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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 .. ...
a0d0e21e 50 right ?:
2ba1f20a 51 right = += -= *= etc. goto last next redo dump
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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
1ca345ed 193Unary "!" performs logical negation, that is, "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
1ca345ed 210Unary "~" performs bitwise negation, that is, 1's complement. For
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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
1ca345ed 256 '\\' =~ q'\\';
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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
1ca345ed 282smallest multiple of C<\$b> that is not less than C<\$a> (that is, 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>
a0d0e21e 319
1ca345ed 320Binary C<+> returns the sum of two numbers.
d74e8afc 321X<+>
a0d0e21e 322
1ca345ed 323Binary C<-> returns the difference of two numbers.
d74e8afc 324X<->
a0d0e21e 325
1ca345ed 326Binary C<.> 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
1ca345ed 335Binary C<<< << >>> returns the value of its left argument shifted left by the
55497cff 336number of bits specified by the right argument. Arguments should be
982ce180 337integers. (See also L<Integer Arithmetic>.)
a0d0e21e 338
1ca345ed 339Binary C<<< >> >>> returns the value of its left argument shifted right by
55497cff 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 C<<< << >>> and C<<< >> >>> in Perl are implemented directly using
344C<<< << >>> and C<<< >> >>> in C. If C<use integer> (see L<Integer Arithmetic>) is
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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
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355If you get tired of being subject to your platform's native integers,
356the C<use bigint> pragma neatly sidesteps the issue altogether:
357
358 print 20 << 20; # 20971520
359 print 20 << 40; # 5120 on 32-bit machines,
360 # 21990232555520 on 64-bit machines
361 use bigint;
362 print 20 << 100; # 25353012004564588029934064107520
363
a0d0e21e 364=head2 Named Unary Operators
d74e8afc 365X<operator, named unary>
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366
367The various named unary operators are treated as functions with one
568e6d8b 368argument, with optional parentheses.
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369
370If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
371is followed by a left parenthesis as the next token, the operator and
372arguments within parentheses are taken to be of highest precedence,
3981b0eb 373just like a normal function call. For example,
1ca345ed 374because named unary operators are higher precedence than C<||>:
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375
376 chdir \$foo || die; # (chdir \$foo) || die
377 chdir(\$foo) || die; # (chdir \$foo) || die
378 chdir (\$foo) || die; # (chdir \$foo) || die
379 chdir +(\$foo) || die; # (chdir \$foo) || die
380
3981b0eb 381but, because * is higher precedence than named operators:
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382
383 chdir \$foo * 20; # chdir (\$foo * 20)
384 chdir(\$foo) * 20; # (chdir \$foo) * 20
385 chdir (\$foo) * 20; # (chdir \$foo) * 20
386 chdir +(\$foo) * 20; # chdir (\$foo * 20)
387
388 rand 10 * 20; # rand (10 * 20)
389 rand(10) * 20; # (rand 10) * 20
390 rand (10) * 20; # (rand 10) * 20
391 rand +(10) * 20; # rand (10 * 20)
392
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393Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are
394treated like named unary operators, but they don't follow this functional
395parenthesis rule. That means, for example, that C<-f(\$file).".bak"> is
396equivalent to C<-f "\$file.bak">.
d74e8afc 397X<-X> X<filetest> X<operator, filetest>
568e6d8b 398
5ba421f6 399See also L<"Terms and List Operators (Leftward)">.
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400
401=head2 Relational Operators
d74e8afc 402X<relational operator> X<operator, relational>
a0d0e21e 403
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404Perl operators that return true or false generally return values
405that can be safely used as numbers. For example, the relational
406operators in this section and the equality operators in the next
407one return C<1> for true and a special version of the defined empty
408string, C<"">, which counts as a zero but is exempt from warnings
409about improper numeric conversions, just as C<"0 but true"> is.
410
35f2feb0 411Binary "<" returns true if the left argument is numerically less than
a0d0e21e 412the right argument.
d74e8afc 413X<< < >>
a0d0e21e 414
35f2feb0 415Binary ">" returns true if the left argument is numerically greater
a0d0e21e 416than the right argument.
d74e8afc 417X<< > >>
a0d0e21e 418
35f2feb0 419Binary "<=" returns true if the left argument is numerically less than
a0d0e21e 420or equal to the right argument.
d74e8afc 421X<< <= >>
a0d0e21e 422
35f2feb0 423Binary ">=" returns true if the left argument is numerically greater
a0d0e21e 424than or equal to the right argument.
d74e8afc 425X<< >= >>
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426
427Binary "lt" returns true if the left argument is stringwise less than
428the right argument.
d74e8afc 429X<< lt >>
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430
431Binary "gt" returns true if the left argument is stringwise greater
432than the right argument.
d74e8afc 433X<< gt >>
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434
435Binary "le" returns true if the left argument is stringwise less than
436or equal to the right argument.
d74e8afc 437X<< le >>
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438
439Binary "ge" returns true if the left argument is stringwise greater
440than or equal to the right argument.
d74e8afc 441X<< ge >>
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442
443=head2 Equality Operators
d74e8afc 444X<equality> X<equal> X<equals> X<operator, equality>
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445
446Binary "==" returns true if the left argument is numerically equal to
447the right argument.
d74e8afc 448X<==>
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449
450Binary "!=" returns true if the left argument is numerically not equal
451to the right argument.
d74e8afc 452X<!=>
a0d0e21e 453
35f2feb0 454Binary "<=>" returns -1, 0, or 1 depending on whether the left
6ee5d4e7 455argument is numerically less than, equal to, or greater than the right
d4ad863d 456argument. If your platform supports NaNs (not-a-numbers) as numeric
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457values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
458"<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
459returns true, as does NaN != anything else. If your platform doesn't
460support NaNs then NaN is just a string with numeric value 0.
d74e8afc 461X<< <=> >> X<spaceship>
7d3a9d88 462
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463 \$ perl -le '\$a = "NaN"; print "No NaN support here" if \$a == \$a'
464 \$ perl -le '\$a = "NaN"; print "NaN support here" if \$a != \$a'
465
466(Note that the L<bigint>, L<bigrat>, and L<bignum> pragmas all
467support "NaN".)
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468
469Binary "eq" returns true if the left argument is stringwise equal to
470the right argument.
d74e8afc 471X<eq>
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472
473Binary "ne" returns true if the left argument is stringwise not equal
474to the right argument.
d74e8afc 475X<ne>
a0d0e21e 476
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477Binary "cmp" returns -1, 0, or 1 depending on whether the left
478argument is stringwise less than, equal to, or greater than the right
479argument.
d74e8afc 480X<cmp>
a0d0e21e 481
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482Binary "~~" does a smartmatch between its arguments. Smart matching
483is described in the next section.
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484X<~~>
485
a034a98d 486"lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
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KW
487by the current locale if a legacy C<use locale> (but not
488C<use locale ':not_characters'>) is in effect. See
1ca345ed
TC
489L<perllocale>. Do not mix these with Unicode, only with legacy binary
490encodings. The standard L<Unicode::Collate> and
491L<Unicode::Collate::Locale> modules offer much more powerful solutions to
492collation issues.
493
494=head2 Smartmatch Operator
495
496First available in Perl 5.10.1 (the 5.10.0 version behaved differently),
497binary C<~~> does a "smartmatch" between its arguments. This is mostly
498used implicitly in the C<when> construct described in L<perlsyn>, although
499not all C<when> clauses call the smartmatch operator. Unique among all of
500Perl's operators, the smartmatch operator can recurse.
501
502It is also unique in that all other Perl operators impose a context
503(usually string or numeric context) on their operands, autoconverting
504those operands to those imposed contexts. In contrast, smartmatch
505I<infers> contexts from the actual types of its operands and uses that
506type information to select a suitable comparison mechanism.
507
508The C<~~> operator compares its operands "polymorphically", determining how
509to compare them according to their actual types (numeric, string, array,
510hash, etc.) Like the equality operators with which it shares the same
511precedence, C<~~> returns 1 for true and C<""> for false. It is often best
512read aloud as "in", "inside of", or "is contained in", because the left
513operand is often looked for I<inside> the right operand. That makes the
40bec8a5 514order of the operands to the smartmatch operand often opposite that of
1ca345ed
TC
515the regular match operator. In other words, the "smaller" thing is usually
516placed in the left operand and the larger one in the right.
517
518The behavior of a smartmatch depends on what type of things its arguments
519are, as determined by the following table. The first row of the table
520whose types apply determines the smartmatch behavior. Because what
521actually happens is mostly determined by the type of the second operand,
522the table is sorted on the right operand instead of on the left.
523
524 Left Right Description and pseudocode
525 ===============================================================
526 Any undef check whether Any is undefined
527 like: !defined Any
528
529 Any Object invoke ~~ overloading on Object, or die
530
531 Right operand is an ARRAY:
532
533 Left Right Description and pseudocode
534 ===============================================================
535 ARRAY1 ARRAY2 recurse on paired elements of ARRAY1 and ARRAY2[2]
536 like: (ARRAY1[0] ~~ ARRAY2[0])
537 && (ARRAY1[1] ~~ ARRAY2[1]) && ...
538 HASH ARRAY any ARRAY elements exist as HASH keys
539 like: grep { exists HASH->{\$_} } ARRAY
540 Regexp ARRAY any ARRAY elements pattern match Regexp
541 like: grep { /Regexp/ } ARRAY
542 undef ARRAY undef in ARRAY
543 like: grep { !defined } ARRAY
40bec8a5 544 Any ARRAY smartmatch each ARRAY element[3]
1ca345ed
TC
545 like: grep { Any ~~ \$_ } ARRAY
546
547 Right operand is a HASH:
548
549 Left Right Description and pseudocode
550 ===============================================================
551 HASH1 HASH2 all same keys in both HASHes
552 like: keys HASH1 ==
553 grep { exists HASH2->{\$_} } keys HASH1
554 ARRAY HASH any ARRAY elements exist as HASH keys
555 like: grep { exists HASH->{\$_} } ARRAY
556 Regexp HASH any HASH keys pattern match Regexp
557 like: grep { /Regexp/ } keys HASH
558 undef HASH always false (undef can't be a key)
559 like: 0 == 1
560 Any HASH HASH key existence
561 like: exists HASH->{Any}
562
563 Right operand is CODE:
f703fc96 564
1ca345ed
TC
565 Left Right Description and pseudocode
566 ===============================================================
567 ARRAY CODE sub returns true on all ARRAY elements[1]
568 like: !grep { !CODE->(\$_) } ARRAY
569 HASH CODE sub returns true on all HASH keys[1]
570 like: !grep { !CODE->(\$_) } keys HASH
571 Any CODE sub passed Any returns true
572 like: CODE->(Any)
573
574Right operand is a Regexp:
575
576 Left Right Description and pseudocode
577 ===============================================================
578 ARRAY Regexp any ARRAY elements match Regexp
579 like: grep { /Regexp/ } ARRAY
580 HASH Regexp any HASH keys match Regexp
581 like: grep { /Regexp/ } keys HASH
582 Any Regexp pattern match
583 like: Any =~ /Regexp/
584
585 Other:
586
587 Left Right Description and pseudocode
588 ===============================================================
589 Object Any invoke ~~ overloading on Object,
590 or fall back to...
591
592 Any Num numeric equality
593 like: Any == Num
594 Num nummy[4] numeric equality
595 like: Num == nummy
596 undef Any check whether undefined
597 like: !defined(Any)
598 Any Any string equality
599 like: Any eq Any
600
601
602Notes:
603
604=over
605
606=item 1.
607Empty hashes or arrays match.
608
609=item 2.
40bec8a5 610That is, each element smartmatches the element of the same index in the other array.[3]
1ca345ed
TC
611
612=item 3.
613If a circular reference is found, fall back to referential equality.
614
615=item 4.
616Either an actual number, or a string that looks like one.
617
618=back
619
620The smartmatch implicitly dereferences any non-blessed hash or array
621reference, so the C<I<HASH>> and C<I<ARRAY>> entries apply in those cases.
622For blessed references, the C<I<Object>> entries apply. Smartmatches
623involving hashes only consider hash keys, never hash values.
624
625The "like" code entry is not always an exact rendition. For example, the
40bec8a5 626smartmatch operator short-circuits whenever possible, but C<grep> does
1ca345ed
TC
627not. Also, C<grep> in scalar context returns the number of matches, but
628C<~~> returns only true or false.
629
630Unlike most operators, the smartmatch operator knows to treat C<undef>
631specially:
632
633 use v5.10.1;
634 @array = (1, 2, 3, undef, 4, 5);
635 say "some elements undefined" if undef ~~ @array;
636
637Each operand is considered in a modified scalar context, the modification
638being that array and hash variables are passed by reference to the
639operator, which implicitly dereferences them. Both elements
640of each pair are the same:
641
642 use v5.10.1;
643
644 my %hash = (red => 1, blue => 2, green => 3,
645 orange => 4, yellow => 5, purple => 6,
646 black => 7, grey => 8, white => 9);
647
648 my @array = qw(red blue green);
649
650 say "some array elements in hash keys" if @array ~~ %hash;
651 say "some array elements in hash keys" if \@array ~~ \%hash;
652
653 say "red in array" if "red" ~~ @array;
654 say "red in array" if "red" ~~ \@array;
655
656 say "some keys end in e" if /e\$/ ~~ %hash;
657 say "some keys end in e" if /e\$/ ~~ \%hash;
658
40bec8a5
TC
659Two arrays smartmatch if each element in the first array smartmatches
660(that is, is "in") the corresponding element in the second array,
661recursively.
1ca345ed
TC
662
663 use v5.10.1;
664 my @little = qw(red blue green);
665 my @bigger = ("red", "blue", [ "orange", "green" ] );
666 if (@little ~~ @bigger) { # true!
667 say "little is contained in bigger";
668 }
669
670Because the smartmatch operator recurses on nested arrays, this
671will still report that "red" is in the array.
672
673 use v5.10.1;
674 my @array = qw(red blue green);
675 my \$nested_array = [[[[[[[ @array ]]]]]]];
676 say "red in array" if "red" ~~ \$nested_array;
677
678If two arrays smartmatch each other, then they are deep
679copies of each others' values, as this example reports:
680
681 use v5.12.0;
682 my @a = (0, 1, 2, [3, [4, 5], 6], 7);
683 my @b = (0, 1, 2, [3, [4, 5], 6], 7);
684
685 if (@a ~~ @b && @b ~~ @a) {
686 say "a and b are deep copies of each other";
687 }
688 elsif (@a ~~ @b) {
689 say "a smartmatches in b";
690 }
691 elsif (@b ~~ @a) {
692 say "b smartmatches in a";
693 }
694 else {
695 say "a and b don't smartmatch each other at all";
696 }
697
698
699If you were to set C<\$b[3] = 4>, then instead of reporting that "a and b
700are deep copies of each other", it now reports that "b smartmatches in a".
701That because the corresponding position in C<@a> contains an array that
702(eventually) has a 4 in it.
703
704Smartmatching one hash against another reports whether both contain the
705same keys, no more and no less. This could be used to see whether two
706records have the same field names, without caring what values those fields
707might have. For example:
708
709 use v5.10.1;
710 sub make_dogtag {
711 state \$REQUIRED_FIELDS = { name=>1, rank=>1, serial_num=>1 };
712
713 my (\$class, \$init_fields) = @_;
714
715 die "Must supply (only) name, rank, and serial number"
716 unless \$init_fields ~~ \$REQUIRED_FIELDS;
717
718 ...
719 }
720
721or, if other non-required fields are allowed, use ARRAY ~~ HASH:
722
723 use v5.10.1;
724 sub make_dogtag {
725 state \$REQUIRED_FIELDS = { name=>1, rank=>1, serial_num=>1 };
726
727 my (\$class, \$init_fields) = @_;
728
729 die "Must supply (at least) name, rank, and serial number"
730 unless [keys %{\$init_fields}] ~~ \$REQUIRED_FIELDS;
731
732 ...
733 }
734
735The smartmatch operator is most often used as the implicit operator of a
736C<when> clause. See the section on "Switch Statements" in L<perlsyn>.
737
738=head3 Smartmatching of Objects
739
40bec8a5
TC
740To avoid relying on an object's underlying representation, if the
741smartmatch's right operand is an object that doesn't overload C<~~>,
742it raises the exception "C<Smartmatching a non-overloaded object
743breaks encapsulation>". That's because one has no business digging
744around to see whether something is "in" an object. These are all
745illegal on objects without a C<~~> overload:
1ca345ed
TC
746
747 %hash ~~ \$object
748 42 ~~ \$object
749 "fred" ~~ \$object
750
751However, you can change the way an object is smartmatched by overloading
752the C<~~> operator. This is allowed to extend the usual smartmatch semantics.
753For objects that do have an C<~~> overload, see L<overload>.
754
755Using an object as the left operand is allowed, although not very useful.
756Smartmatching rules take precedence over overloading, so even if the
757object in the left operand has smartmatch overloading, this will be
758ignored. A left operand that is a non-overloaded object falls back on a
759string or numeric comparison of whatever the C<ref> operator returns. That
760means that
761
762 \$object ~~ X
763
764does I<not> invoke the overload method with C<I<X>> as an argument.
765Instead the above table is consulted as normal, and based on the type of
766C<I<X>>, overloading may or may not be invoked. For simple strings or
767numbers, in becomes equivalent to this:
768
769 \$object ~~ \$number ref(\$object) == \$number
770 \$object ~~ \$string ref(\$object) eq \$string
771
772For example, this reports that the handle smells IOish
773(but please don't really do this!):
774
775 use IO::Handle;
776 my \$fh = IO::Handle->new();
777 if (\$fh ~~ /\bIO\b/) {
778 say "handle smells IOish";
779 }
780
781That's because it treats C<\$fh> as a string like
782C<"IO::Handle=GLOB(0x8039e0)">, then pattern matches against that.
a034a98d 783
a0d0e21e 784=head2 Bitwise And
d74e8afc 785X<operator, bitwise, and> X<bitwise and> X<&>
a0d0e21e 786
2cdc098b 787Binary "&" returns its operands ANDed together bit by bit.
2c268ad5 788(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
a0d0e21e 789
2cdc098b 790Note that "&" has lower priority than relational operators, so for example
1ca345ed 791the parentheses are essential in a test like
2cdc098b 792
1ca345ed 793 print "Even\n" if (\$x & 1) == 0;
2cdc098b 794
a0d0e21e 795=head2 Bitwise Or and Exclusive Or
d74e8afc
ITB
796X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor>
797X<bitwise xor> X<^>
a0d0e21e 798
2cdc098b 799Binary "|" returns its operands ORed together bit by bit.
2c268ad5 800(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
a0d0e21e 801
2cdc098b 802Binary "^" returns its operands XORed together bit by bit.
2c268ad5 803(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
a0d0e21e 804
2cdc098b
MG
805Note that "|" and "^" have lower priority than relational operators, so
806for example the brackets are essential in a test like
807
1ca345ed 808 print "false\n" if (8 | 2) != 10;
2cdc098b 809
a0d0e21e 810=head2 C-style Logical And
d74e8afc 811X<&&> X<logical and> X<operator, logical, and>
a0d0e21e
LW
812
813Binary "&&" performs a short-circuit logical AND operation. That is,
814if the left operand is false, the right operand is not even evaluated.
815Scalar or list context propagates down to the right operand if it
816is evaluated.
817
818=head2 C-style Logical Or
d74e8afc 819X<||> X<operator, logical, or>
a0d0e21e
LW
820
821Binary "||" performs a short-circuit logical OR operation. That is,
822if the left operand is true, the right operand is not even evaluated.
823Scalar or list context propagates down to the right operand if it
824is evaluated.
825
26d9d83b 826=head2 Logical Defined-Or
d74e8afc 827X<//> X<operator, logical, defined-or>
c963b151
BD
828
829Although it has no direct equivalent in C, Perl's C<//> operator is related
89d205f2 830to its C-style or. In fact, it's exactly the same as C<||>, except that it
95bee9ba
A
831tests the left hand side's definedness instead of its truth. Thus,
832C<< EXPR1 // EXPR2 >> returns the value of C<< EXPR1 >> if it's defined,
833otherwise, the value of C<< EXPR2 >> is returned. (C<< EXPR1 >> is evaluated
834in scalar context, C<< EXPR2 >> in the context of C<< // >> itself). Usually,
835this is the same result as C<< defined(EXPR1) ? EXPR1 : EXPR2 >> (except that
836the ternary-operator form can be used as a lvalue, while C<< EXPR1 // EXPR2 >>
837cannot). This is very useful for
bdc7923b
RGS
838providing default values for variables. If you actually want to test if
839at least one of C<\$a> and C<\$b> is defined, use C<defined(\$a // \$b)>.
c963b151 840
d042e63d
MS
841The C<||>, C<//> and C<&&> operators return the last value evaluated
842(unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably
843portable way to find out the home directory might be:
a0d0e21e 844
c543c01b
TC
845 \$home = \$ENV{HOME}
846 // \$ENV{LOGDIR}
847 // (getpwuid(\$<))[7]
848 // die "You're homeless!\n";
a0d0e21e 849
5a964f20
TC
850In particular, this means that you shouldn't use this
851for selecting between two aggregates for assignment:
852
853 @a = @b || @c; # this is wrong
854 @a = scalar(@b) || @c; # really meant this
855 @a = @b ? @b : @c; # this works fine, though
856
1ca345ed 857As alternatives to C<&&> and C<||> when used for
f23102e2
RGS
858control flow, Perl provides the C<and> and C<or> operators (see below).
859The short-circuit behavior is identical. The precedence of "and"
c963b151 860and "or" is much lower, however, so that you can safely use them after a
5a964f20 861list operator without the need for parentheses:
a0d0e21e
LW
862
863 unlink "alpha", "beta", "gamma"
864 or gripe(), next LINE;
865
866With the C-style operators that would have been written like this:
867
868 unlink("alpha", "beta", "gamma")
869 || (gripe(), next LINE);
870
1ca345ed
TC
871It would be even more readable to write that this way:
872
873 unless(unlink("alpha", "beta", "gamma")) {
874 gripe();
875 next LINE;
876 }
877
eeb6a2c9 878Using "or" for assignment is unlikely to do what you want; see below.
5a964f20
TC
879
880=head2 Range Operators
d74e8afc 881X<operator, range> X<range> X<..> X<...>
a0d0e21e
LW
882
883Binary ".." is the range operator, which is really two different
fb53bbb2 884operators depending on the context. In list context, it returns a
54ae734e 885list of values counting (up by ones) from the left value to the right
2cdbc966 886value. If the left value is greater than the right value then it
fb53bbb2 887returns the empty list. The range operator is useful for writing
54ae734e 888C<foreach (1..10)> loops and for doing slice operations on arrays. In
2cdbc966
JD
889the current implementation, no temporary array is created when the
890range operator is used as the expression in C<foreach> loops, but older
891versions of Perl might burn a lot of memory when you write something
892like this:
a0d0e21e
LW
893
894 for (1 .. 1_000_000) {
895 # code
54310121 896 }
a0d0e21e 897
8f0f46f8 898The range operator also works on strings, using the magical
899auto-increment, see below.
54ae734e 900
5a964f20 901In scalar context, ".." returns a boolean value. The operator is
8f0f46f8 902bistable, like a flip-flop, and emulates the line-range (comma)
903operator of B<sed>, B<awk>, and various editors. Each ".." operator
904maintains its own boolean state, even across calls to a subroutine
905that contains it. It is false as long as its left operand is false.
a0d0e21e
LW
906Once the left operand is true, the range operator stays true until the
907right operand is true, I<AFTER> which the range operator becomes false
8f0f46f8 908again. It doesn't become false till the next time the range operator
909is evaluated. It can test the right operand and become false on the
910same evaluation it became true (as in B<awk>), but it still returns
911true once. If you don't want it to test the right operand until the
912next evaluation, as in B<sed>, just use three dots ("...") instead of
19799a22
GS
913two. In all other regards, "..." behaves just like ".." does.
914
915The right operand is not evaluated while the operator is in the
916"false" state, and the left operand is not evaluated while the
917operator is in the "true" state. The precedence is a little lower
918than || and &&. The value returned is either the empty string for
8f0f46f8 919false, or a sequence number (beginning with 1) for true. The sequence
920number is reset for each range encountered. The final sequence number
921in a range has the string "E0" appended to it, which doesn't affect
922its numeric value, but gives you something to search for if you want
923to exclude the endpoint. You can exclude the beginning point by
924waiting for the sequence number to be greater than 1.
df5f8116
CW
925
926If either operand of scalar ".." is a constant expression,
927that operand is considered true if it is equal (C<==>) to the current
928input line number (the C<\$.> variable).
929
930To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>,
931but that is only an issue if you use a floating point expression; when
932implicitly using C<\$.> as described in the previous paragraph, the
933comparison is C<int(EXPR) == int(\$.)> which is only an issue when C<\$.>
934is set to a floating point value and you are not reading from a file.
935Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what
936you want in scalar context because each of the operands are evaluated
937using their integer representation.
938
939Examples:
a0d0e21e
LW
940
941As a scalar operator:
942
df5f8116 943 if (101 .. 200) { print; } # print 2nd hundred lines, short for
950b09ed 944 # if (\$. == 101 .. \$. == 200) { print; }
9f10b797
RGS
945
946 next LINE if (1 .. /^\$/); # skip header lines, short for
f343f960 947 # next LINE if (\$. == 1 .. /^\$/);
9f10b797
RGS
948 # (typically in a loop labeled LINE)
949
950 s/^/> / if (/^\$/ .. eof()); # quote body
a0d0e21e 951
5a964f20
TC
952 # parse mail messages
953 while (<>) {
954 \$in_header = 1 .. /^\$/;
df5f8116
CW
955 \$in_body = /^\$/ .. eof;
956 if (\$in_header) {
f343f960 957 # do something
df5f8116 958 } else { # in body
f343f960 959 # do something else
df5f8116 960 }
5a964f20 961 } continue {
df5f8116 962 close ARGV if eof; # reset \$. each file
5a964f20
TC
963 }
964
acf31ca5
SF
965Here's a simple example to illustrate the difference between
966the two range operators:
967
968 @lines = (" - Foo",
969 "01 - Bar",
970 "1 - Baz",
971 " - Quux");
972
9f10b797
RGS
973 foreach (@lines) {
974 if (/0/ .. /1/) {
acf31ca5
SF
975 print "\$_\n";
976 }
977 }
978
9f10b797
RGS
979This program will print only the line containing "Bar". If
980the range operator is changed to C<...>, it will also print the
acf31ca5
SF
981"Baz" line.
982
983And now some examples as a list operator:
a0d0e21e 984
1ca345ed
TC
985 for (101 .. 200) { print } # print \$_ 100 times
986 @foo = @foo[0 .. \$#foo]; # an expensive no-op
987 @foo = @foo[\$#foo-4 .. \$#foo]; # slice last 5 items
a0d0e21e 988
5a964f20 989The range operator (in list context) makes use of the magical
5f05dabc 990auto-increment algorithm if the operands are strings. You
a0d0e21e
LW
991can say
992
c543c01b 993 @alphabet = ("A" .. "Z");
a0d0e21e 994
54ae734e 995to get all normal letters of the English alphabet, or
a0d0e21e 996
c543c01b 997 \$hexdigit = (0 .. 9, "a" .. "f")[\$num & 15];
a0d0e21e
LW
998
999to get a hexadecimal digit, or
1000
1ca345ed
TC
1001 @z2 = ("01" .. "31");
1002 print \$z2[\$mday];
a0d0e21e 1003
ea4f5703
YST
1004to get dates with leading zeros.
1005
1006If the final value specified is not in the sequence that the magical
1007increment would produce, the sequence goes until the next value would
1008be longer than the final value specified.
1009
1010If the initial value specified isn't part of a magical increment
c543c01b 1011sequence (that is, a non-empty string matching C</^[a-zA-Z]*[0-9]*\z/>),
ea4f5703
YST
1012only the initial value will be returned. So the following will only
1013return an alpha:
1014
c543c01b 1015 use charnames "greek";
ea4f5703
YST
1016 my @greek_small = ("\N{alpha}" .. "\N{omega}");
1017
c543c01b
TC
1018To get the 25 traditional lowercase Greek letters, including both sigmas,
1019you could use this instead:
ea4f5703 1020
c543c01b 1021 use charnames "greek";
1ca345ed
TC
1022 my @greek_small = map { chr } ( ord("\N{alpha}")
1023 ..
1024 ord("\N{omega}")
1025 );
c543c01b
TC
1026
1027However, because there are I<many> other lowercase Greek characters than
1028just those, to match lowercase Greek characters in a regular expression,
1029you would use the pattern C</(?:(?=\p{Greek})\p{Lower})+/>.
a0d0e21e 1030
df5f8116
CW
1031Because each operand is evaluated in integer form, C<2.18 .. 3.14> will
1032return two elements in list context.
1033
1034 @list = (2.18 .. 3.14); # same as @list = (2 .. 3);
1035
a0d0e21e 1036=head2 Conditional Operator
d74e8afc 1037X<operator, conditional> X<operator, ternary> X<ternary> X<?:>
a0d0e21e
LW
1038
1039Ternary "?:" is the conditional operator, just as in C. It works much
1040like an if-then-else. If the argument before the ? is true, the
1041argument before the : is returned, otherwise the argument after the :
cb1a09d0
AD
1042is returned. For example:
1043
54310121 1044 printf "I have %d dog%s.\n", \$n,
c543c01b 1045 (\$n == 1) ? "" : "s";
cb1a09d0
AD
1046
1047Scalar or list context propagates downward into the 2nd
54310121 1048or 3rd argument, whichever is selected.
cb1a09d0
AD
1049
1050 \$a = \$ok ? \$b : \$c; # get a scalar
1051 @a = \$ok ? @b : @c; # get an array
1052 \$a = \$ok ? @b : @c; # oops, that's just a count!
1053
1054The operator may be assigned to if both the 2nd and 3rd arguments are
1055legal lvalues (meaning that you can assign to them):
a0d0e21e
LW
1056
1057 (\$a_or_b ? \$a : \$b) = \$c;
1058
5a964f20
TC
1059Because this operator produces an assignable result, using assignments
1060without parentheses will get you in trouble. For example, this:
1061
1062 \$a % 2 ? \$a += 10 : \$a += 2
1063
1064Really means this:
1065
1066 ((\$a % 2) ? (\$a += 10) : \$a) += 2
1067
1068Rather than this:
1069
1070 (\$a % 2) ? (\$a += 10) : (\$a += 2)
1071
19799a22
GS
1072That should probably be written more simply as:
1073
1074 \$a += (\$a % 2) ? 10 : 2;
1075
4633a7c4 1076=head2 Assignment Operators
d74e8afc 1077X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=>
5ac3b81c 1078X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=>
d74e8afc 1079X<%=> X<^=> X<x=>
a0d0e21e
LW
1080
1081"=" is the ordinary assignment operator.
1082
1083Assignment operators work as in C. That is,
1084
1085 \$a += 2;
1086
1087is equivalent to
1088
1089 \$a = \$a + 2;
1090
1091although without duplicating any side effects that dereferencing the lvalue
54310121
PP
1092might trigger, such as from tie(). Other assignment operators work similarly.
1093The following are recognized:
a0d0e21e
LW
1094
1095 **= += *= &= <<= &&=
9f10b797
RGS
1096 -= /= |= >>= ||=
1097 .= %= ^= //=
1098 x=
a0d0e21e 1099
19799a22 1100Although these are grouped by family, they all have the precedence
a0d0e21e
LW
1101of assignment.
1102
b350dd2f
GS
1103Unlike in C, the scalar assignment operator produces a valid lvalue.
1104Modifying an assignment is equivalent to doing the assignment and
1105then modifying the variable that was assigned to. This is useful
1106for modifying a copy of something, like this:
a0d0e21e 1107
1ca345ed
TC
1108 (\$tmp = \$global) =~ tr/13579/24680/;
1109
1110Although as of 5.14, that can be also be accomplished this way:
1111
1112 use v5.14;
1113 \$tmp = (\$global =~ tr/13579/24680/r);
a0d0e21e
LW
1114
1115Likewise,
1116
1117 (\$a += 2) *= 3;
1118
1119is equivalent to
1120
1121 \$a += 2;
1122 \$a *= 3;
1123
b350dd2f
GS
1124Similarly, a list assignment in list context produces the list of
1125lvalues assigned to, and a list assignment in scalar context returns
1126the number of elements produced by the expression on the right hand
1127side of the assignment.
1128
748a9306 1129=head2 Comma Operator
d74e8afc 1130X<comma> X<operator, comma> X<,>
a0d0e21e 1131
5a964f20 1132Binary "," is the comma operator. In scalar context it evaluates
a0d0e21e
LW
1133its left argument, throws that value away, then evaluates its right
1134argument and returns that value. This is just like C's comma operator.
1135
5a964f20 1136In list context, it's just the list argument separator, and inserts
ed5c6d31
PJ
1137both its arguments into the list. These arguments are also evaluated
1138from left to right.
a0d0e21e 1139
4e1988c6
FC
1140The C<< => >> operator is a synonym for the comma except that it causes a
1141word on its left to be interpreted as a string if it begins with a letter
344f2c40
IG
1142or underscore and is composed only of letters, digits and underscores.
1143This includes operands that might otherwise be interpreted as operators,
1144constants, single number v-strings or function calls. If in doubt about
c543c01b 1145this behavior, the left operand can be quoted explicitly.
344f2c40
IG
1146
1147Otherwise, the C<< => >> operator behaves exactly as the comma operator
1148or list argument separator, according to context.
1149
1150For example:
a44e5664
MS
1151
1152 use constant FOO => "something";
1153
1154 my %h = ( FOO => 23 );
1155
1156is equivalent to:
1157
1158 my %h = ("FOO", 23);
1159
1160It is I<NOT>:
1161
1162 my %h = ("something", 23);
1163
719b43e8
RGS
1164The C<< => >> operator is helpful in documenting the correspondence
1165between keys and values in hashes, and other paired elements in lists.
748a9306 1166
a12b8f3c
FC
1167 %hash = ( \$key => \$value );
1168 login( \$username => \$password );
a44e5664 1169
4e1988c6
FC
1170The special quoting behavior ignores precedence, and hence may apply to
1171I<part> of the left operand:
1172
1173 print time.shift => "bbb";
1174
1175That example prints something like "1314363215shiftbbb", because the
1176C<< => >> implicitly quotes the C<shift> immediately on its left, ignoring
1177the fact that C<time.shift> is the entire left operand.
1178
a0d0e21e 1179=head2 List Operators (Rightward)
d74e8afc 1180X<operator, list, rightward> X<list operator>
a0d0e21e 1181
c543c01b 1182On the right side of a list operator, the comma has very low precedence,
a0d0e21e
LW
1183such that it controls all comma-separated expressions found there.
1184The only operators with lower precedence are the logical operators
1185"and", "or", and "not", which may be used to evaluate calls to list
1ca345ed
TC
1186operators without the need for parentheses:
1187
1188 open HANDLE, "< :utf8", "filename" or die "Can't open: \$!\n";
1189
1190However, some people find that code harder to read than writing
1191it with parentheses:
1192
1193 open(HANDLE, "< :utf8", "filename") or die "Can't open: \$!\n";
1194
1195in which case you might as well just use the more customary "||" operator:
a0d0e21e 1196
1ca345ed 1197 open(HANDLE, "< :utf8", "filename") || die "Can't open: \$!\n";
a0d0e21e 1198
5ba421f6 1199See also discussion of list operators in L<Terms and List Operators (Leftward)>.
a0d0e21e
LW
1200
1201=head2 Logical Not
d74e8afc 1202X<operator, logical, not> X<not>
a0d0e21e
LW
1203
1204Unary "not" returns the logical negation of the expression to its right.
1205It's the equivalent of "!" except for the very low precedence.
1206
1207=head2 Logical And
d74e8afc 1208X<operator, logical, and> X<and>
a0d0e21e
LW
1209
1210Binary "and" returns the logical conjunction of the two surrounding
c543c01b
TC
1211expressions. It's equivalent to C<&&> except for the very low
1212precedence. This means that it short-circuits: the right
a0d0e21e
LW
1213expression is evaluated only if the left expression is true.
1214
59ab9d6e 1215=head2 Logical or and Exclusive Or
f23102e2 1216X<operator, logical, or> X<operator, logical, xor>
59ab9d6e 1217X<operator, logical, exclusive or>
f23102e2 1218X<or> X<xor>
a0d0e21e
LW
1219
1220Binary "or" returns the logical disjunction of the two surrounding
c543c01b
TC
1221expressions. It's equivalent to C<||> except for the very low precedence.
1222This makes it useful for control flow:
5a964f20
TC
1223
1224 print FH \$data or die "Can't write to FH: \$!";
1225
c543c01b
TC
1226This means that it short-circuits: the right expression is evaluated
1227only if the left expression is false. Due to its precedence, you must
1228be careful to avoid using it as replacement for the C<||> operator.
1229It usually works out better for flow control than in assignments:
5a964f20
TC
1230
1231 \$a = \$b or \$c; # bug: this is wrong
1232 (\$a = \$b) or \$c; # really means this
1233 \$a = \$b || \$c; # better written this way
1234
19799a22 1235However, when it's a list-context assignment and you're trying to use
c543c01b 1236C<||> for control flow, you probably need "or" so that the assignment
5a964f20
TC
1237takes higher precedence.
1238
1239 @info = stat(\$file) || die; # oops, scalar sense of stat!
1240 @info = stat(\$file) or die; # better, now @info gets its due
1241
c963b151
BD
1242Then again, you could always use parentheses.
1243
1ca345ed 1244Binary C<xor> returns the exclusive-OR of the two surrounding expressions.
c543c01b 1245It cannot short-circuit (of course).
a0d0e21e 1246
59ab9d6e
MB
1247There is no low precedence operator for defined-OR.
1248
a0d0e21e 1249=head2 C Operators Missing From Perl
d74e8afc
ITB
1250X<operator, missing from perl> X<&> X<*>
1251X<typecasting> X<(TYPE)>
a0d0e21e
LW
1252
1253Here is what C has that Perl doesn't:
1254
1255=over 8
1256
1257=item unary &
1258
1259Address-of operator. (But see the "\" operator for taking a reference.)
1260
1261=item unary *
1262
54310121 1263Dereference-address operator. (Perl's prefix dereferencing
a0d0e21e
LW
1264operators are typed: \$, @, %, and &.)
1265
1266=item (TYPE)
1267
19799a22 1268Type-casting operator.
a0d0e21e
LW
1269
1270=back
1271
5f05dabc 1272=head2 Quote and Quote-like Operators
89d205f2 1273X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m>
d74e8afc
ITB
1274X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>>
1275X<escape sequence> X<escape>
1276
a0d0e21e
LW
1277While we usually think of quotes as literal values, in Perl they
1278function as operators, providing various kinds of interpolating and
1279pattern matching capabilities. Perl provides customary quote characters
1280for these behaviors, but also provides a way for you to choose your
1281quote character for any of them. In the following table, a C<{}> represents
9f10b797 1282any pair of delimiters you choose.
a0d0e21e 1283
2c268ad5
TP
1284 Customary Generic Meaning Interpolates
1285 '' q{} Literal no
1286 "" qq{} Literal yes
af9219ee 1287 `` qx{} Command yes*
2c268ad5 1288 qw{} Word list no
af9219ee
MG
1289 // m{} Pattern match yes*
1290 qr{} Pattern yes*
1291 s{}{} Substitution yes*
2c268ad5 1292 tr{}{} Transliteration no (but see below)
c543c01b 1293 y{}{} Transliteration no (but see below)
7e3b091d 1294 <<EOF here-doc yes*
a0d0e21e 1295
af9219ee
MG
1296 * unless the delimiter is ''.
1297
87275199 1298Non-bracketing delimiters use the same character fore and aft, but the four
c543c01b 1299sorts of ASCII brackets (round, angle, square, curly) all nest, which means
9f10b797 1300that
87275199 1301
c543c01b 1302 q{foo{bar}baz}
35f2feb0 1303
9f10b797 1304is the same as
87275199 1305
c543c01b 1306 'foo{bar}baz'
87275199
GS
1307
1308Note, however, that this does not always work for quoting Perl code:
1309
c543c01b 1310 \$s = q{ if(\$a eq "}") ... }; # WRONG
87275199 1311
c543c01b
TC
1312is a syntax error. The C<Text::Balanced> module (standard as of v5.8,
1313and from CPAN before then) is able to do this properly.
87275199 1314
19799a22 1315There can be whitespace between the operator and the quoting
fb73857a 1316characters, except when C<#> is being used as the quoting character.
19799a22
GS
1317C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
1318operator C<q> followed by a comment. Its argument will be taken
1319from the next line. This allows you to write:
fb73857a
PP
1320
1321 s {foo} # Replace foo
1322 {bar} # with bar.
1323
c543c01b
TC
1324The following escape sequences are available in constructs that interpolate,
1325and in transliterations:
5691ca5f 1326X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N> X<\N{}>
04341565 1327X<\o{}>
5691ca5f 1328
2c4c1ff2
KW
1329 Sequence Note Description
1330 \t tab (HT, TAB)
1331 \n newline (NL)
1332 \r return (CR)
1333 \f form feed (FF)
1334 \b backspace (BS)
1335 \a alarm (bell) (BEL)
1336 \e escape (ESC)
c543c01b 1337 \x{263A} [1,8] hex char (example: SMILEY)
2c4c1ff2 1338 \x1b [2,8] restricted range hex char (example: ESC)
fb121860 1339 \N{name} [3] named Unicode character or character sequence
2c4c1ff2
KW
1340 \N{U+263D} [4,8] Unicode character (example: FIRST QUARTER MOON)
1341 \c[ [5] control char (example: chr(27))
1342 \o{23072} [6,8] octal char (example: SMILEY)
1343 \033 [7,8] restricted range octal char (example: ESC)
5691ca5f
KW
1344
1345=over 4
1346
1347=item [1]
1348
2c4c1ff2
KW
1349The result is the character specified by the hexadecimal number between
1350the braces. See L</[8]> below for details on which character.
96448467
DG
1351
1352Only hexadecimal digits are valid between the braces. If an invalid
1353character is encountered, a warning will be issued and the invalid
1354character and all subsequent characters (valid or invalid) within the
1355braces will be discarded.
1356
1357If there are no valid digits between the braces, the generated character is
1358the NULL character (C<\x{00}>). However, an explicit empty brace (C<\x{}>)
c543c01b 1359will not cause a warning (currently).
40687185
KW
1360
1361=item [2]
1362
2c4c1ff2
KW
1363The result is the character specified by the hexadecimal number in the range
13640x00 to 0xFF. See L</[8]> below for details on which character.
96448467
DG
1365
1366Only hexadecimal digits are valid following C<\x>. When C<\x> is followed
2c4c1ff2 1367by fewer than two valid digits, any valid digits will be zero-padded. This
c543c01b 1368means that C<\x7> will be interpreted as C<\x07>, and a lone <\x> will be
2c4c1ff2 1369interpreted as C<\x00>. Except at the end of a string, having fewer than
c543c01b 1370two valid digits will result in a warning. Note that although the warning
96448467
DG
1371says the illegal character is ignored, it is only ignored as part of the
1372escape and will still be used as the subsequent character in the string.
1373For example:
1374
1375 Original Result Warns?
1376 "\x7" "\x07" no
1377 "\x" "\x00" no
1378 "\x7q" "\x07q" yes
1379 "\xq" "\x00q" yes
1380
40687185
KW
1381=item [3]
1382
fb121860 1383The result is the Unicode character or character sequence given by I<name>.
2c4c1ff2 1384See L<charnames>.
40687185
KW
1385
1386=item [4]
1387
2c4c1ff2
KW
1388C<\N{U+I<hexadecimal number>}> means the Unicode character whose Unicode code
1389point is I<hexadecimal number>.
40687185
KW
1390
1391=item [5]
1392
5691ca5f
KW
1393The character following C<\c> is mapped to some other character as shown in the
1394table:
1395
1396 Sequence Value
1397 \c@ chr(0)
1398 \cA chr(1)
1399 \ca chr(1)
1400 \cB chr(2)
1401 \cb chr(2)
1402 ...
1403 \cZ chr(26)
1404 \cz chr(26)
1405 \c[ chr(27)
1406 \c] chr(29)
1407 \c^ chr(30)
1408 \c? chr(127)
1409
d813941f 1410In other words, it's the character whose code point has had 64 xor'd with
8df27208 1411its uppercase. C<\c?> is DELETE because C<ord("?") ^ 64> is 127, and
d813941f
KW
1412C<\c@> is NULL because the ord of "@" is 64, so xor'ing 64 itself produces 0.
1413
5691ca5f
KW
1414Also, C<\c\I<X>> yields C< chr(28) . "I<X>"> for any I<X>, but cannot come at the
1415end of a string, because the backslash would be parsed as escaping the end
1416quote.
1417
1418On ASCII platforms, the resulting characters from the list above are the
1419complete set of ASCII controls. This isn't the case on EBCDIC platforms; see
1420L<perlebcdic/OPERATOR DIFFERENCES> for the complete list of what these
1421sequences mean on both ASCII and EBCDIC platforms.
1422
1423Use of any other character following the "c" besides those listed above is
17a3df4c 1424discouraged, and some are deprecated with the intention of removing
9fef6a0d 1425those in a later Perl version. What happens for any of these
d813941f
KW
1426other characters currently though, is that the value is derived by xor'ing
1427with the seventh bit, which is 64.
5691ca5f
KW
1428
1429To get platform independent controls, you can use C<\N{...}>.
1430
40687185
KW
1431=item [6]
1432
2c4c1ff2
KW
1433The result is the character specified by the octal number between the braces.
1434See L</[8]> below for details on which character.
04341565
DG
1435
1436If a character that isn't an octal digit is encountered, a warning is raised,
1437and the value is based on the octal digits before it, discarding it and all
1438following characters up to the closing brace. It is a fatal error if there are
1439no octal digits at all.
1440
1441=item [7]
1442
c543c01b 1443The result is the character specified by the three-digit octal number in the
2c4c1ff2
KW
1444range 000 to 777 (but best to not use above 077, see next paragraph). See
1445L</[8]> below for details on which character.
1446
1447Some contexts allow 2 or even 1 digit, but any usage without exactly
40687185 1448three digits, the first being a zero, may give unintended results. (For
5db3e519
FC
1449example, in a regular expression it may be confused with a backreference;
1450see L<perlrebackslash/Octal escapes>.) Starting in Perl 5.14, you may
c543c01b 1451use C<\o{}> instead, which avoids all these problems. Otherwise, it is best to
04341565
DG
1452use this construct only for ordinals C<\077> and below, remembering to pad to
1453the left with zeros to make three digits. For larger ordinals, either use
9fef6a0d 1454C<\o{}>, or convert to something else, such as to hex and use C<\x{}>
04341565 1455instead.
40687185 1456
40687185
KW
1457Having fewer than 3 digits may lead to a misleading warning message that says
1458that what follows is ignored. For example, C<"\128"> in the ASCII character set
1459is equivalent to the two characters C<"\n8">, but the warning C<Illegal octal
5db3e519
FC
1460digit '8' ignored> will be thrown. If C<"\n8"> is what you want, you can
1461avoid this warning by padding your octal number with C<0>'s: C<"\0128">.
5691ca5f 1462
2c4c1ff2
KW
1463=item [8]
1464
c543c01b 1465Several constructs above specify a character by a number. That number
2c4c1ff2 1466gives the character's position in the character set encoding (indexed from 0).
c543c01b 1467This is called synonymously its ordinal, code position, or code point. Perl
2c4c1ff2
KW
1468works on platforms that have a native encoding currently of either ASCII/Latin1
1469or EBCDIC, each of which allow specification of 256 characters. In general, if
1470the number is 255 (0xFF, 0377) or below, Perl interprets this in the platform's
1471native encoding. If the number is 256 (0x100, 0400) or above, Perl interprets
c543c01b 1472it as a Unicode code point and the result is the corresponding Unicode
2c4c1ff2
KW
1473character. For example C<\x{50}> and C<\o{120}> both are the number 80 in
1474decimal, which is less than 256, so the number is interpreted in the native
1475character set encoding. In ASCII the character in the 80th position (indexed
1476from 0) is the letter "P", and in EBCDIC it is the ampersand symbol "&".
1477C<\x{100}> and C<\o{400}> are both 256 in decimal, so the number is interpreted
1478as a Unicode code point no matter what the native encoding is. The name of the
9fef6a0d 1479character in the 256th position (indexed by 0) in Unicode is
2c4c1ff2
KW
1480C<LATIN CAPITAL LETTER A WITH MACRON>.
1481
9fef6a0d 1482There are a couple of exceptions to the above rule. S<C<\N{U+I<hex number>}>> is
2c4c1ff2
KW
1483always interpreted as a Unicode code point, so that C<\N{U+0050}> is "P" even
1484on EBCDIC platforms. And if L<C<S<use encoding>>|encoding> is in effect, the
1485number is considered to be in that encoding, and is translated from that into
1486the platform's native encoding if there is a corresponding native character;
1487otherwise to Unicode.
1488
5691ca5f 1489=back
4c77eaa2 1490
e526e8bb 1491B<NOTE>: Unlike C and other languages, Perl has no C<\v> escape sequence for
8b312c40
KW
1492the vertical tab (VT, which is 11 in both ASCII and EBCDIC), but you may
1493use C<\ck> or
1494C<\x0b>. (C<\v>
e526e8bb
KW
1495does have meaning in regular expression patterns in Perl, see L<perlre>.)
1496
1497The following escape sequences are available in constructs that interpolate,
904501ec 1498but not in transliterations.
628253b8 1499X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q> X<\F>
904501ec 1500
c543c01b
TC
1501 \l lowercase next character only
1502 \u titlecase (not uppercase!) next character only
e4d34742
EB
1503 \L lowercase all characters till \E or end of string
1504 \U uppercase all characters till \E or end of string
628253b8 1505 \F foldcase all characters till \E or end of string
736fe711
KW
1506 \Q quote (disable) pattern metacharacters till \E or
1507 end of string
7e31b643 1508 \E end either case modification or quoted section
c543c01b
TC
1509 (whichever was last seen)
1510
736fe711
KW
1511See L<perlfunc/quotemeta> for the exact definition of characters that
1512are quoted by C<\Q>.
1513
628253b8 1514C<\L>, C<\U>, C<\F>, and C<\Q> can stack, in which case you need one
c543c01b
TC
1515C<\E> for each. For example:
1516
9fef6a0d
KW
1517 say"This \Qquoting \ubusiness \Uhere isn't quite\E done yet,\E is it?";
1518 This quoting\ Business\ HERE\ ISN\'T\ QUITE\ done\ yet\, is it?
a0d0e21e 1519
66cbab2c
KW
1520If C<use locale> is in effect (but not C<use locale ':not_characters'>),
1521the case map used by C<\l>, C<\L>,
c543c01b 1522C<\u>, and C<\U> is taken from the current locale. See L<perllocale>.
b6538e4f 1523If Unicode (for example, C<\N{}> or code points of 0x100 or
c543c01b
TC
1524beyond) is being used, the case map used by C<\l>, C<\L>, C<\u>, and
1525C<\U> is as defined by Unicode. That means that case-mapping
1526a single character can sometimes produce several characters.
628253b8 1527Under C<use locale>, C<\F> produces the same results as C<\L>.
a034a98d 1528
5a964f20
TC
1529All systems use the virtual C<"\n"> to represent a line terminator,
1530called a "newline". There is no such thing as an unvarying, physical
19799a22 1531newline character. It is only an illusion that the operating system,
5a964f20
TC
1532device drivers, C libraries, and Perl all conspire to preserve. Not all
1533systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
c543c01b
TC
1534on the ancient Macs (pre-MacOS X) of yesteryear, these used to be reversed,
1535and on systems without line terminator,
1536printing C<"\n"> might emit no actual data. In general, use C<"\n"> when
5a964f20
TC
1537you mean a "newline" for your system, but use the literal ASCII when you
1538need an exact character. For example, most networking protocols expect
2a380090 1539and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
5a964f20
TC
1540and although they often accept just C<"\012">, they seldom tolerate just
1541C<"\015">. If you get in the habit of using C<"\n"> for networking,
1542you may be burned some day.
d74e8afc
ITB
1543X<newline> X<line terminator> X<eol> X<end of line>
1544X<\n> X<\r> X<\r\n>
5a964f20 1545
904501ec
MG
1546For constructs that do interpolate, variables beginning with "C<\$>"
1547or "C<@>" are interpolated. Subscripted variables such as C<\$a[3]> or
ad0f383a
A
1548C<< \$href->{key}[0] >> are also interpolated, as are array and hash slices.
1549But method calls such as C<< \$obj->meth >> are not.
af9219ee
MG
1550
1551Interpolating an array or slice interpolates the elements in order,
1552separated by the value of C<\$">, so is equivalent to interpolating
c543c01b
TC
1553C<join \$", @array>. "Punctuation" arrays such as C<@*> are usually
1554interpolated only if the name is enclosed in braces C<@{*}>, but the
1555arrays C<@_>, C<@+>, and C<@-> are interpolated even without braces.
af9219ee 1556
bc7b91c6
EB
1557For double-quoted strings, the quoting from C<\Q> is applied after
1558interpolation and escapes are processed.
1559
1560 "abc\Qfoo\tbar\$s\Exyz"
1561
1562is equivalent to
1563
1564 "abc" . quotemeta("foo\tbar\$s") . "xyz"
1565
1566For the pattern of regex operators (C<qr//>, C<m//> and C<s///>),
1567the quoting from C<\Q> is applied after interpolation is processed,
1568but before escapes are processed. This allows the pattern to match
1569literally (except for C<\$> and C<@>). For example, the following matches:
1570
1571 '\s\t' =~ /\Q\s\t/
1572
1573Because C<\$> or C<@> trigger interpolation, you'll need to use something
1574like C</\Quser\E\@\Qhost/> to match them literally.
1d2dff63 1575
a0d0e21e
LW
1576Patterns are subject to an additional level of interpretation as a
1577regular expression. This is done as a second pass, after variables are
1578interpolated, so that regular expressions may be incorporated into the
1579pattern from the variables. If this is not what you want, use C<\Q> to
1580interpolate a variable literally.
1581
19799a22
GS
1582Apart from the behavior described above, Perl does not expand
1583multiple levels of interpolation. In particular, contrary to the
1584expectations of shell programmers, back-quotes do I<NOT> interpolate
1585within double quotes, nor do single quotes impede evaluation of
1586variables when used within double quotes.
a0d0e21e 1587
5f05dabc 1588=head2 Regexp Quote-Like Operators
d74e8afc 1589X<operator, regexp>
cb1a09d0 1590
5f05dabc 1591Here are the quote-like operators that apply to pattern
cb1a09d0
AD
1592matching and related activities.
1593
a0d0e21e
LW
1594=over 8
1595
b6fa137b 1596=item qr/STRING/msixpodual
01c6f5f4 1597X<qr> X</i> X</m> X</o> X</s> X</x> X</p>
a0d0e21e 1598
87e95b7f
YO
1599This operator quotes (and possibly compiles) its I<STRING> as a regular
1600expression. I<STRING> is interpolated the same way as I<PATTERN>
1601in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
1602is done. Returns a Perl value which may be used instead of the
f6050459 1603corresponding C</STRING/msixpodual> expression. The returned value is a
85dd5c8b 1604normalized version of the original pattern. It magically differs from
1c8ee595
CO
1605a string containing the same characters: C<ref(qr/x/)> returns "Regexp";
1606however, dereferencing it is not well defined (you currently get the
1607normalized version of the original pattern, but this may change).
1608
a0d0e21e 1609
87e95b7f
YO
1610For example,
1611
1612 \$rex = qr/my.STRING/is;
85dd5c8b 1613 print \$rex; # prints (?si-xm:my.STRING)
87e95b7f
YO
1614 s/\$rex/foo/;
1615
1616is equivalent to
1617
1618 s/my.STRING/foo/is;
1619
1620The result may be used as a subpattern in a match:
1621
1622 \$re = qr/\$pattern/;
7188ca43
KW
1623 \$string =~ /foo\${re}bar/; # can be interpolated in other
1624 # patterns
87e95b7f
YO
1625 \$string =~ \$re; # or used standalone
1626 \$string =~ /\$re/; # or this way
1627
f6050459 1628Since Perl may compile the pattern at the moment of execution of the qr()
87e95b7f
YO
1629operator, using qr() may have speed advantages in some situations,
1630notably if the result of qr() is used standalone:
1631
1632 sub match {
1633 my \$patterns = shift;
1634 my @compiled = map qr/\$_/i, @\$patterns;
1635 grep {
1636 my \$success = 0;
1637 foreach my \$pat (@compiled) {
1638 \$success = 1, last if /\$pat/;
1639 }
1640 \$success;
1641 } @_;
5a964f20
TC
1642 }
1643
87e95b7f
YO
1644Precompilation of the pattern into an internal representation at
1645the moment of qr() avoids a need to recompile the pattern every
1646time a match C</\$pat/> is attempted. (Perl has many other internal
1647optimizations, but none would be triggered in the above example if
1648we did not use qr() operator.)
1649
765fa144 1650Options (specified by the following modifiers) are:
87e95b7f
YO
1651
1652 m Treat string as multiple lines.
1653 s Treat string as single line. (Make . match a newline)
1654 i Do case-insensitive pattern matching.
1655 x Use extended regular expressions.
1656 p When matching preserve a copy of the matched string so
7188ca43
KW
1657 that \${^PREMATCH}, \${^MATCH}, \${^POSTMATCH} will be
1658 defined.
87e95b7f 1659 o Compile pattern only once.
7188ca43
KW
1660 a ASCII-restrict: Use ASCII for \d, \s, \w; specifying two
1661 a's further restricts /i matching so that no ASCII
48cbae4f
SK
1662 character will match a non-ASCII one.
1663 l Use the locale.
1664 u Use Unicode rules.
1665 d Use Unicode or native charset, as in 5.12 and earlier.
87e95b7f
YO
1666
1667If a precompiled pattern is embedded in a larger pattern then the effect
c543c01b 1668of "msixpluad" will be propagated appropriately. The effect the "o"
87e95b7f
YO
1669modifier has is not propagated, being restricted to those patterns
1670explicitly using it.
1671
b6fa137b 1672The last four modifiers listed above, added in Perl 5.14,
18509dec
KW
1673control the character set semantics, but C</a> is the only one you are likely
1674to want to specify explicitly; the other three are selected
1675automatically by various pragmas.
da392a17 1676
87e95b7f 1677See L<perlre> for additional information on valid syntax for STRING, and
5e2aa8f5 1678for a detailed look at the semantics of regular expressions. In
1ca345ed
TC
1679particular, all modifiers except the largely obsolete C</o> are further
1680explained in L<perlre/Modifiers>. C</o> is described in the next section.
a0d0e21e 1681
b6fa137b 1682=item m/PATTERN/msixpodualgc
89d205f2
YO
1683X<m> X<operator, match>
1684X<regexp, options> X<regexp> X<regex, options> X<regex>
01c6f5f4 1685X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c>
a0d0e21e 1686
b6fa137b 1687=item /PATTERN/msixpodualgc
a0d0e21e 1688
5a964f20 1689Searches a string for a pattern match, and in scalar context returns
19799a22
GS
1690true if it succeeds, false if it fails. If no string is specified
1691via the C<=~> or C<!~> operator, the \$_ string is searched. (The
1692string specified with C<=~> need not be an lvalue--it may be the
1693result of an expression evaluation, but remember the C<=~> binds
006671a6 1694rather tightly.) See also L<perlre>.
a0d0e21e 1695
f6050459 1696Options are as described in C<qr//> above; in addition, the following match
01c6f5f4 1697process modifiers are available:
a0d0e21e 1698
950b09ed 1699 g Match globally, i.e., find all occurrences.
7188ca43
KW
1700 c Do not reset search position on a failed match when /g is
1701 in effect.
a0d0e21e 1702
725a61d7 1703If "/" is the delimiter then the initial C<m> is optional. With the C<m>
c543c01b 1704you can use any pair of non-whitespace (ASCII) characters
725a61d7
Z
1705as delimiters. This is particularly useful for matching path names
1706that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
1707the delimiter, then a match-only-once rule applies,
1708described in C<m?PATTERN?> below.
19799a22 1709If "'" is the delimiter, no interpolation is performed on the PATTERN.
ed02a3bf
DN
1710When using a character valid in an identifier, whitespace is required
1711after the C<m>.
a0d0e21e 1712
532c9e80
KW
1713PATTERN may contain variables, which will be interpolated
1714every time the pattern search is evaluated, except
1f247705
GS
1715for when the delimiter is a single quote. (Note that C<\$(>, C<\$)>, and
1716C<\$|> are not interpolated because they look like end-of-string tests.)
532c9e80
KW
1717Perl will not recompile the pattern unless an interpolated
1718variable that it contains changes. You can force Perl to skip the
1719test and never recompile by adding a C</o> (which stands for "once")
1720after the trailing delimiter.
1721Once upon a time, Perl would recompile regular expressions
1722unnecessarily, and this modifier was useful to tell it not to do so, in the
1723interests of speed. But now, the only reasons to use C</o> are either:
1724
1725=over
1726
1727=item 1
1728
1729The variables are thousands of characters long and you know that they
1730don't change, and you need to wring out the last little bit of speed by
1731having Perl skip testing for that. (There is a maintenance penalty for
1732doing this, as mentioning C</o> constitutes a promise that you won't
18509dec 1733change the variables in the pattern. If you do change them, Perl won't
532c9e80
KW
1734even notice.)
1735
1736=item 2
1737
1738you want the pattern to use the initial values of the variables
1739regardless of whether they change or not. (But there are saner ways
1740of accomplishing this than using C</o>.)
1741
fa9b8686
DM
1742=item 3
1743
1744If the pattern contains embedded code, such as
1745
1746 use re 'eval';
1747 \$code = 'foo(?{ \$x })';
1748 /\$code/
1749
1750then perl will recompile each time, even though the pattern string hasn't
1751changed, to ensure that the current value of C<\$x> is seen each time.
1752Use C</o> if you want to avoid this.
1753
532c9e80 1754=back
a0d0e21e 1755
18509dec
KW
1756The bottom line is that using C</o> is almost never a good idea.
1757
e9d89077
DN
1758=item The empty pattern //
1759
5a964f20 1760If the PATTERN evaluates to the empty string, the last
d65afb4b 1761I<successfully> matched regular expression is used instead. In this
c543c01b 1762case, only the C<g> and C<c> flags on the empty pattern are honored;
d65afb4b
HS
1763the other flags are taken from the original pattern. If no match has
1764previously succeeded, this will (silently) act instead as a genuine
1765empty pattern (which will always match).
a0d0e21e 1766
89d205f2
YO
1767Note that it's possible to confuse Perl into thinking C<//> (the empty
1768regex) is really C<//> (the defined-or operator). Perl is usually pretty
1769good about this, but some pathological cases might trigger this, such as
1770C<\$a///> (is that C<(\$a) / (//)> or C<\$a // />?) and C<print \$fh //>
1771(C<print \$fh(//> or C<print(\$fh //>?). In all of these examples, Perl
1772will assume you meant defined-or. If you meant the empty regex, just
1773use parentheses or spaces to disambiguate, or even prefix the empty
c963b151
BD
1774regex with an C<m> (so C<//> becomes C<m//>).
1775
e9d89077
DN
1776=item Matching in list context
1777
19799a22 1778If the C</g> option is not used, C<m//> in list context returns a
a0d0e21e 1779list consisting of the subexpressions matched by the parentheses in the
3ff8ecf9
BF
1780pattern, that is, (C<\$1>, C<\$2>, C<\$3>...) (Note that here C<\$1> etc. are
1781also set). When there are no parentheses in the pattern, the return
1782value is the list C<(1)> for success.
1783With or without parentheses, an empty list is returned upon failure.
a0d0e21e
LW
1784
1785Examples:
1786
7188ca43
KW
1787 open(TTY, "+</dev/tty")
1788 || die "can't access /dev/tty: \$!";
c543c01b 1789
7188ca43 1790 <TTY> =~ /^y/i && foo(); # do foo if desired
a0d0e21e 1791
7188ca43 1792 if (/Version: *([0-9.]*)/) { \$version = \$1; }
a0d0e21e 1793
7188ca43 1794 next if m#^/usr/spool/uucp#;
a0d0e21e 1795
7188ca43
KW
1796 # poor man's grep
1797 \$arg = shift;
1798 while (<>) {
1799 print if /\$arg/o; # compile only once (no longer needed!)
1800 }
a0d0e21e 1801
7188ca43 1802 if ((\$F1, \$F2, \$Etc) = (\$foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
a0d0e21e
LW
1803
1804This last example splits \$foo into the first two words and the
5f05dabc 1805remainder of the line, and assigns those three fields to \$F1, \$F2, and
c543c01b
TC
1806\$Etc. The conditional is true if any variables were assigned; that is,
1807if the pattern matched.
a0d0e21e 1808
19799a22 1809The C</g> modifier specifies global pattern matching--that is,
3dd93342 1810matching as many times as possible within the string. How it behaves
1811depends on the context. In list context, it returns a list of the
19799a22 1812substrings matched by any capturing parentheses in the regular
3dd93342 1813expression. If there are no parentheses, it returns a list of all
19799a22
GS
1814the matched strings, as if there were parentheses around the whole
1815pattern.
a0d0e21e 1816
7e86de3e 1817In scalar context, each execution of C<m//g> finds the next match,
19799a22 1818returning true if it matches, and false if there is no further match.
3dd93342 1819The position after the last match can be read or set using the C<pos()>
1820function; see L<perlfunc/pos>. A failed match normally resets the
7e86de3e 1821search position to the beginning of the string, but you can avoid that
1ca345ed 1822by adding the C</c> modifier (for example, C<m//gc>). Modifying the target
7e86de3e 1823string also resets the search position.
c90c0ff4 1824
e9d89077
DN
1825=item \G assertion
1826
c90c0ff4 1827You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
3dd93342 1828zero-width assertion that matches the exact position where the
1829previous C<m//g>, if any, left off. Without the C</g> modifier, the
1830C<\G> assertion still anchors at C<pos()> as it was at the start of
1831the operation (see L<perlfunc/pos>), but the match is of course only
1832attempted once. Using C<\G> without C</g> on a target string that has
1833not previously had a C</g> match applied to it is the same as using
1834the C<\A> assertion to match the beginning of the string. Note also
1835that, currently, C<\G> is only properly supported when anchored at the
1836very beginning of the pattern.
c90c0ff4
PP
1837
1838Examples:
a0d0e21e
LW
1839
1840 # list context
1841 (\$one,\$five,\$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
1842
1843 # scalar context
c543c01b
TC
1844 local \$/ = "";
1845 while (\$paragraph = <>) {
1846 while (\$paragraph =~ /\p{Ll}['")]*[.!?]+['")]*\s/g) {
19799a22 1847 \$sentences++;
a0d0e21e
LW
1848 }
1849 }
c543c01b
TC
1850 say \$sentences;
1851
1852Here's another way to check for sentences in a paragraph:
1853
7188ca43
KW
1854 my \$sentence_rx = qr{
1855 (?: (?<= ^ ) | (?<= \s ) ) # after start-of-string or
1856 # whitespace
1857 \p{Lu} # capital letter
1858 .*? # a bunch of anything
1859 (?<= \S ) # that ends in non-
1860 # whitespace
1861 (?<! \b [DMS]r ) # but isn't a common abbr.
1862 (?<! \b Mrs )
1863 (?<! \b Sra )
1864 (?<! \b St )
1865 [.?!] # followed by a sentence
1866 # ender
1867 (?= \$ | \s ) # in front of end-of-string
1868 # or whitespace
1869 }sx;
1870 local \$/ = "";
1871 while (my \$paragraph = <>) {
1872 say "NEW PARAGRAPH";
1873 my \$count = 0;
1874 while (\$paragraph =~ /(\$sentence_rx)/g) {
1875 printf "\tgot sentence %d: <%s>\n", ++\$count, \$1;
c543c01b 1876 }
7188ca43 1877 }
c543c01b
TC
1878
1879Here's how to use C<m//gc> with C<\G>:
a0d0e21e 1880
137443ea 1881 \$_ = "ppooqppqq";
44a8e56a
PP
1882 while (\$i++ < 2) {
1883 print "1: '";
c90c0ff4 1884 print \$1 while /(o)/gc; print "', pos=", pos, "\n";
44a8e56a 1885 print "2: '";
c90c0ff4 1886 print \$1 if /\G(q)/gc; print "', pos=", pos, "\n";
44a8e56a 1887 print "3: '";
c90c0ff4 1888 print \$1 while /(p)/gc; print "', pos=", pos, "\n";
44a8e56a 1889 }
5d43e42d 1890 print "Final: '\$1', pos=",pos,"\n" if /\G(.)/;
44a8e56a
PP
1891
1892The last example should print:
1893
1894 1: 'oo', pos=4
137443ea 1895 2: 'q', pos=5
44a8e56a
PP
1896 3: 'pp', pos=7
1897 1: '', pos=7
137443ea
PP
1898 2: 'q', pos=8
1899 3: '', pos=8
5d43e42d
DC
1900 Final: 'q', pos=8
1901
1902Notice that the final match matched C<q> instead of C<p>, which a match
1903without the C<\G> anchor would have done. Also note that the final match
ac036724 1904did not update C<pos>. C<pos> is only updated on a C</g> match. If the
c543c01b
TC
1905final match did indeed match C<p>, it's a good bet that you're running a
1906very old (pre-5.6.0) version of Perl.
44a8e56a 1907
c90c0ff4 1908A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
e7ea3e70 1909combine several regexps like this to process a string part-by-part,
c90c0ff4
PP
1910doing different actions depending on which regexp matched. Each
1911regexp tries to match where the previous one leaves off.
e7ea3e70 1912
3fe9a6f1 1913 \$_ = <<'EOL';
7188ca43
KW
1914 \$url = URI::URL->new( "http://example.com/" );
1915 die if \$url eq "xXx";
3fe9a6f1 1916 EOL
c543c01b
TC
1917
1918 LOOP: {
950b09ed 1919 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
7188ca43
KW
1920 print(" lowercase"), redo LOOP
1921 if /\G\p{Ll}+\b[,.;]?\s*/gc;
1922 print(" UPPERCASE"), redo LOOP
1923 if /\G\p{Lu}+\b[,.;]?\s*/gc;
1924 print(" Capitalized"), redo LOOP
1925 if /\G\p{Lu}\p{Ll}+\b[,.;]?\s*/gc;
c543c01b 1926 print(" MiXeD"), redo LOOP if /\G\pL+\b[,.;]?\s*/gc;
7188ca43
KW
1927 print(" alphanumeric"), redo LOOP
1928 if /\G[\p{Alpha}\pN]+\b[,.;]?\s*/gc;
c543c01b 1929 print(" line-noise"), redo LOOP if /\G\W+/gc;
950b09ed 1930 print ". That's all!\n";
c543c01b 1931 }
e7ea3e70
IZ
1932
1933Here is the output (split into several lines):
1934
7188ca43
KW
1935 line-noise lowercase line-noise UPPERCASE line-noise UPPERCASE
1936 line-noise lowercase line-noise lowercase line-noise lowercase
1937 lowercase line-noise lowercase lowercase line-noise lowercase
1938 lowercase line-noise MiXeD line-noise. That's all!
44a8e56a 1939
c543c01b 1940=item m?PATTERN?msixpodualgc
725a61d7 1941X<?> X<operator, match-once>
87e95b7f 1942
c543c01b 1943=item ?PATTERN?msixpodualgc
55d389e7 1944
725a61d7
Z
1945This is just like the C<m/PATTERN/> search, except that it matches
1946only once between calls to the reset() operator. This is a useful
87e95b7f 1947optimization when you want to see only the first occurrence of
ceb131e8 1948something in each file of a set of files, for instance. Only C<m??>
87e95b7f
YO
1949patterns local to the current package are reset.
1950
1951 while (<>) {
ceb131e8 1952 if (m?^\$?) {
87e95b7f
YO
1953 # blank line between header and body
1954 }
1955 } continue {
725a61d7 1956 reset if eof; # clear m?? status for next file
87e95b7f
YO
1957 }
1958
c543c01b
TC
1959Another example switched the first "latin1" encoding it finds
1960to "utf8" in a pod file:
1961
1962 s//utf8/ if m? ^ =encoding \h+ \K latin1 ?x;
1963
1964The match-once behavior is controlled by the match delimiter being
725a61d7
Z
1965C<?>; with any other delimiter this is the normal C<m//> operator.
1966
1967For historical reasons, the leading C<m> in C<m?PATTERN?> is optional,
1968but the resulting C<?PATTERN?> syntax is deprecated, will warn on
c543c01b
TC
1969usage and might be removed from a future stable release of Perl (without
1970further notice!).
87e95b7f 1971
b6fa137b 1972=item s/PATTERN/REPLACEMENT/msixpodualgcer
87e95b7f 1973X<substitute> X<substitution> X<replace> X<regexp, replace>
4f4d7508 1974X<regexp, substitute> X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> X</e> X</r>
87e95b7f
YO
1975
1976Searches a string for a pattern, and if found, replaces that pattern
1977with the replacement text and returns the number of substitutions
1978made. Otherwise it returns false (specifically, the empty string).
1979
c543c01b 1980If the C</r> (non-destructive) option is used then it runs the
679563bb
KW
1981substitution on a copy of the string and instead of returning the
1982number of substitutions, it returns the copy whether or not a
c543c01b
TC
1983substitution occurred. The original string is never changed when
1984C</r> is used. The copy will always be a plain string, even if the
1985input is an object or a tied variable.
4f4d7508 1986
87e95b7f 1987If no string is specified via the C<=~> or C<!~> operator, the C<\$_>
c543c01b
TC
1988variable is searched and modified. Unless the C</r> option is used,
1989the string specified must be a scalar variable, an array element, a
1990hash element, or an assignment to one of those; that is, some sort of
1991scalar lvalue.
87e95b7f
YO
1992
1993If the delimiter chosen is a single quote, no interpolation is
1994done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1995PATTERN contains a \$ that looks like a variable rather than an
1996end-of-string test, the variable will be interpolated into the pattern
1997at run-time. If you want the pattern compiled only once the first time
1998the variable is interpolated, use the C</o> option. If the pattern
1999evaluates to the empty string, the last successfully executed regular
2000expression is used instead. See L<perlre> for further explanation on these.
87e95b7f
YO
2001
2002Options are as with m// with the addition of the following replacement
2003specific options:
2004
2005 e Evaluate the right side as an expression.
7188ca43
KW
2006 ee Evaluate the right side as a string then eval the
2007 result.
2008 r Return substitution and leave the original string
2009 untouched.
87e95b7f 2010
ed02a3bf
DN
2011Any non-whitespace delimiter may replace the slashes. Add space after
2012the C<s> when using a character allowed in identifiers. If single quotes
2013are used, no interpretation is done on the replacement string (the C</e>
3ff8ecf9 2014modifier overrides this, however). Note that Perl treats backticks
ed02a3bf
DN
2015as normal delimiters; the replacement text is not evaluated as a command.
2016If the PATTERN is delimited by bracketing quotes, the REPLACEMENT has
1ca345ed 2017its own pair of quotes, which may or may not be bracketing quotes, for example,
87e95b7f
YO
2018C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
2019replacement portion to be treated as a full-fledged Perl expression
2020and evaluated right then and there. It is, however, syntax checked at
2021compile-time. A second C<e> modifier will cause the replacement portion
2022to be C<eval>ed before being run as a Perl expression.
2023
2024Examples:
2025
7188ca43 2026 s/\bgreen\b/mauve/g; # don't change wintergreen
87e95b7f
YO
2027
2028 \$path =~ s|/usr/bin|/usr/local/bin|;
2029
2030 s/Login: \$foo/Login: \$bar/; # run-time pattern
2031
7188ca43
KW
2032 (\$foo = \$bar) =~ s/this/that/; # copy first, then
2033 # change
2034 (\$foo = "\$bar") =~ s/this/that/; # convert to string,
2035 # copy, then change
4f4d7508
DC
2036 \$foo = \$bar =~ s/this/that/r; # Same as above using /r
2037 \$foo = \$bar =~ s/this/that/r
7188ca43
KW
2038 =~ s/that/the other/r; # Chained substitutes
2039 # using /r
2040 @foo = map { s/this/that/r } @bar # /r is very useful in
2041 # maps
87e95b7f 2042
7188ca43 2043 \$count = (\$paragraph =~ s/Mister\b/Mr./g); # get change-cnt
87e95b7f
YO
2044
2045 \$_ = 'abc123xyz';
2046 s/\d+/\$&*2/e; # yields 'abc246xyz'
2047 s/\d+/sprintf("%5d",\$&)/e; # yields 'abc 246xyz'
2048 s/\w/\$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
2049
2050 s/%(.)/\$percent{\$1}/g; # change percent escapes; no /e
2051 s/%(.)/\$percent{\$1} || \$&/ge; # expr now, so /e
2052 s/^=(\w+)/pod(\$1)/ge; # use function call
2053
4f4d7508
DC
2054 \$_ = 'abc123xyz';
2055 \$a = s/abc/def/r; # \$a is 'def123xyz' and
2056 # \$_ remains 'abc123xyz'.
2057
87e95b7f
YO
2058 # expand variables in \$_, but dynamics only, using
2059 # symbolic dereferencing
2060 s/\\$(\w+)/\${\$1}/g;
2061
2062 # Add one to the value of any numbers in the string
2063 s/(\d+)/1 + \$1/eg;
2064
c543c01b
TC
2065 # Titlecase words in the last 30 characters only
2066 substr(\$str, -30) =~ s/\b(\p{Alpha}+)\b/\u\L\$1/g;
2067
87e95b7f
YO
2068 # This will expand any embedded scalar variable
2069 # (including lexicals) in \$_ : First \$1 is interpolated
2070 # to the variable name, and then evaluated
2071 s/(\\$\w+)/\$1/eeg;
2072
2073 # Delete (most) C comments.
2074 \$program =~ s {
2075 /\* # Match the opening delimiter.
2076 .*? # Match a minimal number of characters.
2077 \*/ # Match the closing delimiter.
2078 } []gsx;
2079
7188ca43
KW
2080 s/^\s*(.*?)\s*\$/\$1/; # trim whitespace in \$_,
2081 # expensively
87e95b7f 2082
7188ca43
KW
2083 for (\$variable) { # trim whitespace in \$variable,
2084 # cheap
87e95b7f
YO
2085 s/^\s+//;
2086 s/\s+\$//;
2087 }
2088
2089 s/([^ ]*) *([^ ]*)/\$2 \$1/; # reverse 1st two fields
2090
2091Note the use of \$ instead of \ in the last example. Unlike
2092B<sed>, we use the \<I<digit>> form in only the left hand side.
2093Anywhere else it's \$<I<digit>>.
2094
2095Occasionally, you can't use just a C</g> to get all the changes
2096to occur that you might want. Here are two common cases:
2097
2098 # put commas in the right places in an integer
2099 1 while s/(\d)(\d\d\d)(?!\d)/\$1,\$2/g;
2100
2101 # expand tabs to 8-column spacing
2102 1 while s/\t+/' ' x (length(\$&)*8 - length(\$`)%8)/e;
2103
2104=back
2105
2106=head2 Quote-Like Operators
2107X<operator, quote-like>
2108
01c6f5f4
RGS
2109=over 4
2110
a0d0e21e 2111=item q/STRING/
5d44bfff 2112X<q> X<quote, single> X<'> X<''>
a0d0e21e 2113
5d44bfff 2114=item 'STRING'
a0d0e21e 2115
19799a22 2116A single-quoted, literal string. A backslash represents a backslash
68dc0745
PP
2117unless followed by the delimiter or another backslash, in which case
2118the delimiter or backslash is interpolated.
a0d0e21e
LW
2119
2120 \$foo = q!I said, "You said, 'She said it.'"!;
2121 \$bar = q('This is it.');
68dc0745 2122 \$baz = '\n'; # a two-character string
a0d0e21e
LW
2123
2124=item qq/STRING/
d74e8afc 2125X<qq> X<quote, double> X<"> X<"">
a0d0e21e
LW
2126
2127=item "STRING"
2128
2129A double-quoted, interpolated string.
2130
2131 \$_ .= qq
2132 (*** The previous line contains the naughty word "\$1".\n)
19799a22 2133 if /\b(tcl|java|python)\b/i; # :-)
68dc0745 2134 \$baz = "\n"; # a one-character string
a0d0e21e
LW
2135
2136=item qx/STRING/
d74e8afc 2137X<qx> X<`> X<``> X<backtick>
a0d0e21e
LW
2138
2139=item `STRING`
2140
43dd4d21 2141A string which is (possibly) interpolated and then executed as a
f703fc96 2142system command with F</bin/sh> or its equivalent. Shell wildcards,
43dd4d21
JH
2143pipes, and redirections will be honored. The collected standard
2144output of the command is returned; standard error is unaffected. In
2145scalar context, it comes back as a single (potentially multi-line)
2146string, or undef if the command failed. In list context, returns a
2147list of lines (however you've defined lines with \$/ or
2148\$INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
5a964f20
TC
2149
2150Because backticks do not affect standard error, use shell file descriptor
2151syntax (assuming the shell supports this) if you care to address this.
2152To capture a command's STDERR and STDOUT together:
a0d0e21e 2153
5a964f20
TC
2154 \$output = `cmd 2>&1`;
2155
2156To capture a command's STDOUT but discard its STDERR:
2157
2158 \$output = `cmd 2>/dev/null`;
2159
2160To capture a command's STDERR but discard its STDOUT (ordering is
2161important here):
2162
2163 \$output = `cmd 2>&1 1>/dev/null`;
2164
2165To exchange a command's STDOUT and STDERR in order to capture the STDERR
2166but leave its STDOUT to come out the old STDERR:
2167
2168 \$output = `cmd 3>&1 1>&2 2>&3 3>&-`;
2169
2170To read both a command's STDOUT and its STDERR separately, it's easiest
2359510d
SD
2171to redirect them separately to files, and then read from those files
2172when the program is done:
5a964f20 2173
2359510d 2174 system("program args 1>program.stdout 2>program.stderr");
5a964f20 2175
30398227
SP
2176The STDIN filehandle used by the command is inherited from Perl's STDIN.
2177For example:
2178
c543c01b
TC
2179 open(SPLAT, "stuff") || die "can't open stuff: \$!";
2180 open(STDIN, "<&SPLAT") || die "can't dupe SPLAT: \$!";
40bbb707 2181 print STDOUT `sort`;
30398227 2182
40bbb707 2183will print the sorted contents of the file named F<"stuff">.
30398227 2184
5a964f20
TC
2185Using single-quote as a delimiter protects the command from Perl's
2186double-quote interpolation, passing it on to the shell instead:
2187
2188 \$perl_info = qx(ps \$\$); # that's Perl's \$\$
2189 \$shell_info = qx'ps \$\$'; # that's the new shell's \$\$
2190
19799a22 2191How that string gets evaluated is entirely subject to the command
5a964f20
TC
2192interpreter on your system. On most platforms, you will have to protect
2193shell metacharacters if you want them treated literally. This is in
2194practice difficult to do, as it's unclear how to escape which characters.
2195See L<perlsec> for a clean and safe example of a manual fork() and exec()
2196to emulate backticks safely.
a0d0e21e 2197
bb32b41a
GS
2198On some platforms (notably DOS-like ones), the shell may not be
2199capable of dealing with multiline commands, so putting newlines in
2200the string may not get you what you want. You may be able to evaluate
2201multiple commands in a single line by separating them with the command
1ca345ed
TC
2202separator character, if your shell supports that (for example, C<;> on
2203many Unix shells and C<&> on the Windows NT C<cmd> shell).
bb32b41a 2204
3ff8ecf9 2205Perl will attempt to flush all files opened for
0f897271
GS
2206output before starting the child process, but this may not be supported
2207on some platforms (see L<perlport>). To be safe, you may need to set
2208C<\$|> (\$AUTOFLUSH in English) or call the C<autoflush()> method of
2209C<IO::Handle> on any open handles.
2210
bb32b41a
GS
2211Beware that some command shells may place restrictions on the length
2212of the command line. You must ensure your strings don't exceed this
2213limit after any necessary interpolations. See the platform-specific
2214release notes for more details about your particular environment.
2215
5a964f20
TC
2216Using this operator can lead to programs that are difficult to port,
2217because the shell commands called vary between systems, and may in
2218fact not be present at all. As one example, the C<type> command under
2219the POSIX shell is very different from the C<type> command under DOS.
2220That doesn't mean you should go out of your way to avoid backticks
2221when they're the right way to get something done. Perl was made to be
2222a glue language, and one of the things it glues together is commands.
2223Just understand what you're getting yourself into.
bb32b41a 2224
da87341d 2225See L</"I/O Operators"> for more discussion.
a0d0e21e 2226
945c54fd 2227=item qw/STRING/
d74e8afc 2228X<qw> X<quote, list> X<quote, words>
945c54fd
JH
2229
2230Evaluates to a list of the words extracted out of STRING, using embedded
2231whitespace as the word delimiters. It can be understood as being roughly
2232equivalent to:
2233
c543c01b 2234 split(" ", q/STRING/);
945c54fd 2235
efb1e162
CW
2236the differences being that it generates a real list at compile time, and
2237in scalar context it returns the last element in the list. So
945c54fd
JH
2238this expression:
2239
2240 qw(foo bar baz)
2241
2242is semantically equivalent to the list:
2243
c543c01b 2244 "foo", "bar", "baz"
945c54fd
JH
2245
2246Some frequently seen examples:
2247
2248 use POSIX qw( setlocale localeconv )
2249 @EXPORT = qw( foo bar baz );
2250
2251A common mistake is to try to separate the words with comma or to
2252put comments into a multi-line C<qw>-string. For this reason, the
89d205f2 2253C<use warnings> pragma and the B<-w> switch (that is, the C<\$^W> variable)
945c54fd
JH
2254produces warnings if the STRING contains the "," or the "#" character.
2255
8ff32507 2256=item tr/SEARCHLIST/REPLACEMENTLIST/cdsr
d74e8afc 2257X<tr> X<y> X<transliterate> X</c> X</d> X</s>
a0d0e21e 2258
8ff32507 2259=item y/SEARCHLIST/REPLACEMENTLIST/cdsr
a0d0e21e 2260
2c268ad5 2261Transliterates all occurrences of the characters found in the search list
a0d0e21e
LW
2262with the corresponding character in the replacement list. It returns
2263the number of characters replaced or deleted. If no string is
c543c01b
TC
2264specified via the C<=~> or C<!~> operator, the \$_ string is transliterated.
2265
2266If the C</r> (non-destructive) option is present, a new copy of the string
2267is made and its characters transliterated, and this copy is returned no
2268matter whether it was modified or not: the original string is always
2269left unchanged. The new copy is always a plain string, even if the input
2270string is an object or a tied variable.
8ada0baa 2271
c543c01b
TC
2272Unless the C</r> option is used, the string specified with C<=~> must be a
2273scalar variable, an array element, a hash element, or an assignment to one
2274of those; in other words, an lvalue.
8ff32507 2275
89d205f2 2276A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
2c268ad5 2277does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
54310121
PP
2278For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
2279SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
c543c01b
TC
2280its own pair of quotes, which may or may not be bracketing quotes;
2281for example, C<tr[aeiouy][yuoiea]> or C<tr(+\-*/)/ABCD/>.
2282
2283Note that C<tr> does B<not> do regular expression character classes such as
2284C<\d> or C<\pL>. The C<tr> operator is not equivalent to the tr(1)
2285utility. If you want to map strings between lower/upper cases, see
2286L<perlfunc/lc> and L<perlfunc/uc>, and in general consider using the C<s>
2287operator if you need regular expressions. The C<\U>, C<\u>, C<\L>, and
2288C<\l> string-interpolation escapes on the right side of a substitution
2289operator will perform correct case-mappings, but C<tr[a-z][A-Z]> will not
2290(except sometimes on legacy 7-bit data).
cc255d5f 2291
8ada0baa
JH
2292Note also that the whole range idea is rather unportable between
2293character sets--and even within character sets they may cause results
2294you probably didn't expect. A sound principle is to use only ranges
2295that begin from and end at either alphabets of equal case (a-e, A-E),
2296or digits (0-4). Anything else is unsafe. If in doubt, spell out the
2297character sets in full.
2298
a0d0e21e
LW
2299Options:
2300
2301 c Complement the SEARCHLIST.
2302 d Delete found but unreplaced characters.
2303 s Squash duplicate replaced characters.
8ff32507
FC
2304 r Return the modified string and leave the original string
2305 untouched.
a0d0e21e 2306
19799a22
GS
2307If the C</c> modifier is specified, the SEARCHLIST character set
2308is complemented. If the C</d> modifier is specified, any characters
2309specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
2310(Note that this is slightly more flexible than the behavior of some
2311B<tr> programs, which delete anything they find in the SEARCHLIST,
2312period.) If the C</s> modifier is specified, sequences of characters
2313that were transliterated to the same character are squashed down
2314to a single instance of the character.
a0d0e21e
LW
2315
2316If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
2317exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
2318than the SEARCHLIST, the final character is replicated till it is long
5a964f20 2319enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
a0d0e21e
LW
2320This latter is useful for counting characters in a class or for
2321squashing character sequences in a class.
2322
2323Examples:
2324
c543c01b 2325 \$ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case ASCII
a0d0e21e
LW
2326
2327 \$cnt = tr/*/*/; # count the stars in \$_
2328
2329 \$cnt = \$sky =~ tr/*/*/; # count the stars in \$sky
2330
2331 \$cnt = tr/0-9//; # count the digits in \$_
2332
2333 tr/a-zA-Z//s; # bookkeeper -> bokeper
2334
2335 (\$HOST = \$host) =~ tr/a-z/A-Z/;
c543c01b 2336 \$HOST = \$host =~ tr/a-z/A-Z/r; # same thing
8ff32507 2337
c543c01b 2338 \$HOST = \$host =~ tr/a-z/A-Z/r # chained with s///r
8ff32507 2339 =~ s/:/ -p/r;
a0d0e21e
LW
2340
2341 tr/a-zA-Z/ /cs; # change non-alphas to single space
2342
8ff32507
FC
2343 @stripped = map tr/a-zA-Z/ /csr, @original;
2344 # /r with map
2345
a0d0e21e 2346 tr [\200-\377]
c543c01b 2347 [\000-\177]; # wickedly delete 8th bit
a0d0e21e 2348
19799a22
GS
2349If multiple transliterations are given for a character, only the
2350first one is used:
748a9306
LW
2351
2352 tr/AAA/XYZ/
2353
2c268ad5 2354will transliterate any A to X.
748a9306 2355
19799a22 2356Because the transliteration table is built at compile time, neither
a0d0e21e 2357the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
19799a22
GS
2358interpolation. That means that if you want to use variables, you
2359must use an eval():
a0d0e21e
LW
2360
2361 eval "tr/\$oldlist/\$newlist/";
2362 die \$@ if \$@;
2363
2364 eval "tr/\$oldlist/\$newlist/, 1" or die \$@;
2365
7e3b091d 2366=item <<EOF
d74e8afc 2367X<here-doc> X<heredoc> X<here-document> X<<< << >>>
7e3b091d
DA
2368
2369A line-oriented form of quoting is based on the shell "here-document"
2370syntax. Following a C<< << >> you specify a string to terminate
2371the quoted material, and all lines following the current line down to
89d205f2
YO
2372the terminating string are the value of the item.
2373
2374The terminating string may be either an identifier (a word), or some
2375quoted text. An unquoted identifier works like double quotes.
2376There may not be a space between the C<< << >> and the identifier,
2377unless the identifier is explicitly quoted. (If you put a space it
2378will be treated as a null identifier, which is valid, and matches the
2379first empty line.) The terminating string must appear by itself
2380(unquoted and with no surrounding whitespace) on the terminating line.
2381
2382If the terminating string is quoted, the type of quotes used determine
2383the treatment of the text.
2384
2385=over 4
2386
2387=item Double Quotes
2388
2389Double quotes indicate that the text will be interpolated using exactly
2390the same rules as normal double quoted strings.
7e3b091d
DA
2391
2392 print <<EOF;
2393 The price is \$Price.
2394 EOF
2395
2396 print << "EOF"; # same as above
2397 The price is \$Price.
2398 EOF
2399
89d205f2
YO
2400
2401=item Single Quotes
2402
2403Single quotes indicate the text is to be treated literally with no
2404interpolation of its content. This is similar to single quoted
2405strings except that backslashes have no special meaning, with C<\\>
2406being treated as two backslashes and not one as they would in every
2407other quoting construct.
2408
c543c01b
TC
2409Just as in the shell, a backslashed bareword following the C<<< << >>>
2410means the same thing as a single-quoted string does:
2411
2412 \$cost = <<'VISTA'; # hasta la ...
2413 That'll be \$10 please, ma'am.
2414 VISTA
2415
2416 \$cost = <<\VISTA; # Same thing!
2417 That'll be \$10 please, ma'am.
2418 VISTA
2419
89d205f2
YO
2420This is the only form of quoting in perl where there is no need
2421to worry about escaping content, something that code generators
2422can and do make good use of.
2423
2424=item Backticks
2425
2426The content of the here doc is treated just as it would be if the
2427string were embedded in backticks. Thus the content is interpolated
2428as though it were double quoted and then executed via the shell, with
2429the results of the execution returned.
2430
2431 print << `EOC`; # execute command and get results
7e3b091d 2432 echo hi there
7e3b091d
DA
2433 EOC
2434
89d205f2
YO
2435=back
2436
2437It is possible to stack multiple here-docs in a row:
2438
7e3b091d
DA
2439 print <<"foo", <<"bar"; # you can stack them
2440 I said foo.
2441 foo
2442 I said bar.
2443 bar
2444
2445 myfunc(<< "THIS", 23, <<'THAT');
2446 Here's a line
2447 or two.
2448 THIS
2449 and here's another.
2450 THAT
2451
2452Just don't forget that you have to put a semicolon on the end
2453to finish the statement, as Perl doesn't know you're not going to
2454try to do this:
2455
2456 print <<ABC
2457 179231
2458 ABC
2459 + 20;
2460
872d7e53
ST
2461If you want to remove the line terminator from your here-docs,
2462use C<chomp()>.
2463
2464 chomp(\$string = <<'END');
2465 This is a string.
2466 END
2467
2468If you want your here-docs to be indented with the rest of the code,
2469you'll need to remove leading whitespace from each line manually:
7e3b091d
DA
2470
2471 (\$quote = <<'FINIS') =~ s/^\s+//gm;
89d205f2 2472 The Road goes ever on and on,
7e3b091d
DA
2473 down from the door where it began.
2474 FINIS
2475
2476If you use a here-doc within a delimited construct, such as in C<s///eg>,
1bf48760
FC
2477the quoted material must still come on the line following the
2478C<<< <<FOO >>> marker, which means it may be inside the delimited
2479construct:
7e3b091d
DA
2480
2481 s/this/<<E . 'that'
2482 the other
2483 E
2484 . 'more '/eg;
2485
1bf48760
FC
2486It works this way as of Perl 5.18. Historically, it was inconsistent, and
2487you would have to write
7e3b091d 2488
89d205f2
YO
2489 s/this/<<E . 'that'
2490 . 'more '/eg;
2491 the other
2492 E
7e3b091d 2493
1bf48760
FC
2494outside of string evals.
2495
c543c01b
TC
2496Additionally, quoting rules for the end-of-string identifier are
2497unrelated to Perl's quoting rules. C<q()>, C<qq()>, and the like are not
89d205f2
YO
2498supported in place of C<''> and C<"">, and the only interpolation is for
2499backslashing the quoting character:
7e3b091d
DA
2500
2501 print << "abc\"def";
2502 testing...
2503 abc"def
2504
2505Finally, quoted strings cannot span multiple lines. The general rule is
2506that the identifier must be a string literal. Stick with that, and you
2507should be safe.
2508
a0d0e21e
LW
2509=back
2510
75e14d17 2511=head2 Gory details of parsing quoted constructs
d74e8afc 2512X<quote, gory details>
75e14d17 2513
19799a22
GS
2514When presented with something that might have several different
2515interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
2516principle to pick the most probable interpretation. This strategy
2517is so successful that Perl programmers often do not suspect the
2518ambivalence of what they write. But from time to time, Perl's
2519notions differ substantially from what the author honestly meant.
2520
2521This section hopes to clarify how Perl handles quoted constructs.
2522Although the most common reason to learn this is to unravel labyrinthine
2523regular expressions, because the initial steps of parsing are the
2524same for all quoting operators, they are all discussed together.
2525
2526The most important Perl parsing rule is the first one discussed
2527below: when processing a quoted construct, Perl first finds the end
2528of that construct, then interprets its contents. If you understand
2529this rule, you may skip the rest of this section on the first
2530reading. The other rules are likely to contradict the user's
2531expectations much less frequently than this first one.
2532
2533Some passes discussed below are performed concurrently, but because
2534their results are the same, we consider them individually. For different
2535quoting constructs, Perl performs different numbers of passes, from
6deea57f 2536one to four, but these passes are always performed in the same order.
75e14d17 2537
13a2d996 2538=over 4
75e14d17
IZ
2539
2540=item Finding the end
2541
6deea57f
ST
2542The first pass is finding the end of the quoted construct, where
2543the information about the delimiters is used in parsing.
2544During this search, text between the starting and ending delimiters
2545is copied to a safe location. The text copied gets delimiter-independent.
2546
2547If the construct is a here-doc, the ending delimiter is a line
2548that has a terminating string as the content. Therefore C<<<EOF> is
2549terminated by C<EOF> immediately followed by C<"\n"> and starting
2550from the first column of the terminating line.
2551When searching for the terminating line of a here-doc, nothing
2552is skipped. In other words, lines after the here-doc syntax
2553are compared with the terminating string line by line.
2554
2555For the constructs except here-docs, single characters are used as starting
2556and ending delimiters. If the starting delimiter is an opening punctuation
2557(that is C<(>, C<[>, C<{>, or C<< < >>), the ending delimiter is the
2558corresponding closing punctuation (that is C<)>, C<]>, C<}>, or C<< > >>).
2559If the starting delimiter is an unpaired character like C</> or a closing
2560punctuation, the ending delimiter is same as the starting delimiter.
2561Therefore a C</> terminates a C<qq//> construct, while a C<]> terminates
2562C<qq[]> and C<qq]]> constructs.
2563
2564When searching for single-character delimiters, escaped delimiters
1ca345ed 2565and C<\\> are skipped. For example, while searching for terminating C</>,
6deea57f
ST
2566combinations of C<\\> and C<\/> are skipped. If the delimiters are
2567bracketing, nested pairs are also skipped. For example, while searching
2568for closing C<]> paired with the opening C<[>, combinations of C<\\>, C<\]>,
2569and C<\[> are all skipped, and nested C<[> and C<]> are skipped as well.
2570However, when backslashes are used as the delimiters (like C<qq\\> and
2571C<tr\\\>), nothing is skipped.
32581033 2572During the search for the end, backslashes that escape delimiters or
7188ca43 2573other backslashes are removed (exactly speaking, they are not copied to the
32581033 2574safe location).
75e14d17 2575
19799a22
GS
2576For constructs with three-part delimiters (C<s///>, C<y///>, and
2577C<tr///>), the search is repeated once more.
6deea57f
ST
2578If the first delimiter is not an opening punctuation, three delimiters must
2579be same such as C<s!!!> and C<tr)))>, in which case the second delimiter
2580terminates the left part and starts the right part at once.
b6538e4f 2581If the left part is delimited by bracketing punctuation (that is C<()>,
6deea57f 2582C<[]>, C<{}>, or C<< <> >>), the right part needs another pair of
b6538e4f 2583delimiters such as C<s(){}> and C<tr[]//>. In these cases, whitespace
6deea57f 2584and comments are allowed between both parts, though the comment must follow
b6538e4f
TC
2585at least one whitespace character; otherwise a character expected as the
2586start of the comment may be regarded as the starting delimiter of the right part.
75e14d17 2587
19799a22
GS
2588During this search no attention is paid to the semantics of the construct.
2589Thus:
75e14d17
IZ
2590
2591 "\$hash{"\$foo/\$bar"}"
2592
2a94b7ce 2593or:
75e14d17 2594
89d205f2 2595 m/
2a94b7ce 2596 bar # NOT a comment, this slash / terminated m//!
75e14d17
IZ
2597 /x
2598
19799a22
GS
2599do not form legal quoted expressions. The quoted part ends on the
2600first C<"> and C</>, and the rest happens to be a syntax error.
2601Because the slash that terminated C<m//> was followed by a C<SPACE>,
2602the example above is not C<m//x>, but rather C<m//> with no C</x>
2603modifier. So the embedded C<#> is interpreted as a literal C<#>.
75e14d17 2604
89d205f2
YO
2605Also no attention is paid to C<\c\> (multichar control char syntax) during
2606this search. Thus the second C<\> in C<qq/\c\/> is interpreted as a part
2607of C<\/>, and the following C</> is not recognized as a delimiter.
0d594e51
ST
2608Instead, use C<\034> or C<\x1c> at the end of quoted constructs.
2609
75e14d17 2610=item Interpolation
d74e8afc 2611X<interpolation>
75e14d17 2612
19799a22 2613The next step is interpolation in the text obtained, which is now
89d205f2 2614delimiter-independent. There are multiple cases.
75e14d17 2615
13a2d996 2616=over 4
75e14d17 2617
89d205f2 2618=item C<<<'EOF'>
75e14d17
IZ
2619
2620No interpolation is performed.
6deea57f
ST
2621Note that the combination C<\\> is left intact, since escaped delimiters
2622are not available for here-docs.
75e14d17 2623
6deea57f 2624=item C<m''>, the pattern of C<s'''>
89d205f2 2625
6deea57f
ST
2626No interpolation is performed at this stage.
2627Any backslashed sequences including C<\\> are treated at the stage
2628to L</"parsing regular expressions">.
89d205f2 2629
6deea57f 2630=item C<''>, C<q//>, C<tr'''>, C<y'''>, the replacement of C<s'''>
75e14d17 2631
89d205f2 2632The only interpolation is removal of C<\> from pairs of C<\\>.
6deea57f
ST
2633Therefore C<-> in C<tr'''> and C<y'''> is treated literally
2634as a hyphen and no character range is available.
2635C<\1> in the replacement of C<s'''> does not work as C<\$1>.
89d205f2
YO
2636
2637=item C<tr///>, C<y///>
2638
6deea57f
ST
2639No variable interpolation occurs. String modifying combinations for
2640case and quoting such as C<\Q>, C<\U>, and C<\E> are not recognized.
2641The other escape sequences such as C<\200> and C<\t> and backslashed
2642characters such as C<\\> and C<\-> are converted to appropriate literals.
89d205f2
YO
2643The character C<-> is treated specially and therefore C<\-> is treated
2644as a literal C<->.
75e14d17 2645
89d205f2 2646=item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF">
75e14d17 2647
628253b8 2648C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F> (possibly paired with C<\E>) are
19799a22
GS
2649converted to corresponding Perl constructs. Thus, C<"\$foo\Qbaz\$bar">
2650is converted to C<\$foo . (quotemeta("baz" . \$bar))> internally.
6deea57f
ST
2651The other escape sequences such as C<\200> and C<\t> and backslashed
2652characters such as C<\\> and C<\-> are replaced with appropriate
2653expansions.
2a94b7ce 2654
19799a22
GS
2655Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
2656is interpolated in the usual way. Something like C<"\Q\\E"> has
48cbae4f 2657no C<\E> inside. Instead, it has C<\Q>, C<\\>, and C<E>, so the
19799a22
GS
2658result is the same as for C<"\\\\E">. As a general rule, backslashes
2659between C<\Q> and C<\E> may lead to counterintuitive results. So,
2660C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
2661as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
2a94b7ce
IZ
2662
2663 \$str = '\t';
2664 return "\Q\$str";
2665
2666may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
2667
19799a22 2668Interpolated scalars and arrays are converted internally to the C<join> and
92d29cee 2669C<.> catenation operations. Thus, C<"\$foo XXX '@arr'"> becomes:
75e14d17 2670
19799a22 2671 \$foo . " XXX '" . (join \$", @arr) . "'";
75e14d17 2672
19799a22 2673All operations above are performed simultaneously, left to right.
75e14d17 2674
19799a22
GS
2675Because the result of C<"\Q STRING \E"> has all metacharacters
2676quoted, there is no way to insert a literal C<\$> or C<@> inside a
2677C<\Q\E> pair. If protected by C<\>, C<\$> will be quoted to became
2678C<"\\\\$">; if not, it is interpreted as the start of an interpolated
2679scalar.
75e14d17 2680
19799a22 2681Note also that the interpolation code needs to make a decision on
89d205f2 2682where the interpolated scalar ends. For instance, whether
35f2feb0 2683C<< "a \$b -> {c}" >> really means:
75e14d17
IZ
2684
2685 "a " . \$b . " -> {c}";
2686
2a94b7ce 2687or:
75e14d17
IZ
2688
2689 "a " . \$b -> {c};
2690
19799a22
GS
2691Most of the time, the longest possible text that does not include
2692spaces between components and which contains matching braces or
2693brackets. because the outcome may be determined by voting based
2694on heuristic estimators, the result is not strictly predictable.
2695Fortunately, it's usually correct for ambiguous cases.
75e14d17 2696
6deea57f 2697=item the replacement of C<s///>
75e14d17 2698
628253b8 2699Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F> and interpolation
6deea57f
ST
2700happens as with C<qq//> constructs.
2701
2702It is at this step that C<\1> is begrudgingly converted to C<\$1> in
2703the replacement text of C<s///>, in order to correct the incorrigible
2704I<sed> hackers who haven't picked up the saner idiom yet. A warning
2705is emitted if the C<use warnings> pragma or the B<-w> command-line flag
2706(that is, the C<\$^W> variable) was set.
2707
2708=item C<RE> in C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
2709
628253b8 2710Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F>, C<\E>,
cc74c5bd
ST
2711and interpolation happens (almost) as with C<qq//> constructs.
2712
5d03b57c
KW
2713Processing of C<\N{...}> is also done here, and compiled into an intermediate
2714form for the regex compiler. (This is because, as mentioned below, the regex
2715compilation may be done at execution time, and C<\N{...}> is a compile-time
2716construct.)
2717
cc74c5bd
ST
2718However any other combinations of C<\> followed by a character
2719are not substituted but only skipped, in order to parse them
2720as regular expressions at the following step.
6deea57f 2721As C<\c> is skipped at this step, C<@> of C<\c@> in RE is possibly
1749ea0d 2722treated as an array symbol (for example C<@foo>),
6deea57f 2723even though the same text in C<qq//> gives interpolation of C<\c@>.
6deea57f 2724
e128ab2c
DM
2725Code blocks such as C<(?{BLOCK})> are handled by temporarily passing control
2726back to the perl parser, in a similar way that an interpolated array
2727subscript expression such as C<"foo\$array[1+f("[xyz")]bar"> would be.
2728
6deea57f 2729Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
19799a22
GS
2730a C<#>-comment in a C<//x>-regular expression, no processing is
2731performed whatsoever. This is the first step at which the presence
2732of the C<//x> modifier is relevant.
2733
1749ea0d
ST
2734Interpolation in patterns has several quirks: C<\$|>, C<\$(>, C<\$)>, C<@+>
2735and C<@-> are not interpolated, and constructs C<\$var[SOMETHING]> are
2736voted (by several different estimators) to be either an array element
2737or C<\$var> followed by an RE alternative. This is where the notation
19799a22
GS
2738C<\${arr[\$bar]}> comes handy: C</\${arr[0-9]}/> is interpreted as
2739array element C<-9>, not as a regular expression from the variable
2740C<\$arr> followed by a digit, which would be the interpretation of
2741C</\$arr[0-9]/>. Since voting among different estimators may occur,
2742the result is not predictable.
2743
19799a22
GS
2744The lack of processing of C<\\> creates specific restrictions on
2745the post-processed text. If the delimiter is C</>, one cannot get
2746the combination C<\/> into the result of this step. C</> will
2747finish the regular expression, C<\/> will be stripped to C</> on
2748the previous step, and C<\\/> will be left as is. Because C</> is
2749equivalent to C<\/> inside a regular expression, this does not
2750matter unless the delimiter happens to be character special to the
2751RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
2752alphanumeric char, as in:
2a94b7ce
IZ
2753
2754 m m ^ a \s* b mmx;
2755
19799a22 2756In the RE above, which is intentionally obfuscated for illustration, the
6deea57f 2757delimiter is C<m>, the modifier is C<mx>, and after delimiter-removal the
89d205f2 2758RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
19799a22
GS
2759reason you're encouraged to restrict your delimiters to non-alphanumeric,
2760non-whitespace choices.
75e14d17
IZ
2761
2762=back
2763
19799a22 2764This step is the last one for all constructs except regular expressions,
75e14d17
IZ
2765which are processed further.
2766
6deea57f
ST
2767=item parsing regular expressions
2768X<regexp, parse>
75e14d17 2769
19799a22 2770Previous steps were performed during the compilation of Perl code,
ac036724 2771but this one happens at run time, although it may be optimized to
19799a22 2772be calculated at compile time if appropriate. After preprocessing
6deea57f 2773described above, and possibly after evaluation if concatenation,
19799a22
GS
2774joining, casing translation, or metaquoting are involved, the
2775resulting I<string> is passed to the RE engine for compilation.
2776
2777Whatever happens in the RE engine might be better discussed in L<perlre>,
2778but for the sake of continuity, we shall do so here.
2779
2780This is another step where the presence of the C<//x> modifier is
2781relevant. The RE engine scans the string from left to right and
2782converts it to a finite automaton.
2783
2784Backslashed characters are either replaced with corresponding
2785literal strings (as with C<\{>), or else they generate special nodes
2786in the finite automaton (as with C<\b>). Characters special to the
2787RE engine (such as C<|>) generate corresponding nodes or groups of
2788nodes. C<(?#...)> comments are ignored. All the rest is either
2789converted to literal strings to match, or else is ignored (as is
2790whitespace and C<#>-style comments if C<//x> is present).
2791
2792Parsing of the bracketed character class construct, C<[...]>, is
2793rather different than the rule used for the rest of the pattern.
2794The terminator of this construct is found using the same rules as
2795for finding the terminator of a C<{}>-delimited construct, the only
2796exception being that C<]> immediately following C<[> is treated as
e128ab2c
DM
2797though preceded by a backslash.
2798
2799The terminator of runtime C<(?{...})> is found by temporarily switching
2800control to the perl parser, which should stop at the point where the
2801logically balancing terminating C<}> is found.
19799a22
GS
2802
2803It is possible to inspect both the string given to RE engine and the
2804resulting finite automaton. See the arguments C<debug>/C<debugcolor>
2805in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
4a4eefd0 2806switch documented in L<perlrun/"Command Switches">.
75e14d17
IZ
2807
2808=item Optimization of regular expressions
d74e8afc 2809X<regexp, optimization>
75e14d17 2810
7522fed5 2811This step is listed for completeness only. Since it does not change
75e14d17 2812semantics, details of this step are not documented and are subject
19799a22
GS
2813to change without notice. This step is performed over the finite
2814automaton that was generated during the previous pass.
2a94b7ce 2815
19799a22
GS
2816It is at this stage that C<split()> silently optimizes C</^/> to
2817mean C</^/m>.
75e14d17
IZ
2818
2819=back
2820
a0d0e21e 2821=head2 I/O Operators
d74e8afc
ITB
2822X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle>
2823X<< <> >> X<@ARGV>
a0d0e21e 2824
54310121 2825There are several I/O operators you should know about.
fbad3eb5 2826
7b8d334a 2827A string enclosed by backticks (grave accents) first undergoes
19799a22
GS
2828double-quote interpolation. It is then interpreted as an external
2829command, and the output of that command is the value of the
e9c56f9b
JH
2830backtick string, like in a shell. In scalar context, a single string
2831consisting of all output is returned. In list context, a list of
2832values is returned, one per line of output. (You can set C<\$/> to use
2833a different line terminator.) The command is executed each time the
2834pseudo-literal is evaluated. The status value of the command is
2835returned in C<\$?> (see L<perlvar> for the interpretation of C<\$?>).
2836Unlike in B<csh>, no translation is done on the return data--newlines
2837remain newlines. Unlike in any of the shells, single quotes do not
2838hide variable names in the command from interpretation. To pass a
2839literal dollar-sign through to the shell you need to hide it with a
2840backslash. The generalized form of backticks is C<qx//>. (Because
2841backticks always undergo shell expansion as well, see L<perlsec> for
2842security concerns.)
d74e8afc 2843X<qx> X<`> X<``> X<backtick> X<glob>
19799a22
GS
2844
2845In scalar context, evaluating a filehandle in angle brackets yields
2846the next line from that file (the newline, if any, included), or
2847C<undef> at end-of-file or on error. When C<\$/> is set to C<undef>
2848(sometimes known as file-slurp mode) and the file is empty, it
2849returns C<''> the first time, followed by C<undef> subsequently.
2850
2851Ordinarily you must assign the returned value to a variable, but
2852there is one situation where an automatic assignment happens. If
2853and only if the input symbol is the only thing inside the conditional
2854of a C<while> statement (even if disguised as a C<for(;;)> loop),
2855the value is automatically assigned to the global variable \$_,
2856destroying whatever was there previously. (This may seem like an
2857odd thing to you, but you'll use the construct in almost every Perl
17b829fa 2858script you write.) The \$_ variable is not implicitly localized.
19799a22
GS
2859You'll have to put a C<local \$_;> before the loop if you want that
2860to happen.
2861
2862The following lines are equivalent:
a0d0e21e 2863
748a9306 2864 while (defined(\$_ = <STDIN>)) { print; }
7b8d334a 2865 while (\$_ = <STDIN>) { print; }
a0d0e21e
LW
2866 while (<STDIN>) { print; }
2867 for (;<STDIN>;) { print; }
748a9306 2868 print while defined(\$_ = <STDIN>);
7b8d334a 2869 print while (\$_ = <STDIN>);
a0d0e21e
LW
2870 print while <STDIN>;
2871
1ca345ed
TC
2872This also behaves similarly, but assigns to a lexical variable
2873instead of to C<\$_>:
7b8d334a 2874
89d205f2 2875 while (my \$line = <STDIN>) { print \$line }
7b8d334a 2876
19799a22
GS
2877In these loop constructs, the assigned value (whether assignment
2878is automatic or explicit) is then tested to see whether it is
1ca345ed
TC
2879defined. The defined test avoids problems where the line has a string
2880value that would be treated as false by Perl; for example a "" or
19799a22
GS
2881a "0" with no trailing newline. If you really mean for such values
2882to terminate the loop, they should be tested for explicitly:
7b8d334a
GS
2883
2884 while ((\$_ = <STDIN>) ne '0') { ... }
2885 while (<STDIN>) { last unless \$_; ... }
2886
1ca345ed 2887In other boolean contexts, C<< <FILEHANDLE> >> without an
5ef4d93e 2888explicit C<defined> test or comparison elicits a warning if the
9f1b1f2d 2889C<use warnings> pragma or the B<-w>
19799a22 2890command-line switch (the C<\$^W> variable) is in effect.
7b8d334a 2891
5f05dabc 2892The filehandles STDIN, STDOUT, and STDERR are predefined. (The
19799a22
GS
2893filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
2894in packages, where they would be interpreted as local identifiers
2895rather than global.) Additional filehandles may be created with
2896the open() function, amongst others. See L<perlopentut> and
2897L<perlfunc/open> for details on this.
d74e8afc 2898X<stdin> X<stdout> X<sterr>
a0d0e21e 2899
35f2feb0 2900If a <FILEHANDLE> is used in a context that is looking for
19799a22
GS
2901a list, a list comprising all input lines is returned, one line per
2902list element. It's easy to grow to a rather large data space this
2903way, so use with care.
a0d0e21e 2904
35f2feb0 2905<FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
19799a22 2906See L<perlfunc/readline>.
fbad3eb5 2907
35f2feb0 2908The null filehandle <> is special: it can be used to emulate the
1ca345ed
TC
2909behavior of B<sed> and B<awk>, and any other Unix filter program
2910that takes a list of filenames, doing the same to each line
2911of input from all of them. Input from <> comes either from
a0d0e21e 2912standard input, or from each file listed on the command line. Here's
35f2feb0 2913how it works: the first time <> is evaluated, the @ARGV array is
5a964f20 2914checked, and if it is empty, C<\$ARGV[0]> is set to "-", which when opened
a0d0e21e
LW
2915gives you standard input. The @ARGV array is then processed as a list
2916of filenames. The loop
2917
2918 while (<>) {
2919 ... # code for each line
2920 }
2921
2922is equivalent to the following Perl-like pseudo code:
2923
3e3baf6d 2924 unshift(@ARGV, '-') unless @ARGV;
a0d0e21e
LW
2925 while (\$ARGV = shift) {
2926 open(ARGV, \$ARGV);
2927 while (<ARGV>) {
2928 ... # code for each line
2929 }
2930 }
2931
19799a22
GS
2932except that it isn't so cumbersome to say, and will actually work.
2933It really does shift the @ARGV array and put the current filename
2934into the \$ARGV variable. It also uses filehandle I<ARGV>
ac036724 2935internally. <> is just a synonym for <ARGV>, which
19799a22 2936is magical. (The pseudo code above doesn't work because it treats
35f2feb0 2937<ARGV> as non-magical.)
a0d0e21e 2938
48ab5743
ML
2939Since the null filehandle uses the two argument form of L<perlfunc/open>
2940it interprets special characters, so if you have a script like this:
2941
2942 while (<>) {
2943 print;
2944 }
2945
2946and call it with C<perl dangerous.pl 'rm -rfv *|'>, it actually opens a
2947pipe, executes the C<rm> command and reads C<rm>'s output from that pipe.
2948If you want all items in C<@ARGV> to be interpreted as file names, you
2949can use the module C<ARGV::readonly> from CPAN.
2950
35f2feb0 2951You can modify @ARGV before the first <> as long as the array ends up
a0d0e21e 2952containing the list of filenames you really want. Line numbers (C<\$.>)
19799a22
GS
2953continue as though the input were one big happy file. See the example
2954in L<perlfunc/eof> for how to reset line numbers on each file.
5a964f20 2955
89d205f2 2956If you want to set @ARGV to your own list of files, go right ahead.
5a964f20
TC
2957This sets @ARGV to all plain text files if no @ARGV was given:
2958
2959 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
a0d0e21e 2960
5a964f20
TC
2961You can even set them to pipe commands. For example, this automatically
2962filters compressed arguments through B<gzip>:
2963
2964 @ARGV = map { /\.(gz|Z)\$/ ? "gzip -dc < \$_ |" : \$_ } @ARGV;
2965
2966If you want to pass switches into your script, you can use one of the
a0d0e21e
LW
2967Getopts modules or put a loop on the front like this:
2968
2969 while (\$_ = \$ARGV[0], /^-/) {
2970 shift;
2971 last if /^--\$/;
2972 if (/^-D(.*)/) { \$debug = \$1 }
2973 if (/^-v/) { \$verbose++ }
5a964f20 2974 # ... # other switches
a0d0e21e 2975 }
5a964f20 2976
a0d0e21e 2977 while (<>) {
5a964f20 2978 # ... # code for each line
a0d0e21e
LW
2979 }
2980
89d205f2
YO
2981The <> symbol will return C<undef> for end-of-file only once.
2982If you call it again after this, it will assume you are processing another
19799a22 2983@ARGV list, and if you haven't set @ARGV, will read input from STDIN.
a0d0e21e 2984
1ca345ed 2985If what the angle brackets contain is a simple scalar variable (for example,
35f2feb0 2986<\$foo>), then that variable contains the name of the
19799a22
GS
2987filehandle to input from, or its typeglob, or a reference to the
2988same. For example:
cb1a09d0
AD
2989
2990 \$fh = \*STDIN;
2991 \$line = <\$fh>;
a0d0e21e 2992
5a964f20
TC
2993If what's within the angle brackets is neither a filehandle nor a simple
2994scalar variable containing a filehandle name, typeglob, or typeglob
2995reference, it is interpreted as a filename pattern to be globbed, and
2996either a list of filenames or the next filename in the list is returned,
19799a22 2997depending on context. This distinction is determined on syntactic
35f2feb0
GS
2998grounds alone. That means C<< <\$x> >> is always a readline() from
2999an indirect handle, but C<< <\$hash{key}> >> is always a glob().
5a964f20 3000That's because \$x is a simple scalar variable, but C<\$hash{key}> is
ef191992
YST
3001not--it's a hash element. Even C<< <\$x > >> (note the extra space)
3002is treated as C<glob("\$x ")>, not C<readline(\$x)>.
5a964f20
TC
3003
3004One level of double-quote interpretation is done first, but you can't
35f2feb0 3005say C<< <\$foo> >> because that's an indirect filehandle as explained
5a964f20
TC
3006in the previous paragraph. (In older versions of Perl, programmers
3007would insert curly brackets to force interpretation as a filename glob:
35f2feb0 3008C<< <\${foo}> >>. These days, it's considered cleaner to call the
5a964f20 3009internal function directly as C<glob(\$foo)>, which is probably the right
19799a22 3010way to have done it in the first place.) For example:
a0d0e21e
LW
3011
3012 while (<*.c>) {
3013 chmod 0644, \$_;
3014 }
3015
3a4b19e4 3016is roughly equivalent to:
a0d0e21e
LW
3017
3018 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
3019 while (<FOO>) {
5b3eff12 3020 chomp;
a0d0e21e
LW
3021 chmod 0644, \$_;
3022 }
3023
3a4b19e4
GS
3024except that the globbing is actually done internally using the standard
3025C<File::Glob> extension. Of course, the shortest way to do the above is:
a0d0e21e
LW
3026
3027 chmod 0644, <*.c>;
3028
19799a22
GS
3029A (file)glob evaluates its (embedded) argument only when it is
3030starting a new list. All values must be read before it will start
3031over. In list context, this isn't important because you automatically
3032get them all anyway. However, in scalar context the operator returns
069e01df 3033the next value each time it's called, or C<undef> when the list has
19799a22
GS
3034run out. As with filehandle reads, an automatic C<defined> is
3035generated when the glob occurs in the test part of a C<while>,
1ca345ed
TC
3036because legal glob returns (for example,
3037a file called F<0>) would otherwise
19799a22
GS
3038terminate the loop. Again, C<undef> is returned only once. So if
3039you're expecting a single value from a glob, it is much better to
3040say
4633a7c4
LW
3041
3042 (\$file) = <blurch*>;
3043
3044than
3045
3046 \$file = <blurch*>;
3047
3048because the latter will alternate between returning a filename and
19799a22 3049returning false.
4633a7c4 3050
b159ebd3 3051If you're trying to do variable interpolation, it's definitely better
4633a7c4 3052to use the glob() function, because the older notation can cause people
e37d713d 3053to become confused with the indirect filehandle notation.
4633a7c4
LW
3054
3055 @files = glob("\$dir/*.[ch]");
3056 @files = glob(\$files[\$i]);
3057
a0d0e21e 3058=head2 Constant Folding
d74e8afc 3059X<constant folding> X<folding>
a0d0e21e
LW
3060
3061Like C, Perl does a certain amount of expression evaluation at
19799a22 3062compile time whenever it determines that all arguments to an
a0d0e21e
LW
3063operator are static and have no side effects. In particular, string
3064concatenation happens at compile time between literals that don't do
19799a22 3065variable substitution. Backslash interpolation also happens at
a0d0e21e
LW
3066compile time. You can say
3067
1ca345ed
TC
3068 'Now is the time for all'
3069 . "\n"
3070 . 'good men to come to.'
a0d0e21e 3071
54310121 3072and this all reduces to one string internally. Likewise, if
a0d0e21e
LW
3073you say
3074
3075 foreach \$file (@filenames) {
5a964f20 3076 if (-s \$file > 5 + 100 * 2**16) { }
54310121 3077 }
a0d0e21e 3078
1ca345ed 3079the compiler precomputes the number which that expression
19799a22 3080represents so that the interpreter won't have to.
a0d0e21e 3081
fd1abbef 3082=head2 No-ops
d74e8afc 3083X<no-op> X<nop>
fd1abbef
DN
3084
3085Perl doesn't officially have a no-op operator, but the bare constants
1ca345ed 3086C<0> and C<1> are special-cased not to produce a warning in void
fd1abbef
DN
3087context, so you can for example safely do
3088
3089 1 while foo();
3090
2c268ad5 3091=head2 Bitwise String Operators
d74e8afc 3092X<operator, bitwise, string>
2c268ad5
TP
3093
3094Bitstrings of any size may be manipulated by the bitwise operators
3095(C<~ | & ^>).
3096
19799a22
GS
3097If the operands to a binary bitwise op are strings of different
3098sizes, B<|> and B<^> ops act as though the shorter operand had
3099additional zero bits on the right, while the B<&> op acts as though
3100the longer operand were truncated to the length of the shorter.
3101The granularity for such extension or truncation is one or more
3102bytes.
2c268ad5 3103
89d205f2 3104 # ASCII-based examples
2c268ad5
TP
3105 print "j p \n" ^ " a h"; # prints "JAPH\n"
3106 print "JA" | " ph\n"; # prints "japh\n"
3107 print "japh\nJunk" & '_____'; # prints "JAPH\n";
3108 print 'p N\$' ^ " E<H\n"; # prints "Perl\n";
3109
19799a22 3110If you are intending to manipulate bitstrings, be certain that
2c268ad5 3111you're supplying bitstrings: If an operand is a number, that will imply
19799a22 3112a B<numeric> bitwise operation. You may explicitly show which type of
2c268ad5
TP
3113operation you intend by using C<""> or C<0+>, as in the examples below.
3114
4358a253
SS
3115 \$foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
3116 \$foo = '150' | 105; # yields 255
2c268ad5
TP
3117 \$foo = 150 | '105'; # yields 255
3118 \$foo = '150' | '105'; # yields string '155' (under ASCII)
3119
3120 \$baz = 0+\$foo & 0+\$bar; # both ops explicitly numeric
3121 \$biz = "\$foo" ^ "\$bar"; # both ops explicitly stringy
a0d0e21e 3122
1ae175c8
GS
3123See L<perlfunc/vec> for information on how to manipulate individual bits
3124in a bit vector.
3125
55497cff 3126=head2 Integer Arithmetic
d74e8afc 3127X<integer>
a0d0e21e 3128
19799a22 3129By default, Perl assumes that it must do most of its arithmetic in
a0d0e21e
LW
3130floating point. But by saying
3131
3132 use integer;
3133
3eab78e3
CW
3134you may tell the compiler to use integer operations
3135(see L<integer> for a detailed explanation) from here to the end of
3136the enclosing BLOCK. An inner BLOCK may countermand this by saying
a0d0e21e
LW
3137
3138 no integer;
3139
19799a22 3140which lasts until the end of that BLOCK. Note that this doesn't
3eab78e3
CW
3141mean everything is an integer, merely that Perl will use integer
3142operations for arithmetic, comparison, and bitwise operators. For
3143example, even under C<use integer>, if you take the C<sqrt(2)>, you'll
3144still get C<1.4142135623731> or so.
19799a22
GS
3145
3146Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
89d205f2 3147and ">>") always produce integral results. (But see also
13a2d996 3148L<Bitwise String Operators>.) However, C<use integer> still has meaning for
19799a22
GS
3149them. By default, their results are interpreted as unsigned integers, but
3150if C<use integer> is in effect, their results are interpreted
3151as signed integers. For example, C<~0> usually evaluates to a large
0be96356 3152integral value. However, C<use integer; ~0> is C<-1> on two's-complement
19799a22 3153machines.
68dc0745
PP
3154
3155=head2 Floating-point Arithmetic
06ce2fa3 3156
d74e8afc 3157X<floating-point> X<floating point> X<float> X<real>
68dc0745
PP
3158
3159While C<use integer> provides integer-only arithmetic, there is no
19799a22
GS
3160analogous mechanism to provide automatic rounding or truncation to a
3161certain number of decimal places. For rounding to a certain number
3162of digits, sprintf() or printf() is usually the easiest route.
3163See L<perlfaq4>.
68dc0745 3164
5a964f20
TC
3165Floating-point numbers are only approximations to what a mathematician
3166would call real numbers. There are infinitely more reals than floats,
3167so some corners must be cut. For example:
3168
3169 printf "%.20g\n", 123456789123456789;
3170 # produces 123456789123456784
3171
8548cb57
RGS
3172Testing for exact floating-point equality or inequality is not a
3173good idea. Here's a (relatively expensive) work-around to compare
5a964f20
TC
3174whether two floating-point numbers are equal to a particular number of
3175decimal places. See Knuth, volume II, for a more robust treatment of
3176this topic.
3177
3178 sub fp_equal {
3179 my (\$X, \$Y, \$POINTS) = @_;
3180 my (\$tX, \$tY);
3181 \$tX = sprintf("%.\${POINTS}g", \$X);
3182 \$tY = sprintf("%.\${POINTS}g", \$Y);
3183 return \$tX eq \$tY;
3184 }
3185
68dc0745 3186The POSIX module (part of the standard perl distribution) implements
19799a22
GS
3187ceil(), floor(), and other mathematical and trigonometric functions.
3188The Math::Complex module (part of the standard perl distribution)
3189defines mathematical functions that work on both the reals and the
3190imaginary numbers. Math::Complex not as efficient as POSIX, but
68dc0745
PP
3191POSIX can't work with complex numbers.
3192
3193Rounding in financial applications can have serious implications, and
3194the rounding method used should be specified precisely. In these
3195cases, it probably pays not to trust whichever system rounding is
3196being used by Perl, but to instead implement the rounding function you
3197need yourself.
5a964f20
TC
3198
3199=head2 Bigger Numbers
d74e8afc 3200X<number, arbitrary precision>
5a964f20 3201
c543c01b 3202The standard C<Math::BigInt>, C<Math::BigRat>, and C<Math::BigFloat> modules,
fb1a95c6 3203along with the C<bignum>, C<bigint>, and C<bigrat> pragmas, provide
19799a22 3204variable-precision arithmetic and overloaded operators, although
cd5c4fce 3205they're currently pretty slow. At the cost of some space and
19799a22
GS
3206considerable speed, they avoid the normal pitfalls associated with
3207limited-precision representations.
5a964f20 3208
c543c01b
TC
3209 use 5.010;
3210 use bigint; # easy interface to Math::BigInt
3211 \$x = 123456789123456789;
3212 say \$x * \$x;
3213 +15241578780673678515622620750190521
3214
3215Or with rationals:
3216
3217 use 5.010;
3218 use bigrat;
3219 \$a = 3/22;
3220 \$b = 4/6;
3221 say "a/b is ", \$a/\$b;
3222 say "a*b is ", \$a*\$b;
3223 a/b is 9/44
3224 a*b is 1/11
3225
3226Several modules let you calculate with (bound only by memory and CPU time)
3227unlimited or fixed precision. There are also some non-standard modules that
3228provide faster implementations via external C libraries.
cd5c4fce
T
3229
3230Here is a short, but incomplete summary:
3231
950b09ed
KW
3232 Math::String treat string sequences like numbers
3233 Math::FixedPrecision calculate with a fixed precision
3234 Math::Currency for currency calculations
3235 Bit::Vector manipulate bit vectors fast (uses C)
3236 Math::BigIntFast Bit::Vector wrapper for big numbers
3237 Math::Pari provides access to the Pari C library
70c45be3
FC
3238 Math::Cephes uses the external Cephes C library (no
3239 big numbers)
950b09ed
KW
3240 Math::Cephes::Fraction fractions via the Cephes library
3241 Math::GMP another one using an external C library
70c45be3
FC
3242 Math::GMPz an alternative interface to libgmp's big ints
3243 Math::GMPq an interface to libgmp's fraction numbers
3244 Math::GMPf an interface to libgmp's floating point numbers
cd5c4fce
T
3245
3246Choose wisely.
16070b82
GS
3247
3248=cut