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a0d0e21e 1=head1 NAME
d74e8afc 2X<operator>
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4perlop - Perl operators and precedence
5
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6=head1 DESCRIPTION
7
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8=head2 Operator Precedence and Associativity
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
c963b151 56 left or xor err
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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
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152Note that just as in C, Perl doesn't define B<when> the variable is
153incremented or decremented. You just know it will be done sometime
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<!>
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196
197Unary "-" performs arithmetic negation if the operand is numeric. If
198the operand is an identifier, a string consisting of a minus sign
199concatenated with the identifier is returned. Otherwise, if the string
200starts with a plus or minus, a string starting with the opposite sign
bff5667c 201is returned. One effect of these rules is that -bareword is equivalent
8705167b 202to the string "-bareword". If, however, the string begins with a
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203non-alphabetic character (exluding "+" or "-"), Perl will attempt to convert
204the string to a numeric and the arithmetic negation is performed. If the
205string cannot be cleanly converted to a numeric, Perl will give the warning
206B<Argument "the string" isn't numeric in negation (-) at ...>.
d74e8afc 207X<-> X<negation, arithmetic>
a0d0e21e 208
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209Unary "~" performs bitwise negation, i.e., 1's complement. For
210example, C<0666 & ~027> is 0640. (See also L<Integer Arithmetic> and
211L<Bitwise String Operators>.) Note that the width of the result is
212platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64
213bits wide on a 64-bit platform, so if you are expecting a certain bit
d042e63d 214width, remember to use the & operator to mask off the excess bits.
d74e8afc 215X<~> X<negation, binary>
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216
217Unary "+" has no effect whatsoever, even on strings. It is useful
218syntactically for separating a function name from a parenthesized expression
219that would otherwise be interpreted as the complete list of function
5ba421f6 220arguments. (See examples above under L<Terms and List Operators (Leftward)>.)
d74e8afc 221X<+>
a0d0e21e 222
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223Unary "\" creates a reference to whatever follows it. See L<perlreftut>
224and L<perlref>. Do not confuse this behavior with the behavior of
225backslash within a string, although both forms do convey the notion
226of protecting the next thing from interpolation.
d74e8afc 227X<\> X<reference> X<backslash>
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228
229=head2 Binding Operators
d74e8afc 230X<binding> X<operator, binding> X<=~> X<!~>
a0d0e21e 231
c07a80fd 232Binary "=~" binds a scalar expression to a pattern match. Certain operations
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233search or modify the string $_ by default. This operator makes that kind
234of operation work on some other string. The right argument is a search
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235pattern, substitution, or transliteration. The left argument is what is
236supposed to be searched, substituted, or transliterated instead of the default
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237$_. When used in scalar context, the return value generally indicates the
238success of the operation. Behavior in list context depends on the particular
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239operator. See L</"Regexp Quote-Like Operators"> for details and
240L<perlretut> for examples using these operators.
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241
242If the right argument is an expression rather than a search pattern,
2c268ad5 243substitution, or transliteration, it is interpreted as a search pattern at run
573e01ca 244time.
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245
246Binary "!~" is just like "=~" except the return value is negated in
247the logical sense.
248
249=head2 Multiplicative Operators
d74e8afc 250X<operator, multiplicative>
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251
252Binary "*" multiplies two numbers.
d74e8afc 253X<*>
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254
255Binary "/" divides two numbers.
d74e8afc 256X</> X<slash>
a0d0e21e 257
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258Binary "%" computes the modulus of two numbers. Given integer
259operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
260C<$a> minus the largest multiple of C<$b> that is not greater than
261C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
262smallest multiple of C<$b> that is not less than C<$a> (i.e. the
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263result will be less than or equal to zero). If the operands
264C<$a> and C<$b> are floting point values, only the integer portion
265of C<$a> and C<$b> will be used in the operation.
0412d526 266Note that when C<use integer> is in scope, "%" gives you direct access
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267to the modulus operator as implemented by your C compiler. This
268operator is not as well defined for negative operands, but it will
269execute faster.
d74e8afc 270X<%> X<remainder> X<modulus> X<mod>
55d729e4 271
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272Binary "x" is the repetition operator. In scalar context or if the left
273operand is not enclosed in parentheses, it returns a string consisting
274of the left operand repeated the number of times specified by the right
275operand. In list context, if the left operand is enclosed in
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276parentheses or is a list formed by C<qw/STRING/>, it repeats the list.
277If the right operand is zero or negative, it returns an empty string
278or an empty list, depending on the context.
d74e8afc 279X<x>
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280
281 print '-' x 80; # print row of dashes
282
283 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
284
285 @ones = (1) x 80; # a list of 80 1's
286 @ones = (5) x @ones; # set all elements to 5
287
288
289=head2 Additive Operators
d74e8afc 290X<operator, additive>
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291
292Binary "+" returns the sum of two numbers.
d74e8afc 293X<+>
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294
295Binary "-" returns the difference of two numbers.
d74e8afc 296X<->
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297
298Binary "." concatenates two strings.
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299X<string, concatenation> X<concatenation>
300X<cat> X<concat> X<concatenate> X<.>
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301
302=head2 Shift Operators
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303X<shift operator> X<operator, shift> X<<< << >>>
304X<<< >> >>> X<right shift> X<left shift> X<bitwise shift>
305X<shl> X<shr> X<shift, right> X<shift, left>
a0d0e21e 306
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307Binary "<<" returns the value of its left argument shifted left by the
308number of bits specified by the right argument. Arguments should be
982ce180 309integers. (See also L<Integer Arithmetic>.)
a0d0e21e 310
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311Binary ">>" returns the value of its left argument shifted right by
312the number of bits specified by the right argument. Arguments should
982ce180 313be integers. (See also L<Integer Arithmetic>.)
a0d0e21e 314
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315Note that both "<<" and ">>" in Perl are implemented directly using
316"<<" and ">>" in C. If C<use integer> (see L<Integer Arithmetic>) is
317in force then signed C integers are used, else unsigned C integers are
318used. Either way, the implementation isn't going to generate results
319larger than the size of the integer type Perl was built with (32 bits
320or 64 bits).
321
322The result of overflowing the range of the integers is undefined
323because it is undefined also in C. In other words, using 32-bit
324integers, C<< 1 << 32 >> is undefined. Shifting by a negative number
325of bits is also undefined.
326
a0d0e21e 327=head2 Named Unary Operators
d74e8afc 328X<operator, named unary>
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329
330The various named unary operators are treated as functions with one
568e6d8b 331argument, with optional parentheses.
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332
333If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
334is followed by a left parenthesis as the next token, the operator and
335arguments within parentheses are taken to be of highest precedence,
3981b0eb 336just like a normal function call. For example,
337because named unary operators are higher precedence than ||:
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338
339 chdir $foo || die; # (chdir $foo) || die
340 chdir($foo) || die; # (chdir $foo) || die
341 chdir ($foo) || die; # (chdir $foo) || die
342 chdir +($foo) || die; # (chdir $foo) || die
343
3981b0eb 344but, because * is higher precedence than named operators:
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345
346 chdir $foo * 20; # chdir ($foo * 20)
347 chdir($foo) * 20; # (chdir $foo) * 20
348 chdir ($foo) * 20; # (chdir $foo) * 20
349 chdir +($foo) * 20; # chdir ($foo * 20)
350
351 rand 10 * 20; # rand (10 * 20)
352 rand(10) * 20; # (rand 10) * 20
353 rand (10) * 20; # (rand 10) * 20
354 rand +(10) * 20; # rand (10 * 20)
355
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356Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are
357treated like named unary operators, but they don't follow this functional
358parenthesis rule. That means, for example, that C<-f($file).".bak"> is
359equivalent to C<-f "$file.bak">.
d74e8afc 360X<-X> X<filetest> X<operator, filetest>
568e6d8b 361
5ba421f6 362See also L<"Terms and List Operators (Leftward)">.
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363
364=head2 Relational Operators
d74e8afc 365X<relational operator> X<operator, relational>
a0d0e21e 366
35f2feb0 367Binary "<" returns true if the left argument is numerically less than
a0d0e21e 368the right argument.
d74e8afc 369X<< < >>
a0d0e21e 370
35f2feb0 371Binary ">" returns true if the left argument is numerically greater
a0d0e21e 372than the right argument.
d74e8afc 373X<< > >>
a0d0e21e 374
35f2feb0 375Binary "<=" returns true if the left argument is numerically less than
a0d0e21e 376or equal to the right argument.
d74e8afc 377X<< <= >>
a0d0e21e 378
35f2feb0 379Binary ">=" returns true if the left argument is numerically greater
a0d0e21e 380than or equal to the right argument.
d74e8afc 381X<< >= >>
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382
383Binary "lt" returns true if the left argument is stringwise less than
384the right argument.
d74e8afc 385X<< lt >>
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386
387Binary "gt" returns true if the left argument is stringwise greater
388than the right argument.
d74e8afc 389X<< gt >>
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390
391Binary "le" returns true if the left argument is stringwise less than
392or equal to the right argument.
d74e8afc 393X<< le >>
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394
395Binary "ge" returns true if the left argument is stringwise greater
396than or equal to the right argument.
d74e8afc 397X<< ge >>
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398
399=head2 Equality Operators
d74e8afc 400X<equality> X<equal> X<equals> X<operator, equality>
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401
402Binary "==" returns true if the left argument is numerically equal to
403the right argument.
d74e8afc 404X<==>
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405
406Binary "!=" returns true if the left argument is numerically not equal
407to the right argument.
d74e8afc 408X<!=>
a0d0e21e 409
35f2feb0 410Binary "<=>" returns -1, 0, or 1 depending on whether the left
6ee5d4e7 411argument is numerically less than, equal to, or greater than the right
d4ad863d 412argument. If your platform supports NaNs (not-a-numbers) as numeric
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413values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
414"<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
415returns true, as does NaN != anything else. If your platform doesn't
416support NaNs then NaN is just a string with numeric value 0.
d74e8afc 417X<< <=> >> X<spaceship>
7d3a9d88 418
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419 perl -le '$a = "NaN"; print "No NaN support here" if $a == $a'
420 perl -le '$a = "NaN"; print "NaN support here" if $a != $a'
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421
422Binary "eq" returns true if the left argument is stringwise equal to
423the right argument.
d74e8afc 424X<eq>
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425
426Binary "ne" returns true if the left argument is stringwise not equal
427to the right argument.
d74e8afc 428X<ne>
a0d0e21e 429
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430Binary "cmp" returns -1, 0, or 1 depending on whether the left
431argument is stringwise less than, equal to, or greater than the right
432argument.
d74e8afc 433X<cmp>
a0d0e21e 434
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435Binary "~~" does a smart match between its arguments. Smart matching
436is described in L<perlsyn/"Smart Matching in Detail">.
437This operator is only available if you enable the "~~" feature:
438see L<feature> for more information.
439X<~~>
440
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441"lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
442by the current locale if C<use locale> is in effect. See L<perllocale>.
443
a0d0e21e 444=head2 Bitwise And
d74e8afc 445X<operator, bitwise, and> X<bitwise and> X<&>
a0d0e21e 446
2cdc098b 447Binary "&" returns its operands ANDed together bit by bit.
2c268ad5 448(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
a0d0e21e 449
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450Note that "&" has lower priority than relational operators, so for example
451the brackets are essential in a test like
452
453 print "Even\n" if ($x & 1) == 0;
454
a0d0e21e 455=head2 Bitwise Or and Exclusive Or
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456X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor>
457X<bitwise xor> X<^>
a0d0e21e 458
2cdc098b 459Binary "|" returns its operands ORed together bit by bit.
2c268ad5 460(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
a0d0e21e 461
2cdc098b 462Binary "^" returns its operands XORed together bit by bit.
2c268ad5 463(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
a0d0e21e 464
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465Note that "|" and "^" have lower priority than relational operators, so
466for example the brackets are essential in a test like
467
468 print "false\n" if (8 | 2) != 10;
469
a0d0e21e 470=head2 C-style Logical And
d74e8afc 471X<&&> X<logical and> X<operator, logical, and>
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472
473Binary "&&" performs a short-circuit logical AND operation. That is,
474if the left operand is false, the right operand is not even evaluated.
475Scalar or list context propagates down to the right operand if it
476is evaluated.
477
478=head2 C-style Logical Or
d74e8afc 479X<||> X<operator, logical, or>
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480
481Binary "||" performs a short-circuit logical OR operation. That is,
482if the left operand is true, the right operand is not even evaluated.
483Scalar or list context propagates down to the right operand if it
484is evaluated.
485
c963b151 486=head2 C-style Logical Defined-Or
d74e8afc 487X<//> X<operator, logical, defined-or>
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488
489Although it has no direct equivalent in C, Perl's C<//> operator is related
490to its C-style or. In fact, it's exactly the same as C<||>, except that it
491tests the left hand side's definedness instead of its truth. Thus, C<$a // $b>
492is similar to C<defined($a) || $b> (except that it returns the value of C<$a>
493rather than the value of C<defined($a)>) and is exactly equivalent to
494C<defined($a) ? $a : $b>. This is very useful for providing default values
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495for variables. If you actually want to test if at least one of C<$a> and
496C<$b> is defined, use C<defined($a // $b)>.
c963b151 497
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498The C<||>, C<//> and C<&&> operators return the last value evaluated
499(unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably
500portable way to find out the home directory might be:
a0d0e21e 501
c963b151
BD
502 $home = $ENV{'HOME'} // $ENV{'LOGDIR'} //
503 (getpwuid($<))[7] // die "You're homeless!\n";
a0d0e21e 504
5a964f20
TC
505In particular, this means that you shouldn't use this
506for selecting between two aggregates for assignment:
507
508 @a = @b || @c; # this is wrong
509 @a = scalar(@b) || @c; # really meant this
510 @a = @b ? @b : @c; # this works fine, though
511
c963b151
BD
512As more readable alternatives to C<&&>, C<//> and C<||> when used for
513control flow, Perl provides C<and>, C<err> and C<or> operators (see below).
514The short-circuit behavior is identical. The precedence of "and", "err"
515and "or" is much lower, however, so that you can safely use them after a
5a964f20 516list operator without the need for parentheses:
a0d0e21e
LW
517
518 unlink "alpha", "beta", "gamma"
519 or gripe(), next LINE;
520
521With the C-style operators that would have been written like this:
522
523 unlink("alpha", "beta", "gamma")
524 || (gripe(), next LINE);
525
eeb6a2c9 526Using "or" for assignment is unlikely to do what you want; see below.
5a964f20
TC
527
528=head2 Range Operators
d74e8afc 529X<operator, range> X<range> X<..> X<...>
a0d0e21e
LW
530
531Binary ".." is the range operator, which is really two different
fb53bbb2 532operators depending on the context. In list context, it returns a
54ae734e 533list of values counting (up by ones) from the left value to the right
2cdbc966 534value. If the left value is greater than the right value then it
fb53bbb2 535returns the empty list. The range operator is useful for writing
54ae734e 536C<foreach (1..10)> loops and for doing slice operations on arrays. In
2cdbc966
JD
537the current implementation, no temporary array is created when the
538range operator is used as the expression in C<foreach> loops, but older
539versions of Perl might burn a lot of memory when you write something
540like this:
a0d0e21e
LW
541
542 for (1 .. 1_000_000) {
543 # code
54310121 544 }
a0d0e21e 545
54ae734e
MG
546The range operator also works on strings, using the magical auto-increment,
547see below.
548
5a964f20 549In scalar context, ".." returns a boolean value. The operator is
a0d0e21e
LW
550bistable, like a flip-flop, and emulates the line-range (comma) operator
551of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
552own boolean state. It is false as long as its left operand is false.
553Once the left operand is true, the range operator stays true until the
554right operand is true, I<AFTER> which the range operator becomes false
19799a22 555again. It doesn't become false till the next time the range operator is
a0d0e21e
LW
556evaluated. It can test the right operand and become false on the same
557evaluation it became true (as in B<awk>), but it still returns true once.
19799a22
GS
558If you don't want it to test the right operand till the next
559evaluation, as in B<sed>, just use three dots ("...") instead of
560two. In all other regards, "..." behaves just like ".." does.
561
562The right operand is not evaluated while the operator is in the
563"false" state, and the left operand is not evaluated while the
564operator is in the "true" state. The precedence is a little lower
565than || and &&. The value returned is either the empty string for
566false, or a sequence number (beginning with 1) for true. The
567sequence number is reset for each range encountered. The final
568sequence number in a range has the string "E0" appended to it, which
569doesn't affect its numeric value, but gives you something to search
570for if you want to exclude the endpoint. You can exclude the
571beginning point by waiting for the sequence number to be greater
df5f8116
CW
572than 1.
573
574If either operand of scalar ".." is a constant expression,
575that operand is considered true if it is equal (C<==>) to the current
576input line number (the C<$.> variable).
577
578To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>,
579but that is only an issue if you use a floating point expression; when
580implicitly using C<$.> as described in the previous paragraph, the
581comparison is C<int(EXPR) == int($.)> which is only an issue when C<$.>
582is set to a floating point value and you are not reading from a file.
583Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what
584you want in scalar context because each of the operands are evaluated
585using their integer representation.
586
587Examples:
a0d0e21e
LW
588
589As a scalar operator:
590
df5f8116
CW
591 if (101 .. 200) { print; } # print 2nd hundred lines, short for
592 # if ($. == 101 .. $. == 200) ...
9f10b797
RGS
593
594 next LINE if (1 .. /^$/); # skip header lines, short for
df5f8116 595 # ... if ($. == 1 .. /^$/);
9f10b797
RGS
596 # (typically in a loop labeled LINE)
597
598 s/^/> / if (/^$/ .. eof()); # quote body
a0d0e21e 599
5a964f20
TC
600 # parse mail messages
601 while (<>) {
602 $in_header = 1 .. /^$/;
df5f8116
CW
603 $in_body = /^$/ .. eof;
604 if ($in_header) {
605 # ...
606 } else { # in body
607 # ...
608 }
5a964f20 609 } continue {
df5f8116 610 close ARGV if eof; # reset $. each file
5a964f20
TC
611 }
612
acf31ca5
SF
613Here's a simple example to illustrate the difference between
614the two range operators:
615
616 @lines = (" - Foo",
617 "01 - Bar",
618 "1 - Baz",
619 " - Quux");
620
9f10b797
RGS
621 foreach (@lines) {
622 if (/0/ .. /1/) {
acf31ca5
SF
623 print "$_\n";
624 }
625 }
626
9f10b797
RGS
627This program will print only the line containing "Bar". If
628the range operator is changed to C<...>, it will also print the
acf31ca5
SF
629"Baz" line.
630
631And now some examples as a list operator:
a0d0e21e
LW
632
633 for (101 .. 200) { print; } # print $_ 100 times
3e3baf6d 634 @foo = @foo[0 .. $#foo]; # an expensive no-op
a0d0e21e
LW
635 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
636
5a964f20 637The range operator (in list context) makes use of the magical
5f05dabc 638auto-increment algorithm if the operands are strings. You
a0d0e21e
LW
639can say
640
641 @alphabet = ('A' .. 'Z');
642
54ae734e 643to get all normal letters of the English alphabet, or
a0d0e21e
LW
644
645 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
646
647to get a hexadecimal digit, or
648
649 @z2 = ('01' .. '31'); print $z2[$mday];
650
ea4f5703
YST
651to get dates with leading zeros.
652
653If the final value specified is not in the sequence that the magical
654increment would produce, the sequence goes until the next value would
655be longer than the final value specified.
656
657If the initial value specified isn't part of a magical increment
658sequence (that is, a non-empty string matching "/^[a-zA-Z]*[0-9]*\z/"),
659only the initial value will be returned. So the following will only
660return an alpha:
661
662 use charnames 'greek';
663 my @greek_small = ("\N{alpha}" .. "\N{omega}");
664
665To get lower-case greek letters, use this instead:
666
667 my @greek_small = map { chr } ( ord("\N{alpha}") .. ord("\N{omega}") );
a0d0e21e 668
df5f8116
CW
669Because each operand is evaluated in integer form, C<2.18 .. 3.14> will
670return two elements in list context.
671
672 @list = (2.18 .. 3.14); # same as @list = (2 .. 3);
673
a0d0e21e 674=head2 Conditional Operator
d74e8afc 675X<operator, conditional> X<operator, ternary> X<ternary> X<?:>
a0d0e21e
LW
676
677Ternary "?:" is the conditional operator, just as in C. It works much
678like an if-then-else. If the argument before the ? is true, the
679argument before the : is returned, otherwise the argument after the :
cb1a09d0
AD
680is returned. For example:
681
54310121 682 printf "I have %d dog%s.\n", $n,
cb1a09d0
AD
683 ($n == 1) ? '' : "s";
684
685Scalar or list context propagates downward into the 2nd
54310121 686or 3rd argument, whichever is selected.
cb1a09d0
AD
687
688 $a = $ok ? $b : $c; # get a scalar
689 @a = $ok ? @b : @c; # get an array
690 $a = $ok ? @b : @c; # oops, that's just a count!
691
692The operator may be assigned to if both the 2nd and 3rd arguments are
693legal lvalues (meaning that you can assign to them):
a0d0e21e
LW
694
695 ($a_or_b ? $a : $b) = $c;
696
5a964f20
TC
697Because this operator produces an assignable result, using assignments
698without parentheses will get you in trouble. For example, this:
699
700 $a % 2 ? $a += 10 : $a += 2
701
702Really means this:
703
704 (($a % 2) ? ($a += 10) : $a) += 2
705
706Rather than this:
707
708 ($a % 2) ? ($a += 10) : ($a += 2)
709
19799a22
GS
710That should probably be written more simply as:
711
712 $a += ($a % 2) ? 10 : 2;
713
4633a7c4 714=head2 Assignment Operators
d74e8afc 715X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=>
5ac3b81c 716X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=>
d74e8afc 717X<%=> X<^=> X<x=>
a0d0e21e
LW
718
719"=" is the ordinary assignment operator.
720
721Assignment operators work as in C. That is,
722
723 $a += 2;
724
725is equivalent to
726
727 $a = $a + 2;
728
729although without duplicating any side effects that dereferencing the lvalue
54310121
PP
730might trigger, such as from tie(). Other assignment operators work similarly.
731The following are recognized:
a0d0e21e
LW
732
733 **= += *= &= <<= &&=
9f10b797
RGS
734 -= /= |= >>= ||=
735 .= %= ^= //=
736 x=
a0d0e21e 737
19799a22 738Although these are grouped by family, they all have the precedence
a0d0e21e
LW
739of assignment.
740
b350dd2f
GS
741Unlike in C, the scalar assignment operator produces a valid lvalue.
742Modifying an assignment is equivalent to doing the assignment and
743then modifying the variable that was assigned to. This is useful
744for modifying a copy of something, like this:
a0d0e21e
LW
745
746 ($tmp = $global) =~ tr [A-Z] [a-z];
747
748Likewise,
749
750 ($a += 2) *= 3;
751
752is equivalent to
753
754 $a += 2;
755 $a *= 3;
756
b350dd2f
GS
757Similarly, a list assignment in list context produces the list of
758lvalues assigned to, and a list assignment in scalar context returns
759the number of elements produced by the expression on the right hand
760side of the assignment.
761
748a9306 762=head2 Comma Operator
d74e8afc 763X<comma> X<operator, comma> X<,>
a0d0e21e 764
5a964f20 765Binary "," is the comma operator. In scalar context it evaluates
a0d0e21e
LW
766its left argument, throws that value away, then evaluates its right
767argument and returns that value. This is just like C's comma operator.
768
5a964f20 769In list context, it's just the list argument separator, and inserts
ed5c6d31
PJ
770both its arguments into the list. These arguments are also evaluated
771from left to right.
a0d0e21e 772
d042e63d 773The C<< => >> operator is a synonym for the comma, but forces any word
719b43e8 774(consisting entirely of word characters) to its left to be interpreted
a44e5664
MS
775as a string (as of 5.001). This includes words that might otherwise be
776considered a constant or function call.
777
778 use constant FOO => "something";
779
780 my %h = ( FOO => 23 );
781
782is equivalent to:
783
784 my %h = ("FOO", 23);
785
786It is I<NOT>:
787
788 my %h = ("something", 23);
789
790If the argument on the left is not a word, it is first interpreted as
791an expression, and then the string value of that is used.
719b43e8
RGS
792
793The C<< => >> operator is helpful in documenting the correspondence
794between keys and values in hashes, and other paired elements in lists.
748a9306 795
a44e5664
MS
796 %hash = ( $key => $value );
797 login( $username => $password );
798
a0d0e21e 799=head2 List Operators (Rightward)
d74e8afc 800X<operator, list, rightward> X<list operator>
a0d0e21e
LW
801
802On the right side of a list operator, it has very low precedence,
803such that it controls all comma-separated expressions found there.
804The only operators with lower precedence are the logical operators
805"and", "or", and "not", which may be used to evaluate calls to list
806operators without the need for extra parentheses:
807
808 open HANDLE, "filename"
809 or die "Can't open: $!\n";
810
5ba421f6 811See also discussion of list operators in L<Terms and List Operators (Leftward)>.
a0d0e21e
LW
812
813=head2 Logical Not
d74e8afc 814X<operator, logical, not> X<not>
a0d0e21e
LW
815
816Unary "not" returns the logical negation of the expression to its right.
817It's the equivalent of "!" except for the very low precedence.
818
819=head2 Logical And
d74e8afc 820X<operator, logical, and> X<and>
a0d0e21e
LW
821
822Binary "and" returns the logical conjunction of the two surrounding
823expressions. It's equivalent to && except for the very low
5f05dabc 824precedence. This means that it short-circuits: i.e., the right
a0d0e21e
LW
825expression is evaluated only if the left expression is true.
826
c963b151 827=head2 Logical or, Defined or, and Exclusive Or
d74e8afc
ITB
828X<operator, logical, or> X<operator, logical, xor> X<operator, logical, err>
829X<operator, logical, defined or> X<operator, logical, exclusive or>
830X<or> X<xor> X<err>
a0d0e21e
LW
831
832Binary "or" returns the logical disjunction of the two surrounding
5a964f20
TC
833expressions. It's equivalent to || except for the very low precedence.
834This makes it useful for control flow
835
836 print FH $data or die "Can't write to FH: $!";
837
838This means that it short-circuits: i.e., the right expression is evaluated
839only if the left expression is false. Due to its precedence, you should
840probably avoid using this for assignment, only for control flow.
841
842 $a = $b or $c; # bug: this is wrong
843 ($a = $b) or $c; # really means this
844 $a = $b || $c; # better written this way
845
19799a22 846However, when it's a list-context assignment and you're trying to use
5a964f20
TC
847"||" for control flow, you probably need "or" so that the assignment
848takes higher precedence.
849
850 @info = stat($file) || die; # oops, scalar sense of stat!
851 @info = stat($file) or die; # better, now @info gets its due
852
c963b151
BD
853Then again, you could always use parentheses.
854
9f10b797
RGS
855Binary "err" is equivalent to C<//>--it's just like binary "or", except it
856tests its left argument's definedness instead of its truth. There are two
857ways to remember "err": either because many functions return C<undef> on
858an B<err>or, or as a sort of correction: C<$a = ($b err 'default')>. This
859keyword is only available when the 'err' feature is enabled: see
860L<feature> for more information.
a0d0e21e
LW
861
862Binary "xor" returns the exclusive-OR of the two surrounding expressions.
863It cannot short circuit, of course.
864
865=head2 C Operators Missing From Perl
d74e8afc
ITB
866X<operator, missing from perl> X<&> X<*>
867X<typecasting> X<(TYPE)>
a0d0e21e
LW
868
869Here is what C has that Perl doesn't:
870
871=over 8
872
873=item unary &
874
875Address-of operator. (But see the "\" operator for taking a reference.)
876
877=item unary *
878
54310121 879Dereference-address operator. (Perl's prefix dereferencing
a0d0e21e
LW
880operators are typed: $, @, %, and &.)
881
882=item (TYPE)
883
19799a22 884Type-casting operator.
a0d0e21e
LW
885
886=back
887
5f05dabc 888=head2 Quote and Quote-like Operators
d74e8afc
ITB
889X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m>
890X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>>
891X<escape sequence> X<escape>
892
a0d0e21e
LW
893
894While we usually think of quotes as literal values, in Perl they
895function as operators, providing various kinds of interpolating and
896pattern matching capabilities. Perl provides customary quote characters
897for these behaviors, but also provides a way for you to choose your
898quote character for any of them. In the following table, a C<{}> represents
9f10b797 899any pair of delimiters you choose.
a0d0e21e 900
2c268ad5
TP
901 Customary Generic Meaning Interpolates
902 '' q{} Literal no
903 "" qq{} Literal yes
af9219ee 904 `` qx{} Command yes*
2c268ad5 905 qw{} Word list no
af9219ee
MG
906 // m{} Pattern match yes*
907 qr{} Pattern yes*
908 s{}{} Substitution yes*
2c268ad5 909 tr{}{} Transliteration no (but see below)
7e3b091d 910 <<EOF here-doc yes*
a0d0e21e 911
af9219ee
MG
912 * unless the delimiter is ''.
913
87275199
GS
914Non-bracketing delimiters use the same character fore and aft, but the four
915sorts of brackets (round, angle, square, curly) will all nest, which means
9f10b797 916that
87275199 917
9f10b797 918 q{foo{bar}baz}
35f2feb0 919
9f10b797 920is the same as
87275199
GS
921
922 'foo{bar}baz'
923
924Note, however, that this does not always work for quoting Perl code:
925
926 $s = q{ if($a eq "}") ... }; # WRONG
927
83df6a1d
JH
928is a syntax error. The C<Text::Balanced> module (from CPAN, and
929starting from Perl 5.8 part of the standard distribution) is able
930to do this properly.
87275199 931
19799a22 932There can be whitespace between the operator and the quoting
fb73857a 933characters, except when C<#> is being used as the quoting character.
19799a22
GS
934C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
935operator C<q> followed by a comment. Its argument will be taken
936from the next line. This allows you to write:
fb73857a
PP
937
938 s {foo} # Replace foo
939 {bar} # with bar.
940
904501ec
MG
941The following escape sequences are available in constructs that interpolate
942and in transliterations.
d74e8afc 943X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N>
a0d0e21e 944
6ee5d4e7 945 \t tab (HT, TAB)
5a964f20 946 \n newline (NL)
6ee5d4e7
PP
947 \r return (CR)
948 \f form feed (FF)
949 \b backspace (BS)
950 \a alarm (bell) (BEL)
951 \e escape (ESC)
a0ed51b3
LW
952 \033 octal char (ESC)
953 \x1b hex char (ESC)
954 \x{263a} wide hex char (SMILEY)
19799a22 955 \c[ control char (ESC)
95cc3e0c 956 \N{name} named Unicode character
2c268ad5 957
4c77eaa2
AE
958B<NOTE>: Unlike C and other languages, Perl has no \v escape sequence for
959the vertical tab (VT - ASCII 11).
960
904501ec
MG
961The following escape sequences are available in constructs that interpolate
962but not in transliterations.
d74e8afc 963X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q>
904501ec 964
a0d0e21e
LW
965 \l lowercase next char
966 \u uppercase next char
967 \L lowercase till \E
968 \U uppercase till \E
969 \E end case modification
1d2dff63 970 \Q quote non-word characters till \E
a0d0e21e 971
95cc3e0c
JH
972If C<use locale> is in effect, the case map used by C<\l>, C<\L>,
973C<\u> and C<\U> is taken from the current locale. See L<perllocale>.
974If Unicode (for example, C<\N{}> or wide hex characters of 0x100 or
975beyond) is being used, the case map used by C<\l>, C<\L>, C<\u> and
976C<\U> is as defined by Unicode. For documentation of C<\N{name}>,
977see L<charnames>.
a034a98d 978
5a964f20
TC
979All systems use the virtual C<"\n"> to represent a line terminator,
980called a "newline". There is no such thing as an unvarying, physical
19799a22 981newline character. It is only an illusion that the operating system,
5a964f20
TC
982device drivers, C libraries, and Perl all conspire to preserve. Not all
983systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
984on a Mac, these are reversed, and on systems without line terminator,
985printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
986you mean a "newline" for your system, but use the literal ASCII when you
987need an exact character. For example, most networking protocols expect
2a380090 988and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
5a964f20
TC
989and although they often accept just C<"\012">, they seldom tolerate just
990C<"\015">. If you get in the habit of using C<"\n"> for networking,
991you may be burned some day.
d74e8afc
ITB
992X<newline> X<line terminator> X<eol> X<end of line>
993X<\n> X<\r> X<\r\n>
5a964f20 994
904501ec
MG
995For constructs that do interpolate, variables beginning with "C<$>"
996or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
ad0f383a
A
997C<< $href->{key}[0] >> are also interpolated, as are array and hash slices.
998But method calls such as C<< $obj->meth >> are not.
af9219ee
MG
999
1000Interpolating an array or slice interpolates the elements in order,
1001separated by the value of C<$">, so is equivalent to interpolating
904501ec
MG
1002C<join $", @array>. "Punctuation" arrays such as C<@+> are only
1003interpolated if the name is enclosed in braces C<@{+}>.
af9219ee 1004
1d2dff63
GS
1005You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
1006An unescaped C<$> or C<@> interpolates the corresponding variable,
1007while escaping will cause the literal string C<\$> to be inserted.
1008You'll need to write something like C<m/\Quser\E\@\Qhost/>.
1009
a0d0e21e
LW
1010Patterns are subject to an additional level of interpretation as a
1011regular expression. This is done as a second pass, after variables are
1012interpolated, so that regular expressions may be incorporated into the
1013pattern from the variables. If this is not what you want, use C<\Q> to
1014interpolate a variable literally.
1015
19799a22
GS
1016Apart from the behavior described above, Perl does not expand
1017multiple levels of interpolation. In particular, contrary to the
1018expectations of shell programmers, back-quotes do I<NOT> interpolate
1019within double quotes, nor do single quotes impede evaluation of
1020variables when used within double quotes.
a0d0e21e 1021
5f05dabc 1022=head2 Regexp Quote-Like Operators
d74e8afc 1023X<operator, regexp>
cb1a09d0 1024
5f05dabc 1025Here are the quote-like operators that apply to pattern
cb1a09d0
AD
1026matching and related activities.
1027
a0d0e21e
LW
1028=over 8
1029
1030=item ?PATTERN?
d74e8afc 1031X<?>
a0d0e21e
LW
1032
1033This is just like the C</pattern/> search, except that it matches only
1034once between calls to the reset() operator. This is a useful
5f05dabc 1035optimization when you want to see only the first occurrence of
a0d0e21e
LW
1036something in each file of a set of files, for instance. Only C<??>
1037patterns local to the current package are reset.
1038
5a964f20
TC
1039 while (<>) {
1040 if (?^$?) {
1041 # blank line between header and body
1042 }
1043 } continue {
1044 reset if eof; # clear ?? status for next file
1045 }
1046
483b4840 1047This usage is vaguely deprecated, which means it just might possibly
19799a22
GS
1048be removed in some distant future version of Perl, perhaps somewhere
1049around the year 2168.
a0d0e21e 1050
fb73857a 1051=item m/PATTERN/cgimosx
d74e8afc
ITB
1052X<m> X<operator, match>
1053X<regexp, options> X<regexp> X<regex, options> X<regex>
1054X</c> X</i> X</m> X</o> X</s> X</x>
a0d0e21e 1055
fb73857a 1056=item /PATTERN/cgimosx
a0d0e21e 1057
5a964f20 1058Searches a string for a pattern match, and in scalar context returns
19799a22
GS
1059true if it succeeds, false if it fails. If no string is specified
1060via the C<=~> or C<!~> operator, the $_ string is searched. (The
1061string specified with C<=~> need not be an lvalue--it may be the
1062result of an expression evaluation, but remember the C<=~> binds
1063rather tightly.) See also L<perlre>. See L<perllocale> for
1064discussion of additional considerations that apply when C<use locale>
1065is in effect.
a0d0e21e
LW
1066
1067Options are:
1068
fb73857a 1069 c Do not reset search position on a failed match when /g is in effect.
5f05dabc 1070 g Match globally, i.e., find all occurrences.
a0d0e21e
LW
1071 i Do case-insensitive pattern matching.
1072 m Treat string as multiple lines.
5f05dabc 1073 o Compile pattern only once.
a0d0e21e
LW
1074 s Treat string as single line.
1075 x Use extended regular expressions.
1076
1077If "/" is the delimiter then the initial C<m> is optional. With the C<m>
01ae956f 1078you can use any pair of non-alphanumeric, non-whitespace characters
19799a22
GS
1079as delimiters. This is particularly useful for matching path names
1080that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
7bac28a0 1081the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
19799a22 1082If "'" is the delimiter, no interpolation is performed on the PATTERN.
a0d0e21e
LW
1083
1084PATTERN may contain variables, which will be interpolated (and the
f70b4f9c 1085pattern recompiled) every time the pattern search is evaluated, except
1f247705
GS
1086for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
1087C<$|> are not interpolated because they look like end-of-string tests.)
f70b4f9c
AB
1088If you want such a pattern to be compiled only once, add a C</o> after
1089the trailing delimiter. This avoids expensive run-time recompilations,
1090and is useful when the value you are interpolating won't change over
1091the life of the script. However, mentioning C</o> constitutes a promise
1092that you won't change the variables in the pattern. If you change them,
13a2d996 1093Perl won't even notice. See also L<"qr/STRING/imosx">.
a0d0e21e 1094
5a964f20 1095If the PATTERN evaluates to the empty string, the last
d65afb4b
HS
1096I<successfully> matched regular expression is used instead. In this
1097case, only the C<g> and C<c> flags on the empty pattern is honoured -
1098the other flags are taken from the original pattern. If no match has
1099previously succeeded, this will (silently) act instead as a genuine
1100empty pattern (which will always match).
a0d0e21e 1101
c963b151
BD
1102Note that it's possible to confuse Perl into thinking C<//> (the empty
1103regex) is really C<//> (the defined-or operator). Perl is usually pretty
1104good about this, but some pathological cases might trigger this, such as
1105C<$a///> (is that C<($a) / (//)> or C<$a // />?) and C<print $fh //>
1106(C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl
1107will assume you meant defined-or. If you meant the empty regex, just
1108use parentheses or spaces to disambiguate, or even prefix the empty
1109regex with an C<m> (so C<//> becomes C<m//>).
1110
19799a22 1111If the C</g> option is not used, C<m//> in list context returns a
a0d0e21e 1112list consisting of the subexpressions matched by the parentheses in the
f7e33566
GS
1113pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are
1114also set, and that this differs from Perl 4's behavior.) When there are
1115no parentheses in the pattern, the return value is the list C<(1)> for
1116success. With or without parentheses, an empty list is returned upon
1117failure.
a0d0e21e
LW
1118
1119Examples:
1120
1121 open(TTY, '/dev/tty');
1122 <TTY> =~ /^y/i && foo(); # do foo if desired
1123
1124 if (/Version: *([0-9.]*)/) { $version = $1; }
1125
1126 next if m#^/usr/spool/uucp#;
1127
1128 # poor man's grep
1129 $arg = shift;
1130 while (<>) {
1131 print if /$arg/o; # compile only once
1132 }
1133
1134 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
1135
1136This last example splits $foo into the first two words and the
5f05dabc
PP
1137remainder of the line, and assigns those three fields to $F1, $F2, and
1138$Etc. The conditional is true if any variables were assigned, i.e., if
a0d0e21e
LW
1139the pattern matched.
1140
19799a22
GS
1141The C</g> modifier specifies global pattern matching--that is,
1142matching as many times as possible within the string. How it behaves
1143depends on the context. In list context, it returns a list of the
1144substrings matched by any capturing parentheses in the regular
1145expression. If there are no parentheses, it returns a list of all
1146the matched strings, as if there were parentheses around the whole
1147pattern.
a0d0e21e 1148
7e86de3e 1149In scalar context, each execution of C<m//g> finds the next match,
19799a22 1150returning true if it matches, and false if there is no further match.
7e86de3e
G
1151The position after the last match can be read or set using the pos()
1152function; see L<perlfunc/pos>. A failed match normally resets the
1153search position to the beginning of the string, but you can avoid that
1154by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
1155string also resets the search position.
c90c0ff4
PP
1156
1157You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
1158zero-width assertion that matches the exact position where the previous
5d43e42d
DC
1159C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion
1160still anchors at pos(), but the match is of course only attempted once.
1161Using C<\G> without C</g> on a target string that has not previously had a
1162C</g> match applied to it is the same as using the C<\A> assertion to match
fe4b3f22
RGS
1163the beginning of the string. Note also that, currently, C<\G> is only
1164properly supported when anchored at the very beginning of the pattern.
c90c0ff4
PP
1165
1166Examples:
a0d0e21e
LW
1167
1168 # list context
1169 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
1170
1171 # scalar context
5d43e42d 1172 $/ = "";
19799a22
GS
1173 while (defined($paragraph = <>)) {
1174 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
1175 $sentences++;
a0d0e21e
LW
1176 }
1177 }
1178 print "$sentences\n";
1179
c90c0ff4 1180 # using m//gc with \G
137443ea 1181 $_ = "ppooqppqq";
44a8e56a
PP
1182 while ($i++ < 2) {
1183 print "1: '";
c90c0ff4 1184 print $1 while /(o)/gc; print "', pos=", pos, "\n";
44a8e56a 1185 print "2: '";
c90c0ff4 1186 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
44a8e56a 1187 print "3: '";
c90c0ff4 1188 print $1 while /(p)/gc; print "', pos=", pos, "\n";
44a8e56a 1189 }
5d43e42d 1190 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
44a8e56a
PP
1191
1192The last example should print:
1193
1194 1: 'oo', pos=4
137443ea 1195 2: 'q', pos=5
44a8e56a
PP
1196 3: 'pp', pos=7
1197 1: '', pos=7
137443ea
PP
1198 2: 'q', pos=8
1199 3: '', pos=8
5d43e42d
DC
1200 Final: 'q', pos=8
1201
1202Notice that the final match matched C<q> instead of C<p>, which a match
1203without the C<\G> anchor would have done. Also note that the final match
1204did not update C<pos> -- C<pos> is only updated on a C</g> match. If the
1205final match did indeed match C<p>, it's a good bet that you're running an
1206older (pre-5.6.0) Perl.
44a8e56a 1207
c90c0ff4 1208A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
e7ea3e70 1209combine several regexps like this to process a string part-by-part,
c90c0ff4
PP
1210doing different actions depending on which regexp matched. Each
1211regexp tries to match where the previous one leaves off.
e7ea3e70 1212
3fe9a6f1 1213 $_ = <<'EOL';
e7ea3e70 1214 $url = new URI::URL "http://www/"; die if $url eq "xXx";
3fe9a6f1
PP
1215 EOL
1216 LOOP:
e7ea3e70 1217 {
c90c0ff4
PP
1218 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
1219 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
1220 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
1221 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
1222 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
1223 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
1224 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
e7ea3e70
IZ
1225 print ". That's all!\n";
1226 }
1227
1228Here is the output (split into several lines):
1229
1230 line-noise lowercase line-noise lowercase UPPERCASE line-noise
1231 UPPERCASE line-noise lowercase line-noise lowercase line-noise
1232 lowercase lowercase line-noise lowercase lowercase line-noise
1233 MiXeD line-noise. That's all!
44a8e56a 1234
a0d0e21e 1235=item q/STRING/
5d44bfff 1236X<q> X<quote, single> X<'> X<''>
a0d0e21e 1237
5d44bfff 1238=item 'STRING'
a0d0e21e 1239
19799a22 1240A single-quoted, literal string. A backslash represents a backslash
68dc0745
PP
1241unless followed by the delimiter or another backslash, in which case
1242the delimiter or backslash is interpolated.
a0d0e21e
LW
1243
1244 $foo = q!I said, "You said, 'She said it.'"!;
1245 $bar = q('This is it.');
68dc0745 1246 $baz = '\n'; # a two-character string
a0d0e21e
LW
1247
1248=item qq/STRING/
d74e8afc 1249X<qq> X<quote, double> X<"> X<"">
a0d0e21e
LW
1250
1251=item "STRING"
1252
1253A double-quoted, interpolated string.
1254
1255 $_ .= qq
1256 (*** The previous line contains the naughty word "$1".\n)
19799a22 1257 if /\b(tcl|java|python)\b/i; # :-)
68dc0745 1258 $baz = "\n"; # a one-character string
a0d0e21e 1259
eec2d3df 1260=item qr/STRING/imosx
d74e8afc 1261X<qr> X</i> X</m> X</o> X</s> X</x>
eec2d3df 1262
322edccd 1263This operator quotes (and possibly compiles) its I<STRING> as a regular
19799a22
GS
1264expression. I<STRING> is interpolated the same way as I<PATTERN>
1265in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
1266is done. Returns a Perl value which may be used instead of the
1267corresponding C</STRING/imosx> expression.
4b6a7270
IZ
1268
1269For example,
1270
1271 $rex = qr/my.STRING/is;
1272 s/$rex/foo/;
1273
1274is equivalent to
1275
1276 s/my.STRING/foo/is;
1277
1278The result may be used as a subpattern in a match:
eec2d3df
GS
1279
1280 $re = qr/$pattern/;
0a92e3a8
GS
1281 $string =~ /foo${re}bar/; # can be interpolated in other patterns
1282 $string =~ $re; # or used standalone
4b6a7270
IZ
1283 $string =~ /$re/; # or this way
1284
1285Since Perl may compile the pattern at the moment of execution of qr()
19799a22 1286operator, using qr() may have speed advantages in some situations,
4b6a7270
IZ
1287notably if the result of qr() is used standalone:
1288
1289 sub match {
1290 my $patterns = shift;
1291 my @compiled = map qr/$_/i, @$patterns;
1292 grep {
1293 my $success = 0;
a7665c5e 1294 foreach my $pat (@compiled) {
4b6a7270
IZ
1295 $success = 1, last if /$pat/;
1296 }
1297 $success;
1298 } @_;
1299 }
1300
19799a22
GS
1301Precompilation of the pattern into an internal representation at
1302the moment of qr() avoids a need to recompile the pattern every
1303time a match C</$pat/> is attempted. (Perl has many other internal
1304optimizations, but none would be triggered in the above example if
1305we did not use qr() operator.)
eec2d3df
GS
1306
1307Options are:
1308
1309 i Do case-insensitive pattern matching.
1310 m Treat string as multiple lines.
1311 o Compile pattern only once.
1312 s Treat string as single line.
1313 x Use extended regular expressions.
1314
0a92e3a8
GS
1315See L<perlre> for additional information on valid syntax for STRING, and
1316for a detailed look at the semantics of regular expressions.
1317
a0d0e21e 1318=item qx/STRING/
d74e8afc 1319X<qx> X<`> X<``> X<backtick>
a0d0e21e
LW
1320
1321=item `STRING`
1322
43dd4d21
JH
1323A string which is (possibly) interpolated and then executed as a
1324system command with C</bin/sh> or its equivalent. Shell wildcards,
1325pipes, and redirections will be honored. The collected standard
1326output of the command is returned; standard error is unaffected. In
1327scalar context, it comes back as a single (potentially multi-line)
1328string, or undef if the command failed. In list context, returns a
1329list of lines (however you've defined lines with $/ or
1330$INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
5a964f20
TC
1331
1332Because backticks do not affect standard error, use shell file descriptor
1333syntax (assuming the shell supports this) if you care to address this.
1334To capture a command's STDERR and STDOUT together:
a0d0e21e 1335
5a964f20
TC
1336 $output = `cmd 2>&1`;
1337
1338To capture a command's STDOUT but discard its STDERR:
1339
1340 $output = `cmd 2>/dev/null`;
1341
1342To capture a command's STDERR but discard its STDOUT (ordering is
1343important here):
1344
1345 $output = `cmd 2>&1 1>/dev/null`;
1346
1347To exchange a command's STDOUT and STDERR in order to capture the STDERR
1348but leave its STDOUT to come out the old STDERR:
1349
1350 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
1351
1352To read both a command's STDOUT and its STDERR separately, it's easiest
2359510d
SD
1353to redirect them separately to files, and then read from those files
1354when the program is done:
5a964f20 1355
2359510d 1356 system("program args 1>program.stdout 2>program.stderr");
5a964f20 1357
30398227
SP
1358The STDIN filehandle used by the command is inherited from Perl's STDIN.
1359For example:
1360
1361 open BLAM, "blam" || die "Can't open: $!";
1362 open STDIN, "<&BLAM";
1363 print `sort`;
1364
1365will print the sorted contents of the file "blam".
1366
5a964f20
TC
1367Using single-quote as a delimiter protects the command from Perl's
1368double-quote interpolation, passing it on to the shell instead:
1369
1370 $perl_info = qx(ps $$); # that's Perl's $$
1371 $shell_info = qx'ps $$'; # that's the new shell's $$
1372
19799a22 1373How that string gets evaluated is entirely subject to the command
5a964f20
TC
1374interpreter on your system. On most platforms, you will have to protect
1375shell metacharacters if you want them treated literally. This is in
1376practice difficult to do, as it's unclear how to escape which characters.
1377See L<perlsec> for a clean and safe example of a manual fork() and exec()
1378to emulate backticks safely.
a0d0e21e 1379
bb32b41a
GS
1380On some platforms (notably DOS-like ones), the shell may not be
1381capable of dealing with multiline commands, so putting newlines in
1382the string may not get you what you want. You may be able to evaluate
1383multiple commands in a single line by separating them with the command
1384separator character, if your shell supports that (e.g. C<;> on many Unix
1385shells; C<&> on the Windows NT C<cmd> shell).
1386
0f897271
GS
1387Beginning with v5.6.0, Perl will attempt to flush all files opened for
1388output before starting the child process, but this may not be supported
1389on some platforms (see L<perlport>). To be safe, you may need to set
1390C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1391C<IO::Handle> on any open handles.
1392
bb32b41a
GS
1393Beware that some command shells may place restrictions on the length
1394of the command line. You must ensure your strings don't exceed this
1395limit after any necessary interpolations. See the platform-specific
1396release notes for more details about your particular environment.
1397
5a964f20
TC
1398Using this operator can lead to programs that are difficult to port,
1399because the shell commands called vary between systems, and may in
1400fact not be present at all. As one example, the C<type> command under
1401the POSIX shell is very different from the C<type> command under DOS.
1402That doesn't mean you should go out of your way to avoid backticks
1403when they're the right way to get something done. Perl was made to be
1404a glue language, and one of the things it glues together is commands.
1405Just understand what you're getting yourself into.
bb32b41a 1406
da87341d 1407See L</"I/O Operators"> for more discussion.
a0d0e21e 1408
945c54fd 1409=item qw/STRING/
d74e8afc 1410X<qw> X<quote, list> X<quote, words>
945c54fd
JH
1411
1412Evaluates to a list of the words extracted out of STRING, using embedded
1413whitespace as the word delimiters. It can be understood as being roughly
1414equivalent to:
1415
1416 split(' ', q/STRING/);
1417
efb1e162
CW
1418the differences being that it generates a real list at compile time, and
1419in scalar context it returns the last element in the list. So
945c54fd
JH
1420this expression:
1421
1422 qw(foo bar baz)
1423
1424is semantically equivalent to the list:
1425
1426 'foo', 'bar', 'baz'
1427
1428Some frequently seen examples:
1429
1430 use POSIX qw( setlocale localeconv )
1431 @EXPORT = qw( foo bar baz );
1432
1433A common mistake is to try to separate the words with comma or to
1434put comments into a multi-line C<qw>-string. For this reason, the
1435C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
1436produces warnings if the STRING contains the "," or the "#" character.
1437
a0d0e21e 1438=item s/PATTERN/REPLACEMENT/egimosx
d74e8afc
ITB
1439X<substitute> X<substitution> X<replace> X<regexp, replace>
1440X<regexp, substitute> X</e> X</g> X</i> X</m> X</o> X</s> X</x>
a0d0e21e
LW
1441
1442Searches a string for a pattern, and if found, replaces that pattern
1443with the replacement text and returns the number of substitutions
e37d713d 1444made. Otherwise it returns false (specifically, the empty string).
a0d0e21e
LW
1445
1446If no string is specified via the C<=~> or C<!~> operator, the C<$_>
1447variable is searched and modified. (The string specified with C<=~> must
5a964f20 1448be scalar variable, an array element, a hash element, or an assignment
5f05dabc 1449to one of those, i.e., an lvalue.)
a0d0e21e 1450
19799a22 1451If the delimiter chosen is a single quote, no interpolation is
a0d0e21e
LW
1452done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1453PATTERN contains a $ that looks like a variable rather than an
1454end-of-string test, the variable will be interpolated into the pattern
5f05dabc 1455at run-time. If you want the pattern compiled only once the first time
a0d0e21e 1456the variable is interpolated, use the C</o> option. If the pattern
5a964f20 1457evaluates to the empty string, the last successfully executed regular
a0d0e21e 1458expression is used instead. See L<perlre> for further explanation on these.
5a964f20 1459See L<perllocale> for discussion of additional considerations that apply
a034a98d 1460when C<use locale> is in effect.
a0d0e21e
LW
1461
1462Options are:
1463
1464 e Evaluate the right side as an expression.
5f05dabc 1465 g Replace globally, i.e., all occurrences.
a0d0e21e
LW
1466 i Do case-insensitive pattern matching.
1467 m Treat string as multiple lines.
5f05dabc 1468 o Compile pattern only once.
a0d0e21e
LW
1469 s Treat string as single line.
1470 x Use extended regular expressions.
1471
1472Any non-alphanumeric, non-whitespace delimiter may replace the
1473slashes. If single quotes are used, no interpretation is done on the
e37d713d 1474replacement string (the C</e> modifier overrides this, however). Unlike
54310121 1475Perl 4, Perl 5 treats backticks as normal delimiters; the replacement
e37d713d 1476text is not evaluated as a command. If the
a0d0e21e 1477PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
5f05dabc 1478pair of quotes, which may or may not be bracketing quotes, e.g.,
35f2feb0 1479C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
cec88af6
GS
1480replacement portion to be treated as a full-fledged Perl expression
1481and evaluated right then and there. It is, however, syntax checked at
1482compile-time. A second C<e> modifier will cause the replacement portion
1483to be C<eval>ed before being run as a Perl expression.
a0d0e21e
LW
1484
1485Examples:
1486
1487 s/\bgreen\b/mauve/g; # don't change wintergreen
1488
1489 $path =~ s|/usr/bin|/usr/local/bin|;
1490
1491 s/Login: $foo/Login: $bar/; # run-time pattern
1492
5a964f20 1493 ($foo = $bar) =~ s/this/that/; # copy first, then change
a0d0e21e 1494
5a964f20 1495 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
a0d0e21e
LW
1496
1497 $_ = 'abc123xyz';
1498 s/\d+/$&*2/e; # yields 'abc246xyz'
1499 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
1500 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1501
1502 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
1503 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
1504 s/^=(\w+)/&pod($1)/ge; # use function call
1505
5a964f20
TC
1506 # expand variables in $_, but dynamics only, using
1507 # symbolic dereferencing
1508 s/\$(\w+)/${$1}/g;
1509
cec88af6
GS
1510 # Add one to the value of any numbers in the string
1511 s/(\d+)/1 + $1/eg;
1512
1513 # This will expand any embedded scalar variable
1514 # (including lexicals) in $_ : First $1 is interpolated
1515 # to the variable name, and then evaluated
a0d0e21e
LW
1516 s/(\$\w+)/$1/eeg;
1517
5a964f20 1518 # Delete (most) C comments.
a0d0e21e 1519 $program =~ s {
4633a7c4
LW
1520 /\* # Match the opening delimiter.
1521 .*? # Match a minimal number of characters.
1522 \*/ # Match the closing delimiter.
a0d0e21e
LW
1523 } []gsx;
1524
6b0ac556 1525 s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_, expensively
5a964f20 1526
6b0ac556 1527 for ($variable) { # trim whitespace in $variable, cheap
5a964f20
TC
1528 s/^\s+//;
1529 s/\s+$//;
1530 }
a0d0e21e
LW
1531
1532 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
1533
54310121 1534Note the use of $ instead of \ in the last example. Unlike
35f2feb0
GS
1535B<sed>, we use the \<I<digit>> form in only the left hand side.
1536Anywhere else it's $<I<digit>>.
a0d0e21e 1537
5f05dabc 1538Occasionally, you can't use just a C</g> to get all the changes
19799a22 1539to occur that you might want. Here are two common cases:
a0d0e21e
LW
1540
1541 # put commas in the right places in an integer
19799a22 1542 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
a0d0e21e
LW
1543
1544 # expand tabs to 8-column spacing
1545 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
1546
6940069f 1547=item tr/SEARCHLIST/REPLACEMENTLIST/cds
d74e8afc 1548X<tr> X<y> X<transliterate> X</c> X</d> X</s>
a0d0e21e 1549
6940069f 1550=item y/SEARCHLIST/REPLACEMENTLIST/cds
a0d0e21e 1551
2c268ad5 1552Transliterates all occurrences of the characters found in the search list
a0d0e21e
LW
1553with the corresponding character in the replacement list. It returns
1554the number of characters replaced or deleted. If no string is
2c268ad5 1555specified via the =~ or !~ operator, the $_ string is transliterated. (The
54310121
PP
1556string specified with =~ must be a scalar variable, an array element, a
1557hash element, or an assignment to one of those, i.e., an lvalue.)
8ada0baa 1558
2c268ad5
TP
1559A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
1560does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
54310121
PP
1561For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1562SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1563its own pair of quotes, which may or may not be bracketing quotes,
2c268ad5 1564e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
a0d0e21e 1565
cc255d5f
JH
1566Note that C<tr> does B<not> do regular expression character classes
1567such as C<\d> or C<[:lower:]>. The <tr> operator is not equivalent to
1568the tr(1) utility. If you want to map strings between lower/upper
1569cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider
1570using the C<s> operator if you need regular expressions.
1571
8ada0baa
JH
1572Note also that the whole range idea is rather unportable between
1573character sets--and even within character sets they may cause results
1574you probably didn't expect. A sound principle is to use only ranges
1575that begin from and end at either alphabets of equal case (a-e, A-E),
1576or digits (0-4). Anything else is unsafe. If in doubt, spell out the
1577character sets in full.
1578
a0d0e21e
LW
1579Options:
1580
1581 c Complement the SEARCHLIST.
1582 d Delete found but unreplaced characters.
1583 s Squash duplicate replaced characters.
1584
19799a22
GS
1585If the C</c> modifier is specified, the SEARCHLIST character set
1586is complemented. If the C</d> modifier is specified, any characters
1587specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
1588(Note that this is slightly more flexible than the behavior of some
1589B<tr> programs, which delete anything they find in the SEARCHLIST,
1590period.) If the C</s> modifier is specified, sequences of characters
1591that were transliterated to the same character are squashed down
1592to a single instance of the character.
a0d0e21e
LW
1593
1594If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1595exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1596than the SEARCHLIST, the final character is replicated till it is long
5a964f20 1597enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
a0d0e21e
LW
1598This latter is useful for counting characters in a class or for
1599squashing character sequences in a class.
1600
1601Examples:
1602
1603 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1604
1605 $cnt = tr/*/*/; # count the stars in $_
1606
1607 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1608
1609 $cnt = tr/0-9//; # count the digits in $_
1610
1611 tr/a-zA-Z//s; # bookkeeper -> bokeper
1612
1613 ($HOST = $host) =~ tr/a-z/A-Z/;
1614
1615 tr/a-zA-Z/ /cs; # change non-alphas to single space
1616
1617 tr [\200-\377]
1618 [\000-\177]; # delete 8th bit
1619
19799a22
GS
1620If multiple transliterations are given for a character, only the
1621first one is used:
748a9306
LW
1622
1623 tr/AAA/XYZ/
1624
2c268ad5 1625will transliterate any A to X.
748a9306 1626
19799a22 1627Because the transliteration table is built at compile time, neither
a0d0e21e 1628the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
19799a22
GS
1629interpolation. That means that if you want to use variables, you
1630must use an eval():
a0d0e21e
LW
1631
1632 eval "tr/$oldlist/$newlist/";
1633 die $@ if $@;
1634
1635 eval "tr/$oldlist/$newlist/, 1" or die $@;
1636
7e3b091d 1637=item <<EOF
d74e8afc 1638X<here-doc> X<heredoc> X<here-document> X<<< << >>>
7e3b091d
DA
1639
1640A line-oriented form of quoting is based on the shell "here-document"
1641syntax. Following a C<< << >> you specify a string to terminate
1642the quoted material, and all lines following the current line down to
1643the terminating string are the value of the item. The terminating
1644string may be either an identifier (a word), or some quoted text. If
1645quoted, the type of quotes you use determines the treatment of the
1646text, just as in regular quoting. An unquoted identifier works like
1647double quotes. There must be no space between the C<< << >> and
1648the identifier, unless the identifier is quoted. (If you put a space it
1649will be treated as a null identifier, which is valid, and matches the first
1650empty line.) The terminating string must appear by itself (unquoted and
1651with no surrounding whitespace) on the terminating line.
1652
1653 print <<EOF;
1654 The price is $Price.
1655 EOF
1656
1657 print << "EOF"; # same as above
1658 The price is $Price.
1659 EOF
1660
1661 print << `EOC`; # execute commands
1662 echo hi there
1663 echo lo there
1664 EOC
1665
1666 print <<"foo", <<"bar"; # you can stack them
1667 I said foo.
1668 foo
1669 I said bar.
1670 bar
1671
1672 myfunc(<< "THIS", 23, <<'THAT');
1673 Here's a line
1674 or two.
1675 THIS
1676 and here's another.
1677 THAT
1678
1679Just don't forget that you have to put a semicolon on the end
1680to finish the statement, as Perl doesn't know you're not going to
1681try to do this:
1682
1683 print <<ABC
1684 179231
1685 ABC
1686 + 20;
1687
872d7e53
ST
1688If you want to remove the line terminator from your here-docs,
1689use C<chomp()>.
1690
1691 chomp($string = <<'END');
1692 This is a string.
1693 END
1694
1695If you want your here-docs to be indented with the rest of the code,
1696you'll need to remove leading whitespace from each line manually:
7e3b091d
DA
1697
1698 ($quote = <<'FINIS') =~ s/^\s+//gm;
1699 The Road goes ever on and on,
1700 down from the door where it began.
1701 FINIS
1702
1703If you use a here-doc within a delimited construct, such as in C<s///eg>,
1704the quoted material must come on the lines following the final delimiter.
1705So instead of
1706
1707 s/this/<<E . 'that'
1708 the other
1709 E
1710 . 'more '/eg;
1711
1712you have to write
1713
1714 s/this/<<E . 'that'
1715 . 'more '/eg;
1716 the other
1717 E
1718
1719If the terminating identifier is on the last line of the program, you
1720must be sure there is a newline after it; otherwise, Perl will give the
1721warning B<Can't find string terminator "END" anywhere before EOF...>.
1722
1723Additionally, the quoting rules for the identifier are not related to
1724Perl's quoting rules -- C<q()>, C<qq()>, and the like are not supported
1725in place of C<''> and C<"">, and the only interpolation is for backslashing
1726the quoting character:
1727
1728 print << "abc\"def";
1729 testing...
1730 abc"def
1731
1732Finally, quoted strings cannot span multiple lines. The general rule is
1733that the identifier must be a string literal. Stick with that, and you
1734should be safe.
1735
a0d0e21e
LW
1736=back
1737
75e14d17 1738=head2 Gory details of parsing quoted constructs
d74e8afc 1739X<quote, gory details>
75e14d17 1740
19799a22
GS
1741When presented with something that might have several different
1742interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
1743principle to pick the most probable interpretation. This strategy
1744is so successful that Perl programmers often do not suspect the
1745ambivalence of what they write. But from time to time, Perl's
1746notions differ substantially from what the author honestly meant.
1747
1748This section hopes to clarify how Perl handles quoted constructs.
1749Although the most common reason to learn this is to unravel labyrinthine
1750regular expressions, because the initial steps of parsing are the
1751same for all quoting operators, they are all discussed together.
1752
1753The most important Perl parsing rule is the first one discussed
1754below: when processing a quoted construct, Perl first finds the end
1755of that construct, then interprets its contents. If you understand
1756this rule, you may skip the rest of this section on the first
1757reading. The other rules are likely to contradict the user's
1758expectations much less frequently than this first one.
1759
1760Some passes discussed below are performed concurrently, but because
1761their results are the same, we consider them individually. For different
1762quoting constructs, Perl performs different numbers of passes, from
1763one to five, but these passes are always performed in the same order.
75e14d17 1764
13a2d996 1765=over 4
75e14d17
IZ
1766
1767=item Finding the end
1768
19799a22 1769The first pass is finding the end of the quoted construct, whether
872d7e53 1770it be a multicharacter delimiter C<"EOF\n"> in the C<<<EOF>
19799a22 1771construct, a C</> that terminates a C<qq//> construct, a C<]> which
35f2feb0
GS
1772terminates C<qq[]> construct, or a C<< > >> which terminates a
1773fileglob started with C<< < >>.
75e14d17 1774
19799a22
GS
1775When searching for single-character non-pairing delimiters, such
1776as C</>, combinations of C<\\> and C<\/> are skipped. However,
1777when searching for single-character pairing delimiter like C<[>,
1778combinations of C<\\>, C<\]>, and C<\[> are all skipped, and nested
1779C<[>, C<]> are skipped as well. When searching for multicharacter
872d7e53
ST
1780delimiters like C<"EOF\n">, nothing is skipped, though the delimiter
1781must start from the first column of the terminating line.
75e14d17 1782
19799a22
GS
1783For constructs with three-part delimiters (C<s///>, C<y///>, and
1784C<tr///>), the search is repeated once more.
75e14d17 1785
19799a22
GS
1786During this search no attention is paid to the semantics of the construct.
1787Thus:
75e14d17
IZ
1788
1789 "$hash{"$foo/$bar"}"
1790
2a94b7ce 1791or:
75e14d17
IZ
1792
1793 m/
2a94b7ce 1794 bar # NOT a comment, this slash / terminated m//!
75e14d17
IZ
1795 /x
1796
19799a22
GS
1797do not form legal quoted expressions. The quoted part ends on the
1798first C<"> and C</>, and the rest happens to be a syntax error.
1799Because the slash that terminated C<m//> was followed by a C<SPACE>,
1800the example above is not C<m//x>, but rather C<m//> with no C</x>
1801modifier. So the embedded C<#> is interpreted as a literal C<#>.
75e14d17 1802
0d594e51
ST
1803Also no attention is paid to C<\c\> during this search.
1804Thus the second C<\> in C<qq/\c\/> is interpreted as a part of C<\/>,
1805and the following C</> is not recognized as a delimiter.
1806Instead, use C<\034> or C<\x1c> at the end of quoted constructs.
1807
75e14d17
IZ
1808=item Removal of backslashes before delimiters
1809
19799a22
GS
1810During the second pass, text between the starting and ending
1811delimiters is copied to a safe location, and the C<\> is removed
1812from combinations consisting of C<\> and delimiter--or delimiters,
1813meaning both starting and ending delimiters will should these differ.
1814This removal does not happen for multi-character delimiters.
1815Note that the combination C<\\> is left intact, just as it was.
75e14d17 1816
19799a22
GS
1817Starting from this step no information about the delimiters is
1818used in parsing.
75e14d17
IZ
1819
1820=item Interpolation
d74e8afc 1821X<interpolation>
75e14d17 1822
19799a22
GS
1823The next step is interpolation in the text obtained, which is now
1824delimiter-independent. There are four different cases.
75e14d17 1825
13a2d996 1826=over 4
75e14d17
IZ
1827
1828=item C<<<'EOF'>, C<m''>, C<s'''>, C<tr///>, C<y///>
1829
1830No interpolation is performed.
1831
1832=item C<''>, C<q//>
1833
1834The only interpolation is removal of C<\> from pairs C<\\>.
1835
35f2feb0 1836=item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>
75e14d17 1837
19799a22
GS
1838C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
1839converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
1840is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
1841The other combinations are replaced with appropriate expansions.
2a94b7ce 1842
19799a22
GS
1843Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
1844is interpolated in the usual way. Something like C<"\Q\\E"> has
1845no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
1846result is the same as for C<"\\\\E">. As a general rule, backslashes
1847between C<\Q> and C<\E> may lead to counterintuitive results. So,
1848C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
1849as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
2a94b7ce
IZ
1850
1851 $str = '\t';
1852 return "\Q$str";
1853
1854may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
1855
19799a22 1856Interpolated scalars and arrays are converted internally to the C<join> and
92d29cee 1857C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
75e14d17 1858
19799a22 1859 $foo . " XXX '" . (join $", @arr) . "'";
75e14d17 1860
19799a22 1861All operations above are performed simultaneously, left to right.
75e14d17 1862
19799a22
GS
1863Because the result of C<"\Q STRING \E"> has all metacharacters
1864quoted, there is no way to insert a literal C<$> or C<@> inside a
1865C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
1866C<"\\\$">; if not, it is interpreted as the start of an interpolated
1867scalar.
75e14d17 1868
19799a22
GS
1869Note also that the interpolation code needs to make a decision on
1870where the interpolated scalar ends. For instance, whether
35f2feb0 1871C<< "a $b -> {c}" >> really means:
75e14d17
IZ
1872
1873 "a " . $b . " -> {c}";
1874
2a94b7ce 1875or:
75e14d17
IZ
1876
1877 "a " . $b -> {c};
1878
19799a22
GS
1879Most of the time, the longest possible text that does not include
1880spaces between components and which contains matching braces or
1881brackets. because the outcome may be determined by voting based
1882on heuristic estimators, the result is not strictly predictable.
1883Fortunately, it's usually correct for ambiguous cases.
75e14d17
IZ
1884
1885=item C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
1886
19799a22
GS
1887Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
1888happens (almost) as with C<qq//> constructs, but the substitution
1889of C<\> followed by RE-special chars (including C<\>) is not
1890performed. Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
1891a C<#>-comment in a C<//x>-regular expression, no processing is
1892performed whatsoever. This is the first step at which the presence
1893of the C<//x> modifier is relevant.
1894
1895Interpolation has several quirks: C<$|>, C<$(>, and C<$)> are not
1896interpolated, and constructs C<$var[SOMETHING]> are voted (by several
1897different estimators) to be either an array element or C<$var>
1898followed by an RE alternative. This is where the notation
1899C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
1900array element C<-9>, not as a regular expression from the variable
1901C<$arr> followed by a digit, which would be the interpretation of
1902C</$arr[0-9]/>. Since voting among different estimators may occur,
1903the result is not predictable.
1904
1905It is at this step that C<\1> is begrudgingly converted to C<$1> in
1906the replacement text of C<s///> to correct the incorrigible
1907I<sed> hackers who haven't picked up the saner idiom yet. A warning
9f1b1f2d
GS
1908is emitted if the C<use warnings> pragma or the B<-w> command-line flag
1909(that is, the C<$^W> variable) was set.
19799a22
GS
1910
1911The lack of processing of C<\\> creates specific restrictions on
1912the post-processed text. If the delimiter is C</>, one cannot get
1913the combination C<\/> into the result of this step. C</> will
1914finish the regular expression, C<\/> will be stripped to C</> on
1915the previous step, and C<\\/> will be left as is. Because C</> is
1916equivalent to C<\/> inside a regular expression, this does not
1917matter unless the delimiter happens to be character special to the
1918RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
1919alphanumeric char, as in:
2a94b7ce
IZ
1920
1921 m m ^ a \s* b mmx;
1922
19799a22 1923In the RE above, which is intentionally obfuscated for illustration, the
2a94b7ce 1924delimiter is C<m>, the modifier is C<mx>, and after backslash-removal the
aa863641 1925RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
19799a22
GS
1926reason you're encouraged to restrict your delimiters to non-alphanumeric,
1927non-whitespace choices.
75e14d17
IZ
1928
1929=back
1930
19799a22 1931This step is the last one for all constructs except regular expressions,
75e14d17
IZ
1932which are processed further.
1933
1934=item Interpolation of regular expressions
d74e8afc 1935X<regexp, interpolation>
75e14d17 1936
19799a22
GS
1937Previous steps were performed during the compilation of Perl code,
1938but this one happens at run time--although it may be optimized to
1939be calculated at compile time if appropriate. After preprocessing
1940described above, and possibly after evaluation if catenation,
1941joining, casing translation, or metaquoting are involved, the
1942resulting I<string> is passed to the RE engine for compilation.
1943
1944Whatever happens in the RE engine might be better discussed in L<perlre>,
1945but for the sake of continuity, we shall do so here.
1946
1947This is another step where the presence of the C<//x> modifier is
1948relevant. The RE engine scans the string from left to right and
1949converts it to a finite automaton.
1950
1951Backslashed characters are either replaced with corresponding
1952literal strings (as with C<\{>), or else they generate special nodes
1953in the finite automaton (as with C<\b>). Characters special to the
1954RE engine (such as C<|>) generate corresponding nodes or groups of
1955nodes. C<(?#...)> comments are ignored. All the rest is either
1956converted to literal strings to match, or else is ignored (as is
1957whitespace and C<#>-style comments if C<//x> is present).
1958
1959Parsing of the bracketed character class construct, C<[...]>, is
1960rather different than the rule used for the rest of the pattern.
1961The terminator of this construct is found using the same rules as
1962for finding the terminator of a C<{}>-delimited construct, the only
1963exception being that C<]> immediately following C<[> is treated as
1964though preceded by a backslash. Similarly, the terminator of
1965C<(?{...})> is found using the same rules as for finding the
1966terminator of a C<{}>-delimited construct.
1967
1968It is possible to inspect both the string given to RE engine and the
1969resulting finite automaton. See the arguments C<debug>/C<debugcolor>
1970in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
4a4eefd0 1971switch documented in L<perlrun/"Command Switches">.
75e14d17
IZ
1972
1973=item Optimization of regular expressions
d74e8afc 1974X<regexp, optimization>
75e14d17 1975
7522fed5 1976This step is listed for completeness only. Since it does not change
75e14d17 1977semantics, details of this step are not documented and are subject
19799a22
GS
1978to change without notice. This step is performed over the finite
1979automaton that was generated during the previous pass.
2a94b7ce 1980
19799a22
GS
1981It is at this stage that C<split()> silently optimizes C</^/> to
1982mean C</^/m>.
75e14d17
IZ
1983
1984=back
1985
a0d0e21e 1986=head2 I/O Operators
d74e8afc
ITB
1987X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle>
1988X<< <> >> X<@ARGV>
a0d0e21e 1989
54310121 1990There are several I/O operators you should know about.
fbad3eb5 1991
7b8d334a 1992A string enclosed by backticks (grave accents) first undergoes
19799a22
GS
1993double-quote interpolation. It is then interpreted as an external
1994command, and the output of that command is the value of the
e9c56f9b
JH
1995backtick string, like in a shell. In scalar context, a single string
1996consisting of all output is returned. In list context, a list of
1997values is returned, one per line of output. (You can set C<$/> to use
1998a different line terminator.) The command is executed each time the
1999pseudo-literal is evaluated. The status value of the command is
2000returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
2001Unlike in B<csh>, no translation is done on the return data--newlines
2002remain newlines. Unlike in any of the shells, single quotes do not
2003hide variable names in the command from interpretation. To pass a
2004literal dollar-sign through to the shell you need to hide it with a
2005backslash. The generalized form of backticks is C<qx//>. (Because
2006backticks always undergo shell expansion as well, see L<perlsec> for
2007security concerns.)
d74e8afc 2008X<qx> X<`> X<``> X<backtick> X<glob>
19799a22
GS
2009
2010In scalar context, evaluating a filehandle in angle brackets yields
2011the next line from that file (the newline, if any, included), or
2012C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
2013(sometimes known as file-slurp mode) and the file is empty, it
2014returns C<''> the first time, followed by C<undef> subsequently.
2015
2016Ordinarily you must assign the returned value to a variable, but
2017there is one situation where an automatic assignment happens. If
2018and only if the input symbol is the only thing inside the conditional
2019of a C<while> statement (even if disguised as a C<for(;;)> loop),
2020the value is automatically assigned to the global variable $_,
2021destroying whatever was there previously. (This may seem like an
2022odd thing to you, but you'll use the construct in almost every Perl
17b829fa 2023script you write.) The $_ variable is not implicitly localized.
19799a22
GS
2024You'll have to put a C<local $_;> before the loop if you want that
2025to happen.
2026
2027The following lines are equivalent:
a0d0e21e 2028
748a9306 2029 while (defined($_ = <STDIN>)) { print; }
7b8d334a 2030 while ($_ = <STDIN>) { print; }
a0d0e21e
LW
2031 while (<STDIN>) { print; }
2032 for (;<STDIN>;) { print; }
748a9306 2033 print while defined($_ = <STDIN>);
7b8d334a 2034 print while ($_ = <STDIN>);
a0d0e21e
LW
2035 print while <STDIN>;
2036
19799a22 2037This also behaves similarly, but avoids $_ :
7b8d334a
GS
2038
2039 while (my $line = <STDIN>) { print $line }
2040
19799a22
GS
2041In these loop constructs, the assigned value (whether assignment
2042is automatic or explicit) is then tested to see whether it is
2043defined. The defined test avoids problems where line has a string
2044value that would be treated as false by Perl, for example a "" or
2045a "0" with no trailing newline. If you really mean for such values
2046to terminate the loop, they should be tested for explicitly:
7b8d334a
GS
2047
2048 while (($_ = <STDIN>) ne '0') { ... }
2049 while (<STDIN>) { last unless $_; ... }
2050
35f2feb0 2051In other boolean contexts, C<< <I<filehandle>> >> without an
9f1b1f2d
GS
2052explicit C<defined> test or comparison elicit a warning if the
2053C<use warnings> pragma or the B<-w>
19799a22 2054command-line switch (the C<$^W> variable) is in effect.
7b8d334a 2055
5f05dabc 2056The filehandles STDIN, STDOUT, and STDERR are predefined. (The
19799a22
GS
2057filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
2058in packages, where they would be interpreted as local identifiers
2059rather than global.) Additional filehandles may be created with
2060the open() function, amongst others. See L<perlopentut> and
2061L<perlfunc/open> for details on this.
d74e8afc 2062X<stdin> X<stdout> X<sterr>
a0d0e21e 2063
35f2feb0 2064If a <FILEHANDLE> is used in a context that is looking for
19799a22
GS
2065a list, a list comprising all input lines is returned, one line per
2066list element. It's easy to grow to a rather large data space this
2067way, so use with care.
a0d0e21e 2068
35f2feb0 2069<FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
19799a22 2070See L<perlfunc/readline>.
fbad3eb5 2071
35f2feb0
GS
2072The null filehandle <> is special: it can be used to emulate the
2073behavior of B<sed> and B<awk>. Input from <> comes either from
a0d0e21e 2074standard input, or from each file listed on the command line. Here's
35f2feb0 2075how it works: the first time <> is evaluated, the @ARGV array is
5a964f20 2076checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
a0d0e21e
LW
2077gives you standard input. The @ARGV array is then processed as a list
2078of filenames. The loop
2079
2080 while (<>) {
2081 ... # code for each line
2082 }
2083
2084is equivalent to the following Perl-like pseudo code:
2085
3e3baf6d 2086 unshift(@ARGV, '-') unless @ARGV;
a0d0e21e
LW
2087 while ($ARGV = shift) {
2088 open(ARGV, $ARGV);
2089 while (<ARGV>) {
2090 ... # code for each line
2091 }
2092 }
2093
19799a22
GS
2094except that it isn't so cumbersome to say, and will actually work.
2095It really does shift the @ARGV array and put the current filename
2096into the $ARGV variable. It also uses filehandle I<ARGV>
35f2feb0 2097internally--<> is just a synonym for <ARGV>, which
19799a22 2098is magical. (The pseudo code above doesn't work because it treats
35f2feb0 2099<ARGV> as non-magical.)
a0d0e21e 2100
35f2feb0 2101You can modify @ARGV before the first <> as long as the array ends up
a0d0e21e 2102containing the list of filenames you really want. Line numbers (C<$.>)
19799a22
GS
2103continue as though the input were one big happy file. See the example
2104in L<perlfunc/eof> for how to reset line numbers on each file.
5a964f20
TC
2105
2106If you want to set @ARGV to your own list of files, go right ahead.
2107This sets @ARGV to all plain text files if no @ARGV was given:
2108
2109 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
a0d0e21e 2110
5a964f20
TC
2111You can even set them to pipe commands. For example, this automatically
2112filters compressed arguments through B<gzip>:
2113
2114 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
2115
2116If you want to pass switches into your script, you can use one of the
a0d0e21e
LW
2117Getopts modules or put a loop on the front like this:
2118
2119 while ($_ = $ARGV[0], /^-/) {
2120 shift;
2121 last if /^--$/;
2122 if (/^-D(.*)/) { $debug = $1 }
2123 if (/^-v/) { $verbose++ }
5a964f20 2124 # ... # other switches
a0d0e21e 2125 }
5a964f20 2126
a0d0e21e 2127 while (<>) {
5a964f20 2128 # ... # code for each line
a0d0e21e
LW
2129 }
2130
35f2feb0 2131The <> symbol will return C<undef> for end-of-file only once.
19799a22
GS
2132If you call it again after this, it will assume you are processing another
2133@ARGV list, and if you haven't set @ARGV, will read input from STDIN.
a0d0e21e 2134
b159ebd3 2135If what the angle brackets contain is a simple scalar variable (e.g.,
35f2feb0 2136<$foo>), then that variable contains the name of the
19799a22
GS
2137filehandle to input from, or its typeglob, or a reference to the
2138same. For example:
cb1a09d0
AD
2139
2140 $fh = \*STDIN;
2141 $line = <$fh>;
a0d0e21e 2142
5a964f20
TC
2143If what's within the angle brackets is neither a filehandle nor a simple
2144scalar variable containing a filehandle name, typeglob, or typeglob
2145reference, it is interpreted as a filename pattern to be globbed, and
2146either a list of filenames or the next filename in the list is returned,
19799a22 2147depending on context. This distinction is determined on syntactic
35f2feb0
GS
2148grounds alone. That means C<< <$x> >> is always a readline() from
2149an indirect handle, but C<< <$hash{key}> >> is always a glob().
5a964f20 2150That's because $x is a simple scalar variable, but C<$hash{key}> is
ef191992
YST
2151not--it's a hash element. Even C<< <$x > >> (note the extra space)
2152is treated as C<glob("$x ")>, not C<readline($x)>.
5a964f20
TC
2153
2154One level of double-quote interpretation is done first, but you can't
35f2feb0 2155say C<< <$foo> >> because that's an indirect filehandle as explained
5a964f20
TC
2156in the previous paragraph. (In older versions of Perl, programmers
2157would insert curly brackets to force interpretation as a filename glob:
35f2feb0 2158C<< <${foo}> >>. These days, it's considered cleaner to call the
5a964f20 2159internal function directly as C<glob($foo)>, which is probably the right
19799a22 2160way to have done it in the first place.) For example:
a0d0e21e
LW
2161
2162 while (<*.c>) {
2163 chmod 0644, $_;
2164 }
2165
3a4b19e4 2166is roughly equivalent to:
a0d0e21e
LW
2167
2168 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
2169 while (<FOO>) {
5b3eff12 2170 chomp;
a0d0e21e
LW
2171 chmod 0644, $_;
2172 }
2173
3a4b19e4
GS
2174except that the globbing is actually done internally using the standard
2175C<File::Glob> extension. Of course, the shortest way to do the above is:
a0d0e21e
LW
2176
2177 chmod 0644, <*.c>;
2178
19799a22
GS
2179A (file)glob evaluates its (embedded) argument only when it is
2180starting a new list. All values must be read before it will start
2181over. In list context, this isn't important because you automatically
2182get them all anyway. However, in scalar context the operator returns
069e01df 2183the next value each time it's called, or C<undef> when the list has
19799a22
GS
2184run out. As with filehandle reads, an automatic C<defined> is
2185generated when the glob occurs in the test part of a C<while>,
2186because legal glob returns (e.g. a file called F<0>) would otherwise
2187terminate the loop. Again, C<undef> is returned only once. So if
2188you're expecting a single value from a glob, it is much better to
2189say
4633a7c4
LW
2190
2191 ($file) = <blurch*>;
2192
2193than
2194
2195 $file = <blurch*>;
2196
2197because the latter will alternate between returning a filename and
19799a22 2198returning false.
4633a7c4 2199
b159ebd3 2200If you're trying to do variable interpolation, it's definitely better
4633a7c4 2201to use the glob() function, because the older notation can cause people
e37d713d 2202to become confused with the indirect filehandle notation.
4633a7c4
LW
2203
2204 @files = glob("$dir/*.[ch]");
2205 @files = glob($files[$i]);
2206
a0d0e21e 2207=head2 Constant Folding
d74e8afc 2208X<constant folding> X<folding>
a0d0e21e
LW
2209
2210Like C, Perl does a certain amount of expression evaluation at
19799a22 2211compile time whenever it determines that all arguments to an
a0d0e21e
LW
2212operator are static and have no side effects. In particular, string
2213concatenation happens at compile time between literals that don't do
19799a22 2214variable substitution. Backslash interpolation also happens at
a0d0e21e
LW
2215compile time. You can say
2216
2217 'Now is the time for all' . "\n" .
2218 'good men to come to.'
2219
54310121 2220and this all reduces to one string internally. Likewise, if
a0d0e21e
LW
2221you say
2222
2223 foreach $file (@filenames) {
5a964f20 2224 if (-s $file > 5 + 100 * 2**16) { }
54310121 2225 }
a0d0e21e 2226
19799a22
GS
2227the compiler will precompute the number which that expression
2228represents so that the interpreter won't have to.
a0d0e21e 2229
fd1abbef 2230=head2 No-ops
d74e8afc 2231X<no-op> X<nop>
fd1abbef
DN
2232
2233Perl doesn't officially have a no-op operator, but the bare constants
2234C<0> and C<1> are special-cased to not produce a warning in a void
2235context, so you can for example safely do
2236
2237 1 while foo();
2238
2c268ad5 2239=head2 Bitwise String Operators
d74e8afc 2240X<operator, bitwise, string>
2c268ad5
TP
2241
2242Bitstrings of any size may be manipulated by the bitwise operators
2243(C<~ | & ^>).
2244
19799a22
GS
2245If the operands to a binary bitwise op are strings of different
2246sizes, B<|> and B<^> ops act as though the shorter operand had
2247additional zero bits on the right, while the B<&> op acts as though
2248the longer operand were truncated to the length of the shorter.
2249The granularity for such extension or truncation is one or more
2250bytes.
2c268ad5
TP
2251
2252 # ASCII-based examples
2253 print "j p \n" ^ " a h"; # prints "JAPH\n"
2254 print "JA" | " ph\n"; # prints "japh\n"
2255 print "japh\nJunk" & '_____'; # prints "JAPH\n";
2256 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
2257
19799a22 2258If you are intending to manipulate bitstrings, be certain that
2c268ad5 2259you're supplying bitstrings: If an operand is a number, that will imply
19799a22 2260a B<numeric> bitwise operation. You may explicitly show which type of
2c268ad5
TP
2261operation you intend by using C<""> or C<0+>, as in the examples below.
2262
4358a253
SS
2263 $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
2264 $foo = '150' | 105; # yields 255
2c268ad5
TP
2265 $foo = 150 | '105'; # yields 255
2266 $foo = '150' | '105'; # yields string '155' (under ASCII)
2267
2268 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
2269 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
a0d0e21e 2270
1ae175c8
GS
2271See L<perlfunc/vec> for information on how to manipulate individual bits
2272in a bit vector.
2273
55497cff 2274=head2 Integer Arithmetic
d74e8afc 2275X<integer>
a0d0e21e 2276
19799a22 2277By default, Perl assumes that it must do most of its arithmetic in
a0d0e21e
LW
2278floating point. But by saying
2279
2280 use integer;
2281
2282you may tell the compiler that it's okay to use integer operations
19799a22
GS
2283(if it feels like it) from here to the end of the enclosing BLOCK.
2284An inner BLOCK may countermand this by saying
a0d0e21e
LW
2285
2286 no integer;
2287
19799a22
GS
2288which lasts until the end of that BLOCK. Note that this doesn't
2289mean everything is only an integer, merely that Perl may use integer
2290operations if it is so inclined. For example, even under C<use
2291integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731>
2292or so.
2293
2294Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
13a2d996
SP
2295and ">>") always produce integral results. (But see also
2296L<Bitwise String Operators>.) However, C<use integer> still has meaning for
19799a22
GS
2297them. By default, their results are interpreted as unsigned integers, but
2298if C<use integer> is in effect, their results are interpreted
2299as signed integers. For example, C<~0> usually evaluates to a large
0be96356 2300integral value. However, C<use integer; ~0> is C<-1> on two's-complement
19799a22 2301machines.
68dc0745
PP
2302
2303=head2 Floating-point Arithmetic
d74e8afc 2304X<floating-point> X<floating point> X<float> X<real>
68dc0745
PP
2305
2306While C<use integer> provides integer-only arithmetic, there is no
19799a22
GS
2307analogous mechanism to provide automatic rounding or truncation to a
2308certain number of decimal places. For rounding to a certain number
2309of digits, sprintf() or printf() is usually the easiest route.
2310See L<perlfaq4>.
68dc0745 2311
5a964f20
TC
2312Floating-point numbers are only approximations to what a mathematician
2313would call real numbers. There are infinitely more reals than floats,
2314so some corners must be cut. For example:
2315
2316 printf "%.20g\n", 123456789123456789;
2317 # produces 123456789123456784
2318
2319Testing for exact equality of floating-point equality or inequality is
2320not a good idea. Here's a (relatively expensive) work-around to compare
2321whether two floating-point numbers are equal to a particular number of
2322decimal places. See Knuth, volume II, for a more robust treatment of
2323this topic.
2324
2325 sub fp_equal {
2326 my ($X, $Y, $POINTS) = @_;
2327 my ($tX, $tY);
2328 $tX = sprintf("%.${POINTS}g", $X);
2329 $tY = sprintf("%.${POINTS}g", $Y);
2330 return $tX eq $tY;
2331 }
2332
68dc0745 2333The POSIX module (part of the standard perl distribution) implements
19799a22
GS
2334ceil(), floor(), and other mathematical and trigonometric functions.
2335The Math::Complex module (part of the standard perl distribution)
2336defines mathematical functions that work on both the reals and the
2337imaginary numbers. Math::Complex not as efficient as POSIX, but
68dc0745
PP
2338POSIX can't work with complex numbers.
2339
2340Rounding in financial applications can have serious implications, and
2341the rounding method used should be specified precisely. In these
2342cases, it probably pays not to trust whichever system rounding is
2343being used by Perl, but to instead implement the rounding function you
2344need yourself.
5a964f20
TC
2345
2346=head2 Bigger Numbers
d74e8afc 2347X<number, arbitrary precision>
5a964f20
TC
2348
2349The standard Math::BigInt and Math::BigFloat modules provide
19799a22 2350variable-precision arithmetic and overloaded operators, although
cd5c4fce 2351they're currently pretty slow. At the cost of some space and
19799a22
GS
2352considerable speed, they avoid the normal pitfalls associated with
2353limited-precision representations.
5a964f20
TC
2354
2355 use Math::BigInt;
2356 $x = Math::BigInt->new('123456789123456789');
2357 print $x * $x;
2358
2359 # prints +15241578780673678515622620750190521
19799a22 2360
cd5c4fce
T
2361There are several modules that let you calculate with (bound only by
2362memory and cpu-time) unlimited or fixed precision. There are also
2363some non-standard modules that provide faster implementations via
2364external C libraries.
2365
2366Here is a short, but incomplete summary:
2367
2368 Math::Fraction big, unlimited fractions like 9973 / 12967
2369 Math::String treat string sequences like numbers
2370 Math::FixedPrecision calculate with a fixed precision
2371 Math::Currency for currency calculations
2372 Bit::Vector manipulate bit vectors fast (uses C)
2373 Math::BigIntFast Bit::Vector wrapper for big numbers
2374 Math::Pari provides access to the Pari C library
2375 Math::BigInteger uses an external C library
2376 Math::Cephes uses external Cephes C library (no big numbers)
2377 Math::Cephes::Fraction fractions via the Cephes library
2378 Math::GMP another one using an external C library
2379
2380Choose wisely.
16070b82
GS
2381
2382=cut