Just as in the shell, a backslashed bareword following the C<<< << >>>
2508means the same thing as a single-quoted string does:
2509
2510 $cost = <<'VISTA'; # hasta la ...
2511 That'll be $10 please, ma'am.
2512 VISTA
2513
2514 $cost = <<\VISTA; # Same thing!
2515 That'll be $10 please, ma'am.
2516 VISTA
2517
89d205f2
YO
2518This is the only form of quoting in perl where there is no need
2519to worry about escaping content, something that code generators
2520can and do make good use of.
2521
2522=item Backticks
2523
2524The content of the here doc is treated just as it would be if the
46f8a5ea 2525string were embedded in backticks. Thus the content is interpolated
89d205f2
YO
2526as though it were double quoted and then executed via the shell, with
2527the results of the execution returned.
2528
2529 print << `EOC`; # execute command and get results
7e3b091d 2530 echo hi there
7e3b091d
DA
2531 EOC
2532
89d205f2
YO
2533=back
2534
2535It is possible to stack multiple here-docs in a row:
2536
7e3b091d
DA
2537 print <<"foo", <<"bar"; # you can stack them
2538 I said foo.
2539 foo
2540 I said bar.
2541 bar
2542
2543 myfunc(<< "THIS", 23, <<'THAT');
2544 Here's a line
2545 or two.
2546 THIS
2547 and here's another.
2548 THAT
2549
2550Just don't forget that you have to put a semicolon on the end
2551to finish the statement, as Perl doesn't know you're not going to
2552try to do this:
2553
2554 print <<ABC
2555 179231
2556 ABC
2557 + 20;
2558
872d7e53
ST
2559If you want to remove the line terminator from your here-docs,
2560use C<chomp()>.
2561
2562 chomp($string = <<'END');
2563 This is a string.
2564 END
2565
2566If you want your here-docs to be indented with the rest of the code,
2567you'll need to remove leading whitespace from each line manually:
7e3b091d
DA
2568
2569 ($quote = <<'FINIS') =~ s/^\s+//gm;
89d205f2 2570 The Road goes ever on and on,
7e3b091d
DA
2571 down from the door where it began.
2572 FINIS
2573
2574If you use a here-doc within a delimited construct, such as in C<s///eg>,
1bf48760
FC
2575the quoted material must still come on the line following the
2576C<<< <<FOO >>> marker, which means it may be inside the delimited
2577construct:
7e3b091d
DA
2578
2579 s/this/<<E . 'that'
2580 the other
2581 E
2582 . 'more '/eg;
2583
1bf48760
FC
2584It works this way as of Perl 5.18. Historically, it was inconsistent, and
2585you would have to write
7e3b091d 2586
89d205f2
YO
2587 s/this/<<E . 'that'
2588 . 'more '/eg;
2589 the other
2590 E
7e3b091d 2591
1bf48760
FC
2592outside of string evals.
2593
c543c01b 2594Additionally, quoting rules for the end-of-string identifier are
46f8a5ea 2595unrelated to Perl's quoting rules. C<q()>, C<qq()>, and the like are not
89d205f2
YO
2596supported in place of C<''> and C<"">, and the only interpolation is for
2597backslashing the quoting character:
7e3b091d
DA
2598
2599 print << "abc\"def";
2600 testing...
2601 abc"def
2602
2603Finally, quoted strings cannot span multiple lines. The general rule is
2604that the identifier must be a string literal. Stick with that, and you
2605should be safe.
2606
a0d0e21e
LW
2607=back
2608
75e14d17 2609=head2 Gory details of parsing quoted constructs
d74e8afc 2610X<quote, gory details>
75e14d17 2611
19799a22
GS
2612When presented with something that might have several different
2613interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
2614principle to pick the most probable interpretation. This strategy
2615is so successful that Perl programmers often do not suspect the
2616ambivalence of what they write. But from time to time, Perl's
2617notions differ substantially from what the author honestly meant.
2618
2619This section hopes to clarify how Perl handles quoted constructs.
2620Although the most common reason to learn this is to unravel labyrinthine
2621regular expressions, because the initial steps of parsing are the
2622same for all quoting operators, they are all discussed together.
2623
2624The most important Perl parsing rule is the first one discussed
2625below: when processing a quoted construct, Perl first finds the end
2626of that construct, then interprets its contents. If you understand
2627this rule, you may skip the rest of this section on the first
2628reading. The other rules are likely to contradict the user's
2629expectations much less frequently than this first one.
2630
2631Some passes discussed below are performed concurrently, but because
2632their results are the same, we consider them individually. For different
2633quoting constructs, Perl performs different numbers of passes, from
6deea57f 2634one to four, but these passes are always performed in the same order.
75e14d17 2635
13a2d996 2636=over 4
75e14d17
IZ
2637
2638=item Finding the end
2639
ba7f043c
KW
2640The first pass is finding the end of the quoted construct. This results
2641in saving to a safe location a copy of the text (between the starting
2642and ending delimiters), normalized as necessary to avoid needing to know
2643what the original delimiters were.
6deea57f
ST
2644
2645If the construct is a here-doc, the ending delimiter is a line
46f8a5ea 2646that has a terminating string as the content. Therefore C<<<EOF> is
6deea57f
ST
2647terminated by C<EOF> immediately followed by C<"\n"> and starting
2648from the first column of the terminating line.
2649When searching for the terminating line of a here-doc, nothing
46f8a5ea 2650is skipped. In other words, lines after the here-doc syntax
6deea57f
ST
2651are compared with the terminating string line by line.
2652
2653For the constructs except here-docs, single characters are used as starting
46f8a5ea 2654and ending delimiters. If the starting delimiter is an opening punctuation
6deea57f
ST
2655(that is C<(>, C<[>, C<{>, or C<< < >>), the ending delimiter is the
2656corresponding closing punctuation (that is C<)>, C<]>, C<}>, or C<< > >>).
2657If the starting delimiter is an unpaired character like C</> or a closing
ba7f043c 2658punctuation, the ending delimiter is the same as the starting delimiter.
6deea57f 2659Therefore a C</> terminates a C<qq//> construct, while a C<]> terminates
fc693347 2660both C<qq[]> and C<qq]]> constructs.
6deea57f
ST
2661
2662When searching for single-character delimiters, escaped delimiters
1ca345ed 2663and C<\\> are skipped. For example, while searching for terminating C</>,
6deea57f
ST
2664combinations of C<\\> and C<\/> are skipped. If the delimiters are
2665bracketing, nested pairs are also skipped. For example, while searching
ba7f043c 2666for a closing C<]> paired with the opening C<[>, combinations of C<\\>, C<\]>,
6deea57f
ST
2667and C<\[> are all skipped, and nested C<[> and C<]> are skipped as well.
2668However, when backslashes are used as the delimiters (like C<qq\\> and
2669C<tr\\\>), nothing is skipped.
32581033 2670During the search for the end, backslashes that escape delimiters or
7188ca43 2671other backslashes are removed (exactly speaking, they are not copied to the
32581033 2672safe location).
75e14d17 2673
19799a22
GS
2674For constructs with three-part delimiters (C<s///>, C<y///>, and
2675C<tr///>), the search is repeated once more.
fc693347 2676If the first delimiter is not an opening punctuation, the three delimiters must
d74605e5
FC
2677be the same, such as C<s!!!> and C<tr)))>,
2678in which case the second delimiter
6deea57f 2679terminates the left part and starts the right part at once.
b6538e4f 2680If the left part is delimited by bracketing punctuation (that is C<()>,
6deea57f 2681C<[]>, C<{}>, or C<< <> >>), the right part needs another pair of
b6538e4f 2682delimiters such as C<s(){}> and C<tr[]//>. In these cases, whitespace
ba7f043c 2683and comments are allowed between the two parts, although the comment must follow
b6538e4f
TC
2684at least one whitespace character; otherwise a character expected as the
2685start of the comment may be regarded as the starting delimiter of the right part.
75e14d17 2686
19799a22
GS
2687During this search no attention is paid to the semantics of the construct.
2688Thus:
75e14d17
IZ
2689
2690 "$hash{"$foo/$bar"}"
2691
2a94b7ce 2692or:
75e14d17 2693
89d205f2 2694 m/
2a94b7ce 2695 bar # NOT a comment, this slash / terminated m//!
75e14d17
IZ
2696 /x
2697
19799a22
GS
2698do not form legal quoted expressions. The quoted part ends on the
2699first C<"> and C</>, and the rest happens to be a syntax error.
2700Because the slash that terminated C<m//> was followed by a C<SPACE>,
2701the example above is not C<m//x>, but rather C<m//> with no C</x>
2702modifier. So the embedded C<#> is interpreted as a literal C<#>.
75e14d17 2703
89d205f2 2704Also no attention is paid to C<\c\> (multichar control char syntax) during
46f8a5ea 2705this search. Thus the second C<\> in C<qq/\c\/> is interpreted as a part
89d205f2 2706of C<\/>, and the following C</> is not recognized as a delimiter.
0d594e51
ST
2707Instead, use C<\034> or C<\x1c> at the end of quoted constructs.
2708
75e14d17 2709=item Interpolation
d74e8afc 2710X<interpolation>
75e14d17 2711
19799a22 2712The next step is interpolation in the text obtained, which is now
89d205f2 2713delimiter-independent. There are multiple cases.
75e14d17 2714
13a2d996 2715=over 4
75e14d17 2716
89d205f2 2717=item C<<<'EOF'>
75e14d17
IZ
2718
2719No interpolation is performed.
6deea57f
ST
2720Note that the combination C<\\> is left intact, since escaped delimiters
2721are not available for here-docs.
75e14d17 2722
6deea57f 2723=item C<m''>, the pattern of C<s'''>
89d205f2 2724
6deea57f
ST
2725No interpolation is performed at this stage.
2726Any backslashed sequences including C<\\> are treated at the stage
2727to L</"parsing regular expressions">.
89d205f2 2728
6deea57f 2729=item C<''>, C<q//>, C<tr'''>, C<y'''>, the replacement of C<s'''>
75e14d17 2730
89d205f2 2731The only interpolation is removal of C<\> from pairs of C<\\>.
ba7f043c 2732Therefore C<"-"> in C<tr'''> and C<y'''> is treated literally
6deea57f
ST
2733as a hyphen and no character range is available.
2734C<\1> in the replacement of C<s'''> does not work as C<$1>.
89d205f2
YO
2735
2736=item C<tr///>, C<y///>
2737
6deea57f
ST
2738No variable interpolation occurs. String modifying combinations for
2739case and quoting such as C<\Q>, C<\U>, and C<\E> are not recognized.
2740The other escape sequences such as C<\200> and C<\t> and backslashed
2741characters such as C<\\> and C<\-> are converted to appropriate literals.
ba7f043c
KW
2742The character C<"-"> is treated specially and therefore C<\-> is treated
2743as a literal C<"-">.
75e14d17 2744
89d205f2 2745=item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF">
75e14d17 2746
628253b8 2747C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F> (possibly paired with C<\E>) are
19799a22 2748converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
ba7f043c 2749is converted to S<C<$foo . (quotemeta("baz" . $bar))>> internally.
6deea57f
ST
2750The other escape sequences such as C<\200> and C<\t> and backslashed
2751characters such as C<\\> and C<\-> are replaced with appropriate
2752expansions.
2a94b7ce 2753
19799a22
GS
2754Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
2755is interpolated in the usual way. Something like C<"\Q\\E"> has
48cbae4f 2756no C<\E> inside. Instead, it has C<\Q>, C<\\>, and C<E>, so the
19799a22
GS
2757result is the same as for C<"\\\\E">. As a general rule, backslashes
2758between C<\Q> and C<\E> may lead to counterintuitive results. So,
2759C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
2760as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
2a94b7ce
IZ
2761
2762 $str = '\t';
2763 return "\Q$str";
2764
2765may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
2766
19799a22 2767Interpolated scalars and arrays are converted internally to the C<join> and
ba7f043c 2768C<"."> catenation operations. Thus, S<C<"$foo XXX '@arr'">> becomes:
75e14d17 2769
19799a22 2770 $foo . " XXX '" . (join $", @arr) . "'";
75e14d17 2771
19799a22 2772All operations above are performed simultaneously, left to right.
75e14d17 2773
ba7f043c 2774Because the result of S<C<"\Q I<STRING> \E">> has all metacharacters
19799a22 2775quoted, there is no way to insert a literal C<$> or C<@> inside a
ba7f043c 2776C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to become
19799a22
GS
2777C<"\\\$">; if not, it is interpreted as the start of an interpolated
2778scalar.
75e14d17 2779
19799a22 2780Note also that the interpolation code needs to make a decision on
89d205f2 2781where the interpolated scalar ends. For instance, whether
ba7f043c 2782S<C<< "a $x -> {c}" >>> really means:
75e14d17 2783
db691027 2784 "a " . $x . " -> {c}";
75e14d17 2785
2a94b7ce 2786or:
75e14d17 2787
db691027 2788 "a " . $x -> {c};
75e14d17 2789
19799a22
GS
2790Most of the time, the longest possible text that does not include
2791spaces between components and which contains matching braces or
2792brackets. because the outcome may be determined by voting based
2793on heuristic estimators, the result is not strictly predictable.
2794Fortunately, it's usually correct for ambiguous cases.
75e14d17 2795
6deea57f 2796=item the replacement of C<s///>
75e14d17 2797
628253b8 2798Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F> and interpolation
6deea57f
ST
2799happens as with C<qq//> constructs.
2800
2801It is at this step that C<\1> is begrudgingly converted to C<$1> in
2802the replacement text of C<s///>, in order to correct the incorrigible
2803I<sed> hackers who haven't picked up the saner idiom yet. A warning
ba7f043c 2804is emitted if the S<C<use warnings>> pragma or the B<-w> command-line flag
6deea57f
ST
2805(that is, the C<$^W> variable) was set.
2806
2807=item C<RE> in C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
2808
628253b8 2809Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F>, C<\E>,
cc74c5bd
ST
2810and interpolation happens (almost) as with C<qq//> constructs.
2811
5d03b57c
KW
2812Processing of C<\N{...}> is also done here, and compiled into an intermediate
2813form for the regex compiler. (This is because, as mentioned below, the regex
2814compilation may be done at execution time, and C<\N{...}> is a compile-time
2815construct.)
2816
cc74c5bd
ST
2817However any other combinations of C<\> followed by a character
2818are not substituted but only skipped, in order to parse them
2819as regular expressions at the following step.
6deea57f 2820As C<\c> is skipped at this step, C<@> of C<\c@> in RE is possibly
1749ea0d 2821treated as an array symbol (for example C<@foo>),
6deea57f 2822even though the same text in C<qq//> gives interpolation of C<\c@>.
6deea57f 2823
e128ab2c
DM
2824Code blocks such as C<(?{BLOCK})> are handled by temporarily passing control
2825back to the perl parser, in a similar way that an interpolated array
2826subscript expression such as C<"foo$array[1+f("[xyz")]bar"> would be.
2827
ba7f043c
KW
2828Moreover, inside C<(?{BLOCK})>, S<C<(?# comment )>>, and
2829a C<#>-comment in a C</x>-regular expression, no processing is
19799a22 2830performed whatsoever. This is the first step at which the presence
ba7f043c 2831of the C</x> modifier is relevant.
19799a22 2832
1749ea0d
ST
2833Interpolation in patterns has several quirks: C<$|>, C<$(>, C<$)>, C<@+>
2834and C<@-> are not interpolated, and constructs C<$var[SOMETHING]> are
2835voted (by several different estimators) to be either an array element
2836or C<$var> followed by an RE alternative. This is where the notation
19799a22
GS
2837C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
2838array element C<-9>, not as a regular expression from the variable
2839C<$arr> followed by a digit, which would be the interpretation of
2840C</$arr[0-9]/>. Since voting among different estimators may occur,
2841the result is not predictable.
2842
19799a22
GS
2843The lack of processing of C<\\> creates specific restrictions on
2844the post-processed text. If the delimiter is C</>, one cannot get
2845the combination C<\/> into the result of this step. C</> will
2846finish the regular expression, C<\/> will be stripped to C</> on
2847the previous step, and C<\\/> will be left as is. Because C</> is
2848equivalent to C<\/> inside a regular expression, this does not
2849matter unless the delimiter happens to be character special to the
2850RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
2851alphanumeric char, as in:
2a94b7ce
IZ
2852
2853 m m ^ a \s* b mmx;
2854
19799a22 2855In the RE above, which is intentionally obfuscated for illustration, the
6deea57f 2856delimiter is C<m>, the modifier is C<mx>, and after delimiter-removal the
ba7f043c 2857RE is the same as for S<C<m/ ^ a \s* b /mx>>. There's more than one
19799a22
GS
2858reason you're encouraged to restrict your delimiters to non-alphanumeric,
2859non-whitespace choices.
75e14d17
IZ
2860
2861=back
2862
19799a22 2863This step is the last one for all constructs except regular expressions,
75e14d17
IZ
2864which are processed further.
2865
6deea57f
ST
2866=item parsing regular expressions
2867X<regexp, parse>
75e14d17 2868
19799a22 2869Previous steps were performed during the compilation of Perl code,
ac036724 2870but this one happens at run time, although it may be optimized to
19799a22 2871be calculated at compile time if appropriate. After preprocessing
6deea57f 2872described above, and possibly after evaluation if concatenation,
19799a22
GS
2873joining, casing translation, or metaquoting are involved, the
2874resulting I<string> is passed to the RE engine for compilation.
2875
2876Whatever happens in the RE engine might be better discussed in L<perlre>,
2877but for the sake of continuity, we shall do so here.
2878
ba7f043c 2879This is another step where the presence of the C</x> modifier is
19799a22 2880relevant. The RE engine scans the string from left to right and
ba7f043c 2881converts it into a finite automaton.
19799a22
GS
2882
2883Backslashed characters are either replaced with corresponding
2884literal strings (as with C<\{>), or else they generate special nodes
2885in the finite automaton (as with C<\b>). Characters special to the
2886RE engine (such as C<|>) generate corresponding nodes or groups of
2887nodes. C<(?#...)> comments are ignored. All the rest is either
2888converted to literal strings to match, or else is ignored (as is
ba7f043c 2889whitespace and C<#>-style comments if C</x> is present).
19799a22
GS
2890
2891Parsing of the bracketed character class construct, C<[...]>, is
2892rather different than the rule used for the rest of the pattern.
2893The terminator of this construct is found using the same rules as
2894for finding the terminator of a C<{}>-delimited construct, the only
2895exception being that C<]> immediately following C<[> is treated as
e128ab2c
DM
2896though preceded by a backslash.
2897
2898The terminator of runtime C<(?{...})> is found by temporarily switching
2899control to the perl parser, which should stop at the point where the
2900logically balancing terminating C<}> is found.
19799a22
GS
2901
2902It is possible to inspect both the string given to RE engine and the
2903resulting finite automaton. See the arguments C<debug>/C<debugcolor>
ba7f043c 2904in the S<C<use L<re>>> pragma, as well as Perl's B<-Dr> command-line
4a4eefd0 2905switch documented in L<perlrun/"Command Switches">.
75e14d17
IZ
2906
2907=item Optimization of regular expressions
d74e8afc 2908X<regexp, optimization>
75e14d17 2909
7522fed5 2910This step is listed for completeness only. Since it does not change
75e14d17 2911semantics, details of this step are not documented and are subject
19799a22
GS
2912to change without notice. This step is performed over the finite
2913automaton that was generated during the previous pass.
2a94b7ce 2914
19799a22
GS
2915It is at this stage that C<split()> silently optimizes C</^/> to
2916mean C</^/m>.
75e14d17
IZ
2917
2918=back
2919
a0d0e21e 2920=head2 I/O Operators
d74e8afc 2921X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle>
80a96bfc 2922X<< <> >> X<< <<>> >> X<@ARGV>
a0d0e21e 2923
54310121 2924There are several I/O operators you should know about.
fbad3eb5 2925
7b8d334a 2926A string enclosed by backticks (grave accents) first undergoes
19799a22
GS
2927double-quote interpolation. It is then interpreted as an external
2928command, and the output of that command is the value of the
e9c56f9b
JH
2929backtick string, like in a shell. In scalar context, a single string
2930consisting of all output is returned. In list context, a list of
2931values is returned, one per line of output. (You can set C<$/> to use
2932a different line terminator.) The command is executed each time the
2933pseudo-literal is evaluated. The status value of the command is
2934returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
2935Unlike in B<csh>, no translation is done on the return data--newlines
2936remain newlines. Unlike in any of the shells, single quotes do not
2937hide variable names in the command from interpretation. To pass a
2938literal dollar-sign through to the shell you need to hide it with a
2939backslash. The generalized form of backticks is C<qx//>. (Because
2940backticks always undergo shell expansion as well, see L<perlsec> for
2941security concerns.)
d74e8afc 2942X<qx> X<`> X<``> X<backtick> X<glob>
19799a22
GS
2943
2944In scalar context, evaluating a filehandle in angle brackets yields
2945the next line from that file (the newline, if any, included), or
2946C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
2947(sometimes known as file-slurp mode) and the file is empty, it
2948returns C<''> the first time, followed by C<undef> subsequently.
2949
2950Ordinarily you must assign the returned value to a variable, but
2951there is one situation where an automatic assignment happens. If
2952and only if the input symbol is the only thing inside the conditional
2953of a C<while> statement (even if disguised as a C<for(;;)> loop),
ba7f043c 2954the value is automatically assigned to the global variable C<$_>,
19799a22
GS
2955destroying whatever was there previously. (This may seem like an
2956odd thing to you, but you'll use the construct in almost every Perl
ba7f043c
KW
2957script you write.) The C<$_> variable is not implicitly localized.
2958You'll have to put a S<C<local $_;>> before the loop if you want that
19799a22
GS
2959to happen.
2960
2961The following lines are equivalent:
a0d0e21e 2962
748a9306 2963 while (defined($_ = <STDIN>)) { print; }
7b8d334a 2964 while ($_ = <STDIN>) { print; }
a0d0e21e
LW
2965 while (<STDIN>) { print; }
2966 for (;<STDIN>;) { print; }
748a9306 2967 print while defined($_ = <STDIN>);
7b8d334a 2968 print while ($_ = <STDIN>);
a0d0e21e
LW
2969 print while <STDIN>;
2970
1ca345ed
TC
2971This also behaves similarly, but assigns to a lexical variable
2972instead of to C<$_>:
7b8d334a 2973
89d205f2 2974 while (my $line = <STDIN>) { print $line }
7b8d334a 2975
19799a22
GS
2976In these loop constructs, the assigned value (whether assignment
2977is automatic or explicit) is then tested to see whether it is
1ca345ed
TC
2978defined. The defined test avoids problems where the line has a string
2979value that would be treated as false by Perl; for example a "" or
ba7f043c 2980a C<"0"> with no trailing newline. If you really mean for such values
19799a22 2981to terminate the loop, they should be tested for explicitly:
7b8d334a
GS
2982
2983 while (($_ = <STDIN>) ne '0') { ... }
2984 while (<STDIN>) { last unless $_; ... }
2985
ba7f043c 2986In other boolean contexts, C<< <I<FILEHANDLE>> >> without an
5ef4d93e 2987explicit C<defined> test or comparison elicits a warning if the
ba7f043c 2988S<C<use warnings>> pragma or the B<-w>
19799a22 2989command-line switch (the C<$^W> variable) is in effect.
7b8d334a 2990
5f05dabc 2991The filehandles STDIN, STDOUT, and STDERR are predefined. (The
19799a22
GS
2992filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
2993in packages, where they would be interpreted as local identifiers
2994rather than global.) Additional filehandles may be created with
ba7f043c 2995the C<open()> function, amongst others. See L<perlopentut> and
19799a22 2996L<perlfunc/open> for details on this.
d74e8afc 2997X<stdin> X<stdout> X<sterr>
a0d0e21e 2998
ba7f043c 2999If a C<< <I<FILEHANDLE>> >> is used in a context that is looking for
19799a22
GS
3000a list, a list comprising all input lines is returned, one line per
3001list element. It's easy to grow to a rather large data space this
3002way, so use with care.
a0d0e21e 3003
ba7f043c 3004C<< <I<FILEHANDLE>> >> may also be spelled C<readline(*I<FILEHANDLE>)>.
19799a22 3005See L<perlfunc/readline>.
fbad3eb5 3006
ba7f043c 3007The null filehandle C<< <> >> is special: it can be used to emulate the
1ca345ed
TC
3008behavior of B<sed> and B<awk>, and any other Unix filter program
3009that takes a list of filenames, doing the same to each line
ba7f043c 3010of input from all of them. Input from C<< <> >> comes either from
a0d0e21e 3011standard input, or from each file listed on the command line. Here's
ba7f043c
KW
3012how it works: the first time C<< <> >> is evaluated, the C<@ARGV> array is
3013checked, and if it is empty, C<$ARGV[0]> is set to C<"-">, which when opened
3014gives you standard input. The C<@ARGV> array is then processed as a list
a0d0e21e
LW
3015of filenames. The loop
3016
3017 while (<>) {
3018 ... # code for each line
3019 }
3020
3021is equivalent to the following Perl-like pseudo code:
3022
3e3baf6d 3023 unshift(@ARGV, '-') unless @ARGV;
a0d0e21e
LW
3024 while ($ARGV = shift) {
3025 open(ARGV, $ARGV);
3026 while (<ARGV>) {
3027 ... # code for each line
3028 }
3029 }
3030
19799a22 3031except that it isn't so cumbersome to say, and will actually work.
ba7f043c
KW
3032It really does shift the C<@ARGV> array and put the current filename
3033into the C<$ARGV> variable. It also uses filehandle I<ARGV>
3034internally. C<< <> >> is just a synonym for C<< <ARGV> >>, which
19799a22 3035is magical. (The pseudo code above doesn't work because it treats
ba7f043c 3036C<< <ARGV> >> as non-magical.)
a0d0e21e 3037
48ab5743
ML
3038Since the null filehandle uses the two argument form of L<perlfunc/open>
3039it interprets special characters, so if you have a script like this:
3040
3041 while (<>) {
3042 print;
3043 }
3044
ba7f043c 3045and call it with S<C<perl dangerous.pl 'rm -rfv *|'>>, it actually opens a
48ab5743
ML
3046pipe, executes the C<rm> command and reads C<rm>'s output from that pipe.
3047If you want all items in C<@ARGV> to be interpreted as file names, you
1033ba6e
PM
3048can use the module C<ARGV::readonly> from CPAN, or use the double bracket:
3049
3050 while (<<>>) {
3051 print;
3052 }
3053
3054Using double angle brackets inside of a while causes the open to use the
3055three argument form (with the second argument being C<< < >>), so all
ba7f043c
KW
3056arguments in C<ARGV> are treated as literal filenames (including C<"-">).
3057(Note that for convenience, if you use C<< <<>> >> and if C<@ARGV> is
80a96bfc 3058empty, it will still read from the standard input.)
48ab5743 3059
ba7f043c 3060You can modify C<@ARGV> before the first C<< <> >> as long as the array ends up
a0d0e21e 3061containing the list of filenames you really want. Line numbers (C<$.>)
19799a22
GS
3062continue as though the input were one big happy file. See the example
3063in L<perlfunc/eof> for how to reset line numbers on each file.
5a964f20 3064
ba7f043c
KW
3065If you want to set C<@ARGV> to your own list of files, go right ahead.
3066This sets C<@ARGV> to all plain text files if no C<@ARGV> was given:
5a964f20
TC
3067
3068 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
a0d0e21e 3069
5a964f20
TC
3070You can even set them to pipe commands. For example, this automatically
3071filters compressed arguments through B<gzip>:
3072
3073 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
3074
3075If you want to pass switches into your script, you can use one of the
ba7f043c 3076C<Getopts> modules or put a loop on the front like this:
a0d0e21e
LW
3077
3078 while ($_ = $ARGV[0], /^-/) {
3079 shift;
3080 last if /^--$/;
3081 if (/^-D(.*)/) { $debug = $1 }
3082 if (/^-v/) { $verbose++ }
5a964f20 3083 # ... # other switches
a0d0e21e 3084 }
5a964f20 3085
a0d0e21e 3086 while (<>) {
5a964f20 3087 # ... # code for each line
a0d0e21e
LW
3088 }
3089
ba7f043c 3090The C<< <> >> symbol will return C<undef> for end-of-file only once.
89d205f2 3091If you call it again after this, it will assume you are processing another
ba7f043c 3092C<@ARGV> list, and if you haven't set C<@ARGV>, will read input from STDIN.
a0d0e21e 3093
1ca345ed 3094If what the angle brackets contain is a simple scalar variable (for example,
ba7f043c 3095C<$foo>), then that variable contains the name of the
19799a22
GS
3096filehandle to input from, or its typeglob, or a reference to the
3097same. For example:
cb1a09d0
AD
3098
3099 $fh = \*STDIN;
3100 $line = <$fh>;
a0d0e21e 3101
5a964f20
TC
3102If what's within the angle brackets is neither a filehandle nor a simple
3103scalar variable containing a filehandle name, typeglob, or typeglob
3104reference, it is interpreted as a filename pattern to be globbed, and
3105either a list of filenames or the next filename in the list is returned,
19799a22 3106depending on context. This distinction is determined on syntactic
ba7f043c
KW
3107grounds alone. That means C<< <$x> >> is always a C<readline()> from
3108an indirect handle, but C<< <$hash{key}> >> is always a C<glob()>.
3109That's because C<$x> is a simple scalar variable, but C<$hash{key}> is
ef191992
YST
3110not--it's a hash element. Even C<< <$x > >> (note the extra space)
3111is treated as C<glob("$x ")>, not C<readline($x)>.
5a964f20
TC
3112
3113One level of double-quote interpretation is done first, but you can't
35f2feb0 3114say C<< <$foo> >> because that's an indirect filehandle as explained
5a964f20
TC
3115in the previous paragraph. (In older versions of Perl, programmers
3116would insert curly brackets to force interpretation as a filename glob:
35f2feb0 3117C<< <${foo}> >>. These days, it's considered cleaner to call the
5a964f20 3118internal function directly as C<glob($foo)>, which is probably the right
19799a22 3119way to have done it in the first place.) For example:
a0d0e21e
LW
3120
3121 while (<*.c>) {
3122 chmod 0644, $_;
3123 }
3124
3a4b19e4 3125is roughly equivalent to:
a0d0e21e
LW
3126
3127 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
3128 while (<FOO>) {
5b3eff12 3129 chomp;
a0d0e21e
LW
3130 chmod 0644, $_;
3131 }
3132
3a4b19e4 3133except that the globbing is actually done internally using the standard
ba7f043c 3134C<L<File::Glob>> extension. Of course, the shortest way to do the above is:
a0d0e21e
LW
3135
3136 chmod 0644, <*.c>;
3137
19799a22
GS
3138A (file)glob evaluates its (embedded) argument only when it is
3139starting a new list. All values must be read before it will start
3140over. In list context, this isn't important because you automatically
3141get them all anyway. However, in scalar context the operator returns
069e01df 3142the next value each time it's called, or C<undef> when the list has
19799a22
GS
3143run out. As with filehandle reads, an automatic C<defined> is
3144generated when the glob occurs in the test part of a C<while>,
1ca345ed
TC
3145because legal glob returns (for example,
3146a file called F<0>) would otherwise
19799a22
GS
3147terminate the loop. Again, C<undef> is returned only once. So if
3148you're expecting a single value from a glob, it is much better to
3149say
4633a7c4
LW
3150
3151 ($file) = <blurch*>;
3152
3153than
3154
3155 $file = <blurch*>;
3156
3157because the latter will alternate between returning a filename and
19799a22 3158returning false.
4633a7c4 3159
b159ebd3 3160If you're trying to do variable interpolation, it's definitely better
ba7f043c 3161to use the C<glob()> function, because the older notation can cause people
e37d713d 3162to become confused with the indirect filehandle notation.
4633a7c4
LW
3163
3164 @files = glob("$dir/*.[ch]");
3165 @files = glob($files[$i]);
3166
a0d0e21e 3167=head2 Constant Folding
d74e8afc 3168X<constant folding> X<folding>
a0d0e21e
LW
3169
3170Like C, Perl does a certain amount of expression evaluation at
19799a22 3171compile time whenever it determines that all arguments to an
a0d0e21e
LW
3172operator are static and have no side effects. In particular, string
3173concatenation happens at compile time between literals that don't do
19799a22 3174variable substitution. Backslash interpolation also happens at
a0d0e21e
LW
3175compile time. You can say
3176
1ca345ed
TC
3177 'Now is the time for all'
3178 . "\n"
3179 . 'good men to come to.'
a0d0e21e 3180
54310121 3181and this all reduces to one string internally. Likewise, if
a0d0e21e
LW
3182you say
3183
3184 foreach $file (@filenames) {
5a964f20 3185 if (-s $file > 5 + 100 * 2**16) { }
54310121 3186 }
a0d0e21e 3187
1ca345ed 3188the compiler precomputes the number which that expression
19799a22 3189represents so that the interpreter won't have to.
a0d0e21e 3190
fd1abbef 3191=head2 No-ops
d74e8afc 3192X<no-op> X<nop>
fd1abbef
DN
3193
3194Perl doesn't officially have a no-op operator, but the bare constants
1ca345ed 3195C<0> and C<1> are special-cased not to produce a warning in void
fd1abbef
DN
3196context, so you can for example safely do
3197
3198 1 while foo();
3199
2c268ad5 3200=head2 Bitwise String Operators
fb7054ba 3201X<operator, bitwise, string> X<&.> X<|.> X<^.> X<~.>
2c268ad5
TP
3202
3203Bitstrings of any size may be manipulated by the bitwise operators
3204(C<~ | & ^>).
3205
19799a22
GS
3206If the operands to a binary bitwise op are strings of different
3207sizes, B<|> and B<^> ops act as though the shorter operand had
3208additional zero bits on the right, while the B<&> op acts as though
3209the longer operand were truncated to the length of the shorter.
3210The granularity for such extension or truncation is one or more
3211bytes.
2c268ad5 3212
89d205f2 3213 # ASCII-based examples
2c268ad5
TP
3214 print "j p \n" ^ " a h"; # prints "JAPH\n"
3215 print "JA" | " ph\n"; # prints "japh\n"
3216 print "japh\nJunk" & '_____'; # prints "JAPH\n";
3217 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
3218
19799a22 3219If you are intending to manipulate bitstrings, be certain that
2c268ad5 3220you're supplying bitstrings: If an operand is a number, that will imply
19799a22 3221a B<numeric> bitwise operation. You may explicitly show which type of
2c268ad5
TP
3222operation you intend by using C<""> or C<0+>, as in the examples below.
3223
4358a253
SS
3224 $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
3225 $foo = '150' | 105; # yields 255
2c268ad5
TP
3226 $foo = 150 | '105'; # yields 255
3227 $foo = '150' | '105'; # yields string '155' (under ASCII)
3228
3229 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
3230 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
a0d0e21e 3231
fb7054ba 3232This somewhat unpredictable behavior can be avoided with the experimental
ba7f043c
KW
3233"bitwise" feature, new in Perl 5.22. You can enable it via S<C<use feature
3234'bitwise'>>. By default, it will warn unless the C<"experimental::bitwise">
3235warnings category has been disabled. (S<C<use experimental 'bitwise'>> will
fb7054ba
FC
3236enable the feature and disable the warning.) Under this feature, the four
3237standard bitwise operators (C<~ | & ^>) are always numeric. Adding a dot
3238after each operator (C<~. |. &. ^.>) forces it to treat its operands as
3239strings:
3240
3241 use experimental "bitwise";
3242 $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
3243 $foo = '150' | 105; # yields 255
3244 $foo = 150 | '105'; # yields 255
3245 $foo = '150' | '105'; # yields 255
9f1b8172 3246 $foo = 150 |. 105; # yields string '155'
fb7054ba
FC
3247 $foo = '150' |. 105; # yields string '155'
3248 $foo = 150 |.'105'; # yields string '155'
3249 $foo = '150' |.'105'; # yields string '155'
3250
3251 $baz = $foo & $bar; # both operands numeric
3252 $biz = $foo ^. $bar; # both operands stringy
3253
3254The assignment variants of these operators (C<&= |= ^= &.= |.= ^.=>)
3255behave likewise under the feature.
3256