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d74e8afc 2X<operator>
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4perlop - Perl operators and precedence
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89d205f2 8=head2 Operator Precedence and Associativity
d74e8afc 9X<operator, precedence> X<precedence> X<associativity>
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10
11Operator precedence and associativity work in Perl more or less like
12they do in mathematics.
13
14I<Operator precedence> means some operators are evaluated before
15others. For example, in C<2 + 4 * 5>, the multiplication has higher
16precedence so C<4 * 5> is evaluated first yielding C<2 + 20 ==
1722> and not C<6 * 5 == 30>.
18
19I<Operator associativity> defines what happens if a sequence of the
20same operators is used one after another: whether the evaluator will
21evaluate the left operations first or the right. For example, in C<8
22- 4 - 2>, subtraction is left associative so Perl evaluates the
23expression left to right. C<8 - 4> is evaluated first making the
24expression C<4 - 2 == 2> and not C<8 - 2 == 6>.
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25
26Perl operators have the following associativity and precedence,
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27listed from highest precedence to lowest. Operators borrowed from
28C keep the same precedence relationship with each other, even where
29C's precedence is slightly screwy. (This makes learning Perl easier
30for C folks.) With very few exceptions, these all operate on scalar
31values only, not array values.
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32
33 left terms and list operators (leftward)
34 left ->
35 nonassoc ++ --
36 right **
37 right ! ~ \ and unary + and -
54310121 38 left =~ !~
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39 left * / % x
40 left + - .
41 left << >>
42 nonassoc named unary operators
43 nonassoc < > <= >= lt gt le ge
0d863452 44 nonassoc == != <=> eq ne cmp ~~
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45 left &
46 left | ^
47 left &&
c963b151 48 left || //
137443ea 49 nonassoc .. ...
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50 right ?:
51 right = += -= *= etc.
52 left , =>
53 nonassoc list operators (rightward)
a5f75d66 54 right not
a0d0e21e 55 left and
f23102e2 56 left or xor
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57
58In the following sections, these operators are covered in precedence order.
59
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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
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
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
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154before or after the value is returned. This also means that modifying
155a variable twice in the same statement will lead to undefined behaviour.
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
171 print ++(\$foo = '99'); # prints '100'
172 print ++(\$foo = 'a0'); # prints 'a1'
173 print ++(\$foo = 'Az'); # prints 'Ba'
174 print ++(\$foo = 'zz'); # prints 'aaa'
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
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
d74e8afc 191X<unary operator> X<operator, unary>
a0d0e21e 192
5f05dabc 193Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower
a0d0e21e 194precedence version of this.
d74e8afc 195X<!>
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196
197Unary "-" performs arithmetic negation if the operand is numeric. If
198the operand is an identifier, a string consisting of a minus sign
199concatenated with the identifier is returned. Otherwise, if the string
200starts with a plus or minus, a string starting with the opposite sign
bff5667c 201is returned. One effect of these rules is that -bareword is equivalent
8705167b 202to the string "-bareword". If, however, the string begins with a
353c6505 203non-alphabetic character (excluding "+" or "-"), Perl will attempt to convert
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204the string to a numeric and the arithmetic negation is performed. If the
205string cannot be cleanly converted to a numeric, Perl will give the warning
206B<Argument "the string" isn't numeric in negation (-) at ...>.
d74e8afc 207X<-> X<negation, arithmetic>
a0d0e21e 208
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209Unary "~" performs bitwise negation, i.e., 1's complement. For
211L<Bitwise String Operators>.) Note that the width of the result is
212platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64
213bits wide on a 64-bit platform, so if you are expecting a certain bit
d042e63d 214width, remember to use the & operator to mask off the excess bits.
d74e8afc 215X<~> X<negation, binary>
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216
217Unary "+" has no effect whatsoever, even on strings. It is useful
218syntactically for separating a function name from a parenthesized expression
219that would otherwise be interpreted as the complete list of function
5ba421f6 220arguments. (See examples above under L<Terms and List Operators (Leftward)>.)
d74e8afc 221X<+>
a0d0e21e 222
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223Unary "\" creates a reference to whatever follows it. See L<perlreftut>
224and L<perlref>. Do not confuse this behavior with the behavior of
225backslash within a string, although both forms do convey the notion
226of protecting the next thing from interpolation.
d74e8afc 227X<\> X<reference> X<backslash>
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228
d74e8afc 230X<binding> X<operator, binding> X<=~> X<!~>
a0d0e21e 231
c07a80fd 232Binary "=~" binds a scalar expression to a pattern match. Certain operations
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233search or modify the string \$_ by default. This operator makes that kind
234of operation work on some other string. The right argument is a search
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235pattern, substitution, or transliteration. The left argument is what is
236supposed to be searched, substituted, or transliterated instead of the default
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237\$_. When used in scalar context, the return value generally indicates the
238success of the operation. Behavior in list context depends on the particular
89d205f2 239operator. See L</"Regexp Quote-Like Operators"> for details and
d7782e69 240L<perlretut> for examples using these operators.
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241
242If the right argument is an expression rather than a search pattern,
2c268ad5 243substitution, or transliteration, it is interpreted as a search pattern at run
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244time. Note that this means that its contents will be interpolated twice, so
245
246 '\\' =~ q'\\';
247
248is not ok, as the regex engine will end up trying to compile the
249pattern C<\>, which it will consider a syntax error.
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250
251Binary "!~" is just like "=~" except the return value is negated in
252the logical sense.
253
d74e8afc 255X<operator, multiplicative>
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256
257Binary "*" multiplies two numbers.
d74e8afc 258X<*>
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259
260Binary "/" divides two numbers.
d74e8afc 261X</> X<slash>
a0d0e21e 262
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263Binary "%" is the modulo operator, which computes the division
264remainder of its first argument with respect to its second argument.
265Given integer
54310121 266operands C<\$a> and C<\$b>: If C<\$b> is positive, then C<\$a % \$b> is
f7918450 267C<\$a> minus the largest multiple of C<\$b> less than or equal to
54310121 268C<\$a>. If C<\$b> is negative, then C<\$a % \$b> is C<\$a> minus the
269smallest multiple of C<\$b> that is not less than C<\$a> (i.e. the
89b4f0ad 270result will be less than or equal to zero). If the operands
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271C<\$a> and C<\$b> are floating point values and the absolute value of
272C<\$b> (that is C<abs(\$b)>) is less than C<(UV_MAX + 1)>, only
273the integer portion of C<\$a> and C<\$b> will be used in the operation
274(Note: here C<UV_MAX> means the maximum of the unsigned integer type).
275If the absolute value of the right operand (C<abs(\$b)>) is greater than
276or equal to C<(UV_MAX + 1)>, "%" computes the floating-point remainder
277C<\$r> in the equation C<(\$r = \$a - \$i*\$b)> where C<\$i> is a certain
f7918450 278integer that makes C<\$r> have the same sign as the right operand
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279C<\$b> (B<not> as the left operand C<\$a> like C function C<fmod()>)
280and the absolute value less than that of C<\$b>.
0412d526 281Note that when C<use integer> is in scope, "%" gives you direct access
f7918450 282to the modulo operator as implemented by your C compiler. This
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283operator is not as well defined for negative operands, but it will
284execute faster.
f7918450 285X<%> X<remainder> X<modulo> X<mod>
55d729e4 286
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287Binary "x" is the repetition operator. In scalar context or if the left
288operand is not enclosed in parentheses, it returns a string consisting
289of the left operand repeated the number of times specified by the right
290operand. In list context, if the left operand is enclosed in
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291parentheses or is a list formed by C<qw/STRING/>, it repeats the list.
292If the right operand is zero or negative, it returns an empty string
293or an empty list, depending on the context.
d74e8afc 294X<x>
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295
296 print '-' x 80; # print row of dashes
297
298 print "\t" x (\$tab/8), ' ' x (\$tab%8); # tab over
299
300 @ones = (1) x 80; # a list of 80 1's
301 @ones = (5) x @ones; # set all elements to 5
302
303
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306
307Binary "+" returns the sum of two numbers.
d74e8afc 308X<+>
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309
310Binary "-" returns the difference of two numbers.
d74e8afc 311X<->
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312
313Binary "." concatenates two strings.
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314X<string, concatenation> X<concatenation>
315X<cat> X<concat> X<concatenate> X<.>
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316
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318X<shift operator> X<operator, shift> X<<< << >>>
319X<<< >> >>> X<right shift> X<left shift> X<bitwise shift>
320X<shl> X<shr> X<shift, right> X<shift, left>
a0d0e21e 321
55497cff 322Binary "<<" returns the value of its left argument shifted left by the
323number of bits specified by the right argument. Arguments should be
a0d0e21e 325
55497cff 326Binary ">>" returns the value of its left argument shifted right by
327the number of bits specified by the right argument. Arguments should
a0d0e21e 329
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330Note that both "<<" and ">>" in Perl are implemented directly using
331"<<" and ">>" in C. If C<use integer> (see L<Integer Arithmetic>) is
332in force then signed C integers are used, else unsigned C integers are
333used. Either way, the implementation isn't going to generate results
334larger than the size of the integer type Perl was built with (32 bits
335or 64 bits).
336
337The result of overflowing the range of the integers is undefined
338because it is undefined also in C. In other words, using 32-bit
339integers, C<< 1 << 32 >> is undefined. Shifting by a negative number
340of bits is also undefined.
341
d74e8afc 343X<operator, named unary>
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344
345The various named unary operators are treated as functions with one
568e6d8b 346argument, with optional parentheses.
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347
348If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
349is followed by a left parenthesis as the next token, the operator and
350arguments within parentheses are taken to be of highest precedence,
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351just like a normal function call. For example,
352because named unary operators are higher precedence than ||:
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353
354 chdir \$foo || die; # (chdir \$foo) || die
355 chdir(\$foo) || die; # (chdir \$foo) || die
356 chdir (\$foo) || die; # (chdir \$foo) || die
357 chdir +(\$foo) || die; # (chdir \$foo) || die
358
3981b0eb 359but, because * is higher precedence than named operators:
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360
361 chdir \$foo * 20; # chdir (\$foo * 20)
362 chdir(\$foo) * 20; # (chdir \$foo) * 20
363 chdir (\$foo) * 20; # (chdir \$foo) * 20
364 chdir +(\$foo) * 20; # chdir (\$foo * 20)
365
366 rand 10 * 20; # rand (10 * 20)
367 rand(10) * 20; # (rand 10) * 20
368 rand (10) * 20; # (rand 10) * 20
369 rand +(10) * 20; # rand (10 * 20)
370
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371Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are
372treated like named unary operators, but they don't follow this functional
373parenthesis rule. That means, for example, that C<-f(\$file).".bak"> is
374equivalent to C<-f "\$file.bak">.
d74e8afc 375X<-X> X<filetest> X<operator, filetest>
568e6d8b 376
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378
d74e8afc 380X<relational operator> X<operator, relational>
a0d0e21e 381
35f2feb0 382Binary "<" returns true if the left argument is numerically less than
a0d0e21e 383the right argument.
d74e8afc 384X<< < >>
a0d0e21e 385
35f2feb0 386Binary ">" returns true if the left argument is numerically greater
a0d0e21e 387than the right argument.
d74e8afc 388X<< > >>
a0d0e21e 389
35f2feb0 390Binary "<=" returns true if the left argument is numerically less than
a0d0e21e 391or equal to the right argument.
d74e8afc 392X<< <= >>
a0d0e21e 393
35f2feb0 394Binary ">=" returns true if the left argument is numerically greater
a0d0e21e 395than or equal to the right argument.
d74e8afc 396X<< >= >>
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397
398Binary "lt" returns true if the left argument is stringwise less than
399the right argument.
d74e8afc 400X<< lt >>
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401
402Binary "gt" returns true if the left argument is stringwise greater
403than the right argument.
d74e8afc 404X<< gt >>
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405
406Binary "le" returns true if the left argument is stringwise less than
407or equal to the right argument.
d74e8afc 408X<< le >>
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409
410Binary "ge" returns true if the left argument is stringwise greater
411than or equal to the right argument.
d74e8afc 412X<< ge >>
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413
d74e8afc 415X<equality> X<equal> X<equals> X<operator, equality>
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416
417Binary "==" returns true if the left argument is numerically equal to
418the right argument.
d74e8afc 419X<==>
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420
421Binary "!=" returns true if the left argument is numerically not equal
422to the right argument.
d74e8afc 423X<!=>
a0d0e21e 424
35f2feb0 425Binary "<=>" returns -1, 0, or 1 depending on whether the left
6ee5d4e7 426argument is numerically less than, equal to, or greater than the right
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428values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
429"<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
430returns true, as does NaN != anything else. If your platform doesn't
431support NaNs then NaN is just a string with numeric value 0.
d74e8afc 432X<< <=> >> X<spaceship>
7d3a9d88 433
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434 perl -le '\$a = "NaN"; print "No NaN support here" if \$a == \$a'
435 perl -le '\$a = "NaN"; print "NaN support here" if \$a != \$a'
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436
437Binary "eq" returns true if the left argument is stringwise equal to
438the right argument.
d74e8afc 439X<eq>
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440
441Binary "ne" returns true if the left argument is stringwise not equal
442to the right argument.
d74e8afc 443X<ne>
a0d0e21e 444
JH
445Binary "cmp" returns -1, 0, or 1 depending on whether the left
446argument is stringwise less than, equal to, or greater than the right
447argument.
d74e8afc 448X<cmp>
a0d0e21e 449
0d863452 450Binary "~~" does a smart match between its arguments. Smart matching
0f7107a0 451is described in L<perlsyn/"Smart matching in detail">.
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452X<~~>
453
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454"lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
455by the current locale if C<use locale> is in effect. See L<perllocale>.
456
d74e8afc 458X<operator, bitwise, and> X<bitwise and> X<&>
a0d0e21e 459
2cdc098b 460Binary "&" returns its operands ANDed together bit by bit.
a0d0e21e 462
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463Note that "&" has lower priority than relational operators, so for example
464the brackets are essential in a test like
465
466 print "Even\n" if (\$x & 1) == 0;
467
a0d0e21e 468=head2 Bitwise Or and Exclusive Or
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469X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor>
470X<bitwise xor> X<^>
a0d0e21e 471
2cdc098b 472Binary "|" returns its operands ORed together bit by bit.
a0d0e21e 474
2cdc098b 475Binary "^" returns its operands XORed together bit by bit.
a0d0e21e 477
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478Note that "|" and "^" have lower priority than relational operators, so
479for example the brackets are essential in a test like
480
481 print "false\n" if (8 | 2) != 10;
482
d74e8afc 484X<&&> X<logical and> X<operator, logical, and>
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485
486Binary "&&" performs a short-circuit logical AND operation. That is,
487if the left operand is false, the right operand is not even evaluated.
488Scalar or list context propagates down to the right operand if it
489is evaluated.
490
d74e8afc 492X<||> X<operator, logical, or>
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493
494Binary "||" performs a short-circuit logical OR operation. That is,
495if the left operand is true, the right operand is not even evaluated.
496Scalar or list context propagates down to the right operand if it
497is evaluated.
498
d74e8afc 500X<//> X<operator, logical, defined-or>
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501
502Although it has no direct equivalent in C, Perl's C<//> operator is related
89d205f2 503to its C-style or. In fact, it's exactly the same as C<||>, except that it
c963b151 504tests the left hand side's definedness instead of its truth. Thus, C<\$a // \$b>
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505is similar to C<defined(\$a) || \$b> (except that it returns the value of C<\$a>
506rather than the value of C<defined(\$a)>) and is exactly equivalent to
c963b151 507C<defined(\$a) ? \$a : \$b>. This is very useful for providing default values
89d205f2 508for variables. If you actually want to test if at least one of C<\$a> and
d042e63d 509C<\$b> is defined, use C<defined(\$a // \$b)>.
c963b151 510
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511The C<||>, C<//> and C<&&> operators return the last value evaluated
512(unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably
513portable way to find out the home directory might be:
a0d0e21e 514
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515 \$home = \$ENV{'HOME'} // \$ENV{'LOGDIR'} //
516 (getpwuid(\$<))[7] // die "You're homeless!\n";
a0d0e21e 517
5a964f20
TC
518In particular, this means that you shouldn't use this
519for selecting between two aggregates for assignment:
520
521 @a = @b || @c; # this is wrong
522 @a = scalar(@b) || @c; # really meant this
523 @a = @b ? @b : @c; # this works fine, though
524
f23102e2
RGS
525As more readable alternatives to C<&&> and C<||> when used for
526control flow, Perl provides the C<and> and C<or> operators (see below).
527The short-circuit behavior is identical. The precedence of "and"
c963b151 528and "or" is much lower, however, so that you can safely use them after a
5a964f20 529list operator without the need for parentheses:
a0d0e21e
LW
530
532 or gripe(), next LINE;
533
534With the C-style operators that would have been written like this:
535
537 || (gripe(), next LINE);
538
eeb6a2c9 539Using "or" for assignment is unlikely to do what you want; see below.
5a964f20
TC
540
d74e8afc 542X<operator, range> X<range> X<..> X<...>
a0d0e21e
LW
543
544Binary ".." is the range operator, which is really two different
fb53bbb2 545operators depending on the context. In list context, it returns a
54ae734e 546list of values counting (up by ones) from the left value to the right
2cdbc966 547value. If the left value is greater than the right value then it
fb53bbb2 548returns the empty list. The range operator is useful for writing
54ae734e 549C<foreach (1..10)> loops and for doing slice operations on arrays. In
2cdbc966
JD
550the current implementation, no temporary array is created when the
551range operator is used as the expression in C<foreach> loops, but older
552versions of Perl might burn a lot of memory when you write something
553like this:
a0d0e21e
LW
554
555 for (1 .. 1_000_000) {
556 # code
54310121 557 }
a0d0e21e 558
54ae734e
MG
559The range operator also works on strings, using the magical auto-increment,
560see below.
561
5a964f20 562In scalar context, ".." returns a boolean value. The operator is
a0d0e21e
LW
563bistable, like a flip-flop, and emulates the line-range (comma) operator
564of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
565own boolean state. It is false as long as its left operand is false.
566Once the left operand is true, the range operator stays true until the
567right operand is true, I<AFTER> which the range operator becomes false
19799a22 568again. It doesn't become false till the next time the range operator is
a0d0e21e
LW
569evaluated. It can test the right operand and become false on the same
570evaluation it became true (as in B<awk>), but it still returns true once.
be25f609 571If you don't want it to test the right operand until the next
19799a22
GS
572evaluation, as in B<sed>, just use three dots ("...") instead of
573two. In all other regards, "..." behaves just like ".." does.
574
575The right operand is not evaluated while the operator is in the
576"false" state, and the left operand is not evaluated while the
577operator is in the "true" state. The precedence is a little lower
578than || and &&. The value returned is either the empty string for
579false, or a sequence number (beginning with 1) for true. The
580sequence number is reset for each range encountered. The final
581sequence number in a range has the string "E0" appended to it, which
582doesn't affect its numeric value, but gives you something to search
583for if you want to exclude the endpoint. You can exclude the
584beginning point by waiting for the sequence number to be greater
df5f8116
CW
585than 1.
586
587If either operand of scalar ".." is a constant expression,
588that operand is considered true if it is equal (C<==>) to the current
589input line number (the C<\$.> variable).
590
591To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>,
592but that is only an issue if you use a floating point expression; when
593implicitly using C<\$.> as described in the previous paragraph, the
594comparison is C<int(EXPR) == int(\$.)> which is only an issue when C<\$.>
595is set to a floating point value and you are not reading from a file.
596Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what
597you want in scalar context because each of the operands are evaluated
598using their integer representation.
599
600Examples:
a0d0e21e
LW
601
602As a scalar operator:
603
df5f8116
CW
604 if (101 .. 200) { print; } # print 2nd hundred lines, short for
605 # if (\$. == 101 .. \$. == 200) ...
9f10b797
RGS
606
607 next LINE if (1 .. /^\$/); # skip header lines, short for
df5f8116 608 # ... if (\$. == 1 .. /^\$/);
9f10b797
RGS
609 # (typically in a loop labeled LINE)
610
611 s/^/> / if (/^\$/ .. eof()); # quote body
a0d0e21e 612
5a964f20
TC
613 # parse mail messages
614 while (<>) {
615 \$in_header = 1 .. /^\$/;
df5f8116
CW
616 \$in_body = /^\$/ .. eof;
618 # ...
619 } else { # in body
620 # ...
621 }
5a964f20 622 } continue {
df5f8116 623 close ARGV if eof; # reset \$. each file
5a964f20
TC
624 }
625
acf31ca5
SF
626Here's a simple example to illustrate the difference between
627the two range operators:
628
629 @lines = (" - Foo",
630 "01 - Bar",
631 "1 - Baz",
632 " - Quux");
633
9f10b797
RGS
634 foreach (@lines) {
635 if (/0/ .. /1/) {
acf31ca5
SF
636 print "\$_\n";
637 }
638 }
639
9f10b797
RGS
640This program will print only the line containing "Bar". If
641the range operator is changed to C<...>, it will also print the
acf31ca5
SF
642"Baz" line.
643
644And now some examples as a list operator:
a0d0e21e
LW
645
646 for (101 .. 200) { print; } # print \$_ 100 times
3e3baf6d 647 @foo = @foo[0 .. \$#foo]; # an expensive no-op
a0d0e21e
LW
648 @foo = @foo[\$#foo-4 .. \$#foo]; # slice last 5 items
649
5a964f20 650The range operator (in list context) makes use of the magical
5f05dabc 651auto-increment algorithm if the operands are strings. You
a0d0e21e
LW
652can say
653
654 @alphabet = ('A' .. 'Z');
655
54ae734e 656to get all normal letters of the English alphabet, or
a0d0e21e
LW
657
658 \$hexdigit = (0 .. 9, 'a' .. 'f')[\$num & 15];
659
660to get a hexadecimal digit, or
661
662 @z2 = ('01' .. '31'); print \$z2[\$mday];
663
ea4f5703
YST
664to get dates with leading zeros.
665
666If the final value specified is not in the sequence that the magical
667increment would produce, the sequence goes until the next value would
668be longer than the final value specified.
669
670If the initial value specified isn't part of a magical increment
671sequence (that is, a non-empty string matching "/^[a-zA-Z]*[0-9]*\z/"),
672only the initial value will be returned. So the following will only
673return an alpha:
674
675 use charnames 'greek';
676 my @greek_small = ("\N{alpha}" .. "\N{omega}");
677
678To get lower-case greek letters, use this instead:
679
680 my @greek_small = map { chr } ( ord("\N{alpha}") .. ord("\N{omega}") );
a0d0e21e 681
df5f8116
CW
682Because each operand is evaluated in integer form, C<2.18 .. 3.14> will
683return two elements in list context.
684
685 @list = (2.18 .. 3.14); # same as @list = (2 .. 3);
686
d74e8afc 688X<operator, conditional> X<operator, ternary> X<ternary> X<?:>
a0d0e21e
LW
689
690Ternary "?:" is the conditional operator, just as in C. It works much
691like an if-then-else. If the argument before the ? is true, the
692argument before the : is returned, otherwise the argument after the :
cb1a09d0
693is returned. For example:
694
54310121 695 printf "I have %d dog%s.\n", \$n,
cb1a09d0
696 (\$n == 1) ? '' : "s";
697
698Scalar or list context propagates downward into the 2nd
54310121 699or 3rd argument, whichever is selected.
cb1a09d0
700
701 \$a = \$ok ? \$b : \$c; # get a scalar
702 @a = \$ok ? @b : @c; # get an array
703 \$a = \$ok ? @b : @c; # oops, that's just a count!
704
705The operator may be assigned to if both the 2nd and 3rd arguments are
706legal lvalues (meaning that you can assign to them):
a0d0e21e
LW
707
708 (\$a_or_b ? \$a : \$b) = \$c;
709
5a964f20
TC
710Because this operator produces an assignable result, using assignments
711without parentheses will get you in trouble. For example, this:
712
713 \$a % 2 ? \$a += 10 : \$a += 2
714
715Really means this:
716
717 ((\$a % 2) ? (\$a += 10) : \$a) += 2
718
719Rather than this:
720
721 (\$a % 2) ? (\$a += 10) : (\$a += 2)
722
19799a22
GS
723That should probably be written more simply as:
724
725 \$a += (\$a % 2) ? 10 : 2;
726
d74e8afc 728X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=>
5ac3b81c 729X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=>
d74e8afc 730X<%=> X<^=> X<x=>
a0d0e21e
LW
731
732"=" is the ordinary assignment operator.
733
734Assignment operators work as in C. That is,
735
736 \$a += 2;
737
738is equivalent to
739
740 \$a = \$a + 2;
741
742although without duplicating any side effects that dereferencing the lvalue
54310121 743might trigger, such as from tie(). Other assignment operators work similarly.
744The following are recognized:
a0d0e21e
LW
745
746 **= += *= &= <<= &&=
9f10b797
RGS
747 -= /= |= >>= ||=
748 .= %= ^= //=
749 x=
a0d0e21e 750
19799a22 751Although these are grouped by family, they all have the precedence
a0d0e21e
LW
752of assignment.
753
b350dd2f
GS
754Unlike in C, the scalar assignment operator produces a valid lvalue.
755Modifying an assignment is equivalent to doing the assignment and
756then modifying the variable that was assigned to. This is useful
757for modifying a copy of something, like this:
a0d0e21e
LW
758
759 (\$tmp = \$global) =~ tr [A-Z] [a-z];
760
761Likewise,
762
763 (\$a += 2) *= 3;
764
765is equivalent to
766
767 \$a += 2;
768 \$a *= 3;
769
b350dd2f
GS
770Similarly, a list assignment in list context produces the list of
771lvalues assigned to, and a list assignment in scalar context returns
772the number of elements produced by the expression on the right hand
773side of the assignment.
774
d74e8afc 776X<comma> X<operator, comma> X<,>
a0d0e21e 777
5a964f20 778Binary "," is the comma operator. In scalar context it evaluates
a0d0e21e
LW
779its left argument, throws that value away, then evaluates its right
780argument and returns that value. This is just like C's comma operator.
781
5a964f20 782In list context, it's just the list argument separator, and inserts
ed5c6d31
PJ
783both its arguments into the list. These arguments are also evaluated
784from left to right.
a0d0e21e 785
d042e63d 786The C<< => >> operator is a synonym for the comma, but forces any word
719b43e8 787(consisting entirely of word characters) to its left to be interpreted
a44e5664
MS
788as a string (as of 5.001). This includes words that might otherwise be
789considered a constant or function call.
790
791 use constant FOO => "something";
792
793 my %h = ( FOO => 23 );
794
795is equivalent to:
796
797 my %h = ("FOO", 23);
798
799It is I<NOT>:
800
801 my %h = ("something", 23);
802
803If the argument on the left is not a word, it is first interpreted as
804an expression, and then the string value of that is used.
719b43e8
RGS
805
806The C<< => >> operator is helpful in documenting the correspondence
807between keys and values in hashes, and other paired elements in lists.
748a9306 808
a44e5664
MS
809 %hash = ( \$key => \$value );
811
813X<...> X<... operator> X<!!!> X<!!! operator> X<???> X<??? operator>
815
816The yada yada operators are placeholders for code. They parse without error,
817but when executed either throw an exception or a warning.
818
819The C<...> operator takes no arguments. When executed, it throws an exception
820with the text C<Unimplemented>:
821
822 sub foo { ... }
823 foo();
824
825 Unimplemented at <file> line <line number>.
826
827The C<!!!> operator is similar, but it takes one argument, a string to use as
828the text of the exception:
829
830 sub bar { !!! "Don't call me, Ishmael!" }
831 bar();
832
833 Don't call me, Ishmael! at <file> line <line number>.
834
835The C<???> operator also takes one argument, but it emits a warning instead of
836throwing an exception:
837
d3c2e021 838 sub baz { ??? "Who are you? What do you want?" }
be25f609 839 baz();
840 say "Why are you here?";
841
842 Who are you? What do you want? at <file> line <line number>.
843 Why are you here?
844
d74e8afc 846X<operator, list, rightward> X<list operator>
a0d0e21e
LW
847
848On the right side of a list operator, it has very low precedence,
849such that it controls all comma-separated expressions found there.
850The only operators with lower precedence are the logical operators
851"and", "or", and "not", which may be used to evaluate calls to list
852operators without the need for extra parentheses:
853
854 open HANDLE, "filename"
855 or die "Can't open: \$!\n";
856
5ba421f6 857See also discussion of list operators in L<Terms and List Operators (Leftward)>.
a0d0e21e
LW
858
d74e8afc 860X<operator, logical, not> X<not>
a0d0e21e
LW
861
862Unary "not" returns the logical negation of the expression to its right.
863It's the equivalent of "!" except for the very low precedence.
864
d74e8afc 866X<operator, logical, and> X<and>
a0d0e21e
LW
867
868Binary "and" returns the logical conjunction of the two surrounding
869expressions. It's equivalent to && except for the very low
5f05dabc 870precedence. This means that it short-circuits: i.e., the right
a0d0e21e
LW
871expression is evaluated only if the left expression is true.
872
c963b151 873=head2 Logical or, Defined or, and Exclusive Or
f23102e2 874X<operator, logical, or> X<operator, logical, xor>
d74e8afc 875X<operator, logical, defined or> X<operator, logical, exclusive or>
f23102e2 876X<or> X<xor>
a0d0e21e
LW
877
878Binary "or" returns the logical disjunction of the two surrounding
5a964f20
TC
879expressions. It's equivalent to || except for the very low precedence.
880This makes it useful for control flow
881
882 print FH \$data or die "Can't write to FH: \$!";
883
884This means that it short-circuits: i.e., the right expression is evaluated
885only if the left expression is false. Due to its precedence, you should
886probably avoid using this for assignment, only for control flow.
887
888 \$a = \$b or \$c; # bug: this is wrong
889 (\$a = \$b) or \$c; # really means this
890 \$a = \$b || \$c; # better written this way
891
19799a22 892However, when it's a list-context assignment and you're trying to use
5a964f20
TC
893"||" for control flow, you probably need "or" so that the assignment
894takes higher precedence.
895
896 @info = stat(\$file) || die; # oops, scalar sense of stat!
897 @info = stat(\$file) or die; # better, now @info gets its due
898
c963b151
BD
899Then again, you could always use parentheses.
900
a0d0e21e
LW
901Binary "xor" returns the exclusive-OR of the two surrounding expressions.
902It cannot short circuit, of course.
903
904=head2 C Operators Missing From Perl
d74e8afc
ITB
905X<operator, missing from perl> X<&> X<*>
906X<typecasting> X<(TYPE)>
a0d0e21e
LW
907
908Here is what C has that Perl doesn't:
909
910=over 8
911
912=item unary &
913
914Address-of operator. (But see the "\" operator for taking a reference.)
915
916=item unary *
917
54310121 918Dereference-address operator. (Perl's prefix dereferencing
a0d0e21e
LW
919operators are typed: \$, @, %, and &.)
920
921=item (TYPE)
922
19799a22 923Type-casting operator.
a0d0e21e
LW
924
925=back
926
5f05dabc 927=head2 Quote and Quote-like Operators
89d205f2 928X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m>
d74e8afc
ITB
929X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>>
930X<escape sequence> X<escape>
931
a0d0e21e
LW
932
933While we usually think of quotes as literal values, in Perl they
934function as operators, providing various kinds of interpolating and
935pattern matching capabilities. Perl provides customary quote characters
936for these behaviors, but also provides a way for you to choose your
937quote character for any of them. In the following table, a C<{}> represents
9f10b797 938any pair of delimiters you choose.
a0d0e21e 939
TP
940 Customary Generic Meaning Interpolates
941 '' q{} Literal no
942 "" qq{} Literal yes
af9219ee 943 `` qx{} Command yes*
2c268ad5 944 qw{} Word list no
af9219ee
MG
945 // m{} Pattern match yes*
946 qr{} Pattern yes*
947 s{}{} Substitution yes*
2c268ad5 948 tr{}{} Transliteration no (but see below)
7e3b091d 949 <<EOF here-doc yes*
a0d0e21e 950
af9219ee
MG
951 * unless the delimiter is ''.
952
87275199
GS
953Non-bracketing delimiters use the same character fore and aft, but the four
954sorts of brackets (round, angle, square, curly) will all nest, which means
9f10b797 955that
87275199 956
9f10b797 957 q{foo{bar}baz}
35f2feb0 958
9f10b797 959is the same as
87275199
GS
960
961 'foo{bar}baz'
962
963Note, however, that this does not always work for quoting Perl code:
964
965 \$s = q{ if(\$a eq "}") ... }; # WRONG
966
83df6a1d
JH
967is a syntax error. The C<Text::Balanced> module (from CPAN, and
968starting from Perl 5.8 part of the standard distribution) is able
969to do this properly.
87275199 970
19799a22 971There can be whitespace between the operator and the quoting
fb73857a 972characters, except when C<#> is being used as the quoting character.
19799a22
GS
973C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
974operator C<q> followed by a comment. Its argument will be taken
975from the next line. This allows you to write:
fb73857a 976
977 s {foo} # Replace foo
978 {bar} # with bar.
979
904501ec
MG
980The following escape sequences are available in constructs that interpolate
981and in transliterations.
d74e8afc 982X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N>
a0d0e21e 983
6ee5d4e7 984 \t tab (HT, TAB)
5a964f20 985 \n newline (NL)
6ee5d4e7 986 \r return (CR)
987 \f form feed (FF)
988 \b backspace (BS)
989 \a alarm (bell) (BEL)
990 \e escape (ESC)
ee9f418e
WL
991 \033 octal char (example: ESC)
992 \x1b hex char (example: ESC)
993 \x{263a} wide hex char (example: SMILEY)
994 \c[ control char (example: ESC)
95cc3e0c 995 \N{name} named Unicode character
ee9f418e
WL
997The character following C<\c> is mapped to some other character by
998converting letters to upper case and then (on ASCII systems) by inverting
999the 7th bit (0x40). The most interesting range is from '@' to '_'
1000(0x40 through 0x5F), resulting in a control character from 0x00
1001through 0x1F. A '?' maps to the DEL character. On EBCDIC systems only
1002'@', the letters, '[', '\', ']', '^', '_' and '?' will work, resulting
1003in 0x00 through 0x1F and 0x7F.
1004
4c77eaa2 1005B<NOTE>: Unlike C and other languages, Perl has no \v escape sequence for
ee9f418e 1006the vertical tab (VT - ASCII 11), but you may use C<\ck> or C<\x0b>.
4c77eaa2 1007
904501ec
MG
1008The following escape sequences are available in constructs that interpolate
1009but not in transliterations.
d74e8afc 1010X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q>
904501ec 1011
a0d0e21e
LW
1012 \l lowercase next char
1013 \u uppercase next char
1014 \L lowercase till \E
1015 \U uppercase till \E
1016 \E end case modification
1d2dff63 1017 \Q quote non-word characters till \E
a0d0e21e 1018
95cc3e0c
JH
1019If C<use locale> is in effect, the case map used by C<\l>, C<\L>,
1020C<\u> and C<\U> is taken from the current locale. See L<perllocale>.
1021If Unicode (for example, C<\N{}> or wide hex characters of 0x100 or
1022beyond) is being used, the case map used by C<\l>, C<\L>, C<\u> and
1023C<\U> is as defined by Unicode. For documentation of C<\N{name}>,
1024see L<charnames>.
a034a98d 1025
5a964f20
TC
1026All systems use the virtual C<"\n"> to represent a line terminator,
1027called a "newline". There is no such thing as an unvarying, physical
19799a22 1028newline character. It is only an illusion that the operating system,
5a964f20
TC
1029device drivers, C libraries, and Perl all conspire to preserve. Not all
1030systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
1031on a Mac, these are reversed, and on systems without line terminator,
1032printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
1033you mean a "newline" for your system, but use the literal ASCII when you
1034need an exact character. For example, most networking protocols expect
2a380090 1035and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
5a964f20
TC
1036and although they often accept just C<"\012">, they seldom tolerate just
1037C<"\015">. If you get in the habit of using C<"\n"> for networking,
1038you may be burned some day.
d74e8afc
ITB
1039X<newline> X<line terminator> X<eol> X<end of line>
1040X<\n> X<\r> X<\r\n>
5a964f20 1041
904501ec
MG
1042For constructs that do interpolate, variables beginning with "C<\$>"
1043or "C<@>" are interpolated. Subscripted variables such as C<\$a[3]> or
A
1044C<< \$href->{key}[0] >> are also interpolated, as are array and hash slices.
1045But method calls such as C<< \$obj->meth >> are not.
af9219ee
MG
1046
1047Interpolating an array or slice interpolates the elements in order,
1048separated by the value of C<\$">, so is equivalent to interpolating
6deea57f
TS
1049C<join \$", @array>. "Punctuation" arrays such as C<@*> are only
1050interpolated if the name is enclosed in braces C<@{*}>, but special
1051arrays C<@_>, C<@+>, and C<@-> are interpolated, even without braces.
af9219ee 1052
89d205f2
YO
1053You cannot include a literal C<\$> or C<@> within a C<\Q> sequence.
1054An unescaped C<\$> or C<@> interpolates the corresponding variable,
1d2dff63 1055while escaping will cause the literal string C<\\$> to be inserted.
89d205f2 1056You'll need to write something like C<m/\Quser\E\@\Qhost/>.
1d2dff63 1057
a0d0e21e
LW
1058Patterns are subject to an additional level of interpretation as a
1059regular expression. This is done as a second pass, after variables are
1060interpolated, so that regular expressions may be incorporated into the
1061pattern from the variables. If this is not what you want, use C<\Q> to
1062interpolate a variable literally.
1063
19799a22
GS
1064Apart from the behavior described above, Perl does not expand
1065multiple levels of interpolation. In particular, contrary to the
1066expectations of shell programmers, back-quotes do I<NOT> interpolate
1067within double quotes, nor do single quotes impede evaluation of
1068variables when used within double quotes.
a0d0e21e 1069
d74e8afc 1071X<operator, regexp>
cb1a09d0 1072
5f05dabc 1073Here are the quote-like operators that apply to pattern
cb1a09d0
1074matching and related activities.
1075
a0d0e21e
LW
1076=over 8
1077
87e95b7f 1078=item qr/STRING/msixpo
01c6f5f4 1079X<qr> X</i> X</m> X</o> X</s> X</x> X</p>
a0d0e21e 1080
87e95b7f
YO
1081This operator quotes (and possibly compiles) its I<STRING> as a regular
1082expression. I<STRING> is interpolated the same way as I<PATTERN>
1083in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
1084is done. Returns a Perl value which may be used instead of the
64c5a566 1085corresponding C</STRING/msixpo> expression. The returned value is a
85dd5c8b 1086normalized version of the original pattern. It magically differs from
64c5a566 1087a string containing the same characters: C<ref(qr/x/)> returns "Regexp",
85dd5c8b 1088even though dereferencing the result returns undef.
a0d0e21e 1089
87e95b7f
YO
1090For example,
1091
1092 \$rex = qr/my.STRING/is;
85dd5c8b 1093 print \$rex; # prints (?si-xm:my.STRING)
87e95b7f
YO
1094 s/\$rex/foo/;
1095
1096is equivalent to
1097
1098 s/my.STRING/foo/is;
1099
1100The result may be used as a subpattern in a match:
1101
1102 \$re = qr/\$pattern/;
1103 \$string =~ /foo\${re}bar/; # can be interpolated in other patterns
1104 \$string =~ \$re; # or used standalone
1105 \$string =~ /\$re/; # or this way
1106
1107Since Perl may compile the pattern at the moment of execution of qr()
1108operator, using qr() may have speed advantages in some situations,
1109notably if the result of qr() is used standalone:
1110
1111 sub match {
1112 my \$patterns = shift;
1113 my @compiled = map qr/\$_/i, @\$patterns;
1114 grep {
1115 my \$success = 0;
1116 foreach my \$pat (@compiled) {
1117 \$success = 1, last if /\$pat/;
1118 }
1119 \$success;
1120 } @_;
5a964f20
TC
1121 }
1122
87e95b7f
YO
1123Precompilation of the pattern into an internal representation at
1124the moment of qr() avoids a need to recompile the pattern every
1125time a match C</\$pat/> is attempted. (Perl has many other internal
1126optimizations, but none would be triggered in the above example if
1127we did not use qr() operator.)
1128
1129Options are:
1130
1131 m Treat string as multiple lines.
1132 s Treat string as single line. (Make . match a newline)
1133 i Do case-insensitive pattern matching.
1134 x Use extended regular expressions.
1135 p When matching preserve a copy of the matched string so
1136 that \${^PREMATCH}, \${^MATCH}, \${^POSTMATCH} will be defined.
1137 o Compile pattern only once.
1138
1139If a precompiled pattern is embedded in a larger pattern then the effect
1140of 'msixp' will be propagated appropriately. The effect of the 'o'
1141modifier has is not propagated, being restricted to those patterns
1142explicitly using it.
1143
1144See L<perlre> for additional information on valid syntax for STRING, and
1145for a detailed look at the semantics of regular expressions.
a0d0e21e 1146
87e95b7f 1147=item m/PATTERN/msixpogc
89d205f2
YO
1148X<m> X<operator, match>
1149X<regexp, options> X<regexp> X<regex, options> X<regex>
01c6f5f4 1150X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c>
a0d0e21e 1151
87e95b7f 1152=item /PATTERN/msixpogc
a0d0e21e 1153
5a964f20 1154Searches a string for a pattern match, and in scalar context returns
19799a22
GS
1155true if it succeeds, false if it fails. If no string is specified
1156via the C<=~> or C<!~> operator, the \$_ string is searched. (The
1157string specified with C<=~> need not be an lvalue--it may be the
1158result of an expression evaluation, but remember the C<=~> binds
1160discussion of additional considerations that apply when C<use locale>
1161is in effect.
a0d0e21e 1162
01c6f5f4
RGS
1163Options are as described in C<qr//>; in addition, the following match
1164process modifiers are available:
a0d0e21e 1165
cde0cee5
YO
1166 g Match globally, i.e., find all occurrences.
1167 c Do not reset search position on a failed match when /g is in effect.
a0d0e21e
LW
1168
1169If "/" is the delimiter then the initial C<m> is optional. With the C<m>
89d205f2 1170you can use any pair of non-alphanumeric, non-whitespace characters
19799a22
GS
1171as delimiters. This is particularly useful for matching path names
1172that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
7bac28a0 1173the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
19799a22 1174If "'" is the delimiter, no interpolation is performed on the PATTERN.
a0d0e21e
LW
1175
1176PATTERN may contain variables, which will be interpolated (and the
f70b4f9c 1177pattern recompiled) every time the pattern search is evaluated, except
1f247705
GS
1178for when the delimiter is a single quote. (Note that C<\$(>, C<\$)>, and
1179C<\$|> are not interpolated because they look like end-of-string tests.)
f70b4f9c
AB
1180If you want such a pattern to be compiled only once, add a C</o> after
1181the trailing delimiter. This avoids expensive run-time recompilations,
1182and is useful when the value you are interpolating won't change over
1183the life of the script. However, mentioning C</o> constitutes a promise
1184that you won't change the variables in the pattern. If you change them,
a0d0e21e 1186
e9d89077
DN
1187=item The empty pattern //
1188
5a964f20 1189If the PATTERN evaluates to the empty string, the last
d65afb4b
HS
1190I<successfully> matched regular expression is used instead. In this
1191case, only the C<g> and C<c> flags on the empty pattern is honoured -
1192the other flags are taken from the original pattern. If no match has
1193previously succeeded, this will (silently) act instead as a genuine
1194empty pattern (which will always match).
a0d0e21e 1195
89d205f2
YO
1196Note that it's possible to confuse Perl into thinking C<//> (the empty
1197regex) is really C<//> (the defined-or operator). Perl is usually pretty
1199C<\$a///> (is that C<(\$a) / (//)> or C<\$a // />?) and C<print \$fh //>
1200(C<print \$fh(//> or C<print(\$fh //>?). In all of these examples, Perl
1201will assume you meant defined-or. If you meant the empty regex, just
1202use parentheses or spaces to disambiguate, or even prefix the empty
c963b151
BD
1203regex with an C<m> (so C<//> becomes C<m//>).
1204
e9d89077
DN
1205=item Matching in list context
1206
19799a22 1207If the C</g> option is not used, C<m//> in list context returns a
a0d0e21e 1208list consisting of the subexpressions matched by the parentheses in the
f7e33566
GS
1209pattern, i.e., (C<\$1>, C<\$2>, C<\$3>...). (Note that here C<\$1> etc. are
1210also set, and that this differs from Perl 4's behavior.) When there are
1211no parentheses in the pattern, the return value is the list C<(1)> for
1212success. With or without parentheses, an empty list is returned upon
1213failure.
a0d0e21e
LW
1214
1215Examples:
1216
1217 open(TTY, '/dev/tty');
1218 <TTY> =~ /^y/i && foo(); # do foo if desired
1219
1220 if (/Version: *([0-9.]*)/) { \$version = \$1; }
1221
1222 next if m#^/usr/spool/uucp#;
1223
1224 # poor man's grep
1225 \$arg = shift;
1226 while (<>) {
1227 print if /\$arg/o; # compile only once
1228 }
1229
1230 if ((\$F1, \$F2, \$Etc) = (\$foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
1231
1232This last example splits \$foo into the first two words and the
5f05dabc 1233remainder of the line, and assigns those three fields to \$F1, \$F2, and
1234\$Etc. The conditional is true if any variables were assigned, i.e., if
a0d0e21e
LW
1235the pattern matched.
1236
19799a22
GS
1237The C</g> modifier specifies global pattern matching--that is,
1238matching as many times as possible within the string. How it behaves
1239depends on the context. In list context, it returns a list of the
1240substrings matched by any capturing parentheses in the regular
1241expression. If there are no parentheses, it returns a list of all
1242the matched strings, as if there were parentheses around the whole
1243pattern.
a0d0e21e 1244
7e86de3e 1245In scalar context, each execution of C<m//g> finds the next match,
19799a22 1246returning true if it matches, and false if there is no further match.
7e86de3e
MG
1247The position after the last match can be read or set using the pos()
1248function; see L<perlfunc/pos>. A failed match normally resets the
1249search position to the beginning of the string, but you can avoid that
1250by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
1251string also resets the search position.
c90c0ff4 1252
e9d89077
DN
1253=item \G assertion
1254
c90c0ff4 1255You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
1256zero-width assertion that matches the exact position where the previous
5d43e42d
DC
1257C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion
1258still anchors at pos(), but the match is of course only attempted once.
1259Using C<\G> without C</g> on a target string that has not previously had a
1260C</g> match applied to it is the same as using the C<\A> assertion to match
fe4b3f22
RGS
1261the beginning of the string. Note also that, currently, C<\G> is only
1262properly supported when anchored at the very beginning of the pattern.
c90c0ff4 1263
1264Examples:
a0d0e21e
LW
1265
1266 # list context
1267 (\$one,\$five,\$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
1268
1269 # scalar context
5d43e42d 1270 \$/ = "";
19799a22
GS
1271 while (defined(\$paragraph = <>)) {
1272 while (\$paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
1273 \$sentences++;
a0d0e21e
LW
1274 }
1275 }
1276 print "\$sentences\n";
1277
c90c0ff4 1278 # using m//gc with \G
137443ea 1279 \$_ = "ppooqppqq";
44a8e56a 1280 while (\$i++ < 2) {
1281 print "1: '";
c90c0ff4 1282 print \$1 while /(o)/gc; print "', pos=", pos, "\n";
44a8e56a 1283 print "2: '";
c90c0ff4 1284 print \$1 if /\G(q)/gc; print "', pos=", pos, "\n";
44a8e56a 1285 print "3: '";
c90c0ff4 1286 print \$1 while /(p)/gc; print "', pos=", pos, "\n";
44a8e56a 1287 }
5d43e42d 1288 print "Final: '\$1', pos=",pos,"\n" if /\G(.)/;
44a8e56a 1289
1290The last example should print:
1291
1292 1: 'oo', pos=4
137443ea 1293 2: 'q', pos=5
44a8e56a 1294 3: 'pp', pos=7
1295 1: '', pos=7
137443ea 1296 2: 'q', pos=8
1297 3: '', pos=8
5d43e42d
DC
1298 Final: 'q', pos=8
1299
1300Notice that the final match matched C<q> instead of C<p>, which a match
1301without the C<\G> anchor would have done. Also note that the final match
1302did not update C<pos> -- C<pos> is only updated on a C</g> match. If the
1303final match did indeed match C<p>, it's a good bet that you're running an
1304older (pre-5.6.0) Perl.
44a8e56a 1305
c90c0ff4 1306A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
e7ea3e70 1307combine several regexps like this to process a string part-by-part,
c90c0ff4 1308doing different actions depending on which regexp matched. Each
1309regexp tries to match where the previous one leaves off.
e7ea3e70 1310
3fe9a6f1 1311 \$_ = <<'EOL';
63acfd00 1312 \$url = URI::URL->new( "http://www/" ); die if \$url eq "xXx";
3fe9a6f1 1313 EOL
1314 LOOP:
e7ea3e70 1315 {
c90c0ff4 1316 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
1317 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
1318 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
1319 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
1320 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
1321 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
1322 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
e7ea3e70
IZ
1323 print ". That's all!\n";
1324 }
1325
1326Here is the output (split into several lines):
1327
1328 line-noise lowercase line-noise lowercase UPPERCASE line-noise
1329 UPPERCASE line-noise lowercase line-noise lowercase line-noise
1330 lowercase lowercase line-noise lowercase lowercase line-noise
1331 MiXeD line-noise. That's all!
44a8e56a 1332
87e95b7f
YO
1333=item ?PATTERN?
1334X<?>
1335
1336This is just like the C</pattern/> search, except that it matches only
1337once between calls to the reset() operator. This is a useful
1338optimization when you want to see only the first occurrence of
1339something in each file of a set of files, for instance. Only C<??>
1340patterns local to the current package are reset.
1341
1342 while (<>) {
1343 if (?^\$?) {
1344 # blank line between header and body
1345 }
1346 } continue {
1347 reset if eof; # clear ?? status for next file
1348 }
1349
1350This usage is vaguely deprecated, which means it just might possibly
1351be removed in some distant future version of Perl, perhaps somewhere
1352around the year 2168.
1353
1354=item s/PATTERN/REPLACEMENT/msixpogce
1355X<substitute> X<substitution> X<replace> X<regexp, replace>
01c6f5f4 1356X<regexp, substitute> X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> X</e>
87e95b7f
YO
1357
1358Searches a string for a pattern, and if found, replaces that pattern
1359with the replacement text and returns the number of substitutions
1360made. Otherwise it returns false (specifically, the empty string).
1361
1362If no string is specified via the C<=~> or C<!~> operator, the C<\$_>
1363variable is searched and modified. (The string specified with C<=~> must
1364be scalar variable, an array element, a hash element, or an assignment
1365to one of those, i.e., an lvalue.)
1366
1367If the delimiter chosen is a single quote, no interpolation is
1368done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1369PATTERN contains a \$ that looks like a variable rather than an
1370end-of-string test, the variable will be interpolated into the pattern
1371at run-time. If you want the pattern compiled only once the first time
1372the variable is interpolated, use the C</o> option. If the pattern
1373evaluates to the empty string, the last successfully executed regular
1374expression is used instead. See L<perlre> for further explanation on these.
1375See L<perllocale> for discussion of additional considerations that apply
1376when C<use locale> is in effect.
1377
1378Options are as with m// with the addition of the following replacement
1379specific options:
1380
1381 e Evaluate the right side as an expression.
1382 ee Evaluate the right side as a string then eval the result
1383
1384Any non-alphanumeric, non-whitespace delimiter may replace the
1385slashes. If single quotes are used, no interpretation is done on the
1386replacement string (the C</e> modifier overrides this, however). Unlike
1387Perl 4, Perl 5 treats backticks as normal delimiters; the replacement
1388text is not evaluated as a command. If the
1389PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
1390pair of quotes, which may or may not be bracketing quotes, e.g.,
1391C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
1392replacement portion to be treated as a full-fledged Perl expression
1393and evaluated right then and there. It is, however, syntax checked at
1394compile-time. A second C<e> modifier will cause the replacement portion
1395to be C<eval>ed before being run as a Perl expression.
1396
1397Examples:
1398
1399 s/\bgreen\b/mauve/g; # don't change wintergreen
1400
1401 \$path =~ s|/usr/bin|/usr/local/bin|;
1402
1404
1405 (\$foo = \$bar) =~ s/this/that/; # copy first, then change
1406
1407 \$count = (\$paragraph =~ s/Mister\b/Mr./g); # get change-count
1408
1409 \$_ = 'abc123xyz';
1410 s/\d+/\$&*2/e; # yields 'abc246xyz'
1411 s/\d+/sprintf("%5d",\$&)/e; # yields 'abc 246xyz'
1412 s/\w/\$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1413
1414 s/%(.)/\$percent{\$1}/g; # change percent escapes; no /e
1415 s/%(.)/\$percent{\$1} || \$&/ge; # expr now, so /e
1416 s/^=(\w+)/pod(\$1)/ge; # use function call
1417
1418 # expand variables in \$_, but dynamics only, using
1419 # symbolic dereferencing
1420 s/\\$(\w+)/\${\$1}/g;
1421
1422 # Add one to the value of any numbers in the string
1423 s/(\d+)/1 + \$1/eg;
1424
1425 # This will expand any embedded scalar variable
1426 # (including lexicals) in \$_ : First \$1 is interpolated
1427 # to the variable name, and then evaluated
1428 s/(\\$\w+)/\$1/eeg;
1429
1430 # Delete (most) C comments.
1431 \$program =~ s {
1432 /\* # Match the opening delimiter.
1433 .*? # Match a minimal number of characters.
1434 \*/ # Match the closing delimiter.
1435 } []gsx;
1436
1437 s/^\s*(.*?)\s*\$/\$1/; # trim whitespace in \$_, expensively
1438
1439 for (\$variable) { # trim whitespace in \$variable, cheap
1440 s/^\s+//;
1441 s/\s+\$//;
1442 }
1443
1444 s/([^ ]*) *([^ ]*)/\$2 \$1/; # reverse 1st two fields
1445
1446Note the use of \$ instead of \ in the last example. Unlike
1447B<sed>, we use the \<I<digit>> form in only the left hand side.
1448Anywhere else it's \$<I<digit>>.
1449
1450Occasionally, you can't use just a C</g> to get all the changes
1451to occur that you might want. Here are two common cases:
1452
1453 # put commas in the right places in an integer
1454 1 while s/(\d)(\d\d\d)(?!\d)/\$1,\$2/g;
1455
1456 # expand tabs to 8-column spacing
1457 1 while s/\t+/' ' x (length(\$&)*8 - length(\$`)%8)/e;
1458
1459=back
1460
1462X<operator, quote-like>
1463
01c6f5f4
RGS
1464=over 4
1465
a0d0e21e 1466=item q/STRING/
5d44bfff 1467X<q> X<quote, single> X<'> X<''>
a0d0e21e 1468
5d44bfff 1469=item 'STRING'
a0d0e21e 1470
19799a22 1471A single-quoted, literal string. A backslash represents a backslash
68dc0745 1472unless followed by the delimiter or another backslash, in which case
1473the delimiter or backslash is interpolated.
a0d0e21e
LW
1474
1475 \$foo = q!I said, "You said, 'She said it.'"!;
1476 \$bar = q('This is it.');
68dc0745 1477 \$baz = '\n'; # a two-character string
a0d0e21e
LW
1478
1479=item qq/STRING/
d74e8afc 1480X<qq> X<quote, double> X<"> X<"">
a0d0e21e
LW
1481
1482=item "STRING"
1483
1484A double-quoted, interpolated string.
1485
1486 \$_ .= qq
1487 (*** The previous line contains the naughty word "\$1".\n)
19799a22 1488 if /\b(tcl|java|python)\b/i; # :-)
68dc0745 1489 \$baz = "\n"; # a one-character string
a0d0e21e
LW
1490
1491=item qx/STRING/
d74e8afc 1492X<qx> X<`> X<``> X<backtick>
a0d0e21e
LW
1493
1494=item `STRING`
1495
43dd4d21
JH
1496A string which is (possibly) interpolated and then executed as a
1497system command with C</bin/sh> or its equivalent. Shell wildcards,
1498pipes, and redirections will be honored. The collected standard
1499output of the command is returned; standard error is unaffected. In
1500scalar context, it comes back as a single (potentially multi-line)
1501string, or undef if the command failed. In list context, returns a
1502list of lines (however you've defined lines with \$/ or
1503\$INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
5a964f20
TC
1504
1505Because backticks do not affect standard error, use shell file descriptor
1506syntax (assuming the shell supports this) if you care to address this.
1507To capture a command's STDERR and STDOUT together:
a0d0e21e 1508
5a964f20
TC
1509 \$output = `cmd 2>&1`;
1510
1511To capture a command's STDOUT but discard its STDERR:
1512
1513 \$output = `cmd 2>/dev/null`;
1514
1515To capture a command's STDERR but discard its STDOUT (ordering is
1516important here):
1517
1518 \$output = `cmd 2>&1 1>/dev/null`;
1519
1520To exchange a command's STDOUT and STDERR in order to capture the STDERR
1521but leave its STDOUT to come out the old STDERR:
1522
1523 \$output = `cmd 3>&1 1>&2 2>&3 3>&-`;
1524
1525To read both a command's STDOUT and its STDERR separately, it's easiest
2359510d
SD
1526to redirect them separately to files, and then read from those files
1527when the program is done:
5a964f20 1528
2359510d 1529 system("program args 1>program.stdout 2>program.stderr");
5a964f20 1530
30398227
SP
1531The STDIN filehandle used by the command is inherited from Perl's STDIN.
1532For example:
1533
1534 open BLAM, "blam" || die "Can't open: \$!";
1535 open STDIN, "<&BLAM";
1536 print `sort`;
1537
1538will print the sorted contents of the file "blam".
1539
5a964f20
TC
1540Using single-quote as a delimiter protects the command from Perl's
1541double-quote interpolation, passing it on to the shell instead:
1542
1543 \$perl_info = qx(ps \$\$); # that's Perl's \$\$
1544 \$shell_info = qx'ps \$\$'; # that's the new shell's \$\$
1545
19799a22 1546How that string gets evaluated is entirely subject to the command
5a964f20
TC
1547interpreter on your system. On most platforms, you will have to protect
1548shell metacharacters if you want them treated literally. This is in
1549practice difficult to do, as it's unclear how to escape which characters.
1550See L<perlsec> for a clean and safe example of a manual fork() and exec()
1551to emulate backticks safely.
a0d0e21e 1552
bb32b41a
GS
1553On some platforms (notably DOS-like ones), the shell may not be
1554capable of dealing with multiline commands, so putting newlines in
1555the string may not get you what you want. You may be able to evaluate
1556multiple commands in a single line by separating them with the command
1557separator character, if your shell supports that (e.g. C<;> on many Unix
1558shells; C<&> on the Windows NT C<cmd> shell).
1559
0f897271
GS
1560Beginning with v5.6.0, Perl will attempt to flush all files opened for
1561output before starting the child process, but this may not be supported
1562on some platforms (see L<perlport>). To be safe, you may need to set
1563C<\$|> (\$AUTOFLUSH in English) or call the C<autoflush()> method of
1564C<IO::Handle> on any open handles.
1565
bb32b41a
GS
1566Beware that some command shells may place restrictions on the length
1567of the command line. You must ensure your strings don't exceed this
1568limit after any necessary interpolations. See the platform-specific
1570
5a964f20
TC
1571Using this operator can lead to programs that are difficult to port,
1572because the shell commands called vary between systems, and may in
1573fact not be present at all. As one example, the C<type> command under
1574the POSIX shell is very different from the C<type> command under DOS.
1575That doesn't mean you should go out of your way to avoid backticks
1576when they're the right way to get something done. Perl was made to be
1577a glue language, and one of the things it glues together is commands.
1578Just understand what you're getting yourself into.
bb32b41a 1579
da87341d 1580See L</"I/O Operators"> for more discussion.
a0d0e21e 1581
945c54fd 1582=item qw/STRING/
d74e8afc 1583X<qw> X<quote, list> X<quote, words>
945c54fd
JH
1584
1585Evaluates to a list of the words extracted out of STRING, using embedded
1586whitespace as the word delimiters. It can be understood as being roughly
1587equivalent to:
1588
1589 split(' ', q/STRING/);
1590
efb1e162
CW
1591the differences being that it generates a real list at compile time, and
1592in scalar context it returns the last element in the list. So
945c54fd
JH
1593this expression:
1594
1595 qw(foo bar baz)
1596
1597is semantically equivalent to the list:
1598
1599 'foo', 'bar', 'baz'
1600
1601Some frequently seen examples:
1602
1603 use POSIX qw( setlocale localeconv )
1604 @EXPORT = qw( foo bar baz );
1605
1606A common mistake is to try to separate the words with comma or to
1607put comments into a multi-line C<qw>-string. For this reason, the
89d205f2 1608C<use warnings> pragma and the B<-w> switch (that is, the C<\$^W> variable)
945c54fd
JH
1609produces warnings if the STRING contains the "," or the "#" character.
1610
a0d0e21e 1611
6940069f 1612=item tr/SEARCHLIST/REPLACEMENTLIST/cds
d74e8afc 1613X<tr> X<y> X<transliterate> X</c> X</d> X</s>
a0d0e21e 1614
6940069f 1615=item y/SEARCHLIST/REPLACEMENTLIST/cds
a0d0e21e 1616
2c268ad5 1617Transliterates all occurrences of the characters found in the search list
a0d0e21e
LW
1618with the corresponding character in the replacement list. It returns
1619the number of characters replaced or deleted. If no string is
2c268ad5 1620specified via the =~ or !~ operator, the \$_ string is transliterated. (The
54310121 1621string specified with =~ must be a scalar variable, an array element, a
1622hash element, or an assignment to one of those, i.e., an lvalue.)
89d205f2 1624A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
2c268ad5 1625does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
54310121 1626For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1627SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1628its own pair of quotes, which may or may not be bracketing quotes,
a0d0e21e 1630
cc255d5f 1631Note that C<tr> does B<not> do regular expression character classes
e0c83546 1632such as C<\d> or C<[:lower:]>. The C<tr> operator is not equivalent to
cc255d5f
JH
1633the tr(1) utility. If you want to map strings between lower/upper
1634cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider
1635using the C<s> operator if you need regular expressions.
1636
JH
1637Note also that the whole range idea is rather unportable between
1638character sets--and even within character sets they may cause results
1639you probably didn't expect. A sound principle is to use only ranges
1640that begin from and end at either alphabets of equal case (a-e, A-E),
1641or digits (0-4). Anything else is unsafe. If in doubt, spell out the
1642character sets in full.
1643
a0d0e21e
LW
1644Options:
1645
1646 c Complement the SEARCHLIST.
1647 d Delete found but unreplaced characters.
1648 s Squash duplicate replaced characters.
1649
19799a22
GS
1650If the C</c> modifier is specified, the SEARCHLIST character set
1651is complemented. If the C</d> modifier is specified, any characters
1653(Note that this is slightly more flexible than the behavior of some
1654B<tr> programs, which delete anything they find in the SEARCHLIST,
1655period.) If the C</s> modifier is specified, sequences of characters
1656that were transliterated to the same character are squashed down
1657to a single instance of the character.
a0d0e21e
LW
1658
1659If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1660exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1661than the SEARCHLIST, the final character is replicated till it is long
5a964f20 1662enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
a0d0e21e
LW
1663This latter is useful for counting characters in a class or for
1664squashing character sequences in a class.
1665
1666Examples:
1667
1668 \$ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1669
1670 \$cnt = tr/*/*/; # count the stars in \$_
1671
1672 \$cnt = \$sky =~ tr/*/*/; # count the stars in \$sky
1673
1674 \$cnt = tr/0-9//; # count the digits in \$_
1675
1676 tr/a-zA-Z//s; # bookkeeper -> bokeper
1677
1678 (\$HOST = \$host) =~ tr/a-z/A-Z/;
1679
1680 tr/a-zA-Z/ /cs; # change non-alphas to single space
1681
1682 tr [\200-\377]
1683 [\000-\177]; # delete 8th bit
1684
19799a22
GS
1685If multiple transliterations are given for a character, only the
1686first one is used:
748a9306
LW
1687
1688 tr/AAA/XYZ/
1689
2c268ad5 1690will transliterate any A to X.
748a9306 1691
19799a22 1692Because the transliteration table is built at compile time, neither
a0d0e21e 1693the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
19799a22
GS
1694interpolation. That means that if you want to use variables, you
1695must use an eval():
a0d0e21e
LW
1696
1697 eval "tr/\$oldlist/\$newlist/";
1698 die \$@ if \$@;
1699
1700 eval "tr/\$oldlist/\$newlist/, 1" or die \$@;
1701
7e3b091d 1702=item <<EOF
d74e8afc 1703X<here-doc> X<heredoc> X<here-document> X<<< << >>>
7e3b091d
DA
1704
1705A line-oriented form of quoting is based on the shell "here-document"
1706syntax. Following a C<< << >> you specify a string to terminate
1707the quoted material, and all lines following the current line down to
89d205f2
YO
1708the terminating string are the value of the item.
1709
1710The terminating string may be either an identifier (a word), or some
1711quoted text. An unquoted identifier works like double quotes.
1712There may not be a space between the C<< << >> and the identifier,
1713unless the identifier is explicitly quoted. (If you put a space it
1714will be treated as a null identifier, which is valid, and matches the
1715first empty line.) The terminating string must appear by itself
1716(unquoted and with no surrounding whitespace) on the terminating line.
1717
1718If the terminating string is quoted, the type of quotes used determine
1719the treatment of the text.
1720
1721=over 4
1722
1723=item Double Quotes
1724
1725Double quotes indicate that the text will be interpolated using exactly
1726the same rules as normal double quoted strings.
7e3b091d
DA
1727
1728 print <<EOF;
1729 The price is \$Price.
1730 EOF
1731
1732 print << "EOF"; # same as above
1733 The price is \$Price.
1734 EOF
1735
89d205f2
YO
1736
1737=item Single Quotes
1738
1739Single quotes indicate the text is to be treated literally with no
1740interpolation of its content. This is similar to single quoted
1741strings except that backslashes have no special meaning, with C<\\>
1742being treated as two backslashes and not one as they would in every
1743other quoting construct.
1744
1745This is the only form of quoting in perl where there is no need
1746to worry about escaping content, something that code generators
1747can and do make good use of.
1748
1749=item Backticks
1750
1751The content of the here doc is treated just as it would be if the
1752string were embedded in backticks. Thus the content is interpolated
1753as though it were double quoted and then executed via the shell, with
1754the results of the execution returned.
1755
1756 print << `EOC`; # execute command and get results
7e3b091d 1757 echo hi there
7e3b091d
DA
1758 EOC
1759
89d205f2
YO
1760=back
1761
1762It is possible to stack multiple here-docs in a row:
1763
7e3b091d
DA
1764 print <<"foo", <<"bar"; # you can stack them
1765 I said foo.
1766 foo
1767 I said bar.
1768 bar
1769
1770 myfunc(<< "THIS", 23, <<'THAT');
1771 Here's a line
1772 or two.
1773 THIS
1774 and here's another.
1775 THAT
1776
1777Just don't forget that you have to put a semicolon on the end
1778to finish the statement, as Perl doesn't know you're not going to
1779try to do this:
1780
1781 print <<ABC
1782 179231
1783 ABC
1784 + 20;
1785
872d7e53
TS
1786If you want to remove the line terminator from your here-docs,
1787use C<chomp()>.
1788
1789 chomp(\$string = <<'END');
1790 This is a string.
1791 END
1792
1793If you want your here-docs to be indented with the rest of the code,
1794you'll need to remove leading whitespace from each line manually:
7e3b091d
DA
1795
1796 (\$quote = <<'FINIS') =~ s/^\s+//gm;
89d205f2 1797 The Road goes ever on and on,
7e3b091d
DA
1798 down from the door where it began.
1799 FINIS
1800
1801If you use a here-doc within a delimited construct, such as in C<s///eg>,
1802the quoted material must come on the lines following the final delimiter.
1804
1805 s/this/<<E . 'that'
1806 the other
1807 E
1808 . 'more '/eg;
1809
1810you have to write
1811
89d205f2
YO
1812 s/this/<<E . 'that'
1813 . 'more '/eg;
1814 the other
1815 E
7e3b091d
DA
1816
1817If the terminating identifier is on the last line of the program, you
1818must be sure there is a newline after it; otherwise, Perl will give the
1819warning B<Can't find string terminator "END" anywhere before EOF...>.
1820
89d205f2
YO
1821Additionally, the quoting rules for the end of string identifier are not
1822related to Perl's quoting rules -- C<q()>, C<qq()>, and the like are not
1823supported in place of C<''> and C<"">, and the only interpolation is for
1824backslashing the quoting character:
7e3b091d
DA
1825
1826 print << "abc\"def";
1827 testing...
1828 abc"def
1829
1830Finally, quoted strings cannot span multiple lines. The general rule is
1831that the identifier must be a string literal. Stick with that, and you
1832should be safe.
1833
a0d0e21e
LW
1834=back
1835
75e14d17 1836=head2 Gory details of parsing quoted constructs
d74e8afc 1837X<quote, gory details>
75e14d17 1838
19799a22
GS
1839When presented with something that might have several different
1840interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
1841principle to pick the most probable interpretation. This strategy
1842is so successful that Perl programmers often do not suspect the
1843ambivalence of what they write. But from time to time, Perl's
1844notions differ substantially from what the author honestly meant.
1845
1846This section hopes to clarify how Perl handles quoted constructs.
1847Although the most common reason to learn this is to unravel labyrinthine
1848regular expressions, because the initial steps of parsing are the
1849same for all quoting operators, they are all discussed together.
1850
1851The most important Perl parsing rule is the first one discussed
1852below: when processing a quoted construct, Perl first finds the end
1853of that construct, then interprets its contents. If you understand
1854this rule, you may skip the rest of this section on the first
1856expectations much less frequently than this first one.
1857
1858Some passes discussed below are performed concurrently, but because
1859their results are the same, we consider them individually. For different
1860quoting constructs, Perl performs different numbers of passes, from
6deea57f 1861one to four, but these passes are always performed in the same order.
75e14d17 1862
13a2d996 1863=over 4
75e14d17
IZ
1864
1865=item Finding the end
1866
6deea57f
TS
1867The first pass is finding the end of the quoted construct, where
1868the information about the delimiters is used in parsing.
1869During this search, text between the starting and ending delimiters
1870is copied to a safe location. The text copied gets delimiter-independent.
1871
1872If the construct is a here-doc, the ending delimiter is a line
1873that has a terminating string as the content. Therefore C<<<EOF> is
1874terminated by C<EOF> immediately followed by C<"\n"> and starting
1875from the first column of the terminating line.
1876When searching for the terminating line of a here-doc, nothing
1877is skipped. In other words, lines after the here-doc syntax
1878are compared with the terminating string line by line.
1879
1880For the constructs except here-docs, single characters are used as starting
1881and ending delimiters. If the starting delimiter is an opening punctuation
1882(that is C<(>, C<[>, C<{>, or C<< < >>), the ending delimiter is the
1883corresponding closing punctuation (that is C<)>, C<]>, C<}>, or C<< > >>).
1884If the starting delimiter is an unpaired character like C</> or a closing
1885punctuation, the ending delimiter is same as the starting delimiter.
1886Therefore a C</> terminates a C<qq//> construct, while a C<]> terminates
1887C<qq[]> and C<qq]]> constructs.
1888
1889When searching for single-character delimiters, escaped delimiters
1890and C<\\> are skipped. For example, while searching for terminating C</>,
1891combinations of C<\\> and C<\/> are skipped. If the delimiters are
1892bracketing, nested pairs are also skipped. For example, while searching
1893for closing C<]> paired with the opening C<[>, combinations of C<\\>, C<\]>,
1894and C<\[> are all skipped, and nested C<[> and C<]> are skipped as well.
1895However, when backslashes are used as the delimiters (like C<qq\\> and
1896C<tr\\\>), nothing is skipped.
1897During the search for the end, backslashes that escape delimiters
1898are removed (exactly speaking, they are not copied to the safe location).
75e14d17 1899
19799a22
GS
1900For constructs with three-part delimiters (C<s///>, C<y///>, and
1901C<tr///>), the search is repeated once more.
6deea57f
TS
1902If the first delimiter is not an opening punctuation, three delimiters must
1903be same such as C<s!!!> and C<tr)))>, in which case the second delimiter
1904terminates the left part and starts the right part at once.
1905If the left part is delimited by bracketing punctuations (that is C<()>,
1906C<[]>, C<{}>, or C<< <> >>), the right part needs another pair of
1907delimiters such as C<s(){}> and C<tr[]//>. In these cases, whitespaces
1908and comments are allowed between both parts, though the comment must follow
1909at least one whitespace; otherwise a character expected as the start of
1910the comment may be regarded as the starting delimiter of the right part.
75e14d17 1911
19799a22
GS
1912During this search no attention is paid to the semantics of the construct.
1913Thus:
75e14d17
IZ
1914
1915 "\$hash{"\$foo/\$bar"}"
1916
2a94b7ce 1917or:
75e14d17 1918
89d205f2 1919 m/
2a94b7ce 1920 bar # NOT a comment, this slash / terminated m//!
75e14d17
IZ
1921 /x
1922
19799a22
GS
1923do not form legal quoted expressions. The quoted part ends on the
1924first C<"> and C</>, and the rest happens to be a syntax error.
1925Because the slash that terminated C<m//> was followed by a C<SPACE>,
1926the example above is not C<m//x>, but rather C<m//> with no C</x>
1927modifier. So the embedded C<#> is interpreted as a literal C<#>.
75e14d17 1928
89d205f2
YO
1929Also no attention is paid to C<\c\> (multichar control char syntax) during
1930this search. Thus the second C<\> in C<qq/\c\/> is interpreted as a part
1931of C<\/>, and the following C</> is not recognized as a delimiter.
0d594e51
TS
1932Instead, use C<\034> or C<\x1c> at the end of quoted constructs.
1933
75e14d17 1934=item Interpolation
d74e8afc 1935X<interpolation>
75e14d17 1936
19799a22 1937The next step is interpolation in the text obtained, which is now
89d205f2 1938delimiter-independent. There are multiple cases.
75e14d17 1939
13a2d996 1940=over 4
75e14d17 1941
89d205f2 1942=item C<<<'EOF'>
75e14d17
IZ
1943
1944No interpolation is performed.
6deea57f
TS
1945Note that the combination C<\\> is left intact, since escaped delimiters
1946are not available for here-docs.
75e14d17 1947
6deea57f 1948=item C<m''>, the pattern of C<s'''>
89d205f2 1949
6deea57f
TS
1950No interpolation is performed at this stage.
1951Any backslashed sequences including C<\\> are treated at the stage
1952to L</"parsing regular expressions">.
89d205f2 1953
6deea57f 1954=item C<''>, C<q//>, C<tr'''>, C<y'''>, the replacement of C<s'''>
75e14d17 1955
89d205f2 1956The only interpolation is removal of C<\> from pairs of C<\\>.
6deea57f
TS
1957Therefore C<-> in C<tr'''> and C<y'''> is treated literally
1958as a hyphen and no character range is available.
1959C<\1> in the replacement of C<s'''> does not work as C<\$1>.
89d205f2
YO
1960
1961=item C<tr///>, C<y///>
1962
6deea57f
TS
1963No variable interpolation occurs. String modifying combinations for
1964case and quoting such as C<\Q>, C<\U>, and C<\E> are not recognized.
1965The other escape sequences such as C<\200> and C<\t> and backslashed
1966characters such as C<\\> and C<\-> are converted to appropriate literals.
89d205f2
YO
1967The character C<-> is treated specially and therefore C<\-> is treated
1968as a literal C<->.
75e14d17 1969
89d205f2 1970=item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF">
75e14d17 1971
19799a22
GS
1972C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
1973converted to corresponding Perl constructs. Thus, C<"\$foo\Qbaz\$bar">
1974is converted to C<\$foo . (quotemeta("baz" . \$bar))> internally.
6deea57f
TS
1975The other escape sequences such as C<\200> and C<\t> and backslashed
1976characters such as C<\\> and C<\-> are replaced with appropriate
1977expansions.
2a94b7ce 1978
19799a22
GS
1979Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
1980is interpolated in the usual way. Something like C<"\Q\\E"> has
1981no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
1982result is the same as for C<"\\\\E">. As a general rule, backslashes
1983between C<\Q> and C<\E> may lead to counterintuitive results. So,
1984C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
1985as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
2a94b7ce
IZ
1986
1987 \$str = '\t';
1988 return "\Q\$str";
1989
1990may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
1991
19799a22 1992Interpolated scalars and arrays are converted internally to the C<join> and
92d29cee 1993C<.> catenation operations. Thus, C<"\$foo XXX '@arr'"> becomes:
75e14d17 1994
19799a22 1995 \$foo . " XXX '" . (join \$", @arr) . "'";
75e14d17 1996
19799a22 1997All operations above are performed simultaneously, left to right.
75e14d17 1998
19799a22
GS
1999Because the result of C<"\Q STRING \E"> has all metacharacters
2000quoted, there is no way to insert a literal C<\$> or C<@> inside a
2001C<\Q\E> pair. If protected by C<\>, C<\$> will be quoted to became
2002C<"\\\\$">; if not, it is interpreted as the start of an interpolated
2003scalar.
75e14d17 2004
19799a22 2005Note also that the interpolation code needs to make a decision on
89d205f2 2006where the interpolated scalar ends. For instance, whether
35f2feb0 2007C<< "a \$b -> {c}" >> really means:
75e14d17
IZ
2008
2009 "a " . \$b . " -> {c}";
2010
2a94b7ce 2011or:
75e14d17
IZ
2012
2013 "a " . \$b -> {c};
2014
19799a22
GS
2015Most of the time, the longest possible text that does not include
2016spaces between components and which contains matching braces or
2017brackets. because the outcome may be determined by voting based
2018on heuristic estimators, the result is not strictly predictable.
2019Fortunately, it's usually correct for ambiguous cases.
75e14d17 2020
6deea57f 2021=item the replacement of C<s///>
75e14d17 2022
19799a22 2023Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
6deea57f
TS
2024happens as with C<qq//> constructs.
2025
2026It is at this step that C<\1> is begrudgingly converted to C<\$1> in
2027the replacement text of C<s///>, in order to correct the incorrigible
2028I<sed> hackers who haven't picked up the saner idiom yet. A warning
2029is emitted if the C<use warnings> pragma or the B<-w> command-line flag
2030(that is, the C<\$^W> variable) was set.
2031
2032=item C<RE> in C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
2033
cc74c5bd
TS
2034Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\E>,
2035and interpolation happens (almost) as with C<qq//> constructs.
2036
2037However any other combinations of C<\> followed by a character
2038are not substituted but only skipped, in order to parse them
2039as regular expressions at the following step.
6deea57f 2040As C<\c> is skipped at this step, C<@> of C<\c@> in RE is possibly
1749ea0d 2041treated as an array symbol (for example C<@foo>),
6deea57f 2042even though the same text in C<qq//> gives interpolation of C<\c@>.
6deea57f
TS
2043
2044Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
19799a22
GS
2045a C<#>-comment in a C<//x>-regular expression, no processing is
2046performed whatsoever. This is the first step at which the presence
2047of the C<//x> modifier is relevant.
2048
1749ea0d
TS
2049Interpolation in patterns has several quirks: C<\$|>, C<\$(>, C<\$)>, C<@+>
2050and C<@-> are not interpolated, and constructs C<\$var[SOMETHING]> are
2051voted (by several different estimators) to be either an array element
2052or C<\$var> followed by an RE alternative. This is where the notation
19799a22
GS
2053C<\${arr[\$bar]}> comes handy: C</\${arr[0-9]}/> is interpreted as
2054array element C<-9>, not as a regular expression from the variable
2055C<\$arr> followed by a digit, which would be the interpretation of
2056C</\$arr[0-9]/>. Since voting among different estimators may occur,
2057the result is not predictable.
2058
19799a22
GS
2059The lack of processing of C<\\> creates specific restrictions on
2060the post-processed text. If the delimiter is C</>, one cannot get
2061the combination C<\/> into the result of this step. C</> will
2062finish the regular expression, C<\/> will be stripped to C</> on
2063the previous step, and C<\\/> will be left as is. Because C</> is
2064equivalent to C<\/> inside a regular expression, this does not
2065matter unless the delimiter happens to be character special to the
2066RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
2067alphanumeric char, as in:
2a94b7ce
IZ
2068
2069 m m ^ a \s* b mmx;
2070
19799a22 2071In the RE above, which is intentionally obfuscated for illustration, the
6deea57f 2072delimiter is C<m>, the modifier is C<mx>, and after delimiter-removal the
89d205f2 2073RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
19799a22
GS
2074reason you're encouraged to restrict your delimiters to non-alphanumeric,
2075non-whitespace choices.
75e14d17
IZ
2076
2077=back
2078
19799a22 2079This step is the last one for all constructs except regular expressions,
75e14d17
IZ
2080which are processed further.
2081
6deea57f
TS
2082=item parsing regular expressions
2083X<regexp, parse>
75e14d17 2084
19799a22
GS
2085Previous steps were performed during the compilation of Perl code,
2086but this one happens at run time--although it may be optimized to
2087be calculated at compile time if appropriate. After preprocessing
6deea57f 2088described above, and possibly after evaluation if concatenation,
19799a22
GS
2089joining, casing translation, or metaquoting are involved, the
2090resulting I<string> is passed to the RE engine for compilation.
2091
2092Whatever happens in the RE engine might be better discussed in L<perlre>,
2093but for the sake of continuity, we shall do so here.
2094
2095This is another step where the presence of the C<//x> modifier is
2096relevant. The RE engine scans the string from left to right and
2097converts it to a finite automaton.
2098
2099Backslashed characters are either replaced with corresponding
2100literal strings (as with C<\{>), or else they generate special nodes
2101in the finite automaton (as with C<\b>). Characters special to the
2102RE engine (such as C<|>) generate corresponding nodes or groups of
2103nodes. C<(?#...)> comments are ignored. All the rest is either
2104converted to literal strings to match, or else is ignored (as is
2105whitespace and C<#>-style comments if C<//x> is present).
2106
2107Parsing of the bracketed character class construct, C<[...]>, is
2108rather different than the rule used for the rest of the pattern.
2109The terminator of this construct is found using the same rules as
2110for finding the terminator of a C<{}>-delimited construct, the only
2111exception being that C<]> immediately following C<[> is treated as
2112though preceded by a backslash. Similarly, the terminator of
2113C<(?{...})> is found using the same rules as for finding the
2114terminator of a C<{}>-delimited construct.
2115
2116It is possible to inspect both the string given to RE engine and the
2117resulting finite automaton. See the arguments C<debug>/C<debugcolor>
2118in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
4a4eefd0 2119switch documented in L<perlrun/"Command Switches">.
75e14d17
IZ
2120
2121=item Optimization of regular expressions
d74e8afc 2122X<regexp, optimization>
75e14d17 2123
7522fed5 2124This step is listed for completeness only. Since it does not change
75e14d17 2125semantics, details of this step are not documented and are subject
19799a22
GS
2126to change without notice. This step is performed over the finite
2127automaton that was generated during the previous pass.
2a94b7ce 2128
19799a22
GS
2129It is at this stage that C<split()> silently optimizes C</^/> to
2130mean C</^/m>.
75e14d17
IZ
2131
2132=back
2133
d74e8afc
ITB
2135X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle>
2136X<< <> >> X<@ARGV>
a0d0e21e 2137
54310121 2138There are several I/O operators you should know about.
7b8d334a 2140A string enclosed by backticks (grave accents) first undergoes
19799a22
GS
2141double-quote interpolation. It is then interpreted as an external
2142command, and the output of that command is the value of the
e9c56f9b
JH
2143backtick string, like in a shell. In scalar context, a single string
2144consisting of all output is returned. In list context, a list of
2145values is returned, one per line of output. (You can set C<\$/> to use
2146a different line terminator.) The command is executed each time the
2147pseudo-literal is evaluated. The status value of the command is
2148returned in C<\$?> (see L<perlvar> for the interpretation of C<\$?>).
2149Unlike in B<csh>, no translation is done on the return data--newlines
2150remain newlines. Unlike in any of the shells, single quotes do not
2151hide variable names in the command from interpretation. To pass a
2152literal dollar-sign through to the shell you need to hide it with a
2153backslash. The generalized form of backticks is C<qx//>. (Because
2154backticks always undergo shell expansion as well, see L<perlsec> for
2155security concerns.)
d74e8afc 2156X<qx> X<`> X<``> X<backtick> X<glob>
19799a22
GS
2157
2158In scalar context, evaluating a filehandle in angle brackets yields
2159the next line from that file (the newline, if any, included), or
2160C<undef> at end-of-file or on error. When C<\$/> is set to C<undef>
2161(sometimes known as file-slurp mode) and the file is empty, it
2162returns C<''> the first time, followed by C<undef> subsequently.
2163
2164Ordinarily you must assign the returned value to a variable, but
2165there is one situation where an automatic assignment happens. If
2166and only if the input symbol is the only thing inside the conditional
2167of a C<while> statement (even if disguised as a C<for(;;)> loop),
2168the value is automatically assigned to the global variable \$_,
2169destroying whatever was there previously. (This may seem like an
2170odd thing to you, but you'll use the construct in almost every Perl
17b829fa 2171script you write.) The \$_ variable is not implicitly localized.
19799a22
GS
2172You'll have to put a C<local \$_;> before the loop if you want that
2173to happen.
2174
2175The following lines are equivalent:
a0d0e21e 2176
748a9306 2177 while (defined(\$_ = <STDIN>)) { print; }
7b8d334a 2178 while (\$_ = <STDIN>) { print; }
a0d0e21e
LW
2179 while (<STDIN>) { print; }
2180 for (;<STDIN>;) { print; }
748a9306 2181 print while defined(\$_ = <STDIN>);
7b8d334a 2182 print while (\$_ = <STDIN>);
a0d0e21e
LW
2183 print while <STDIN>;
2184
19799a22 2185This also behaves similarly, but avoids \$_ :
7b8d334a 2186
89d205f2 2187 while (my \$line = <STDIN>) { print \$line }
7b8d334a 2188
19799a22
GS
2189In these loop constructs, the assigned value (whether assignment
2190is automatic or explicit) is then tested to see whether it is
2191defined. The defined test avoids problems where line has a string
2192value that would be treated as false by Perl, for example a "" or
2193a "0" with no trailing newline. If you really mean for such values
2194to terminate the loop, they should be tested for explicitly:
7b8d334a
GS
2195
2196 while ((\$_ = <STDIN>) ne '0') { ... }
2197 while (<STDIN>) { last unless \$_; ... }
2198
35f2feb0 2199In other boolean contexts, C<< <I<filehandle>> >> without an
89d205f2 2200explicit C<defined> test or comparison elicit a warning if the
9f1b1f2d 2201C<use warnings> pragma or the B<-w>
19799a22 2202command-line switch (the C<\$^W> variable) is in effect.
7b8d334a 2203
5f05dabc 2204The filehandles STDIN, STDOUT, and STDERR are predefined. (The
19799a22
GS
2205filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
2206in packages, where they would be interpreted as local identifiers
2207rather than global.) Additional filehandles may be created with
2208the open() function, amongst others. See L<perlopentut> and
2209L<perlfunc/open> for details on this.
d74e8afc 2210X<stdin> X<stdout> X<sterr>
a0d0e21e 2211
35f2feb0 2212If a <FILEHANDLE> is used in a context that is looking for
19799a22
GS
2213a list, a list comprising all input lines is returned, one line per
2214list element. It's easy to grow to a rather large data space this
2215way, so use with care.
a0d0e21e 2216
35f2feb0 2217<FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
35f2feb0
GS
2220The null filehandle <> is special: it can be used to emulate the
2221behavior of B<sed> and B<awk>. Input from <> comes either from
a0d0e21e 2222standard input, or from each file listed on the command line. Here's
35f2feb0 2223how it works: the first time <> is evaluated, the @ARGV array is
5a964f20 2224checked, and if it is empty, C<\$ARGV[0]> is set to "-", which when opened
a0d0e21e
LW
2225gives you standard input. The @ARGV array is then processed as a list
2226of filenames. The loop
2227
2228 while (<>) {
2229 ... # code for each line
2230 }
2231
2232is equivalent to the following Perl-like pseudo code:
2233
3e3baf6d 2234 unshift(@ARGV, '-') unless @ARGV;
a0d0e21e
LW
2235 while (\$ARGV = shift) {
2236 open(ARGV, \$ARGV);
2237 while (<ARGV>) {
2238 ... # code for each line
2239 }
2240 }
2241
19799a22
GS
2242except that it isn't so cumbersome to say, and will actually work.
2243It really does shift the @ARGV array and put the current filename
2244into the \$ARGV variable. It also uses filehandle I<ARGV>
35f2feb0 2245internally--<> is just a synonym for <ARGV>, which
19799a22 2246is magical. (The pseudo code above doesn't work because it treats
35f2feb0 2247<ARGV> as non-magical.)
a0d0e21e 2248
35f2feb0 2249You can modify @ARGV before the first <> as long as the array ends up
a0d0e21e 2250containing the list of filenames you really want. Line numbers (C<\$.>)
19799a22
GS
2251continue as though the input were one big happy file. See the example
2252in L<perlfunc/eof> for how to reset line numbers on each file.
5a964f20 2253
89d205f2 2254If you want to set @ARGV to your own list of files, go right ahead.
5a964f20
TC
2255This sets @ARGV to all plain text files if no @ARGV was given:
2256
2257 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
a0d0e21e 2258
5a964f20
TC
2259You can even set them to pipe commands. For example, this automatically
2260filters compressed arguments through B<gzip>:
2261
2262 @ARGV = map { /\.(gz|Z)\$/ ? "gzip -dc < \$_ |" : \$_ } @ARGV;
2263
2264If you want to pass switches into your script, you can use one of the
a0d0e21e
LW
2265Getopts modules or put a loop on the front like this:
2266
2267 while (\$_ = \$ARGV[0], /^-/) {
2268 shift;
2269 last if /^--\$/;
2270 if (/^-D(.*)/) { \$debug = \$1 }
2271 if (/^-v/) { \$verbose++ }
5a964f20 2272 # ... # other switches
a0d0e21e 2273 }
5a964f20 2274
a0d0e21e 2275 while (<>) {
5a964f20 2276 # ... # code for each line
a0d0e21e
LW
2277 }
2278
89d205f2
YO
2279The <> symbol will return C<undef> for end-of-file only once.
2280If you call it again after this, it will assume you are processing another
19799a22 2281@ARGV list, and if you haven't set @ARGV, will read input from STDIN.
a0d0e21e 2282
b159ebd3 2283If what the angle brackets contain is a simple scalar variable (e.g.,
35f2feb0 2284<\$foo>), then that variable contains the name of the
19799a22
GS
2285filehandle to input from, or its typeglob, or a reference to the
2286same. For example:
cb1a09d0
2287
2288 \$fh = \*STDIN;
2289 \$line = <\$fh>;
a0d0e21e 2290
5a964f20
TC
2291If what's within the angle brackets is neither a filehandle nor a simple
2292scalar variable containing a filehandle name, typeglob, or typeglob
2293reference, it is interpreted as a filename pattern to be globbed, and
2294either a list of filenames or the next filename in the list is returned,
19799a22 2295depending on context. This distinction is determined on syntactic
35f2feb0
GS
2296grounds alone. That means C<< <\$x> >> is always a readline() from
2297an indirect handle, but C<< <\$hash{key}> >> is always a glob().
5a964f20 2298That's because \$x is a simple scalar variable, but C<\$hash{key}> is
ef191992
YST
2299not--it's a hash element. Even C<< <\$x > >> (note the extra space)
2300is treated as C<glob("\$x ")>, not C<readline(\$x)>.
5a964f20
TC
2301
2302One level of double-quote interpretation is done first, but you can't
35f2feb0 2303say C<< <\$foo> >> because that's an indirect filehandle as explained
5a964f20
TC
2304in the previous paragraph. (In older versions of Perl, programmers
2305would insert curly brackets to force interpretation as a filename glob:
35f2feb0 2306C<< <\${foo}> >>. These days, it's considered cleaner to call the
5a964f20 2307internal function directly as C<glob(\$foo)>, which is probably the right
19799a22 2308way to have done it in the first place.) For example:
a0d0e21e
LW
2309
2310 while (<*.c>) {
2311 chmod 0644, \$_;
2312 }
2313
3a4b19e4 2314is roughly equivalent to:
a0d0e21e
LW
2315
2316 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
2317 while (<FOO>) {
5b3eff12 2318 chomp;
a0d0e21e
LW
2319 chmod 0644, \$_;
2320 }
2321
3a4b19e4
GS
2322except that the globbing is actually done internally using the standard
2323C<File::Glob> extension. Of course, the shortest way to do the above is:
a0d0e21e
LW
2324
2325 chmod 0644, <*.c>;
2326
19799a22
GS
2327A (file)glob evaluates its (embedded) argument only when it is
2328starting a new list. All values must be read before it will start
2329over. In list context, this isn't important because you automatically
2330get them all anyway. However, in scalar context the operator returns
069e01df 2331the next value each time it's called, or C<undef> when the list has
19799a22
GS
2332run out. As with filehandle reads, an automatic C<defined> is
2333generated when the glob occurs in the test part of a C<while>,
2334because legal glob returns (e.g. a file called F<0>) would otherwise
2335terminate the loop. Again, C<undef> is returned only once. So if
2336you're expecting a single value from a glob, it is much better to
2337say
4633a7c4
LW
2338
2339 (\$file) = <blurch*>;
2340
2341than
2342
2343 \$file = <blurch*>;
2344
2345because the latter will alternate between returning a filename and
19799a22 2346returning false.
4633a7c4 2347
b159ebd3 2348If you're trying to do variable interpolation, it's definitely better
4633a7c4 2349to use the glob() function, because the older notation can cause people
e37d713d 2350to become confused with the indirect filehandle notation.
4633a7c4
LW
2351
2352 @files = glob("\$dir/*.[ch]");
2353 @files = glob(\$files[\$i]);
2354
d74e8afc 2356X<constant folding> X<folding>
a0d0e21e
LW
2357
2358Like C, Perl does a certain amount of expression evaluation at
19799a22 2359compile time whenever it determines that all arguments to an
a0d0e21e
LW
2360operator are static and have no side effects. In particular, string
2361concatenation happens at compile time between literals that don't do
19799a22 2362variable substitution. Backslash interpolation also happens at
a0d0e21e
LW
2363compile time. You can say
2364
2365 'Now is the time for all' . "\n" .
2366 'good men to come to.'
2367
54310121 2368and this all reduces to one string internally. Likewise, if
a0d0e21e
LW
2369you say
2370
2371 foreach \$file (@filenames) {
5a964f20 2372 if (-s \$file > 5 + 100 * 2**16) { }
54310121 2373 }
a0d0e21e 2374
19799a22
GS
2375the compiler will precompute the number which that expression
2376represents so that the interpreter won't have to.
a0d0e21e 2377
d74e8afc 2379X<no-op> X<nop>
fd1abbef
DN
2380
2381Perl doesn't officially have a no-op operator, but the bare constants
2382C<0> and C<1> are special-cased to not produce a warning in a void
2383context, so you can for example safely do
2384
2385 1 while foo();
2386
d74e8afc 2388X<operator, bitwise, string>
TP
2389
2390Bitstrings of any size may be manipulated by the bitwise operators
2391(C<~ | & ^>).
2392
19799a22
GS
2393If the operands to a binary bitwise op are strings of different
2394sizes, B<|> and B<^> ops act as though the shorter operand had
2395additional zero bits on the right, while the B<&> op acts as though
2396the longer operand were truncated to the length of the shorter.
2397The granularity for such extension or truncation is one or more
2398bytes.
89d205f2 2400 # ASCII-based examples
TP
2401 print "j p \n" ^ " a h"; # prints "JAPH\n"
2402 print "JA" | " ph\n"; # prints "japh\n"
2403 print "japh\nJunk" & '_____'; # prints "JAPH\n";
2404 print 'p N\$' ^ " E<H\n"; # prints "Perl\n";
2405
19799a22 2406If you are intending to manipulate bitstrings, be certain that
2c268ad5 2407you're supplying bitstrings: If an operand is a number, that will imply
19799a22 2408a B<numeric> bitwise operation. You may explicitly show which type of
TP
2409operation you intend by using C<""> or C<0+>, as in the examples below.
2410
4358a253
SS
2411 \$foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
2412 \$foo = '150' | 105; # yields 255
TP
2413 \$foo = 150 | '105'; # yields 255
2414 \$foo = '150' | '105'; # yields string '155' (under ASCII)
2415
2416 \$baz = 0+\$foo & 0+\$bar; # both ops explicitly numeric
2417 \$biz = "\$foo" ^ "\$bar"; # both ops explicitly stringy
a0d0e21e 2418
1ae175c8
GS
2419See L<perlfunc/vec> for information on how to manipulate individual bits
2420in a bit vector.
2421
d74e8afc 2423X<integer>
a0d0e21e 2424
19799a22 2425By default, Perl assumes that it must do most of its arithmetic in
a0d0e21e
LW
2426floating point. But by saying
2427
2428 use integer;
2429
2430you may tell the compiler that it's okay to use integer operations
19799a22
GS
2431(if it feels like it) from here to the end of the enclosing BLOCK.
2432An inner BLOCK may countermand this by saying
a0d0e21e
LW
2433
2434 no integer;
2435
19799a22
GS
2436which lasts until the end of that BLOCK. Note that this doesn't
2437mean everything is only an integer, merely that Perl may use integer
2438operations if it is so inclined. For example, even under C<use
2439integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731>
2440or so.
2441
2442Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
13a2d996 2444L<Bitwise String Operators>.) However, C<use integer> still has meaning for
19799a22
GS
2445them. By default, their results are interpreted as unsigned integers, but
2446if C<use integer> is in effect, their results are interpreted
2447as signed integers. For example, C<~0> usually evaluates to a large
0be96356 2448integral value. However, C<use integer; ~0> is C<-1> on two's-complement
19799a22 2449machines.
68dc0745 2450
d74e8afc 2452X<floating-point> X<floating point> X<float> X<real>
68dc0745 2453
2454While C<use integer> provides integer-only arithmetic, there is no
19799a22
GS
2455analogous mechanism to provide automatic rounding or truncation to a
2456certain number of decimal places. For rounding to a certain number
2457of digits, sprintf() or printf() is usually the easiest route.
2458See L<perlfaq4>.
68dc0745 2459
5a964f20
TC
2460Floating-point numbers are only approximations to what a mathematician
2461would call real numbers. There are infinitely more reals than floats,
2462so some corners must be cut. For example:
2463
2464 printf "%.20g\n", 123456789123456789;
2465 # produces 123456789123456784
2466
2467Testing for exact equality of floating-point equality or inequality is
2468not a good idea. Here's a (relatively expensive) work-around to compare
2469whether two floating-point numbers are equal to a particular number of
2470decimal places. See Knuth, volume II, for a more robust treatment of
2471this topic.
2472
2473 sub fp_equal {
2474 my (\$X, \$Y, \$POINTS) = @_;
2475 my (\$tX, \$tY);
2476 \$tX = sprintf("%.\${POINTS}g", \$X);
2477 \$tY = sprintf("%.\${POINTS}g", \$Y);
2478 return \$tX eq \$tY;
2479 }
2480
68dc0745 2481The POSIX module (part of the standard perl distribution) implements
19799a22
GS
2482ceil(), floor(), and other mathematical and trigonometric functions.
2483The Math::Complex module (part of the standard perl distribution)
2484defines mathematical functions that work on both the reals and the
2485imaginary numbers. Math::Complex not as efficient as POSIX, but
68dc0745 2486POSIX can't work with complex numbers.
2487
2488Rounding in financial applications can have serious implications, and
2489the rounding method used should be specified precisely. In these
2490cases, it probably pays not to trust whichever system rounding is
2491being used by Perl, but to instead implement the rounding function you
2492need yourself.
5a964f20
TC
2493
d74e8afc 2495X<number, arbitrary precision>
5a964f20
TC
2496
2497The standard Math::BigInt and Math::BigFloat modules provide
19799a22 2498variable-precision arithmetic and overloaded operators, although
cd5c4fce 2499they're currently pretty slow. At the cost of some space and
19799a22
GS
2500considerable speed, they avoid the normal pitfalls associated with
2501limited-precision representations.
5a964f20
TC
2502
2503 use Math::BigInt;
2504 \$x = Math::BigInt->new('123456789123456789');
2505 print \$x * \$x;
2506
2507 # prints +15241578780673678515622620750190521
19799a22 2508
cd5c4fce
T
2509There are several modules that let you calculate with (bound only by
2510memory and cpu-time) unlimited or fixed precision. There are also
2511some non-standard modules that provide faster implementations via
2512external C libraries.
2513
2514Here is a short, but incomplete summary:
2515
2516 Math::Fraction big, unlimited fractions like 9973 / 12967
2517 Math::String treat string sequences like numbers
2518 Math::FixedPrecision calculate with a fixed precision
2519 Math::Currency for currency calculations
2520 Bit::Vector manipulate bit vectors fast (uses C)
2521 Math::BigIntFast Bit::Vector wrapper for big numbers