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a0d0e21e 1=head1 NAME
d74e8afc 2X<operator>
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3
4perlop - Perl operators and precedence
5
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6=head1 DESCRIPTION
7
89d205f2 8=head2 Operator Precedence and Associativity
d74e8afc 9X<operator, precedence> X<precedence> X<associativity>
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10
11Operator precedence and associativity work in Perl more or less like
12they do in mathematics.
13
14I<Operator precedence> means some operators are evaluated before
15others. For example, in C<2 + 4 * 5>, the multiplication has higher
16precedence so C<4 * 5> is evaluated first yielding C<2 + 20 ==
1722> and not C<6 * 5 == 30>.
18
19I<Operator associativity> defines what happens if a sequence of the
20same operators is used one after another: whether the evaluator will
21evaluate the left operations first or the right. For example, in C<8
22- 4 - 2>, subtraction is left associative so Perl evaluates the
23expression left to right. C<8 - 4> is evaluated first making the
24expression C<4 - 2 == 2> and not C<8 - 2 == 6>.
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25
26Perl operators have the following associativity and precedence,
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27listed from highest precedence to lowest. Operators borrowed from
28C keep the same precedence relationship with each other, even where
29C's precedence is slightly screwy. (This makes learning Perl easier
30for C folks.) With very few exceptions, these all operate on scalar
31values only, not array values.
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32
33 left terms and list operators (leftward)
34 left ->
35 nonassoc ++ --
36 right **
37 right ! ~ \ and unary + and -
54310121 38 left =~ !~
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39 left * / % x
40 left + - .
41 left << >>
42 nonassoc named unary operators
43 nonassoc < > <= >= lt gt le ge
0d863452 44 nonassoc == != <=> eq ne cmp ~~
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45 left &
46 left | ^
47 left &&
c963b151 48 left || //
137443ea 49 nonassoc .. ...
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50 right ?:
51 right = += -= *= etc.
52 left , =>
53 nonassoc list operators (rightward)
a5f75d66 54 right not
a0d0e21e 55 left and
f23102e2 56 left or xor
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57
58In the following sections, these operators are covered in precedence order.
59
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60Many operators can be overloaded for objects. See L<overload>.
61
a0d0e21e 62=head2 Terms and List Operators (Leftward)
d74e8afc 63X<list operator> X<operator, list> X<term>
a0d0e21e 64
62c18ce2 65A TERM has the highest precedence in Perl. They include variables,
5f05dabc 66quote and quote-like operators, any expression in parentheses,
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67and any function whose arguments are parenthesized. Actually, there
68aren't really functions in this sense, just list operators and unary
69operators behaving as functions because you put parentheses around
70the arguments. These are all documented in L<perlfunc>.
71
72If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
73is followed by a left parenthesis as the next token, the operator and
74arguments within parentheses are taken to be of highest precedence,
75just like a normal function call.
76
77In the absence of parentheses, the precedence of list operators such as
78C<print>, C<sort>, or C<chmod> is either very high or very low depending on
54310121 79whether you are looking at the left side or the right side of the operator.
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80For example, in
81
82 @ary = (1, 3, sort 4, 2);
83 print @ary; # prints 1324
84
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85the commas on the right of the sort are evaluated before the sort,
86but the commas on the left are evaluated after. In other words,
87list operators tend to gobble up all arguments that follow, and
a0d0e21e 88then act like a simple TERM with regard to the preceding expression.
19799a22 89Be careful with parentheses:
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90
91 # These evaluate exit before doing the print:
92 print($foo, exit); # Obviously not what you want.
93 print $foo, exit; # Nor is this.
94
95 # These do the print before evaluating exit:
96 (print $foo), exit; # This is what you want.
97 print($foo), exit; # Or this.
98 print ($foo), exit; # Or even this.
99
100Also note that
101
102 print ($foo & 255) + 1, "\n";
103
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104probably doesn't do what you expect at first glance. The parentheses
105enclose the argument list for C<print> which is evaluated (printing
106the result of C<$foo & 255>). Then one is added to the return value
107of C<print> (usually 1). The result is something like this:
108
109 1 + 1, "\n"; # Obviously not what you meant.
110
111To do what you meant properly, you must write:
112
113 print(($foo & 255) + 1, "\n");
114
115See L<Named Unary Operators> for more discussion of this.
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116
117Also parsed as terms are the C<do {}> and C<eval {}> constructs, as
54310121 118well as subroutine and method calls, and the anonymous
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119constructors C<[]> and C<{}>.
120
2ae324a7 121See also L<Quote and Quote-like Operators> toward the end of this section,
da87341d 122as well as L</"I/O Operators">.
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123
124=head2 The Arrow Operator
d74e8afc 125X<arrow> X<dereference> X<< -> >>
a0d0e21e 126
35f2feb0 127"C<< -> >>" is an infix dereference operator, just as it is in C
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128and C++. If the right side is either a C<[...]>, C<{...}>, or a
129C<(...)> subscript, then the left side must be either a hard or
130symbolic reference to an array, a hash, or a subroutine respectively.
131(Or technically speaking, a location capable of holding a hard
132reference, if it's an array or hash reference being used for
133assignment.) See L<perlreftut> and L<perlref>.
a0d0e21e 134
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135Otherwise, the right side is a method name or a simple scalar
136variable containing either the method name or a subroutine reference,
137and the left side must be either an object (a blessed reference)
138or a class name (that is, a package name). See L<perlobj>.
a0d0e21e 139
5f05dabc 140=head2 Auto-increment and Auto-decrement
d74e8afc 141X<increment> X<auto-increment> X<++> X<decrement> X<auto-decrement> X<-->
a0d0e21e 142
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143"++" and "--" work as in C. That is, if placed before a variable,
144they increment or decrement the variable by one before returning the
145value, and if placed after, increment or decrement after returning the
146value.
147
148 $i = 0; $j = 0;
149 print $i++; # prints 0
150 print ++$j; # prints 1
a0d0e21e 151
b033823e 152Note that just as in C, Perl doesn't define B<when> the variable is
89d205f2 153incremented or decremented. You just know it will be done sometime
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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
182=head2 Exponentiation
d74e8afc 183X<**> X<exponentiation> X<power>
a0d0e21e 184
19799a22 185Binary "**" is the exponentiation operator. It binds even more
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186tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is
187implemented using C's pow(3) function, which actually works on doubles
188internally.)
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189
190=head2 Symbolic Unary Operators
d74e8afc 191X<unary operator> X<operator, unary>
a0d0e21e 192
5f05dabc 193Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower
a0d0e21e 194precedence version of this.
d74e8afc 195X<!>
a0d0e21e 196
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197Unary "-" performs arithmetic negation if the operand is numeric,
198including any string that looks like a number. If the operand is
199an identifier, a string consisting of a minus sign concatenated
200with the identifier is returned. Otherwise, if the string starts
201with a plus or minus, a string starting with the opposite sign is
202returned. One effect of these rules is that -bareword is equivalent
8705167b 203to the string "-bareword". If, however, the string begins with a
353c6505 204non-alphabetic character (excluding "+" or "-"), Perl will attempt to convert
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205the string to a numeric and the arithmetic negation is performed. If the
206string cannot be cleanly converted to a numeric, Perl will give the warning
207B<Argument "the string" isn't numeric in negation (-) at ...>.
d74e8afc 208X<-> X<negation, arithmetic>
a0d0e21e 209
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210Unary "~" performs bitwise negation, i.e., 1's complement. For
211example, C<0666 & ~027> is 0640. (See also L<Integer Arithmetic> and
212L<Bitwise String Operators>.) Note that the width of the result is
213platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64
214bits wide on a 64-bit platform, so if you are expecting a certain bit
d042e63d 215width, remember to use the & operator to mask off the excess bits.
d74e8afc 216X<~> X<negation, binary>
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217
218Unary "+" has no effect whatsoever, even on strings. It is useful
219syntactically for separating a function name from a parenthesized expression
220that would otherwise be interpreted as the complete list of function
5ba421f6 221arguments. (See examples above under L<Terms and List Operators (Leftward)>.)
d74e8afc 222X<+>
a0d0e21e 223
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224Unary "\" creates a reference to whatever follows it. See L<perlreftut>
225and L<perlref>. Do not confuse this behavior with the behavior of
226backslash within a string, although both forms do convey the notion
227of protecting the next thing from interpolation.
d74e8afc 228X<\> X<reference> X<backslash>
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229
230=head2 Binding Operators
d74e8afc 231X<binding> X<operator, binding> X<=~> X<!~>
a0d0e21e 232
c07a80fd 233Binary "=~" binds a scalar expression to a pattern match. Certain operations
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234search or modify the string $_ by default. This operator makes that kind
235of operation work on some other string. The right argument is a search
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236pattern, substitution, or transliteration. The left argument is what is
237supposed to be searched, substituted, or transliterated instead of the default
f8bab1e9 238$_. When used in scalar context, the return value generally indicates the
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239success of the operation. The exceptions are substitution (s///)
240and transliteration (y///) with the C</r> (non-destructive) option,
241which cause the B<r>eturn value to be the result of the substitution.
242Behavior in list context depends on the particular operator.
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243See L</"Regexp Quote-Like Operators"> for details and L<perlretut> for
244examples using these operators.
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245
246If the right argument is an expression rather than a search pattern,
2c268ad5 247substitution, or transliteration, it is interpreted as a search pattern at run
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248time. Note that this means that its contents will be interpolated twice, so
249
250 '\\' =~ q'\\';
251
252is not ok, as the regex engine will end up trying to compile the
253pattern C<\>, which it will consider a syntax error.
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254
255Binary "!~" is just like "=~" except the return value is negated in
256the logical sense.
257
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258Binary "!~" with a non-destructive substitution (s///r) or transliteration
259(y///r) is a syntax error.
4f4d7508 260
a0d0e21e 261=head2 Multiplicative Operators
d74e8afc 262X<operator, multiplicative>
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263
264Binary "*" multiplies two numbers.
d74e8afc 265X<*>
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266
267Binary "/" divides two numbers.
d74e8afc 268X</> X<slash>
a0d0e21e 269
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270Binary "%" is the modulo operator, which computes the division
271remainder of its first argument with respect to its second argument.
272Given integer
54310121 273operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
f7918450 274C<$a> minus the largest multiple of C<$b> less than or equal to
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275C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
276smallest multiple of C<$b> that is not less than C<$a> (i.e. the
89b4f0ad 277result will be less than or equal to zero). If the operands
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278C<$a> and C<$b> are floating point values and the absolute value of
279C<$b> (that is C<abs($b)>) is less than C<(UV_MAX + 1)>, only
280the integer portion of C<$a> and C<$b> will be used in the operation
281(Note: here C<UV_MAX> means the maximum of the unsigned integer type).
282If the absolute value of the right operand (C<abs($b)>) is greater than
283or equal to C<(UV_MAX + 1)>, "%" computes the floating-point remainder
284C<$r> in the equation C<($r = $a - $i*$b)> where C<$i> is a certain
f7918450 285integer that makes C<$r> have the same sign as the right operand
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286C<$b> (B<not> as the left operand C<$a> like C function C<fmod()>)
287and the absolute value less than that of C<$b>.
0412d526 288Note that when C<use integer> is in scope, "%" gives you direct access
f7918450 289to the modulo operator as implemented by your C compiler. This
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290operator is not as well defined for negative operands, but it will
291execute faster.
f7918450 292X<%> X<remainder> X<modulo> X<mod>
55d729e4 293
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294Binary "x" is the repetition operator. In scalar context or if the left
295operand is not enclosed in parentheses, it returns a string consisting
296of the left operand repeated the number of times specified by the right
297operand. In list context, if the left operand is enclosed in
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298parentheses or is a list formed by C<qw/STRING/>, it repeats the list.
299If the right operand is zero or negative, it returns an empty string
300or an empty list, depending on the context.
d74e8afc 301X<x>
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302
303 print '-' x 80; # print row of dashes
304
305 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
306
307 @ones = (1) x 80; # a list of 80 1's
308 @ones = (5) x @ones; # set all elements to 5
309
310
311=head2 Additive Operators
d74e8afc 312X<operator, additive>
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313
314Binary "+" returns the sum of two numbers.
d74e8afc 315X<+>
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316
317Binary "-" returns the difference of two numbers.
d74e8afc 318X<->
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319
320Binary "." concatenates two strings.
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321X<string, concatenation> X<concatenation>
322X<cat> X<concat> X<concatenate> X<.>
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323
324=head2 Shift Operators
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325X<shift operator> X<operator, shift> X<<< << >>>
326X<<< >> >>> X<right shift> X<left shift> X<bitwise shift>
327X<shl> X<shr> X<shift, right> X<shift, left>
a0d0e21e 328
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329Binary "<<" returns the value of its left argument shifted left by the
330number of bits specified by the right argument. Arguments should be
982ce180 331integers. (See also L<Integer Arithmetic>.)
a0d0e21e 332
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333Binary ">>" returns the value of its left argument shifted right by
334the number of bits specified by the right argument. Arguments should
982ce180 335be integers. (See also L<Integer Arithmetic>.)
a0d0e21e 336
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337Note that both "<<" and ">>" in Perl are implemented directly using
338"<<" and ">>" in C. If C<use integer> (see L<Integer Arithmetic>) is
339in force then signed C integers are used, else unsigned C integers are
340used. Either way, the implementation isn't going to generate results
341larger than the size of the integer type Perl was built with (32 bits
342or 64 bits).
343
344The result of overflowing the range of the integers is undefined
345because it is undefined also in C. In other words, using 32-bit
346integers, C<< 1 << 32 >> is undefined. Shifting by a negative number
347of bits is also undefined.
348
a0d0e21e 349=head2 Named Unary Operators
d74e8afc 350X<operator, named unary>
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351
352The various named unary operators are treated as functions with one
568e6d8b 353argument, with optional parentheses.
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354
355If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
356is followed by a left parenthesis as the next token, the operator and
357arguments within parentheses are taken to be of highest precedence,
3981b0eb 358just like a normal function call. For example,
359because named unary operators are higher precedence than ||:
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360
361 chdir $foo || die; # (chdir $foo) || die
362 chdir($foo) || die; # (chdir $foo) || die
363 chdir ($foo) || die; # (chdir $foo) || die
364 chdir +($foo) || die; # (chdir $foo) || die
365
3981b0eb 366but, because * is higher precedence than named operators:
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367
368 chdir $foo * 20; # chdir ($foo * 20)
369 chdir($foo) * 20; # (chdir $foo) * 20
370 chdir ($foo) * 20; # (chdir $foo) * 20
371 chdir +($foo) * 20; # chdir ($foo * 20)
372
373 rand 10 * 20; # rand (10 * 20)
374 rand(10) * 20; # (rand 10) * 20
375 rand (10) * 20; # (rand 10) * 20
376 rand +(10) * 20; # rand (10 * 20)
377
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378Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are
379treated like named unary operators, but they don't follow this functional
380parenthesis rule. That means, for example, that C<-f($file).".bak"> is
381equivalent to C<-f "$file.bak">.
d74e8afc 382X<-X> X<filetest> X<operator, filetest>
568e6d8b 383
5ba421f6 384See also L<"Terms and List Operators (Leftward)">.
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385
386=head2 Relational Operators
d74e8afc 387X<relational operator> X<operator, relational>
a0d0e21e 388
35f2feb0 389Binary "<" returns true if the left argument is numerically less than
a0d0e21e 390the right argument.
d74e8afc 391X<< < >>
a0d0e21e 392
35f2feb0 393Binary ">" returns true if the left argument is numerically greater
a0d0e21e 394than the right argument.
d74e8afc 395X<< > >>
a0d0e21e 396
35f2feb0 397Binary "<=" returns true if the left argument is numerically less than
a0d0e21e 398or equal to the right argument.
d74e8afc 399X<< <= >>
a0d0e21e 400
35f2feb0 401Binary ">=" returns true if the left argument is numerically greater
a0d0e21e 402than or equal to the right argument.
d74e8afc 403X<< >= >>
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404
405Binary "lt" returns true if the left argument is stringwise less than
406the right argument.
d74e8afc 407X<< lt >>
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408
409Binary "gt" returns true if the left argument is stringwise greater
410than the right argument.
d74e8afc 411X<< gt >>
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412
413Binary "le" returns true if the left argument is stringwise less than
414or equal to the right argument.
d74e8afc 415X<< le >>
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416
417Binary "ge" returns true if the left argument is stringwise greater
418than or equal to the right argument.
d74e8afc 419X<< ge >>
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420
421=head2 Equality Operators
d74e8afc 422X<equality> X<equal> X<equals> X<operator, equality>
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423
424Binary "==" returns true if the left argument is numerically equal to
425the right argument.
d74e8afc 426X<==>
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427
428Binary "!=" returns true if the left argument is numerically not equal
429to the right argument.
d74e8afc 430X<!=>
a0d0e21e 431
35f2feb0 432Binary "<=>" returns -1, 0, or 1 depending on whether the left
6ee5d4e7 433argument is numerically less than, equal to, or greater than the right
d4ad863d 434argument. If your platform supports NaNs (not-a-numbers) as numeric
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435values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
436"<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
437returns true, as does NaN != anything else. If your platform doesn't
438support NaNs then NaN is just a string with numeric value 0.
d74e8afc 439X<< <=> >> X<spaceship>
7d3a9d88 440
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441 perl -le '$a = "NaN"; print "No NaN support here" if $a == $a'
442 perl -le '$a = "NaN"; print "NaN support here" if $a != $a'
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443
444Binary "eq" returns true if the left argument is stringwise equal to
445the right argument.
d74e8afc 446X<eq>
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447
448Binary "ne" returns true if the left argument is stringwise not equal
449to the right argument.
d74e8afc 450X<ne>
a0d0e21e 451
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452Binary "cmp" returns -1, 0, or 1 depending on whether the left
453argument is stringwise less than, equal to, or greater than the right
454argument.
d74e8afc 455X<cmp>
a0d0e21e 456
0d863452 457Binary "~~" does a smart match between its arguments. Smart matching
0f7107a0 458is described in L<perlsyn/"Smart matching in detail">.
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459X<~~>
460
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461"lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
462by the current locale if C<use locale> is in effect. See L<perllocale>.
463
a0d0e21e 464=head2 Bitwise And
d74e8afc 465X<operator, bitwise, and> X<bitwise and> X<&>
a0d0e21e 466
2cdc098b 467Binary "&" returns its operands ANDed together bit by bit.
2c268ad5 468(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
a0d0e21e 469
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470Note that "&" has lower priority than relational operators, so for example
471the brackets are essential in a test like
472
473 print "Even\n" if ($x & 1) == 0;
474
a0d0e21e 475=head2 Bitwise Or and Exclusive Or
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476X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor>
477X<bitwise xor> X<^>
a0d0e21e 478
2cdc098b 479Binary "|" returns its operands ORed together bit by bit.
2c268ad5 480(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
a0d0e21e 481
2cdc098b 482Binary "^" returns its operands XORed together bit by bit.
2c268ad5 483(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
a0d0e21e 484
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485Note that "|" and "^" have lower priority than relational operators, so
486for example the brackets are essential in a test like
487
488 print "false\n" if (8 | 2) != 10;
489
a0d0e21e 490=head2 C-style Logical And
d74e8afc 491X<&&> X<logical and> X<operator, logical, and>
a0d0e21e
LW
492
493Binary "&&" performs a short-circuit logical AND operation. That is,
494if the left operand is false, the right operand is not even evaluated.
495Scalar or list context propagates down to the right operand if it
496is evaluated.
497
498=head2 C-style Logical Or
d74e8afc 499X<||> X<operator, logical, or>
a0d0e21e
LW
500
501Binary "||" performs a short-circuit logical OR operation. That is,
502if the left operand is true, the right operand is not even evaluated.
503Scalar or list context propagates down to the right operand if it
504is evaluated.
505
c963b151 506=head2 C-style Logical Defined-Or
d74e8afc 507X<//> X<operator, logical, defined-or>
c963b151
BD
508
509Although it has no direct equivalent in C, Perl's C<//> operator is related
89d205f2 510to its C-style or. In fact, it's exactly the same as C<||>, except that it
c963b151 511tests the left hand side's definedness instead of its truth. Thus, C<$a // $b>
89d205f2 512is similar to C<defined($a) || $b> (except that it returns the value of C<$a>
d90d5a38 513rather than the value of C<defined($a)>) and yields the same result as
bdc7923b
RGS
514C<defined($a) ? $a : $b> (except that the ternary-operator form can be
515used as a lvalue, while C<$a // $b> cannot). This is very useful for
516providing default values for variables. If you actually want to test if
517at least one of C<$a> and C<$b> is defined, use C<defined($a // $b)>.
c963b151 518
d042e63d
MS
519The C<||>, C<//> and C<&&> operators return the last value evaluated
520(unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably
521portable way to find out the home directory might be:
a0d0e21e 522
c963b151
BD
523 $home = $ENV{'HOME'} // $ENV{'LOGDIR'} //
524 (getpwuid($<))[7] // die "You're homeless!\n";
a0d0e21e 525
5a964f20
TC
526In particular, this means that you shouldn't use this
527for selecting between two aggregates for assignment:
528
529 @a = @b || @c; # this is wrong
530 @a = scalar(@b) || @c; # really meant this
531 @a = @b ? @b : @c; # this works fine, though
532
f23102e2
RGS
533As more readable alternatives to C<&&> and C<||> when used for
534control flow, Perl provides the C<and> and C<or> operators (see below).
535The short-circuit behavior is identical. The precedence of "and"
c963b151 536and "or" is much lower, however, so that you can safely use them after a
5a964f20 537list operator without the need for parentheses:
a0d0e21e
LW
538
539 unlink "alpha", "beta", "gamma"
540 or gripe(), next LINE;
541
542With the C-style operators that would have been written like this:
543
544 unlink("alpha", "beta", "gamma")
545 || (gripe(), next LINE);
546
eeb6a2c9 547Using "or" for assignment is unlikely to do what you want; see below.
5a964f20
TC
548
549=head2 Range Operators
d74e8afc 550X<operator, range> X<range> X<..> X<...>
a0d0e21e
LW
551
552Binary ".." is the range operator, which is really two different
fb53bbb2 553operators depending on the context. In list context, it returns a
54ae734e 554list of values counting (up by ones) from the left value to the right
2cdbc966 555value. If the left value is greater than the right value then it
fb53bbb2 556returns the empty list. The range operator is useful for writing
54ae734e 557C<foreach (1..10)> loops and for doing slice operations on arrays. In
2cdbc966
JD
558the current implementation, no temporary array is created when the
559range operator is used as the expression in C<foreach> loops, but older
560versions of Perl might burn a lot of memory when you write something
561like this:
a0d0e21e
LW
562
563 for (1 .. 1_000_000) {
564 # code
54310121 565 }
a0d0e21e 566
8f0f46f8 567The range operator also works on strings, using the magical
568auto-increment, see below.
54ae734e 569
5a964f20 570In scalar context, ".." returns a boolean value. The operator is
8f0f46f8 571bistable, like a flip-flop, and emulates the line-range (comma)
572operator of B<sed>, B<awk>, and various editors. Each ".." operator
573maintains its own boolean state, even across calls to a subroutine
574that contains it. It is false as long as its left operand is false.
a0d0e21e
LW
575Once the left operand is true, the range operator stays true until the
576right operand is true, I<AFTER> which the range operator becomes false
8f0f46f8 577again. It doesn't become false till the next time the range operator
578is evaluated. It can test the right operand and become false on the
579same evaluation it became true (as in B<awk>), but it still returns
580true once. If you don't want it to test the right operand until the
581next evaluation, as in B<sed>, just use three dots ("...") instead of
19799a22
GS
582two. In all other regards, "..." behaves just like ".." does.
583
584The right operand is not evaluated while the operator is in the
585"false" state, and the left operand is not evaluated while the
586operator is in the "true" state. The precedence is a little lower
587than || and &&. The value returned is either the empty string for
8f0f46f8 588false, or a sequence number (beginning with 1) for true. The sequence
589number is reset for each range encountered. The final sequence number
590in a range has the string "E0" appended to it, which doesn't affect
591its numeric value, but gives you something to search for if you want
592to exclude the endpoint. You can exclude the beginning point by
593waiting for the sequence number to be greater than 1.
df5f8116
CW
594
595If either operand of scalar ".." is a constant expression,
596that operand is considered true if it is equal (C<==>) to the current
597input line number (the C<$.> variable).
598
599To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>,
600but that is only an issue if you use a floating point expression; when
601implicitly using C<$.> as described in the previous paragraph, the
602comparison is C<int(EXPR) == int($.)> which is only an issue when C<$.>
603is set to a floating point value and you are not reading from a file.
604Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what
605you want in scalar context because each of the operands are evaluated
606using their integer representation.
607
608Examples:
a0d0e21e
LW
609
610As a scalar operator:
611
df5f8116 612 if (101 .. 200) { print; } # print 2nd hundred lines, short for
950b09ed 613 # if ($. == 101 .. $. == 200) { print; }
9f10b797
RGS
614
615 next LINE if (1 .. /^$/); # skip header lines, short for
f343f960 616 # next LINE if ($. == 1 .. /^$/);
9f10b797
RGS
617 # (typically in a loop labeled LINE)
618
619 s/^/> / if (/^$/ .. eof()); # quote body
a0d0e21e 620
5a964f20
TC
621 # parse mail messages
622 while (<>) {
623 $in_header = 1 .. /^$/;
df5f8116
CW
624 $in_body = /^$/ .. eof;
625 if ($in_header) {
f343f960 626 # do something
df5f8116 627 } else { # in body
f343f960 628 # do something else
df5f8116 629 }
5a964f20 630 } continue {
df5f8116 631 close ARGV if eof; # reset $. each file
5a964f20
TC
632 }
633
acf31ca5
SF
634Here's a simple example to illustrate the difference between
635the two range operators:
636
637 @lines = (" - Foo",
638 "01 - Bar",
639 "1 - Baz",
640 " - Quux");
641
9f10b797
RGS
642 foreach (@lines) {
643 if (/0/ .. /1/) {
acf31ca5
SF
644 print "$_\n";
645 }
646 }
647
9f10b797
RGS
648This program will print only the line containing "Bar". If
649the range operator is changed to C<...>, it will also print the
acf31ca5
SF
650"Baz" line.
651
652And now some examples as a list operator:
a0d0e21e
LW
653
654 for (101 .. 200) { print; } # print $_ 100 times
3e3baf6d 655 @foo = @foo[0 .. $#foo]; # an expensive no-op
a0d0e21e
LW
656 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
657
5a964f20 658The range operator (in list context) makes use of the magical
5f05dabc 659auto-increment algorithm if the operands are strings. You
a0d0e21e
LW
660can say
661
662 @alphabet = ('A' .. 'Z');
663
54ae734e 664to get all normal letters of the English alphabet, or
a0d0e21e
LW
665
666 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
667
668to get a hexadecimal digit, or
669
670 @z2 = ('01' .. '31'); print $z2[$mday];
671
ea4f5703
YST
672to get dates with leading zeros.
673
674If the final value specified is not in the sequence that the magical
675increment would produce, the sequence goes until the next value would
676be longer than the final value specified.
677
678If the initial value specified isn't part of a magical increment
679sequence (that is, a non-empty string matching "/^[a-zA-Z]*[0-9]*\z/"),
680only the initial value will be returned. So the following will only
681return an alpha:
682
683 use charnames 'greek';
684 my @greek_small = ("\N{alpha}" .. "\N{omega}");
685
686To get lower-case greek letters, use this instead:
687
950b09ed
KW
688 my @greek_small = map { chr } ( ord("\N{alpha}") ..
689 ord("\N{omega}") );
a0d0e21e 690
df5f8116
CW
691Because each operand is evaluated in integer form, C<2.18 .. 3.14> will
692return two elements in list context.
693
694 @list = (2.18 .. 3.14); # same as @list = (2 .. 3);
695
a0d0e21e 696=head2 Conditional Operator
d74e8afc 697X<operator, conditional> X<operator, ternary> X<ternary> X<?:>
a0d0e21e
LW
698
699Ternary "?:" is the conditional operator, just as in C. It works much
700like an if-then-else. If the argument before the ? is true, the
701argument before the : is returned, otherwise the argument after the :
cb1a09d0
AD
702is returned. For example:
703
54310121 704 printf "I have %d dog%s.\n", $n,
cb1a09d0
AD
705 ($n == 1) ? '' : "s";
706
707Scalar or list context propagates downward into the 2nd
54310121 708or 3rd argument, whichever is selected.
cb1a09d0
AD
709
710 $a = $ok ? $b : $c; # get a scalar
711 @a = $ok ? @b : @c; # get an array
712 $a = $ok ? @b : @c; # oops, that's just a count!
713
714The operator may be assigned to if both the 2nd and 3rd arguments are
715legal lvalues (meaning that you can assign to them):
a0d0e21e
LW
716
717 ($a_or_b ? $a : $b) = $c;
718
5a964f20
TC
719Because this operator produces an assignable result, using assignments
720without parentheses will get you in trouble. For example, this:
721
722 $a % 2 ? $a += 10 : $a += 2
723
724Really means this:
725
726 (($a % 2) ? ($a += 10) : $a) += 2
727
728Rather than this:
729
730 ($a % 2) ? ($a += 10) : ($a += 2)
731
19799a22
GS
732That should probably be written more simply as:
733
734 $a += ($a % 2) ? 10 : 2;
735
4633a7c4 736=head2 Assignment Operators
d74e8afc 737X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=>
5ac3b81c 738X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=>
d74e8afc 739X<%=> X<^=> X<x=>
a0d0e21e
LW
740
741"=" is the ordinary assignment operator.
742
743Assignment operators work as in C. That is,
744
745 $a += 2;
746
747is equivalent to
748
749 $a = $a + 2;
750
751although without duplicating any side effects that dereferencing the lvalue
54310121
PP
752might trigger, such as from tie(). Other assignment operators work similarly.
753The following are recognized:
a0d0e21e
LW
754
755 **= += *= &= <<= &&=
9f10b797
RGS
756 -= /= |= >>= ||=
757 .= %= ^= //=
758 x=
a0d0e21e 759
19799a22 760Although these are grouped by family, they all have the precedence
a0d0e21e
LW
761of assignment.
762
b350dd2f
GS
763Unlike in C, the scalar assignment operator produces a valid lvalue.
764Modifying an assignment is equivalent to doing the assignment and
765then modifying the variable that was assigned to. This is useful
766for modifying a copy of something, like this:
a0d0e21e
LW
767
768 ($tmp = $global) =~ tr [A-Z] [a-z];
769
770Likewise,
771
772 ($a += 2) *= 3;
773
774is equivalent to
775
776 $a += 2;
777 $a *= 3;
778
b350dd2f
GS
779Similarly, a list assignment in list context produces the list of
780lvalues assigned to, and a list assignment in scalar context returns
781the number of elements produced by the expression on the right hand
782side of the assignment.
783
748a9306 784=head2 Comma Operator
d74e8afc 785X<comma> X<operator, comma> X<,>
a0d0e21e 786
5a964f20 787Binary "," is the comma operator. In scalar context it evaluates
a0d0e21e
LW
788its left argument, throws that value away, then evaluates its right
789argument and returns that value. This is just like C's comma operator.
790
5a964f20 791In list context, it's just the list argument separator, and inserts
ed5c6d31
PJ
792both its arguments into the list. These arguments are also evaluated
793from left to right.
a0d0e21e 794
344f2c40
IG
795The C<< => >> operator is a synonym for the comma except that it causes
796its left operand to be interpreted as a string if it begins with a letter
797or underscore and is composed only of letters, digits and underscores.
798This includes operands that might otherwise be interpreted as operators,
799constants, single number v-strings or function calls. If in doubt about
800this behaviour, the left operand can be quoted explicitly.
801
802Otherwise, the C<< => >> operator behaves exactly as the comma operator
803or list argument separator, according to context.
804
805For example:
a44e5664
MS
806
807 use constant FOO => "something";
808
809 my %h = ( FOO => 23 );
810
811is equivalent to:
812
813 my %h = ("FOO", 23);
814
815It is I<NOT>:
816
817 my %h = ("something", 23);
818
719b43e8
RGS
819The C<< => >> operator is helpful in documenting the correspondence
820between keys and values in hashes, and other paired elements in lists.
748a9306 821
a44e5664
MS
822 %hash = ( $key => $value );
823 login( $username => $password );
824
678ae90b
RGS
825=head2 Yada Yada Operator
826X<...> X<... operator> X<yada yada operator>
be25f609 827
e8163f9b 828The yada yada operator (noted C<...>) is a placeholder for code. Perl
829parses it without error, but when you try to execute a yada yada, it
830throws an exception with the text C<Unimplemented>:
831
832 sub unimplemented { ... }
833
834 eval { unimplemented() };
835 if( $@ eq 'Unimplemented' ) {
836 print "I found the yada yada!\n";
837 }
838
839You can only use the yada yada to stand in for a complete statement.
840These examples of the yada yada work:
841
842 { ... }
843
844 sub foo { ... }
845
846 ...;
847
848 eval { ... };
849
850 sub foo {
851 my( $self ) = shift;
852
853 ...;
854 }
855
856 do { my $n; ...; print 'Hurrah!' };
857
858The yada yada cannot stand in for an expression that is part of a
859larger statement since the C<...> is also the three-dot version of the
860range operator (see L<Range Operators>). These examples of the yada
861yada are still syntax errors:
862
863 print ...;
864
865 open my($fh), '>', '/dev/passwd' or ...;
866
867 if( $condition && ... ) { print "Hello\n" };
868
869There are some cases where Perl can't immediately tell the difference
870between an expression and a statement. For instance, the syntax for a
871block and an anonymous hash reference constructor look the same unless
872there's something in the braces that give Perl a hint. The yada yada
873is a syntax error if Perl doesn't guess that the C<{ ... }> is a
874block. In that case, it doesn't think the C<...> is the yada yada
875because it's expecting an expression instead of a statement:
876
877 my @transformed = map { ... } @input; # syntax error
878
879You can use a C<;> inside your block to denote that the C<{ ... }> is
880a block and not a hash reference constructor. Now the yada yada works:
881
882 my @transformed = map {; ... } @input; # ; disambiguates
883
884 my @transformed = map { ...; } @input; # ; disambiguates
be25f609 885
a0d0e21e 886=head2 List Operators (Rightward)
d74e8afc 887X<operator, list, rightward> X<list operator>
a0d0e21e
LW
888
889On the right side of a list operator, it has very low precedence,
890such that it controls all comma-separated expressions found there.
891The only operators with lower precedence are the logical operators
892"and", "or", and "not", which may be used to evaluate calls to list
893operators without the need for extra parentheses:
894
895 open HANDLE, "filename"
896 or die "Can't open: $!\n";
897
5ba421f6 898See also discussion of list operators in L<Terms and List Operators (Leftward)>.
a0d0e21e
LW
899
900=head2 Logical Not
d74e8afc 901X<operator, logical, not> X<not>
a0d0e21e
LW
902
903Unary "not" returns the logical negation of the expression to its right.
904It's the equivalent of "!" except for the very low precedence.
905
906=head2 Logical And
d74e8afc 907X<operator, logical, and> X<and>
a0d0e21e
LW
908
909Binary "and" returns the logical conjunction of the two surrounding
910expressions. It's equivalent to && except for the very low
5f05dabc 911precedence. This means that it short-circuits: i.e., the right
a0d0e21e
LW
912expression is evaluated only if the left expression is true.
913
c963b151 914=head2 Logical or, Defined or, and Exclusive Or
f23102e2 915X<operator, logical, or> X<operator, logical, xor>
d74e8afc 916X<operator, logical, defined or> X<operator, logical, exclusive or>
f23102e2 917X<or> X<xor>
a0d0e21e
LW
918
919Binary "or" returns the logical disjunction of the two surrounding
5a964f20
TC
920expressions. It's equivalent to || except for the very low precedence.
921This makes it useful for control flow
922
923 print FH $data or die "Can't write to FH: $!";
924
925This means that it short-circuits: i.e., the right expression is evaluated
926only if the left expression is false. Due to its precedence, you should
927probably avoid using this for assignment, only for control flow.
928
929 $a = $b or $c; # bug: this is wrong
930 ($a = $b) or $c; # really means this
931 $a = $b || $c; # better written this way
932
19799a22 933However, when it's a list-context assignment and you're trying to use
5a964f20
TC
934"||" for control flow, you probably need "or" so that the assignment
935takes higher precedence.
936
937 @info = stat($file) || die; # oops, scalar sense of stat!
938 @info = stat($file) or die; # better, now @info gets its due
939
c963b151
BD
940Then again, you could always use parentheses.
941
a0d0e21e
LW
942Binary "xor" returns the exclusive-OR of the two surrounding expressions.
943It cannot short circuit, of course.
944
945=head2 C Operators Missing From Perl
d74e8afc
ITB
946X<operator, missing from perl> X<&> X<*>
947X<typecasting> X<(TYPE)>
a0d0e21e
LW
948
949Here is what C has that Perl doesn't:
950
951=over 8
952
953=item unary &
954
955Address-of operator. (But see the "\" operator for taking a reference.)
956
957=item unary *
958
54310121 959Dereference-address operator. (Perl's prefix dereferencing
a0d0e21e
LW
960operators are typed: $, @, %, and &.)
961
962=item (TYPE)
963
19799a22 964Type-casting operator.
a0d0e21e
LW
965
966=back
967
5f05dabc 968=head2 Quote and Quote-like Operators
89d205f2 969X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m>
d74e8afc
ITB
970X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>>
971X<escape sequence> X<escape>
972
a0d0e21e
LW
973
974While we usually think of quotes as literal values, in Perl they
975function as operators, providing various kinds of interpolating and
976pattern matching capabilities. Perl provides customary quote characters
977for these behaviors, but also provides a way for you to choose your
978quote character for any of them. In the following table, a C<{}> represents
9f10b797 979any pair of delimiters you choose.
a0d0e21e 980
2c268ad5
TP
981 Customary Generic Meaning Interpolates
982 '' q{} Literal no
983 "" qq{} Literal yes
af9219ee 984 `` qx{} Command yes*
2c268ad5 985 qw{} Word list no
af9219ee
MG
986 // m{} Pattern match yes*
987 qr{} Pattern yes*
988 s{}{} Substitution yes*
2c268ad5 989 tr{}{} Transliteration no (but see below)
7e3b091d 990 <<EOF here-doc yes*
a0d0e21e 991
af9219ee
MG
992 * unless the delimiter is ''.
993
87275199 994Non-bracketing delimiters use the same character fore and aft, but the four
93a274bf 995sorts of ASCII brackets (round, angle, square, curly) will all nest, which means
9f10b797 996that
87275199 997
9f10b797 998 q{foo{bar}baz}
35f2feb0 999
9f10b797 1000is the same as
87275199
GS
1001
1002 'foo{bar}baz'
1003
1004Note, however, that this does not always work for quoting Perl code:
1005
1006 $s = q{ if($a eq "}") ... }; # WRONG
1007
83df6a1d
JH
1008is a syntax error. The C<Text::Balanced> module (from CPAN, and
1009starting from Perl 5.8 part of the standard distribution) is able
1010to do this properly.
87275199 1011
19799a22 1012There can be whitespace between the operator and the quoting
fb73857a 1013characters, except when C<#> is being used as the quoting character.
19799a22
GS
1014C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
1015operator C<q> followed by a comment. Its argument will be taken
1016from the next line. This allows you to write:
fb73857a
PP
1017
1018 s {foo} # Replace foo
1019 {bar} # with bar.
1020
904501ec
MG
1021The following escape sequences are available in constructs that interpolate
1022and in transliterations.
5691ca5f 1023X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N> X<\N{}>
04341565 1024X<\o{}>
5691ca5f 1025
2c4c1ff2
KW
1026 Sequence Note Description
1027 \t tab (HT, TAB)
1028 \n newline (NL)
1029 \r return (CR)
1030 \f form feed (FF)
1031 \b backspace (BS)
1032 \a alarm (bell) (BEL)
1033 \e escape (ESC)
1034 \x{263a} [1,8] hex char (example: SMILEY)
1035 \x1b [2,8] restricted range hex char (example: ESC)
fb121860 1036 \N{name} [3] named Unicode character or character sequence
2c4c1ff2
KW
1037 \N{U+263D} [4,8] Unicode character (example: FIRST QUARTER MOON)
1038 \c[ [5] control char (example: chr(27))
1039 \o{23072} [6,8] octal char (example: SMILEY)
1040 \033 [7,8] restricted range octal char (example: ESC)
5691ca5f
KW
1041
1042=over 4
1043
1044=item [1]
1045
2c4c1ff2
KW
1046The result is the character specified by the hexadecimal number between
1047the braces. See L</[8]> below for details on which character.
96448467
DG
1048
1049Only hexadecimal digits are valid between the braces. If an invalid
1050character is encountered, a warning will be issued and the invalid
1051character and all subsequent characters (valid or invalid) within the
1052braces will be discarded.
1053
1054If there are no valid digits between the braces, the generated character is
1055the NULL character (C<\x{00}>). However, an explicit empty brace (C<\x{}>)
1056will not cause a warning.
40687185
KW
1057
1058=item [2]
1059
2c4c1ff2
KW
1060The result is the character specified by the hexadecimal number in the range
10610x00 to 0xFF. See L</[8]> below for details on which character.
96448467
DG
1062
1063Only hexadecimal digits are valid following C<\x>. When C<\x> is followed
2c4c1ff2 1064by fewer than two valid digits, any valid digits will be zero-padded. This
96448467 1065means that C<\x7> will be interpreted as C<\x07> and C<\x> alone will be
2c4c1ff2 1066interpreted as C<\x00>. Except at the end of a string, having fewer than
96448467
DG
1067two valid digits will result in a warning. Note that while the warning
1068says the illegal character is ignored, it is only ignored as part of the
1069escape and will still be used as the subsequent character in the string.
1070For example:
1071
1072 Original Result Warns?
1073 "\x7" "\x07" no
1074 "\x" "\x00" no
1075 "\x7q" "\x07q" yes
1076 "\xq" "\x00q" yes
1077
40687185
KW
1078=item [3]
1079
fb121860 1080The result is the Unicode character or character sequence given by I<name>.
2c4c1ff2 1081See L<charnames>.
40687185
KW
1082
1083=item [4]
1084
2c4c1ff2
KW
1085C<\N{U+I<hexadecimal number>}> means the Unicode character whose Unicode code
1086point is I<hexadecimal number>.
40687185
KW
1087
1088=item [5]
1089
5691ca5f
KW
1090The character following C<\c> is mapped to some other character as shown in the
1091table:
1092
1093 Sequence Value
1094 \c@ chr(0)
1095 \cA chr(1)
1096 \ca chr(1)
1097 \cB chr(2)
1098 \cb chr(2)
1099 ...
1100 \cZ chr(26)
1101 \cz chr(26)
1102 \c[ chr(27)
1103 \c] chr(29)
1104 \c^ chr(30)
1105 \c? chr(127)
1106
d813941f
KW
1107In other words, it's the character whose code point has had 64 xor'd with
1108its uppercase. C<\c?> is DELETE because C<ord("@") ^ 64> is 127, and
1109C<\c@> is NULL because the ord of "@" is 64, so xor'ing 64 itself produces 0.
1110
5691ca5f
KW
1111Also, C<\c\I<X>> yields C< chr(28) . "I<X>"> for any I<X>, but cannot come at the
1112end of a string, because the backslash would be parsed as escaping the end
1113quote.
1114
1115On ASCII platforms, the resulting characters from the list above are the
1116complete set of ASCII controls. This isn't the case on EBCDIC platforms; see
1117L<perlebcdic/OPERATOR DIFFERENCES> for the complete list of what these
1118sequences mean on both ASCII and EBCDIC platforms.
1119
1120Use of any other character following the "c" besides those listed above is
17a3df4c
KW
1121discouraged, and some are deprecated with the intention of removing
1122those in Perl 5.16. What happens for any of these
d813941f
KW
1123other characters currently though, is that the value is derived by xor'ing
1124with the seventh bit, which is 64.
5691ca5f
KW
1125
1126To get platform independent controls, you can use C<\N{...}>.
1127
40687185
KW
1128=item [6]
1129
2c4c1ff2
KW
1130The result is the character specified by the octal number between the braces.
1131See L</[8]> below for details on which character.
04341565
DG
1132
1133If a character that isn't an octal digit is encountered, a warning is raised,
1134and the value is based on the octal digits before it, discarding it and all
1135following characters up to the closing brace. It is a fatal error if there are
1136no octal digits at all.
1137
1138=item [7]
1139
2c4c1ff2
KW
1140The result is the character specified by the three digit octal number in the
1141range 000 to 777 (but best to not use above 077, see next paragraph). See
1142L</[8]> below for details on which character.
1143
1144Some contexts allow 2 or even 1 digit, but any usage without exactly
40687185 1145three digits, the first being a zero, may give unintended results. (For
04341565
DG
1146example, see L<perlrebackslash/Octal escapes>.) Starting in Perl 5.14, you may
1147use C<\o{}> instead which avoids all these problems. Otherwise, it is best to
1148use this construct only for ordinals C<\077> and below, remembering to pad to
1149the left with zeros to make three digits. For larger ordinals, either use
d90d5a38 1150C<\o{}> , or convert to something else, such as to hex and use C<\x{}>
04341565 1151instead.
40687185 1152
40687185
KW
1153Having fewer than 3 digits may lead to a misleading warning message that says
1154that what follows is ignored. For example, C<"\128"> in the ASCII character set
1155is equivalent to the two characters C<"\n8">, but the warning C<Illegal octal
1156digit '8' ignored> will be thrown. To avoid this warning, make sure to pad
f6050459 1157your octal number with C<0>'s: C<"\0128">.
5691ca5f 1158
2c4c1ff2
KW
1159=item [8]
1160
1161Several of the constructs above specify a character by a number. That number
1162gives the character's position in the character set encoding (indexed from 0).
1163This is called synonymously its ordinal, code position, or code point). Perl
1164works on platforms that have a native encoding currently of either ASCII/Latin1
1165or EBCDIC, each of which allow specification of 256 characters. In general, if
1166the number is 255 (0xFF, 0377) or below, Perl interprets this in the platform's
1167native encoding. If the number is 256 (0x100, 0400) or above, Perl interprets
1168it as as a Unicode code point and the result is the corresponding Unicode
1169character. For example C<\x{50}> and C<\o{120}> both are the number 80 in
1170decimal, which is less than 256, so the number is interpreted in the native
1171character set encoding. In ASCII the character in the 80th position (indexed
1172from 0) is the letter "P", and in EBCDIC it is the ampersand symbol "&".
1173C<\x{100}> and C<\o{400}> are both 256 in decimal, so the number is interpreted
1174as a Unicode code point no matter what the native encoding is. The name of the
1175character in the 100th position (indexed by 0) in Unicode is
1176C<LATIN CAPITAL LETTER A WITH MACRON>.
1177
1178There are a couple of exceptions to the above rule. C<\N{U+I<hex number>}> is
1179always interpreted as a Unicode code point, so that C<\N{U+0050}> is "P" even
1180on EBCDIC platforms. And if L<C<S<use encoding>>|encoding> is in effect, the
1181number is considered to be in that encoding, and is translated from that into
1182the platform's native encoding if there is a corresponding native character;
1183otherwise to Unicode.
1184
5691ca5f 1185=back
4c77eaa2 1186
e526e8bb
KW
1187B<NOTE>: Unlike C and other languages, Perl has no C<\v> escape sequence for
1188the vertical tab (VT - ASCII 11), but you may use C<\ck> or C<\x0b>. (C<\v>
1189does have meaning in regular expression patterns in Perl, see L<perlre>.)
1190
1191The following escape sequences are available in constructs that interpolate,
904501ec 1192but not in transliterations.
d74e8afc 1193X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q>
904501ec 1194
a0d0e21e
LW
1195 \l lowercase next char
1196 \u uppercase next char
1197 \L lowercase till \E
1198 \U uppercase till \E
1d2dff63 1199 \Q quote non-word characters till \E
7e31b643 1200 \E end either case modification or quoted section
a0d0e21e 1201
95cc3e0c
JH
1202If C<use locale> is in effect, the case map used by C<\l>, C<\L>,
1203C<\u> and C<\U> is taken from the current locale. See L<perllocale>.
b6538e4f 1204If Unicode (for example, C<\N{}> or code points of 0x100 or
95cc3e0c 1205beyond) is being used, the case map used by C<\l>, C<\L>, C<\u> and
e526e8bb 1206C<\U> is as defined by Unicode.
a034a98d 1207
5a964f20
TC
1208All systems use the virtual C<"\n"> to represent a line terminator,
1209called a "newline". There is no such thing as an unvarying, physical
19799a22 1210newline character. It is only an illusion that the operating system,
5a964f20
TC
1211device drivers, C libraries, and Perl all conspire to preserve. Not all
1212systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
1213on a Mac, these are reversed, and on systems without line terminator,
1214printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
1215you mean a "newline" for your system, but use the literal ASCII when you
1216need an exact character. For example, most networking protocols expect
2a380090 1217and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
5a964f20
TC
1218and although they often accept just C<"\012">, they seldom tolerate just
1219C<"\015">. If you get in the habit of using C<"\n"> for networking,
1220you may be burned some day.
d74e8afc
ITB
1221X<newline> X<line terminator> X<eol> X<end of line>
1222X<\n> X<\r> X<\r\n>
5a964f20 1223
904501ec
MG
1224For constructs that do interpolate, variables beginning with "C<$>"
1225or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
ad0f383a
A
1226C<< $href->{key}[0] >> are also interpolated, as are array and hash slices.
1227But method calls such as C<< $obj->meth >> are not.
af9219ee
MG
1228
1229Interpolating an array or slice interpolates the elements in order,
1230separated by the value of C<$">, so is equivalent to interpolating
6deea57f
ST
1231C<join $", @array>. "Punctuation" arrays such as C<@*> are only
1232interpolated if the name is enclosed in braces C<@{*}>, but special
1233arrays C<@_>, C<@+>, and C<@-> are interpolated, even without braces.
af9219ee 1234
bc7b91c6
EB
1235For double-quoted strings, the quoting from C<\Q> is applied after
1236interpolation and escapes are processed.
1237
1238 "abc\Qfoo\tbar$s\Exyz"
1239
1240is equivalent to
1241
1242 "abc" . quotemeta("foo\tbar$s") . "xyz"
1243
1244For the pattern of regex operators (C<qr//>, C<m//> and C<s///>),
1245the quoting from C<\Q> is applied after interpolation is processed,
1246but before escapes are processed. This allows the pattern to match
1247literally (except for C<$> and C<@>). For example, the following matches:
1248
1249 '\s\t' =~ /\Q\s\t/
1250
1251Because C<$> or C<@> trigger interpolation, you'll need to use something
1252like C</\Quser\E\@\Qhost/> to match them literally.
1d2dff63 1253
a0d0e21e
LW
1254Patterns are subject to an additional level of interpretation as a
1255regular expression. This is done as a second pass, after variables are
1256interpolated, so that regular expressions may be incorporated into the
1257pattern from the variables. If this is not what you want, use C<\Q> to
1258interpolate a variable literally.
1259
19799a22
GS
1260Apart from the behavior described above, Perl does not expand
1261multiple levels of interpolation. In particular, contrary to the
1262expectations of shell programmers, back-quotes do I<NOT> interpolate
1263within double quotes, nor do single quotes impede evaluation of
1264variables when used within double quotes.
a0d0e21e 1265
5f05dabc 1266=head2 Regexp Quote-Like Operators
d74e8afc 1267X<operator, regexp>
cb1a09d0 1268
5f05dabc 1269Here are the quote-like operators that apply to pattern
cb1a09d0
AD
1270matching and related activities.
1271
a0d0e21e
LW
1272=over 8
1273
b6fa137b 1274=item qr/STRING/msixpodual
01c6f5f4 1275X<qr> X</i> X</m> X</o> X</s> X</x> X</p>
a0d0e21e 1276
87e95b7f
YO
1277This operator quotes (and possibly compiles) its I<STRING> as a regular
1278expression. I<STRING> is interpolated the same way as I<PATTERN>
1279in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
1280is done. Returns a Perl value which may be used instead of the
f6050459 1281corresponding C</STRING/msixpodual> expression. The returned value is a
85dd5c8b 1282normalized version of the original pattern. It magically differs from
64c5a566 1283a string containing the same characters: C<ref(qr/x/)> returns "Regexp",
85dd5c8b 1284even though dereferencing the result returns undef.
a0d0e21e 1285
87e95b7f
YO
1286For example,
1287
1288 $rex = qr/my.STRING/is;
85dd5c8b 1289 print $rex; # prints (?si-xm:my.STRING)
87e95b7f
YO
1290 s/$rex/foo/;
1291
1292is equivalent to
1293
1294 s/my.STRING/foo/is;
1295
1296The result may be used as a subpattern in a match:
1297
1298 $re = qr/$pattern/;
1299 $string =~ /foo${re}bar/; # can be interpolated in other patterns
1300 $string =~ $re; # or used standalone
1301 $string =~ /$re/; # or this way
1302
f6050459 1303Since Perl may compile the pattern at the moment of execution of the qr()
87e95b7f
YO
1304operator, using qr() may have speed advantages in some situations,
1305notably if the result of qr() is used standalone:
1306
1307 sub match {
1308 my $patterns = shift;
1309 my @compiled = map qr/$_/i, @$patterns;
1310 grep {
1311 my $success = 0;
1312 foreach my $pat (@compiled) {
1313 $success = 1, last if /$pat/;
1314 }
1315 $success;
1316 } @_;
5a964f20
TC
1317 }
1318
87e95b7f
YO
1319Precompilation of the pattern into an internal representation at
1320the moment of qr() avoids a need to recompile the pattern every
1321time a match C</$pat/> is attempted. (Perl has many other internal
1322optimizations, but none would be triggered in the above example if
1323we did not use qr() operator.)
1324
765fa144 1325Options (specified by the following modifiers) are:
87e95b7f
YO
1326
1327 m Treat string as multiple lines.
1328 s Treat string as single line. (Make . match a newline)
1329 i Do case-insensitive pattern matching.
1330 x Use extended regular expressions.
1331 p When matching preserve a copy of the matched string so
1332 that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be defined.
1333 o Compile pattern only once.
b6fa137b 1334 l Use the locale
5e2aa8f5
KW
1335 u Use Unicode rules
1336 a Use ASCII for \d, \s, \w; specifying two a's further restricts
1337 /i matching so that no ASCII character will match a non-ASCII
1338 one
b6fa137b 1339 d Use Unicode or native charset, as in 5.12 and earlier
87e95b7f
YO
1340
1341If a precompiled pattern is embedded in a larger pattern then the effect
f6050459 1342of 'msixpluad' will be propagated appropriately. The effect the 'o'
87e95b7f
YO
1343modifier has is not propagated, being restricted to those patterns
1344explicitly using it.
1345
b6fa137b 1346The last four modifiers listed above, added in Perl 5.14,
5e2aa8f5 1347control the character set semantics.
da392a17 1348
87e95b7f 1349See L<perlre> for additional information on valid syntax for STRING, and
5e2aa8f5 1350for a detailed look at the semantics of regular expressions. In
f6050459
KW
1351particular, all the modifiers execpt C</o> are further explained in
1352L<perlre/Modifiers>. C</o> is described in the next section.
a0d0e21e 1353
b6fa137b 1354=item m/PATTERN/msixpodualgc
89d205f2
YO
1355X<m> X<operator, match>
1356X<regexp, options> X<regexp> X<regex, options> X<regex>
01c6f5f4 1357X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c>
a0d0e21e 1358
b6fa137b 1359=item /PATTERN/msixpodualgc
a0d0e21e 1360
5a964f20 1361Searches a string for a pattern match, and in scalar context returns
19799a22
GS
1362true if it succeeds, false if it fails. If no string is specified
1363via the C<=~> or C<!~> operator, the $_ string is searched. (The
1364string specified with C<=~> need not be an lvalue--it may be the
1365result of an expression evaluation, but remember the C<=~> binds
006671a6 1366rather tightly.) See also L<perlre>.
a0d0e21e 1367
f6050459 1368Options are as described in C<qr//> above; in addition, the following match
01c6f5f4 1369process modifiers are available:
a0d0e21e 1370
950b09ed
KW
1371 g Match globally, i.e., find all occurrences.
1372 c Do not reset search position on a failed match when /g is in effect.
a0d0e21e 1373
725a61d7
Z
1374If "/" is the delimiter then the initial C<m> is optional. With the C<m>
1375you can use any pair of non-whitespace characters
1376as delimiters. This is particularly useful for matching path names
1377that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
1378the delimiter, then a match-only-once rule applies,
1379described in C<m?PATTERN?> below.
19799a22 1380If "'" is the delimiter, no interpolation is performed on the PATTERN.
ed02a3bf
DN
1381When using a character valid in an identifier, whitespace is required
1382after the C<m>.
a0d0e21e 1383
532c9e80
KW
1384PATTERN may contain variables, which will be interpolated
1385every time the pattern search is evaluated, except
1f247705
GS
1386for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
1387C<$|> are not interpolated because they look like end-of-string tests.)
532c9e80
KW
1388Perl will not recompile the pattern unless an interpolated
1389variable that it contains changes. You can force Perl to skip the
1390test and never recompile by adding a C</o> (which stands for "once")
1391after the trailing delimiter.
1392Once upon a time, Perl would recompile regular expressions
1393unnecessarily, and this modifier was useful to tell it not to do so, in the
1394interests of speed. But now, the only reasons to use C</o> are either:
1395
1396=over
1397
1398=item 1
1399
1400The variables are thousands of characters long and you know that they
1401don't change, and you need to wring out the last little bit of speed by
1402having Perl skip testing for that. (There is a maintenance penalty for
1403doing this, as mentioning C</o> constitutes a promise that you won't
1404change the variables in the pattern. If you change them, Perl won't
1405even notice.)
1406
1407=item 2
1408
1409you want the pattern to use the initial values of the variables
1410regardless of whether they change or not. (But there are saner ways
1411of accomplishing this than using C</o>.)
1412
1413=back
a0d0e21e 1414
e9d89077
DN
1415=item The empty pattern //
1416
5a964f20 1417If the PATTERN evaluates to the empty string, the last
d65afb4b
HS
1418I<successfully> matched regular expression is used instead. In this
1419case, only the C<g> and C<c> flags on the empty pattern is honoured -
1420the other flags are taken from the original pattern. If no match has
1421previously succeeded, this will (silently) act instead as a genuine
1422empty pattern (which will always match).
a0d0e21e 1423
89d205f2
YO
1424Note that it's possible to confuse Perl into thinking C<//> (the empty
1425regex) is really C<//> (the defined-or operator). Perl is usually pretty
1426good about this, but some pathological cases might trigger this, such as
1427C<$a///> (is that C<($a) / (//)> or C<$a // />?) and C<print $fh //>
1428(C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl
1429will assume you meant defined-or. If you meant the empty regex, just
1430use parentheses or spaces to disambiguate, or even prefix the empty
c963b151
BD
1431regex with an C<m> (so C<//> becomes C<m//>).
1432
e9d89077
DN
1433=item Matching in list context
1434
19799a22 1435If the C</g> option is not used, C<m//> in list context returns a
a0d0e21e 1436list consisting of the subexpressions matched by the parentheses in the
f7e33566
GS
1437pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are
1438also set, and that this differs from Perl 4's behavior.) When there are
1439no parentheses in the pattern, the return value is the list C<(1)> for
1440success. With or without parentheses, an empty list is returned upon
1441failure.
a0d0e21e
LW
1442
1443Examples:
1444
1445 open(TTY, '/dev/tty');
1446 <TTY> =~ /^y/i && foo(); # do foo if desired
1447
1448 if (/Version: *([0-9.]*)/) { $version = $1; }
1449
1450 next if m#^/usr/spool/uucp#;
1451
1452 # poor man's grep
1453 $arg = shift;
1454 while (<>) {
1455 print if /$arg/o; # compile only once
1456 }
1457
1458 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
1459
1460This last example splits $foo into the first two words and the
5f05dabc
PP
1461remainder of the line, and assigns those three fields to $F1, $F2, and
1462$Etc. The conditional is true if any variables were assigned, i.e., if
a0d0e21e
LW
1463the pattern matched.
1464
19799a22 1465The C</g> modifier specifies global pattern matching--that is,
3dd93342 1466matching as many times as possible within the string. How it behaves
1467depends on the context. In list context, it returns a list of the
19799a22 1468substrings matched by any capturing parentheses in the regular
3dd93342 1469expression. If there are no parentheses, it returns a list of all
19799a22
GS
1470the matched strings, as if there were parentheses around the whole
1471pattern.
a0d0e21e 1472
7e86de3e 1473In scalar context, each execution of C<m//g> finds the next match,
19799a22 1474returning true if it matches, and false if there is no further match.
3dd93342 1475The position after the last match can be read or set using the C<pos()>
1476function; see L<perlfunc/pos>. A failed match normally resets the
7e86de3e 1477search position to the beginning of the string, but you can avoid that
3dd93342 1478by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
7e86de3e 1479string also resets the search position.
c90c0ff4 1480
e9d89077
DN
1481=item \G assertion
1482
c90c0ff4 1483You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
3dd93342 1484zero-width assertion that matches the exact position where the
1485previous C<m//g>, if any, left off. Without the C</g> modifier, the
1486C<\G> assertion still anchors at C<pos()> as it was at the start of
1487the operation (see L<perlfunc/pos>), but the match is of course only
1488attempted once. Using C<\G> without C</g> on a target string that has
1489not previously had a C</g> match applied to it is the same as using
1490the C<\A> assertion to match the beginning of the string. Note also
1491that, currently, C<\G> is only properly supported when anchored at the
1492very beginning of the pattern.
c90c0ff4
PP
1493
1494Examples:
a0d0e21e
LW
1495
1496 # list context
1497 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
1498
1499 # scalar context
5d43e42d 1500 $/ = "";
19799a22
GS
1501 while (defined($paragraph = <>)) {
1502 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
1503 $sentences++;
a0d0e21e
LW
1504 }
1505 }
1506 print "$sentences\n";
1507
c90c0ff4 1508 # using m//gc with \G
137443ea 1509 $_ = "ppooqppqq";
44a8e56a
PP
1510 while ($i++ < 2) {
1511 print "1: '";
c90c0ff4 1512 print $1 while /(o)/gc; print "', pos=", pos, "\n";
44a8e56a 1513 print "2: '";
c90c0ff4 1514 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
44a8e56a 1515 print "3: '";
c90c0ff4 1516 print $1 while /(p)/gc; print "', pos=", pos, "\n";
44a8e56a 1517 }
5d43e42d 1518 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
44a8e56a
PP
1519
1520The last example should print:
1521
1522 1: 'oo', pos=4
137443ea 1523 2: 'q', pos=5
44a8e56a
PP
1524 3: 'pp', pos=7
1525 1: '', pos=7
137443ea
PP
1526 2: 'q', pos=8
1527 3: '', pos=8
5d43e42d
DC
1528 Final: 'q', pos=8
1529
1530Notice that the final match matched C<q> instead of C<p>, which a match
1531without the C<\G> anchor would have done. Also note that the final match
ac036724 1532did not update C<pos>. C<pos> is only updated on a C</g> match. If the
5d43e42d
DC
1533final match did indeed match C<p>, it's a good bet that you're running an
1534older (pre-5.6.0) Perl.
44a8e56a 1535
c90c0ff4 1536A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
e7ea3e70 1537combine several regexps like this to process a string part-by-part,
c90c0ff4
PP
1538doing different actions depending on which regexp matched. Each
1539regexp tries to match where the previous one leaves off.
e7ea3e70 1540
3fe9a6f1 1541 $_ = <<'EOL';
950b09ed 1542 $url = URI::URL->new( "http://example.com/" ); die if $url eq "xXx";
3fe9a6f1
PP
1543 EOL
1544 LOOP:
e7ea3e70 1545 {
950b09ed
KW
1546 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
1547 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
1548 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
1549 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
1550 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
1551 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
1552 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
1553 print ". That's all!\n";
e7ea3e70
IZ
1554 }
1555
1556Here is the output (split into several lines):
1557
51622cce 1558 line-noise lowercase line-noise UPPERCASE line-noise UPPERCASE
1559 line-noise lowercase line-noise lowercase line-noise lowercase
1560 lowercase line-noise lowercase lowercase line-noise lowercase
1561 lowercase line-noise MiXeD line-noise. That's all!
44a8e56a 1562
ceb131e8 1563=item m?PATTERN?
725a61d7 1564X<?> X<operator, match-once>
87e95b7f 1565
55d389e7
DG
1566=item ?PATTERN?
1567
725a61d7
Z
1568This is just like the C<m/PATTERN/> search, except that it matches
1569only once between calls to the reset() operator. This is a useful
87e95b7f 1570optimization when you want to see only the first occurrence of
ceb131e8 1571something in each file of a set of files, for instance. Only C<m??>
87e95b7f
YO
1572patterns local to the current package are reset.
1573
1574 while (<>) {
ceb131e8 1575 if (m?^$?) {
87e95b7f
YO
1576 # blank line between header and body
1577 }
1578 } continue {
725a61d7 1579 reset if eof; # clear m?? status for next file
87e95b7f
YO
1580 }
1581
725a61d7
Z
1582The match-once behaviour is controlled by the match delimiter being
1583C<?>; with any other delimiter this is the normal C<m//> operator.
1584
1585For historical reasons, the leading C<m> in C<m?PATTERN?> is optional,
1586but the resulting C<?PATTERN?> syntax is deprecated, will warn on
1587usage and may be removed from a future stable release of Perl without
55d389e7 1588further notice.
87e95b7f 1589
b6fa137b 1590=item s/PATTERN/REPLACEMENT/msixpodualgcer
87e95b7f 1591X<substitute> X<substitution> X<replace> X<regexp, replace>
4f4d7508 1592X<regexp, substitute> X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> X</e> X</r>
87e95b7f
YO
1593
1594Searches a string for a pattern, and if found, replaces that pattern
1595with the replacement text and returns the number of substitutions
1596made. Otherwise it returns false (specifically, the empty string).
1597
4f4d7508 1598If the C</r> (non-destructive) option is used then it will perform the
679563bb
KW
1599substitution on a copy of the string and instead of returning the
1600number of substitutions, it returns the copy whether or not a
4f4d7508 1601substitution occurred. The original string will always remain unchanged in
c8dbf8cd 1602this case. The copy will always be a plain string, even if the input is an
4f4d7508
DC
1603object or a tied variable.
1604
87e95b7f
YO
1605If no string is specified via the C<=~> or C<!~> operator, the C<$_>
1606variable is searched and modified. (The string specified with C<=~> must
1607be scalar variable, an array element, a hash element, or an assignment
1608to one of those, i.e., an lvalue.)
1609
1610If the delimiter chosen is a single quote, no interpolation is
1611done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1612PATTERN contains a $ that looks like a variable rather than an
1613end-of-string test, the variable will be interpolated into the pattern
1614at run-time. If you want the pattern compiled only once the first time
1615the variable is interpolated, use the C</o> option. If the pattern
1616evaluates to the empty string, the last successfully executed regular
1617expression is used instead. See L<perlre> for further explanation on these.
87e95b7f
YO
1618
1619Options are as with m// with the addition of the following replacement
1620specific options:
1621
1622 e Evaluate the right side as an expression.
4f4d7508
DC
1623 ee Evaluate the right side as a string then eval the result.
1624 r Return substitution and leave the original string untouched.
87e95b7f 1625
ed02a3bf
DN
1626Any non-whitespace delimiter may replace the slashes. Add space after
1627the C<s> when using a character allowed in identifiers. If single quotes
1628are used, no interpretation is done on the replacement string (the C</e>
1629modifier overrides this, however). Unlike Perl 4, Perl 5 treats backticks
1630as normal delimiters; the replacement text is not evaluated as a command.
1631If the PATTERN is delimited by bracketing quotes, the REPLACEMENT has
1632its own pair of quotes, which may or may not be bracketing quotes, e.g.,
87e95b7f
YO
1633C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
1634replacement portion to be treated as a full-fledged Perl expression
1635and evaluated right then and there. It is, however, syntax checked at
1636compile-time. A second C<e> modifier will cause the replacement portion
1637to be C<eval>ed before being run as a Perl expression.
1638
1639Examples:
1640
1641 s/\bgreen\b/mauve/g; # don't change wintergreen
1642
1643 $path =~ s|/usr/bin|/usr/local/bin|;
1644
1645 s/Login: $foo/Login: $bar/; # run-time pattern
1646
1647 ($foo = $bar) =~ s/this/that/; # copy first, then change
4f4d7508
DC
1648 ($foo = "$bar") =~ s/this/that/; # convert to string, copy, then change
1649 $foo = $bar =~ s/this/that/r; # Same as above using /r
1650 $foo = $bar =~ s/this/that/r
1651 =~ s/that/the other/r; # Chained substitutes using /r
1652 @foo = map { s/this/that/r } @bar # /r is very useful in maps
87e95b7f
YO
1653
1654 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
1655
1656 $_ = 'abc123xyz';
1657 s/\d+/$&*2/e; # yields 'abc246xyz'
1658 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
1659 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1660
1661 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
1662 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
1663 s/^=(\w+)/pod($1)/ge; # use function call
1664
4f4d7508
DC
1665 $_ = 'abc123xyz';
1666 $a = s/abc/def/r; # $a is 'def123xyz' and
1667 # $_ remains 'abc123xyz'.
1668
87e95b7f
YO
1669 # expand variables in $_, but dynamics only, using
1670 # symbolic dereferencing
1671 s/\$(\w+)/${$1}/g;
1672
1673 # Add one to the value of any numbers in the string
1674 s/(\d+)/1 + $1/eg;
1675
1676 # This will expand any embedded scalar variable
1677 # (including lexicals) in $_ : First $1 is interpolated
1678 # to the variable name, and then evaluated
1679 s/(\$\w+)/$1/eeg;
1680
1681 # Delete (most) C comments.
1682 $program =~ s {
1683 /\* # Match the opening delimiter.
1684 .*? # Match a minimal number of characters.
1685 \*/ # Match the closing delimiter.
1686 } []gsx;
1687
1688 s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_, expensively
1689
1690 for ($variable) { # trim whitespace in $variable, cheap
1691 s/^\s+//;
1692 s/\s+$//;
1693 }
1694
1695 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
1696
1697Note the use of $ instead of \ in the last example. Unlike
1698B<sed>, we use the \<I<digit>> form in only the left hand side.
1699Anywhere else it's $<I<digit>>.
1700
1701Occasionally, you can't use just a C</g> to get all the changes
1702to occur that you might want. Here are two common cases:
1703
1704 # put commas in the right places in an integer
1705 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
1706
1707 # expand tabs to 8-column spacing
1708 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
1709
1c424184
FC
1710C<s///le> is treated as a substitution followed by the C<le> operator, not
1711the C</le> flags. This may change in a future version of Perl. It
1712produces a warning if warnings are enabled. To disambiguate, use a space
1713or change the order of the flags:
1714
1715 s/foo/bar/ le 5; # "le" infix operator
1716 s/foo/bar/el; # "e" and "l" flags
1717
87e95b7f
YO
1718=back
1719
1720=head2 Quote-Like Operators
1721X<operator, quote-like>
1722
01c6f5f4
RGS
1723=over 4
1724
a0d0e21e 1725=item q/STRING/
5d44bfff 1726X<q> X<quote, single> X<'> X<''>
a0d0e21e 1727
5d44bfff 1728=item 'STRING'
a0d0e21e 1729
19799a22 1730A single-quoted, literal string. A backslash represents a backslash
68dc0745
PP
1731unless followed by the delimiter or another backslash, in which case
1732the delimiter or backslash is interpolated.
a0d0e21e
LW
1733
1734 $foo = q!I said, "You said, 'She said it.'"!;
1735 $bar = q('This is it.');
68dc0745 1736 $baz = '\n'; # a two-character string
a0d0e21e
LW
1737
1738=item qq/STRING/
d74e8afc 1739X<qq> X<quote, double> X<"> X<"">
a0d0e21e
LW
1740
1741=item "STRING"
1742
1743A double-quoted, interpolated string.
1744
1745 $_ .= qq
1746 (*** The previous line contains the naughty word "$1".\n)
19799a22 1747 if /\b(tcl|java|python)\b/i; # :-)
68dc0745 1748 $baz = "\n"; # a one-character string
a0d0e21e
LW
1749
1750=item qx/STRING/
d74e8afc 1751X<qx> X<`> X<``> X<backtick>
a0d0e21e
LW
1752
1753=item `STRING`
1754
43dd4d21
JH
1755A string which is (possibly) interpolated and then executed as a
1756system command with C</bin/sh> or its equivalent. Shell wildcards,
1757pipes, and redirections will be honored. The collected standard
1758output of the command is returned; standard error is unaffected. In
1759scalar context, it comes back as a single (potentially multi-line)
1760string, or undef if the command failed. In list context, returns a
1761list of lines (however you've defined lines with $/ or
1762$INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
5a964f20
TC
1763
1764Because backticks do not affect standard error, use shell file descriptor
1765syntax (assuming the shell supports this) if you care to address this.
1766To capture a command's STDERR and STDOUT together:
a0d0e21e 1767
5a964f20
TC
1768 $output = `cmd 2>&1`;
1769
1770To capture a command's STDOUT but discard its STDERR:
1771
1772 $output = `cmd 2>/dev/null`;
1773
1774To capture a command's STDERR but discard its STDOUT (ordering is
1775important here):
1776
1777 $output = `cmd 2>&1 1>/dev/null`;
1778
1779To exchange a command's STDOUT and STDERR in order to capture the STDERR
1780but leave its STDOUT to come out the old STDERR:
1781
1782 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
1783
1784To read both a command's STDOUT and its STDERR separately, it's easiest
2359510d
SD
1785to redirect them separately to files, and then read from those files
1786when the program is done:
5a964f20 1787
2359510d 1788 system("program args 1>program.stdout 2>program.stderr");
5a964f20 1789
30398227
SP
1790The STDIN filehandle used by the command is inherited from Perl's STDIN.
1791For example:
1792
40bbb707
MB
1793 open SPLAT, "stuff" or die "can't open stuff: $!";
1794 open STDIN, "<&SPLAT" or die "can't dupe SPLAT: $!";
1795 print STDOUT `sort`;
30398227 1796
40bbb707 1797will print the sorted contents of the file named F<"stuff">.
30398227 1798
5a964f20
TC
1799Using single-quote as a delimiter protects the command from Perl's
1800double-quote interpolation, passing it on to the shell instead:
1801
1802 $perl_info = qx(ps $$); # that's Perl's $$
1803 $shell_info = qx'ps $$'; # that's the new shell's $$
1804
19799a22 1805How that string gets evaluated is entirely subject to the command
5a964f20
TC
1806interpreter on your system. On most platforms, you will have to protect
1807shell metacharacters if you want them treated literally. This is in
1808practice difficult to do, as it's unclear how to escape which characters.
1809See L<perlsec> for a clean and safe example of a manual fork() and exec()
1810to emulate backticks safely.
a0d0e21e 1811
bb32b41a
GS
1812On some platforms (notably DOS-like ones), the shell may not be
1813capable of dealing with multiline commands, so putting newlines in
1814the string may not get you what you want. You may be able to evaluate
1815multiple commands in a single line by separating them with the command
1816separator character, if your shell supports that (e.g. C<;> on many Unix
1817shells; C<&> on the Windows NT C<cmd> shell).
1818
0f897271
GS
1819Beginning with v5.6.0, Perl will attempt to flush all files opened for
1820output before starting the child process, but this may not be supported
1821on some platforms (see L<perlport>). To be safe, you may need to set
1822C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1823C<IO::Handle> on any open handles.
1824
bb32b41a
GS
1825Beware that some command shells may place restrictions on the length
1826of the command line. You must ensure your strings don't exceed this
1827limit after any necessary interpolations. See the platform-specific
1828release notes for more details about your particular environment.
1829
5a964f20
TC
1830Using this operator can lead to programs that are difficult to port,
1831because the shell commands called vary between systems, and may in
1832fact not be present at all. As one example, the C<type> command under
1833the POSIX shell is very different from the C<type> command under DOS.
1834That doesn't mean you should go out of your way to avoid backticks
1835when they're the right way to get something done. Perl was made to be
1836a glue language, and one of the things it glues together is commands.
1837Just understand what you're getting yourself into.
bb32b41a 1838
da87341d 1839See L</"I/O Operators"> for more discussion.
a0d0e21e 1840
945c54fd 1841=item qw/STRING/
d74e8afc 1842X<qw> X<quote, list> X<quote, words>
945c54fd
JH
1843
1844Evaluates to a list of the words extracted out of STRING, using embedded
1845whitespace as the word delimiters. It can be understood as being roughly
1846equivalent to:
1847
1848 split(' ', q/STRING/);
1849
efb1e162
CW
1850the differences being that it generates a real list at compile time, and
1851in scalar context it returns the last element in the list. So
945c54fd
JH
1852this expression:
1853
1854 qw(foo bar baz)
1855
1856is semantically equivalent to the list:
1857
1858 'foo', 'bar', 'baz'
1859
1860Some frequently seen examples:
1861
1862 use POSIX qw( setlocale localeconv )
1863 @EXPORT = qw( foo bar baz );
1864
1865A common mistake is to try to separate the words with comma or to
1866put comments into a multi-line C<qw>-string. For this reason, the
89d205f2 1867C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
945c54fd
JH
1868produces warnings if the STRING contains the "," or the "#" character.
1869
a0d0e21e 1870
8ff32507 1871=item tr/SEARCHLIST/REPLACEMENTLIST/cdsr
d74e8afc 1872X<tr> X<y> X<transliterate> X</c> X</d> X</s>
a0d0e21e 1873
8ff32507 1874=item y/SEARCHLIST/REPLACEMENTLIST/cdsr
a0d0e21e 1875
2c268ad5 1876Transliterates all occurrences of the characters found in the search list
a0d0e21e
LW
1877with the corresponding character in the replacement list. It returns
1878the number of characters replaced or deleted. If no string is
2c268ad5 1879specified via the =~ or !~ operator, the $_ string is transliterated. (The
54310121
PP
1880string specified with =~ must be a scalar variable, an array element, a
1881hash element, or an assignment to one of those, i.e., an lvalue.)
8ada0baa 1882
8ff32507
FC
1883If the C</r> (non-destructive) option is used then it will perform the
1884replacement on a copy of the string and return the copy whether or not it
1885was modified. The original string will always remain unchanged in
1886this case. The copy will always be a plain string, even if the input is an
1887object or a tied variable.
1888
89d205f2 1889A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
2c268ad5 1890does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
54310121
PP
1891For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1892SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1893its own pair of quotes, which may or may not be bracketing quotes,
2c268ad5 1894e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
a0d0e21e 1895
cc255d5f 1896Note that C<tr> does B<not> do regular expression character classes
e0c83546 1897such as C<\d> or C<[:lower:]>. The C<tr> operator is not equivalent to
cc255d5f
JH
1898the tr(1) utility. If you want to map strings between lower/upper
1899cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider
1900using the C<s> operator if you need regular expressions.
1901
8ada0baa
JH
1902Note also that the whole range idea is rather unportable between
1903character sets--and even within character sets they may cause results
1904you probably didn't expect. A sound principle is to use only ranges
1905that begin from and end at either alphabets of equal case (a-e, A-E),
1906or digits (0-4). Anything else is unsafe. If in doubt, spell out the
1907character sets in full.
1908
a0d0e21e
LW
1909Options:
1910
1911 c Complement the SEARCHLIST.
1912 d Delete found but unreplaced characters.
1913 s Squash duplicate replaced characters.
8ff32507
FC
1914 r Return the modified string and leave the original string
1915 untouched.
a0d0e21e 1916
19799a22
GS
1917If the C</c> modifier is specified, the SEARCHLIST character set
1918is complemented. If the C</d> modifier is specified, any characters
1919specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
1920(Note that this is slightly more flexible than the behavior of some
1921B<tr> programs, which delete anything they find in the SEARCHLIST,
1922period.) If the C</s> modifier is specified, sequences of characters
1923that were transliterated to the same character are squashed down
1924to a single instance of the character.
a0d0e21e
LW
1925
1926If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1927exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1928than the SEARCHLIST, the final character is replicated till it is long
5a964f20 1929enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
a0d0e21e
LW
1930This latter is useful for counting characters in a class or for
1931squashing character sequences in a class.
1932
1933Examples:
1934
1935 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1936
1937 $cnt = tr/*/*/; # count the stars in $_
1938
1939 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1940
1941 $cnt = tr/0-9//; # count the digits in $_
1942
1943 tr/a-zA-Z//s; # bookkeeper -> bokeper
1944
1945 ($HOST = $host) =~ tr/a-z/A-Z/;
8ff32507
FC
1946 $HOST = $host =~ tr/a-z/A-Z/r; # same thing
1947
1948 $HOST = $host =~ tr/a-z/A-Z/r # chained with s///
1949 =~ s/:/ -p/r;
a0d0e21e
LW
1950
1951 tr/a-zA-Z/ /cs; # change non-alphas to single space
1952
8ff32507
FC
1953 @stripped = map tr/a-zA-Z/ /csr, @original;
1954 # /r with map
1955
a0d0e21e
LW
1956 tr [\200-\377]
1957 [\000-\177]; # delete 8th bit
1958
19799a22
GS
1959If multiple transliterations are given for a character, only the
1960first one is used:
748a9306
LW
1961
1962 tr/AAA/XYZ/
1963
2c268ad5 1964will transliterate any A to X.
748a9306 1965
19799a22 1966Because the transliteration table is built at compile time, neither
a0d0e21e 1967the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
19799a22
GS
1968interpolation. That means that if you want to use variables, you
1969must use an eval():
a0d0e21e
LW
1970
1971 eval "tr/$oldlist/$newlist/";
1972 die $@ if $@;
1973
1974 eval "tr/$oldlist/$newlist/, 1" or die $@;
1975
7e3b091d 1976=item <<EOF
d74e8afc 1977X<here-doc> X<heredoc> X<here-document> X<<< << >>>
7e3b091d
DA
1978
1979A line-oriented form of quoting is based on the shell "here-document"
1980syntax. Following a C<< << >> you specify a string to terminate
1981the quoted material, and all lines following the current line down to
89d205f2
YO
1982the terminating string are the value of the item.
1983
1984The terminating string may be either an identifier (a word), or some
1985quoted text. An unquoted identifier works like double quotes.
1986There may not be a space between the C<< << >> and the identifier,
1987unless the identifier is explicitly quoted. (If you put a space it
1988will be treated as a null identifier, which is valid, and matches the
1989first empty line.) The terminating string must appear by itself
1990(unquoted and with no surrounding whitespace) on the terminating line.
1991
1992If the terminating string is quoted, the type of quotes used determine
1993the treatment of the text.
1994
1995=over 4
1996
1997=item Double Quotes
1998
1999Double quotes indicate that the text will be interpolated using exactly
2000the same rules as normal double quoted strings.
7e3b091d
DA
2001
2002 print <<EOF;
2003 The price is $Price.
2004 EOF
2005
2006 print << "EOF"; # same as above
2007 The price is $Price.
2008 EOF
2009
89d205f2
YO
2010
2011=item Single Quotes
2012
2013Single quotes indicate the text is to be treated literally with no
2014interpolation of its content. This is similar to single quoted
2015strings except that backslashes have no special meaning, with C<\\>
2016being treated as two backslashes and not one as they would in every
2017other quoting construct.
2018
2019This is the only form of quoting in perl where there is no need
2020to worry about escaping content, something that code generators
2021can and do make good use of.
2022
2023=item Backticks
2024
2025The content of the here doc is treated just as it would be if the
2026string were embedded in backticks. Thus the content is interpolated
2027as though it were double quoted and then executed via the shell, with
2028the results of the execution returned.
2029
2030 print << `EOC`; # execute command and get results
7e3b091d 2031 echo hi there
7e3b091d
DA
2032 EOC
2033
89d205f2
YO
2034=back
2035
2036It is possible to stack multiple here-docs in a row:
2037
7e3b091d
DA
2038 print <<"foo", <<"bar"; # you can stack them
2039 I said foo.
2040 foo
2041 I said bar.
2042 bar
2043
2044 myfunc(<< "THIS", 23, <<'THAT');
2045 Here's a line
2046 or two.
2047 THIS
2048 and here's another.
2049 THAT
2050
2051Just don't forget that you have to put a semicolon on the end
2052to finish the statement, as Perl doesn't know you're not going to
2053try to do this:
2054
2055 print <<ABC
2056 179231
2057 ABC
2058 + 20;
2059
872d7e53
ST
2060If you want to remove the line terminator from your here-docs,
2061use C<chomp()>.
2062
2063 chomp($string = <<'END');
2064 This is a string.
2065 END
2066
2067If you want your here-docs to be indented with the rest of the code,
2068you'll need to remove leading whitespace from each line manually:
7e3b091d
DA
2069
2070 ($quote = <<'FINIS') =~ s/^\s+//gm;
89d205f2 2071 The Road goes ever on and on,
7e3b091d
DA
2072 down from the door where it began.
2073 FINIS
2074
2075If you use a here-doc within a delimited construct, such as in C<s///eg>,
2076the quoted material must come on the lines following the final delimiter.
2077So instead of
2078
2079 s/this/<<E . 'that'
2080 the other
2081 E
2082 . 'more '/eg;
2083
2084you have to write
2085
89d205f2
YO
2086 s/this/<<E . 'that'
2087 . 'more '/eg;
2088 the other
2089 E
7e3b091d
DA
2090
2091If the terminating identifier is on the last line of the program, you
2092must be sure there is a newline after it; otherwise, Perl will give the
2093warning B<Can't find string terminator "END" anywhere before EOF...>.
2094
89d205f2 2095Additionally, the quoting rules for the end of string identifier are not
ac036724 2096related to Perl's quoting rules. C<q()>, C<qq()>, and the like are not
89d205f2
YO
2097supported in place of C<''> and C<"">, and the only interpolation is for
2098backslashing the quoting character:
7e3b091d
DA
2099
2100 print << "abc\"def";
2101 testing...
2102 abc"def
2103
2104Finally, quoted strings cannot span multiple lines. The general rule is
2105that the identifier must be a string literal. Stick with that, and you
2106should be safe.
2107
a0d0e21e
LW
2108=back
2109
75e14d17 2110=head2 Gory details of parsing quoted constructs
d74e8afc 2111X<quote, gory details>
75e14d17 2112
19799a22
GS
2113When presented with something that might have several different
2114interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
2115principle to pick the most probable interpretation. This strategy
2116is so successful that Perl programmers often do not suspect the
2117ambivalence of what they write. But from time to time, Perl's
2118notions differ substantially from what the author honestly meant.
2119
2120This section hopes to clarify how Perl handles quoted constructs.
2121Although the most common reason to learn this is to unravel labyrinthine
2122regular expressions, because the initial steps of parsing are the
2123same for all quoting operators, they are all discussed together.
2124
2125The most important Perl parsing rule is the first one discussed
2126below: when processing a quoted construct, Perl first finds the end
2127of that construct, then interprets its contents. If you understand
2128this rule, you may skip the rest of this section on the first
2129reading. The other rules are likely to contradict the user's
2130expectations much less frequently than this first one.
2131
2132Some passes discussed below are performed concurrently, but because
2133their results are the same, we consider them individually. For different
2134quoting constructs, Perl performs different numbers of passes, from
6deea57f 2135one to four, but these passes are always performed in the same order.
75e14d17 2136
13a2d996 2137=over 4
75e14d17
IZ
2138
2139=item Finding the end
2140
6deea57f
ST
2141The first pass is finding the end of the quoted construct, where
2142the information about the delimiters is used in parsing.
2143During this search, text between the starting and ending delimiters
2144is copied to a safe location. The text copied gets delimiter-independent.
2145
2146If the construct is a here-doc, the ending delimiter is a line
2147that has a terminating string as the content. Therefore C<<<EOF> is
2148terminated by C<EOF> immediately followed by C<"\n"> and starting
2149from the first column of the terminating line.
2150When searching for the terminating line of a here-doc, nothing
2151is skipped. In other words, lines after the here-doc syntax
2152are compared with the terminating string line by line.
2153
2154For the constructs except here-docs, single characters are used as starting
2155and ending delimiters. If the starting delimiter is an opening punctuation
2156(that is C<(>, C<[>, C<{>, or C<< < >>), the ending delimiter is the
2157corresponding closing punctuation (that is C<)>, C<]>, C<}>, or C<< > >>).
2158If the starting delimiter is an unpaired character like C</> or a closing
2159punctuation, the ending delimiter is same as the starting delimiter.
2160Therefore a C</> terminates a C<qq//> construct, while a C<]> terminates
2161C<qq[]> and C<qq]]> constructs.
2162
2163When searching for single-character delimiters, escaped delimiters
2164and C<\\> are skipped. For example, while searching for terminating C</>,
2165combinations of C<\\> and C<\/> are skipped. If the delimiters are
2166bracketing, nested pairs are also skipped. For example, while searching
2167for closing C<]> paired with the opening C<[>, combinations of C<\\>, C<\]>,
2168and C<\[> are all skipped, and nested C<[> and C<]> are skipped as well.
2169However, when backslashes are used as the delimiters (like C<qq\\> and
2170C<tr\\\>), nothing is skipped.
2171During the search for the end, backslashes that escape delimiters
2172are removed (exactly speaking, they are not copied to the safe location).
75e14d17 2173
19799a22
GS
2174For constructs with three-part delimiters (C<s///>, C<y///>, and
2175C<tr///>), the search is repeated once more.
6deea57f
ST
2176If the first delimiter is not an opening punctuation, three delimiters must
2177be same such as C<s!!!> and C<tr)))>, in which case the second delimiter
2178terminates the left part and starts the right part at once.
b6538e4f 2179If the left part is delimited by bracketing punctuation (that is C<()>,
6deea57f 2180C<[]>, C<{}>, or C<< <> >>), the right part needs another pair of
b6538e4f 2181delimiters such as C<s(){}> and C<tr[]//>. In these cases, whitespace
6deea57f 2182and comments are allowed between both parts, though the comment must follow
b6538e4f
TC
2183at least one whitespace character; otherwise a character expected as the
2184start of the comment may be regarded as the starting delimiter of the right part.
75e14d17 2185
19799a22
GS
2186During this search no attention is paid to the semantics of the construct.
2187Thus:
75e14d17
IZ
2188
2189 "$hash{"$foo/$bar"}"
2190
2a94b7ce 2191or:
75e14d17 2192
89d205f2 2193 m/
2a94b7ce 2194 bar # NOT a comment, this slash / terminated m//!
75e14d17
IZ
2195 /x
2196
19799a22
GS
2197do not form legal quoted expressions. The quoted part ends on the
2198first C<"> and C</>, and the rest happens to be a syntax error.
2199Because the slash that terminated C<m//> was followed by a C<SPACE>,
2200the example above is not C<m//x>, but rather C<m//> with no C</x>
2201modifier. So the embedded C<#> is interpreted as a literal C<#>.
75e14d17 2202
89d205f2
YO
2203Also no attention is paid to C<\c\> (multichar control char syntax) during
2204this search. Thus the second C<\> in C<qq/\c\/> is interpreted as a part
2205of C<\/>, and the following C</> is not recognized as a delimiter.
0d594e51
ST
2206Instead, use C<\034> or C<\x1c> at the end of quoted constructs.
2207
75e14d17 2208=item Interpolation
d74e8afc 2209X<interpolation>
75e14d17 2210
19799a22 2211The next step is interpolation in the text obtained, which is now
89d205f2 2212delimiter-independent. There are multiple cases.
75e14d17 2213
13a2d996 2214=over 4
75e14d17 2215
89d205f2 2216=item C<<<'EOF'>
75e14d17
IZ
2217
2218No interpolation is performed.
6deea57f
ST
2219Note that the combination C<\\> is left intact, since escaped delimiters
2220are not available for here-docs.
75e14d17 2221
6deea57f 2222=item C<m''>, the pattern of C<s'''>
89d205f2 2223
6deea57f
ST
2224No interpolation is performed at this stage.
2225Any backslashed sequences including C<\\> are treated at the stage
2226to L</"parsing regular expressions">.
89d205f2 2227
6deea57f 2228=item C<''>, C<q//>, C<tr'''>, C<y'''>, the replacement of C<s'''>
75e14d17 2229
89d205f2 2230The only interpolation is removal of C<\> from pairs of C<\\>.
6deea57f
ST
2231Therefore C<-> in C<tr'''> and C<y'''> is treated literally
2232as a hyphen and no character range is available.
2233C<\1> in the replacement of C<s'''> does not work as C<$1>.
89d205f2
YO
2234
2235=item C<tr///>, C<y///>
2236
6deea57f
ST
2237No variable interpolation occurs. String modifying combinations for
2238case and quoting such as C<\Q>, C<\U>, and C<\E> are not recognized.
2239The other escape sequences such as C<\200> and C<\t> and backslashed
2240characters such as C<\\> and C<\-> are converted to appropriate literals.
89d205f2
YO
2241The character C<-> is treated specially and therefore C<\-> is treated
2242as a literal C<->.
75e14d17 2243
89d205f2 2244=item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF">
75e14d17 2245
19799a22
GS
2246C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
2247converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
2248is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
6deea57f
ST
2249The other escape sequences such as C<\200> and C<\t> and backslashed
2250characters such as C<\\> and C<\-> are replaced with appropriate
2251expansions.
2a94b7ce 2252
19799a22
GS
2253Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
2254is interpolated in the usual way. Something like C<"\Q\\E"> has
2255no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
2256result is the same as for C<"\\\\E">. As a general rule, backslashes
2257between C<\Q> and C<\E> may lead to counterintuitive results. So,
2258C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
2259as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
2a94b7ce
IZ
2260
2261 $str = '\t';
2262 return "\Q$str";
2263
2264may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
2265
19799a22 2266Interpolated scalars and arrays are converted internally to the C<join> and
92d29cee 2267C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
75e14d17 2268
19799a22 2269 $foo . " XXX '" . (join $", @arr) . "'";
75e14d17 2270
19799a22 2271All operations above are performed simultaneously, left to right.
75e14d17 2272
19799a22
GS
2273Because the result of C<"\Q STRING \E"> has all metacharacters
2274quoted, there is no way to insert a literal C<$> or C<@> inside a
2275C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
2276C<"\\\$">; if not, it is interpreted as the start of an interpolated
2277scalar.
75e14d17 2278
19799a22 2279Note also that the interpolation code needs to make a decision on
89d205f2 2280where the interpolated scalar ends. For instance, whether
35f2feb0 2281C<< "a $b -> {c}" >> really means:
75e14d17
IZ
2282
2283 "a " . $b . " -> {c}";
2284
2a94b7ce 2285or:
75e14d17
IZ
2286
2287 "a " . $b -> {c};
2288
19799a22
GS
2289Most of the time, the longest possible text that does not include
2290spaces between components and which contains matching braces or
2291brackets. because the outcome may be determined by voting based
2292on heuristic estimators, the result is not strictly predictable.
2293Fortunately, it's usually correct for ambiguous cases.
75e14d17 2294
6deea57f 2295=item the replacement of C<s///>
75e14d17 2296
19799a22 2297Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
6deea57f
ST
2298happens as with C<qq//> constructs.
2299
2300It is at this step that C<\1> is begrudgingly converted to C<$1> in
2301the replacement text of C<s///>, in order to correct the incorrigible
2302I<sed> hackers who haven't picked up the saner idiom yet. A warning
2303is emitted if the C<use warnings> pragma or the B<-w> command-line flag
2304(that is, the C<$^W> variable) was set.
2305
2306=item C<RE> in C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
2307
cc74c5bd
ST
2308Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\E>,
2309and interpolation happens (almost) as with C<qq//> constructs.
2310
5d03b57c
KW
2311Processing of C<\N{...}> is also done here, and compiled into an intermediate
2312form for the regex compiler. (This is because, as mentioned below, the regex
2313compilation may be done at execution time, and C<\N{...}> is a compile-time
2314construct.)
2315
cc74c5bd
ST
2316However any other combinations of C<\> followed by a character
2317are not substituted but only skipped, in order to parse them
2318as regular expressions at the following step.
6deea57f 2319As C<\c> is skipped at this step, C<@> of C<\c@> in RE is possibly
1749ea0d 2320treated as an array symbol (for example C<@foo>),
6deea57f 2321even though the same text in C<qq//> gives interpolation of C<\c@>.
6deea57f
ST
2322
2323Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
19799a22
GS
2324a C<#>-comment in a C<//x>-regular expression, no processing is
2325performed whatsoever. This is the first step at which the presence
2326of the C<//x> modifier is relevant.
2327
1749ea0d
ST
2328Interpolation in patterns has several quirks: C<$|>, C<$(>, C<$)>, C<@+>
2329and C<@-> are not interpolated, and constructs C<$var[SOMETHING]> are
2330voted (by several different estimators) to be either an array element
2331or C<$var> followed by an RE alternative. This is where the notation
19799a22
GS
2332C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
2333array element C<-9>, not as a regular expression from the variable
2334C<$arr> followed by a digit, which would be the interpretation of
2335C</$arr[0-9]/>. Since voting among different estimators may occur,
2336the result is not predictable.
2337
19799a22
GS
2338The lack of processing of C<\\> creates specific restrictions on
2339the post-processed text. If the delimiter is C</>, one cannot get
2340the combination C<\/> into the result of this step. C</> will
2341finish the regular expression, C<\/> will be stripped to C</> on
2342the previous step, and C<\\/> will be left as is. Because C</> is
2343equivalent to C<\/> inside a regular expression, this does not
2344matter unless the delimiter happens to be character special to the
2345RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
2346alphanumeric char, as in:
2a94b7ce
IZ
2347
2348 m m ^ a \s* b mmx;
2349
19799a22 2350In the RE above, which is intentionally obfuscated for illustration, the
6deea57f 2351delimiter is C<m>, the modifier is C<mx>, and after delimiter-removal the
89d205f2 2352RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
19799a22
GS
2353reason you're encouraged to restrict your delimiters to non-alphanumeric,
2354non-whitespace choices.
75e14d17
IZ
2355
2356=back
2357
19799a22 2358This step is the last one for all constructs except regular expressions,
75e14d17
IZ
2359which are processed further.
2360
6deea57f
ST
2361=item parsing regular expressions
2362X<regexp, parse>
75e14d17 2363
19799a22 2364Previous steps were performed during the compilation of Perl code,
ac036724 2365but this one happens at run time, although it may be optimized to
19799a22 2366be calculated at compile time if appropriate. After preprocessing
6deea57f 2367described above, and possibly after evaluation if concatenation,
19799a22
GS
2368joining, casing translation, or metaquoting are involved, the
2369resulting I<string> is passed to the RE engine for compilation.
2370
2371Whatever happens in the RE engine might be better discussed in L<perlre>,
2372but for the sake of continuity, we shall do so here.
2373
2374This is another step where the presence of the C<//x> modifier is
2375relevant. The RE engine scans the string from left to right and
2376converts it to a finite automaton.
2377
2378Backslashed characters are either replaced with corresponding
2379literal strings (as with C<\{>), or else they generate special nodes
2380in the finite automaton (as with C<\b>). Characters special to the
2381RE engine (such as C<|>) generate corresponding nodes or groups of
2382nodes. C<(?#...)> comments are ignored. All the rest is either
2383converted to literal strings to match, or else is ignored (as is
2384whitespace and C<#>-style comments if C<//x> is present).
2385
2386Parsing of the bracketed character class construct, C<[...]>, is
2387rather different than the rule used for the rest of the pattern.
2388The terminator of this construct is found using the same rules as
2389for finding the terminator of a C<{}>-delimited construct, the only
2390exception being that C<]> immediately following C<[> is treated as
2391though preceded by a backslash. Similarly, the terminator of
2392C<(?{...})> is found using the same rules as for finding the
2393terminator of a C<{}>-delimited construct.
2394
2395It is possible to inspect both the string given to RE engine and the
2396resulting finite automaton. See the arguments C<debug>/C<debugcolor>
2397in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
4a4eefd0 2398switch documented in L<perlrun/"Command Switches">.
75e14d17
IZ
2399
2400=item Optimization of regular expressions
d74e8afc 2401X<regexp, optimization>
75e14d17 2402
7522fed5 2403This step is listed for completeness only. Since it does not change
75e14d17 2404semantics, details of this step are not documented and are subject
19799a22
GS
2405to change without notice. This step is performed over the finite
2406automaton that was generated during the previous pass.
2a94b7ce 2407
19799a22
GS
2408It is at this stage that C<split()> silently optimizes C</^/> to
2409mean C</^/m>.
75e14d17
IZ
2410
2411=back
2412
a0d0e21e 2413=head2 I/O Operators
d74e8afc
ITB
2414X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle>
2415X<< <> >> X<@ARGV>
a0d0e21e 2416
54310121 2417There are several I/O operators you should know about.
fbad3eb5 2418
7b8d334a 2419A string enclosed by backticks (grave accents) first undergoes
19799a22
GS
2420double-quote interpolation. It is then interpreted as an external
2421command, and the output of that command is the value of the
e9c56f9b
JH
2422backtick string, like in a shell. In scalar context, a single string
2423consisting of all output is returned. In list context, a list of
2424values is returned, one per line of output. (You can set C<$/> to use
2425a different line terminator.) The command is executed each time the
2426pseudo-literal is evaluated. The status value of the command is
2427returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
2428Unlike in B<csh>, no translation is done on the return data--newlines
2429remain newlines. Unlike in any of the shells, single quotes do not
2430hide variable names in the command from interpretation. To pass a
2431literal dollar-sign through to the shell you need to hide it with a
2432backslash. The generalized form of backticks is C<qx//>. (Because
2433backticks always undergo shell expansion as well, see L<perlsec> for
2434security concerns.)
d74e8afc 2435X<qx> X<`> X<``> X<backtick> X<glob>
19799a22
GS
2436
2437In scalar context, evaluating a filehandle in angle brackets yields
2438the next line from that file (the newline, if any, included), or
2439C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
2440(sometimes known as file-slurp mode) and the file is empty, it
2441returns C<''> the first time, followed by C<undef> subsequently.
2442
2443Ordinarily you must assign the returned value to a variable, but
2444there is one situation where an automatic assignment happens. If
2445and only if the input symbol is the only thing inside the conditional
2446of a C<while> statement (even if disguised as a C<for(;;)> loop),
2447the value is automatically assigned to the global variable $_,
2448destroying whatever was there previously. (This may seem like an
2449odd thing to you, but you'll use the construct in almost every Perl
17b829fa 2450script you write.) The $_ variable is not implicitly localized.
19799a22
GS
2451You'll have to put a C<local $_;> before the loop if you want that
2452to happen.
2453
2454The following lines are equivalent:
a0d0e21e 2455
748a9306 2456 while (defined($_ = <STDIN>)) { print; }
7b8d334a 2457 while ($_ = <STDIN>) { print; }
a0d0e21e
LW
2458 while (<STDIN>) { print; }
2459 for (;<STDIN>;) { print; }
748a9306 2460 print while defined($_ = <STDIN>);
7b8d334a 2461 print while ($_ = <STDIN>);
a0d0e21e
LW
2462 print while <STDIN>;
2463
19799a22 2464This also behaves similarly, but avoids $_ :
7b8d334a 2465
89d205f2 2466 while (my $line = <STDIN>) { print $line }
7b8d334a 2467
19799a22
GS
2468In these loop constructs, the assigned value (whether assignment
2469is automatic or explicit) is then tested to see whether it is
2470defined. The defined test avoids problems where line has a string
2471value that would be treated as false by Perl, for example a "" or
2472a "0" with no trailing newline. If you really mean for such values
2473to terminate the loop, they should be tested for explicitly:
7b8d334a
GS
2474
2475 while (($_ = <STDIN>) ne '0') { ... }
2476 while (<STDIN>) { last unless $_; ... }
2477
5ef4d93e 2478In other boolean contexts, C<< <filehandle> >> without an
2479explicit C<defined> test or comparison elicits a warning if the
9f1b1f2d 2480C<use warnings> pragma or the B<-w>
19799a22 2481command-line switch (the C<$^W> variable) is in effect.
7b8d334a 2482
5f05dabc 2483The filehandles STDIN, STDOUT, and STDERR are predefined. (The
19799a22
GS
2484filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
2485in packages, where they would be interpreted as local identifiers
2486rather than global.) Additional filehandles may be created with
2487the open() function, amongst others. See L<perlopentut> and
2488L<perlfunc/open> for details on this.
d74e8afc 2489X<stdin> X<stdout> X<sterr>
a0d0e21e 2490
35f2feb0 2491If a <FILEHANDLE> is used in a context that is looking for
19799a22
GS
2492a list, a list comprising all input lines is returned, one line per
2493list element. It's easy to grow to a rather large data space this
2494way, so use with care.
a0d0e21e 2495
35f2feb0 2496<FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
19799a22 2497See L<perlfunc/readline>.
fbad3eb5 2498
35f2feb0
GS
2499The null filehandle <> is special: it can be used to emulate the
2500behavior of B<sed> and B<awk>. Input from <> comes either from
a0d0e21e 2501standard input, or from each file listed on the command line. Here's
35f2feb0 2502how it works: the first time <> is evaluated, the @ARGV array is
5a964f20 2503checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
a0d0e21e
LW
2504gives you standard input. The @ARGV array is then processed as a list
2505of filenames. The loop
2506
2507 while (<>) {
2508 ... # code for each line
2509 }
2510
2511is equivalent to the following Perl-like pseudo code:
2512
3e3baf6d 2513 unshift(@ARGV, '-') unless @ARGV;
a0d0e21e
LW
2514 while ($ARGV = shift) {
2515 open(ARGV, $ARGV);
2516 while (<ARGV>) {
2517 ... # code for each line
2518 }
2519 }
2520
19799a22
GS
2521except that it isn't so cumbersome to say, and will actually work.
2522It really does shift the @ARGV array and put the current filename
2523into the $ARGV variable. It also uses filehandle I<ARGV>
ac036724 2524internally. <> is just a synonym for <ARGV>, which
19799a22 2525is magical. (The pseudo code above doesn't work because it treats
35f2feb0 2526<ARGV> as non-magical.)
a0d0e21e 2527
48ab5743
ML
2528Since the null filehandle uses the two argument form of L<perlfunc/open>
2529it interprets special characters, so if you have a script like this:
2530
2531 while (<>) {
2532 print;
2533 }
2534
2535and call it with C<perl dangerous.pl 'rm -rfv *|'>, it actually opens a
2536pipe, executes the C<rm> command and reads C<rm>'s output from that pipe.
2537If you want all items in C<@ARGV> to be interpreted as file names, you
2538can use the module C<ARGV::readonly> from CPAN.
2539
35f2feb0 2540You can modify @ARGV before the first <> as long as the array ends up
a0d0e21e 2541containing the list of filenames you really want. Line numbers (C<$.>)
19799a22
GS
2542continue as though the input were one big happy file. See the example
2543in L<perlfunc/eof> for how to reset line numbers on each file.
5a964f20 2544
89d205f2 2545If you want to set @ARGV to your own list of files, go right ahead.
5a964f20
TC
2546This sets @ARGV to all plain text files if no @ARGV was given:
2547
2548 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
a0d0e21e 2549
5a964f20
TC
2550You can even set them to pipe commands. For example, this automatically
2551filters compressed arguments through B<gzip>:
2552
2553 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
2554
2555If you want to pass switches into your script, you can use one of the
a0d0e21e
LW
2556Getopts modules or put a loop on the front like this:
2557
2558 while ($_ = $ARGV[0], /^-/) {
2559 shift;
2560 last if /^--$/;
2561 if (/^-D(.*)/) { $debug = $1 }
2562 if (/^-v/) { $verbose++ }
5a964f20 2563 # ... # other switches
a0d0e21e 2564 }
5a964f20 2565
a0d0e21e 2566 while (<>) {
5a964f20 2567 # ... # code for each line
a0d0e21e
LW
2568 }
2569
89d205f2
YO
2570The <> symbol will return C<undef> for end-of-file only once.
2571If you call it again after this, it will assume you are processing another
19799a22 2572@ARGV list, and if you haven't set @ARGV, will read input from STDIN.
a0d0e21e 2573
b159ebd3 2574If what the angle brackets contain is a simple scalar variable (e.g.,
35f2feb0 2575<$foo>), then that variable contains the name of the
19799a22
GS
2576filehandle to input from, or its typeglob, or a reference to the
2577same. For example:
cb1a09d0
AD
2578
2579 $fh = \*STDIN;
2580 $line = <$fh>;
a0d0e21e 2581
5a964f20
TC
2582If what's within the angle brackets is neither a filehandle nor a simple
2583scalar variable containing a filehandle name, typeglob, or typeglob
2584reference, it is interpreted as a filename pattern to be globbed, and
2585either a list of filenames or the next filename in the list is returned,
19799a22 2586depending on context. This distinction is determined on syntactic
35f2feb0
GS
2587grounds alone. That means C<< <$x> >> is always a readline() from
2588an indirect handle, but C<< <$hash{key}> >> is always a glob().
5a964f20 2589That's because $x is a simple scalar variable, but C<$hash{key}> is
ef191992
YST
2590not--it's a hash element. Even C<< <$x > >> (note the extra space)
2591is treated as C<glob("$x ")>, not C<readline($x)>.
5a964f20
TC
2592
2593One level of double-quote interpretation is done first, but you can't
35f2feb0 2594say C<< <$foo> >> because that's an indirect filehandle as explained
5a964f20
TC
2595in the previous paragraph. (In older versions of Perl, programmers
2596would insert curly brackets to force interpretation as a filename glob:
35f2feb0 2597C<< <${foo}> >>. These days, it's considered cleaner to call the
5a964f20 2598internal function directly as C<glob($foo)>, which is probably the right
19799a22 2599way to have done it in the first place.) For example:
a0d0e21e
LW
2600
2601 while (<*.c>) {
2602 chmod 0644, $_;
2603 }
2604
3a4b19e4 2605is roughly equivalent to:
a0d0e21e
LW
2606
2607 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
2608 while (<FOO>) {
5b3eff12 2609 chomp;
a0d0e21e
LW
2610 chmod 0644, $_;
2611 }
2612
3a4b19e4
GS
2613except that the globbing is actually done internally using the standard
2614C<File::Glob> extension. Of course, the shortest way to do the above is:
a0d0e21e
LW
2615
2616 chmod 0644, <*.c>;
2617
19799a22
GS
2618A (file)glob evaluates its (embedded) argument only when it is
2619starting a new list. All values must be read before it will start
2620over. In list context, this isn't important because you automatically
2621get them all anyway. However, in scalar context the operator returns
069e01df 2622the next value each time it's called, or C<undef> when the list has
19799a22
GS
2623run out. As with filehandle reads, an automatic C<defined> is
2624generated when the glob occurs in the test part of a C<while>,
2625because legal glob returns (e.g. a file called F<0>) would otherwise
2626terminate the loop. Again, C<undef> is returned only once. So if
2627you're expecting a single value from a glob, it is much better to
2628say
4633a7c4
LW
2629
2630 ($file) = <blurch*>;
2631
2632than
2633
2634 $file = <blurch*>;
2635
2636because the latter will alternate between returning a filename and
19799a22 2637returning false.
4633a7c4 2638
b159ebd3 2639If you're trying to do variable interpolation, it's definitely better
4633a7c4 2640to use the glob() function, because the older notation can cause people
e37d713d 2641to become confused with the indirect filehandle notation.
4633a7c4
LW
2642
2643 @files = glob("$dir/*.[ch]");
2644 @files = glob($files[$i]);
2645
a0d0e21e 2646=head2 Constant Folding
d74e8afc 2647X<constant folding> X<folding>
a0d0e21e
LW
2648
2649Like C, Perl does a certain amount of expression evaluation at
19799a22 2650compile time whenever it determines that all arguments to an
a0d0e21e
LW
2651operator are static and have no side effects. In particular, string
2652concatenation happens at compile time between literals that don't do
19799a22 2653variable substitution. Backslash interpolation also happens at
a0d0e21e
LW
2654compile time. You can say
2655
2656 'Now is the time for all' . "\n" .
2657 'good men to come to.'
2658
54310121 2659and this all reduces to one string internally. Likewise, if
a0d0e21e
LW
2660you say
2661
2662 foreach $file (@filenames) {
5a964f20 2663 if (-s $file > 5 + 100 * 2**16) { }
54310121 2664 }
a0d0e21e 2665
19799a22
GS
2666the compiler will precompute the number which that expression
2667represents so that the interpreter won't have to.
a0d0e21e 2668
fd1abbef 2669=head2 No-ops
d74e8afc 2670X<no-op> X<nop>
fd1abbef
DN
2671
2672Perl doesn't officially have a no-op operator, but the bare constants
2673C<0> and C<1> are special-cased to not produce a warning in a void
2674context, so you can for example safely do
2675
2676 1 while foo();
2677
2c268ad5 2678=head2 Bitwise String Operators
d74e8afc 2679X<operator, bitwise, string>
2c268ad5
TP
2680
2681Bitstrings of any size may be manipulated by the bitwise operators
2682(C<~ | & ^>).
2683
19799a22
GS
2684If the operands to a binary bitwise op are strings of different
2685sizes, B<|> and B<^> ops act as though the shorter operand had
2686additional zero bits on the right, while the B<&> op acts as though
2687the longer operand were truncated to the length of the shorter.
2688The granularity for such extension or truncation is one or more
2689bytes.
2c268ad5 2690
89d205f2 2691 # ASCII-based examples
2c268ad5
TP
2692 print "j p \n" ^ " a h"; # prints "JAPH\n"
2693 print "JA" | " ph\n"; # prints "japh\n"
2694 print "japh\nJunk" & '_____'; # prints "JAPH\n";
2695 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
2696
19799a22 2697If you are intending to manipulate bitstrings, be certain that
2c268ad5 2698you're supplying bitstrings: If an operand is a number, that will imply
19799a22 2699a B<numeric> bitwise operation. You may explicitly show which type of
2c268ad5
TP
2700operation you intend by using C<""> or C<0+>, as in the examples below.
2701
4358a253
SS
2702 $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
2703 $foo = '150' | 105; # yields 255
2c268ad5
TP
2704 $foo = 150 | '105'; # yields 255
2705 $foo = '150' | '105'; # yields string '155' (under ASCII)
2706
2707 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
2708 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
a0d0e21e 2709
1ae175c8
GS
2710See L<perlfunc/vec> for information on how to manipulate individual bits
2711in a bit vector.
2712
55497cff 2713=head2 Integer Arithmetic
d74e8afc 2714X<integer>
a0d0e21e 2715
19799a22 2716By default, Perl assumes that it must do most of its arithmetic in
a0d0e21e
LW
2717floating point. But by saying
2718
2719 use integer;
2720
3eab78e3
CW
2721you may tell the compiler to use integer operations
2722(see L<integer> for a detailed explanation) from here to the end of
2723the enclosing BLOCK. An inner BLOCK may countermand this by saying
a0d0e21e
LW
2724
2725 no integer;
2726
19799a22 2727which lasts until the end of that BLOCK. Note that this doesn't
3eab78e3
CW
2728mean everything is an integer, merely that Perl will use integer
2729operations for arithmetic, comparison, and bitwise operators. For
2730example, even under C<use integer>, if you take the C<sqrt(2)>, you'll
2731still get C<1.4142135623731> or so.
19799a22
GS
2732
2733Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
89d205f2 2734and ">>") always produce integral results. (But see also
13a2d996 2735L<Bitwise String Operators>.) However, C<use integer> still has meaning for
19799a22
GS
2736them. By default, their results are interpreted as unsigned integers, but
2737if C<use integer> is in effect, their results are interpreted
2738as signed integers. For example, C<~0> usually evaluates to a large
0be96356 2739integral value. However, C<use integer; ~0> is C<-1> on two's-complement
19799a22 2740machines.
68dc0745
PP
2741
2742=head2 Floating-point Arithmetic
06ce2fa3 2743
d74e8afc 2744X<floating-point> X<floating point> X<float> X<real>
68dc0745
PP
2745
2746While C<use integer> provides integer-only arithmetic, there is no
19799a22
GS
2747analogous mechanism to provide automatic rounding or truncation to a
2748certain number of decimal places. For rounding to a certain number
2749of digits, sprintf() or printf() is usually the easiest route.
2750See L<perlfaq4>.
68dc0745 2751
5a964f20
TC
2752Floating-point numbers are only approximations to what a mathematician
2753would call real numbers. There are infinitely more reals than floats,
2754so some corners must be cut. For example:
2755
2756 printf "%.20g\n", 123456789123456789;
2757 # produces 123456789123456784
2758
8548cb57
RGS
2759Testing for exact floating-point equality or inequality is not a
2760good idea. Here's a (relatively expensive) work-around to compare
5a964f20
TC
2761whether two floating-point numbers are equal to a particular number of
2762decimal places. See Knuth, volume II, for a more robust treatment of
2763this topic.
2764
2765 sub fp_equal {
2766 my ($X, $Y, $POINTS) = @_;
2767 my ($tX, $tY);
2768 $tX = sprintf("%.${POINTS}g", $X);
2769 $tY = sprintf("%.${POINTS}g", $Y);
2770 return $tX eq $tY;
2771 }
2772
68dc0745 2773The POSIX module (part of the standard perl distribution) implements
19799a22
GS
2774ceil(), floor(), and other mathematical and trigonometric functions.
2775The Math::Complex module (part of the standard perl distribution)
2776defines mathematical functions that work on both the reals and the
2777imaginary numbers. Math::Complex not as efficient as POSIX, but
68dc0745
PP
2778POSIX can't work with complex numbers.
2779
2780Rounding in financial applications can have serious implications, and
2781the rounding method used should be specified precisely. In these
2782cases, it probably pays not to trust whichever system rounding is
2783being used by Perl, but to instead implement the rounding function you
2784need yourself.
5a964f20
TC
2785
2786=head2 Bigger Numbers
d74e8afc 2787X<number, arbitrary precision>
5a964f20
TC
2788
2789The standard Math::BigInt and Math::BigFloat modules provide
19799a22 2790variable-precision arithmetic and overloaded operators, although
cd5c4fce 2791they're currently pretty slow. At the cost of some space and
19799a22
GS
2792considerable speed, they avoid the normal pitfalls associated with
2793limited-precision representations.
5a964f20
TC
2794
2795 use Math::BigInt;
2796 $x = Math::BigInt->new('123456789123456789');
2797 print $x * $x;
2798
2799 # prints +15241578780673678515622620750190521
19799a22 2800
cd5c4fce
T
2801There are several modules that let you calculate with (bound only by
2802memory and cpu-time) unlimited or fixed precision. There are also
2803some non-standard modules that provide faster implementations via
2804external C libraries.
2805
2806Here is a short, but incomplete summary:
2807
950b09ed
KW
2808 Math::Fraction big, unlimited fractions like 9973 / 12967
2809 Math::String treat string sequences like numbers
2810 Math::FixedPrecision calculate with a fixed precision
2811 Math::Currency for currency calculations
2812 Bit::Vector manipulate bit vectors fast (uses C)
2813 Math::BigIntFast Bit::Vector wrapper for big numbers
2814 Math::Pari provides access to the Pari C library
2815 Math::BigInteger uses an external C library
2816 Math::Cephes uses external Cephes C library (no big numbers)
2817 Math::Cephes::Fraction fractions via the Cephes library
2818 Math::GMP another one using an external C library
cd5c4fce
T
2819
2820Choose wisely.
16070b82
GS
2821
2822=cut