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a0d0e21e 1=head1 NAME
d74e8afc 2X<operator>
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4perlop - Perl operators and precedence
5
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6=head1 DESCRIPTION
7
89d205f2 8=head2 Operator Precedence and Associativity
d74e8afc 9X<operator, precedence> X<precedence> X<associativity>
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10
11Operator precedence and associativity work in Perl more or less like
12they do in mathematics.
13
14I<Operator precedence> means some operators are evaluated before
15others. For example, in C<2 + 4 * 5>, the multiplication has higher
16precedence so C<4 * 5> is evaluated first yielding C<2 + 20 ==
1722> and not C<6 * 5 == 30>.
18
19I<Operator associativity> defines what happens if a sequence of the
20same operators is used one after another: whether the evaluator will
21evaluate the left operations first or the right. For example, in C<8
22- 4 - 2>, subtraction is left associative so Perl evaluates the
23expression left to right. C<8 - 4> is evaluated first making the
24expression C<4 - 2 == 2> and not C<8 - 2 == 6>.
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25
26Perl operators have the following associativity and precedence,
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27listed from highest precedence to lowest. Operators borrowed from
28C keep the same precedence relationship with each other, even where
29C's precedence is slightly screwy. (This makes learning Perl easier
30for C folks.) With very few exceptions, these all operate on scalar
31values only, not array values.
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32
33 left terms and list operators (leftward)
34 left ->
35 nonassoc ++ --
36 right **
37 right ! ~ \ and unary + and -
54310121 38 left =~ !~
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39 left * / % x
40 left + - .
41 left << >>
42 nonassoc named unary operators
43 nonassoc < > <= >= lt gt le ge
0d863452 44 nonassoc == != <=> eq ne cmp ~~
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45 left &
46 left | ^
47 left &&
c963b151 48 left || //
137443ea 49 nonassoc .. ...
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50 right ?:
51 right = += -= *= etc.
52 left , =>
53 nonassoc list operators (rightward)
a5f75d66 54 right not
a0d0e21e 55 left and
f23102e2 56 left or xor
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57
58In the following sections, these operators are covered in precedence order.
59
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60Many operators can be overloaded for objects. See L<overload>.
61
a0d0e21e 62=head2 Terms and List Operators (Leftward)
d74e8afc 63X<list operator> X<operator, list> X<term>
a0d0e21e 64
62c18ce2 65A TERM has the highest precedence in Perl. They include variables,
5f05dabc 66quote and quote-like operators, any expression in parentheses,
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67and any function whose arguments are parenthesized. Actually, there
68aren't really functions in this sense, just list operators and unary
69operators behaving as functions because you put parentheses around
70the arguments. These are all documented in L<perlfunc>.
71
72If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
73is followed by a left parenthesis as the next token, the operator and
74arguments within parentheses are taken to be of highest precedence,
75just like a normal function call.
76
77In the absence of parentheses, the precedence of list operators such as
78C<print>, C<sort>, or C<chmod> is either very high or very low depending on
54310121 79whether you are looking at the left side or the right side of the operator.
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80For example, in
81
82 @ary = (1, 3, sort 4, 2);
83 print @ary; # prints 1324
84
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85the commas on the right of the sort are evaluated before the sort,
86but the commas on the left are evaluated after. In other words,
87list operators tend to gobble up all arguments that follow, and
a0d0e21e 88then act like a simple TERM with regard to the preceding expression.
19799a22 89Be careful with parentheses:
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90
91 # These evaluate exit before doing the print:
92 print($foo, exit); # Obviously not what you want.
93 print $foo, exit; # Nor is this.
94
95 # These do the print before evaluating exit:
96 (print $foo), exit; # This is what you want.
97 print($foo), exit; # Or this.
98 print ($foo), exit; # Or even this.
99
100Also note that
101
102 print ($foo & 255) + 1, "\n";
103
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104probably doesn't do what you expect at first glance. The parentheses
105enclose the argument list for C<print> which is evaluated (printing
106the result of C<$foo & 255>). Then one is added to the return value
107of C<print> (usually 1). The result is something like this:
108
109 1 + 1, "\n"; # Obviously not what you meant.
110
111To do what you meant properly, you must write:
112
113 print(($foo & 255) + 1, "\n");
114
115See L<Named Unary Operators> for more discussion of this.
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116
117Also parsed as terms are the C<do {}> and C<eval {}> constructs, as
54310121 118well as subroutine and method calls, and the anonymous
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119constructors C<[]> and C<{}>.
120
2ae324a7 121See also L<Quote and Quote-like Operators> toward the end of this section,
da87341d 122as well as L</"I/O Operators">.
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123
124=head2 The Arrow Operator
d74e8afc 125X<arrow> X<dereference> X<< -> >>
a0d0e21e 126
35f2feb0 127"C<< -> >>" is an infix dereference operator, just as it is in C
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128and C++. If the right side is either a C<[...]>, C<{...}>, or a
129C<(...)> subscript, then the left side must be either a hard or
130symbolic reference to an array, a hash, or a subroutine respectively.
131(Or technically speaking, a location capable of holding a hard
132reference, if it's an array or hash reference being used for
133assignment.) See L<perlreftut> and L<perlref>.
a0d0e21e 134
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135Otherwise, the right side is a method name or a simple scalar
136variable containing either the method name or a subroutine reference,
137and the left side must be either an object (a blessed reference)
138or a class name (that is, a package name). See L<perlobj>.
a0d0e21e 139
5f05dabc 140=head2 Auto-increment and Auto-decrement
d74e8afc 141X<increment> X<auto-increment> X<++> X<decrement> X<auto-decrement> X<-->
a0d0e21e 142
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143"++" and "--" work as in C. That is, if placed before a variable,
144they increment or decrement the variable by one before returning the
145value, and if placed after, increment or decrement after returning the
146value.
147
148 $i = 0; $j = 0;
149 print $i++; # prints 0
150 print ++$j; # prints 1
a0d0e21e 151
b033823e 152Note that just as in C, Perl doesn't define B<when> the variable is
89d205f2 153incremented or decremented. You just know it will be done sometime
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154before or after the value is returned. This also means that modifying
155a variable twice in the same statement will lead to undefined behaviour.
156Avoid statements like:
157
158 $i = $i ++;
159 print ++ $i + $i ++;
160
161Perl will not guarantee what the result of the above statements is.
162
54310121 163The auto-increment operator has a little extra builtin magic to it. If
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164you increment a variable that is numeric, or that has ever been used in
165a numeric context, you get a normal increment. If, however, the
5f05dabc 166variable has been used in only string contexts since it was set, and
5a964f20 167has a value that is not the empty string and matches the pattern
9c0670e1 168C</^[a-zA-Z]*[0-9]*\z/>, the increment is done as a string, preserving each
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169character within its range, with carry:
170
171 print ++($foo = '99'); # prints '100'
172 print ++($foo = 'a0'); # prints 'a1'
173 print ++($foo = 'Az'); # prints 'Ba'
174 print ++($foo = 'zz'); # prints 'aaa'
175
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176C<undef> is always treated as numeric, and in particular is changed
177to C<0> before incrementing (so that a post-increment of an undef value
178will return C<0> rather than C<undef>).
179
5f05dabc 180The auto-decrement operator is not magical.
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181
182=head2 Exponentiation
d74e8afc 183X<**> X<exponentiation> X<power>
a0d0e21e 184
19799a22 185Binary "**" is the exponentiation operator. It binds even more
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186tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is
187implemented using C's pow(3) function, which actually works on doubles
188internally.)
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189
190=head2 Symbolic Unary Operators
d74e8afc 191X<unary operator> X<operator, unary>
a0d0e21e 192
5f05dabc 193Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower
a0d0e21e 194precedence version of this.
d74e8afc 195X<!>
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196
197Unary "-" performs arithmetic negation if the operand is numeric. If
198the operand is an identifier, a string consisting of a minus sign
199concatenated with the identifier is returned. Otherwise, if the string
200starts with a plus or minus, a string starting with the opposite sign
bff5667c 201is returned. One effect of these rules is that -bareword is equivalent
8705167b 202to the string "-bareword". If, however, the string begins with a
353c6505 203non-alphabetic character (excluding "+" or "-"), Perl will attempt to convert
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204the string to a numeric and the arithmetic negation is performed. If the
205string cannot be cleanly converted to a numeric, Perl will give the warning
206B<Argument "the string" isn't numeric in negation (-) at ...>.
d74e8afc 207X<-> X<negation, arithmetic>
a0d0e21e 208
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209Unary "~" performs bitwise negation, i.e., 1's complement. For
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 $_ by default. This operator makes that kind
234of operation work on some other string. The right argument is a search
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235pattern, substitution, or transliteration. The left argument is what is
236supposed to be searched, substituted, or transliterated instead of the default
f8bab1e9 237$_. When used in scalar context, the return value generally indicates the
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238success of the operation. The exception is substitution with the C</r>
239(non-destructive) option, which causes the return value to be the result of
240the substition. Behavior in list context depends on the particular operator.
241See L</"Regexp Quote-Like Operators"> for details and L<perlretut> for
242examples using these operators.
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243
244If the right argument is an expression rather than a search pattern,
2c268ad5 245substitution, or transliteration, it is interpreted as a search pattern at run
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246time. Note that this means that its contents will be interpolated twice, so
247
248 '\\' =~ q'\\';
249
250is not ok, as the regex engine will end up trying to compile the
251pattern C<\>, which it will consider a syntax error.
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252
253Binary "!~" is just like "=~" except the return value is negated in
254the logical sense.
255
000c65fc 256Binary "!~" with a non-destructive substitution (s///r) is a syntax error.
4f4d7508 257
a0d0e21e 258=head2 Multiplicative Operators
d74e8afc 259X<operator, multiplicative>
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260
261Binary "*" multiplies two numbers.
d74e8afc 262X<*>
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263
264Binary "/" divides two numbers.
d74e8afc 265X</> X<slash>
a0d0e21e 266
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267Binary "%" is the modulo operator, which computes the division
268remainder of its first argument with respect to its second argument.
269Given integer
54310121 270operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
f7918450 271C<$a> minus the largest multiple of C<$b> less than or equal to
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272C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
273smallest multiple of C<$b> that is not less than C<$a> (i.e. the
89b4f0ad 274result will be less than or equal to zero). If the operands
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275C<$a> and C<$b> are floating point values and the absolute value of
276C<$b> (that is C<abs($b)>) is less than C<(UV_MAX + 1)>, only
277the integer portion of C<$a> and C<$b> will be used in the operation
278(Note: here C<UV_MAX> means the maximum of the unsigned integer type).
279If the absolute value of the right operand (C<abs($b)>) is greater than
280or equal to C<(UV_MAX + 1)>, "%" computes the floating-point remainder
281C<$r> in the equation C<($r = $a - $i*$b)> where C<$i> is a certain
f7918450 282integer that makes C<$r> have the same sign as the right operand
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283C<$b> (B<not> as the left operand C<$a> like C function C<fmod()>)
284and the absolute value less than that of C<$b>.
0412d526 285Note that when C<use integer> is in scope, "%" gives you direct access
f7918450 286to the modulo operator as implemented by your C compiler. This
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287operator is not as well defined for negative operands, but it will
288execute faster.
f7918450 289X<%> X<remainder> X<modulo> X<mod>
55d729e4 290
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291Binary "x" is the repetition operator. In scalar context or if the left
292operand is not enclosed in parentheses, it returns a string consisting
293of the left operand repeated the number of times specified by the right
294operand. In list context, if the left operand is enclosed in
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295parentheses or is a list formed by C<qw/STRING/>, it repeats the list.
296If the right operand is zero or negative, it returns an empty string
297or an empty list, depending on the context.
d74e8afc 298X<x>
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299
300 print '-' x 80; # print row of dashes
301
302 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
303
304 @ones = (1) x 80; # a list of 80 1's
305 @ones = (5) x @ones; # set all elements to 5
306
307
308=head2 Additive Operators
d74e8afc 309X<operator, additive>
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310
311Binary "+" returns the sum of two numbers.
d74e8afc 312X<+>
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313
314Binary "-" returns the difference of two numbers.
d74e8afc 315X<->
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316
317Binary "." concatenates two strings.
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318X<string, concatenation> X<concatenation>
319X<cat> X<concat> X<concatenate> X<.>
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320
321=head2 Shift Operators
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322X<shift operator> X<operator, shift> X<<< << >>>
323X<<< >> >>> X<right shift> X<left shift> X<bitwise shift>
324X<shl> X<shr> X<shift, right> X<shift, left>
a0d0e21e 325
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326Binary "<<" returns the value of its left argument shifted left by the
327number of bits specified by the right argument. Arguments should be
982ce180 328integers. (See also L<Integer Arithmetic>.)
a0d0e21e 329
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330Binary ">>" returns the value of its left argument shifted right by
331the number of bits specified by the right argument. Arguments should
982ce180 332be integers. (See also L<Integer Arithmetic>.)
a0d0e21e 333
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334Note that both "<<" and ">>" in Perl are implemented directly using
335"<<" and ">>" in C. If C<use integer> (see L<Integer Arithmetic>) is
336in force then signed C integers are used, else unsigned C integers are
337used. Either way, the implementation isn't going to generate results
338larger than the size of the integer type Perl was built with (32 bits
339or 64 bits).
340
341The result of overflowing the range of the integers is undefined
342because it is undefined also in C. In other words, using 32-bit
343integers, C<< 1 << 32 >> is undefined. Shifting by a negative number
344of bits is also undefined.
345
a0d0e21e 346=head2 Named Unary Operators
d74e8afc 347X<operator, named unary>
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348
349The various named unary operators are treated as functions with one
568e6d8b 350argument, with optional parentheses.
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351
352If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
353is followed by a left parenthesis as the next token, the operator and
354arguments within parentheses are taken to be of highest precedence,
3981b0eb 355just like a normal function call. For example,
356because named unary operators are higher precedence than ||:
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357
358 chdir $foo || die; # (chdir $foo) || die
359 chdir($foo) || die; # (chdir $foo) || die
360 chdir ($foo) || die; # (chdir $foo) || die
361 chdir +($foo) || die; # (chdir $foo) || die
362
3981b0eb 363but, because * is higher precedence than named operators:
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364
365 chdir $foo * 20; # chdir ($foo * 20)
366 chdir($foo) * 20; # (chdir $foo) * 20
367 chdir ($foo) * 20; # (chdir $foo) * 20
368 chdir +($foo) * 20; # chdir ($foo * 20)
369
370 rand 10 * 20; # rand (10 * 20)
371 rand(10) * 20; # (rand 10) * 20
372 rand (10) * 20; # (rand 10) * 20
373 rand +(10) * 20; # rand (10 * 20)
374
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375Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are
376treated like named unary operators, but they don't follow this functional
377parenthesis rule. That means, for example, that C<-f($file).".bak"> is
378equivalent to C<-f "$file.bak">.
d74e8afc 379X<-X> X<filetest> X<operator, filetest>
568e6d8b 380
5ba421f6 381See also L<"Terms and List Operators (Leftward)">.
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382
383=head2 Relational Operators
d74e8afc 384X<relational operator> X<operator, relational>
a0d0e21e 385
35f2feb0 386Binary "<" returns true if the left argument is numerically less than
a0d0e21e 387the right argument.
d74e8afc 388X<< < >>
a0d0e21e 389
35f2feb0 390Binary ">" returns true if the left argument is numerically greater
a0d0e21e 391than the right argument.
d74e8afc 392X<< > >>
a0d0e21e 393
35f2feb0 394Binary "<=" returns true if the left argument is numerically less than
a0d0e21e 395or equal to the right argument.
d74e8afc 396X<< <= >>
a0d0e21e 397
35f2feb0 398Binary ">=" returns true if the left argument is numerically greater
a0d0e21e 399than or equal to the right argument.
d74e8afc 400X<< >= >>
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401
402Binary "lt" returns true if the left argument is stringwise less than
403the right argument.
d74e8afc 404X<< lt >>
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405
406Binary "gt" returns true if the left argument is stringwise greater
407than the right argument.
d74e8afc 408X<< gt >>
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409
410Binary "le" returns true if the left argument is stringwise less than
411or equal to the right argument.
d74e8afc 412X<< le >>
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413
414Binary "ge" returns true if the left argument is stringwise greater
415than or equal to the right argument.
d74e8afc 416X<< ge >>
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417
418=head2 Equality Operators
d74e8afc 419X<equality> X<equal> X<equals> X<operator, equality>
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420
421Binary "==" returns true if the left argument is numerically equal to
422the right argument.
d74e8afc 423X<==>
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424
425Binary "!=" returns true if the left argument is numerically not equal
426to the right argument.
d74e8afc 427X<!=>
a0d0e21e 428
35f2feb0 429Binary "<=>" returns -1, 0, or 1 depending on whether the left
6ee5d4e7 430argument is numerically less than, equal to, or greater than the right
d4ad863d 431argument. If your platform supports NaNs (not-a-numbers) as numeric
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432values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
433"<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
434returns true, as does NaN != anything else. If your platform doesn't
435support NaNs then NaN is just a string with numeric value 0.
d74e8afc 436X<< <=> >> X<spaceship>
7d3a9d88 437
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438 perl -le '$a = "NaN"; print "No NaN support here" if $a == $a'
439 perl -le '$a = "NaN"; print "NaN support here" if $a != $a'
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440
441Binary "eq" returns true if the left argument is stringwise equal to
442the right argument.
d74e8afc 443X<eq>
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444
445Binary "ne" returns true if the left argument is stringwise not equal
446to the right argument.
d74e8afc 447X<ne>
a0d0e21e 448
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449Binary "cmp" returns -1, 0, or 1 depending on whether the left
450argument is stringwise less than, equal to, or greater than the right
451argument.
d74e8afc 452X<cmp>
a0d0e21e 453
0d863452 454Binary "~~" does a smart match between its arguments. Smart matching
0f7107a0 455is described in L<perlsyn/"Smart matching in detail">.
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456X<~~>
457
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458"lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
459by the current locale if C<use locale> is in effect. See L<perllocale>.
460
a0d0e21e 461=head2 Bitwise And
d74e8afc 462X<operator, bitwise, and> X<bitwise and> X<&>
a0d0e21e 463
2cdc098b 464Binary "&" returns its operands ANDed together bit by bit.
2c268ad5 465(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
a0d0e21e 466
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467Note that "&" has lower priority than relational operators, so for example
468the brackets are essential in a test like
469
470 print "Even\n" if ($x & 1) == 0;
471
a0d0e21e 472=head2 Bitwise Or and Exclusive Or
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473X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor>
474X<bitwise xor> X<^>
a0d0e21e 475
2cdc098b 476Binary "|" returns its operands ORed together bit by bit.
2c268ad5 477(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
a0d0e21e 478
2cdc098b 479Binary "^" returns its operands XORed together bit by bit.
2c268ad5 480(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
a0d0e21e 481
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482Note that "|" and "^" have lower priority than relational operators, so
483for example the brackets are essential in a test like
484
485 print "false\n" if (8 | 2) != 10;
486
a0d0e21e 487=head2 C-style Logical And
d74e8afc 488X<&&> X<logical and> X<operator, logical, and>
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489
490Binary "&&" performs a short-circuit logical AND operation. That is,
491if the left operand is false, the right operand is not even evaluated.
492Scalar or list context propagates down to the right operand if it
493is evaluated.
494
495=head2 C-style Logical Or
d74e8afc 496X<||> X<operator, logical, or>
a0d0e21e
LW
497
498Binary "||" performs a short-circuit logical OR operation. That is,
499if the left operand is true, the right operand is not even evaluated.
500Scalar or list context propagates down to the right operand if it
501is evaluated.
502
c963b151 503=head2 C-style Logical Defined-Or
d74e8afc 504X<//> X<operator, logical, defined-or>
c963b151
BD
505
506Although it has no direct equivalent in C, Perl's C<//> operator is related
89d205f2 507to its C-style or. In fact, it's exactly the same as C<||>, except that it
c963b151 508tests the left hand side's definedness instead of its truth. Thus, C<$a // $b>
89d205f2
YO
509is similar to C<defined($a) || $b> (except that it returns the value of C<$a>
510rather than the value of C<defined($a)>) and is exactly equivalent to
c963b151 511C<defined($a) ? $a : $b>. This is very useful for providing default values
89d205f2 512for variables. If you actually want to test if at least one of C<$a> and
d042e63d 513C<$b> is defined, use C<defined($a // $b)>.
c963b151 514
d042e63d
MS
515The C<||>, C<//> and C<&&> operators return the last value evaluated
516(unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably
517portable way to find out the home directory might be:
a0d0e21e 518
c963b151
BD
519 $home = $ENV{'HOME'} // $ENV{'LOGDIR'} //
520 (getpwuid($<))[7] // die "You're homeless!\n";
a0d0e21e 521
5a964f20
TC
522In particular, this means that you shouldn't use this
523for selecting between two aggregates for assignment:
524
525 @a = @b || @c; # this is wrong
526 @a = scalar(@b) || @c; # really meant this
527 @a = @b ? @b : @c; # this works fine, though
528
f23102e2
RGS
529As more readable alternatives to C<&&> and C<||> when used for
530control flow, Perl provides the C<and> and C<or> operators (see below).
531The short-circuit behavior is identical. The precedence of "and"
c963b151 532and "or" is much lower, however, so that you can safely use them after a
5a964f20 533list operator without the need for parentheses:
a0d0e21e
LW
534
535 unlink "alpha", "beta", "gamma"
536 or gripe(), next LINE;
537
538With the C-style operators that would have been written like this:
539
540 unlink("alpha", "beta", "gamma")
541 || (gripe(), next LINE);
542
eeb6a2c9 543Using "or" for assignment is unlikely to do what you want; see below.
5a964f20
TC
544
545=head2 Range Operators
d74e8afc 546X<operator, range> X<range> X<..> X<...>
a0d0e21e
LW
547
548Binary ".." is the range operator, which is really two different
fb53bbb2 549operators depending on the context. In list context, it returns a
54ae734e 550list of values counting (up by ones) from the left value to the right
2cdbc966 551value. If the left value is greater than the right value then it
fb53bbb2 552returns the empty list. The range operator is useful for writing
54ae734e 553C<foreach (1..10)> loops and for doing slice operations on arrays. In
2cdbc966
JD
554the current implementation, no temporary array is created when the
555range operator is used as the expression in C<foreach> loops, but older
556versions of Perl might burn a lot of memory when you write something
557like this:
a0d0e21e
LW
558
559 for (1 .. 1_000_000) {
560 # code
54310121 561 }
a0d0e21e 562
8f0f46f8 563The range operator also works on strings, using the magical
564auto-increment, see below.
54ae734e 565
5a964f20 566In scalar context, ".." returns a boolean value. The operator is
8f0f46f8 567bistable, like a flip-flop, and emulates the line-range (comma)
568operator of B<sed>, B<awk>, and various editors. Each ".." operator
569maintains its own boolean state, even across calls to a subroutine
570that contains it. It is false as long as its left operand is false.
a0d0e21e
LW
571Once the left operand is true, the range operator stays true until the
572right operand is true, I<AFTER> which the range operator becomes false
8f0f46f8 573again. It doesn't become false till the next time the range operator
574is evaluated. It can test the right operand and become false on the
575same evaluation it became true (as in B<awk>), but it still returns
576true once. If you don't want it to test the right operand until the
577next evaluation, as in B<sed>, just use three dots ("...") instead of
19799a22
GS
578two. In all other regards, "..." behaves just like ".." does.
579
580The right operand is not evaluated while the operator is in the
581"false" state, and the left operand is not evaluated while the
582operator is in the "true" state. The precedence is a little lower
583than || and &&. The value returned is either the empty string for
8f0f46f8 584false, or a sequence number (beginning with 1) for true. The sequence
585number is reset for each range encountered. The final sequence number
586in a range has the string "E0" appended to it, which doesn't affect
587its numeric value, but gives you something to search for if you want
588to exclude the endpoint. You can exclude the beginning point by
589waiting for the sequence number to be greater than 1.
df5f8116
CW
590
591If either operand of scalar ".." is a constant expression,
592that operand is considered true if it is equal (C<==>) to the current
593input line number (the C<$.> variable).
594
595To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>,
596but that is only an issue if you use a floating point expression; when
597implicitly using C<$.> as described in the previous paragraph, the
598comparison is C<int(EXPR) == int($.)> which is only an issue when C<$.>
599is set to a floating point value and you are not reading from a file.
600Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what
601you want in scalar context because each of the operands are evaluated
602using their integer representation.
603
604Examples:
a0d0e21e
LW
605
606As a scalar operator:
607
df5f8116 608 if (101 .. 200) { print; } # print 2nd hundred lines, short for
950b09ed 609 # if ($. == 101 .. $. == 200) { print; }
9f10b797
RGS
610
611 next LINE if (1 .. /^$/); # skip header lines, short for
f343f960 612 # next LINE if ($. == 1 .. /^$/);
9f10b797
RGS
613 # (typically in a loop labeled LINE)
614
615 s/^/> / if (/^$/ .. eof()); # quote body
a0d0e21e 616
5a964f20
TC
617 # parse mail messages
618 while (<>) {
619 $in_header = 1 .. /^$/;
df5f8116
CW
620 $in_body = /^$/ .. eof;
621 if ($in_header) {
f343f960 622 # do something
df5f8116 623 } else { # in body
f343f960 624 # do something else
df5f8116 625 }
5a964f20 626 } continue {
df5f8116 627 close ARGV if eof; # reset $. each file
5a964f20
TC
628 }
629
acf31ca5
SF
630Here's a simple example to illustrate the difference between
631the two range operators:
632
633 @lines = (" - Foo",
634 "01 - Bar",
635 "1 - Baz",
636 " - Quux");
637
9f10b797
RGS
638 foreach (@lines) {
639 if (/0/ .. /1/) {
acf31ca5
SF
640 print "$_\n";
641 }
642 }
643
9f10b797
RGS
644This program will print only the line containing "Bar". If
645the range operator is changed to C<...>, it will also print the
acf31ca5
SF
646"Baz" line.
647
648And now some examples as a list operator:
a0d0e21e
LW
649
650 for (101 .. 200) { print; } # print $_ 100 times
3e3baf6d 651 @foo = @foo[0 .. $#foo]; # an expensive no-op
a0d0e21e
LW
652 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
653
5a964f20 654The range operator (in list context) makes use of the magical
5f05dabc 655auto-increment algorithm if the operands are strings. You
a0d0e21e
LW
656can say
657
658 @alphabet = ('A' .. 'Z');
659
54ae734e 660to get all normal letters of the English alphabet, or
a0d0e21e
LW
661
662 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
663
664to get a hexadecimal digit, or
665
666 @z2 = ('01' .. '31'); print $z2[$mday];
667
ea4f5703
YST
668to get dates with leading zeros.
669
670If the final value specified is not in the sequence that the magical
671increment would produce, the sequence goes until the next value would
672be longer than the final value specified.
673
674If the initial value specified isn't part of a magical increment
675sequence (that is, a non-empty string matching "/^[a-zA-Z]*[0-9]*\z/"),
676only the initial value will be returned. So the following will only
677return an alpha:
678
679 use charnames 'greek';
680 my @greek_small = ("\N{alpha}" .. "\N{omega}");
681
682To get lower-case greek letters, use this instead:
683
950b09ed
KW
684 my @greek_small = map { chr } ( ord("\N{alpha}") ..
685 ord("\N{omega}") );
a0d0e21e 686
df5f8116
CW
687Because each operand is evaluated in integer form, C<2.18 .. 3.14> will
688return two elements in list context.
689
690 @list = (2.18 .. 3.14); # same as @list = (2 .. 3);
691
a0d0e21e 692=head2 Conditional Operator
d74e8afc 693X<operator, conditional> X<operator, ternary> X<ternary> X<?:>
a0d0e21e
LW
694
695Ternary "?:" is the conditional operator, just as in C. It works much
696like an if-then-else. If the argument before the ? is true, the
697argument before the : is returned, otherwise the argument after the :
cb1a09d0
AD
698is returned. For example:
699
54310121 700 printf "I have %d dog%s.\n", $n,
cb1a09d0
AD
701 ($n == 1) ? '' : "s";
702
703Scalar or list context propagates downward into the 2nd
54310121 704or 3rd argument, whichever is selected.
cb1a09d0
AD
705
706 $a = $ok ? $b : $c; # get a scalar
707 @a = $ok ? @b : @c; # get an array
708 $a = $ok ? @b : @c; # oops, that's just a count!
709
710The operator may be assigned to if both the 2nd and 3rd arguments are
711legal lvalues (meaning that you can assign to them):
a0d0e21e
LW
712
713 ($a_or_b ? $a : $b) = $c;
714
5a964f20
TC
715Because this operator produces an assignable result, using assignments
716without parentheses will get you in trouble. For example, this:
717
718 $a % 2 ? $a += 10 : $a += 2
719
720Really means this:
721
722 (($a % 2) ? ($a += 10) : $a) += 2
723
724Rather than this:
725
726 ($a % 2) ? ($a += 10) : ($a += 2)
727
19799a22
GS
728That should probably be written more simply as:
729
730 $a += ($a % 2) ? 10 : 2;
731
4633a7c4 732=head2 Assignment Operators
d74e8afc 733X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=>
5ac3b81c 734X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=>
d74e8afc 735X<%=> X<^=> X<x=>
a0d0e21e
LW
736
737"=" is the ordinary assignment operator.
738
739Assignment operators work as in C. That is,
740
741 $a += 2;
742
743is equivalent to
744
745 $a = $a + 2;
746
747although without duplicating any side effects that dereferencing the lvalue
54310121
PP
748might trigger, such as from tie(). Other assignment operators work similarly.
749The following are recognized:
a0d0e21e
LW
750
751 **= += *= &= <<= &&=
9f10b797
RGS
752 -= /= |= >>= ||=
753 .= %= ^= //=
754 x=
a0d0e21e 755
19799a22 756Although these are grouped by family, they all have the precedence
a0d0e21e
LW
757of assignment.
758
b350dd2f
GS
759Unlike in C, the scalar assignment operator produces a valid lvalue.
760Modifying an assignment is equivalent to doing the assignment and
761then modifying the variable that was assigned to. This is useful
762for modifying a copy of something, like this:
a0d0e21e
LW
763
764 ($tmp = $global) =~ tr [A-Z] [a-z];
765
766Likewise,
767
768 ($a += 2) *= 3;
769
770is equivalent to
771
772 $a += 2;
773 $a *= 3;
774
b350dd2f
GS
775Similarly, a list assignment in list context produces the list of
776lvalues assigned to, and a list assignment in scalar context returns
777the number of elements produced by the expression on the right hand
778side of the assignment.
779
748a9306 780=head2 Comma Operator
d74e8afc 781X<comma> X<operator, comma> X<,>
a0d0e21e 782
5a964f20 783Binary "," is the comma operator. In scalar context it evaluates
a0d0e21e
LW
784its left argument, throws that value away, then evaluates its right
785argument and returns that value. This is just like C's comma operator.
786
5a964f20 787In list context, it's just the list argument separator, and inserts
ed5c6d31
PJ
788both its arguments into the list. These arguments are also evaluated
789from left to right.
a0d0e21e 790
344f2c40
IG
791The C<< => >> operator is a synonym for the comma except that it causes
792its left operand to be interpreted as a string if it begins with a letter
793or underscore and is composed only of letters, digits and underscores.
794This includes operands that might otherwise be interpreted as operators,
795constants, single number v-strings or function calls. If in doubt about
796this behaviour, the left operand can be quoted explicitly.
797
798Otherwise, the C<< => >> operator behaves exactly as the comma operator
799or list argument separator, according to context.
800
801For example:
a44e5664
MS
802
803 use constant FOO => "something";
804
805 my %h = ( FOO => 23 );
806
807is equivalent to:
808
809 my %h = ("FOO", 23);
810
811It is I<NOT>:
812
813 my %h = ("something", 23);
814
719b43e8
RGS
815The C<< => >> operator is helpful in documenting the correspondence
816between keys and values in hashes, and other paired elements in lists.
748a9306 817
a44e5664
MS
818 %hash = ( $key => $value );
819 login( $username => $password );
820
678ae90b
RGS
821=head2 Yada Yada Operator
822X<...> X<... operator> X<yada yada operator>
be25f609 823
e8163f9b 824The yada yada operator (noted C<...>) is a placeholder for code. Perl
825parses it without error, but when you try to execute a yada yada, it
826throws an exception with the text C<Unimplemented>:
827
828 sub unimplemented { ... }
829
830 eval { unimplemented() };
831 if( $@ eq 'Unimplemented' ) {
832 print "I found the yada yada!\n";
833 }
834
835You can only use the yada yada to stand in for a complete statement.
836These examples of the yada yada work:
837
838 { ... }
839
840 sub foo { ... }
841
842 ...;
843
844 eval { ... };
845
846 sub foo {
847 my( $self ) = shift;
848
849 ...;
850 }
851
852 do { my $n; ...; print 'Hurrah!' };
853
854The yada yada cannot stand in for an expression that is part of a
855larger statement since the C<...> is also the three-dot version of the
856range operator (see L<Range Operators>). These examples of the yada
857yada are still syntax errors:
858
859 print ...;
860
861 open my($fh), '>', '/dev/passwd' or ...;
862
863 if( $condition && ... ) { print "Hello\n" };
864
865There are some cases where Perl can't immediately tell the difference
866between an expression and a statement. For instance, the syntax for a
867block and an anonymous hash reference constructor look the same unless
868there's something in the braces that give Perl a hint. The yada yada
869is a syntax error if Perl doesn't guess that the C<{ ... }> is a
870block. In that case, it doesn't think the C<...> is the yada yada
871because it's expecting an expression instead of a statement:
872
873 my @transformed = map { ... } @input; # syntax error
874
875You can use a C<;> inside your block to denote that the C<{ ... }> is
876a block and not a hash reference constructor. Now the yada yada works:
877
878 my @transformed = map {; ... } @input; # ; disambiguates
879
880 my @transformed = map { ...; } @input; # ; disambiguates
be25f609 881
a0d0e21e 882=head2 List Operators (Rightward)
d74e8afc 883X<operator, list, rightward> X<list operator>
a0d0e21e
LW
884
885On the right side of a list operator, it has very low precedence,
886such that it controls all comma-separated expressions found there.
887The only operators with lower precedence are the logical operators
888"and", "or", and "not", which may be used to evaluate calls to list
889operators without the need for extra parentheses:
890
891 open HANDLE, "filename"
892 or die "Can't open: $!\n";
893
5ba421f6 894See also discussion of list operators in L<Terms and List Operators (Leftward)>.
a0d0e21e
LW
895
896=head2 Logical Not
d74e8afc 897X<operator, logical, not> X<not>
a0d0e21e
LW
898
899Unary "not" returns the logical negation of the expression to its right.
900It's the equivalent of "!" except for the very low precedence.
901
902=head2 Logical And
d74e8afc 903X<operator, logical, and> X<and>
a0d0e21e
LW
904
905Binary "and" returns the logical conjunction of the two surrounding
906expressions. It's equivalent to && except for the very low
5f05dabc 907precedence. This means that it short-circuits: i.e., the right
a0d0e21e
LW
908expression is evaluated only if the left expression is true.
909
c963b151 910=head2 Logical or, Defined or, and Exclusive Or
f23102e2 911X<operator, logical, or> X<operator, logical, xor>
d74e8afc 912X<operator, logical, defined or> X<operator, logical, exclusive or>
f23102e2 913X<or> X<xor>
a0d0e21e
LW
914
915Binary "or" returns the logical disjunction of the two surrounding
5a964f20
TC
916expressions. It's equivalent to || except for the very low precedence.
917This makes it useful for control flow
918
919 print FH $data or die "Can't write to FH: $!";
920
921This means that it short-circuits: i.e., the right expression is evaluated
922only if the left expression is false. Due to its precedence, you should
923probably avoid using this for assignment, only for control flow.
924
925 $a = $b or $c; # bug: this is wrong
926 ($a = $b) or $c; # really means this
927 $a = $b || $c; # better written this way
928
19799a22 929However, when it's a list-context assignment and you're trying to use
5a964f20
TC
930"||" for control flow, you probably need "or" so that the assignment
931takes higher precedence.
932
933 @info = stat($file) || die; # oops, scalar sense of stat!
934 @info = stat($file) or die; # better, now @info gets its due
935
c963b151
BD
936Then again, you could always use parentheses.
937
a0d0e21e
LW
938Binary "xor" returns the exclusive-OR of the two surrounding expressions.
939It cannot short circuit, of course.
940
941=head2 C Operators Missing From Perl
d74e8afc
ITB
942X<operator, missing from perl> X<&> X<*>
943X<typecasting> X<(TYPE)>
a0d0e21e
LW
944
945Here is what C has that Perl doesn't:
946
947=over 8
948
949=item unary &
950
951Address-of operator. (But see the "\" operator for taking a reference.)
952
953=item unary *
954
54310121 955Dereference-address operator. (Perl's prefix dereferencing
a0d0e21e
LW
956operators are typed: $, @, %, and &.)
957
958=item (TYPE)
959
19799a22 960Type-casting operator.
a0d0e21e
LW
961
962=back
963
5f05dabc 964=head2 Quote and Quote-like Operators
89d205f2 965X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m>
d74e8afc
ITB
966X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>>
967X<escape sequence> X<escape>
968
a0d0e21e
LW
969
970While we usually think of quotes as literal values, in Perl they
971function as operators, providing various kinds of interpolating and
972pattern matching capabilities. Perl provides customary quote characters
973for these behaviors, but also provides a way for you to choose your
974quote character for any of them. In the following table, a C<{}> represents
9f10b797 975any pair of delimiters you choose.
a0d0e21e 976
2c268ad5
TP
977 Customary Generic Meaning Interpolates
978 '' q{} Literal no
979 "" qq{} Literal yes
af9219ee 980 `` qx{} Command yes*
2c268ad5 981 qw{} Word list no
af9219ee
MG
982 // m{} Pattern match yes*
983 qr{} Pattern yes*
984 s{}{} Substitution yes*
2c268ad5 985 tr{}{} Transliteration no (but see below)
7e3b091d 986 <<EOF here-doc yes*
a0d0e21e 987
af9219ee
MG
988 * unless the delimiter is ''.
989
87275199
GS
990Non-bracketing delimiters use the same character fore and aft, but the four
991sorts of brackets (round, angle, square, curly) will all nest, which means
9f10b797 992that
87275199 993
9f10b797 994 q{foo{bar}baz}
35f2feb0 995
9f10b797 996is the same as
87275199
GS
997
998 'foo{bar}baz'
999
1000Note, however, that this does not always work for quoting Perl code:
1001
1002 $s = q{ if($a eq "}") ... }; # WRONG
1003
83df6a1d
JH
1004is a syntax error. The C<Text::Balanced> module (from CPAN, and
1005starting from Perl 5.8 part of the standard distribution) is able
1006to do this properly.
87275199 1007
19799a22 1008There can be whitespace between the operator and the quoting
fb73857a 1009characters, except when C<#> is being used as the quoting character.
19799a22
GS
1010C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
1011operator C<q> followed by a comment. Its argument will be taken
1012from the next line. This allows you to write:
fb73857a
PP
1013
1014 s {foo} # Replace foo
1015 {bar} # with bar.
1016
904501ec
MG
1017The following escape sequences are available in constructs that interpolate
1018and in transliterations.
5691ca5f 1019X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N> X<\N{}>
04341565 1020X<\o{}>
5691ca5f
KW
1021
1022 Sequence Note Description
04341565
DG
1023 \t tab (HT, TAB)
1024 \n newline (NL)
1025 \r return (CR)
1026 \f form feed (FF)
1027 \b backspace (BS)
1028 \a alarm (bell) (BEL)
1029 \e escape (ESC)
1030 \x{263a} [1] hex char (example: SMILEY)
1031 \x1b [2] restricted range hex char (example: ESC)
40687185 1032 \N{name} [3] named Unicode character
04341565
DG
1033 \N{U+263D} [4] Unicode character (example: FIRST QUARTER MOON)
1034 \c[ [5] control char (example: chr(27))
1035 \o{23072} [6] octal char (example: SMILEY)
1036 \033 [7] restricted range octal char (example: ESC)
5691ca5f
KW
1037
1038=over 4
1039
1040=item [1]
1041
96448467
DG
1042The result is the character whose ordinal is the hexadecimal number between
1043the braces. If the ordinal is 0x100 and above, the character will be the
1044Unicode character corresponding to the ordinal. If the ordinal is between
10450 and 0xFF, the rules for which character it represents are the same as for
1046L<restricted hex chars|/[2]>.
1047
1048Only hexadecimal digits are valid between the braces. If an invalid
1049character is encountered, a warning will be issued and the invalid
1050character and all subsequent characters (valid or invalid) within the
1051braces will be discarded.
1052
1053If there are no valid digits between the braces, the generated character is
1054the NULL character (C<\x{00}>). However, an explicit empty brace (C<\x{}>)
1055will not cause a warning.
40687185
KW
1056
1057=item [2]
1058
96448467
DG
1059The result is a single-byte character whose ordinal is in the range 0x00 to
10600xFF.
1061
1062Only hexadecimal digits are valid following C<\x>. When C<\x> is followed
1063by less than two valid digits, any valid digits will be zero-padded. This
1064means that C<\x7> will be interpreted as C<\x07> and C<\x> alone will be
1065interpreted as C<\x00>. Except at the end of a string, having less than
1066two valid digits will result in a warning. Note that while the warning
1067says the illegal character is ignored, it is only ignored as part of the
1068escape and will still be used as the subsequent character in the string.
1069For example:
1070
1071 Original Result Warns?
1072 "\x7" "\x07" no
1073 "\x" "\x00" no
1074 "\x7q" "\x07q" yes
1075 "\xq" "\x00q" yes
1076
1077The B<run-time> interpretation of single-byte characters depends on the
1078platform and on pragmata in effect. On EBCDIC platforms the character is
1079treated as native to the platform's code page. On other platforms, the
1080representation and semantics (sort order and which characters are upper
1081case, lower case, digit, non-digit, etc.) depends on the current
1082L<S<C<locale>>|perllocale> settings at run-time.
1083
1084However, when L<C<S<use feature 'unicode_strings'>>|feature> is in effect
1085and both L<C<S<use bytes>>|bytes> and L<C<S<use locale>>|locale> are not,
1086characters from 0x80 to 0xff are treated as Unicode code points from
1087the Latin-1 Supplement block.
1088
1089Note that the locale semantics of single-byte characters in a regular
1090expression are determined when the regular expression is compiled, not when
1091the regular expression is used. When a regular expression is interpolated
1092into another regular expression -- any prior semantics are ignored and only
1093current locale matters for the resulting regular expression.
40687185
KW
1094
1095=item [3]
1096
1097For documentation of C<\N{name}>, see L<charnames>.
1098
1099=item [4]
1100
1101C<\N{U+I<wide hex char>}> means the Unicode character whose Unicode ordinal
1102number is I<wide hex char>.
1103
1104=item [5]
1105
5691ca5f
KW
1106The character following C<\c> is mapped to some other character as shown in the
1107table:
1108
1109 Sequence Value
1110 \c@ chr(0)
1111 \cA chr(1)
1112 \ca chr(1)
1113 \cB chr(2)
1114 \cb chr(2)
1115 ...
1116 \cZ chr(26)
1117 \cz chr(26)
1118 \c[ chr(27)
1119 \c] chr(29)
1120 \c^ chr(30)
1121 \c? chr(127)
1122
1123Also, C<\c\I<X>> yields C< chr(28) . "I<X>"> for any I<X>, but cannot come at the
1124end of a string, because the backslash would be parsed as escaping the end
1125quote.
1126
1127On ASCII platforms, the resulting characters from the list above are the
1128complete set of ASCII controls. This isn't the case on EBCDIC platforms; see
1129L<perlebcdic/OPERATOR DIFFERENCES> for the complete list of what these
1130sequences mean on both ASCII and EBCDIC platforms.
1131
1132Use of any other character following the "c" besides those listed above is
6ae9f32a
KW
1133discouraged, and may become deprecated or forbidden. What happens for those
1134other characters currently though, is that the value is derived by inverting
1135the 7th bit (0x40).
5691ca5f
KW
1136
1137To get platform independent controls, you can use C<\N{...}>.
1138
40687185
KW
1139=item [6]
1140
04341565
DG
1141The result is the character whose ordinal is the octal number between the
1142braces.
1143
1144If a character that isn't an octal digit is encountered, a warning is raised,
1145and the value is based on the octal digits before it, discarding it and all
1146following characters up to the closing brace. It is a fatal error if there are
1147no octal digits at all.
1148
1149=item [7]
1150
40687185
KW
1151The result is the character whose ordinal is the given three digit octal
1152number. Some contexts allow 2 or even 1 digit, but any usage without exactly
1153three digits, the first being a zero, may give unintended results. (For
04341565
DG
1154example, see L<perlrebackslash/Octal escapes>.) Starting in Perl 5.14, you may
1155use C<\o{}> instead which avoids all these problems. Otherwise, it is best to
1156use this construct only for ordinals C<\077> and below, remembering to pad to
1157the left with zeros to make three digits. For larger ordinals, either use
1158C<\o{}> , or convert to someething else, such as to hex and use C<\x{}>
1159instead.
40687185
KW
1160
1161A backslash followed by a non-octal digit in a bracketed character class
1162(C<[\8]> or C<[\9]>) will be interpreted as a NULL character and the digit.
1163Having fewer than 3 digits may lead to a misleading warning message that says
1164that what follows is ignored. For example, C<"\128"> in the ASCII character set
1165is equivalent to the two characters C<"\n8">, but the warning C<Illegal octal
1166digit '8' ignored> will be thrown. To avoid this warning, make sure to pad
1167your octal number with C<0>s: C<"\0128">.
5691ca5f
KW
1168
1169=back
4c77eaa2 1170
e526e8bb
KW
1171B<NOTE>: Unlike C and other languages, Perl has no C<\v> escape sequence for
1172the vertical tab (VT - ASCII 11), but you may use C<\ck> or C<\x0b>. (C<\v>
1173does have meaning in regular expression patterns in Perl, see L<perlre>.)
1174
1175The following escape sequences are available in constructs that interpolate,
904501ec 1176but not in transliterations.
d74e8afc 1177X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q>
904501ec 1178
a0d0e21e
LW
1179 \l lowercase next char
1180 \u uppercase next char
1181 \L lowercase till \E
1182 \U uppercase till \E
1183 \E end case modification
1d2dff63 1184 \Q quote non-word characters till \E
a0d0e21e 1185
95cc3e0c
JH
1186If C<use locale> is in effect, the case map used by C<\l>, C<\L>,
1187C<\u> and C<\U> is taken from the current locale. See L<perllocale>.
1188If Unicode (for example, C<\N{}> or wide hex characters of 0x100 or
1189beyond) is being used, the case map used by C<\l>, C<\L>, C<\u> and
e526e8bb 1190C<\U> is as defined by Unicode.
a034a98d 1191
5a964f20
TC
1192All systems use the virtual C<"\n"> to represent a line terminator,
1193called a "newline". There is no such thing as an unvarying, physical
19799a22 1194newline character. It is only an illusion that the operating system,
5a964f20
TC
1195device drivers, C libraries, and Perl all conspire to preserve. Not all
1196systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
1197on a Mac, these are reversed, and on systems without line terminator,
1198printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
1199you mean a "newline" for your system, but use the literal ASCII when you
1200need an exact character. For example, most networking protocols expect
2a380090 1201and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
5a964f20
TC
1202and although they often accept just C<"\012">, they seldom tolerate just
1203C<"\015">. If you get in the habit of using C<"\n"> for networking,
1204you may be burned some day.
d74e8afc
ITB
1205X<newline> X<line terminator> X<eol> X<end of line>
1206X<\n> X<\r> X<\r\n>
5a964f20 1207
904501ec
MG
1208For constructs that do interpolate, variables beginning with "C<$>"
1209or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
ad0f383a
A
1210C<< $href->{key}[0] >> are also interpolated, as are array and hash slices.
1211But method calls such as C<< $obj->meth >> are not.
af9219ee
MG
1212
1213Interpolating an array or slice interpolates the elements in order,
1214separated by the value of C<$">, so is equivalent to interpolating
6deea57f
ST
1215C<join $", @array>. "Punctuation" arrays such as C<@*> are only
1216interpolated if the name is enclosed in braces C<@{*}>, but special
1217arrays C<@_>, C<@+>, and C<@-> are interpolated, even without braces.
af9219ee 1218
89d205f2
YO
1219You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
1220An unescaped C<$> or C<@> interpolates the corresponding variable,
1d2dff63 1221while escaping will cause the literal string C<\$> to be inserted.
89d205f2 1222You'll need to write something like C<m/\Quser\E\@\Qhost/>.
1d2dff63 1223
a0d0e21e
LW
1224Patterns are subject to an additional level of interpretation as a
1225regular expression. This is done as a second pass, after variables are
1226interpolated, so that regular expressions may be incorporated into the
1227pattern from the variables. If this is not what you want, use C<\Q> to
1228interpolate a variable literally.
1229
19799a22
GS
1230Apart from the behavior described above, Perl does not expand
1231multiple levels of interpolation. In particular, contrary to the
1232expectations of shell programmers, back-quotes do I<NOT> interpolate
1233within double quotes, nor do single quotes impede evaluation of
1234variables when used within double quotes.
a0d0e21e 1235
5f05dabc 1236=head2 Regexp Quote-Like Operators
d74e8afc 1237X<operator, regexp>
cb1a09d0 1238
5f05dabc 1239Here are the quote-like operators that apply to pattern
cb1a09d0
AD
1240matching and related activities.
1241
a0d0e21e
LW
1242=over 8
1243
87e95b7f 1244=item qr/STRING/msixpo
01c6f5f4 1245X<qr> X</i> X</m> X</o> X</s> X</x> X</p>
a0d0e21e 1246
87e95b7f
YO
1247This operator quotes (and possibly compiles) its I<STRING> as a regular
1248expression. I<STRING> is interpolated the same way as I<PATTERN>
1249in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
1250is done. Returns a Perl value which may be used instead of the
64c5a566 1251corresponding C</STRING/msixpo> expression. The returned value is a
85dd5c8b 1252normalized version of the original pattern. It magically differs from
64c5a566 1253a string containing the same characters: C<ref(qr/x/)> returns "Regexp",
85dd5c8b 1254even though dereferencing the result returns undef.
a0d0e21e 1255
87e95b7f
YO
1256For example,
1257
1258 $rex = qr/my.STRING/is;
85dd5c8b 1259 print $rex; # prints (?si-xm:my.STRING)
87e95b7f
YO
1260 s/$rex/foo/;
1261
1262is equivalent to
1263
1264 s/my.STRING/foo/is;
1265
1266The result may be used as a subpattern in a match:
1267
1268 $re = qr/$pattern/;
1269 $string =~ /foo${re}bar/; # can be interpolated in other patterns
1270 $string =~ $re; # or used standalone
1271 $string =~ /$re/; # or this way
1272
1273Since Perl may compile the pattern at the moment of execution of qr()
1274operator, using qr() may have speed advantages in some situations,
1275notably if the result of qr() is used standalone:
1276
1277 sub match {
1278 my $patterns = shift;
1279 my @compiled = map qr/$_/i, @$patterns;
1280 grep {
1281 my $success = 0;
1282 foreach my $pat (@compiled) {
1283 $success = 1, last if /$pat/;
1284 }
1285 $success;
1286 } @_;
5a964f20
TC
1287 }
1288
87e95b7f
YO
1289Precompilation of the pattern into an internal representation at
1290the moment of qr() avoids a need to recompile the pattern every
1291time a match C</$pat/> is attempted. (Perl has many other internal
1292optimizations, but none would be triggered in the above example if
1293we did not use qr() operator.)
1294
1295Options are:
1296
1297 m Treat string as multiple lines.
1298 s Treat string as single line. (Make . match a newline)
1299 i Do case-insensitive pattern matching.
1300 x Use extended regular expressions.
1301 p When matching preserve a copy of the matched string so
1302 that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be defined.
1303 o Compile pattern only once.
1304
1305If a precompiled pattern is embedded in a larger pattern then the effect
1306of 'msixp' will be propagated appropriately. The effect of the 'o'
1307modifier has is not propagated, being restricted to those patterns
1308explicitly using it.
1309
1310See L<perlre> for additional information on valid syntax for STRING, and
1311for a detailed look at the semantics of regular expressions.
a0d0e21e 1312
87e95b7f 1313=item m/PATTERN/msixpogc
89d205f2
YO
1314X<m> X<operator, match>
1315X<regexp, options> X<regexp> X<regex, options> X<regex>
01c6f5f4 1316X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c>
a0d0e21e 1317
87e95b7f 1318=item /PATTERN/msixpogc
a0d0e21e 1319
5a964f20 1320Searches a string for a pattern match, and in scalar context returns
19799a22
GS
1321true if it succeeds, false if it fails. If no string is specified
1322via the C<=~> or C<!~> operator, the $_ string is searched. (The
1323string specified with C<=~> need not be an lvalue--it may be the
1324result of an expression evaluation, but remember the C<=~> binds
1325rather tightly.) See also L<perlre>. See L<perllocale> for
1326discussion of additional considerations that apply when C<use locale>
1327is in effect.
a0d0e21e 1328
01c6f5f4
RGS
1329Options are as described in C<qr//>; in addition, the following match
1330process modifiers are available:
a0d0e21e 1331
950b09ed
KW
1332 g Match globally, i.e., find all occurrences.
1333 c Do not reset search position on a failed match when /g is in effect.
a0d0e21e
LW
1334
1335If "/" is the delimiter then the initial C<m> is optional. With the C<m>
ed02a3bf 1336you can use any pair of non-whitespace characters
19799a22
GS
1337as delimiters. This is particularly useful for matching path names
1338that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
7bac28a0 1339the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
19799a22 1340If "'" is the delimiter, no interpolation is performed on the PATTERN.
ed02a3bf
DN
1341When using a character valid in an identifier, whitespace is required
1342after the C<m>.
a0d0e21e
LW
1343
1344PATTERN may contain variables, which will be interpolated (and the
f70b4f9c 1345pattern recompiled) every time the pattern search is evaluated, except
1f247705
GS
1346for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
1347C<$|> are not interpolated because they look like end-of-string tests.)
f70b4f9c
AB
1348If you want such a pattern to be compiled only once, add a C</o> after
1349the trailing delimiter. This avoids expensive run-time recompilations,
1350and is useful when the value you are interpolating won't change over
1351the life of the script. However, mentioning C</o> constitutes a promise
1352that you won't change the variables in the pattern. If you change them,
01c6f5f4 1353Perl won't even notice. See also L<"qr/STRING/msixpo">.
a0d0e21e 1354
e9d89077
DN
1355=item The empty pattern //
1356
5a964f20 1357If the PATTERN evaluates to the empty string, the last
d65afb4b
HS
1358I<successfully> matched regular expression is used instead. In this
1359case, only the C<g> and C<c> flags on the empty pattern is honoured -
1360the other flags are taken from the original pattern. If no match has
1361previously succeeded, this will (silently) act instead as a genuine
1362empty pattern (which will always match).
a0d0e21e 1363
89d205f2
YO
1364Note that it's possible to confuse Perl into thinking C<//> (the empty
1365regex) is really C<//> (the defined-or operator). Perl is usually pretty
1366good about this, but some pathological cases might trigger this, such as
1367C<$a///> (is that C<($a) / (//)> or C<$a // />?) and C<print $fh //>
1368(C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl
1369will assume you meant defined-or. If you meant the empty regex, just
1370use parentheses or spaces to disambiguate, or even prefix the empty
c963b151
BD
1371regex with an C<m> (so C<//> becomes C<m//>).
1372
e9d89077
DN
1373=item Matching in list context
1374
19799a22 1375If the C</g> option is not used, C<m//> in list context returns a
a0d0e21e 1376list consisting of the subexpressions matched by the parentheses in the
f7e33566
GS
1377pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are
1378also set, and that this differs from Perl 4's behavior.) When there are
1379no parentheses in the pattern, the return value is the list C<(1)> for
1380success. With or without parentheses, an empty list is returned upon
1381failure.
a0d0e21e
LW
1382
1383Examples:
1384
1385 open(TTY, '/dev/tty');
1386 <TTY> =~ /^y/i && foo(); # do foo if desired
1387
1388 if (/Version: *([0-9.]*)/) { $version = $1; }
1389
1390 next if m#^/usr/spool/uucp#;
1391
1392 # poor man's grep
1393 $arg = shift;
1394 while (<>) {
1395 print if /$arg/o; # compile only once
1396 }
1397
1398 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
1399
1400This last example splits $foo into the first two words and the
5f05dabc
PP
1401remainder of the line, and assigns those three fields to $F1, $F2, and
1402$Etc. The conditional is true if any variables were assigned, i.e., if
a0d0e21e
LW
1403the pattern matched.
1404
19799a22
GS
1405The C</g> modifier specifies global pattern matching--that is,
1406matching as many times as possible within the string. How it behaves
1407depends on the context. In list context, it returns a list of the
1408substrings matched by any capturing parentheses in the regular
1409expression. If there are no parentheses, it returns a list of all
1410the matched strings, as if there were parentheses around the whole
1411pattern.
a0d0e21e 1412
7e86de3e 1413In scalar context, each execution of C<m//g> finds the next match,
19799a22 1414returning true if it matches, and false if there is no further match.
7e86de3e
G
1415The position after the last match can be read or set using the pos()
1416function; see L<perlfunc/pos>. A failed match normally resets the
1417search position to the beginning of the string, but you can avoid that
1418by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
1419string also resets the search position.
c90c0ff4 1420
e9d89077
DN
1421=item \G assertion
1422
c90c0ff4
PP
1423You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
1424zero-width assertion that matches the exact position where the previous
5d43e42d
DC
1425C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion
1426still anchors at pos(), but the match is of course only attempted once.
1427Using C<\G> without C</g> on a target string that has not previously had a
1428C</g> match applied to it is the same as using the C<\A> assertion to match
fe4b3f22
RGS
1429the beginning of the string. Note also that, currently, C<\G> is only
1430properly supported when anchored at the very beginning of the pattern.
c90c0ff4
PP
1431
1432Examples:
a0d0e21e
LW
1433
1434 # list context
1435 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
1436
1437 # scalar context
5d43e42d 1438 $/ = "";
19799a22
GS
1439 while (defined($paragraph = <>)) {
1440 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
1441 $sentences++;
a0d0e21e
LW
1442 }
1443 }
1444 print "$sentences\n";
1445
c90c0ff4 1446 # using m//gc with \G
137443ea 1447 $_ = "ppooqppqq";
44a8e56a
PP
1448 while ($i++ < 2) {
1449 print "1: '";
c90c0ff4 1450 print $1 while /(o)/gc; print "', pos=", pos, "\n";
44a8e56a 1451 print "2: '";
c90c0ff4 1452 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
44a8e56a 1453 print "3: '";
c90c0ff4 1454 print $1 while /(p)/gc; print "', pos=", pos, "\n";
44a8e56a 1455 }
5d43e42d 1456 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
44a8e56a
PP
1457
1458The last example should print:
1459
1460 1: 'oo', pos=4
137443ea 1461 2: 'q', pos=5
44a8e56a
PP
1462 3: 'pp', pos=7
1463 1: '', pos=7
137443ea
PP
1464 2: 'q', pos=8
1465 3: '', pos=8
5d43e42d
DC
1466 Final: 'q', pos=8
1467
1468Notice that the final match matched C<q> instead of C<p>, which a match
1469without the C<\G> anchor would have done. Also note that the final match
ac036724 1470did not update C<pos>. C<pos> is only updated on a C</g> match. If the
5d43e42d
DC
1471final match did indeed match C<p>, it's a good bet that you're running an
1472older (pre-5.6.0) Perl.
44a8e56a 1473
c90c0ff4 1474A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
e7ea3e70 1475combine several regexps like this to process a string part-by-part,
c90c0ff4
PP
1476doing different actions depending on which regexp matched. Each
1477regexp tries to match where the previous one leaves off.
e7ea3e70 1478
3fe9a6f1 1479 $_ = <<'EOL';
950b09ed 1480 $url = URI::URL->new( "http://example.com/" ); die if $url eq "xXx";
3fe9a6f1
PP
1481 EOL
1482 LOOP:
e7ea3e70 1483 {
950b09ed
KW
1484 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
1485 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
1486 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
1487 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
1488 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
1489 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
1490 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
1491 print ". That's all!\n";
e7ea3e70
IZ
1492 }
1493
1494Here is the output (split into several lines):
1495
1496 line-noise lowercase line-noise lowercase UPPERCASE line-noise
1497 UPPERCASE line-noise lowercase line-noise lowercase line-noise
1498 lowercase lowercase line-noise lowercase lowercase line-noise
1499 MiXeD line-noise. That's all!
44a8e56a 1500
87e95b7f
YO
1501=item ?PATTERN?
1502X<?>
1503
1504This is just like the C</pattern/> search, except that it matches only
1505once between calls to the reset() operator. This is a useful
1506optimization when you want to see only the first occurrence of
1507something in each file of a set of files, for instance. Only C<??>
1508patterns local to the current package are reset.
1509
1510 while (<>) {
1511 if (?^$?) {
1512 # blank line between header and body
1513 }
1514 } continue {
1515 reset if eof; # clear ?? status for next file
1516 }
1517
1518This usage is vaguely deprecated, which means it just might possibly
1519be removed in some distant future version of Perl, perhaps somewhere
1520around the year 2168.
1521
4f4d7508 1522=item s/PATTERN/REPLACEMENT/msixpogcer
87e95b7f 1523X<substitute> X<substitution> X<replace> X<regexp, replace>
4f4d7508 1524X<regexp, substitute> X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> X</e> X</r>
87e95b7f
YO
1525
1526Searches a string for a pattern, and if found, replaces that pattern
1527with the replacement text and returns the number of substitutions
1528made. Otherwise it returns false (specifically, the empty string).
1529
4f4d7508
DC
1530If the C</r> (non-destructive) option is used then it will perform the
1531substitution on a copy of the string and return the copy whether or not a
1532substitution occurred. The original string will always remain unchanged in
c8dbf8cd 1533this case. The copy will always be a plain string, even if the input is an
4f4d7508
DC
1534object or a tied variable.
1535
87e95b7f
YO
1536If no string is specified via the C<=~> or C<!~> operator, the C<$_>
1537variable is searched and modified. (The string specified with C<=~> must
1538be scalar variable, an array element, a hash element, or an assignment
1539to one of those, i.e., an lvalue.)
1540
1541If the delimiter chosen is a single quote, no interpolation is
1542done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1543PATTERN contains a $ that looks like a variable rather than an
1544end-of-string test, the variable will be interpolated into the pattern
1545at run-time. If you want the pattern compiled only once the first time
1546the variable is interpolated, use the C</o> option. If the pattern
1547evaluates to the empty string, the last successfully executed regular
1548expression is used instead. See L<perlre> for further explanation on these.
1549See L<perllocale> for discussion of additional considerations that apply
1550when C<use locale> is in effect.
1551
1552Options are as with m// with the addition of the following replacement
1553specific options:
1554
1555 e Evaluate the right side as an expression.
4f4d7508
DC
1556 ee Evaluate the right side as a string then eval the result.
1557 r Return substitution and leave the original string untouched.
87e95b7f 1558
ed02a3bf
DN
1559Any non-whitespace delimiter may replace the slashes. Add space after
1560the C<s> when using a character allowed in identifiers. If single quotes
1561are used, no interpretation is done on the replacement string (the C</e>
1562modifier overrides this, however). Unlike Perl 4, Perl 5 treats backticks
1563as normal delimiters; the replacement text is not evaluated as a command.
1564If the PATTERN is delimited by bracketing quotes, the REPLACEMENT has
1565its own pair of quotes, which may or may not be bracketing quotes, e.g.,
87e95b7f
YO
1566C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
1567replacement portion to be treated as a full-fledged Perl expression
1568and evaluated right then and there. It is, however, syntax checked at
1569compile-time. A second C<e> modifier will cause the replacement portion
1570to be C<eval>ed before being run as a Perl expression.
1571
1572Examples:
1573
1574 s/\bgreen\b/mauve/g; # don't change wintergreen
1575
1576 $path =~ s|/usr/bin|/usr/local/bin|;
1577
1578 s/Login: $foo/Login: $bar/; # run-time pattern
1579
1580 ($foo = $bar) =~ s/this/that/; # copy first, then change
4f4d7508
DC
1581 ($foo = "$bar") =~ s/this/that/; # convert to string, copy, then change
1582 $foo = $bar =~ s/this/that/r; # Same as above using /r
1583 $foo = $bar =~ s/this/that/r
1584 =~ s/that/the other/r; # Chained substitutes using /r
1585 @foo = map { s/this/that/r } @bar # /r is very useful in maps
87e95b7f
YO
1586
1587 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
1588
1589 $_ = 'abc123xyz';
1590 s/\d+/$&*2/e; # yields 'abc246xyz'
1591 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
1592 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1593
1594 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
1595 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
1596 s/^=(\w+)/pod($1)/ge; # use function call
1597
4f4d7508
DC
1598 $_ = 'abc123xyz';
1599 $a = s/abc/def/r; # $a is 'def123xyz' and
1600 # $_ remains 'abc123xyz'.
1601
87e95b7f
YO
1602 # expand variables in $_, but dynamics only, using
1603 # symbolic dereferencing
1604 s/\$(\w+)/${$1}/g;
1605
1606 # Add one to the value of any numbers in the string
1607 s/(\d+)/1 + $1/eg;
1608
1609 # This will expand any embedded scalar variable
1610 # (including lexicals) in $_ : First $1 is interpolated
1611 # to the variable name, and then evaluated
1612 s/(\$\w+)/$1/eeg;
1613
1614 # Delete (most) C comments.
1615 $program =~ s {
1616 /\* # Match the opening delimiter.
1617 .*? # Match a minimal number of characters.
1618 \*/ # Match the closing delimiter.
1619 } []gsx;
1620
1621 s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_, expensively
1622
1623 for ($variable) { # trim whitespace in $variable, cheap
1624 s/^\s+//;
1625 s/\s+$//;
1626 }
1627
1628 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
1629
1630Note the use of $ instead of \ in the last example. Unlike
1631B<sed>, we use the \<I<digit>> form in only the left hand side.
1632Anywhere else it's $<I<digit>>.
1633
1634Occasionally, you can't use just a C</g> to get all the changes
1635to occur that you might want. Here are two common cases:
1636
1637 # put commas in the right places in an integer
1638 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
1639
1640 # expand tabs to 8-column spacing
1641 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
1642
1643=back
1644
1645=head2 Quote-Like Operators
1646X<operator, quote-like>
1647
01c6f5f4
RGS
1648=over 4
1649
a0d0e21e 1650=item q/STRING/
5d44bfff 1651X<q> X<quote, single> X<'> X<''>
a0d0e21e 1652
5d44bfff 1653=item 'STRING'
a0d0e21e 1654
19799a22 1655A single-quoted, literal string. A backslash represents a backslash
68dc0745
PP
1656unless followed by the delimiter or another backslash, in which case
1657the delimiter or backslash is interpolated.
a0d0e21e
LW
1658
1659 $foo = q!I said, "You said, 'She said it.'"!;
1660 $bar = q('This is it.');
68dc0745 1661 $baz = '\n'; # a two-character string
a0d0e21e
LW
1662
1663=item qq/STRING/
d74e8afc 1664X<qq> X<quote, double> X<"> X<"">
a0d0e21e
LW
1665
1666=item "STRING"
1667
1668A double-quoted, interpolated string.
1669
1670 $_ .= qq
1671 (*** The previous line contains the naughty word "$1".\n)
19799a22 1672 if /\b(tcl|java|python)\b/i; # :-)
68dc0745 1673 $baz = "\n"; # a one-character string
a0d0e21e
LW
1674
1675=item qx/STRING/
d74e8afc 1676X<qx> X<`> X<``> X<backtick>
a0d0e21e
LW
1677
1678=item `STRING`
1679
43dd4d21
JH
1680A string which is (possibly) interpolated and then executed as a
1681system command with C</bin/sh> or its equivalent. Shell wildcards,
1682pipes, and redirections will be honored. The collected standard
1683output of the command is returned; standard error is unaffected. In
1684scalar context, it comes back as a single (potentially multi-line)
1685string, or undef if the command failed. In list context, returns a
1686list of lines (however you've defined lines with $/ or
1687$INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
5a964f20
TC
1688
1689Because backticks do not affect standard error, use shell file descriptor
1690syntax (assuming the shell supports this) if you care to address this.
1691To capture a command's STDERR and STDOUT together:
a0d0e21e 1692
5a964f20
TC
1693 $output = `cmd 2>&1`;
1694
1695To capture a command's STDOUT but discard its STDERR:
1696
1697 $output = `cmd 2>/dev/null`;
1698
1699To capture a command's STDERR but discard its STDOUT (ordering is
1700important here):
1701
1702 $output = `cmd 2>&1 1>/dev/null`;
1703
1704To exchange a command's STDOUT and STDERR in order to capture the STDERR
1705but leave its STDOUT to come out the old STDERR:
1706
1707 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
1708
1709To read both a command's STDOUT and its STDERR separately, it's easiest
2359510d
SD
1710to redirect them separately to files, and then read from those files
1711when the program is done:
5a964f20 1712
2359510d 1713 system("program args 1>program.stdout 2>program.stderr");
5a964f20 1714
30398227
SP
1715The STDIN filehandle used by the command is inherited from Perl's STDIN.
1716For example:
1717
1718 open BLAM, "blam" || die "Can't open: $!";
1719 open STDIN, "<&BLAM";
1720 print `sort`;
1721
1722will print the sorted contents of the file "blam".
1723
5a964f20
TC
1724Using single-quote as a delimiter protects the command from Perl's
1725double-quote interpolation, passing it on to the shell instead:
1726
1727 $perl_info = qx(ps $$); # that's Perl's $$
1728 $shell_info = qx'ps $$'; # that's the new shell's $$
1729
19799a22 1730How that string gets evaluated is entirely subject to the command
5a964f20
TC
1731interpreter on your system. On most platforms, you will have to protect
1732shell metacharacters if you want them treated literally. This is in
1733practice difficult to do, as it's unclear how to escape which characters.
1734See L<perlsec> for a clean and safe example of a manual fork() and exec()
1735to emulate backticks safely.
a0d0e21e 1736
bb32b41a
GS
1737On some platforms (notably DOS-like ones), the shell may not be
1738capable of dealing with multiline commands, so putting newlines in
1739the string may not get you what you want. You may be able to evaluate
1740multiple commands in a single line by separating them with the command
1741separator character, if your shell supports that (e.g. C<;> on many Unix
1742shells; C<&> on the Windows NT C<cmd> shell).
1743
0f897271
GS
1744Beginning with v5.6.0, Perl will attempt to flush all files opened for
1745output before starting the child process, but this may not be supported
1746on some platforms (see L<perlport>). To be safe, you may need to set
1747C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1748C<IO::Handle> on any open handles.
1749
bb32b41a
GS
1750Beware that some command shells may place restrictions on the length
1751of the command line. You must ensure your strings don't exceed this
1752limit after any necessary interpolations. See the platform-specific
1753release notes for more details about your particular environment.
1754
5a964f20
TC
1755Using this operator can lead to programs that are difficult to port,
1756because the shell commands called vary between systems, and may in
1757fact not be present at all. As one example, the C<type> command under
1758the POSIX shell is very different from the C<type> command under DOS.
1759That doesn't mean you should go out of your way to avoid backticks
1760when they're the right way to get something done. Perl was made to be
1761a glue language, and one of the things it glues together is commands.
1762Just understand what you're getting yourself into.
bb32b41a 1763
da87341d 1764See L</"I/O Operators"> for more discussion.
a0d0e21e 1765
945c54fd 1766=item qw/STRING/
d74e8afc 1767X<qw> X<quote, list> X<quote, words>
945c54fd
JH
1768
1769Evaluates to a list of the words extracted out of STRING, using embedded
1770whitespace as the word delimiters. It can be understood as being roughly
1771equivalent to:
1772
1773 split(' ', q/STRING/);
1774
efb1e162
CW
1775the differences being that it generates a real list at compile time, and
1776in scalar context it returns the last element in the list. So
945c54fd
JH
1777this expression:
1778
1779 qw(foo bar baz)
1780
1781is semantically equivalent to the list:
1782
1783 'foo', 'bar', 'baz'
1784
1785Some frequently seen examples:
1786
1787 use POSIX qw( setlocale localeconv )
1788 @EXPORT = qw( foo bar baz );
1789
1790A common mistake is to try to separate the words with comma or to
1791put comments into a multi-line C<qw>-string. For this reason, the
89d205f2 1792C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
945c54fd
JH
1793produces warnings if the STRING contains the "," or the "#" character.
1794
a0d0e21e 1795
6940069f 1796=item tr/SEARCHLIST/REPLACEMENTLIST/cds
d74e8afc 1797X<tr> X<y> X<transliterate> X</c> X</d> X</s>
a0d0e21e 1798
6940069f 1799=item y/SEARCHLIST/REPLACEMENTLIST/cds
a0d0e21e 1800
2c268ad5 1801Transliterates all occurrences of the characters found in the search list
a0d0e21e
LW
1802with the corresponding character in the replacement list. It returns
1803the number of characters replaced or deleted. If no string is
2c268ad5 1804specified via the =~ or !~ operator, the $_ string is transliterated. (The
54310121
PP
1805string specified with =~ must be a scalar variable, an array element, a
1806hash element, or an assignment to one of those, i.e., an lvalue.)
8ada0baa 1807
89d205f2 1808A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
2c268ad5 1809does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
54310121
PP
1810For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1811SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1812its own pair of quotes, which may or may not be bracketing quotes,
2c268ad5 1813e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
a0d0e21e 1814
cc255d5f 1815Note that C<tr> does B<not> do regular expression character classes
e0c83546 1816such as C<\d> or C<[:lower:]>. The C<tr> operator is not equivalent to
cc255d5f
JH
1817the tr(1) utility. If you want to map strings between lower/upper
1818cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider
1819using the C<s> operator if you need regular expressions.
1820
8ada0baa
JH
1821Note also that the whole range idea is rather unportable between
1822character sets--and even within character sets they may cause results
1823you probably didn't expect. A sound principle is to use only ranges
1824that begin from and end at either alphabets of equal case (a-e, A-E),
1825or digits (0-4). Anything else is unsafe. If in doubt, spell out the
1826character sets in full.
1827
a0d0e21e
LW
1828Options:
1829
1830 c Complement the SEARCHLIST.
1831 d Delete found but unreplaced characters.
1832 s Squash duplicate replaced characters.
1833
19799a22
GS
1834If the C</c> modifier is specified, the SEARCHLIST character set
1835is complemented. If the C</d> modifier is specified, any characters
1836specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
1837(Note that this is slightly more flexible than the behavior of some
1838B<tr> programs, which delete anything they find in the SEARCHLIST,
1839period.) If the C</s> modifier is specified, sequences of characters
1840that were transliterated to the same character are squashed down
1841to a single instance of the character.
a0d0e21e
LW
1842
1843If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1844exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1845than the SEARCHLIST, the final character is replicated till it is long
5a964f20 1846enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
a0d0e21e
LW
1847This latter is useful for counting characters in a class or for
1848squashing character sequences in a class.
1849
1850Examples:
1851
1852 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1853
1854 $cnt = tr/*/*/; # count the stars in $_
1855
1856 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1857
1858 $cnt = tr/0-9//; # count the digits in $_
1859
1860 tr/a-zA-Z//s; # bookkeeper -> bokeper
1861
1862 ($HOST = $host) =~ tr/a-z/A-Z/;
1863
1864 tr/a-zA-Z/ /cs; # change non-alphas to single space
1865
1866 tr [\200-\377]
1867 [\000-\177]; # delete 8th bit
1868
19799a22
GS
1869If multiple transliterations are given for a character, only the
1870first one is used:
748a9306
LW
1871
1872 tr/AAA/XYZ/
1873
2c268ad5 1874will transliterate any A to X.
748a9306 1875
19799a22 1876Because the transliteration table is built at compile time, neither
a0d0e21e 1877the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
19799a22
GS
1878interpolation. That means that if you want to use variables, you
1879must use an eval():
a0d0e21e
LW
1880
1881 eval "tr/$oldlist/$newlist/";
1882 die $@ if $@;
1883
1884 eval "tr/$oldlist/$newlist/, 1" or die $@;
1885
7e3b091d 1886=item <<EOF
d74e8afc 1887X<here-doc> X<heredoc> X<here-document> X<<< << >>>
7e3b091d
DA
1888
1889A line-oriented form of quoting is based on the shell "here-document"
1890syntax. Following a C<< << >> you specify a string to terminate
1891the quoted material, and all lines following the current line down to
89d205f2
YO
1892the terminating string are the value of the item.
1893
1894The terminating string may be either an identifier (a word), or some
1895quoted text. An unquoted identifier works like double quotes.
1896There may not be a space between the C<< << >> and the identifier,
1897unless the identifier is explicitly quoted. (If you put a space it
1898will be treated as a null identifier, which is valid, and matches the
1899first empty line.) The terminating string must appear by itself
1900(unquoted and with no surrounding whitespace) on the terminating line.
1901
1902If the terminating string is quoted, the type of quotes used determine
1903the treatment of the text.
1904
1905=over 4
1906
1907=item Double Quotes
1908
1909Double quotes indicate that the text will be interpolated using exactly
1910the same rules as normal double quoted strings.
7e3b091d
DA
1911
1912 print <<EOF;
1913 The price is $Price.
1914 EOF
1915
1916 print << "EOF"; # same as above
1917 The price is $Price.
1918 EOF
1919
89d205f2
YO
1920
1921=item Single Quotes
1922
1923Single quotes indicate the text is to be treated literally with no
1924interpolation of its content. This is similar to single quoted
1925strings except that backslashes have no special meaning, with C<\\>
1926being treated as two backslashes and not one as they would in every
1927other quoting construct.
1928
1929This is the only form of quoting in perl where there is no need
1930to worry about escaping content, something that code generators
1931can and do make good use of.
1932
1933=item Backticks
1934
1935The content of the here doc is treated just as it would be if the
1936string were embedded in backticks. Thus the content is interpolated
1937as though it were double quoted and then executed via the shell, with
1938the results of the execution returned.
1939
1940 print << `EOC`; # execute command and get results
7e3b091d 1941 echo hi there
7e3b091d
DA
1942 EOC
1943
89d205f2
YO
1944=back
1945
1946It is possible to stack multiple here-docs in a row:
1947
7e3b091d
DA
1948 print <<"foo", <<"bar"; # you can stack them
1949 I said foo.
1950 foo
1951 I said bar.
1952 bar
1953
1954 myfunc(<< "THIS", 23, <<'THAT');
1955 Here's a line
1956 or two.
1957 THIS
1958 and here's another.
1959 THAT
1960
1961Just don't forget that you have to put a semicolon on the end
1962to finish the statement, as Perl doesn't know you're not going to
1963try to do this:
1964
1965 print <<ABC
1966 179231
1967 ABC
1968 + 20;
1969
872d7e53
ST
1970If you want to remove the line terminator from your here-docs,
1971use C<chomp()>.
1972
1973 chomp($string = <<'END');
1974 This is a string.
1975 END
1976
1977If you want your here-docs to be indented with the rest of the code,
1978you'll need to remove leading whitespace from each line manually:
7e3b091d
DA
1979
1980 ($quote = <<'FINIS') =~ s/^\s+//gm;
89d205f2 1981 The Road goes ever on and on,
7e3b091d
DA
1982 down from the door where it began.
1983 FINIS
1984
1985If you use a here-doc within a delimited construct, such as in C<s///eg>,
1986the quoted material must come on the lines following the final delimiter.
1987So instead of
1988
1989 s/this/<<E . 'that'
1990 the other
1991 E
1992 . 'more '/eg;
1993
1994you have to write
1995
89d205f2
YO
1996 s/this/<<E . 'that'
1997 . 'more '/eg;
1998 the other
1999 E
7e3b091d
DA
2000
2001If the terminating identifier is on the last line of the program, you
2002must be sure there is a newline after it; otherwise, Perl will give the
2003warning B<Can't find string terminator "END" anywhere before EOF...>.
2004
89d205f2 2005Additionally, the quoting rules for the end of string identifier are not
ac036724 2006related to Perl's quoting rules. C<q()>, C<qq()>, and the like are not
89d205f2
YO
2007supported in place of C<''> and C<"">, and the only interpolation is for
2008backslashing the quoting character:
7e3b091d
DA
2009
2010 print << "abc\"def";
2011 testing...
2012 abc"def
2013
2014Finally, quoted strings cannot span multiple lines. The general rule is
2015that the identifier must be a string literal. Stick with that, and you
2016should be safe.
2017
a0d0e21e
LW
2018=back
2019
75e14d17 2020=head2 Gory details of parsing quoted constructs
d74e8afc 2021X<quote, gory details>
75e14d17 2022
19799a22
GS
2023When presented with something that might have several different
2024interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
2025principle to pick the most probable interpretation. This strategy
2026is so successful that Perl programmers often do not suspect the
2027ambivalence of what they write. But from time to time, Perl's
2028notions differ substantially from what the author honestly meant.
2029
2030This section hopes to clarify how Perl handles quoted constructs.
2031Although the most common reason to learn this is to unravel labyrinthine
2032regular expressions, because the initial steps of parsing are the
2033same for all quoting operators, they are all discussed together.
2034
2035The most important Perl parsing rule is the first one discussed
2036below: when processing a quoted construct, Perl first finds the end
2037of that construct, then interprets its contents. If you understand
2038this rule, you may skip the rest of this section on the first
2039reading. The other rules are likely to contradict the user's
2040expectations much less frequently than this first one.
2041
2042Some passes discussed below are performed concurrently, but because
2043their results are the same, we consider them individually. For different
2044quoting constructs, Perl performs different numbers of passes, from
6deea57f 2045one to four, but these passes are always performed in the same order.
75e14d17 2046
13a2d996 2047=over 4
75e14d17
IZ
2048
2049=item Finding the end
2050
6deea57f
ST
2051The first pass is finding the end of the quoted construct, where
2052the information about the delimiters is used in parsing.
2053During this search, text between the starting and ending delimiters
2054is copied to a safe location. The text copied gets delimiter-independent.
2055
2056If the construct is a here-doc, the ending delimiter is a line
2057that has a terminating string as the content. Therefore C<<<EOF> is
2058terminated by C<EOF> immediately followed by C<"\n"> and starting
2059from the first column of the terminating line.
2060When searching for the terminating line of a here-doc, nothing
2061is skipped. In other words, lines after the here-doc syntax
2062are compared with the terminating string line by line.
2063
2064For the constructs except here-docs, single characters are used as starting
2065and ending delimiters. If the starting delimiter is an opening punctuation
2066(that is C<(>, C<[>, C<{>, or C<< < >>), the ending delimiter is the
2067corresponding closing punctuation (that is C<)>, C<]>, C<}>, or C<< > >>).
2068If the starting delimiter is an unpaired character like C</> or a closing
2069punctuation, the ending delimiter is same as the starting delimiter.
2070Therefore a C</> terminates a C<qq//> construct, while a C<]> terminates
2071C<qq[]> and C<qq]]> constructs.
2072
2073When searching for single-character delimiters, escaped delimiters
2074and C<\\> are skipped. For example, while searching for terminating C</>,
2075combinations of C<\\> and C<\/> are skipped. If the delimiters are
2076bracketing, nested pairs are also skipped. For example, while searching
2077for closing C<]> paired with the opening C<[>, combinations of C<\\>, C<\]>,
2078and C<\[> are all skipped, and nested C<[> and C<]> are skipped as well.
2079However, when backslashes are used as the delimiters (like C<qq\\> and
2080C<tr\\\>), nothing is skipped.
2081During the search for the end, backslashes that escape delimiters
2082are removed (exactly speaking, they are not copied to the safe location).
75e14d17 2083
19799a22
GS
2084For constructs with three-part delimiters (C<s///>, C<y///>, and
2085C<tr///>), the search is repeated once more.
6deea57f
ST
2086If the first delimiter is not an opening punctuation, three delimiters must
2087be same such as C<s!!!> and C<tr)))>, in which case the second delimiter
2088terminates the left part and starts the right part at once.
2089If the left part is delimited by bracketing punctuations (that is C<()>,
2090C<[]>, C<{}>, or C<< <> >>), the right part needs another pair of
2091delimiters such as C<s(){}> and C<tr[]//>. In these cases, whitespaces
2092and comments are allowed between both parts, though the comment must follow
2093at least one whitespace; otherwise a character expected as the start of
2094the comment may be regarded as the starting delimiter of the right part.
75e14d17 2095
19799a22
GS
2096During this search no attention is paid to the semantics of the construct.
2097Thus:
75e14d17
IZ
2098
2099 "$hash{"$foo/$bar"}"
2100
2a94b7ce 2101or:
75e14d17 2102
89d205f2 2103 m/
2a94b7ce 2104 bar # NOT a comment, this slash / terminated m//!
75e14d17
IZ
2105 /x
2106
19799a22
GS
2107do not form legal quoted expressions. The quoted part ends on the
2108first C<"> and C</>, and the rest happens to be a syntax error.
2109Because the slash that terminated C<m//> was followed by a C<SPACE>,
2110the example above is not C<m//x>, but rather C<m//> with no C</x>
2111modifier. So the embedded C<#> is interpreted as a literal C<#>.
75e14d17 2112
89d205f2
YO
2113Also no attention is paid to C<\c\> (multichar control char syntax) during
2114this search. Thus the second C<\> in C<qq/\c\/> is interpreted as a part
2115of C<\/>, and the following C</> is not recognized as a delimiter.
0d594e51
ST
2116Instead, use C<\034> or C<\x1c> at the end of quoted constructs.
2117
75e14d17 2118=item Interpolation
d74e8afc 2119X<interpolation>
75e14d17 2120
19799a22 2121The next step is interpolation in the text obtained, which is now
89d205f2 2122delimiter-independent. There are multiple cases.
75e14d17 2123
13a2d996 2124=over 4
75e14d17 2125
89d205f2 2126=item C<<<'EOF'>
75e14d17
IZ
2127
2128No interpolation is performed.
6deea57f
ST
2129Note that the combination C<\\> is left intact, since escaped delimiters
2130are not available for here-docs.
75e14d17 2131
6deea57f 2132=item C<m''>, the pattern of C<s'''>
89d205f2 2133
6deea57f
ST
2134No interpolation is performed at this stage.
2135Any backslashed sequences including C<\\> are treated at the stage
2136to L</"parsing regular expressions">.
89d205f2 2137
6deea57f 2138=item C<''>, C<q//>, C<tr'''>, C<y'''>, the replacement of C<s'''>
75e14d17 2139
89d205f2 2140The only interpolation is removal of C<\> from pairs of C<\\>.
6deea57f
ST
2141Therefore C<-> in C<tr'''> and C<y'''> is treated literally
2142as a hyphen and no character range is available.
2143C<\1> in the replacement of C<s'''> does not work as C<$1>.
89d205f2
YO
2144
2145=item C<tr///>, C<y///>
2146
6deea57f
ST
2147No variable interpolation occurs. String modifying combinations for
2148case and quoting such as C<\Q>, C<\U>, and C<\E> are not recognized.
2149The other escape sequences such as C<\200> and C<\t> and backslashed
2150characters such as C<\\> and C<\-> are converted to appropriate literals.
89d205f2
YO
2151The character C<-> is treated specially and therefore C<\-> is treated
2152as a literal C<->.
75e14d17 2153
89d205f2 2154=item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF">
75e14d17 2155
19799a22
GS
2156C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
2157converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
2158is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
6deea57f
ST
2159The other escape sequences such as C<\200> and C<\t> and backslashed
2160characters such as C<\\> and C<\-> are replaced with appropriate
2161expansions.
2a94b7ce 2162
19799a22
GS
2163Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
2164is interpolated in the usual way. Something like C<"\Q\\E"> has
2165no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
2166result is the same as for C<"\\\\E">. As a general rule, backslashes
2167between C<\Q> and C<\E> may lead to counterintuitive results. So,
2168C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
2169as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
2a94b7ce
IZ
2170
2171 $str = '\t';
2172 return "\Q$str";
2173
2174may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
2175
19799a22 2176Interpolated scalars and arrays are converted internally to the C<join> and
92d29cee 2177C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
75e14d17 2178
19799a22 2179 $foo . " XXX '" . (join $", @arr) . "'";
75e14d17 2180
19799a22 2181All operations above are performed simultaneously, left to right.
75e14d17 2182
19799a22
GS
2183Because the result of C<"\Q STRING \E"> has all metacharacters
2184quoted, there is no way to insert a literal C<$> or C<@> inside a
2185C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
2186C<"\\\$">; if not, it is interpreted as the start of an interpolated
2187scalar.
75e14d17 2188
19799a22 2189Note also that the interpolation code needs to make a decision on
89d205f2 2190where the interpolated scalar ends. For instance, whether
35f2feb0 2191C<< "a $b -> {c}" >> really means:
75e14d17
IZ
2192
2193 "a " . $b . " -> {c}";
2194
2a94b7ce 2195or:
75e14d17
IZ
2196
2197 "a " . $b -> {c};
2198
19799a22
GS
2199Most of the time, the longest possible text that does not include
2200spaces between components and which contains matching braces or
2201brackets. because the outcome may be determined by voting based
2202on heuristic estimators, the result is not strictly predictable.
2203Fortunately, it's usually correct for ambiguous cases.
75e14d17 2204
6deea57f 2205=item the replacement of C<s///>
75e14d17 2206
19799a22 2207Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
6deea57f
ST
2208happens as with C<qq//> constructs.
2209
2210It is at this step that C<\1> is begrudgingly converted to C<$1> in
2211the replacement text of C<s///>, in order to correct the incorrigible
2212I<sed> hackers who haven't picked up the saner idiom yet. A warning
2213is emitted if the C<use warnings> pragma or the B<-w> command-line flag
2214(that is, the C<$^W> variable) was set.
2215
2216=item C<RE> in C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
2217
cc74c5bd
ST
2218Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\E>,
2219and interpolation happens (almost) as with C<qq//> constructs.
2220
5d03b57c
KW
2221Processing of C<\N{...}> is also done here, and compiled into an intermediate
2222form for the regex compiler. (This is because, as mentioned below, the regex
2223compilation may be done at execution time, and C<\N{...}> is a compile-time
2224construct.)
2225
cc74c5bd
ST
2226However any other combinations of C<\> followed by a character
2227are not substituted but only skipped, in order to parse them
2228as regular expressions at the following step.
6deea57f 2229As C<\c> is skipped at this step, C<@> of C<\c@> in RE is possibly
1749ea0d 2230treated as an array symbol (for example C<@foo>),
6deea57f 2231even though the same text in C<qq//> gives interpolation of C<\c@>.
6deea57f
ST
2232
2233Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
19799a22
GS
2234a C<#>-comment in a C<//x>-regular expression, no processing is
2235performed whatsoever. This is the first step at which the presence
2236of the C<//x> modifier is relevant.
2237
1749ea0d
ST
2238Interpolation in patterns has several quirks: C<$|>, C<$(>, C<$)>, C<@+>
2239and C<@-> are not interpolated, and constructs C<$var[SOMETHING]> are
2240voted (by several different estimators) to be either an array element
2241or C<$var> followed by an RE alternative. This is where the notation
19799a22
GS
2242C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
2243array element C<-9>, not as a regular expression from the variable
2244C<$arr> followed by a digit, which would be the interpretation of
2245C</$arr[0-9]/>. Since voting among different estimators may occur,
2246the result is not predictable.
2247
19799a22
GS
2248The lack of processing of C<\\> creates specific restrictions on
2249the post-processed text. If the delimiter is C</>, one cannot get
2250the combination C<\/> into the result of this step. C</> will
2251finish the regular expression, C<\/> will be stripped to C</> on
2252the previous step, and C<\\/> will be left as is. Because C</> is
2253equivalent to C<\/> inside a regular expression, this does not
2254matter unless the delimiter happens to be character special to the
2255RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
2256alphanumeric char, as in:
2a94b7ce
IZ
2257
2258 m m ^ a \s* b mmx;
2259
19799a22 2260In the RE above, which is intentionally obfuscated for illustration, the
6deea57f 2261delimiter is C<m>, the modifier is C<mx>, and after delimiter-removal the
89d205f2 2262RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
19799a22
GS
2263reason you're encouraged to restrict your delimiters to non-alphanumeric,
2264non-whitespace choices.
75e14d17
IZ
2265
2266=back
2267
19799a22 2268This step is the last one for all constructs except regular expressions,
75e14d17
IZ
2269which are processed further.
2270
6deea57f
ST
2271=item parsing regular expressions
2272X<regexp, parse>
75e14d17 2273
19799a22 2274Previous steps were performed during the compilation of Perl code,
ac036724 2275but this one happens at run time, although it may be optimized to
19799a22 2276be calculated at compile time if appropriate. After preprocessing
6deea57f 2277described above, and possibly after evaluation if concatenation,
19799a22
GS
2278joining, casing translation, or metaquoting are involved, the
2279resulting I<string> is passed to the RE engine for compilation.
2280
2281Whatever happens in the RE engine might be better discussed in L<perlre>,
2282but for the sake of continuity, we shall do so here.
2283
2284This is another step where the presence of the C<//x> modifier is
2285relevant. The RE engine scans the string from left to right and
2286converts it to a finite automaton.
2287
2288Backslashed characters are either replaced with corresponding
2289literal strings (as with C<\{>), or else they generate special nodes
2290in the finite automaton (as with C<\b>). Characters special to the
2291RE engine (such as C<|>) generate corresponding nodes or groups of
2292nodes. C<(?#...)> comments are ignored. All the rest is either
2293converted to literal strings to match, or else is ignored (as is
2294whitespace and C<#>-style comments if C<//x> is present).
2295
2296Parsing of the bracketed character class construct, C<[...]>, is
2297rather different than the rule used for the rest of the pattern.
2298The terminator of this construct is found using the same rules as
2299for finding the terminator of a C<{}>-delimited construct, the only
2300exception being that C<]> immediately following C<[> is treated as
2301though preceded by a backslash. Similarly, the terminator of
2302C<(?{...})> is found using the same rules as for finding the
2303terminator of a C<{}>-delimited construct.
2304
2305It is possible to inspect both the string given to RE engine and the
2306resulting finite automaton. See the arguments C<debug>/C<debugcolor>
2307in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
4a4eefd0 2308switch documented in L<perlrun/"Command Switches">.
75e14d17
IZ
2309
2310=item Optimization of regular expressions
d74e8afc 2311X<regexp, optimization>
75e14d17 2312
7522fed5 2313This step is listed for completeness only. Since it does not change
75e14d17 2314semantics, details of this step are not documented and are subject
19799a22
GS
2315to change without notice. This step is performed over the finite
2316automaton that was generated during the previous pass.
2a94b7ce 2317
19799a22
GS
2318It is at this stage that C<split()> silently optimizes C</^/> to
2319mean C</^/m>.
75e14d17
IZ
2320
2321=back
2322
a0d0e21e 2323=head2 I/O Operators
d74e8afc
ITB
2324X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle>
2325X<< <> >> X<@ARGV>
a0d0e21e 2326
54310121 2327There are several I/O operators you should know about.
fbad3eb5 2328
7b8d334a 2329A string enclosed by backticks (grave accents) first undergoes
19799a22
GS
2330double-quote interpolation. It is then interpreted as an external
2331command, and the output of that command is the value of the
e9c56f9b
JH
2332backtick string, like in a shell. In scalar context, a single string
2333consisting of all output is returned. In list context, a list of
2334values is returned, one per line of output. (You can set C<$/> to use
2335a different line terminator.) The command is executed each time the
2336pseudo-literal is evaluated. The status value of the command is
2337returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
2338Unlike in B<csh>, no translation is done on the return data--newlines
2339remain newlines. Unlike in any of the shells, single quotes do not
2340hide variable names in the command from interpretation. To pass a
2341literal dollar-sign through to the shell you need to hide it with a
2342backslash. The generalized form of backticks is C<qx//>. (Because
2343backticks always undergo shell expansion as well, see L<perlsec> for
2344security concerns.)
d74e8afc 2345X<qx> X<`> X<``> X<backtick> X<glob>
19799a22
GS
2346
2347In scalar context, evaluating a filehandle in angle brackets yields
2348the next line from that file (the newline, if any, included), or
2349C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
2350(sometimes known as file-slurp mode) and the file is empty, it
2351returns C<''> the first time, followed by C<undef> subsequently.
2352
2353Ordinarily you must assign the returned value to a variable, but
2354there is one situation where an automatic assignment happens. If
2355and only if the input symbol is the only thing inside the conditional
2356of a C<while> statement (even if disguised as a C<for(;;)> loop),
2357the value is automatically assigned to the global variable $_,
2358destroying whatever was there previously. (This may seem like an
2359odd thing to you, but you'll use the construct in almost every Perl
17b829fa 2360script you write.) The $_ variable is not implicitly localized.
19799a22
GS
2361You'll have to put a C<local $_;> before the loop if you want that
2362to happen.
2363
2364The following lines are equivalent:
a0d0e21e 2365
748a9306 2366 while (defined($_ = <STDIN>)) { print; }
7b8d334a 2367 while ($_ = <STDIN>) { print; }
a0d0e21e
LW
2368 while (<STDIN>) { print; }
2369 for (;<STDIN>;) { print; }
748a9306 2370 print while defined($_ = <STDIN>);
7b8d334a 2371 print while ($_ = <STDIN>);
a0d0e21e
LW
2372 print while <STDIN>;
2373
19799a22 2374This also behaves similarly, but avoids $_ :
7b8d334a 2375
89d205f2 2376 while (my $line = <STDIN>) { print $line }
7b8d334a 2377
19799a22
GS
2378In these loop constructs, the assigned value (whether assignment
2379is automatic or explicit) is then tested to see whether it is
2380defined. The defined test avoids problems where line has a string
2381value that would be treated as false by Perl, for example a "" or
2382a "0" with no trailing newline. If you really mean for such values
2383to terminate the loop, they should be tested for explicitly:
7b8d334a
GS
2384
2385 while (($_ = <STDIN>) ne '0') { ... }
2386 while (<STDIN>) { last unless $_; ... }
2387
5ef4d93e 2388In other boolean contexts, C<< <filehandle> >> without an
2389explicit C<defined> test or comparison elicits a warning if the
9f1b1f2d 2390C<use warnings> pragma or the B<-w>
19799a22 2391command-line switch (the C<$^W> variable) is in effect.
7b8d334a 2392
5f05dabc 2393The filehandles STDIN, STDOUT, and STDERR are predefined. (The
19799a22
GS
2394filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
2395in packages, where they would be interpreted as local identifiers
2396rather than global.) Additional filehandles may be created with
2397the open() function, amongst others. See L<perlopentut> and
2398L<perlfunc/open> for details on this.
d74e8afc 2399X<stdin> X<stdout> X<sterr>
a0d0e21e 2400
35f2feb0 2401If a <FILEHANDLE> is used in a context that is looking for
19799a22
GS
2402a list, a list comprising all input lines is returned, one line per
2403list element. It's easy to grow to a rather large data space this
2404way, so use with care.
a0d0e21e 2405
35f2feb0 2406<FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
19799a22 2407See L<perlfunc/readline>.
fbad3eb5 2408
35f2feb0
GS
2409The null filehandle <> is special: it can be used to emulate the
2410behavior of B<sed> and B<awk>. Input from <> comes either from
a0d0e21e 2411standard input, or from each file listed on the command line. Here's
35f2feb0 2412how it works: the first time <> is evaluated, the @ARGV array is
5a964f20 2413checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
a0d0e21e
LW
2414gives you standard input. The @ARGV array is then processed as a list
2415of filenames. The loop
2416
2417 while (<>) {
2418 ... # code for each line
2419 }
2420
2421is equivalent to the following Perl-like pseudo code:
2422
3e3baf6d 2423 unshift(@ARGV, '-') unless @ARGV;
a0d0e21e
LW
2424 while ($ARGV = shift) {
2425 open(ARGV, $ARGV);
2426 while (<ARGV>) {
2427 ... # code for each line
2428 }
2429 }
2430
19799a22
GS
2431except that it isn't so cumbersome to say, and will actually work.
2432It really does shift the @ARGV array and put the current filename
2433into the $ARGV variable. It also uses filehandle I<ARGV>
ac036724 2434internally. <> is just a synonym for <ARGV>, which
19799a22 2435is magical. (The pseudo code above doesn't work because it treats
35f2feb0 2436<ARGV> as non-magical.)
a0d0e21e 2437
48ab5743
ML
2438Since the null filehandle uses the two argument form of L<perlfunc/open>
2439it interprets special characters, so if you have a script like this:
2440
2441 while (<>) {
2442 print;
2443 }
2444
2445and call it with C<perl dangerous.pl 'rm -rfv *|'>, it actually opens a
2446pipe, executes the C<rm> command and reads C<rm>'s output from that pipe.
2447If you want all items in C<@ARGV> to be interpreted as file names, you
2448can use the module C<ARGV::readonly> from CPAN.
2449
35f2feb0 2450You can modify @ARGV before the first <> as long as the array ends up
a0d0e21e 2451containing the list of filenames you really want. Line numbers (C<$.>)
19799a22
GS
2452continue as though the input were one big happy file. See the example
2453in L<perlfunc/eof> for how to reset line numbers on each file.
5a964f20 2454
89d205f2 2455If you want to set @ARGV to your own list of files, go right ahead.
5a964f20
TC
2456This sets @ARGV to all plain text files if no @ARGV was given:
2457
2458 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
a0d0e21e 2459
5a964f20
TC
2460You can even set them to pipe commands. For example, this automatically
2461filters compressed arguments through B<gzip>:
2462
2463 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
2464
2465If you want to pass switches into your script, you can use one of the
a0d0e21e
LW
2466Getopts modules or put a loop on the front like this:
2467
2468 while ($_ = $ARGV[0], /^-/) {
2469 shift;
2470 last if /^--$/;
2471 if (/^-D(.*)/) { $debug = $1 }
2472 if (/^-v/) { $verbose++ }
5a964f20 2473 # ... # other switches
a0d0e21e 2474 }
5a964f20 2475
a0d0e21e 2476 while (<>) {
5a964f20 2477 # ... # code for each line
a0d0e21e
LW
2478 }
2479
89d205f2
YO
2480The <> symbol will return C<undef> for end-of-file only once.
2481If you call it again after this, it will assume you are processing another
19799a22 2482@ARGV list, and if you haven't set @ARGV, will read input from STDIN.
a0d0e21e 2483
b159ebd3 2484If what the angle brackets contain is a simple scalar variable (e.g.,
35f2feb0 2485<$foo>), then that variable contains the name of the
19799a22
GS
2486filehandle to input from, or its typeglob, or a reference to the
2487same. For example:
cb1a09d0
AD
2488
2489 $fh = \*STDIN;
2490 $line = <$fh>;
a0d0e21e 2491
5a964f20
TC
2492If what's within the angle brackets is neither a filehandle nor a simple
2493scalar variable containing a filehandle name, typeglob, or typeglob
2494reference, it is interpreted as a filename pattern to be globbed, and
2495either a list of filenames or the next filename in the list is returned,
19799a22 2496depending on context. This distinction is determined on syntactic
35f2feb0
GS
2497grounds alone. That means C<< <$x> >> is always a readline() from
2498an indirect handle, but C<< <$hash{key}> >> is always a glob().
5a964f20 2499That's because $x is a simple scalar variable, but C<$hash{key}> is
ef191992
YST
2500not--it's a hash element. Even C<< <$x > >> (note the extra space)
2501is treated as C<glob("$x ")>, not C<readline($x)>.
5a964f20
TC
2502
2503One level of double-quote interpretation is done first, but you can't
35f2feb0 2504say C<< <$foo> >> because that's an indirect filehandle as explained
5a964f20
TC
2505in the previous paragraph. (In older versions of Perl, programmers
2506would insert curly brackets to force interpretation as a filename glob:
35f2feb0 2507C<< <${foo}> >>. These days, it's considered cleaner to call the
5a964f20 2508internal function directly as C<glob($foo)>, which is probably the right
19799a22 2509way to have done it in the first place.) For example:
a0d0e21e
LW
2510
2511 while (<*.c>) {
2512 chmod 0644, $_;
2513 }
2514
3a4b19e4 2515is roughly equivalent to:
a0d0e21e
LW
2516
2517 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
2518 while (<FOO>) {
5b3eff12 2519 chomp;
a0d0e21e
LW
2520 chmod 0644, $_;
2521 }
2522
3a4b19e4
GS
2523except that the globbing is actually done internally using the standard
2524C<File::Glob> extension. Of course, the shortest way to do the above is:
a0d0e21e
LW
2525
2526 chmod 0644, <*.c>;
2527
19799a22
GS
2528A (file)glob evaluates its (embedded) argument only when it is
2529starting a new list. All values must be read before it will start
2530over. In list context, this isn't important because you automatically
2531get them all anyway. However, in scalar context the operator returns
069e01df 2532the next value each time it's called, or C<undef> when the list has
19799a22
GS
2533run out. As with filehandle reads, an automatic C<defined> is
2534generated when the glob occurs in the test part of a C<while>,
2535because legal glob returns (e.g. a file called F<0>) would otherwise
2536terminate the loop. Again, C<undef> is returned only once. So if
2537you're expecting a single value from a glob, it is much better to
2538say
4633a7c4
LW
2539
2540 ($file) = <blurch*>;
2541
2542than
2543
2544 $file = <blurch*>;
2545
2546because the latter will alternate between returning a filename and
19799a22 2547returning false.
4633a7c4 2548
b159ebd3 2549If you're trying to do variable interpolation, it's definitely better
4633a7c4 2550to use the glob() function, because the older notation can cause people
e37d713d 2551to become confused with the indirect filehandle notation.
4633a7c4
LW
2552
2553 @files = glob("$dir/*.[ch]");
2554 @files = glob($files[$i]);
2555
a0d0e21e 2556=head2 Constant Folding
d74e8afc 2557X<constant folding> X<folding>
a0d0e21e
LW
2558
2559Like C, Perl does a certain amount of expression evaluation at
19799a22 2560compile time whenever it determines that all arguments to an
a0d0e21e
LW
2561operator are static and have no side effects. In particular, string
2562concatenation happens at compile time between literals that don't do
19799a22 2563variable substitution. Backslash interpolation also happens at
a0d0e21e
LW
2564compile time. You can say
2565
2566 'Now is the time for all' . "\n" .
2567 'good men to come to.'
2568
54310121 2569and this all reduces to one string internally. Likewise, if
a0d0e21e
LW
2570you say
2571
2572 foreach $file (@filenames) {
5a964f20 2573 if (-s $file > 5 + 100 * 2**16) { }
54310121 2574 }
a0d0e21e 2575
19799a22
GS
2576the compiler will precompute the number which that expression
2577represents so that the interpreter won't have to.
a0d0e21e 2578
fd1abbef 2579=head2 No-ops
d74e8afc 2580X<no-op> X<nop>
fd1abbef
DN
2581
2582Perl doesn't officially have a no-op operator, but the bare constants
2583C<0> and C<1> are special-cased to not produce a warning in a void
2584context, so you can for example safely do
2585
2586 1 while foo();
2587
2c268ad5 2588=head2 Bitwise String Operators
d74e8afc 2589X<operator, bitwise, string>
2c268ad5
TP
2590
2591Bitstrings of any size may be manipulated by the bitwise operators
2592(C<~ | & ^>).
2593
19799a22
GS
2594If the operands to a binary bitwise op are strings of different
2595sizes, B<|> and B<^> ops act as though the shorter operand had
2596additional zero bits on the right, while the B<&> op acts as though
2597the longer operand were truncated to the length of the shorter.
2598The granularity for such extension or truncation is one or more
2599bytes.
2c268ad5 2600
89d205f2 2601 # ASCII-based examples
2c268ad5
TP
2602 print "j p \n" ^ " a h"; # prints "JAPH\n"
2603 print "JA" | " ph\n"; # prints "japh\n"
2604 print "japh\nJunk" & '_____'; # prints "JAPH\n";
2605 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
2606
19799a22 2607If you are intending to manipulate bitstrings, be certain that
2c268ad5 2608you're supplying bitstrings: If an operand is a number, that will imply
19799a22 2609a B<numeric> bitwise operation. You may explicitly show which type of
2c268ad5
TP
2610operation you intend by using C<""> or C<0+>, as in the examples below.
2611
4358a253
SS
2612 $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
2613 $foo = '150' | 105; # yields 255
2c268ad5
TP
2614 $foo = 150 | '105'; # yields 255
2615 $foo = '150' | '105'; # yields string '155' (under ASCII)
2616
2617 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
2618 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
a0d0e21e 2619
1ae175c8
GS
2620See L<perlfunc/vec> for information on how to manipulate individual bits
2621in a bit vector.
2622
55497cff 2623=head2 Integer Arithmetic
d74e8afc 2624X<integer>
a0d0e21e 2625
19799a22 2626By default, Perl assumes that it must do most of its arithmetic in
a0d0e21e
LW
2627floating point. But by saying
2628
2629 use integer;
2630
2631you may tell the compiler that it's okay to use integer operations
19799a22
GS
2632(if it feels like it) from here to the end of the enclosing BLOCK.
2633An inner BLOCK may countermand this by saying
a0d0e21e
LW
2634
2635 no integer;
2636
19799a22
GS
2637which lasts until the end of that BLOCK. Note that this doesn't
2638mean everything is only an integer, merely that Perl may use integer
2639operations if it is so inclined. For example, even under C<use
2640integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731>
2641or so.
2642
2643Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
89d205f2 2644and ">>") always produce integral results. (But see also
13a2d996 2645L<Bitwise String Operators>.) However, C<use integer> still has meaning for
19799a22
GS
2646them. By default, their results are interpreted as unsigned integers, but
2647if C<use integer> is in effect, their results are interpreted
2648as signed integers. For example, C<~0> usually evaluates to a large
0be96356 2649integral value. However, C<use integer; ~0> is C<-1> on two's-complement
19799a22 2650machines.
68dc0745
PP
2651
2652=head2 Floating-point Arithmetic
d74e8afc 2653X<floating-point> X<floating point> X<float> X<real>
68dc0745
PP
2654
2655While C<use integer> provides integer-only arithmetic, there is no
19799a22
GS
2656analogous mechanism to provide automatic rounding or truncation to a
2657certain number of decimal places. For rounding to a certain number
2658of digits, sprintf() or printf() is usually the easiest route.
2659See L<perlfaq4>.
68dc0745 2660
5a964f20
TC
2661Floating-point numbers are only approximations to what a mathematician
2662would call real numbers. There are infinitely more reals than floats,
2663so some corners must be cut. For example:
2664
2665 printf "%.20g\n", 123456789123456789;
2666 # produces 123456789123456784
2667
8548cb57
RGS
2668Testing for exact floating-point equality or inequality is not a
2669good idea. Here's a (relatively expensive) work-around to compare
5a964f20
TC
2670whether two floating-point numbers are equal to a particular number of
2671decimal places. See Knuth, volume II, for a more robust treatment of
2672this topic.
2673
2674 sub fp_equal {
2675 my ($X, $Y, $POINTS) = @_;
2676 my ($tX, $tY);
2677 $tX = sprintf("%.${POINTS}g", $X);
2678 $tY = sprintf("%.${POINTS}g", $Y);
2679 return $tX eq $tY;
2680 }
2681
68dc0745 2682The POSIX module (part of the standard perl distribution) implements
19799a22
GS
2683ceil(), floor(), and other mathematical and trigonometric functions.
2684The Math::Complex module (part of the standard perl distribution)
2685defines mathematical functions that work on both the reals and the
2686imaginary numbers. Math::Complex not as efficient as POSIX, but
68dc0745
PP
2687POSIX can't work with complex numbers.
2688
2689Rounding in financial applications can have serious implications, and
2690the rounding method used should be specified precisely. In these
2691cases, it probably pays not to trust whichever system rounding is
2692being used by Perl, but to instead implement the rounding function you
2693need yourself.
5a964f20
TC
2694
2695=head2 Bigger Numbers
d74e8afc 2696X<number, arbitrary precision>
5a964f20
TC
2697
2698The standard Math::BigInt and Math::BigFloat modules provide
19799a22 2699variable-precision arithmetic and overloaded operators, although
cd5c4fce 2700they're currently pretty slow. At the cost of some space and
19799a22
GS
2701considerable speed, they avoid the normal pitfalls associated with
2702limited-precision representations.
5a964f20
TC
2703
2704 use Math::BigInt;
2705 $x = Math::BigInt->new('123456789123456789');
2706 print $x * $x;
2707
2708 # prints +15241578780673678515622620750190521
19799a22 2709
cd5c4fce
T
2710There are several modules that let you calculate with (bound only by
2711memory and cpu-time) unlimited or fixed precision. There are also
2712some non-standard modules that provide faster implementations via
2713external C libraries.
2714
2715Here is a short, but incomplete summary:
2716
950b09ed
KW
2717 Math::Fraction big, unlimited fractions like 9973 / 12967
2718 Math::String treat string sequences like numbers
2719 Math::FixedPrecision calculate with a fixed precision
2720 Math::Currency for currency calculations
2721 Bit::Vector manipulate bit vectors fast (uses C)
2722 Math::BigIntFast Bit::Vector wrapper for big numbers
2723 Math::Pari provides access to the Pari C library
2724 Math::BigInteger uses an external C library
2725 Math::Cephes uses external Cephes C library (no big numbers)
2726 Math::Cephes::Fraction fractions via the Cephes library
2727 Math::GMP another one using an external C library
cd5c4fce
T
2728
2729Choose wisely.
16070b82
GS
2730
2731=cut