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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
89d205f2 8=head2 Operator Precedence and Associativity
d74e8afc 9X<operator, precedence> X<precedence> X<associativity>
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10
11Operator precedence and associativity work in Perl more or less like
12they do in mathematics.
13
14I<Operator precedence> means some operators are evaluated before
15others. For example, in C<2 + 4 * 5>, the multiplication has higher
16precedence so C<4 * 5> is evaluated first yielding C<2 + 20 ==
1722> and not C<6 * 5 == 30>.
18
19I<Operator associativity> defines what happens if a sequence of the
20same operators is used one after another: whether the evaluator will
21evaluate the left operations first or the right. For example, in C<8
22- 4 - 2>, subtraction is left associative so Perl evaluates the
23expression left to right. C<8 - 4> is evaluated first making the
24expression C<4 - 2 == 2> and not C<8 - 2 == 6>.
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25
26Perl operators have the following associativity and precedence,
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27listed from highest precedence to lowest. Operators borrowed from
28C keep the same precedence relationship with each other, even where
29C's precedence is slightly screwy. (This makes learning Perl easier
30for C folks.) With very few exceptions, these all operate on scalar
31values only, not array values.
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32
33 left terms and list operators (leftward)
34 left ->
35 nonassoc ++ --
36 right **
37 right ! ~ \ and unary + and -
54310121 38 left =~ !~
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39 left * / % x
40 left + - .
41 left << >>
42 nonassoc named unary operators
43 nonassoc < > <= >= lt gt le ge
0d863452 44 nonassoc == != <=> eq ne cmp ~~
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45 left &
46 left | ^
47 left &&
c963b151 48 left || //
137443ea 49 nonassoc .. ...
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50 right ?:
51 right = += -= *= etc.
52 left , =>
53 nonassoc list operators (rightward)
a5f75d66 54 right not
a0d0e21e 55 left and
f23102e2 56 left or xor
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57
58In the following sections, these operators are covered in precedence order.
59
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60Many operators can be overloaded for objects. See L<overload>.
61
a0d0e21e 62=head2 Terms and List Operators (Leftward)
d74e8afc 63X<list operator> X<operator, list> X<term>
a0d0e21e 64
62c18ce2 65A TERM has the highest precedence in Perl. They include variables,
5f05dabc 66quote and quote-like operators, any expression in parentheses,
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67and any function whose arguments are parenthesized. Actually, there
68aren't really functions in this sense, just list operators and unary
69operators behaving as functions because you put parentheses around
70the arguments. These are all documented in L<perlfunc>.
71
72If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
73is followed by a left parenthesis as the next token, the operator and
74arguments within parentheses are taken to be of highest precedence,
75just like a normal function call.
76
77In the absence of parentheses, the precedence of list operators such as
78C<print>, C<sort>, or C<chmod> is either very high or very low depending on
54310121 79whether you are looking at the left side or the right side of the operator.
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80For example, in
81
82 @ary = (1, 3, sort 4, 2);
83 print @ary; # prints 1324
84
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85the commas on the right of the sort are evaluated before the sort,
86but the commas on the left are evaluated after. In other words,
87list operators tend to gobble up all arguments that follow, and
a0d0e21e 88then act like a simple TERM with regard to the preceding expression.
19799a22 89Be careful with parentheses:
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90
91 # These evaluate exit before doing the print:
92 print($foo, exit); # Obviously not what you want.
93 print $foo, exit; # Nor is this.
94
95 # These do the print before evaluating exit:
96 (print $foo), exit; # This is what you want.
97 print($foo), exit; # Or this.
98 print ($foo), exit; # Or even this.
99
100Also note that
101
102 print ($foo & 255) + 1, "\n";
103
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104probably doesn't do what you expect at first glance. The parentheses
105enclose the argument list for C<print> which is evaluated (printing
106the result of C<$foo & 255>). Then one is added to the return value
107of C<print> (usually 1). The result is something like this:
108
109 1 + 1, "\n"; # Obviously not what you meant.
110
111To do what you meant properly, you must write:
112
113 print(($foo & 255) + 1, "\n");
114
115See L<Named Unary Operators> for more discussion of this.
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116
117Also parsed as terms are the C<do {}> and C<eval {}> constructs, as
54310121 118well as subroutine and method calls, and the anonymous
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119constructors C<[]> and C<{}>.
120
2ae324a7 121See also L<Quote and Quote-like Operators> toward the end of this section,
da87341d 122as well as L</"I/O Operators">.
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123
124=head2 The Arrow Operator
d74e8afc 125X<arrow> X<dereference> X<< -> >>
a0d0e21e 126
35f2feb0 127"C<< -> >>" is an infix dereference operator, just as it is in C
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128and C++. If the right side is either a C<[...]>, C<{...}>, or a
129C<(...)> subscript, then the left side must be either a hard or
130symbolic reference to an array, a hash, or a subroutine respectively.
131(Or technically speaking, a location capable of holding a hard
132reference, if it's an array or hash reference being used for
133assignment.) See L<perlreftut> and L<perlref>.
a0d0e21e 134
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135Otherwise, the right side is a method name or a simple scalar
136variable containing either the method name or a subroutine reference,
137and the left side must be either an object (a blessed reference)
138or a class name (that is, a package name). See L<perlobj>.
a0d0e21e 139
5f05dabc 140=head2 Auto-increment and Auto-decrement
d74e8afc 141X<increment> X<auto-increment> X<++> X<decrement> X<auto-decrement> X<-->
a0d0e21e 142
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143"++" and "--" work as in C. That is, if placed before a variable,
144they increment or decrement the variable by one before returning the
145value, and if placed after, increment or decrement after returning the
146value.
147
148 $i = 0; $j = 0;
149 print $i++; # prints 0
150 print ++$j; # prints 1
a0d0e21e 151
b033823e 152Note that just as in C, Perl doesn't define B<when> the variable is
89d205f2 153incremented or decremented. You just know it will be done sometime
b033823e 154before or after the value is returned. This also means that modifying
c543c01b 155a variable twice in the same statement will lead to undefined behavior.
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156Avoid statements like:
157
158 $i = $i ++;
159 print ++ $i + $i ++;
160
161Perl will not guarantee what the result of the above statements is.
162
54310121 163The auto-increment operator has a little extra builtin magic to it. If
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164you increment a variable that is numeric, or that has ever been used in
165a numeric context, you get a normal increment. If, however, the
5f05dabc 166variable has been used in only string contexts since it was set, and
5a964f20 167has a value that is not the empty string and matches the pattern
9c0670e1 168C</^[a-zA-Z]*[0-9]*\z/>, the increment is done as a string, preserving each
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169character within its range, with carry:
170
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171 print ++($foo = "99"); # prints "100"
172 print ++($foo = "a0"); # prints "a1"
173 print ++($foo = "Az"); # prints "Ba"
174 print ++($foo = "zz"); # prints "aaa"
a0d0e21e 175
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176C<undef> is always treated as numeric, and in particular is changed
177to C<0> before incrementing (so that a post-increment of an undef value
178will return C<0> rather than C<undef>).
179
5f05dabc 180The auto-decrement operator is not magical.
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181
182=head2 Exponentiation
d74e8afc 183X<**> X<exponentiation> X<power>
a0d0e21e 184
19799a22 185Binary "**" is the exponentiation operator. It binds even more
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186tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is
187implemented using C's pow(3) function, which actually works on doubles
188internally.)
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189
190=head2 Symbolic Unary Operators
d74e8afc 191X<unary operator> X<operator, unary>
a0d0e21e 192
5f05dabc 193Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower
a0d0e21e 194precedence version of this.
d74e8afc 195X<!>
a0d0e21e 196
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197Unary "-" performs arithmetic negation if the operand is numeric,
198including any string that looks like a number. If the operand is
199an identifier, a string consisting of a minus sign concatenated
200with the identifier is returned. Otherwise, if the string starts
201with a plus or minus, a string starting with the opposite sign is
202returned. One effect of these rules is that -bareword is equivalent
8705167b 203to the string "-bareword". If, however, the string begins with a
353c6505 204non-alphabetic character (excluding "+" or "-"), Perl will attempt to convert
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205the string to a numeric and the arithmetic negation is performed. If the
206string cannot be cleanly converted to a numeric, Perl will give the warning
207B<Argument "the string" isn't numeric in negation (-) at ...>.
d74e8afc 208X<-> X<negation, arithmetic>
a0d0e21e 209
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210Unary "~" performs bitwise negation, i.e., 1's complement. For
211example, C<0666 & ~027> is 0640. (See also L<Integer Arithmetic> and
212L<Bitwise String Operators>.) Note that the width of the result is
213platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64
214bits wide on a 64-bit platform, so if you are expecting a certain bit
f113cf86 215width, remember to use the "&" operator to mask off the excess bits.
d74e8afc 216X<~> X<negation, binary>
a0d0e21e 217
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218When complementing strings, if all characters have ordinal values under
219256, then their complements will, also. But if they do not, all
220characters will be in either 32- or 64-bit complements, depending on your
221architecture. So for example, C<~"\x{3B1}"> is C<"\x{FFFF_FC4E}"> on
22232-bit machines and C<"\x{FFFF_FFFF_FFFF_FC4E}"> on 64-bit machines.
223
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224Unary "+" has no effect whatsoever, even on strings. It is useful
225syntactically for separating a function name from a parenthesized expression
226that would otherwise be interpreted as the complete list of function
5ba421f6 227arguments. (See examples above under L<Terms and List Operators (Leftward)>.)
d74e8afc 228X<+>
a0d0e21e 229
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230Unary "\" creates a reference to whatever follows it. See L<perlreftut>
231and L<perlref>. Do not confuse this behavior with the behavior of
232backslash within a string, although both forms do convey the notion
233of protecting the next thing from interpolation.
d74e8afc 234X<\> X<reference> X<backslash>
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235
236=head2 Binding Operators
d74e8afc 237X<binding> X<operator, binding> X<=~> X<!~>
a0d0e21e 238
c07a80fd 239Binary "=~" binds a scalar expression to a pattern match. Certain operations
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240search or modify the string $_ by default. This operator makes that kind
241of operation work on some other string. The right argument is a search
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242pattern, substitution, or transliteration. The left argument is what is
243supposed to be searched, substituted, or transliterated instead of the default
f8bab1e9 244$_. When used in scalar context, the return value generally indicates the
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245success of the operation. The exceptions are substitution (s///)
246and transliteration (y///) with the C</r> (non-destructive) option,
247which cause the B<r>eturn value to be the result of the substitution.
248Behavior in list context depends on the particular operator.
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249See L</"Regexp Quote-Like Operators"> for details and L<perlretut> for
250examples using these operators.
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251
252If the right argument is an expression rather than a search pattern,
2c268ad5 253substitution, or transliteration, it is interpreted as a search pattern at run
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254time. Note that this means that its contents will be interpolated twice, so
255
256 '\\' =~ q'\\';
257
258is not ok, as the regex engine will end up trying to compile the
259pattern C<\>, which it will consider a syntax error.
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260
261Binary "!~" is just like "=~" except the return value is negated in
262the logical sense.
263
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264Binary "!~" with a non-destructive substitution (s///r) or transliteration
265(y///r) is a syntax error.
4f4d7508 266
a0d0e21e 267=head2 Multiplicative Operators
d74e8afc 268X<operator, multiplicative>
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269
270Binary "*" multiplies two numbers.
d74e8afc 271X<*>
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272
273Binary "/" divides two numbers.
d74e8afc 274X</> X<slash>
a0d0e21e 275
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276Binary "%" is the modulo operator, which computes the division
277remainder of its first argument with respect to its second argument.
278Given integer
54310121 279operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
f7918450 280C<$a> minus the largest multiple of C<$b> less than or equal to
54310121 281C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
282smallest multiple of C<$b> that is not less than C<$a> (i.e. the
89b4f0ad 283result will be less than or equal to zero). If the operands
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284C<$a> and C<$b> are floating point values and the absolute value of
285C<$b> (that is C<abs($b)>) is less than C<(UV_MAX + 1)>, only
286the integer portion of C<$a> and C<$b> will be used in the operation
287(Note: here C<UV_MAX> means the maximum of the unsigned integer type).
288If the absolute value of the right operand (C<abs($b)>) is greater than
289or equal to C<(UV_MAX + 1)>, "%" computes the floating-point remainder
290C<$r> in the equation C<($r = $a - $i*$b)> where C<$i> is a certain
f7918450 291integer that makes C<$r> have the same sign as the right operand
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292C<$b> (B<not> as the left operand C<$a> like C function C<fmod()>)
293and the absolute value less than that of C<$b>.
0412d526 294Note that when C<use integer> is in scope, "%" gives you direct access
f7918450 295to the modulo operator as implemented by your C compiler. This
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296operator is not as well defined for negative operands, but it will
297execute faster.
f7918450 298X<%> X<remainder> X<modulo> X<mod>
55d729e4 299
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300Binary "x" is the repetition operator. In scalar context or if the left
301operand is not enclosed in parentheses, it returns a string consisting
302of the left operand repeated the number of times specified by the right
303operand. In list context, if the left operand is enclosed in
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304parentheses or is a list formed by C<qw/STRING/>, it repeats the list.
305If the right operand is zero or negative, it returns an empty string
306or an empty list, depending on the context.
d74e8afc 307X<x>
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308
309 print '-' x 80; # print row of dashes
310
311 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
312
313 @ones = (1) x 80; # a list of 80 1's
314 @ones = (5) x @ones; # set all elements to 5
315
316
317=head2 Additive Operators
d74e8afc 318X<operator, additive>
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319
320Binary "+" returns the sum of two numbers.
d74e8afc 321X<+>
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322
323Binary "-" returns the difference of two numbers.
d74e8afc 324X<->
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325
326Binary "." concatenates two strings.
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327X<string, concatenation> X<concatenation>
328X<cat> X<concat> X<concatenate> X<.>
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329
330=head2 Shift Operators
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331X<shift operator> X<operator, shift> X<<< << >>>
332X<<< >> >>> X<right shift> X<left shift> X<bitwise shift>
333X<shl> X<shr> X<shift, right> X<shift, left>
a0d0e21e 334
55497cff 335Binary "<<" returns the value of its left argument shifted left by the
336number of bits specified by the right argument. Arguments should be
982ce180 337integers. (See also L<Integer Arithmetic>.)
a0d0e21e 338
55497cff 339Binary ">>" returns the value of its left argument shifted right by
340the number of bits specified by the right argument. Arguments should
982ce180 341be integers. (See also L<Integer Arithmetic>.)
a0d0e21e 342
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343Note that both "<<" and ">>" in Perl are implemented directly using
344"<<" and ">>" in C. If C<use integer> (see L<Integer Arithmetic>) is
345in force then signed C integers are used, else unsigned C integers are
346used. Either way, the implementation isn't going to generate results
347larger than the size of the integer type Perl was built with (32 bits
348or 64 bits).
349
350The result of overflowing the range of the integers is undefined
351because it is undefined also in C. In other words, using 32-bit
352integers, C<< 1 << 32 >> is undefined. Shifting by a negative number
353of bits is also undefined.
354
a0d0e21e 355=head2 Named Unary Operators
d74e8afc 356X<operator, named unary>
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357
358The various named unary operators are treated as functions with one
568e6d8b 359argument, with optional parentheses.
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360
361If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
362is followed by a left parenthesis as the next token, the operator and
363arguments within parentheses are taken to be of highest precedence,
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364just like a normal function call. For example,
365because named unary operators are higher precedence than ||:
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366
367 chdir $foo || die; # (chdir $foo) || die
368 chdir($foo) || die; # (chdir $foo) || die
369 chdir ($foo) || die; # (chdir $foo) || die
370 chdir +($foo) || die; # (chdir $foo) || die
371
3981b0eb 372but, because * is higher precedence than named operators:
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373
374 chdir $foo * 20; # chdir ($foo * 20)
375 chdir($foo) * 20; # (chdir $foo) * 20
376 chdir ($foo) * 20; # (chdir $foo) * 20
377 chdir +($foo) * 20; # chdir ($foo * 20)
378
379 rand 10 * 20; # rand (10 * 20)
380 rand(10) * 20; # (rand 10) * 20
381 rand (10) * 20; # (rand 10) * 20
382 rand +(10) * 20; # rand (10 * 20)
383
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384Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are
385treated like named unary operators, but they don't follow this functional
386parenthesis rule. That means, for example, that C<-f($file).".bak"> is
387equivalent to C<-f "$file.bak">.
d74e8afc 388X<-X> X<filetest> X<operator, filetest>
568e6d8b 389
5ba421f6 390See also L<"Terms and List Operators (Leftward)">.
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391
392=head2 Relational Operators
d74e8afc 393X<relational operator> X<operator, relational>
a0d0e21e 394
35f2feb0 395Binary "<" returns true if the left argument is numerically less than
a0d0e21e 396the right argument.
d74e8afc 397X<< < >>
a0d0e21e 398
35f2feb0 399Binary ">" returns true if the left argument is numerically greater
a0d0e21e 400than the right argument.
d74e8afc 401X<< > >>
a0d0e21e 402
35f2feb0 403Binary "<=" returns true if the left argument is numerically less than
a0d0e21e 404or equal to the right argument.
d74e8afc 405X<< <= >>
a0d0e21e 406
35f2feb0 407Binary ">=" returns true if the left argument is numerically greater
a0d0e21e 408than or equal to the right argument.
d74e8afc 409X<< >= >>
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410
411Binary "lt" returns true if the left argument is stringwise less than
412the right argument.
d74e8afc 413X<< lt >>
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414
415Binary "gt" returns true if the left argument is stringwise greater
416than the right argument.
d74e8afc 417X<< gt >>
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418
419Binary "le" returns true if the left argument is stringwise less than
420or equal to the right argument.
d74e8afc 421X<< le >>
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422
423Binary "ge" returns true if the left argument is stringwise greater
424than or equal to the right argument.
d74e8afc 425X<< ge >>
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426
427=head2 Equality Operators
d74e8afc 428X<equality> X<equal> X<equals> X<operator, equality>
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429
430Binary "==" returns true if the left argument is numerically equal to
431the right argument.
d74e8afc 432X<==>
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433
434Binary "!=" returns true if the left argument is numerically not equal
435to the right argument.
d74e8afc 436X<!=>
a0d0e21e 437
35f2feb0 438Binary "<=>" returns -1, 0, or 1 depending on whether the left
6ee5d4e7 439argument is numerically less than, equal to, or greater than the right
d4ad863d 440argument. If your platform supports NaNs (not-a-numbers) as numeric
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441values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
442"<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
443returns true, as does NaN != anything else. If your platform doesn't
444support NaNs then NaN is just a string with numeric value 0.
d74e8afc 445X<< <=> >> X<spaceship>
7d3a9d88 446
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447 perl -le '$a = "NaN"; print "No NaN support here" if $a == $a'
448 perl -le '$a = "NaN"; print "NaN support here" if $a != $a'
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449
450Binary "eq" returns true if the left argument is stringwise equal to
451the right argument.
d74e8afc 452X<eq>
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453
454Binary "ne" returns true if the left argument is stringwise not equal
455to the right argument.
d74e8afc 456X<ne>
a0d0e21e 457
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458Binary "cmp" returns -1, 0, or 1 depending on whether the left
459argument is stringwise less than, equal to, or greater than the right
460argument.
d74e8afc 461X<cmp>
a0d0e21e 462
0d863452 463Binary "~~" does a smart match between its arguments. Smart matching
0f7107a0 464is described in L<perlsyn/"Smart matching in detail">.
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465X<~~>
466
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467"lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
468by the current locale if C<use locale> is in effect. See L<perllocale>.
469
a0d0e21e 470=head2 Bitwise And
d74e8afc 471X<operator, bitwise, and> X<bitwise and> X<&>
a0d0e21e 472
2cdc098b 473Binary "&" returns its operands ANDed together bit by bit.
2c268ad5 474(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
a0d0e21e 475
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476Note that "&" has lower priority than relational operators, so for example
477the brackets are essential in a test like
478
479 print "Even\n" if ($x & 1) == 0;
480
a0d0e21e 481=head2 Bitwise Or and Exclusive Or
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482X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor>
483X<bitwise xor> X<^>
a0d0e21e 484
2cdc098b 485Binary "|" returns its operands ORed together bit by bit.
2c268ad5 486(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
a0d0e21e 487
2cdc098b 488Binary "^" returns its operands XORed together bit by bit.
2c268ad5 489(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
a0d0e21e 490
2cdc098b
MG
491Note that "|" and "^" have lower priority than relational operators, so
492for example the brackets are essential in a test like
493
494 print "false\n" if (8 | 2) != 10;
495
a0d0e21e 496=head2 C-style Logical And
d74e8afc 497X<&&> X<logical and> X<operator, logical, and>
a0d0e21e
LW
498
499Binary "&&" performs a short-circuit logical AND operation. That is,
500if the left operand is false, the right operand is not even evaluated.
501Scalar or list context propagates down to the right operand if it
502is evaluated.
503
504=head2 C-style Logical Or
d74e8afc 505X<||> X<operator, logical, or>
a0d0e21e
LW
506
507Binary "||" performs a short-circuit logical OR operation. That is,
508if the left operand is true, the right operand is not even evaluated.
509Scalar or list context propagates down to the right operand if it
510is evaluated.
511
c963b151 512=head2 C-style Logical Defined-Or
d74e8afc 513X<//> X<operator, logical, defined-or>
c963b151
BD
514
515Although it has no direct equivalent in C, Perl's C<//> operator is related
89d205f2 516to its C-style or. In fact, it's exactly the same as C<||>, except that it
c963b151 517tests the left hand side's definedness instead of its truth. Thus, C<$a // $b>
89d205f2 518is similar to C<defined($a) || $b> (except that it returns the value of C<$a>
d90d5a38 519rather than the value of C<defined($a)>) and yields the same result as
bdc7923b
RGS
520C<defined($a) ? $a : $b> (except that the ternary-operator form can be
521used as a lvalue, while C<$a // $b> cannot). This is very useful for
522providing default values for variables. If you actually want to test if
523at least one of C<$a> and C<$b> is defined, use C<defined($a // $b)>.
c963b151 524
d042e63d
MS
525The C<||>, C<//> and C<&&> operators return the last value evaluated
526(unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably
527portable way to find out the home directory might be:
a0d0e21e 528
c543c01b
TC
529 $home = $ENV{HOME}
530 // $ENV{LOGDIR}
531 // (getpwuid($<))[7]
532 // die "You're homeless!\n";
a0d0e21e 533
5a964f20
TC
534In particular, this means that you shouldn't use this
535for selecting between two aggregates for assignment:
536
537 @a = @b || @c; # this is wrong
538 @a = scalar(@b) || @c; # really meant this
539 @a = @b ? @b : @c; # this works fine, though
540
f23102e2
RGS
541As more readable alternatives to C<&&> and C<||> when used for
542control flow, Perl provides the C<and> and C<or> operators (see below).
543The short-circuit behavior is identical. The precedence of "and"
c963b151 544and "or" is much lower, however, so that you can safely use them after a
5a964f20 545list operator without the need for parentheses:
a0d0e21e
LW
546
547 unlink "alpha", "beta", "gamma"
548 or gripe(), next LINE;
549
550With the C-style operators that would have been written like this:
551
552 unlink("alpha", "beta", "gamma")
553 || (gripe(), next LINE);
554
eeb6a2c9 555Using "or" for assignment is unlikely to do what you want; see below.
5a964f20
TC
556
557=head2 Range Operators
d74e8afc 558X<operator, range> X<range> X<..> X<...>
a0d0e21e
LW
559
560Binary ".." is the range operator, which is really two different
fb53bbb2 561operators depending on the context. In list context, it returns a
54ae734e 562list of values counting (up by ones) from the left value to the right
2cdbc966 563value. If the left value is greater than the right value then it
fb53bbb2 564returns the empty list. The range operator is useful for writing
54ae734e 565C<foreach (1..10)> loops and for doing slice operations on arrays. In
2cdbc966
JD
566the current implementation, no temporary array is created when the
567range operator is used as the expression in C<foreach> loops, but older
568versions of Perl might burn a lot of memory when you write something
569like this:
a0d0e21e
LW
570
571 for (1 .. 1_000_000) {
572 # code
54310121 573 }
a0d0e21e 574
8f0f46f8 575The range operator also works on strings, using the magical
576auto-increment, see below.
54ae734e 577
5a964f20 578In scalar context, ".." returns a boolean value. The operator is
8f0f46f8 579bistable, like a flip-flop, and emulates the line-range (comma)
580operator of B<sed>, B<awk>, and various editors. Each ".." operator
581maintains its own boolean state, even across calls to a subroutine
582that contains it. It is false as long as its left operand is false.
a0d0e21e
LW
583Once the left operand is true, the range operator stays true until the
584right operand is true, I<AFTER> which the range operator becomes false
8f0f46f8 585again. It doesn't become false till the next time the range operator
586is evaluated. It can test the right operand and become false on the
587same evaluation it became true (as in B<awk>), but it still returns
588true once. If you don't want it to test the right operand until the
589next evaluation, as in B<sed>, just use three dots ("...") instead of
19799a22
GS
590two. In all other regards, "..." behaves just like ".." does.
591
592The right operand is not evaluated while the operator is in the
593"false" state, and the left operand is not evaluated while the
594operator is in the "true" state. The precedence is a little lower
595than || and &&. The value returned is either the empty string for
8f0f46f8 596false, or a sequence number (beginning with 1) for true. The sequence
597number is reset for each range encountered. The final sequence number
598in a range has the string "E0" appended to it, which doesn't affect
599its numeric value, but gives you something to search for if you want
600to exclude the endpoint. You can exclude the beginning point by
601waiting for the sequence number to be greater than 1.
df5f8116
CW
602
603If either operand of scalar ".." is a constant expression,
604that operand is considered true if it is equal (C<==>) to the current
605input line number (the C<$.> variable).
606
607To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>,
608but that is only an issue if you use a floating point expression; when
609implicitly using C<$.> as described in the previous paragraph, the
610comparison is C<int(EXPR) == int($.)> which is only an issue when C<$.>
611is set to a floating point value and you are not reading from a file.
612Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what
613you want in scalar context because each of the operands are evaluated
614using their integer representation.
615
616Examples:
a0d0e21e
LW
617
618As a scalar operator:
619
df5f8116 620 if (101 .. 200) { print; } # print 2nd hundred lines, short for
950b09ed 621 # if ($. == 101 .. $. == 200) { print; }
9f10b797
RGS
622
623 next LINE if (1 .. /^$/); # skip header lines, short for
f343f960 624 # next LINE if ($. == 1 .. /^$/);
9f10b797
RGS
625 # (typically in a loop labeled LINE)
626
627 s/^/> / if (/^$/ .. eof()); # quote body
a0d0e21e 628
5a964f20
TC
629 # parse mail messages
630 while (<>) {
631 $in_header = 1 .. /^$/;
df5f8116
CW
632 $in_body = /^$/ .. eof;
633 if ($in_header) {
f343f960 634 # do something
df5f8116 635 } else { # in body
f343f960 636 # do something else
df5f8116 637 }
5a964f20 638 } continue {
df5f8116 639 close ARGV if eof; # reset $. each file
5a964f20
TC
640 }
641
acf31ca5
SF
642Here's a simple example to illustrate the difference between
643the two range operators:
644
645 @lines = (" - Foo",
646 "01 - Bar",
647 "1 - Baz",
648 " - Quux");
649
9f10b797
RGS
650 foreach (@lines) {
651 if (/0/ .. /1/) {
acf31ca5
SF
652 print "$_\n";
653 }
654 }
655
9f10b797
RGS
656This program will print only the line containing "Bar". If
657the range operator is changed to C<...>, it will also print the
acf31ca5
SF
658"Baz" line.
659
660And now some examples as a list operator:
a0d0e21e
LW
661
662 for (101 .. 200) { print; } # print $_ 100 times
3e3baf6d 663 @foo = @foo[0 .. $#foo]; # an expensive no-op
a0d0e21e
LW
664 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
665
5a964f20 666The range operator (in list context) makes use of the magical
5f05dabc 667auto-increment algorithm if the operands are strings. You
a0d0e21e
LW
668can say
669
c543c01b 670 @alphabet = ("A" .. "Z");
a0d0e21e 671
54ae734e 672to get all normal letters of the English alphabet, or
a0d0e21e 673
c543c01b 674 $hexdigit = (0 .. 9, "a" .. "f")[$num & 15];
a0d0e21e
LW
675
676to get a hexadecimal digit, or
677
c543c01b 678 @z2 = ("01" .. "31"); print $z2[$mday];
a0d0e21e 679
ea4f5703
YST
680to get dates with leading zeros.
681
682If the final value specified is not in the sequence that the magical
683increment would produce, the sequence goes until the next value would
684be longer than the final value specified.
685
686If the initial value specified isn't part of a magical increment
c543c01b 687sequence (that is, a non-empty string matching C</^[a-zA-Z]*[0-9]*\z/>),
ea4f5703
YST
688only the initial value will be returned. So the following will only
689return an alpha:
690
c543c01b 691 use charnames "greek";
ea4f5703
YST
692 my @greek_small = ("\N{alpha}" .. "\N{omega}");
693
c543c01b
TC
694To get the 25 traditional lowercase Greek letters, including both sigmas,
695you could use this instead:
ea4f5703 696
c543c01b
TC
697 use charnames "greek";
698 my @greek_small = map { chr }
699 ord "\N{alpha}" .. ord "\N{omega}";
700
701However, because there are I<many> other lowercase Greek characters than
702just those, to match lowercase Greek characters in a regular expression,
703you would use the pattern C</(?:(?=\p{Greek})\p{Lower})+/>.
a0d0e21e 704
df5f8116
CW
705Because each operand is evaluated in integer form, C<2.18 .. 3.14> will
706return two elements in list context.
707
708 @list = (2.18 .. 3.14); # same as @list = (2 .. 3);
709
a0d0e21e 710=head2 Conditional Operator
d74e8afc 711X<operator, conditional> X<operator, ternary> X<ternary> X<?:>
a0d0e21e
LW
712
713Ternary "?:" is the conditional operator, just as in C. It works much
714like an if-then-else. If the argument before the ? is true, the
715argument before the : is returned, otherwise the argument after the :
cb1a09d0
AD
716is returned. For example:
717
54310121 718 printf "I have %d dog%s.\n", $n,
c543c01b 719 ($n == 1) ? "" : "s";
cb1a09d0
AD
720
721Scalar or list context propagates downward into the 2nd
54310121 722or 3rd argument, whichever is selected.
cb1a09d0
AD
723
724 $a = $ok ? $b : $c; # get a scalar
725 @a = $ok ? @b : @c; # get an array
726 $a = $ok ? @b : @c; # oops, that's just a count!
727
728The operator may be assigned to if both the 2nd and 3rd arguments are
729legal lvalues (meaning that you can assign to them):
a0d0e21e
LW
730
731 ($a_or_b ? $a : $b) = $c;
732
5a964f20
TC
733Because this operator produces an assignable result, using assignments
734without parentheses will get you in trouble. For example, this:
735
736 $a % 2 ? $a += 10 : $a += 2
737
738Really means this:
739
740 (($a % 2) ? ($a += 10) : $a) += 2
741
742Rather than this:
743
744 ($a % 2) ? ($a += 10) : ($a += 2)
745
19799a22
GS
746That should probably be written more simply as:
747
748 $a += ($a % 2) ? 10 : 2;
749
4633a7c4 750=head2 Assignment Operators
d74e8afc 751X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=>
5ac3b81c 752X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=>
d74e8afc 753X<%=> X<^=> X<x=>
a0d0e21e
LW
754
755"=" is the ordinary assignment operator.
756
757Assignment operators work as in C. That is,
758
759 $a += 2;
760
761is equivalent to
762
763 $a = $a + 2;
764
765although without duplicating any side effects that dereferencing the lvalue
54310121 766might trigger, such as from tie(). Other assignment operators work similarly.
767The following are recognized:
a0d0e21e
LW
768
769 **= += *= &= <<= &&=
9f10b797
RGS
770 -= /= |= >>= ||=
771 .= %= ^= //=
772 x=
a0d0e21e 773
19799a22 774Although these are grouped by family, they all have the precedence
a0d0e21e
LW
775of assignment.
776
b350dd2f
GS
777Unlike in C, the scalar assignment operator produces a valid lvalue.
778Modifying an assignment is equivalent to doing the assignment and
779then modifying the variable that was assigned to. This is useful
780for modifying a copy of something, like this:
a0d0e21e 781
c543c01b 782 ($tmp = $global) =~ tr [0-9] [a-j];
a0d0e21e
LW
783
784Likewise,
785
786 ($a += 2) *= 3;
787
788is equivalent to
789
790 $a += 2;
791 $a *= 3;
792
b350dd2f
GS
793Similarly, a list assignment in list context produces the list of
794lvalues assigned to, and a list assignment in scalar context returns
795the number of elements produced by the expression on the right hand
796side of the assignment.
797
c543c01b
TC
798=head2 The Triple-Dot Operator
799X<...> X<... operator> X<yada-yada operator> X<whatever operator>
800X<triple-dot operator>
801
802The triple-dot operator, C<...>, sometimes called the "whatever operator", the
803"yada-yada operator", or the "I<et cetera>" operator, is a placeholder for
804code. Perl parses it without error, but when you try to execute a whatever,
805it throws an exception with the text C<Unimplemented>:
806
807 sub unimplemented { ... }
808
809 eval { unimplemented() };
810 if ($@ eq "Unimplemented" ) {
811 say "Oh look, an exception--whatever.";
812 }
813
814You can only use the triple-dot operator to stand in for a complete statement.
815These examples of the triple-dot work:
816
817 { ... }
818
819 sub foo { ... }
820
821 ...;
822
823 eval { ... };
824
825 sub foo {
826 my ($self) = shift;
827 ...;
828 }
829
830 do {
831 my $variable;
832 ...;
833 say "Hurrah!";
834 } while $cheering;
835
836The yada-yada--or whatever--cannot stand in for an expression that is
837part of a larger statement since the C<...> is also the three-dot version
838of the binary range operator (see L<Range Operators>). These examples of
839the whatever operator are still syntax errors:
840
841 print ...;
842
843 open(PASSWD, ">", "/dev/passwd") or ...;
844
845 if ($condition && ...) { say "Hello" }
846
847There are some cases where Perl can't immediately tell the difference
848between an expression and a statement. For instance, the syntax for a
849block and an anonymous hash reference constructor look the same unless
850there's something in the braces that give Perl a hint. The whatever
851is a syntax error if Perl doesn't guess that the C<{ ... }> is a
852block. In that case, it doesn't think the C<...> is the whatever
853because it's expecting an expression instead of a statement:
854
855 my @transformed = map { ... } @input; # syntax error
856
857You can use a C<;> inside your block to denote that the C<{ ... }> is
858a block and not a hash reference constructor. Now the whatever works:
859
860 my @transformed = map {; ... } @input; # ; disambiguates
861
862 my @transformed = map { ...; } @input; # ; disambiguates
863
748a9306 864=head2 Comma Operator
d74e8afc 865X<comma> X<operator, comma> X<,>
a0d0e21e 866
5a964f20 867Binary "," is the comma operator. In scalar context it evaluates
a0d0e21e
LW
868its left argument, throws that value away, then evaluates its right
869argument and returns that value. This is just like C's comma operator.
870
5a964f20 871In list context, it's just the list argument separator, and inserts
ed5c6d31
PJ
872both its arguments into the list. These arguments are also evaluated
873from left to right.
a0d0e21e 874
344f2c40
IG
875The C<< => >> operator is a synonym for the comma except that it causes
876its left operand to be interpreted as a string if it begins with a letter
877or underscore and is composed only of letters, digits and underscores.
878This includes operands that might otherwise be interpreted as operators,
879constants, single number v-strings or function calls. If in doubt about
c543c01b 880this behavior, the left operand can be quoted explicitly.
344f2c40
IG
881
882Otherwise, the C<< => >> operator behaves exactly as the comma operator
883or list argument separator, according to context.
884
885For example:
a44e5664
MS
886
887 use constant FOO => "something";
888
889 my %h = ( FOO => 23 );
890
891is equivalent to:
892
893 my %h = ("FOO", 23);
894
895It is I<NOT>:
896
897 my %h = ("something", 23);
898
719b43e8
RGS
899The C<< => >> operator is helpful in documenting the correspondence
900between keys and values in hashes, and other paired elements in lists.
748a9306 901
a44e5664
MS
902 %hash = ( $key => $value );
903 login( $username => $password );
904
a0d0e21e 905=head2 List Operators (Rightward)
d74e8afc 906X<operator, list, rightward> X<list operator>
a0d0e21e 907
c543c01b 908On the right side of a list operator, the comma has very low precedence,
a0d0e21e
LW
909such that it controls all comma-separated expressions found there.
910The only operators with lower precedence are the logical operators
911"and", "or", and "not", which may be used to evaluate calls to list
912operators without the need for extra parentheses:
913
c543c01b
TC
914 open HANDLE, "< $file"
915 or die "Can't open $file: $!\n";
a0d0e21e 916
5ba421f6 917See also discussion of list operators in L<Terms and List Operators (Leftward)>.
a0d0e21e
LW
918
919=head2 Logical Not
d74e8afc 920X<operator, logical, not> X<not>
a0d0e21e
LW
921
922Unary "not" returns the logical negation of the expression to its right.
923It's the equivalent of "!" except for the very low precedence.
924
925=head2 Logical And
d74e8afc 926X<operator, logical, and> X<and>
a0d0e21e
LW
927
928Binary "and" returns the logical conjunction of the two surrounding
c543c01b
TC
929expressions. It's equivalent to C<&&> except for the very low
930precedence. This means that it short-circuits: the right
a0d0e21e
LW
931expression is evaluated only if the left expression is true.
932
c963b151 933=head2 Logical or, Defined or, and Exclusive Or
f23102e2 934X<operator, logical, or> X<operator, logical, xor>
d74e8afc 935X<operator, logical, defined or> X<operator, logical, exclusive or>
f23102e2 936X<or> X<xor>
a0d0e21e
LW
937
938Binary "or" returns the logical disjunction of the two surrounding
c543c01b
TC
939expressions. It's equivalent to C<||> except for the very low precedence.
940This makes it useful for control flow:
5a964f20
TC
941
942 print FH $data or die "Can't write to FH: $!";
943
c543c01b
TC
944This means that it short-circuits: the right expression is evaluated
945only if the left expression is false. Due to its precedence, you must
946be careful to avoid using it as replacement for the C<||> operator.
947It usually works out better for flow control than in assignments:
5a964f20
TC
948
949 $a = $b or $c; # bug: this is wrong
950 ($a = $b) or $c; # really means this
951 $a = $b || $c; # better written this way
952
19799a22 953However, when it's a list-context assignment and you're trying to use
c543c01b 954C<||> for control flow, you probably need "or" so that the assignment
5a964f20
TC
955takes higher precedence.
956
957 @info = stat($file) || die; # oops, scalar sense of stat!
958 @info = stat($file) or die; # better, now @info gets its due
959
c963b151
BD
960Then again, you could always use parentheses.
961
a0d0e21e 962Binary "xor" returns the exclusive-OR of the two surrounding expressions.
c543c01b 963It cannot short-circuit (of course).
a0d0e21e
LW
964
965=head2 C Operators Missing From Perl
d74e8afc
ITB
966X<operator, missing from perl> X<&> X<*>
967X<typecasting> X<(TYPE)>
a0d0e21e
LW
968
969Here is what C has that Perl doesn't:
970
971=over 8
972
973=item unary &
974
975Address-of operator. (But see the "\" operator for taking a reference.)
976
977=item unary *
978
54310121 979Dereference-address operator. (Perl's prefix dereferencing
a0d0e21e
LW
980operators are typed: $, @, %, and &.)
981
982=item (TYPE)
983
19799a22 984Type-casting operator.
a0d0e21e
LW
985
986=back
987
5f05dabc 988=head2 Quote and Quote-like Operators
89d205f2 989X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m>
d74e8afc
ITB
990X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>>
991X<escape sequence> X<escape>
992
a0d0e21e
LW
993While we usually think of quotes as literal values, in Perl they
994function as operators, providing various kinds of interpolating and
995pattern matching capabilities. Perl provides customary quote characters
996for these behaviors, but also provides a way for you to choose your
997quote character for any of them. In the following table, a C<{}> represents
9f10b797 998any pair of delimiters you choose.
a0d0e21e 999
2c268ad5
TP
1000 Customary Generic Meaning Interpolates
1001 '' q{} Literal no
1002 "" qq{} Literal yes
af9219ee 1003 `` qx{} Command yes*
2c268ad5 1004 qw{} Word list no
af9219ee
MG
1005 // m{} Pattern match yes*
1006 qr{} Pattern yes*
1007 s{}{} Substitution yes*
2c268ad5 1008 tr{}{} Transliteration no (but see below)
c543c01b 1009 y{}{} Transliteration no (but see below)
7e3b091d 1010 <<EOF here-doc yes*
a0d0e21e 1011
af9219ee
MG
1012 * unless the delimiter is ''.
1013
87275199 1014Non-bracketing delimiters use the same character fore and aft, but the four
c543c01b 1015sorts of ASCII brackets (round, angle, square, curly) all nest, which means
9f10b797 1016that
87275199 1017
c543c01b 1018 q{foo{bar}baz}
35f2feb0 1019
9f10b797 1020is the same as
87275199 1021
c543c01b 1022 'foo{bar}baz'
87275199
GS
1023
1024Note, however, that this does not always work for quoting Perl code:
1025
c543c01b 1026 $s = q{ if($a eq "}") ... }; # WRONG
87275199 1027
c543c01b
TC
1028is a syntax error. The C<Text::Balanced> module (standard as of v5.8,
1029and from CPAN before then) is able to do this properly.
87275199 1030
19799a22 1031There can be whitespace between the operator and the quoting
fb73857a 1032characters, except when C<#> is being used as the quoting character.
19799a22
GS
1033C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
1034operator C<q> followed by a comment. Its argument will be taken
1035from the next line. This allows you to write:
fb73857a 1036
1037 s {foo} # Replace foo
1038 {bar} # with bar.
1039
c543c01b
TC
1040The following escape sequences are available in constructs that interpolate,
1041and in transliterations:
5691ca5f 1042X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N> X<\N{}>
04341565 1043X<\o{}>
5691ca5f 1044
2c4c1ff2
KW
1045 Sequence Note Description
1046 \t tab (HT, TAB)
1047 \n newline (NL)
1048 \r return (CR)
1049 \f form feed (FF)
1050 \b backspace (BS)
1051 \a alarm (bell) (BEL)
1052 \e escape (ESC)
c543c01b 1053 \x{263A} [1,8] hex char (example: SMILEY)
2c4c1ff2 1054 \x1b [2,8] restricted range hex char (example: ESC)
fb121860 1055 \N{name} [3] named Unicode character or character sequence
2c4c1ff2
KW
1056 \N{U+263D} [4,8] Unicode character (example: FIRST QUARTER MOON)
1057 \c[ [5] control char (example: chr(27))
1058 \o{23072} [6,8] octal char (example: SMILEY)
1059 \033 [7,8] restricted range octal char (example: ESC)
5691ca5f
KW
1060
1061=over 4
1062
1063=item [1]
1064
2c4c1ff2
KW
1065The result is the character specified by the hexadecimal number between
1066the braces. See L</[8]> below for details on which character.
96448467
DG
1067
1068Only hexadecimal digits are valid between the braces. If an invalid
1069character is encountered, a warning will be issued and the invalid
1070character and all subsequent characters (valid or invalid) within the
1071braces will be discarded.
1072
1073If there are no valid digits between the braces, the generated character is
1074the NULL character (C<\x{00}>). However, an explicit empty brace (C<\x{}>)
c543c01b 1075will not cause a warning (currently).
40687185
KW
1076
1077=item [2]
1078
2c4c1ff2
KW
1079The result is the character specified by the hexadecimal number in the range
10800x00 to 0xFF. See L</[8]> below for details on which character.
96448467
DG
1081
1082Only hexadecimal digits are valid following C<\x>. When C<\x> is followed
2c4c1ff2 1083by fewer than two valid digits, any valid digits will be zero-padded. This
c543c01b 1084means that C<\x7> will be interpreted as C<\x07>, and a lone <\x> will be
2c4c1ff2 1085interpreted as C<\x00>. Except at the end of a string, having fewer than
c543c01b 1086two valid digits will result in a warning. Note that although the warning
96448467
DG
1087says the illegal character is ignored, it is only ignored as part of the
1088escape and will still be used as the subsequent character in the string.
1089For example:
1090
1091 Original Result Warns?
1092 "\x7" "\x07" no
1093 "\x" "\x00" no
1094 "\x7q" "\x07q" yes
1095 "\xq" "\x00q" yes
1096
40687185
KW
1097=item [3]
1098
fb121860 1099The result is the Unicode character or character sequence given by I<name>.
2c4c1ff2 1100See L<charnames>.
40687185
KW
1101
1102=item [4]
1103
2c4c1ff2
KW
1104C<\N{U+I<hexadecimal number>}> means the Unicode character whose Unicode code
1105point is I<hexadecimal number>.
40687185
KW
1106
1107=item [5]
1108
5691ca5f
KW
1109The character following C<\c> is mapped to some other character as shown in the
1110table:
1111
1112 Sequence Value
1113 \c@ chr(0)
1114 \cA chr(1)
1115 \ca chr(1)
1116 \cB chr(2)
1117 \cb chr(2)
1118 ...
1119 \cZ chr(26)
1120 \cz chr(26)
1121 \c[ chr(27)
1122 \c] chr(29)
1123 \c^ chr(30)
1124 \c? chr(127)
1125
d813941f
KW
1126In other words, it's the character whose code point has had 64 xor'd with
1127its uppercase. C<\c?> is DELETE because C<ord("@") ^ 64> is 127, and
1128C<\c@> is NULL because the ord of "@" is 64, so xor'ing 64 itself produces 0.
1129
5691ca5f
KW
1130Also, C<\c\I<X>> yields C< chr(28) . "I<X>"> for any I<X>, but cannot come at the
1131end of a string, because the backslash would be parsed as escaping the end
1132quote.
1133
1134On ASCII platforms, the resulting characters from the list above are the
1135complete set of ASCII controls. This isn't the case on EBCDIC platforms; see
1136L<perlebcdic/OPERATOR DIFFERENCES> for the complete list of what these
1137sequences mean on both ASCII and EBCDIC platforms.
1138
1139Use of any other character following the "c" besides those listed above is
17a3df4c
KW
1140discouraged, and some are deprecated with the intention of removing
1141those in Perl 5.16. What happens for any of these
d813941f
KW
1142other characters currently though, is that the value is derived by xor'ing
1143with the seventh bit, which is 64.
5691ca5f
KW
1144
1145To get platform independent controls, you can use C<\N{...}>.
1146
40687185
KW
1147=item [6]
1148
2c4c1ff2
KW
1149The result is the character specified by the octal number between the braces.
1150See L</[8]> below for details on which character.
04341565
DG
1151
1152If a character that isn't an octal digit is encountered, a warning is raised,
1153and the value is based on the octal digits before it, discarding it and all
1154following characters up to the closing brace. It is a fatal error if there are
1155no octal digits at all.
1156
1157=item [7]
1158
c543c01b 1159The result is the character specified by the three-digit octal number in the
2c4c1ff2
KW
1160range 000 to 777 (but best to not use above 077, see next paragraph). See
1161L</[8]> below for details on which character.
1162
1163Some contexts allow 2 or even 1 digit, but any usage without exactly
40687185 1164three digits, the first being a zero, may give unintended results. (For
04341565 1165example, see L<perlrebackslash/Octal escapes>.) Starting in Perl 5.14, you may
c543c01b 1166use C<\o{}> instead, which avoids all these problems. Otherwise, it is best to
04341565
DG
1167use this construct only for ordinals C<\077> and below, remembering to pad to
1168the left with zeros to make three digits. For larger ordinals, either use
d90d5a38 1169C<\o{}> , or convert to something else, such as to hex and use C<\x{}>
04341565 1170instead.
40687185 1171
40687185
KW
1172Having fewer than 3 digits may lead to a misleading warning message that says
1173that what follows is ignored. For example, C<"\128"> in the ASCII character set
1174is equivalent to the two characters C<"\n8">, but the warning C<Illegal octal
1175digit '8' ignored> will be thrown. To avoid this warning, make sure to pad
f6050459 1176your octal number with C<0>'s: C<"\0128">.
5691ca5f 1177
2c4c1ff2
KW
1178=item [8]
1179
c543c01b 1180Several constructs above specify a character by a number. That number
2c4c1ff2 1181gives the character's position in the character set encoding (indexed from 0).
c543c01b 1182This is called synonymously its ordinal, code position, or code point. Perl
2c4c1ff2
KW
1183works on platforms that have a native encoding currently of either ASCII/Latin1
1184or EBCDIC, each of which allow specification of 256 characters. In general, if
1185the number is 255 (0xFF, 0377) or below, Perl interprets this in the platform's
1186native encoding. If the number is 256 (0x100, 0400) or above, Perl interprets
c543c01b 1187it as a Unicode code point and the result is the corresponding Unicode
2c4c1ff2
KW
1188character. For example C<\x{50}> and C<\o{120}> both are the number 80 in
1189decimal, which is less than 256, so the number is interpreted in the native
1190character set encoding. In ASCII the character in the 80th position (indexed
1191from 0) is the letter "P", and in EBCDIC it is the ampersand symbol "&".
1192C<\x{100}> and C<\o{400}> are both 256 in decimal, so the number is interpreted
1193as a Unicode code point no matter what the native encoding is. The name of the
1194character in the 100th position (indexed by 0) in Unicode is
1195C<LATIN CAPITAL LETTER A WITH MACRON>.
1196
1197There are a couple of exceptions to the above rule. C<\N{U+I<hex number>}> is
1198always interpreted as a Unicode code point, so that C<\N{U+0050}> is "P" even
1199on EBCDIC platforms. And if L<C<S<use encoding>>|encoding> is in effect, the
1200number is considered to be in that encoding, and is translated from that into
1201the platform's native encoding if there is a corresponding native character;
1202otherwise to Unicode.
1203
5691ca5f 1204=back
4c77eaa2 1205
e526e8bb
KW
1206B<NOTE>: Unlike C and other languages, Perl has no C<\v> escape sequence for
1207the vertical tab (VT - ASCII 11), but you may use C<\ck> or C<\x0b>. (C<\v>
1208does have meaning in regular expression patterns in Perl, see L<perlre>.)
1209
1210The following escape sequences are available in constructs that interpolate,
904501ec 1211but not in transliterations.
d74e8afc 1212X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q>
904501ec 1213
c543c01b
TC
1214 \l lowercase next character only
1215 \u titlecase (not uppercase!) next character only
1216 \L lowercase all characters till \E seen
1217 \U uppercase all characters till \E seen
1d2dff63 1218 \Q quote non-word characters till \E
7e31b643 1219 \E end either case modification or quoted section
c543c01b
TC
1220 (whichever was last seen)
1221
1222C<\L>, C<\U>, and C<\Q> can stack, in which case you need one
1223C<\E> for each. For example:
1224
1225 say "This \Qquoting \ubusiness \Uhere isn't quite\E done yet,\E is it?";
1226 This quoting\ Business\ HERE\ ISN\'T\ QUITE\ done\ yet\, is it?
a0d0e21e 1227
95cc3e0c 1228If C<use locale> is in effect, the case map used by C<\l>, C<\L>,
c543c01b 1229C<\u>, and C<\U> is taken from the current locale. See L<perllocale>.
b6538e4f 1230If Unicode (for example, C<\N{}> or code points of 0x100 or
c543c01b
TC
1231beyond) is being used, the case map used by C<\l>, C<\L>, C<\u>, and
1232C<\U> is as defined by Unicode. That means that case-mapping
1233a single character can sometimes produce several characters.
a034a98d 1234
5a964f20
TC
1235All systems use the virtual C<"\n"> to represent a line terminator,
1236called a "newline". There is no such thing as an unvarying, physical
19799a22 1237newline character. It is only an illusion that the operating system,
5a964f20
TC
1238device drivers, C libraries, and Perl all conspire to preserve. Not all
1239systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
c543c01b
TC
1240on the ancient Macs (pre-MacOS X) of yesteryear, these used to be reversed,
1241and on systems without line terminator,
1242printing C<"\n"> might emit no actual data. In general, use C<"\n"> when
5a964f20
TC
1243you mean a "newline" for your system, but use the literal ASCII when you
1244need an exact character. For example, most networking protocols expect
2a380090 1245and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
5a964f20
TC
1246and although they often accept just C<"\012">, they seldom tolerate just
1247C<"\015">. If you get in the habit of using C<"\n"> for networking,
1248you may be burned some day.
d74e8afc
ITB
1249X<newline> X<line terminator> X<eol> X<end of line>
1250X<\n> X<\r> X<\r\n>
5a964f20 1251
904501ec
MG
1252For constructs that do interpolate, variables beginning with "C<$>"
1253or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
ad0f383a
A
1254C<< $href->{key}[0] >> are also interpolated, as are array and hash slices.
1255But method calls such as C<< $obj->meth >> are not.
af9219ee
MG
1256
1257Interpolating an array or slice interpolates the elements in order,
1258separated by the value of C<$">, so is equivalent to interpolating
c543c01b
TC
1259C<join $", @array>. "Punctuation" arrays such as C<@*> are usually
1260interpolated only if the name is enclosed in braces C<@{*}>, but the
1261arrays C<@_>, C<@+>, and C<@-> are interpolated even without braces.
af9219ee 1262
bc7b91c6
EB
1263For double-quoted strings, the quoting from C<\Q> is applied after
1264interpolation and escapes are processed.
1265
1266 "abc\Qfoo\tbar$s\Exyz"
1267
1268is equivalent to
1269
1270 "abc" . quotemeta("foo\tbar$s") . "xyz"
1271
1272For the pattern of regex operators (C<qr//>, C<m//> and C<s///>),
1273the quoting from C<\Q> is applied after interpolation is processed,
1274but before escapes are processed. This allows the pattern to match
1275literally (except for C<$> and C<@>). For example, the following matches:
1276
1277 '\s\t' =~ /\Q\s\t/
1278
1279Because C<$> or C<@> trigger interpolation, you'll need to use something
1280like C</\Quser\E\@\Qhost/> to match them literally.
1d2dff63 1281
a0d0e21e
LW
1282Patterns are subject to an additional level of interpretation as a
1283regular expression. This is done as a second pass, after variables are
1284interpolated, so that regular expressions may be incorporated into the
1285pattern from the variables. If this is not what you want, use C<\Q> to
1286interpolate a variable literally.
1287
19799a22
GS
1288Apart from the behavior described above, Perl does not expand
1289multiple levels of interpolation. In particular, contrary to the
1290expectations of shell programmers, back-quotes do I<NOT> interpolate
1291within double quotes, nor do single quotes impede evaluation of
1292variables when used within double quotes.
a0d0e21e 1293
5f05dabc 1294=head2 Regexp Quote-Like Operators
d74e8afc 1295X<operator, regexp>
cb1a09d0 1296
5f05dabc 1297Here are the quote-like operators that apply to pattern
cb1a09d0
AD
1298matching and related activities.
1299
a0d0e21e
LW
1300=over 8
1301
b6fa137b 1302=item qr/STRING/msixpodual
01c6f5f4 1303X<qr> X</i> X</m> X</o> X</s> X</x> X</p>
a0d0e21e 1304
87e95b7f
YO
1305This operator quotes (and possibly compiles) its I<STRING> as a regular
1306expression. I<STRING> is interpolated the same way as I<PATTERN>
1307in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
1308is done. Returns a Perl value which may be used instead of the
f6050459 1309corresponding C</STRING/msixpodual> expression. The returned value is a
85dd5c8b 1310normalized version of the original pattern. It magically differs from
1c8ee595
CO
1311a string containing the same characters: C<ref(qr/x/)> returns "Regexp";
1312however, dereferencing it is not well defined (you currently get the
1313normalized version of the original pattern, but this may change).
1314
a0d0e21e 1315
87e95b7f
YO
1316For example,
1317
1318 $rex = qr/my.STRING/is;
85dd5c8b 1319 print $rex; # prints (?si-xm:my.STRING)
87e95b7f
YO
1320 s/$rex/foo/;
1321
1322is equivalent to
1323
1324 s/my.STRING/foo/is;
1325
1326The result may be used as a subpattern in a match:
1327
1328 $re = qr/$pattern/;
1329 $string =~ /foo${re}bar/; # can be interpolated in other patterns
1330 $string =~ $re; # or used standalone
1331 $string =~ /$re/; # or this way
1332
f6050459 1333Since Perl may compile the pattern at the moment of execution of the qr()
87e95b7f
YO
1334operator, using qr() may have speed advantages in some situations,
1335notably if the result of qr() is used standalone:
1336
1337 sub match {
1338 my $patterns = shift;
1339 my @compiled = map qr/$_/i, @$patterns;
1340 grep {
1341 my $success = 0;
1342 foreach my $pat (@compiled) {
1343 $success = 1, last if /$pat/;
1344 }
1345 $success;
1346 } @_;
5a964f20
TC
1347 }
1348
87e95b7f
YO
1349Precompilation of the pattern into an internal representation at
1350the moment of qr() avoids a need to recompile the pattern every
1351time a match C</$pat/> is attempted. (Perl has many other internal
1352optimizations, but none would be triggered in the above example if
1353we did not use qr() operator.)
1354
765fa144 1355Options (specified by the following modifiers) are:
87e95b7f
YO
1356
1357 m Treat string as multiple lines.
1358 s Treat string as single line. (Make . match a newline)
1359 i Do case-insensitive pattern matching.
1360 x Use extended regular expressions.
1361 p When matching preserve a copy of the matched string so
1362 that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be defined.
1363 o Compile pattern only once.
b5c53fdc
KW
1364 a ASCII-restrict: Use ASCII for \d, \s, \w; specifying two a's
1365 further restricts /i matching so that no ASCII character will
1366 match a non-ASCII one
18509dec
KW
1367 l Use the locale
1368 u Use Unicode rules
b6fa137b 1369 d Use Unicode or native charset, as in 5.12 and earlier
87e95b7f
YO
1370
1371If a precompiled pattern is embedded in a larger pattern then the effect
c543c01b 1372of "msixpluad" will be propagated appropriately. The effect the "o"
87e95b7f
YO
1373modifier has is not propagated, being restricted to those patterns
1374explicitly using it.
1375
b6fa137b 1376The last four modifiers listed above, added in Perl 5.14,
18509dec
KW
1377control the character set semantics, but C</a> is the only one you are likely
1378to want to specify explicitly; the other three are selected
1379automatically by various pragmas.
da392a17 1380
87e95b7f 1381See L<perlre> for additional information on valid syntax for STRING, and
5e2aa8f5 1382for a detailed look at the semantics of regular expressions. In
f6050459
KW
1383particular, all the modifiers execpt C</o> are further explained in
1384L<perlre/Modifiers>. C</o> is described in the next section.
a0d0e21e 1385
b6fa137b 1386=item m/PATTERN/msixpodualgc
89d205f2
YO
1387X<m> X<operator, match>
1388X<regexp, options> X<regexp> X<regex, options> X<regex>
01c6f5f4 1389X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c>
a0d0e21e 1390
b6fa137b 1391=item /PATTERN/msixpodualgc
a0d0e21e 1392
5a964f20 1393Searches a string for a pattern match, and in scalar context returns
19799a22
GS
1394true if it succeeds, false if it fails. If no string is specified
1395via the C<=~> or C<!~> operator, the $_ string is searched. (The
1396string specified with C<=~> need not be an lvalue--it may be the
1397result of an expression evaluation, but remember the C<=~> binds
006671a6 1398rather tightly.) See also L<perlre>.
a0d0e21e 1399
f6050459 1400Options are as described in C<qr//> above; in addition, the following match
01c6f5f4 1401process modifiers are available:
a0d0e21e 1402
950b09ed
KW
1403 g Match globally, i.e., find all occurrences.
1404 c Do not reset search position on a failed match when /g is in effect.
a0d0e21e 1405
725a61d7 1406If "/" is the delimiter then the initial C<m> is optional. With the C<m>
c543c01b 1407you can use any pair of non-whitespace (ASCII) characters
725a61d7
Z
1408as delimiters. This is particularly useful for matching path names
1409that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
1410the delimiter, then a match-only-once rule applies,
1411described in C<m?PATTERN?> below.
19799a22 1412If "'" is the delimiter, no interpolation is performed on the PATTERN.
ed02a3bf
DN
1413When using a character valid in an identifier, whitespace is required
1414after the C<m>.
a0d0e21e 1415
532c9e80
KW
1416PATTERN may contain variables, which will be interpolated
1417every time the pattern search is evaluated, except
1f247705
GS
1418for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
1419C<$|> are not interpolated because they look like end-of-string tests.)
532c9e80
KW
1420Perl will not recompile the pattern unless an interpolated
1421variable that it contains changes. You can force Perl to skip the
1422test and never recompile by adding a C</o> (which stands for "once")
1423after the trailing delimiter.
1424Once upon a time, Perl would recompile regular expressions
1425unnecessarily, and this modifier was useful to tell it not to do so, in the
1426interests of speed. But now, the only reasons to use C</o> are either:
1427
1428=over
1429
1430=item 1
1431
1432The variables are thousands of characters long and you know that they
1433don't change, and you need to wring out the last little bit of speed by
1434having Perl skip testing for that. (There is a maintenance penalty for
1435doing this, as mentioning C</o> constitutes a promise that you won't
18509dec 1436change the variables in the pattern. If you do change them, Perl won't
532c9e80
KW
1437even notice.)
1438
1439=item 2
1440
1441you want the pattern to use the initial values of the variables
1442regardless of whether they change or not. (But there are saner ways
1443of accomplishing this than using C</o>.)
1444
1445=back
a0d0e21e 1446
18509dec
KW
1447The bottom line is that using C</o> is almost never a good idea.
1448
e9d89077
DN
1449=item The empty pattern //
1450
5a964f20 1451If the PATTERN evaluates to the empty string, the last
d65afb4b 1452I<successfully> matched regular expression is used instead. In this
c543c01b 1453case, only the C<g> and C<c> flags on the empty pattern are honored;
d65afb4b
HS
1454the other flags are taken from the original pattern. If no match has
1455previously succeeded, this will (silently) act instead as a genuine
1456empty pattern (which will always match).
a0d0e21e 1457
89d205f2
YO
1458Note that it's possible to confuse Perl into thinking C<//> (the empty
1459regex) is really C<//> (the defined-or operator). Perl is usually pretty
1460good about this, but some pathological cases might trigger this, such as
1461C<$a///> (is that C<($a) / (//)> or C<$a // />?) and C<print $fh //>
1462(C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl
1463will assume you meant defined-or. If you meant the empty regex, just
1464use parentheses or spaces to disambiguate, or even prefix the empty
c963b151
BD
1465regex with an C<m> (so C<//> becomes C<m//>).
1466
e9d89077
DN
1467=item Matching in list context
1468
19799a22 1469If the C</g> option is not used, C<m//> in list context returns a
a0d0e21e 1470list consisting of the subexpressions matched by the parentheses in the
f7e33566
GS
1471pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are
1472also set, and that this differs from Perl 4's behavior.) When there are
1473no parentheses in the pattern, the return value is the list C<(1)> for
1474success. With or without parentheses, an empty list is returned upon
1475failure.
a0d0e21e
LW
1476
1477Examples:
1478
c543c01b
TC
1479 open(TTY, "+>/dev/tty")
1480 || die "can't access /dev/tty: $!";
1481
a0d0e21e
LW
1482 <TTY> =~ /^y/i && foo(); # do foo if desired
1483
1484 if (/Version: *([0-9.]*)/) { $version = $1; }
1485
1486 next if m#^/usr/spool/uucp#;
1487
1488 # poor man's grep
1489 $arg = shift;
1490 while (<>) {
c543c01b 1491 print if /$arg/o; # compile only once (no longer needed!)
a0d0e21e
LW
1492 }
1493
1494 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
1495
1496This last example splits $foo into the first two words and the
5f05dabc 1497remainder of the line, and assigns those three fields to $F1, $F2, and
c543c01b
TC
1498$Etc. The conditional is true if any variables were assigned; that is,
1499if the pattern matched.
a0d0e21e 1500
19799a22 1501The C</g> modifier specifies global pattern matching--that is,
3dd93342 1502matching as many times as possible within the string. How it behaves
1503depends on the context. In list context, it returns a list of the
19799a22 1504substrings matched by any capturing parentheses in the regular
3dd93342 1505expression. If there are no parentheses, it returns a list of all
19799a22
GS
1506the matched strings, as if there were parentheses around the whole
1507pattern.
a0d0e21e 1508
7e86de3e 1509In scalar context, each execution of C<m//g> finds the next match,
19799a22 1510returning true if it matches, and false if there is no further match.
3dd93342 1511The position after the last match can be read or set using the C<pos()>
1512function; see L<perlfunc/pos>. A failed match normally resets the
7e86de3e 1513search position to the beginning of the string, but you can avoid that
3dd93342 1514by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
7e86de3e 1515string also resets the search position.
c90c0ff4 1516
e9d89077
DN
1517=item \G assertion
1518
c90c0ff4 1519You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
3dd93342 1520zero-width assertion that matches the exact position where the
1521previous C<m//g>, if any, left off. Without the C</g> modifier, the
1522C<\G> assertion still anchors at C<pos()> as it was at the start of
1523the operation (see L<perlfunc/pos>), but the match is of course only
1524attempted once. Using C<\G> without C</g> on a target string that has
1525not previously had a C</g> match applied to it is the same as using
1526the C<\A> assertion to match the beginning of the string. Note also
1527that, currently, C<\G> is only properly supported when anchored at the
1528very beginning of the pattern.
c90c0ff4 1529
1530Examples:
a0d0e21e
LW
1531
1532 # list context
1533 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
1534
1535 # scalar context
c543c01b
TC
1536 local $/ = "";
1537 while ($paragraph = <>) {
1538 while ($paragraph =~ /\p{Ll}['")]*[.!?]+['")]*\s/g) {
19799a22 1539 $sentences++;
a0d0e21e
LW
1540 }
1541 }
c543c01b
TC
1542 say $sentences;
1543
1544Here's another way to check for sentences in a paragraph:
1545
1546 my $sentence_rx = qr{
1547 (?: (?<= ^ ) | (?<= \s ) ) # after start-of-string or whitespace
1548 \p{Lu} # capital letter
1549 .*? # a bunch of anything
1550 (?<= \S ) # that ends in non-whitespace
1551 (?<! \b [DMS]r ) # but isn't a common abbreviation
1552 (?<! \b Mrs )
1553 (?<! \b Sra )
1554 (?<! \b St )
1555 [.?!] # followed by a sentence ender
1556 (?= $ | \s ) # in front of end-of-string or whitespace
1557 }sx;
1558 local $/ = "";
1559 while (my $paragraph = <>) {
1560 say "NEW PARAGRAPH";
1561 my $count = 0;
1562 while ($paragraph =~ /($sentence_rx)/g) {
1563 printf "\tgot sentence %d: <%s>\n", ++$count, $1;
1564 }
1565 }
1566
1567Here's how to use C<m//gc> with C<\G>:
a0d0e21e 1568
137443ea 1569 $_ = "ppooqppqq";
44a8e56a 1570 while ($i++ < 2) {
1571 print "1: '";
c90c0ff4 1572 print $1 while /(o)/gc; print "', pos=", pos, "\n";
44a8e56a 1573 print "2: '";
c90c0ff4 1574 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
44a8e56a 1575 print "3: '";
c90c0ff4 1576 print $1 while /(p)/gc; print "', pos=", pos, "\n";
44a8e56a 1577 }
5d43e42d 1578 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
44a8e56a 1579
1580The last example should print:
1581
1582 1: 'oo', pos=4
137443ea 1583 2: 'q', pos=5
44a8e56a 1584 3: 'pp', pos=7
1585 1: '', pos=7
137443ea 1586 2: 'q', pos=8
1587 3: '', pos=8
5d43e42d
DC
1588 Final: 'q', pos=8
1589
1590Notice that the final match matched C<q> instead of C<p>, which a match
1591without the C<\G> anchor would have done. Also note that the final match
ac036724 1592did not update C<pos>. C<pos> is only updated on a C</g> match. If the
c543c01b
TC
1593final match did indeed match C<p>, it's a good bet that you're running a
1594very old (pre-5.6.0) version of Perl.
44a8e56a 1595
c90c0ff4 1596A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
e7ea3e70 1597combine several regexps like this to process a string part-by-part,
c90c0ff4 1598doing different actions depending on which regexp matched. Each
1599regexp tries to match where the previous one leaves off.
e7ea3e70 1600
3fe9a6f1 1601 $_ = <<'EOL';
950b09ed 1602 $url = URI::URL->new( "http://example.com/" ); die if $url eq "xXx";
3fe9a6f1 1603 EOL
c543c01b
TC
1604
1605 LOOP: {
950b09ed 1606 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
c543c01b
TC
1607 print(" lowercase"), redo LOOP if /\G\p{Ll}+\b[,.;]?\s*/gc;
1608 print(" UPPERCASE"), redo LOOP if /\G\p{Lu}+\b[,.;]?\s*/gc;
1609 print(" Capitalized"), redo LOOP if /\G\p{Lu}\p{Ll}+\b[,.;]?\s*/gc;
1610 print(" MiXeD"), redo LOOP if /\G\pL+\b[,.;]?\s*/gc;
1611 print(" alphanumeric"), redo LOOP if /\G[\p{Alpha}\pN]+\b[,.;]?\s*/gc;
1612 print(" line-noise"), redo LOOP if /\G\W+/gc;
950b09ed 1613 print ". That's all!\n";
c543c01b 1614 }
e7ea3e70
IZ
1615
1616Here is the output (split into several lines):
1617
c543c01b
TC
1618 line-noise lowercase line-noise UPPERCASE line-noise UPPERCASE
1619 line-noise lowercase line-noise lowercase line-noise lowercase
1620 lowercase line-noise lowercase lowercase line-noise lowercase
1621 lowercase line-noise MiXeD line-noise. That's all!
44a8e56a 1622
c543c01b 1623=item m?PATTERN?msixpodualgc
725a61d7 1624X<?> X<operator, match-once>
87e95b7f 1625
c543c01b 1626=item ?PATTERN?msixpodualgc
55d389e7 1627
725a61d7
Z
1628This is just like the C<m/PATTERN/> search, except that it matches
1629only once between calls to the reset() operator. This is a useful
87e95b7f 1630optimization when you want to see only the first occurrence of
ceb131e8 1631something in each file of a set of files, for instance. Only C<m??>
87e95b7f
YO
1632patterns local to the current package are reset.
1633
1634 while (<>) {
ceb131e8 1635 if (m?^$?) {
87e95b7f
YO
1636 # blank line between header and body
1637 }
1638 } continue {
725a61d7 1639 reset if eof; # clear m?? status for next file
87e95b7f
YO
1640 }
1641
c543c01b
TC
1642Another example switched the first "latin1" encoding it finds
1643to "utf8" in a pod file:
1644
1645 s//utf8/ if m? ^ =encoding \h+ \K latin1 ?x;
1646
1647The match-once behavior is controlled by the match delimiter being
725a61d7
Z
1648C<?>; with any other delimiter this is the normal C<m//> operator.
1649
1650For historical reasons, the leading C<m> in C<m?PATTERN?> is optional,
1651but the resulting C<?PATTERN?> syntax is deprecated, will warn on
c543c01b
TC
1652usage and might be removed from a future stable release of Perl (without
1653further notice!).
87e95b7f 1654
b6fa137b 1655=item s/PATTERN/REPLACEMENT/msixpodualgcer
87e95b7f 1656X<substitute> X<substitution> X<replace> X<regexp, replace>
4f4d7508 1657X<regexp, substitute> X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> X</e> X</r>
87e95b7f
YO
1658
1659Searches a string for a pattern, and if found, replaces that pattern
1660with the replacement text and returns the number of substitutions
1661made. Otherwise it returns false (specifically, the empty string).
1662
c543c01b 1663If the C</r> (non-destructive) option is used then it runs the
679563bb
KW
1664substitution on a copy of the string and instead of returning the
1665number of substitutions, it returns the copy whether or not a
c543c01b
TC
1666substitution occurred. The original string is never changed when
1667C</r> is used. The copy will always be a plain string, even if the
1668input is an object or a tied variable.
4f4d7508 1669
87e95b7f 1670If no string is specified via the C<=~> or C<!~> operator, the C<$_>
c543c01b
TC
1671variable is searched and modified. Unless the C</r> option is used,
1672the string specified must be a scalar variable, an array element, a
1673hash element, or an assignment to one of those; that is, some sort of
1674scalar lvalue.
87e95b7f
YO
1675
1676If the delimiter chosen is a single quote, no interpolation is
1677done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1678PATTERN contains a $ that looks like a variable rather than an
1679end-of-string test, the variable will be interpolated into the pattern
1680at run-time. If you want the pattern compiled only once the first time
1681the variable is interpolated, use the C</o> option. If the pattern
1682evaluates to the empty string, the last successfully executed regular
1683expression is used instead. See L<perlre> for further explanation on these.
87e95b7f
YO
1684
1685Options are as with m// with the addition of the following replacement
1686specific options:
1687
1688 e Evaluate the right side as an expression.
4f4d7508
DC
1689 ee Evaluate the right side as a string then eval the result.
1690 r Return substitution and leave the original string untouched.
87e95b7f 1691
ed02a3bf
DN
1692Any non-whitespace delimiter may replace the slashes. Add space after
1693the C<s> when using a character allowed in identifiers. If single quotes
1694are used, no interpretation is done on the replacement string (the C</e>
1695modifier overrides this, however). Unlike Perl 4, Perl 5 treats backticks
1696as normal delimiters; the replacement text is not evaluated as a command.
1697If the PATTERN is delimited by bracketing quotes, the REPLACEMENT has
1698its own pair of quotes, which may or may not be bracketing quotes, e.g.,
87e95b7f
YO
1699C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
1700replacement portion to be treated as a full-fledged Perl expression
1701and evaluated right then and there. It is, however, syntax checked at
1702compile-time. A second C<e> modifier will cause the replacement portion
1703to be C<eval>ed before being run as a Perl expression.
1704
1705Examples:
1706
1707 s/\bgreen\b/mauve/g; # don't change wintergreen
1708
1709 $path =~ s|/usr/bin|/usr/local/bin|;
1710
1711 s/Login: $foo/Login: $bar/; # run-time pattern
1712
1713 ($foo = $bar) =~ s/this/that/; # copy first, then change
4f4d7508
DC
1714 ($foo = "$bar") =~ s/this/that/; # convert to string, copy, then change
1715 $foo = $bar =~ s/this/that/r; # Same as above using /r
1716 $foo = $bar =~ s/this/that/r
1717 =~ s/that/the other/r; # Chained substitutes using /r
1718 @foo = map { s/this/that/r } @bar # /r is very useful in maps
87e95b7f
YO
1719
1720 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
1721
1722 $_ = 'abc123xyz';
1723 s/\d+/$&*2/e; # yields 'abc246xyz'
1724 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
1725 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1726
1727 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
1728 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
1729 s/^=(\w+)/pod($1)/ge; # use function call
1730
4f4d7508
DC
1731 $_ = 'abc123xyz';
1732 $a = s/abc/def/r; # $a is 'def123xyz' and
1733 # $_ remains 'abc123xyz'.
1734
87e95b7f
YO
1735 # expand variables in $_, but dynamics only, using
1736 # symbolic dereferencing
1737 s/\$(\w+)/${$1}/g;
1738
1739 # Add one to the value of any numbers in the string
1740 s/(\d+)/1 + $1/eg;
1741
c543c01b
TC
1742 # Titlecase words in the last 30 characters only
1743 substr($str, -30) =~ s/\b(\p{Alpha}+)\b/\u\L$1/g;
1744
87e95b7f
YO
1745 # This will expand any embedded scalar variable
1746 # (including lexicals) in $_ : First $1 is interpolated
1747 # to the variable name, and then evaluated
1748 s/(\$\w+)/$1/eeg;
1749
1750 # Delete (most) C comments.
1751 $program =~ s {
1752 /\* # Match the opening delimiter.
1753 .*? # Match a minimal number of characters.
1754 \*/ # Match the closing delimiter.
1755 } []gsx;
1756
1757 s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_, expensively
1758
1759 for ($variable) { # trim whitespace in $variable, cheap
1760 s/^\s+//;
1761 s/\s+$//;
1762 }
1763
1764 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
1765
1766Note the use of $ instead of \ in the last example. Unlike
1767B<sed>, we use the \<I<digit>> form in only the left hand side.
1768Anywhere else it's $<I<digit>>.
1769
1770Occasionally, you can't use just a C</g> to get all the changes
1771to occur that you might want. Here are two common cases:
1772
1773 # put commas in the right places in an integer
1774 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
1775
1776 # expand tabs to 8-column spacing
1777 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
1778
1c424184
FC
1779C<s///le> is treated as a substitution followed by the C<le> operator, not
1780the C</le> flags. This may change in a future version of Perl. It
1781produces a warning if warnings are enabled. To disambiguate, use a space
1782or change the order of the flags:
1783
1784 s/foo/bar/ le 5; # "le" infix operator
1785 s/foo/bar/el; # "e" and "l" flags
1786
87e95b7f
YO
1787=back
1788
1789=head2 Quote-Like Operators
1790X<operator, quote-like>
1791
01c6f5f4
RGS
1792=over 4
1793
a0d0e21e 1794=item q/STRING/
5d44bfff 1795X<q> X<quote, single> X<'> X<''>
a0d0e21e 1796
5d44bfff 1797=item 'STRING'
a0d0e21e 1798
19799a22 1799A single-quoted, literal string. A backslash represents a backslash
68dc0745 1800unless followed by the delimiter or another backslash, in which case
1801the delimiter or backslash is interpolated.
a0d0e21e
LW
1802
1803 $foo = q!I said, "You said, 'She said it.'"!;
1804 $bar = q('This is it.');
68dc0745 1805 $baz = '\n'; # a two-character string
a0d0e21e
LW
1806
1807=item qq/STRING/
d74e8afc 1808X<qq> X<quote, double> X<"> X<"">
a0d0e21e
LW
1809
1810=item "STRING"
1811
1812A double-quoted, interpolated string.
1813
1814 $_ .= qq
1815 (*** The previous line contains the naughty word "$1".\n)
19799a22 1816 if /\b(tcl|java|python)\b/i; # :-)
68dc0745 1817 $baz = "\n"; # a one-character string
a0d0e21e
LW
1818
1819=item qx/STRING/
d74e8afc 1820X<qx> X<`> X<``> X<backtick>
a0d0e21e
LW
1821
1822=item `STRING`
1823
43dd4d21
JH
1824A string which is (possibly) interpolated and then executed as a
1825system command with C</bin/sh> or its equivalent. Shell wildcards,
1826pipes, and redirections will be honored. The collected standard
1827output of the command is returned; standard error is unaffected. In
1828scalar context, it comes back as a single (potentially multi-line)
1829string, or undef if the command failed. In list context, returns a
1830list of lines (however you've defined lines with $/ or
1831$INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
5a964f20
TC
1832
1833Because backticks do not affect standard error, use shell file descriptor
1834syntax (assuming the shell supports this) if you care to address this.
1835To capture a command's STDERR and STDOUT together:
a0d0e21e 1836
5a964f20
TC
1837 $output = `cmd 2>&1`;
1838
1839To capture a command's STDOUT but discard its STDERR:
1840
1841 $output = `cmd 2>/dev/null`;
1842
1843To capture a command's STDERR but discard its STDOUT (ordering is
1844important here):
1845
1846 $output = `cmd 2>&1 1>/dev/null`;
1847
1848To exchange a command's STDOUT and STDERR in order to capture the STDERR
1849but leave its STDOUT to come out the old STDERR:
1850
1851 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
1852
1853To read both a command's STDOUT and its STDERR separately, it's easiest
2359510d
SD
1854to redirect them separately to files, and then read from those files
1855when the program is done:
5a964f20 1856
2359510d 1857 system("program args 1>program.stdout 2>program.stderr");
5a964f20 1858
30398227
SP
1859The STDIN filehandle used by the command is inherited from Perl's STDIN.
1860For example:
1861
c543c01b
TC
1862 open(SPLAT, "stuff") || die "can't open stuff: $!";
1863 open(STDIN, "<&SPLAT") || die "can't dupe SPLAT: $!";
40bbb707 1864 print STDOUT `sort`;
30398227 1865
40bbb707 1866will print the sorted contents of the file named F<"stuff">.
30398227 1867
5a964f20
TC
1868Using single-quote as a delimiter protects the command from Perl's
1869double-quote interpolation, passing it on to the shell instead:
1870
1871 $perl_info = qx(ps $$); # that's Perl's $$
1872 $shell_info = qx'ps $$'; # that's the new shell's $$
1873
19799a22 1874How that string gets evaluated is entirely subject to the command
5a964f20
TC
1875interpreter on your system. On most platforms, you will have to protect
1876shell metacharacters if you want them treated literally. This is in
1877practice difficult to do, as it's unclear how to escape which characters.
1878See L<perlsec> for a clean and safe example of a manual fork() and exec()
1879to emulate backticks safely.
a0d0e21e 1880
bb32b41a
GS
1881On some platforms (notably DOS-like ones), the shell may not be
1882capable of dealing with multiline commands, so putting newlines in
1883the string may not get you what you want. You may be able to evaluate
1884multiple commands in a single line by separating them with the command
1885separator character, if your shell supports that (e.g. C<;> on many Unix
1886shells; C<&> on the Windows NT C<cmd> shell).
1887
0f897271
GS
1888Beginning with v5.6.0, Perl will attempt to flush all files opened for
1889output before starting the child process, but this may not be supported
1890on some platforms (see L<perlport>). To be safe, you may need to set
1891C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1892C<IO::Handle> on any open handles.
1893
bb32b41a
GS
1894Beware that some command shells may place restrictions on the length
1895of the command line. You must ensure your strings don't exceed this
1896limit after any necessary interpolations. See the platform-specific
1897release notes for more details about your particular environment.
1898
5a964f20
TC
1899Using this operator can lead to programs that are difficult to port,
1900because the shell commands called vary between systems, and may in
1901fact not be present at all. As one example, the C<type> command under
1902the POSIX shell is very different from the C<type> command under DOS.
1903That doesn't mean you should go out of your way to avoid backticks
1904when they're the right way to get something done. Perl was made to be
1905a glue language, and one of the things it glues together is commands.
1906Just understand what you're getting yourself into.
bb32b41a 1907
da87341d 1908See L</"I/O Operators"> for more discussion.
a0d0e21e 1909
945c54fd 1910=item qw/STRING/
d74e8afc 1911X<qw> X<quote, list> X<quote, words>
945c54fd
JH
1912
1913Evaluates to a list of the words extracted out of STRING, using embedded
1914whitespace as the word delimiters. It can be understood as being roughly
1915equivalent to:
1916
c543c01b 1917 split(" ", q/STRING/);
945c54fd 1918
efb1e162
CW
1919the differences being that it generates a real list at compile time, and
1920in scalar context it returns the last element in the list. So
945c54fd
JH
1921this expression:
1922
1923 qw(foo bar baz)
1924
1925is semantically equivalent to the list:
1926
c543c01b 1927 "foo", "bar", "baz"
945c54fd
JH
1928
1929Some frequently seen examples:
1930
1931 use POSIX qw( setlocale localeconv )
1932 @EXPORT = qw( foo bar baz );
1933
1934A common mistake is to try to separate the words with comma or to
1935put comments into a multi-line C<qw>-string. For this reason, the
89d205f2 1936C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
945c54fd
JH
1937produces warnings if the STRING contains the "," or the "#" character.
1938
8ff32507 1939=item tr/SEARCHLIST/REPLACEMENTLIST/cdsr
d74e8afc 1940X<tr> X<y> X<transliterate> X</c> X</d> X</s>
a0d0e21e 1941
8ff32507 1942=item y/SEARCHLIST/REPLACEMENTLIST/cdsr
a0d0e21e 1943
2c268ad5 1944Transliterates all occurrences of the characters found in the search list
a0d0e21e
LW
1945with the corresponding character in the replacement list. It returns
1946the number of characters replaced or deleted. If no string is
c543c01b
TC
1947specified via the C<=~> or C<!~> operator, the $_ string is transliterated.
1948
1949If the C</r> (non-destructive) option is present, a new copy of the string
1950is made and its characters transliterated, and this copy is returned no
1951matter whether it was modified or not: the original string is always
1952left unchanged. The new copy is always a plain string, even if the input
1953string is an object or a tied variable.
8ada0baa 1954
c543c01b
TC
1955Unless the C</r> option is used, the string specified with C<=~> must be a
1956scalar variable, an array element, a hash element, or an assignment to one
1957of those; in other words, an lvalue.
8ff32507 1958
89d205f2 1959A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
2c268ad5 1960does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
54310121 1961For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1962SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
c543c01b
TC
1963its own pair of quotes, which may or may not be bracketing quotes;
1964for example, C<tr[aeiouy][yuoiea]> or C<tr(+\-*/)/ABCD/>.
1965
1966Note that C<tr> does B<not> do regular expression character classes such as
1967C<\d> or C<\pL>. The C<tr> operator is not equivalent to the tr(1)
1968utility. If you want to map strings between lower/upper cases, see
1969L<perlfunc/lc> and L<perlfunc/uc>, and in general consider using the C<s>
1970operator if you need regular expressions. The C<\U>, C<\u>, C<\L>, and
1971C<\l> string-interpolation escapes on the right side of a substitution
1972operator will perform correct case-mappings, but C<tr[a-z][A-Z]> will not
1973(except sometimes on legacy 7-bit data).
cc255d5f 1974
8ada0baa
JH
1975Note also that the whole range idea is rather unportable between
1976character sets--and even within character sets they may cause results
1977you probably didn't expect. A sound principle is to use only ranges
1978that begin from and end at either alphabets of equal case (a-e, A-E),
1979or digits (0-4). Anything else is unsafe. If in doubt, spell out the
1980character sets in full.
1981
a0d0e21e
LW
1982Options:
1983
1984 c Complement the SEARCHLIST.
1985 d Delete found but unreplaced characters.
1986 s Squash duplicate replaced characters.
8ff32507
FC
1987 r Return the modified string and leave the original string
1988 untouched.
a0d0e21e 1989
19799a22
GS
1990If the C</c> modifier is specified, the SEARCHLIST character set
1991is complemented. If the C</d> modifier is specified, any characters
1992specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
1993(Note that this is slightly more flexible than the behavior of some
1994B<tr> programs, which delete anything they find in the SEARCHLIST,
1995period.) If the C</s> modifier is specified, sequences of characters
1996that were transliterated to the same character are squashed down
1997to a single instance of the character.
a0d0e21e
LW
1998
1999If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
2000exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
2001than the SEARCHLIST, the final character is replicated till it is long
5a964f20 2002enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
a0d0e21e
LW
2003This latter is useful for counting characters in a class or for
2004squashing character sequences in a class.
2005
2006Examples:
2007
c543c01b 2008 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case ASCII
a0d0e21e
LW
2009
2010 $cnt = tr/*/*/; # count the stars in $_
2011
2012 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
2013
2014 $cnt = tr/0-9//; # count the digits in $_
2015
2016 tr/a-zA-Z//s; # bookkeeper -> bokeper
2017
2018 ($HOST = $host) =~ tr/a-z/A-Z/;
c543c01b 2019 $HOST = $host =~ tr/a-z/A-Z/r; # same thing
8ff32507 2020
c543c01b 2021 $HOST = $host =~ tr/a-z/A-Z/r # chained with s///r
8ff32507 2022 =~ s/:/ -p/r;
a0d0e21e
LW
2023
2024 tr/a-zA-Z/ /cs; # change non-alphas to single space
2025
8ff32507
FC
2026 @stripped = map tr/a-zA-Z/ /csr, @original;
2027 # /r with map
2028
a0d0e21e 2029 tr [\200-\377]
c543c01b 2030 [\000-\177]; # wickedly delete 8th bit
a0d0e21e 2031
19799a22
GS
2032If multiple transliterations are given for a character, only the
2033first one is used:
748a9306
LW
2034
2035 tr/AAA/XYZ/
2036
2c268ad5 2037will transliterate any A to X.
748a9306 2038
19799a22 2039Because the transliteration table is built at compile time, neither
a0d0e21e 2040the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
19799a22
GS
2041interpolation. That means that if you want to use variables, you
2042must use an eval():
a0d0e21e
LW
2043
2044 eval "tr/$oldlist/$newlist/";
2045 die $@ if $@;
2046
2047 eval "tr/$oldlist/$newlist/, 1" or die $@;
2048
7e3b091d 2049=item <<EOF
d74e8afc 2050X<here-doc> X<heredoc> X<here-document> X<<< << >>>
7e3b091d
DA
2051
2052A line-oriented form of quoting is based on the shell "here-document"
2053syntax. Following a C<< << >> you specify a string to terminate
2054the quoted material, and all lines following the current line down to
89d205f2
YO
2055the terminating string are the value of the item.
2056
2057The terminating string may be either an identifier (a word), or some
2058quoted text. An unquoted identifier works like double quotes.
2059There may not be a space between the C<< << >> and the identifier,
2060unless the identifier is explicitly quoted. (If you put a space it
2061will be treated as a null identifier, which is valid, and matches the
2062first empty line.) The terminating string must appear by itself
2063(unquoted and with no surrounding whitespace) on the terminating line.
2064
2065If the terminating string is quoted, the type of quotes used determine
2066the treatment of the text.
2067
2068=over 4
2069
2070=item Double Quotes
2071
2072Double quotes indicate that the text will be interpolated using exactly
2073the same rules as normal double quoted strings.
7e3b091d
DA
2074
2075 print <<EOF;
2076 The price is $Price.
2077 EOF
2078
2079 print << "EOF"; # same as above
2080 The price is $Price.
2081 EOF
2082
89d205f2
YO
2083
2084=item Single Quotes
2085
2086Single quotes indicate the text is to be treated literally with no
2087interpolation of its content. This is similar to single quoted
2088strings except that backslashes have no special meaning, with C<\\>
2089being treated as two backslashes and not one as they would in every
2090other quoting construct.
2091
c543c01b
TC
2092Just as in the shell, a backslashed bareword following the C<<< << >>>
2093means the same thing as a single-quoted string does:
2094
2095 $cost = <<'VISTA'; # hasta la ...
2096 That'll be $10 please, ma'am.
2097 VISTA
2098
2099 $cost = <<\VISTA; # Same thing!
2100 That'll be $10 please, ma'am.
2101 VISTA
2102
89d205f2
YO
2103This is the only form of quoting in perl where there is no need
2104to worry about escaping content, something that code generators
2105can and do make good use of.
2106
2107=item Backticks
2108
2109The content of the here doc is treated just as it would be if the
2110string were embedded in backticks. Thus the content is interpolated
2111as though it were double quoted and then executed via the shell, with
2112the results of the execution returned.
2113
2114 print << `EOC`; # execute command and get results
7e3b091d 2115 echo hi there
7e3b091d
DA
2116 EOC
2117
89d205f2
YO
2118=back
2119
2120It is possible to stack multiple here-docs in a row:
2121
7e3b091d
DA
2122 print <<"foo", <<"bar"; # you can stack them
2123 I said foo.
2124 foo
2125 I said bar.
2126 bar
2127
2128 myfunc(<< "THIS", 23, <<'THAT');
2129 Here's a line
2130 or two.
2131 THIS
2132 and here's another.
2133 THAT
2134
2135Just don't forget that you have to put a semicolon on the end
2136to finish the statement, as Perl doesn't know you're not going to
2137try to do this:
2138
2139 print <<ABC
2140 179231
2141 ABC
2142 + 20;
2143
872d7e53
TS
2144If you want to remove the line terminator from your here-docs,
2145use C<chomp()>.
2146
2147 chomp($string = <<'END');
2148 This is a string.
2149 END
2150
2151If you want your here-docs to be indented with the rest of the code,
2152you'll need to remove leading whitespace from each line manually:
7e3b091d
DA
2153
2154 ($quote = <<'FINIS') =~ s/^\s+//gm;
89d205f2 2155 The Road goes ever on and on,
7e3b091d
DA
2156 down from the door where it began.
2157 FINIS
2158
2159If you use a here-doc within a delimited construct, such as in C<s///eg>,
2160the quoted material must come on the lines following the final delimiter.
2161So instead of
2162
2163 s/this/<<E . 'that'
2164 the other
2165 E
2166 . 'more '/eg;
2167
2168you have to write
2169
89d205f2
YO
2170 s/this/<<E . 'that'
2171 . 'more '/eg;
2172 the other
2173 E
7e3b091d
DA
2174
2175If the terminating identifier is on the last line of the program, you
2176must be sure there is a newline after it; otherwise, Perl will give the
2177warning B<Can't find string terminator "END" anywhere before EOF...>.
2178
c543c01b
TC
2179Additionally, quoting rules for the end-of-string identifier are
2180unrelated to Perl's quoting rules. C<q()>, C<qq()>, and the like are not
89d205f2
YO
2181supported in place of C<''> and C<"">, and the only interpolation is for
2182backslashing the quoting character:
7e3b091d
DA
2183
2184 print << "abc\"def";
2185 testing...
2186 abc"def
2187
2188Finally, quoted strings cannot span multiple lines. The general rule is
2189that the identifier must be a string literal. Stick with that, and you
2190should be safe.
2191
a0d0e21e
LW
2192=back
2193
75e14d17 2194=head2 Gory details of parsing quoted constructs
d74e8afc 2195X<quote, gory details>
75e14d17 2196
19799a22
GS
2197When presented with something that might have several different
2198interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
2199principle to pick the most probable interpretation. This strategy
2200is so successful that Perl programmers often do not suspect the
2201ambivalence of what they write. But from time to time, Perl's
2202notions differ substantially from what the author honestly meant.
2203
2204This section hopes to clarify how Perl handles quoted constructs.
2205Although the most common reason to learn this is to unravel labyrinthine
2206regular expressions, because the initial steps of parsing are the
2207same for all quoting operators, they are all discussed together.
2208
2209The most important Perl parsing rule is the first one discussed
2210below: when processing a quoted construct, Perl first finds the end
2211of that construct, then interprets its contents. If you understand
2212this rule, you may skip the rest of this section on the first
2213reading. The other rules are likely to contradict the user's
2214expectations much less frequently than this first one.
2215
2216Some passes discussed below are performed concurrently, but because
2217their results are the same, we consider them individually. For different
2218quoting constructs, Perl performs different numbers of passes, from
6deea57f 2219one to four, but these passes are always performed in the same order.
75e14d17 2220
13a2d996 2221=over 4
75e14d17
IZ
2222
2223=item Finding the end
2224
6deea57f
TS
2225The first pass is finding the end of the quoted construct, where
2226the information about the delimiters is used in parsing.
2227During this search, text between the starting and ending delimiters
2228is copied to a safe location. The text copied gets delimiter-independent.
2229
2230If the construct is a here-doc, the ending delimiter is a line
2231that has a terminating string as the content. Therefore C<<<EOF> is
2232terminated by C<EOF> immediately followed by C<"\n"> and starting
2233from the first column of the terminating line.
2234When searching for the terminating line of a here-doc, nothing
2235is skipped. In other words, lines after the here-doc syntax
2236are compared with the terminating string line by line.
2237
2238For the constructs except here-docs, single characters are used as starting
2239and ending delimiters. If the starting delimiter is an opening punctuation
2240(that is C<(>, C<[>, C<{>, or C<< < >>), the ending delimiter is the
2241corresponding closing punctuation (that is C<)>, C<]>, C<}>, or C<< > >>).
2242If the starting delimiter is an unpaired character like C</> or a closing
2243punctuation, the ending delimiter is same as the starting delimiter.
2244Therefore a C</> terminates a C<qq//> construct, while a C<]> terminates
2245C<qq[]> and C<qq]]> constructs.
2246
2247When searching for single-character delimiters, escaped delimiters
2248and C<\\> are skipped. For example, while searching for terminating C</>,
2249combinations of C<\\> and C<\/> are skipped. If the delimiters are
2250bracketing, nested pairs are also skipped. For example, while searching
2251for closing C<]> paired with the opening C<[>, combinations of C<\\>, C<\]>,
2252and C<\[> are all skipped, and nested C<[> and C<]> are skipped as well.
2253However, when backslashes are used as the delimiters (like C<qq\\> and
2254C<tr\\\>), nothing is skipped.
2255During the search for the end, backslashes that escape delimiters
2256are removed (exactly speaking, they are not copied to the safe location).
75e14d17 2257
19799a22
GS
2258For constructs with three-part delimiters (C<s///>, C<y///>, and
2259C<tr///>), the search is repeated once more.
6deea57f
TS
2260If the first delimiter is not an opening punctuation, three delimiters must
2261be same such as C<s!!!> and C<tr)))>, in which case the second delimiter
2262terminates the left part and starts the right part at once.
b6538e4f 2263If the left part is delimited by bracketing punctuation (that is C<()>,
6deea57f 2264C<[]>, C<{}>, or C<< <> >>), the right part needs another pair of
b6538e4f 2265delimiters such as C<s(){}> and C<tr[]//>. In these cases, whitespace
6deea57f 2266and comments are allowed between both parts, though the comment must follow
b6538e4f
TC
2267at least one whitespace character; otherwise a character expected as the
2268start of the comment may be regarded as the starting delimiter of the right part.
75e14d17 2269
19799a22
GS
2270During this search no attention is paid to the semantics of the construct.
2271Thus:
75e14d17
IZ
2272
2273 "$hash{"$foo/$bar"}"
2274
2a94b7ce 2275or:
75e14d17 2276
89d205f2 2277 m/
2a94b7ce 2278 bar # NOT a comment, this slash / terminated m//!
75e14d17
IZ
2279 /x
2280
19799a22
GS
2281do not form legal quoted expressions. The quoted part ends on the
2282first C<"> and C</>, and the rest happens to be a syntax error.
2283Because the slash that terminated C<m//> was followed by a C<SPACE>,
2284the example above is not C<m//x>, but rather C<m//> with no C</x>
2285modifier. So the embedded C<#> is interpreted as a literal C<#>.
75e14d17 2286
89d205f2
YO
2287Also no attention is paid to C<\c\> (multichar control char syntax) during
2288this search. Thus the second C<\> in C<qq/\c\/> is interpreted as a part
2289of C<\/>, and the following C</> is not recognized as a delimiter.
0d594e51
TS
2290Instead, use C<\034> or C<\x1c> at the end of quoted constructs.
2291
75e14d17 2292=item Interpolation
d74e8afc 2293X<interpolation>
75e14d17 2294
19799a22 2295The next step is interpolation in the text obtained, which is now
89d205f2 2296delimiter-independent. There are multiple cases.
75e14d17 2297
13a2d996 2298=over 4
75e14d17 2299
89d205f2 2300=item C<<<'EOF'>
75e14d17
IZ
2301
2302No interpolation is performed.
6deea57f
TS
2303Note that the combination C<\\> is left intact, since escaped delimiters
2304are not available for here-docs.
75e14d17 2305
6deea57f 2306=item C<m''>, the pattern of C<s'''>
89d205f2 2307
6deea57f
TS
2308No interpolation is performed at this stage.
2309Any backslashed sequences including C<\\> are treated at the stage
2310to L</"parsing regular expressions">.
89d205f2 2311
6deea57f 2312=item C<''>, C<q//>, C<tr'''>, C<y'''>, the replacement of C<s'''>
75e14d17 2313
89d205f2 2314The only interpolation is removal of C<\> from pairs of C<\\>.
6deea57f
TS
2315Therefore C<-> in C<tr'''> and C<y'''> is treated literally
2316as a hyphen and no character range is available.
2317C<\1> in the replacement of C<s'''> does not work as C<$1>.
89d205f2
YO
2318
2319=item C<tr///>, C<y///>
2320
6deea57f
TS
2321No variable interpolation occurs. String modifying combinations for
2322case and quoting such as C<\Q>, C<\U>, and C<\E> are not recognized.
2323The other escape sequences such as C<\200> and C<\t> and backslashed
2324characters such as C<\\> and C<\-> are converted to appropriate literals.
89d205f2
YO
2325The character C<-> is treated specially and therefore C<\-> is treated
2326as a literal C<->.
75e14d17 2327
89d205f2 2328=item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF">
75e14d17 2329
19799a22
GS
2330C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
2331converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
2332is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
6deea57f
TS
2333The other escape sequences such as C<\200> and C<\t> and backslashed
2334characters such as C<\\> and C<\-> are replaced with appropriate
2335expansions.
2a94b7ce 2336
19799a22
GS
2337Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
2338is interpolated in the usual way. Something like C<"\Q\\E"> has
2339no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
2340result is the same as for C<"\\\\E">. As a general rule, backslashes
2341between C<\Q> and C<\E> may lead to counterintuitive results. So,
2342C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
2343as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
2a94b7ce
IZ
2344
2345 $str = '\t';
2346 return "\Q$str";
2347
2348may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
2349
19799a22 2350Interpolated scalars and arrays are converted internally to the C<join> and
92d29cee 2351C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
75e14d17 2352
19799a22 2353 $foo . " XXX '" . (join $", @arr) . "'";
75e14d17 2354
19799a22 2355All operations above are performed simultaneously, left to right.
75e14d17 2356
19799a22
GS
2357Because the result of C<"\Q STRING \E"> has all metacharacters
2358quoted, there is no way to insert a literal C<$> or C<@> inside a
2359C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
2360C<"\\\$">; if not, it is interpreted as the start of an interpolated
2361scalar.
75e14d17 2362
19799a22 2363Note also that the interpolation code needs to make a decision on
89d205f2 2364where the interpolated scalar ends. For instance, whether
35f2feb0 2365C<< "a $b -> {c}" >> really means:
75e14d17
IZ
2366
2367 "a " . $b . " -> {c}";
2368
2a94b7ce 2369or:
75e14d17
IZ
2370
2371 "a " . $b -> {c};
2372
19799a22
GS
2373Most of the time, the longest possible text that does not include
2374spaces between components and which contains matching braces or
2375brackets. because the outcome may be determined by voting based
2376on heuristic estimators, the result is not strictly predictable.
2377Fortunately, it's usually correct for ambiguous cases.
75e14d17 2378
6deea57f 2379=item the replacement of C<s///>
75e14d17 2380
19799a22 2381Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
6deea57f
TS
2382happens as with C<qq//> constructs.
2383
2384It is at this step that C<\1> is begrudgingly converted to C<$1> in
2385the replacement text of C<s///>, in order to correct the incorrigible
2386I<sed> hackers who haven't picked up the saner idiom yet. A warning
2387is emitted if the C<use warnings> pragma or the B<-w> command-line flag
2388(that is, the C<$^W> variable) was set.
2389
2390=item C<RE> in C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
2391
cc74c5bd
TS
2392Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\E>,
2393and interpolation happens (almost) as with C<qq//> constructs.
2394
5d03b57c
KW
2395Processing of C<\N{...}> is also done here, and compiled into an intermediate
2396form for the regex compiler. (This is because, as mentioned below, the regex
2397compilation may be done at execution time, and C<\N{...}> is a compile-time
2398construct.)
2399
cc74c5bd
TS
2400However any other combinations of C<\> followed by a character
2401are not substituted but only skipped, in order to parse them
2402as regular expressions at the following step.
6deea57f 2403As C<\c> is skipped at this step, C<@> of C<\c@> in RE is possibly
1749ea0d 2404treated as an array symbol (for example C<@foo>),
6deea57f 2405even though the same text in C<qq//> gives interpolation of C<\c@>.
6deea57f
TS
2406
2407Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
19799a22
GS
2408a C<#>-comment in a C<//x>-regular expression, no processing is
2409performed whatsoever. This is the first step at which the presence
2410of the C<//x> modifier is relevant.
2411
1749ea0d
TS
2412Interpolation in patterns has several quirks: C<$|>, C<$(>, C<$)>, C<@+>
2413and C<@-> are not interpolated, and constructs C<$var[SOMETHING]> are
2414voted (by several different estimators) to be either an array element
2415or C<$var> followed by an RE alternative. This is where the notation
19799a22
GS
2416C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
2417array element C<-9>, not as a regular expression from the variable
2418C<$arr> followed by a digit, which would be the interpretation of
2419C</$arr[0-9]/>. Since voting among different estimators may occur,
2420the result is not predictable.
2421
19799a22
GS
2422The lack of processing of C<\\> creates specific restrictions on
2423the post-processed text. If the delimiter is C</>, one cannot get
2424the combination C<\/> into the result of this step. C</> will
2425finish the regular expression, C<\/> will be stripped to C</> on
2426the previous step, and C<\\/> will be left as is. Because C</> is
2427equivalent to C<\/> inside a regular expression, this does not
2428matter unless the delimiter happens to be character special to the
2429RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
2430alphanumeric char, as in:
2a94b7ce
IZ
2431
2432 m m ^ a \s* b mmx;
2433
19799a22 2434In the RE above, which is intentionally obfuscated for illustration, the
6deea57f 2435delimiter is C<m>, the modifier is C<mx>, and after delimiter-removal the
89d205f2 2436RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
19799a22
GS
2437reason you're encouraged to restrict your delimiters to non-alphanumeric,
2438non-whitespace choices.
75e14d17
IZ
2439
2440=back
2441
19799a22 2442This step is the last one for all constructs except regular expressions,
75e14d17
IZ
2443which are processed further.
2444
6deea57f
TS
2445=item parsing regular expressions
2446X<regexp, parse>
75e14d17 2447
19799a22 2448Previous steps were performed during the compilation of Perl code,
ac036724 2449but this one happens at run time, although it may be optimized to
19799a22 2450be calculated at compile time if appropriate. After preprocessing
6deea57f 2451described above, and possibly after evaluation if concatenation,
19799a22
GS
2452joining, casing translation, or metaquoting are involved, the
2453resulting I<string> is passed to the RE engine for compilation.
2454
2455Whatever happens in the RE engine might be better discussed in L<perlre>,
2456but for the sake of continuity, we shall do so here.
2457
2458This is another step where the presence of the C<//x> modifier is
2459relevant. The RE engine scans the string from left to right and
2460converts it to a finite automaton.
2461
2462Backslashed characters are either replaced with corresponding
2463literal strings (as with C<\{>), or else they generate special nodes
2464in the finite automaton (as with C<\b>). Characters special to the
2465RE engine (such as C<|>) generate corresponding nodes or groups of
2466nodes. C<(?#...)> comments are ignored. All the rest is either
2467converted to literal strings to match, or else is ignored (as is
2468whitespace and C<#>-style comments if C<//x> is present).
2469
2470Parsing of the bracketed character class construct, C<[...]>, is
2471rather different than the rule used for the rest of the pattern.
2472The terminator of this construct is found using the same rules as
2473for finding the terminator of a C<{}>-delimited construct, the only
2474exception being that C<]> immediately following C<[> is treated as
2475though preceded by a backslash. Similarly, the terminator of
2476C<(?{...})> is found using the same rules as for finding the
2477terminator of a C<{}>-delimited construct.
2478
2479It is possible to inspect both the string given to RE engine and the
2480resulting finite automaton. See the arguments C<debug>/C<debugcolor>
2481in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
4a4eefd0 2482switch documented in L<perlrun/"Command Switches">.
75e14d17
IZ
2483
2484=item Optimization of regular expressions
d74e8afc 2485X<regexp, optimization>
75e14d17 2486
7522fed5 2487This step is listed for completeness only. Since it does not change
75e14d17 2488semantics, details of this step are not documented and are subject
19799a22
GS
2489to change without notice. This step is performed over the finite
2490automaton that was generated during the previous pass.
2a94b7ce 2491
19799a22
GS
2492It is at this stage that C<split()> silently optimizes C</^/> to
2493mean C</^/m>.
75e14d17
IZ
2494
2495=back
2496
a0d0e21e 2497=head2 I/O Operators
d74e8afc
ITB
2498X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle>
2499X<< <> >> X<@ARGV>
a0d0e21e 2500
54310121 2501There are several I/O operators you should know about.
fbad3eb5 2502
7b8d334a 2503A string enclosed by backticks (grave accents) first undergoes
19799a22
GS
2504double-quote interpolation. It is then interpreted as an external
2505command, and the output of that command is the value of the
e9c56f9b
JH
2506backtick string, like in a shell. In scalar context, a single string
2507consisting of all output is returned. In list context, a list of
2508values is returned, one per line of output. (You can set C<$/> to use
2509a different line terminator.) The command is executed each time the
2510pseudo-literal is evaluated. The status value of the command is
2511returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
2512Unlike in B<csh>, no translation is done on the return data--newlines
2513remain newlines. Unlike in any of the shells, single quotes do not
2514hide variable names in the command from interpretation. To pass a
2515literal dollar-sign through to the shell you need to hide it with a
2516backslash. The generalized form of backticks is C<qx//>. (Because
2517backticks always undergo shell expansion as well, see L<perlsec> for
2518security concerns.)
d74e8afc 2519X<qx> X<`> X<``> X<backtick> X<glob>
19799a22
GS
2520
2521In scalar context, evaluating a filehandle in angle brackets yields
2522the next line from that file (the newline, if any, included), or
2523C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
2524(sometimes known as file-slurp mode) and the file is empty, it
2525returns C<''> the first time, followed by C<undef> subsequently.
2526
2527Ordinarily you must assign the returned value to a variable, but
2528there is one situation where an automatic assignment happens. If
2529and only if the input symbol is the only thing inside the conditional
2530of a C<while> statement (even if disguised as a C<for(;;)> loop),
2531the value is automatically assigned to the global variable $_,
2532destroying whatever was there previously. (This may seem like an
2533odd thing to you, but you'll use the construct in almost every Perl
17b829fa 2534script you write.) The $_ variable is not implicitly localized.
19799a22
GS
2535You'll have to put a C<local $_;> before the loop if you want that
2536to happen.
2537
2538The following lines are equivalent:
a0d0e21e 2539
748a9306 2540 while (defined($_ = <STDIN>)) { print; }
7b8d334a 2541 while ($_ = <STDIN>) { print; }
a0d0e21e
LW
2542 while (<STDIN>) { print; }
2543 for (;<STDIN>;) { print; }
748a9306 2544 print while defined($_ = <STDIN>);
7b8d334a 2545 print while ($_ = <STDIN>);
a0d0e21e
LW
2546 print while <STDIN>;
2547
19799a22 2548This also behaves similarly, but avoids $_ :
7b8d334a 2549
89d205f2 2550 while (my $line = <STDIN>) { print $line }
7b8d334a 2551
19799a22
GS
2552In these loop constructs, the assigned value (whether assignment
2553is automatic or explicit) is then tested to see whether it is
2554defined. The defined test avoids problems where line has a string
2555value that would be treated as false by Perl, for example a "" or
2556a "0" with no trailing newline. If you really mean for such values
2557to terminate the loop, they should be tested for explicitly:
7b8d334a
GS
2558
2559 while (($_ = <STDIN>) ne '0') { ... }
2560 while (<STDIN>) { last unless $_; ... }
2561
5ef4d93e 2562In other boolean contexts, C<< <filehandle> >> without an
2563explicit C<defined> test or comparison elicits a warning if the
9f1b1f2d 2564C<use warnings> pragma or the B<-w>
19799a22 2565command-line switch (the C<$^W> variable) is in effect.
7b8d334a 2566
5f05dabc 2567The filehandles STDIN, STDOUT, and STDERR are predefined. (The
19799a22
GS
2568filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
2569in packages, where they would be interpreted as local identifiers
2570rather than global.) Additional filehandles may be created with
2571the open() function, amongst others. See L<perlopentut> and
2572L<perlfunc/open> for details on this.
d74e8afc 2573X<stdin> X<stdout> X<sterr>
a0d0e21e 2574
35f2feb0 2575If a <FILEHANDLE> is used in a context that is looking for
19799a22
GS
2576a list, a list comprising all input lines is returned, one line per
2577list element. It's easy to grow to a rather large data space this
2578way, so use with care.
a0d0e21e 2579
35f2feb0 2580<FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
19799a22 2581See L<perlfunc/readline>.
fbad3eb5 2582
35f2feb0
GS
2583The null filehandle <> is special: it can be used to emulate the
2584behavior of B<sed> and B<awk>. Input from <> comes either from
a0d0e21e 2585standard input, or from each file listed on the command line. Here's
35f2feb0 2586how it works: the first time <> is evaluated, the @ARGV array is
5a964f20 2587checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
a0d0e21e
LW
2588gives you standard input. The @ARGV array is then processed as a list
2589of filenames. The loop
2590
2591 while (<>) {
2592 ... # code for each line
2593 }
2594
2595is equivalent to the following Perl-like pseudo code:
2596
3e3baf6d 2597 unshift(@ARGV, '-') unless @ARGV;
a0d0e21e
LW
2598 while ($ARGV = shift) {
2599 open(ARGV, $ARGV);
2600 while (<ARGV>) {
2601 ... # code for each line
2602 }
2603 }
2604
19799a22
GS
2605except that it isn't so cumbersome to say, and will actually work.
2606It really does shift the @ARGV array and put the current filename
2607into the $ARGV variable. It also uses filehandle I<ARGV>
ac036724 2608internally. <> is just a synonym for <ARGV>, which
19799a22 2609is magical. (The pseudo code above doesn't work because it treats
35f2feb0 2610<ARGV> as non-magical.)
a0d0e21e 2611
48ab5743
ML
2612Since the null filehandle uses the two argument form of L<perlfunc/open>
2613it interprets special characters, so if you have a script like this:
2614
2615 while (<>) {
2616 print;
2617 }
2618
2619and call it with C<perl dangerous.pl 'rm -rfv *|'>, it actually opens a
2620pipe, executes the C<rm> command and reads C<rm>'s output from that pipe.
2621If you want all items in C<@ARGV> to be interpreted as file names, you
2622can use the module C<ARGV::readonly> from CPAN.
2623
35f2feb0 2624You can modify @ARGV before the first <> as long as the array ends up
a0d0e21e 2625containing the list of filenames you really want. Line numbers (C<$.>)
19799a22
GS
2626continue as though the input were one big happy file. See the example
2627in L<perlfunc/eof> for how to reset line numbers on each file.
5a964f20 2628
89d205f2 2629If you want to set @ARGV to your own list of files, go right ahead.
5a964f20
TC
2630This sets @ARGV to all plain text files if no @ARGV was given:
2631
2632 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
a0d0e21e 2633
5a964f20
TC
2634You can even set them to pipe commands. For example, this automatically
2635filters compressed arguments through B<gzip>:
2636
2637 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
2638
2639If you want to pass switches into your script, you can use one of the
a0d0e21e
LW
2640Getopts modules or put a loop on the front like this:
2641
2642 while ($_ = $ARGV[0], /^-/) {
2643 shift;
2644 last if /^--$/;
2645 if (/^-D(.*)/) { $debug = $1 }
2646 if (/^-v/) { $verbose++ }
5a964f20 2647 # ... # other switches
a0d0e21e 2648 }
5a964f20 2649
a0d0e21e 2650 while (<>) {
5a964f20 2651 # ... # code for each line
a0d0e21e
LW
2652 }
2653
89d205f2
YO
2654The <> symbol will return C<undef> for end-of-file only once.
2655If you call it again after this, it will assume you are processing another
19799a22 2656@ARGV list, and if you haven't set @ARGV, will read input from STDIN.
a0d0e21e 2657
b159ebd3 2658If what the angle brackets contain is a simple scalar variable (e.g.,
35f2feb0 2659<$foo>), then that variable contains the name of the
19799a22
GS
2660filehandle to input from, or its typeglob, or a reference to the
2661same. For example:
cb1a09d0
AD
2662
2663 $fh = \*STDIN;
2664 $line = <$fh>;
a0d0e21e 2665
5a964f20
TC
2666If what's within the angle brackets is neither a filehandle nor a simple
2667scalar variable containing a filehandle name, typeglob, or typeglob
2668reference, it is interpreted as a filename pattern to be globbed, and
2669either a list of filenames or the next filename in the list is returned,
19799a22 2670depending on context. This distinction is determined on syntactic
35f2feb0
GS
2671grounds alone. That means C<< <$x> >> is always a readline() from
2672an indirect handle, but C<< <$hash{key}> >> is always a glob().
5a964f20 2673That's because $x is a simple scalar variable, but C<$hash{key}> is
ef191992
YST
2674not--it's a hash element. Even C<< <$x > >> (note the extra space)
2675is treated as C<glob("$x ")>, not C<readline($x)>.
5a964f20
TC
2676
2677One level of double-quote interpretation is done first, but you can't
35f2feb0 2678say C<< <$foo> >> because that's an indirect filehandle as explained
5a964f20
TC
2679in the previous paragraph. (In older versions of Perl, programmers
2680would insert curly brackets to force interpretation as a filename glob:
35f2feb0 2681C<< <${foo}> >>. These days, it's considered cleaner to call the
5a964f20 2682internal function directly as C<glob($foo)>, which is probably the right
19799a22 2683way to have done it in the first place.) For example:
a0d0e21e
LW
2684
2685 while (<*.c>) {
2686 chmod 0644, $_;
2687 }
2688
3a4b19e4 2689is roughly equivalent to:
a0d0e21e
LW
2690
2691 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
2692 while (<FOO>) {
5b3eff12 2693 chomp;
a0d0e21e
LW
2694 chmod 0644, $_;
2695 }
2696
3a4b19e4
GS
2697except that the globbing is actually done internally using the standard
2698C<File::Glob> extension. Of course, the shortest way to do the above is:
a0d0e21e
LW
2699
2700 chmod 0644, <*.c>;
2701
19799a22
GS
2702A (file)glob evaluates its (embedded) argument only when it is
2703starting a new list. All values must be read before it will start
2704over. In list context, this isn't important because you automatically
2705get them all anyway. However, in scalar context the operator returns
069e01df 2706the next value each time it's called, or C<undef> when the list has
19799a22
GS
2707run out. As with filehandle reads, an automatic C<defined> is
2708generated when the glob occurs in the test part of a C<while>,
2709because legal glob returns (e.g. a file called F<0>) would otherwise
2710terminate the loop. Again, C<undef> is returned only once. So if
2711you're expecting a single value from a glob, it is much better to
2712say
4633a7c4
LW
2713
2714 ($file) = <blurch*>;
2715
2716than
2717
2718 $file = <blurch*>;
2719
2720because the latter will alternate between returning a filename and
19799a22 2721returning false.
4633a7c4 2722
b159ebd3 2723If you're trying to do variable interpolation, it's definitely better
4633a7c4 2724to use the glob() function, because the older notation can cause people
e37d713d 2725to become confused with the indirect filehandle notation.
4633a7c4
LW
2726
2727 @files = glob("$dir/*.[ch]");
2728 @files = glob($files[$i]);
2729
a0d0e21e 2730=head2 Constant Folding
d74e8afc 2731X<constant folding> X<folding>
a0d0e21e
LW
2732
2733Like C, Perl does a certain amount of expression evaluation at
19799a22 2734compile time whenever it determines that all arguments to an
a0d0e21e
LW
2735operator are static and have no side effects. In particular, string
2736concatenation happens at compile time between literals that don't do
19799a22 2737variable substitution. Backslash interpolation also happens at
a0d0e21e
LW
2738compile time. You can say
2739
2740 'Now is the time for all' . "\n" .
2741 'good men to come to.'
2742
54310121 2743and this all reduces to one string internally. Likewise, if
a0d0e21e
LW
2744you say
2745
2746 foreach $file (@filenames) {
5a964f20 2747 if (-s $file > 5 + 100 * 2**16) { }
54310121 2748 }
a0d0e21e 2749
19799a22
GS
2750the compiler will precompute the number which that expression
2751represents so that the interpreter won't have to.
a0d0e21e 2752
fd1abbef 2753=head2 No-ops
d74e8afc 2754X<no-op> X<nop>
fd1abbef
DN
2755
2756Perl doesn't officially have a no-op operator, but the bare constants
2757C<0> and C<1> are special-cased to not produce a warning in a void
2758context, so you can for example safely do
2759
2760 1 while foo();
2761
2c268ad5 2762=head2 Bitwise String Operators
d74e8afc 2763X<operator, bitwise, string>
2c268ad5
TP
2764
2765Bitstrings of any size may be manipulated by the bitwise operators
2766(C<~ | & ^>).
2767
19799a22
GS
2768If the operands to a binary bitwise op are strings of different
2769sizes, B<|> and B<^> ops act as though the shorter operand had
2770additional zero bits on the right, while the B<&> op acts as though
2771the longer operand were truncated to the length of the shorter.
2772The granularity for such extension or truncation is one or more
2773bytes.
2c268ad5 2774
89d205f2 2775 # ASCII-based examples
2c268ad5
TP
2776 print "j p \n" ^ " a h"; # prints "JAPH\n"
2777 print "JA" | " ph\n"; # prints "japh\n"
2778 print "japh\nJunk" & '_____'; # prints "JAPH\n";
2779 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
2780
19799a22 2781If you are intending to manipulate bitstrings, be certain that
2c268ad5 2782you're supplying bitstrings: If an operand is a number, that will imply
19799a22 2783a B<numeric> bitwise operation. You may explicitly show which type of
2c268ad5
TP
2784operation you intend by using C<""> or C<0+>, as in the examples below.
2785
4358a253
SS
2786 $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
2787 $foo = '150' | 105; # yields 255
2c268ad5
TP
2788 $foo = 150 | '105'; # yields 255
2789 $foo = '150' | '105'; # yields string '155' (under ASCII)
2790
2791 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
2792 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
a0d0e21e 2793
1ae175c8
GS
2794See L<perlfunc/vec> for information on how to manipulate individual bits
2795in a bit vector.
2796
55497cff 2797=head2 Integer Arithmetic
d74e8afc 2798X<integer>
a0d0e21e 2799
19799a22 2800By default, Perl assumes that it must do most of its arithmetic in
a0d0e21e
LW
2801floating point. But by saying
2802
2803 use integer;
2804
3eab78e3
CW
2805you may tell the compiler to use integer operations
2806(see L<integer> for a detailed explanation) from here to the end of
2807the enclosing BLOCK. An inner BLOCK may countermand this by saying
a0d0e21e
LW
2808
2809 no integer;
2810
19799a22 2811which lasts until the end of that BLOCK. Note that this doesn't
3eab78e3
CW
2812mean everything is an integer, merely that Perl will use integer
2813operations for arithmetic, comparison, and bitwise operators. For
2814example, even under C<use integer>, if you take the C<sqrt(2)>, you'll
2815still get C<1.4142135623731> or so.
19799a22
GS
2816
2817Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
89d205f2 2818and ">>") always produce integral results. (But see also
13a2d996 2819L<Bitwise String Operators>.) However, C<use integer> still has meaning for
19799a22
GS
2820them. By default, their results are interpreted as unsigned integers, but
2821if C<use integer> is in effect, their results are interpreted
2822as signed integers. For example, C<~0> usually evaluates to a large
0be96356 2823integral value. However, C<use integer; ~0> is C<-1> on two's-complement
19799a22 2824machines.
68dc0745 2825
2826=head2 Floating-point Arithmetic
06ce2fa3 2827
d74e8afc 2828X<floating-point> X<floating point> X<float> X<real>
68dc0745 2829
2830While C<use integer> provides integer-only arithmetic, there is no
19799a22
GS
2831analogous mechanism to provide automatic rounding or truncation to a
2832certain number of decimal places. For rounding to a certain number
2833of digits, sprintf() or printf() is usually the easiest route.
2834See L<perlfaq4>.
68dc0745 2835
5a964f20
TC
2836Floating-point numbers are only approximations to what a mathematician
2837would call real numbers. There are infinitely more reals than floats,
2838so some corners must be cut. For example:
2839
2840 printf "%.20g\n", 123456789123456789;
2841 # produces 123456789123456784
2842
8548cb57
RGS
2843Testing for exact floating-point equality or inequality is not a
2844good idea. Here's a (relatively expensive) work-around to compare
5a964f20
TC
2845whether two floating-point numbers are equal to a particular number of
2846decimal places. See Knuth, volume II, for a more robust treatment of
2847this topic.
2848
2849 sub fp_equal {
2850 my ($X, $Y, $POINTS) = @_;
2851 my ($tX, $tY);
2852 $tX = sprintf("%.${POINTS}g", $X);
2853 $tY = sprintf("%.${POINTS}g", $Y);
2854 return $tX eq $tY;
2855 }
2856
68dc0745 2857The POSIX module (part of the standard perl distribution) implements
19799a22
GS
2858ceil(), floor(), and other mathematical and trigonometric functions.
2859The Math::Complex module (part of the standard perl distribution)
2860defines mathematical functions that work on both the reals and the
2861imaginary numbers. Math::Complex not as efficient as POSIX, but
68dc0745 2862POSIX can't work with complex numbers.
2863
2864Rounding in financial applications can have serious implications, and
2865the rounding method used should be specified precisely. In these
2866cases, it probably pays not to trust whichever system rounding is
2867being used by Perl, but to instead implement the rounding function you
2868need yourself.
5a964f20
TC
2869
2870=head2 Bigger Numbers
d74e8afc 2871X<number, arbitrary precision>
5a964f20 2872
c543c01b
TC
2873The standard C<Math::BigInt>, C<Math::BigRat>, and C<Math::BigFloat> modules,
2874along with the C<bigint>, C<bigrat>, and C<bitfloat> pragmas, provide
19799a22 2875variable-precision arithmetic and overloaded operators, although
cd5c4fce 2876they're currently pretty slow. At the cost of some space and
19799a22
GS
2877considerable speed, they avoid the normal pitfalls associated with
2878limited-precision representations.
5a964f20 2879
c543c01b
TC
2880 use 5.010;
2881 use bigint; # easy interface to Math::BigInt
2882 $x = 123456789123456789;
2883 say $x * $x;
2884 +15241578780673678515622620750190521
2885
2886Or with rationals:
2887
2888 use 5.010;
2889 use bigrat;
2890 $a = 3/22;
2891 $b = 4/6;
2892 say "a/b is ", $a/$b;
2893 say "a*b is ", $a*$b;
2894 a/b is 9/44
2895 a*b is 1/11
2896
2897Several modules let you calculate with (bound only by memory and CPU time)
2898unlimited or fixed precision. There are also some non-standard modules that
2899provide faster implementations via external C libraries.
cd5c4fce
T
2900
2901Here is a short, but incomplete summary:
2902
950b09ed
KW
2903 Math::Fraction big, unlimited fractions like 9973 / 12967
2904 Math::String treat string sequences like numbers
2905 Math::FixedPrecision calculate with a fixed precision
2906 Math::Currency for currency calculations
2907 Bit::Vector manipulate bit vectors fast (uses C)
2908 Math::BigIntFast Bit::Vector wrapper for big numbers
2909 Math::Pari provides access to the Pari C library
2910 Math::BigInteger uses an external C library
2911 Math::Cephes uses external Cephes C library (no big numbers)
2912 Math::Cephes::Fraction fractions via the Cephes library
2913 Math::GMP another one using an external C library
cd5c4fce
T
2914
2915Choose wisely.
16070b82
GS
2916
2917=cut