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Re: [perl #41215] % on scalars sometimes throws away fractions
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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
c963b151 56 left or xor err
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57
58In the following sections, these operators are covered in precedence order.
59
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60Many operators can be overloaded for objects. See L<overload>.
61
a0d0e21e 62=head2 Terms and List Operators (Leftward)
d74e8afc 63X<list operator> X<operator, list> X<term>
a0d0e21e 64
62c18ce2 65A TERM has the highest precedence in Perl. They include variables,
5f05dabc 66quote and quote-like operators, any expression in parentheses,
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67and any function whose arguments are parenthesized. Actually, there
68aren't really functions in this sense, just list operators and unary
69operators behaving as functions because you put parentheses around
70the arguments. These are all documented in L<perlfunc>.
71
72If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
73is followed by a left parenthesis as the next token, the operator and
74arguments within parentheses are taken to be of highest precedence,
75just like a normal function call.
76
77In the absence of parentheses, the precedence of list operators such as
78C<print>, C<sort>, or C<chmod> is either very high or very low depending on
54310121 79whether you are looking at the left side or the right side of the operator.
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80For example, in
81
82 @ary = (1, 3, sort 4, 2);
83 print @ary; # prints 1324
84
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85the commas on the right of the sort are evaluated before the sort,
86but the commas on the left are evaluated after. In other words,
87list operators tend to gobble up all arguments that follow, and
a0d0e21e 88then act like a simple TERM with regard to the preceding expression.
19799a22 89Be careful with parentheses:
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90
91 # These evaluate exit before doing the print:
92 print($foo, exit); # Obviously not what you want.
93 print $foo, exit; # Nor is this.
94
95 # These do the print before evaluating exit:
96 (print $foo), exit; # This is what you want.
97 print($foo), exit; # Or this.
98 print ($foo), exit; # Or even this.
99
100Also note that
101
102 print ($foo & 255) + 1, "\n";
103
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104probably doesn't do what you expect at first glance. The parentheses
105enclose the argument list for C<print> which is evaluated (printing
106the result of C<$foo & 255>). Then one is added to the return value
107of C<print> (usually 1). The result is something like this:
108
109 1 + 1, "\n"; # Obviously not what you meant.
110
111To do what you meant properly, you must write:
112
113 print(($foo & 255) + 1, "\n");
114
115See L<Named Unary Operators> for more discussion of this.
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116
117Also parsed as terms are the C<do {}> and C<eval {}> constructs, as
54310121 118well as subroutine and method calls, and the anonymous
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119constructors C<[]> and C<{}>.
120
2ae324a7 121See also L<Quote and Quote-like Operators> toward the end of this section,
da87341d 122as well as L</"I/O Operators">.
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123
124=head2 The Arrow Operator
d74e8afc 125X<arrow> X<dereference> X<< -> >>
a0d0e21e 126
35f2feb0 127"C<< -> >>" is an infix dereference operator, just as it is in C
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128and C++. If the right side is either a C<[...]>, C<{...}>, or a
129C<(...)> subscript, then the left side must be either a hard or
130symbolic reference to an array, a hash, or a subroutine respectively.
131(Or technically speaking, a location capable of holding a hard
132reference, if it's an array or hash reference being used for
133assignment.) See L<perlreftut> and L<perlref>.
a0d0e21e 134
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135Otherwise, the right side is a method name or a simple scalar
136variable containing either the method name or a subroutine reference,
137and the left side must be either an object (a blessed reference)
138or a class name (that is, a package name). See L<perlobj>.
a0d0e21e 139
5f05dabc 140=head2 Auto-increment and Auto-decrement
d74e8afc 141X<increment> X<auto-increment> X<++> X<decrement> X<auto-decrement> X<-->
a0d0e21e 142
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143"++" and "--" work as in C. That is, if placed before a variable,
144they increment or decrement the variable by one before returning the
145value, and if placed after, increment or decrement after returning the
146value.
147
148 $i = 0; $j = 0;
149 print $i++; # prints 0
150 print ++$j; # prints 1
a0d0e21e 151
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
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203non-alphabetic character (exluding "+" or "-"), Perl will attempt to convert
204the string to a numeric and the arithmetic negation is performed. If the
205string cannot be cleanly converted to a numeric, Perl will give the warning
206B<Argument "the string" isn't numeric in negation (-) at ...>.
d74e8afc 207X<-> X<negation, arithmetic>
a0d0e21e 208
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209Unary "~" performs bitwise negation, i.e., 1's complement. For
210example, C<0666 & ~027> is 0640. (See also L<Integer Arithmetic> and
211L<Bitwise String Operators>.) Note that the width of the result is
212platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64
213bits wide on a 64-bit platform, so if you are expecting a certain bit
d042e63d 214width, remember to use the & operator to mask off the excess bits.
d74e8afc 215X<~> X<negation, binary>
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216
217Unary "+" has no effect whatsoever, even on strings. It is useful
218syntactically for separating a function name from a parenthesized expression
219that would otherwise be interpreted as the complete list of function
5ba421f6 220arguments. (See examples above under L<Terms and List Operators (Leftward)>.)
d74e8afc 221X<+>
a0d0e21e 222
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223Unary "\" creates a reference to whatever follows it. See L<perlreftut>
224and L<perlref>. Do not confuse this behavior with the behavior of
225backslash within a string, although both forms do convey the notion
226of protecting the next thing from interpolation.
d74e8afc 227X<\> X<reference> X<backslash>
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228
229=head2 Binding Operators
d74e8afc 230X<binding> X<operator, binding> X<=~> X<!~>
a0d0e21e 231
c07a80fd 232Binary "=~" binds a scalar expression to a pattern match. Certain operations
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233search or modify the string $_ by default. This operator makes that kind
234of operation work on some other string. The right argument is a search
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235pattern, substitution, or transliteration. The left argument is what is
236supposed to be searched, substituted, or transliterated instead of the default
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237$_. When used in scalar context, the return value generally indicates the
238success of the operation. Behavior in list context depends on the particular
89d205f2 239operator. See L</"Regexp Quote-Like Operators"> for details and
d7782e69 240L<perlretut> for examples using these operators.
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241
242If the right argument is an expression rather than a search pattern,
2c268ad5 243substitution, or transliteration, it is interpreted as a search pattern at run
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244time. Note that this means that its contents will be interpolated twice, so
245
246 '\\' =~ q'\\';
247
248is not ok, as the regex engine will end up trying to compile the
249pattern C<\>, which it will consider a syntax error.
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250
251Binary "!~" is just like "=~" except the return value is negated in
252the logical sense.
253
254=head2 Multiplicative Operators
d74e8afc 255X<operator, multiplicative>
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256
257Binary "*" multiplies two numbers.
d74e8afc 258X<*>
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259
260Binary "/" divides two numbers.
d74e8afc 261X</> X<slash>
a0d0e21e 262
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263Binary "%" computes the modulus of two numbers. Given integer
264operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
265C<$a> minus the largest multiple of C<$b> that is not greater than
266C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
267smallest multiple of C<$b> that is not less than C<$a> (i.e. the
89b4f0ad 268result will be less than or equal to zero). If the operands
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269C<$a> and C<$b> are floating point values and the absolute value of
270C<$b> (that is C<abs($b)>) is less than C<(UV_MAX + 1)>, only
271the integer portion of C<$a> and C<$b> will be used in the operation
272(Note: here C<UV_MAX> means the maximum of the unsigned integer type).
273If the absolute value of the right operand (C<abs($b)>) is greater than
274or equal to C<(UV_MAX + 1)>, "%" computes the floating-point remainder
275C<$r> in the equation C<($r = $a - $i*$b)> where C<$i> is a certain
276integer that makes C<$r> should have the same sign as the right operand
277C<$b> (B<not> as the left operand C<$a> like C function C<fmod()>)
278and the absolute value less than that of C<$b>.
0412d526 279Note that when C<use integer> is in scope, "%" gives you direct access
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280to the modulus operator as implemented by your C compiler. This
281operator is not as well defined for negative operands, but it will
282execute faster.
d74e8afc 283X<%> X<remainder> X<modulus> X<mod>
55d729e4 284
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285Binary "x" is the repetition operator. In scalar context or if the left
286operand is not enclosed in parentheses, it returns a string consisting
287of the left operand repeated the number of times specified by the right
288operand. In list context, if the left operand is enclosed in
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289parentheses or is a list formed by C<qw/STRING/>, it repeats the list.
290If the right operand is zero or negative, it returns an empty string
291or an empty list, depending on the context.
d74e8afc 292X<x>
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293
294 print '-' x 80; # print row of dashes
295
296 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
297
298 @ones = (1) x 80; # a list of 80 1's
299 @ones = (5) x @ones; # set all elements to 5
300
301
302=head2 Additive Operators
d74e8afc 303X<operator, additive>
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304
305Binary "+" returns the sum of two numbers.
d74e8afc 306X<+>
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307
308Binary "-" returns the difference of two numbers.
d74e8afc 309X<->
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310
311Binary "." concatenates two strings.
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312X<string, concatenation> X<concatenation>
313X<cat> X<concat> X<concatenate> X<.>
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314
315=head2 Shift Operators
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316X<shift operator> X<operator, shift> X<<< << >>>
317X<<< >> >>> X<right shift> X<left shift> X<bitwise shift>
318X<shl> X<shr> X<shift, right> X<shift, left>
a0d0e21e 319
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320Binary "<<" returns the value of its left argument shifted left by the
321number of bits specified by the right argument. Arguments should be
982ce180 322integers. (See also L<Integer Arithmetic>.)
a0d0e21e 323
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324Binary ">>" returns the value of its left argument shifted right by
325the number of bits specified by the right argument. Arguments should
982ce180 326be integers. (See also L<Integer Arithmetic>.)
a0d0e21e 327
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328Note that both "<<" and ">>" in Perl are implemented directly using
329"<<" and ">>" in C. If C<use integer> (see L<Integer Arithmetic>) is
330in force then signed C integers are used, else unsigned C integers are
331used. Either way, the implementation isn't going to generate results
332larger than the size of the integer type Perl was built with (32 bits
333or 64 bits).
334
335The result of overflowing the range of the integers is undefined
336because it is undefined also in C. In other words, using 32-bit
337integers, C<< 1 << 32 >> is undefined. Shifting by a negative number
338of bits is also undefined.
339
a0d0e21e 340=head2 Named Unary Operators
d74e8afc 341X<operator, named unary>
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342
343The various named unary operators are treated as functions with one
568e6d8b 344argument, with optional parentheses.
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345
346If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
347is followed by a left parenthesis as the next token, the operator and
348arguments within parentheses are taken to be of highest precedence,
3981b0eb 349just like a normal function call. For example,
350because named unary operators are higher precedence than ||:
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351
352 chdir $foo || die; # (chdir $foo) || die
353 chdir($foo) || die; # (chdir $foo) || die
354 chdir ($foo) || die; # (chdir $foo) || die
355 chdir +($foo) || die; # (chdir $foo) || die
356
3981b0eb 357but, because * is higher precedence than named operators:
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358
359 chdir $foo * 20; # chdir ($foo * 20)
360 chdir($foo) * 20; # (chdir $foo) * 20
361 chdir ($foo) * 20; # (chdir $foo) * 20
362 chdir +($foo) * 20; # chdir ($foo * 20)
363
364 rand 10 * 20; # rand (10 * 20)
365 rand(10) * 20; # (rand 10) * 20
366 rand (10) * 20; # (rand 10) * 20
367 rand +(10) * 20; # rand (10 * 20)
368
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369Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are
370treated like named unary operators, but they don't follow this functional
371parenthesis rule. That means, for example, that C<-f($file).".bak"> is
372equivalent to C<-f "$file.bak">.
d74e8afc 373X<-X> X<filetest> X<operator, filetest>
568e6d8b 374
5ba421f6 375See also L<"Terms and List Operators (Leftward)">.
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376
377=head2 Relational Operators
d74e8afc 378X<relational operator> X<operator, relational>
a0d0e21e 379
35f2feb0 380Binary "<" returns true if the left argument is numerically less than
a0d0e21e 381the right argument.
d74e8afc 382X<< < >>
a0d0e21e 383
35f2feb0 384Binary ">" returns true if the left argument is numerically greater
a0d0e21e 385than the right argument.
d74e8afc 386X<< > >>
a0d0e21e 387
35f2feb0 388Binary "<=" returns true if the left argument is numerically less than
a0d0e21e 389or equal to the right argument.
d74e8afc 390X<< <= >>
a0d0e21e 391
35f2feb0 392Binary ">=" returns true if the left argument is numerically greater
a0d0e21e 393than or equal to the right argument.
d74e8afc 394X<< >= >>
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395
396Binary "lt" returns true if the left argument is stringwise less than
397the right argument.
d74e8afc 398X<< lt >>
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399
400Binary "gt" returns true if the left argument is stringwise greater
401than the right argument.
d74e8afc 402X<< gt >>
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403
404Binary "le" returns true if the left argument is stringwise less than
405or equal to the right argument.
d74e8afc 406X<< le >>
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407
408Binary "ge" returns true if the left argument is stringwise greater
409than or equal to the right argument.
d74e8afc 410X<< ge >>
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411
412=head2 Equality Operators
d74e8afc 413X<equality> X<equal> X<equals> X<operator, equality>
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414
415Binary "==" returns true if the left argument is numerically equal to
416the right argument.
d74e8afc 417X<==>
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418
419Binary "!=" returns true if the left argument is numerically not equal
420to the right argument.
d74e8afc 421X<!=>
a0d0e21e 422
35f2feb0 423Binary "<=>" returns -1, 0, or 1 depending on whether the left
6ee5d4e7 424argument is numerically less than, equal to, or greater than the right
d4ad863d 425argument. If your platform supports NaNs (not-a-numbers) as numeric
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426values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
427"<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
428returns true, as does NaN != anything else. If your platform doesn't
429support NaNs then NaN is just a string with numeric value 0.
d74e8afc 430X<< <=> >> X<spaceship>
7d3a9d88 431
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432 perl -le '$a = "NaN"; print "No NaN support here" if $a == $a'
433 perl -le '$a = "NaN"; print "NaN support here" if $a != $a'
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434
435Binary "eq" returns true if the left argument is stringwise equal to
436the right argument.
d74e8afc 437X<eq>
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438
439Binary "ne" returns true if the left argument is stringwise not equal
440to the right argument.
d74e8afc 441X<ne>
a0d0e21e 442
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443Binary "cmp" returns -1, 0, or 1 depending on whether the left
444argument is stringwise less than, equal to, or greater than the right
445argument.
d74e8afc 446X<cmp>
a0d0e21e 447
0d863452 448Binary "~~" does a smart match between its arguments. Smart matching
0f7107a0 449is described in L<perlsyn/"Smart matching in detail">.
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450This operator is only available if you enable the "~~" feature:
451see L<feature> for more information.
452X<~~>
453
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454"lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
455by the current locale if C<use locale> is in effect. See L<perllocale>.
456
a0d0e21e 457=head2 Bitwise And
d74e8afc 458X<operator, bitwise, and> X<bitwise and> X<&>
a0d0e21e 459
2cdc098b 460Binary "&" returns its operands ANDed together bit by bit.
2c268ad5 461(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
a0d0e21e 462
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463Note that "&" has lower priority than relational operators, so for example
464the brackets are essential in a test like
465
466 print "Even\n" if ($x & 1) == 0;
467
a0d0e21e 468=head2 Bitwise Or and Exclusive Or
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469X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor>
470X<bitwise xor> X<^>
a0d0e21e 471
2cdc098b 472Binary "|" returns its operands ORed together bit by bit.
2c268ad5 473(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
a0d0e21e 474
2cdc098b 475Binary "^" returns its operands XORed together bit by bit.
2c268ad5 476(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
a0d0e21e 477
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478Note that "|" and "^" have lower priority than relational operators, so
479for example the brackets are essential in a test like
480
481 print "false\n" if (8 | 2) != 10;
482
a0d0e21e 483=head2 C-style Logical And
d74e8afc 484X<&&> X<logical and> X<operator, logical, and>
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485
486Binary "&&" performs a short-circuit logical AND operation. That is,
487if the left operand is false, the right operand is not even evaluated.
488Scalar or list context propagates down to the right operand if it
489is evaluated.
490
491=head2 C-style Logical Or
d74e8afc 492X<||> X<operator, logical, or>
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493
494Binary "||" performs a short-circuit logical OR operation. That is,
495if the left operand is true, the right operand is not even evaluated.
496Scalar or list context propagates down to the right operand if it
497is evaluated.
498
c963b151 499=head2 C-style Logical Defined-Or
d74e8afc 500X<//> X<operator, logical, defined-or>
c963b151
BD
501
502Although it has no direct equivalent in C, Perl's C<//> operator is related
89d205f2 503to its C-style or. In fact, it's exactly the same as C<||>, except that it
c963b151 504tests the left hand side's definedness instead of its truth. Thus, C<$a // $b>
89d205f2
YO
505is similar to C<defined($a) || $b> (except that it returns the value of C<$a>
506rather than the value of C<defined($a)>) and is exactly equivalent to
c963b151 507C<defined($a) ? $a : $b>. This is very useful for providing default values
89d205f2 508for variables. If you actually want to test if at least one of C<$a> and
d042e63d 509C<$b> is defined, use C<defined($a // $b)>.
c963b151 510
d042e63d
MS
511The C<||>, C<//> and C<&&> operators return the last value evaluated
512(unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably
513portable way to find out the home directory might be:
a0d0e21e 514
c963b151
BD
515 $home = $ENV{'HOME'} // $ENV{'LOGDIR'} //
516 (getpwuid($<))[7] // die "You're homeless!\n";
a0d0e21e 517
5a964f20
TC
518In particular, this means that you shouldn't use this
519for selecting between two aggregates for assignment:
520
521 @a = @b || @c; # this is wrong
522 @a = scalar(@b) || @c; # really meant this
523 @a = @b ? @b : @c; # this works fine, though
524
c963b151
BD
525As more readable alternatives to C<&&>, C<//> and C<||> when used for
526control flow, Perl provides C<and>, C<err> and C<or> operators (see below).
89d205f2 527The short-circuit behavior is identical. The precedence of "and", "err"
c963b151 528and "or" is much lower, however, so that you can safely use them after a
5a964f20 529list operator without the need for parentheses:
a0d0e21e
LW
530
531 unlink "alpha", "beta", "gamma"
532 or gripe(), next LINE;
533
534With the C-style operators that would have been written like this:
535
536 unlink("alpha", "beta", "gamma")
537 || (gripe(), next LINE);
538
eeb6a2c9 539Using "or" for assignment is unlikely to do what you want; see below.
5a964f20
TC
540
541=head2 Range Operators
d74e8afc 542X<operator, range> X<range> X<..> X<...>
a0d0e21e
LW
543
544Binary ".." is the range operator, which is really two different
fb53bbb2 545operators depending on the context. In list context, it returns a
54ae734e 546list of values counting (up by ones) from the left value to the right
2cdbc966 547value. If the left value is greater than the right value then it
fb53bbb2 548returns the empty list. The range operator is useful for writing
54ae734e 549C<foreach (1..10)> loops and for doing slice operations on arrays. In
2cdbc966
JD
550the current implementation, no temporary array is created when the
551range operator is used as the expression in C<foreach> loops, but older
552versions of Perl might burn a lot of memory when you write something
553like this:
a0d0e21e
LW
554
555 for (1 .. 1_000_000) {
556 # code
54310121 557 }
a0d0e21e 558
54ae734e
MG
559The range operator also works on strings, using the magical auto-increment,
560see below.
561
5a964f20 562In scalar context, ".." returns a boolean value. The operator is
a0d0e21e
LW
563bistable, like a flip-flop, and emulates the line-range (comma) operator
564of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
565own boolean state. It is false as long as its left operand is false.
566Once the left operand is true, the range operator stays true until the
567right operand is true, I<AFTER> which the range operator becomes false
19799a22 568again. It doesn't become false till the next time the range operator is
a0d0e21e
LW
569evaluated. It can test the right operand and become false on the same
570evaluation it became true (as in B<awk>), but it still returns true once.
19799a22
GS
571If you don't want it to test the right operand till the next
572evaluation, as in B<sed>, just use three dots ("...") instead of
573two. In all other regards, "..." behaves just like ".." does.
574
575The right operand is not evaluated while the operator is in the
576"false" state, and the left operand is not evaluated while the
577operator is in the "true" state. The precedence is a little lower
578than || and &&. The value returned is either the empty string for
579false, or a sequence number (beginning with 1) for true. The
580sequence number is reset for each range encountered. The final
581sequence number in a range has the string "E0" appended to it, which
582doesn't affect its numeric value, but gives you something to search
583for if you want to exclude the endpoint. You can exclude the
584beginning point by waiting for the sequence number to be greater
df5f8116
CW
585than 1.
586
587If either operand of scalar ".." is a constant expression,
588that operand is considered true if it is equal (C<==>) to the current
589input line number (the C<$.> variable).
590
591To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>,
592but that is only an issue if you use a floating point expression; when
593implicitly using C<$.> as described in the previous paragraph, the
594comparison is C<int(EXPR) == int($.)> which is only an issue when C<$.>
595is set to a floating point value and you are not reading from a file.
596Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what
597you want in scalar context because each of the operands are evaluated
598using their integer representation.
599
600Examples:
a0d0e21e
LW
601
602As a scalar operator:
603
df5f8116
CW
604 if (101 .. 200) { print; } # print 2nd hundred lines, short for
605 # if ($. == 101 .. $. == 200) ...
9f10b797
RGS
606
607 next LINE if (1 .. /^$/); # skip header lines, short for
df5f8116 608 # ... if ($. == 1 .. /^$/);
9f10b797
RGS
609 # (typically in a loop labeled LINE)
610
611 s/^/> / if (/^$/ .. eof()); # quote body
a0d0e21e 612
5a964f20
TC
613 # parse mail messages
614 while (<>) {
615 $in_header = 1 .. /^$/;
df5f8116
CW
616 $in_body = /^$/ .. eof;
617 if ($in_header) {
618 # ...
619 } else { # in body
620 # ...
621 }
5a964f20 622 } continue {
df5f8116 623 close ARGV if eof; # reset $. each file
5a964f20
TC
624 }
625
acf31ca5
SF
626Here's a simple example to illustrate the difference between
627the two range operators:
628
629 @lines = (" - Foo",
630 "01 - Bar",
631 "1 - Baz",
632 " - Quux");
633
9f10b797
RGS
634 foreach (@lines) {
635 if (/0/ .. /1/) {
acf31ca5
SF
636 print "$_\n";
637 }
638 }
639
9f10b797
RGS
640This program will print only the line containing "Bar". If
641the range operator is changed to C<...>, it will also print the
acf31ca5
SF
642"Baz" line.
643
644And now some examples as a list operator:
a0d0e21e
LW
645
646 for (101 .. 200) { print; } # print $_ 100 times
3e3baf6d 647 @foo = @foo[0 .. $#foo]; # an expensive no-op
a0d0e21e
LW
648 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
649
5a964f20 650The range operator (in list context) makes use of the magical
5f05dabc 651auto-increment algorithm if the operands are strings. You
a0d0e21e
LW
652can say
653
654 @alphabet = ('A' .. 'Z');
655
54ae734e 656to get all normal letters of the English alphabet, or
a0d0e21e
LW
657
658 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
659
660to get a hexadecimal digit, or
661
662 @z2 = ('01' .. '31'); print $z2[$mday];
663
ea4f5703
YST
664to get dates with leading zeros.
665
666If the final value specified is not in the sequence that the magical
667increment would produce, the sequence goes until the next value would
668be longer than the final value specified.
669
670If the initial value specified isn't part of a magical increment
671sequence (that is, a non-empty string matching "/^[a-zA-Z]*[0-9]*\z/"),
672only the initial value will be returned. So the following will only
673return an alpha:
674
675 use charnames 'greek';
676 my @greek_small = ("\N{alpha}" .. "\N{omega}");
677
678To get lower-case greek letters, use this instead:
679
680 my @greek_small = map { chr } ( ord("\N{alpha}") .. ord("\N{omega}") );
a0d0e21e 681
df5f8116
CW
682Because each operand is evaluated in integer form, C<2.18 .. 3.14> will
683return two elements in list context.
684
685 @list = (2.18 .. 3.14); # same as @list = (2 .. 3);
686
a0d0e21e 687=head2 Conditional Operator
d74e8afc 688X<operator, conditional> X<operator, ternary> X<ternary> X<?:>
a0d0e21e
LW
689
690Ternary "?:" is the conditional operator, just as in C. It works much
691like an if-then-else. If the argument before the ? is true, the
692argument before the : is returned, otherwise the argument after the :
cb1a09d0
AD
693is returned. For example:
694
54310121 695 printf "I have %d dog%s.\n", $n,
cb1a09d0
AD
696 ($n == 1) ? '' : "s";
697
698Scalar or list context propagates downward into the 2nd
54310121 699or 3rd argument, whichever is selected.
cb1a09d0
AD
700
701 $a = $ok ? $b : $c; # get a scalar
702 @a = $ok ? @b : @c; # get an array
703 $a = $ok ? @b : @c; # oops, that's just a count!
704
705The operator may be assigned to if both the 2nd and 3rd arguments are
706legal lvalues (meaning that you can assign to them):
a0d0e21e
LW
707
708 ($a_or_b ? $a : $b) = $c;
709
5a964f20
TC
710Because this operator produces an assignable result, using assignments
711without parentheses will get you in trouble. For example, this:
712
713 $a % 2 ? $a += 10 : $a += 2
714
715Really means this:
716
717 (($a % 2) ? ($a += 10) : $a) += 2
718
719Rather than this:
720
721 ($a % 2) ? ($a += 10) : ($a += 2)
722
19799a22
GS
723That should probably be written more simply as:
724
725 $a += ($a % 2) ? 10 : 2;
726
4633a7c4 727=head2 Assignment Operators
d74e8afc 728X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=>
5ac3b81c 729X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=>
d74e8afc 730X<%=> X<^=> X<x=>
a0d0e21e
LW
731
732"=" is the ordinary assignment operator.
733
734Assignment operators work as in C. That is,
735
736 $a += 2;
737
738is equivalent to
739
740 $a = $a + 2;
741
742although without duplicating any side effects that dereferencing the lvalue
54310121
PP
743might trigger, such as from tie(). Other assignment operators work similarly.
744The following are recognized:
a0d0e21e
LW
745
746 **= += *= &= <<= &&=
9f10b797
RGS
747 -= /= |= >>= ||=
748 .= %= ^= //=
749 x=
a0d0e21e 750
19799a22 751Although these are grouped by family, they all have the precedence
a0d0e21e
LW
752of assignment.
753
b350dd2f
GS
754Unlike in C, the scalar assignment operator produces a valid lvalue.
755Modifying an assignment is equivalent to doing the assignment and
756then modifying the variable that was assigned to. This is useful
757for modifying a copy of something, like this:
a0d0e21e
LW
758
759 ($tmp = $global) =~ tr [A-Z] [a-z];
760
761Likewise,
762
763 ($a += 2) *= 3;
764
765is equivalent to
766
767 $a += 2;
768 $a *= 3;
769
b350dd2f
GS
770Similarly, a list assignment in list context produces the list of
771lvalues assigned to, and a list assignment in scalar context returns
772the number of elements produced by the expression on the right hand
773side of the assignment.
774
748a9306 775=head2 Comma Operator
d74e8afc 776X<comma> X<operator, comma> X<,>
a0d0e21e 777
5a964f20 778Binary "," is the comma operator. In scalar context it evaluates
a0d0e21e
LW
779its left argument, throws that value away, then evaluates its right
780argument and returns that value. This is just like C's comma operator.
781
5a964f20 782In list context, it's just the list argument separator, and inserts
ed5c6d31
PJ
783both its arguments into the list. These arguments are also evaluated
784from left to right.
a0d0e21e 785
d042e63d 786The C<< => >> operator is a synonym for the comma, but forces any word
719b43e8 787(consisting entirely of word characters) to its left to be interpreted
a44e5664
MS
788as a string (as of 5.001). This includes words that might otherwise be
789considered a constant or function call.
790
791 use constant FOO => "something";
792
793 my %h = ( FOO => 23 );
794
795is equivalent to:
796
797 my %h = ("FOO", 23);
798
799It is I<NOT>:
800
801 my %h = ("something", 23);
802
803If the argument on the left is not a word, it is first interpreted as
804an expression, and then the string value of that is used.
719b43e8
RGS
805
806The C<< => >> operator is helpful in documenting the correspondence
807between keys and values in hashes, and other paired elements in lists.
748a9306 808
a44e5664
MS
809 %hash = ( $key => $value );
810 login( $username => $password );
811
a0d0e21e 812=head2 List Operators (Rightward)
d74e8afc 813X<operator, list, rightward> X<list operator>
a0d0e21e
LW
814
815On the right side of a list operator, it has very low precedence,
816such that it controls all comma-separated expressions found there.
817The only operators with lower precedence are the logical operators
818"and", "or", and "not", which may be used to evaluate calls to list
819operators without the need for extra parentheses:
820
821 open HANDLE, "filename"
822 or die "Can't open: $!\n";
823
5ba421f6 824See also discussion of list operators in L<Terms and List Operators (Leftward)>.
a0d0e21e
LW
825
826=head2 Logical Not
d74e8afc 827X<operator, logical, not> X<not>
a0d0e21e
LW
828
829Unary "not" returns the logical negation of the expression to its right.
830It's the equivalent of "!" except for the very low precedence.
831
832=head2 Logical And
d74e8afc 833X<operator, logical, and> X<and>
a0d0e21e
LW
834
835Binary "and" returns the logical conjunction of the two surrounding
836expressions. It's equivalent to && except for the very low
5f05dabc 837precedence. This means that it short-circuits: i.e., the right
a0d0e21e
LW
838expression is evaluated only if the left expression is true.
839
c963b151 840=head2 Logical or, Defined or, and Exclusive Or
d74e8afc
ITB
841X<operator, logical, or> X<operator, logical, xor> X<operator, logical, err>
842X<operator, logical, defined or> X<operator, logical, exclusive or>
843X<or> X<xor> X<err>
a0d0e21e
LW
844
845Binary "or" returns the logical disjunction of the two surrounding
5a964f20
TC
846expressions. It's equivalent to || except for the very low precedence.
847This makes it useful for control flow
848
849 print FH $data or die "Can't write to FH: $!";
850
851This means that it short-circuits: i.e., the right expression is evaluated
852only if the left expression is false. Due to its precedence, you should
853probably avoid using this for assignment, only for control flow.
854
855 $a = $b or $c; # bug: this is wrong
856 ($a = $b) or $c; # really means this
857 $a = $b || $c; # better written this way
858
19799a22 859However, when it's a list-context assignment and you're trying to use
5a964f20
TC
860"||" for control flow, you probably need "or" so that the assignment
861takes higher precedence.
862
863 @info = stat($file) || die; # oops, scalar sense of stat!
864 @info = stat($file) or die; # better, now @info gets its due
865
c963b151
BD
866Then again, you could always use parentheses.
867
9f10b797
RGS
868Binary "err" is equivalent to C<//>--it's just like binary "or", except it
869tests its left argument's definedness instead of its truth. There are two
870ways to remember "err": either because many functions return C<undef> on
871an B<err>or, or as a sort of correction: C<$a = ($b err 'default')>. This
872keyword is only available when the 'err' feature is enabled: see
873L<feature> for more information.
a0d0e21e
LW
874
875Binary "xor" returns the exclusive-OR of the two surrounding expressions.
876It cannot short circuit, of course.
877
878=head2 C Operators Missing From Perl
d74e8afc
ITB
879X<operator, missing from perl> X<&> X<*>
880X<typecasting> X<(TYPE)>
a0d0e21e
LW
881
882Here is what C has that Perl doesn't:
883
884=over 8
885
886=item unary &
887
888Address-of operator. (But see the "\" operator for taking a reference.)
889
890=item unary *
891
54310121 892Dereference-address operator. (Perl's prefix dereferencing
a0d0e21e
LW
893operators are typed: $, @, %, and &.)
894
895=item (TYPE)
896
19799a22 897Type-casting operator.
a0d0e21e
LW
898
899=back
900
5f05dabc 901=head2 Quote and Quote-like Operators
89d205f2 902X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m>
d74e8afc
ITB
903X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>>
904X<escape sequence> X<escape>
905
a0d0e21e
LW
906
907While we usually think of quotes as literal values, in Perl they
908function as operators, providing various kinds of interpolating and
909pattern matching capabilities. Perl provides customary quote characters
910for these behaviors, but also provides a way for you to choose your
911quote character for any of them. In the following table, a C<{}> represents
9f10b797 912any pair of delimiters you choose.
a0d0e21e 913
2c268ad5
TP
914 Customary Generic Meaning Interpolates
915 '' q{} Literal no
916 "" qq{} Literal yes
af9219ee 917 `` qx{} Command yes*
2c268ad5 918 qw{} Word list no
af9219ee
MG
919 // m{} Pattern match yes*
920 qr{} Pattern yes*
921 s{}{} Substitution yes*
2c268ad5 922 tr{}{} Transliteration no (but see below)
7e3b091d 923 <<EOF here-doc yes*
a0d0e21e 924
af9219ee
MG
925 * unless the delimiter is ''.
926
87275199
GS
927Non-bracketing delimiters use the same character fore and aft, but the four
928sorts of brackets (round, angle, square, curly) will all nest, which means
9f10b797 929that
87275199 930
9f10b797 931 q{foo{bar}baz}
35f2feb0 932
9f10b797 933is the same as
87275199
GS
934
935 'foo{bar}baz'
936
937Note, however, that this does not always work for quoting Perl code:
938
939 $s = q{ if($a eq "}") ... }; # WRONG
940
83df6a1d
JH
941is a syntax error. The C<Text::Balanced> module (from CPAN, and
942starting from Perl 5.8 part of the standard distribution) is able
943to do this properly.
87275199 944
19799a22 945There can be whitespace between the operator and the quoting
fb73857a 946characters, except when C<#> is being used as the quoting character.
19799a22
GS
947C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
948operator C<q> followed by a comment. Its argument will be taken
949from the next line. This allows you to write:
fb73857a
PP
950
951 s {foo} # Replace foo
952 {bar} # with bar.
953
904501ec
MG
954The following escape sequences are available in constructs that interpolate
955and in transliterations.
d74e8afc 956X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N>
a0d0e21e 957
6ee5d4e7 958 \t tab (HT, TAB)
5a964f20 959 \n newline (NL)
6ee5d4e7
PP
960 \r return (CR)
961 \f form feed (FF)
962 \b backspace (BS)
963 \a alarm (bell) (BEL)
964 \e escape (ESC)
a0ed51b3
LW
965 \033 octal char (ESC)
966 \x1b hex char (ESC)
967 \x{263a} wide hex char (SMILEY)
19799a22 968 \c[ control char (ESC)
95cc3e0c 969 \N{name} named Unicode character
2c268ad5 970
4c77eaa2
AE
971B<NOTE>: Unlike C and other languages, Perl has no \v escape sequence for
972the vertical tab (VT - ASCII 11).
973
904501ec
MG
974The following escape sequences are available in constructs that interpolate
975but not in transliterations.
d74e8afc 976X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q>
904501ec 977
a0d0e21e
LW
978 \l lowercase next char
979 \u uppercase next char
980 \L lowercase till \E
981 \U uppercase till \E
982 \E end case modification
1d2dff63 983 \Q quote non-word characters till \E
a0d0e21e 984
95cc3e0c
JH
985If C<use locale> is in effect, the case map used by C<\l>, C<\L>,
986C<\u> and C<\U> is taken from the current locale. See L<perllocale>.
987If Unicode (for example, C<\N{}> or wide hex characters of 0x100 or
988beyond) is being used, the case map used by C<\l>, C<\L>, C<\u> and
989C<\U> is as defined by Unicode. For documentation of C<\N{name}>,
990see L<charnames>.
a034a98d 991
5a964f20
TC
992All systems use the virtual C<"\n"> to represent a line terminator,
993called a "newline". There is no such thing as an unvarying, physical
19799a22 994newline character. It is only an illusion that the operating system,
5a964f20
TC
995device drivers, C libraries, and Perl all conspire to preserve. Not all
996systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
997on a Mac, these are reversed, and on systems without line terminator,
998printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
999you mean a "newline" for your system, but use the literal ASCII when you
1000need an exact character. For example, most networking protocols expect
2a380090 1001and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
5a964f20
TC
1002and although they often accept just C<"\012">, they seldom tolerate just
1003C<"\015">. If you get in the habit of using C<"\n"> for networking,
1004you may be burned some day.
d74e8afc
ITB
1005X<newline> X<line terminator> X<eol> X<end of line>
1006X<\n> X<\r> X<\r\n>
5a964f20 1007
904501ec
MG
1008For constructs that do interpolate, variables beginning with "C<$>"
1009or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
ad0f383a
A
1010C<< $href->{key}[0] >> are also interpolated, as are array and hash slices.
1011But method calls such as C<< $obj->meth >> are not.
af9219ee
MG
1012
1013Interpolating an array or slice interpolates the elements in order,
1014separated by the value of C<$">, so is equivalent to interpolating
6deea57f
ST
1015C<join $", @array>. "Punctuation" arrays such as C<@*> are only
1016interpolated if the name is enclosed in braces C<@{*}>, but special
1017arrays C<@_>, C<@+>, and C<@-> are interpolated, even without braces.
af9219ee 1018
89d205f2
YO
1019You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
1020An unescaped C<$> or C<@> interpolates the corresponding variable,
1d2dff63 1021while escaping will cause the literal string C<\$> to be inserted.
89d205f2 1022You'll need to write something like C<m/\Quser\E\@\Qhost/>.
1d2dff63 1023
a0d0e21e
LW
1024Patterns are subject to an additional level of interpretation as a
1025regular expression. This is done as a second pass, after variables are
1026interpolated, so that regular expressions may be incorporated into the
1027pattern from the variables. If this is not what you want, use C<\Q> to
1028interpolate a variable literally.
1029
19799a22
GS
1030Apart from the behavior described above, Perl does not expand
1031multiple levels of interpolation. In particular, contrary to the
1032expectations of shell programmers, back-quotes do I<NOT> interpolate
1033within double quotes, nor do single quotes impede evaluation of
1034variables when used within double quotes.
a0d0e21e 1035
5f05dabc 1036=head2 Regexp Quote-Like Operators
d74e8afc 1037X<operator, regexp>
cb1a09d0 1038
5f05dabc 1039Here are the quote-like operators that apply to pattern
cb1a09d0
AD
1040matching and related activities.
1041
a0d0e21e
LW
1042=over 8
1043
1044=item ?PATTERN?
d74e8afc 1045X<?>
a0d0e21e
LW
1046
1047This is just like the C</pattern/> search, except that it matches only
1048once between calls to the reset() operator. This is a useful
5f05dabc 1049optimization when you want to see only the first occurrence of
a0d0e21e
LW
1050something in each file of a set of files, for instance. Only C<??>
1051patterns local to the current package are reset.
1052
5a964f20
TC
1053 while (<>) {
1054 if (?^$?) {
1055 # blank line between header and body
1056 }
1057 } continue {
1058 reset if eof; # clear ?? status for next file
1059 }
1060
483b4840 1061This usage is vaguely deprecated, which means it just might possibly
19799a22
GS
1062be removed in some distant future version of Perl, perhaps somewhere
1063around the year 2168.
a0d0e21e 1064
fb73857a 1065=item m/PATTERN/cgimosx
89d205f2
YO
1066X<m> X<operator, match>
1067X<regexp, options> X<regexp> X<regex, options> X<regex>
d74e8afc 1068X</c> X</i> X</m> X</o> X</s> X</x>
a0d0e21e 1069
fb73857a 1070=item /PATTERN/cgimosx
a0d0e21e 1071
5a964f20 1072Searches a string for a pattern match, and in scalar context returns
19799a22
GS
1073true if it succeeds, false if it fails. If no string is specified
1074via the C<=~> or C<!~> operator, the $_ string is searched. (The
1075string specified with C<=~> need not be an lvalue--it may be the
1076result of an expression evaluation, but remember the C<=~> binds
1077rather tightly.) See also L<perlre>. See L<perllocale> for
1078discussion of additional considerations that apply when C<use locale>
1079is in effect.
a0d0e21e
LW
1080
1081Options are:
1082
fb73857a 1083 c Do not reset search position on a failed match when /g is in effect.
5f05dabc 1084 g Match globally, i.e., find all occurrences.
a0d0e21e
LW
1085 i Do case-insensitive pattern matching.
1086 m Treat string as multiple lines.
5f05dabc 1087 o Compile pattern only once.
a0d0e21e
LW
1088 s Treat string as single line.
1089 x Use extended regular expressions.
1090
1091If "/" is the delimiter then the initial C<m> is optional. With the C<m>
89d205f2 1092you can use any pair of non-alphanumeric, non-whitespace characters
19799a22
GS
1093as delimiters. This is particularly useful for matching path names
1094that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
7bac28a0 1095the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
19799a22 1096If "'" is the delimiter, no interpolation is performed on the PATTERN.
a0d0e21e
LW
1097
1098PATTERN may contain variables, which will be interpolated (and the
f70b4f9c 1099pattern recompiled) every time the pattern search is evaluated, except
1f247705
GS
1100for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
1101C<$|> are not interpolated because they look like end-of-string tests.)
f70b4f9c
AB
1102If you want such a pattern to be compiled only once, add a C</o> after
1103the trailing delimiter. This avoids expensive run-time recompilations,
1104and is useful when the value you are interpolating won't change over
1105the life of the script. However, mentioning C</o> constitutes a promise
1106that you won't change the variables in the pattern. If you change them,
13a2d996 1107Perl won't even notice. See also L<"qr/STRING/imosx">.
a0d0e21e 1108
5a964f20 1109If the PATTERN evaluates to the empty string, the last
d65afb4b
HS
1110I<successfully> matched regular expression is used instead. In this
1111case, only the C<g> and C<c> flags on the empty pattern is honoured -
1112the other flags are taken from the original pattern. If no match has
1113previously succeeded, this will (silently) act instead as a genuine
1114empty pattern (which will always match).
a0d0e21e 1115
89d205f2
YO
1116Note that it's possible to confuse Perl into thinking C<//> (the empty
1117regex) is really C<//> (the defined-or operator). Perl is usually pretty
1118good about this, but some pathological cases might trigger this, such as
1119C<$a///> (is that C<($a) / (//)> or C<$a // />?) and C<print $fh //>
1120(C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl
1121will assume you meant defined-or. If you meant the empty regex, just
1122use parentheses or spaces to disambiguate, or even prefix the empty
c963b151
BD
1123regex with an C<m> (so C<//> becomes C<m//>).
1124
19799a22 1125If the C</g> option is not used, C<m//> in list context returns a
a0d0e21e 1126list consisting of the subexpressions matched by the parentheses in the
f7e33566
GS
1127pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are
1128also set, and that this differs from Perl 4's behavior.) When there are
1129no parentheses in the pattern, the return value is the list C<(1)> for
1130success. With or without parentheses, an empty list is returned upon
1131failure.
a0d0e21e
LW
1132
1133Examples:
1134
1135 open(TTY, '/dev/tty');
1136 <TTY> =~ /^y/i && foo(); # do foo if desired
1137
1138 if (/Version: *([0-9.]*)/) { $version = $1; }
1139
1140 next if m#^/usr/spool/uucp#;
1141
1142 # poor man's grep
1143 $arg = shift;
1144 while (<>) {
1145 print if /$arg/o; # compile only once
1146 }
1147
1148 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
1149
1150This last example splits $foo into the first two words and the
5f05dabc
PP
1151remainder of the line, and assigns those three fields to $F1, $F2, and
1152$Etc. The conditional is true if any variables were assigned, i.e., if
a0d0e21e
LW
1153the pattern matched.
1154
19799a22
GS
1155The C</g> modifier specifies global pattern matching--that is,
1156matching as many times as possible within the string. How it behaves
1157depends on the context. In list context, it returns a list of the
1158substrings matched by any capturing parentheses in the regular
1159expression. If there are no parentheses, it returns a list of all
1160the matched strings, as if there were parentheses around the whole
1161pattern.
a0d0e21e 1162
7e86de3e 1163In scalar context, each execution of C<m//g> finds the next match,
19799a22 1164returning true if it matches, and false if there is no further match.
7e86de3e
G
1165The position after the last match can be read or set using the pos()
1166function; see L<perlfunc/pos>. A failed match normally resets the
1167search position to the beginning of the string, but you can avoid that
1168by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
1169string also resets the search position.
c90c0ff4
PP
1170
1171You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
1172zero-width assertion that matches the exact position where the previous
5d43e42d
DC
1173C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion
1174still anchors at pos(), but the match is of course only attempted once.
1175Using C<\G> without C</g> on a target string that has not previously had a
1176C</g> match applied to it is the same as using the C<\A> assertion to match
fe4b3f22
RGS
1177the beginning of the string. Note also that, currently, C<\G> is only
1178properly supported when anchored at the very beginning of the pattern.
c90c0ff4
PP
1179
1180Examples:
a0d0e21e
LW
1181
1182 # list context
1183 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
1184
1185 # scalar context
5d43e42d 1186 $/ = "";
19799a22
GS
1187 while (defined($paragraph = <>)) {
1188 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
1189 $sentences++;
a0d0e21e
LW
1190 }
1191 }
1192 print "$sentences\n";
1193
c90c0ff4 1194 # using m//gc with \G
137443ea 1195 $_ = "ppooqppqq";
44a8e56a
PP
1196 while ($i++ < 2) {
1197 print "1: '";
c90c0ff4 1198 print $1 while /(o)/gc; print "', pos=", pos, "\n";
44a8e56a 1199 print "2: '";
c90c0ff4 1200 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
44a8e56a 1201 print "3: '";
c90c0ff4 1202 print $1 while /(p)/gc; print "', pos=", pos, "\n";
44a8e56a 1203 }
5d43e42d 1204 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
44a8e56a
PP
1205
1206The last example should print:
1207
1208 1: 'oo', pos=4
137443ea 1209 2: 'q', pos=5
44a8e56a
PP
1210 3: 'pp', pos=7
1211 1: '', pos=7
137443ea
PP
1212 2: 'q', pos=8
1213 3: '', pos=8
5d43e42d
DC
1214 Final: 'q', pos=8
1215
1216Notice that the final match matched C<q> instead of C<p>, which a match
1217without the C<\G> anchor would have done. Also note that the final match
1218did not update C<pos> -- C<pos> is only updated on a C</g> match. If the
1219final match did indeed match C<p>, it's a good bet that you're running an
1220older (pre-5.6.0) Perl.
44a8e56a 1221
c90c0ff4 1222A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
e7ea3e70 1223combine several regexps like this to process a string part-by-part,
c90c0ff4
PP
1224doing different actions depending on which regexp matched. Each
1225regexp tries to match where the previous one leaves off.
e7ea3e70 1226
3fe9a6f1 1227 $_ = <<'EOL';
63acfd00 1228 $url = URI::URL->new( "http://www/" ); die if $url eq "xXx";
3fe9a6f1
PP
1229 EOL
1230 LOOP:
e7ea3e70 1231 {
c90c0ff4
PP
1232 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
1233 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
1234 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
1235 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
1236 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
1237 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
1238 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
e7ea3e70
IZ
1239 print ". That's all!\n";
1240 }
1241
1242Here is the output (split into several lines):
1243
1244 line-noise lowercase line-noise lowercase UPPERCASE line-noise
1245 UPPERCASE line-noise lowercase line-noise lowercase line-noise
1246 lowercase lowercase line-noise lowercase lowercase line-noise
1247 MiXeD line-noise. That's all!
44a8e56a 1248
a0d0e21e 1249=item q/STRING/
5d44bfff 1250X<q> X<quote, single> X<'> X<''>
a0d0e21e 1251
5d44bfff 1252=item 'STRING'
a0d0e21e 1253
19799a22 1254A single-quoted, literal string. A backslash represents a backslash
68dc0745
PP
1255unless followed by the delimiter or another backslash, in which case
1256the delimiter or backslash is interpolated.
a0d0e21e
LW
1257
1258 $foo = q!I said, "You said, 'She said it.'"!;
1259 $bar = q('This is it.');
68dc0745 1260 $baz = '\n'; # a two-character string
a0d0e21e
LW
1261
1262=item qq/STRING/
d74e8afc 1263X<qq> X<quote, double> X<"> X<"">
a0d0e21e
LW
1264
1265=item "STRING"
1266
1267A double-quoted, interpolated string.
1268
1269 $_ .= qq
1270 (*** The previous line contains the naughty word "$1".\n)
19799a22 1271 if /\b(tcl|java|python)\b/i; # :-)
68dc0745 1272 $baz = "\n"; # a one-character string
a0d0e21e 1273
eec2d3df 1274=item qr/STRING/imosx
d74e8afc 1275X<qr> X</i> X</m> X</o> X</s> X</x>
eec2d3df 1276
322edccd 1277This operator quotes (and possibly compiles) its I<STRING> as a regular
19799a22
GS
1278expression. I<STRING> is interpolated the same way as I<PATTERN>
1279in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
1280is done. Returns a Perl value which may be used instead of the
1281corresponding C</STRING/imosx> expression.
4b6a7270
IZ
1282
1283For example,
1284
1285 $rex = qr/my.STRING/is;
1286 s/$rex/foo/;
1287
1288is equivalent to
1289
1290 s/my.STRING/foo/is;
1291
1292The result may be used as a subpattern in a match:
eec2d3df
GS
1293
1294 $re = qr/$pattern/;
0a92e3a8
GS
1295 $string =~ /foo${re}bar/; # can be interpolated in other patterns
1296 $string =~ $re; # or used standalone
4b6a7270
IZ
1297 $string =~ /$re/; # or this way
1298
1299Since Perl may compile the pattern at the moment of execution of qr()
19799a22 1300operator, using qr() may have speed advantages in some situations,
4b6a7270
IZ
1301notably if the result of qr() is used standalone:
1302
1303 sub match {
1304 my $patterns = shift;
1305 my @compiled = map qr/$_/i, @$patterns;
1306 grep {
1307 my $success = 0;
a7665c5e 1308 foreach my $pat (@compiled) {
4b6a7270
IZ
1309 $success = 1, last if /$pat/;
1310 }
1311 $success;
1312 } @_;
1313 }
1314
19799a22
GS
1315Precompilation of the pattern into an internal representation at
1316the moment of qr() avoids a need to recompile the pattern every
1317time a match C</$pat/> is attempted. (Perl has many other internal
1318optimizations, but none would be triggered in the above example if
1319we did not use qr() operator.)
eec2d3df
GS
1320
1321Options are:
1322
1323 i Do case-insensitive pattern matching.
1324 m Treat string as multiple lines.
1325 o Compile pattern only once.
1326 s Treat string as single line.
1327 x Use extended regular expressions.
1328
0a92e3a8
GS
1329See L<perlre> for additional information on valid syntax for STRING, and
1330for a detailed look at the semantics of regular expressions.
1331
a0d0e21e 1332=item qx/STRING/
d74e8afc 1333X<qx> X<`> X<``> X<backtick>
a0d0e21e
LW
1334
1335=item `STRING`
1336
43dd4d21
JH
1337A string which is (possibly) interpolated and then executed as a
1338system command with C</bin/sh> or its equivalent. Shell wildcards,
1339pipes, and redirections will be honored. The collected standard
1340output of the command is returned; standard error is unaffected. In
1341scalar context, it comes back as a single (potentially multi-line)
1342string, or undef if the command failed. In list context, returns a
1343list of lines (however you've defined lines with $/ or
1344$INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
5a964f20
TC
1345
1346Because backticks do not affect standard error, use shell file descriptor
1347syntax (assuming the shell supports this) if you care to address this.
1348To capture a command's STDERR and STDOUT together:
a0d0e21e 1349
5a964f20
TC
1350 $output = `cmd 2>&1`;
1351
1352To capture a command's STDOUT but discard its STDERR:
1353
1354 $output = `cmd 2>/dev/null`;
1355
1356To capture a command's STDERR but discard its STDOUT (ordering is
1357important here):
1358
1359 $output = `cmd 2>&1 1>/dev/null`;
1360
1361To exchange a command's STDOUT and STDERR in order to capture the STDERR
1362but leave its STDOUT to come out the old STDERR:
1363
1364 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
1365
1366To read both a command's STDOUT and its STDERR separately, it's easiest
2359510d
SD
1367to redirect them separately to files, and then read from those files
1368when the program is done:
5a964f20 1369
2359510d 1370 system("program args 1>program.stdout 2>program.stderr");
5a964f20 1371
30398227
SP
1372The STDIN filehandle used by the command is inherited from Perl's STDIN.
1373For example:
1374
1375 open BLAM, "blam" || die "Can't open: $!";
1376 open STDIN, "<&BLAM";
1377 print `sort`;
1378
1379will print the sorted contents of the file "blam".
1380
5a964f20
TC
1381Using single-quote as a delimiter protects the command from Perl's
1382double-quote interpolation, passing it on to the shell instead:
1383
1384 $perl_info = qx(ps $$); # that's Perl's $$
1385 $shell_info = qx'ps $$'; # that's the new shell's $$
1386
19799a22 1387How that string gets evaluated is entirely subject to the command
5a964f20
TC
1388interpreter on your system. On most platforms, you will have to protect
1389shell metacharacters if you want them treated literally. This is in
1390practice difficult to do, as it's unclear how to escape which characters.
1391See L<perlsec> for a clean and safe example of a manual fork() and exec()
1392to emulate backticks safely.
a0d0e21e 1393
bb32b41a
GS
1394On some platforms (notably DOS-like ones), the shell may not be
1395capable of dealing with multiline commands, so putting newlines in
1396the string may not get you what you want. You may be able to evaluate
1397multiple commands in a single line by separating them with the command
1398separator character, if your shell supports that (e.g. C<;> on many Unix
1399shells; C<&> on the Windows NT C<cmd> shell).
1400
0f897271
GS
1401Beginning with v5.6.0, Perl will attempt to flush all files opened for
1402output before starting the child process, but this may not be supported
1403on some platforms (see L<perlport>). To be safe, you may need to set
1404C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1405C<IO::Handle> on any open handles.
1406
bb32b41a
GS
1407Beware that some command shells may place restrictions on the length
1408of the command line. You must ensure your strings don't exceed this
1409limit after any necessary interpolations. See the platform-specific
1410release notes for more details about your particular environment.
1411
5a964f20
TC
1412Using this operator can lead to programs that are difficult to port,
1413because the shell commands called vary between systems, and may in
1414fact not be present at all. As one example, the C<type> command under
1415the POSIX shell is very different from the C<type> command under DOS.
1416That doesn't mean you should go out of your way to avoid backticks
1417when they're the right way to get something done. Perl was made to be
1418a glue language, and one of the things it glues together is commands.
1419Just understand what you're getting yourself into.
bb32b41a 1420
da87341d 1421See L</"I/O Operators"> for more discussion.
a0d0e21e 1422
945c54fd 1423=item qw/STRING/
d74e8afc 1424X<qw> X<quote, list> X<quote, words>
945c54fd
JH
1425
1426Evaluates to a list of the words extracted out of STRING, using embedded
1427whitespace as the word delimiters. It can be understood as being roughly
1428equivalent to:
1429
1430 split(' ', q/STRING/);
1431
efb1e162
CW
1432the differences being that it generates a real list at compile time, and
1433in scalar context it returns the last element in the list. So
945c54fd
JH
1434this expression:
1435
1436 qw(foo bar baz)
1437
1438is semantically equivalent to the list:
1439
1440 'foo', 'bar', 'baz'
1441
1442Some frequently seen examples:
1443
1444 use POSIX qw( setlocale localeconv )
1445 @EXPORT = qw( foo bar baz );
1446
1447A common mistake is to try to separate the words with comma or to
1448put comments into a multi-line C<qw>-string. For this reason, the
89d205f2 1449C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
945c54fd
JH
1450produces warnings if the STRING contains the "," or the "#" character.
1451
a0d0e21e 1452=item s/PATTERN/REPLACEMENT/egimosx
d74e8afc
ITB
1453X<substitute> X<substitution> X<replace> X<regexp, replace>
1454X<regexp, substitute> X</e> X</g> X</i> X</m> X</o> X</s> X</x>
a0d0e21e
LW
1455
1456Searches a string for a pattern, and if found, replaces that pattern
1457with the replacement text and returns the number of substitutions
e37d713d 1458made. Otherwise it returns false (specifically, the empty string).
a0d0e21e
LW
1459
1460If no string is specified via the C<=~> or C<!~> operator, the C<$_>
1461variable is searched and modified. (The string specified with C<=~> must
5a964f20 1462be scalar variable, an array element, a hash element, or an assignment
5f05dabc 1463to one of those, i.e., an lvalue.)
a0d0e21e 1464
19799a22 1465If the delimiter chosen is a single quote, no interpolation is
a0d0e21e
LW
1466done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1467PATTERN contains a $ that looks like a variable rather than an
1468end-of-string test, the variable will be interpolated into the pattern
5f05dabc 1469at run-time. If you want the pattern compiled only once the first time
a0d0e21e 1470the variable is interpolated, use the C</o> option. If the pattern
5a964f20 1471evaluates to the empty string, the last successfully executed regular
a0d0e21e 1472expression is used instead. See L<perlre> for further explanation on these.
5a964f20 1473See L<perllocale> for discussion of additional considerations that apply
a034a98d 1474when C<use locale> is in effect.
a0d0e21e
LW
1475
1476Options are:
1477
1478 e Evaluate the right side as an expression.
5f05dabc 1479 g Replace globally, i.e., all occurrences.
a0d0e21e
LW
1480 i Do case-insensitive pattern matching.
1481 m Treat string as multiple lines.
5f05dabc 1482 o Compile pattern only once.
a0d0e21e
LW
1483 s Treat string as single line.
1484 x Use extended regular expressions.
1485
1486Any non-alphanumeric, non-whitespace delimiter may replace the
1487slashes. If single quotes are used, no interpretation is done on the
e37d713d 1488replacement string (the C</e> modifier overrides this, however). Unlike
54310121 1489Perl 4, Perl 5 treats backticks as normal delimiters; the replacement
e37d713d 1490text is not evaluated as a command. If the
a0d0e21e 1491PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
5f05dabc 1492pair of quotes, which may or may not be bracketing quotes, e.g.,
35f2feb0 1493C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
cec88af6
GS
1494replacement portion to be treated as a full-fledged Perl expression
1495and evaluated right then and there. It is, however, syntax checked at
1496compile-time. A second C<e> modifier will cause the replacement portion
1497to be C<eval>ed before being run as a Perl expression.
a0d0e21e
LW
1498
1499Examples:
1500
1501 s/\bgreen\b/mauve/g; # don't change wintergreen
1502
1503 $path =~ s|/usr/bin|/usr/local/bin|;
1504
1505 s/Login: $foo/Login: $bar/; # run-time pattern
1506
5a964f20 1507 ($foo = $bar) =~ s/this/that/; # copy first, then change
a0d0e21e 1508
5a964f20 1509 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
a0d0e21e
LW
1510
1511 $_ = 'abc123xyz';
1512 s/\d+/$&*2/e; # yields 'abc246xyz'
1513 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
1514 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1515
1516 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
1517 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
023a2b40 1518 s/^=(\w+)/pod($1)/ge; # use function call
a0d0e21e 1519
5a964f20
TC
1520 # expand variables in $_, but dynamics only, using
1521 # symbolic dereferencing
1522 s/\$(\w+)/${$1}/g;
1523
cec88af6
GS
1524 # Add one to the value of any numbers in the string
1525 s/(\d+)/1 + $1/eg;
1526
1527 # This will expand any embedded scalar variable
1528 # (including lexicals) in $_ : First $1 is interpolated
1529 # to the variable name, and then evaluated
a0d0e21e
LW
1530 s/(\$\w+)/$1/eeg;
1531
5a964f20 1532 # Delete (most) C comments.
a0d0e21e 1533 $program =~ s {
4633a7c4
LW
1534 /\* # Match the opening delimiter.
1535 .*? # Match a minimal number of characters.
1536 \*/ # Match the closing delimiter.
a0d0e21e
LW
1537 } []gsx;
1538
6b0ac556 1539 s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_, expensively
5a964f20 1540
6b0ac556 1541 for ($variable) { # trim whitespace in $variable, cheap
5a964f20
TC
1542 s/^\s+//;
1543 s/\s+$//;
1544 }
a0d0e21e
LW
1545
1546 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
1547
54310121 1548Note the use of $ instead of \ in the last example. Unlike
35f2feb0
GS
1549B<sed>, we use the \<I<digit>> form in only the left hand side.
1550Anywhere else it's $<I<digit>>.
a0d0e21e 1551
5f05dabc 1552Occasionally, you can't use just a C</g> to get all the changes
19799a22 1553to occur that you might want. Here are two common cases:
a0d0e21e
LW
1554
1555 # put commas in the right places in an integer
89d205f2 1556 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
a0d0e21e
LW
1557
1558 # expand tabs to 8-column spacing
1559 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
1560
6940069f 1561=item tr/SEARCHLIST/REPLACEMENTLIST/cds
d74e8afc 1562X<tr> X<y> X<transliterate> X</c> X</d> X</s>
a0d0e21e 1563
6940069f 1564=item y/SEARCHLIST/REPLACEMENTLIST/cds
a0d0e21e 1565
2c268ad5 1566Transliterates all occurrences of the characters found in the search list
a0d0e21e
LW
1567with the corresponding character in the replacement list. It returns
1568the number of characters replaced or deleted. If no string is
2c268ad5 1569specified via the =~ or !~ operator, the $_ string is transliterated. (The
54310121
PP
1570string specified with =~ must be a scalar variable, an array element, a
1571hash element, or an assignment to one of those, i.e., an lvalue.)
8ada0baa 1572
89d205f2 1573A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
2c268ad5 1574does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
54310121
PP
1575For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1576SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1577its own pair of quotes, which may or may not be bracketing quotes,
2c268ad5 1578e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
a0d0e21e 1579
cc255d5f 1580Note that C<tr> does B<not> do regular expression character classes
e0c83546 1581such as C<\d> or C<[:lower:]>. The C<tr> operator is not equivalent to
cc255d5f
JH
1582the tr(1) utility. If you want to map strings between lower/upper
1583cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider
1584using the C<s> operator if you need regular expressions.
1585
8ada0baa
JH
1586Note also that the whole range idea is rather unportable between
1587character sets--and even within character sets they may cause results
1588you probably didn't expect. A sound principle is to use only ranges
1589that begin from and end at either alphabets of equal case (a-e, A-E),
1590or digits (0-4). Anything else is unsafe. If in doubt, spell out the
1591character sets in full.
1592
a0d0e21e
LW
1593Options:
1594
1595 c Complement the SEARCHLIST.
1596 d Delete found but unreplaced characters.
1597 s Squash duplicate replaced characters.
1598
19799a22
GS
1599If the C</c> modifier is specified, the SEARCHLIST character set
1600is complemented. If the C</d> modifier is specified, any characters
1601specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
1602(Note that this is slightly more flexible than the behavior of some
1603B<tr> programs, which delete anything they find in the SEARCHLIST,
1604period.) If the C</s> modifier is specified, sequences of characters
1605that were transliterated to the same character are squashed down
1606to a single instance of the character.
a0d0e21e
LW
1607
1608If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1609exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1610than the SEARCHLIST, the final character is replicated till it is long
5a964f20 1611enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
a0d0e21e
LW
1612This latter is useful for counting characters in a class or for
1613squashing character sequences in a class.
1614
1615Examples:
1616
1617 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1618
1619 $cnt = tr/*/*/; # count the stars in $_
1620
1621 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1622
1623 $cnt = tr/0-9//; # count the digits in $_
1624
1625 tr/a-zA-Z//s; # bookkeeper -> bokeper
1626
1627 ($HOST = $host) =~ tr/a-z/A-Z/;
1628
1629 tr/a-zA-Z/ /cs; # change non-alphas to single space
1630
1631 tr [\200-\377]
1632 [\000-\177]; # delete 8th bit
1633
19799a22
GS
1634If multiple transliterations are given for a character, only the
1635first one is used:
748a9306
LW
1636
1637 tr/AAA/XYZ/
1638
2c268ad5 1639will transliterate any A to X.
748a9306 1640
19799a22 1641Because the transliteration table is built at compile time, neither
a0d0e21e 1642the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
19799a22
GS
1643interpolation. That means that if you want to use variables, you
1644must use an eval():
a0d0e21e
LW
1645
1646 eval "tr/$oldlist/$newlist/";
1647 die $@ if $@;
1648
1649 eval "tr/$oldlist/$newlist/, 1" or die $@;
1650
7e3b091d 1651=item <<EOF
d74e8afc 1652X<here-doc> X<heredoc> X<here-document> X<<< << >>>
7e3b091d
DA
1653
1654A line-oriented form of quoting is based on the shell "here-document"
1655syntax. Following a C<< << >> you specify a string to terminate
1656the quoted material, and all lines following the current line down to
89d205f2
YO
1657the terminating string are the value of the item.
1658
1659The terminating string may be either an identifier (a word), or some
1660quoted text. An unquoted identifier works like double quotes.
1661There may not be a space between the C<< << >> and the identifier,
1662unless the identifier is explicitly quoted. (If you put a space it
1663will be treated as a null identifier, which is valid, and matches the
1664first empty line.) The terminating string must appear by itself
1665(unquoted and with no surrounding whitespace) on the terminating line.
1666
1667If the terminating string is quoted, the type of quotes used determine
1668the treatment of the text.
1669
1670=over 4
1671
1672=item Double Quotes
1673
1674Double quotes indicate that the text will be interpolated using exactly
1675the same rules as normal double quoted strings.
7e3b091d
DA
1676
1677 print <<EOF;
1678 The price is $Price.
1679 EOF
1680
1681 print << "EOF"; # same as above
1682 The price is $Price.
1683 EOF
1684
89d205f2
YO
1685
1686=item Single Quotes
1687
1688Single quotes indicate the text is to be treated literally with no
1689interpolation of its content. This is similar to single quoted
1690strings except that backslashes have no special meaning, with C<\\>
1691being treated as two backslashes and not one as they would in every
1692other quoting construct.
1693
1694This is the only form of quoting in perl where there is no need
1695to worry about escaping content, something that code generators
1696can and do make good use of.
1697
1698=item Backticks
1699
1700The content of the here doc is treated just as it would be if the
1701string were embedded in backticks. Thus the content is interpolated
1702as though it were double quoted and then executed via the shell, with
1703the results of the execution returned.
1704
1705 print << `EOC`; # execute command and get results
7e3b091d 1706 echo hi there
7e3b091d
DA
1707 EOC
1708
89d205f2
YO
1709=back
1710
1711It is possible to stack multiple here-docs in a row:
1712
7e3b091d
DA
1713 print <<"foo", <<"bar"; # you can stack them
1714 I said foo.
1715 foo
1716 I said bar.
1717 bar
1718
1719 myfunc(<< "THIS", 23, <<'THAT');
1720 Here's a line
1721 or two.
1722 THIS
1723 and here's another.
1724 THAT
1725
1726Just don't forget that you have to put a semicolon on the end
1727to finish the statement, as Perl doesn't know you're not going to
1728try to do this:
1729
1730 print <<ABC
1731 179231
1732 ABC
1733 + 20;
1734
872d7e53
ST
1735If you want to remove the line terminator from your here-docs,
1736use C<chomp()>.
1737
1738 chomp($string = <<'END');
1739 This is a string.
1740 END
1741
1742If you want your here-docs to be indented with the rest of the code,
1743you'll need to remove leading whitespace from each line manually:
7e3b091d
DA
1744
1745 ($quote = <<'FINIS') =~ s/^\s+//gm;
89d205f2 1746 The Road goes ever on and on,
7e3b091d
DA
1747 down from the door where it began.
1748 FINIS
1749
1750If you use a here-doc within a delimited construct, such as in C<s///eg>,
1751the quoted material must come on the lines following the final delimiter.
1752So instead of
1753
1754 s/this/<<E . 'that'
1755 the other
1756 E
1757 . 'more '/eg;
1758
1759you have to write
1760
89d205f2
YO
1761 s/this/<<E . 'that'
1762 . 'more '/eg;
1763 the other
1764 E
7e3b091d
DA
1765
1766If the terminating identifier is on the last line of the program, you
1767must be sure there is a newline after it; otherwise, Perl will give the
1768warning B<Can't find string terminator "END" anywhere before EOF...>.
1769
89d205f2
YO
1770Additionally, the quoting rules for the end of string identifier are not
1771related to Perl's quoting rules -- C<q()>, C<qq()>, and the like are not
1772supported in place of C<''> and C<"">, and the only interpolation is for
1773backslashing the quoting character:
7e3b091d
DA
1774
1775 print << "abc\"def";
1776 testing...
1777 abc"def
1778
1779Finally, quoted strings cannot span multiple lines. The general rule is
1780that the identifier must be a string literal. Stick with that, and you
1781should be safe.
1782
a0d0e21e
LW
1783=back
1784
75e14d17 1785=head2 Gory details of parsing quoted constructs
d74e8afc 1786X<quote, gory details>
75e14d17 1787
19799a22
GS
1788When presented with something that might have several different
1789interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
1790principle to pick the most probable interpretation. This strategy
1791is so successful that Perl programmers often do not suspect the
1792ambivalence of what they write. But from time to time, Perl's
1793notions differ substantially from what the author honestly meant.
1794
1795This section hopes to clarify how Perl handles quoted constructs.
1796Although the most common reason to learn this is to unravel labyrinthine
1797regular expressions, because the initial steps of parsing are the
1798same for all quoting operators, they are all discussed together.
1799
1800The most important Perl parsing rule is the first one discussed
1801below: when processing a quoted construct, Perl first finds the end
1802of that construct, then interprets its contents. If you understand
1803this rule, you may skip the rest of this section on the first
1804reading. The other rules are likely to contradict the user's
1805expectations much less frequently than this first one.
1806
1807Some passes discussed below are performed concurrently, but because
1808their results are the same, we consider them individually. For different
1809quoting constructs, Perl performs different numbers of passes, from
6deea57f 1810one to four, but these passes are always performed in the same order.
75e14d17 1811
13a2d996 1812=over 4
75e14d17
IZ
1813
1814=item Finding the end
1815
6deea57f
ST
1816The first pass is finding the end of the quoted construct, where
1817the information about the delimiters is used in parsing.
1818During this search, text between the starting and ending delimiters
1819is copied to a safe location. The text copied gets delimiter-independent.
1820
1821If the construct is a here-doc, the ending delimiter is a line
1822that has a terminating string as the content. Therefore C<<<EOF> is
1823terminated by C<EOF> immediately followed by C<"\n"> and starting
1824from the first column of the terminating line.
1825When searching for the terminating line of a here-doc, nothing
1826is skipped. In other words, lines after the here-doc syntax
1827are compared with the terminating string line by line.
1828
1829For the constructs except here-docs, single characters are used as starting
1830and ending delimiters. If the starting delimiter is an opening punctuation
1831(that is C<(>, C<[>, C<{>, or C<< < >>), the ending delimiter is the
1832corresponding closing punctuation (that is C<)>, C<]>, C<}>, or C<< > >>).
1833If the starting delimiter is an unpaired character like C</> or a closing
1834punctuation, the ending delimiter is same as the starting delimiter.
1835Therefore a C</> terminates a C<qq//> construct, while a C<]> terminates
1836C<qq[]> and C<qq]]> constructs.
1837
1838When searching for single-character delimiters, escaped delimiters
1839and C<\\> are skipped. For example, while searching for terminating C</>,
1840combinations of C<\\> and C<\/> are skipped. If the delimiters are
1841bracketing, nested pairs are also skipped. For example, while searching
1842for closing C<]> paired with the opening C<[>, combinations of C<\\>, C<\]>,
1843and C<\[> are all skipped, and nested C<[> and C<]> are skipped as well.
1844However, when backslashes are used as the delimiters (like C<qq\\> and
1845C<tr\\\>), nothing is skipped.
1846During the search for the end, backslashes that escape delimiters
1847are removed (exactly speaking, they are not copied to the safe location).
75e14d17 1848
19799a22
GS
1849For constructs with three-part delimiters (C<s///>, C<y///>, and
1850C<tr///>), the search is repeated once more.
6deea57f
ST
1851If the first delimiter is not an opening punctuation, three delimiters must
1852be same such as C<s!!!> and C<tr)))>, in which case the second delimiter
1853terminates the left part and starts the right part at once.
1854If the left part is delimited by bracketing punctuations (that is C<()>,
1855C<[]>, C<{}>, or C<< <> >>), the right part needs another pair of
1856delimiters such as C<s(){}> and C<tr[]//>. In these cases, whitespaces
1857and comments are allowed between both parts, though the comment must follow
1858at least one whitespace; otherwise a character expected as the start of
1859the comment may be regarded as the starting delimiter of the right part.
75e14d17 1860
19799a22
GS
1861During this search no attention is paid to the semantics of the construct.
1862Thus:
75e14d17
IZ
1863
1864 "$hash{"$foo/$bar"}"
1865
2a94b7ce 1866or:
75e14d17 1867
89d205f2 1868 m/
2a94b7ce 1869 bar # NOT a comment, this slash / terminated m//!
75e14d17
IZ
1870 /x
1871
19799a22
GS
1872do not form legal quoted expressions. The quoted part ends on the
1873first C<"> and C</>, and the rest happens to be a syntax error.
1874Because the slash that terminated C<m//> was followed by a C<SPACE>,
1875the example above is not C<m//x>, but rather C<m//> with no C</x>
1876modifier. So the embedded C<#> is interpreted as a literal C<#>.
75e14d17 1877
89d205f2
YO
1878Also no attention is paid to C<\c\> (multichar control char syntax) during
1879this search. Thus the second C<\> in C<qq/\c\/> is interpreted as a part
1880of C<\/>, and the following C</> is not recognized as a delimiter.
0d594e51
ST
1881Instead, use C<\034> or C<\x1c> at the end of quoted constructs.
1882
75e14d17 1883=item Interpolation
d74e8afc 1884X<interpolation>
75e14d17 1885
19799a22 1886The next step is interpolation in the text obtained, which is now
89d205f2 1887delimiter-independent. There are multiple cases.
75e14d17 1888
13a2d996 1889=over 4
75e14d17 1890
89d205f2 1891=item C<<<'EOF'>
75e14d17
IZ
1892
1893No interpolation is performed.
6deea57f
ST
1894Note that the combination C<\\> is left intact, since escaped delimiters
1895are not available for here-docs.
75e14d17 1896
6deea57f 1897=item C<m''>, the pattern of C<s'''>
89d205f2 1898
6deea57f
ST
1899No interpolation is performed at this stage.
1900Any backslashed sequences including C<\\> are treated at the stage
1901to L</"parsing regular expressions">.
89d205f2 1902
6deea57f 1903=item C<''>, C<q//>, C<tr'''>, C<y'''>, the replacement of C<s'''>
75e14d17 1904
89d205f2 1905The only interpolation is removal of C<\> from pairs of C<\\>.
6deea57f
ST
1906Therefore C<-> in C<tr'''> and C<y'''> is treated literally
1907as a hyphen and no character range is available.
1908C<\1> in the replacement of C<s'''> does not work as C<$1>.
89d205f2
YO
1909
1910=item C<tr///>, C<y///>
1911
6deea57f
ST
1912No variable interpolation occurs. String modifying combinations for
1913case and quoting such as C<\Q>, C<\U>, and C<\E> are not recognized.
1914The other escape sequences such as C<\200> and C<\t> and backslashed
1915characters such as C<\\> and C<\-> are converted to appropriate literals.
89d205f2
YO
1916The character C<-> is treated specially and therefore C<\-> is treated
1917as a literal C<->.
75e14d17 1918
89d205f2 1919=item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF">
75e14d17 1920
19799a22
GS
1921C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
1922converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
1923is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
6deea57f
ST
1924The other escape sequences such as C<\200> and C<\t> and backslashed
1925characters such as C<\\> and C<\-> are replaced with appropriate
1926expansions.
2a94b7ce 1927
19799a22
GS
1928Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
1929is interpolated in the usual way. Something like C<"\Q\\E"> has
1930no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
1931result is the same as for C<"\\\\E">. As a general rule, backslashes
1932between C<\Q> and C<\E> may lead to counterintuitive results. So,
1933C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
1934as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
2a94b7ce
IZ
1935
1936 $str = '\t';
1937 return "\Q$str";
1938
1939may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
1940
19799a22 1941Interpolated scalars and arrays are converted internally to the C<join> and
92d29cee 1942C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
75e14d17 1943
19799a22 1944 $foo . " XXX '" . (join $", @arr) . "'";
75e14d17 1945
19799a22 1946All operations above are performed simultaneously, left to right.
75e14d17 1947
19799a22
GS
1948Because the result of C<"\Q STRING \E"> has all metacharacters
1949quoted, there is no way to insert a literal C<$> or C<@> inside a
1950C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
1951C<"\\\$">; if not, it is interpreted as the start of an interpolated
1952scalar.
75e14d17 1953
19799a22 1954Note also that the interpolation code needs to make a decision on
89d205f2 1955where the interpolated scalar ends. For instance, whether
35f2feb0 1956C<< "a $b -> {c}" >> really means:
75e14d17
IZ
1957
1958 "a " . $b . " -> {c}";
1959
2a94b7ce 1960or:
75e14d17
IZ
1961
1962 "a " . $b -> {c};
1963
19799a22
GS
1964Most of the time, the longest possible text that does not include
1965spaces between components and which contains matching braces or
1966brackets. because the outcome may be determined by voting based
1967on heuristic estimators, the result is not strictly predictable.
1968Fortunately, it's usually correct for ambiguous cases.
75e14d17 1969
6deea57f 1970=item the replacement of C<s///>
75e14d17 1971
19799a22 1972Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
6deea57f
ST
1973happens as with C<qq//> constructs.
1974
1975It is at this step that C<\1> is begrudgingly converted to C<$1> in
1976the replacement text of C<s///>, in order to correct the incorrigible
1977I<sed> hackers who haven't picked up the saner idiom yet. A warning
1978is emitted if the C<use warnings> pragma or the B<-w> command-line flag
1979(that is, the C<$^W> variable) was set.
1980
1981=item C<RE> in C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
1982
cc74c5bd
ST
1983Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\E>,
1984and interpolation happens (almost) as with C<qq//> constructs.
1985
1986However any other combinations of C<\> followed by a character
1987are not substituted but only skipped, in order to parse them
1988as regular expressions at the following step.
6deea57f 1989As C<\c> is skipped at this step, C<@> of C<\c@> in RE is possibly
1749ea0d 1990treated as an array symbol (for example C<@foo>),
6deea57f 1991even though the same text in C<qq//> gives interpolation of C<\c@>.
6deea57f
ST
1992
1993Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
19799a22
GS
1994a C<#>-comment in a C<//x>-regular expression, no processing is
1995performed whatsoever. This is the first step at which the presence
1996of the C<//x> modifier is relevant.
1997
1749ea0d
ST
1998Interpolation in patterns has several quirks: C<$|>, C<$(>, C<$)>, C<@+>
1999and C<@-> are not interpolated, and constructs C<$var[SOMETHING]> are
2000voted (by several different estimators) to be either an array element
2001or C<$var> followed by an RE alternative. This is where the notation
19799a22
GS
2002C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
2003array element C<-9>, not as a regular expression from the variable
2004C<$arr> followed by a digit, which would be the interpretation of
2005C</$arr[0-9]/>. Since voting among different estimators may occur,
2006the result is not predictable.
2007
19799a22
GS
2008The lack of processing of C<\\> creates specific restrictions on
2009the post-processed text. If the delimiter is C</>, one cannot get
2010the combination C<\/> into the result of this step. C</> will
2011finish the regular expression, C<\/> will be stripped to C</> on
2012the previous step, and C<\\/> will be left as is. Because C</> is
2013equivalent to C<\/> inside a regular expression, this does not
2014matter unless the delimiter happens to be character special to the
2015RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
2016alphanumeric char, as in:
2a94b7ce
IZ
2017
2018 m m ^ a \s* b mmx;
2019
19799a22 2020In the RE above, which is intentionally obfuscated for illustration, the
6deea57f 2021delimiter is C<m>, the modifier is C<mx>, and after delimiter-removal the
89d205f2 2022RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
19799a22
GS
2023reason you're encouraged to restrict your delimiters to non-alphanumeric,
2024non-whitespace choices.
75e14d17
IZ
2025
2026=back
2027
19799a22 2028This step is the last one for all constructs except regular expressions,
75e14d17
IZ
2029which are processed further.
2030
6deea57f
ST
2031=item parsing regular expressions
2032X<regexp, parse>
75e14d17 2033
19799a22
GS
2034Previous steps were performed during the compilation of Perl code,
2035but this one happens at run time--although it may be optimized to
2036be calculated at compile time if appropriate. After preprocessing
6deea57f 2037described above, and possibly after evaluation if concatenation,
19799a22
GS
2038joining, casing translation, or metaquoting are involved, the
2039resulting I<string> is passed to the RE engine for compilation.
2040
2041Whatever happens in the RE engine might be better discussed in L<perlre>,
2042but for the sake of continuity, we shall do so here.
2043
2044This is another step where the presence of the C<//x> modifier is
2045relevant. The RE engine scans the string from left to right and
2046converts it to a finite automaton.
2047
2048Backslashed characters are either replaced with corresponding
2049literal strings (as with C<\{>), or else they generate special nodes
2050in the finite automaton (as with C<\b>). Characters special to the
2051RE engine (such as C<|>) generate corresponding nodes or groups of
2052nodes. C<(?#...)> comments are ignored. All the rest is either
2053converted to literal strings to match, or else is ignored (as is
2054whitespace and C<#>-style comments if C<//x> is present).
2055
2056Parsing of the bracketed character class construct, C<[...]>, is
2057rather different than the rule used for the rest of the pattern.
2058The terminator of this construct is found using the same rules as
2059for finding the terminator of a C<{}>-delimited construct, the only
2060exception being that C<]> immediately following C<[> is treated as
2061though preceded by a backslash. Similarly, the terminator of
2062C<(?{...})> is found using the same rules as for finding the
2063terminator of a C<{}>-delimited construct.
2064
2065It is possible to inspect both the string given to RE engine and the
2066resulting finite automaton. See the arguments C<debug>/C<debugcolor>
2067in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
4a4eefd0 2068switch documented in L<perlrun/"Command Switches">.
75e14d17
IZ
2069
2070=item Optimization of regular expressions
d74e8afc 2071X<regexp, optimization>
75e14d17 2072
7522fed5 2073This step is listed for completeness only. Since it does not change
75e14d17 2074semantics, details of this step are not documented and are subject
19799a22
GS
2075to change without notice. This step is performed over the finite
2076automaton that was generated during the previous pass.
2a94b7ce 2077
19799a22
GS
2078It is at this stage that C<split()> silently optimizes C</^/> to
2079mean C</^/m>.
75e14d17
IZ
2080
2081=back
2082
a0d0e21e 2083=head2 I/O Operators
d74e8afc
ITB
2084X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle>
2085X<< <> >> X<@ARGV>
a0d0e21e 2086
54310121 2087There are several I/O operators you should know about.
fbad3eb5 2088
7b8d334a 2089A string enclosed by backticks (grave accents) first undergoes
19799a22
GS
2090double-quote interpolation. It is then interpreted as an external
2091command, and the output of that command is the value of the
e9c56f9b
JH
2092backtick string, like in a shell. In scalar context, a single string
2093consisting of all output is returned. In list context, a list of
2094values is returned, one per line of output. (You can set C<$/> to use
2095a different line terminator.) The command is executed each time the
2096pseudo-literal is evaluated. The status value of the command is
2097returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
2098Unlike in B<csh>, no translation is done on the return data--newlines
2099remain newlines. Unlike in any of the shells, single quotes do not
2100hide variable names in the command from interpretation. To pass a
2101literal dollar-sign through to the shell you need to hide it with a
2102backslash. The generalized form of backticks is C<qx//>. (Because
2103backticks always undergo shell expansion as well, see L<perlsec> for
2104security concerns.)
d74e8afc 2105X<qx> X<`> X<``> X<backtick> X<glob>
19799a22
GS
2106
2107In scalar context, evaluating a filehandle in angle brackets yields
2108the next line from that file (the newline, if any, included), or
2109C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
2110(sometimes known as file-slurp mode) and the file is empty, it
2111returns C<''> the first time, followed by C<undef> subsequently.
2112
2113Ordinarily you must assign the returned value to a variable, but
2114there is one situation where an automatic assignment happens. If
2115and only if the input symbol is the only thing inside the conditional
2116of a C<while> statement (even if disguised as a C<for(;;)> loop),
2117the value is automatically assigned to the global variable $_,
2118destroying whatever was there previously. (This may seem like an
2119odd thing to you, but you'll use the construct in almost every Perl
17b829fa 2120script you write.) The $_ variable is not implicitly localized.
19799a22
GS
2121You'll have to put a C<local $_;> before the loop if you want that
2122to happen.
2123
2124The following lines are equivalent:
a0d0e21e 2125
748a9306 2126 while (defined($_ = <STDIN>)) { print; }
7b8d334a 2127 while ($_ = <STDIN>) { print; }
a0d0e21e
LW
2128 while (<STDIN>) { print; }
2129 for (;<STDIN>;) { print; }
748a9306 2130 print while defined($_ = <STDIN>);
7b8d334a 2131 print while ($_ = <STDIN>);
a0d0e21e
LW
2132 print while <STDIN>;
2133
19799a22 2134This also behaves similarly, but avoids $_ :
7b8d334a 2135
89d205f2 2136 while (my $line = <STDIN>) { print $line }
7b8d334a 2137
19799a22
GS
2138In these loop constructs, the assigned value (whether assignment
2139is automatic or explicit) is then tested to see whether it is
2140defined. The defined test avoids problems where line has a string
2141value that would be treated as false by Perl, for example a "" or
2142a "0" with no trailing newline. If you really mean for such values
2143to terminate the loop, they should be tested for explicitly:
7b8d334a
GS
2144
2145 while (($_ = <STDIN>) ne '0') { ... }
2146 while (<STDIN>) { last unless $_; ... }
2147
35f2feb0 2148In other boolean contexts, C<< <I<filehandle>> >> without an
89d205f2 2149explicit C<defined> test or comparison elicit a warning if the
9f1b1f2d 2150C<use warnings> pragma or the B<-w>
19799a22 2151command-line switch (the C<$^W> variable) is in effect.
7b8d334a 2152
5f05dabc 2153The filehandles STDIN, STDOUT, and STDERR are predefined. (The
19799a22
GS
2154filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
2155in packages, where they would be interpreted as local identifiers
2156rather than global.) Additional filehandles may be created with
2157the open() function, amongst others. See L<perlopentut> and
2158L<perlfunc/open> for details on this.
d74e8afc 2159X<stdin> X<stdout> X<sterr>
a0d0e21e 2160
35f2feb0 2161If a <FILEHANDLE> is used in a context that is looking for
19799a22
GS
2162a list, a list comprising all input lines is returned, one line per
2163list element. It's easy to grow to a rather large data space this
2164way, so use with care.
a0d0e21e 2165
35f2feb0 2166<FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
19799a22 2167See L<perlfunc/readline>.
fbad3eb5 2168
35f2feb0
GS
2169The null filehandle <> is special: it can be used to emulate the
2170behavior of B<sed> and B<awk>. Input from <> comes either from
a0d0e21e 2171standard input, or from each file listed on the command line. Here's
35f2feb0 2172how it works: the first time <> is evaluated, the @ARGV array is
5a964f20 2173checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
a0d0e21e
LW
2174gives you standard input. The @ARGV array is then processed as a list
2175of filenames. The loop
2176
2177 while (<>) {
2178 ... # code for each line
2179 }
2180
2181is equivalent to the following Perl-like pseudo code:
2182
3e3baf6d 2183 unshift(@ARGV, '-') unless @ARGV;
a0d0e21e
LW
2184 while ($ARGV = shift) {
2185 open(ARGV, $ARGV);
2186 while (<ARGV>) {
2187 ... # code for each line
2188 }
2189 }
2190
19799a22
GS
2191except that it isn't so cumbersome to say, and will actually work.
2192It really does shift the @ARGV array and put the current filename
2193into the $ARGV variable. It also uses filehandle I<ARGV>
35f2feb0 2194internally--<> is just a synonym for <ARGV>, which
19799a22 2195is magical. (The pseudo code above doesn't work because it treats
35f2feb0 2196<ARGV> as non-magical.)
a0d0e21e 2197
35f2feb0 2198You can modify @ARGV before the first <> as long as the array ends up
a0d0e21e 2199containing the list of filenames you really want. Line numbers (C<$.>)
19799a22
GS
2200continue as though the input were one big happy file. See the example
2201in L<perlfunc/eof> for how to reset line numbers on each file.
5a964f20 2202
89d205f2 2203If you want to set @ARGV to your own list of files, go right ahead.
5a964f20
TC
2204This sets @ARGV to all plain text files if no @ARGV was given:
2205
2206 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
a0d0e21e 2207
5a964f20
TC
2208You can even set them to pipe commands. For example, this automatically
2209filters compressed arguments through B<gzip>:
2210
2211 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
2212
2213If you want to pass switches into your script, you can use one of the
a0d0e21e
LW
2214Getopts modules or put a loop on the front like this:
2215
2216 while ($_ = $ARGV[0], /^-/) {
2217 shift;
2218 last if /^--$/;
2219 if (/^-D(.*)/) { $debug = $1 }
2220 if (/^-v/) { $verbose++ }
5a964f20 2221 # ... # other switches
a0d0e21e 2222 }
5a964f20 2223
a0d0e21e 2224 while (<>) {
5a964f20 2225 # ... # code for each line
a0d0e21e
LW
2226 }
2227
89d205f2
YO
2228The <> symbol will return C<undef> for end-of-file only once.
2229If you call it again after this, it will assume you are processing another
19799a22 2230@ARGV list, and if you haven't set @ARGV, will read input from STDIN.
a0d0e21e 2231
b159ebd3 2232If what the angle brackets contain is a simple scalar variable (e.g.,
35f2feb0 2233<$foo>), then that variable contains the name of the
19799a22
GS
2234filehandle to input from, or its typeglob, or a reference to the
2235same. For example:
cb1a09d0
AD
2236
2237 $fh = \*STDIN;
2238 $line = <$fh>;
a0d0e21e 2239
5a964f20
TC
2240If what's within the angle brackets is neither a filehandle nor a simple
2241scalar variable containing a filehandle name, typeglob, or typeglob
2242reference, it is interpreted as a filename pattern to be globbed, and
2243either a list of filenames or the next filename in the list is returned,
19799a22 2244depending on context. This distinction is determined on syntactic
35f2feb0
GS
2245grounds alone. That means C<< <$x> >> is always a readline() from
2246an indirect handle, but C<< <$hash{key}> >> is always a glob().
5a964f20 2247That's because $x is a simple scalar variable, but C<$hash{key}> is
ef191992
YST
2248not--it's a hash element. Even C<< <$x > >> (note the extra space)
2249is treated as C<glob("$x ")>, not C<readline($x)>.
5a964f20
TC
2250
2251One level of double-quote interpretation is done first, but you can't
35f2feb0 2252say C<< <$foo> >> because that's an indirect filehandle as explained
5a964f20
TC
2253in the previous paragraph. (In older versions of Perl, programmers
2254would insert curly brackets to force interpretation as a filename glob:
35f2feb0 2255C<< <${foo}> >>. These days, it's considered cleaner to call the
5a964f20 2256internal function directly as C<glob($foo)>, which is probably the right
19799a22 2257way to have done it in the first place.) For example:
a0d0e21e
LW
2258
2259 while (<*.c>) {
2260 chmod 0644, $_;
2261 }
2262
3a4b19e4 2263is roughly equivalent to:
a0d0e21e
LW
2264
2265 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
2266 while (<FOO>) {
5b3eff12 2267 chomp;
a0d0e21e
LW
2268 chmod 0644, $_;
2269 }
2270
3a4b19e4
GS
2271except that the globbing is actually done internally using the standard
2272C<File::Glob> extension. Of course, the shortest way to do the above is:
a0d0e21e
LW
2273
2274 chmod 0644, <*.c>;
2275
19799a22
GS
2276A (file)glob evaluates its (embedded) argument only when it is
2277starting a new list. All values must be read before it will start
2278over. In list context, this isn't important because you automatically
2279get them all anyway. However, in scalar context the operator returns
069e01df 2280the next value each time it's called, or C<undef> when the list has
19799a22
GS
2281run out. As with filehandle reads, an automatic C<defined> is
2282generated when the glob occurs in the test part of a C<while>,
2283because legal glob returns (e.g. a file called F<0>) would otherwise
2284terminate the loop. Again, C<undef> is returned only once. So if
2285you're expecting a single value from a glob, it is much better to
2286say
4633a7c4
LW
2287
2288 ($file) = <blurch*>;
2289
2290than
2291
2292 $file = <blurch*>;
2293
2294because the latter will alternate between returning a filename and
19799a22 2295returning false.
4633a7c4 2296
b159ebd3 2297If you're trying to do variable interpolation, it's definitely better
4633a7c4 2298to use the glob() function, because the older notation can cause people
e37d713d 2299to become confused with the indirect filehandle notation.
4633a7c4
LW
2300
2301 @files = glob("$dir/*.[ch]");
2302 @files = glob($files[$i]);
2303
a0d0e21e 2304=head2 Constant Folding
d74e8afc 2305X<constant folding> X<folding>
a0d0e21e
LW
2306
2307Like C, Perl does a certain amount of expression evaluation at
19799a22 2308compile time whenever it determines that all arguments to an
a0d0e21e
LW
2309operator are static and have no side effects. In particular, string
2310concatenation happens at compile time between literals that don't do
19799a22 2311variable substitution. Backslash interpolation also happens at
a0d0e21e
LW
2312compile time. You can say
2313
2314 'Now is the time for all' . "\n" .
2315 'good men to come to.'
2316
54310121 2317and this all reduces to one string internally. Likewise, if
a0d0e21e
LW
2318you say
2319
2320 foreach $file (@filenames) {
5a964f20 2321 if (-s $file > 5 + 100 * 2**16) { }
54310121 2322 }
a0d0e21e 2323
19799a22
GS
2324the compiler will precompute the number which that expression
2325represents so that the interpreter won't have to.
a0d0e21e 2326
fd1abbef 2327=head2 No-ops
d74e8afc 2328X<no-op> X<nop>
fd1abbef
DN
2329
2330Perl doesn't officially have a no-op operator, but the bare constants
2331C<0> and C<1> are special-cased to not produce a warning in a void
2332context, so you can for example safely do
2333
2334 1 while foo();
2335
2c268ad5 2336=head2 Bitwise String Operators
d74e8afc 2337X<operator, bitwise, string>
2c268ad5
TP
2338
2339Bitstrings of any size may be manipulated by the bitwise operators
2340(C<~ | & ^>).
2341
19799a22
GS
2342If the operands to a binary bitwise op are strings of different
2343sizes, B<|> and B<^> ops act as though the shorter operand had
2344additional zero bits on the right, while the B<&> op acts as though
2345the longer operand were truncated to the length of the shorter.
2346The granularity for such extension or truncation is one or more
2347bytes.
2c268ad5 2348
89d205f2 2349 # ASCII-based examples
2c268ad5
TP
2350 print "j p \n" ^ " a h"; # prints "JAPH\n"
2351 print "JA" | " ph\n"; # prints "japh\n"
2352 print "japh\nJunk" & '_____'; # prints "JAPH\n";
2353 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
2354
19799a22 2355If you are intending to manipulate bitstrings, be certain that
2c268ad5 2356you're supplying bitstrings: If an operand is a number, that will imply
19799a22 2357a B<numeric> bitwise operation. You may explicitly show which type of
2c268ad5
TP
2358operation you intend by using C<""> or C<0+>, as in the examples below.
2359
4358a253
SS
2360 $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
2361 $foo = '150' | 105; # yields 255
2c268ad5
TP
2362 $foo = 150 | '105'; # yields 255
2363 $foo = '150' | '105'; # yields string '155' (under ASCII)
2364
2365 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
2366 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
a0d0e21e 2367
1ae175c8
GS
2368See L<perlfunc/vec> for information on how to manipulate individual bits
2369in a bit vector.
2370
55497cff 2371=head2 Integer Arithmetic
d74e8afc 2372X<integer>
a0d0e21e 2373
19799a22 2374By default, Perl assumes that it must do most of its arithmetic in
a0d0e21e
LW
2375floating point. But by saying
2376
2377 use integer;
2378
2379you may tell the compiler that it's okay to use integer operations
19799a22
GS
2380(if it feels like it) from here to the end of the enclosing BLOCK.
2381An inner BLOCK may countermand this by saying
a0d0e21e
LW
2382
2383 no integer;
2384
19799a22
GS
2385which lasts until the end of that BLOCK. Note that this doesn't
2386mean everything is only an integer, merely that Perl may use integer
2387operations if it is so inclined. For example, even under C<use
2388integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731>
2389or so.
2390
2391Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
89d205f2 2392and ">>") always produce integral results. (But see also
13a2d996 2393L<Bitwise String Operators>.) However, C<use integer> still has meaning for
19799a22
GS
2394them. By default, their results are interpreted as unsigned integers, but
2395if C<use integer> is in effect, their results are interpreted
2396as signed integers. For example, C<~0> usually evaluates to a large
0be96356 2397integral value. However, C<use integer; ~0> is C<-1> on two's-complement
19799a22 2398machines.
68dc0745
PP
2399
2400=head2 Floating-point Arithmetic
d74e8afc 2401X<floating-point> X<floating point> X<float> X<real>
68dc0745
PP
2402
2403While C<use integer> provides integer-only arithmetic, there is no
19799a22
GS
2404analogous mechanism to provide automatic rounding or truncation to a
2405certain number of decimal places. For rounding to a certain number
2406of digits, sprintf() or printf() is usually the easiest route.
2407See L<perlfaq4>.
68dc0745 2408
5a964f20
TC
2409Floating-point numbers are only approximations to what a mathematician
2410would call real numbers. There are infinitely more reals than floats,
2411so some corners must be cut. For example:
2412
2413 printf "%.20g\n", 123456789123456789;
2414 # produces 123456789123456784
2415
2416Testing for exact equality of floating-point equality or inequality is
2417not a good idea. Here's a (relatively expensive) work-around to compare
2418whether two floating-point numbers are equal to a particular number of
2419decimal places. See Knuth, volume II, for a more robust treatment of
2420this topic.
2421
2422 sub fp_equal {
2423 my ($X, $Y, $POINTS) = @_;
2424 my ($tX, $tY);
2425 $tX = sprintf("%.${POINTS}g", $X);
2426 $tY = sprintf("%.${POINTS}g", $Y);
2427 return $tX eq $tY;
2428 }
2429
68dc0745 2430The POSIX module (part of the standard perl distribution) implements
19799a22
GS
2431ceil(), floor(), and other mathematical and trigonometric functions.
2432The Math::Complex module (part of the standard perl distribution)
2433defines mathematical functions that work on both the reals and the
2434imaginary numbers. Math::Complex not as efficient as POSIX, but
68dc0745
PP
2435POSIX can't work with complex numbers.
2436
2437Rounding in financial applications can have serious implications, and
2438the rounding method used should be specified precisely. In these
2439cases, it probably pays not to trust whichever system rounding is
2440being used by Perl, but to instead implement the rounding function you
2441need yourself.
5a964f20
TC
2442
2443=head2 Bigger Numbers
d74e8afc 2444X<number, arbitrary precision>
5a964f20
TC
2445
2446The standard Math::BigInt and Math::BigFloat modules provide
19799a22 2447variable-precision arithmetic and overloaded operators, although
cd5c4fce 2448they're currently pretty slow. At the cost of some space and
19799a22
GS
2449considerable speed, they avoid the normal pitfalls associated with
2450limited-precision representations.
5a964f20
TC
2451
2452 use Math::BigInt;
2453 $x = Math::BigInt->new('123456789123456789');
2454 print $x * $x;
2455
2456 # prints +15241578780673678515622620750190521
19799a22 2457
cd5c4fce
T
2458There are several modules that let you calculate with (bound only by
2459memory and cpu-time) unlimited or fixed precision. There are also
2460some non-standard modules that provide faster implementations via
2461external C libraries.
2462
2463Here is a short, but incomplete summary:
2464
2465 Math::Fraction big, unlimited fractions like 9973 / 12967
2466 Math::String treat string sequences like numbers
2467 Math::FixedPrecision calculate with a fixed precision
2468 Math::Currency for currency calculations
2469 Bit::Vector manipulate bit vectors fast (uses C)
2470 Math::BigIntFast Bit::Vector wrapper for big numbers
2471 Math::Pari provides access to the Pari C library
2472 Math::BigInteger uses an external C library
2473 Math::Cephes uses external Cephes C library (no big numbers)
2474 Math::Cephes::Fraction fractions via the Cephes library
2475 Math::GMP another one using an external C library
2476
2477Choose wisely.
16070b82
GS
2478
2479=cut