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perlop.pod: Reword escapes that have ordinals
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a0d0e21e 1=head1 NAME
d74e8afc 2X<operator>
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4perlop - Perl operators and precedence
5
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6=head1 DESCRIPTION
7
89d205f2 8=head2 Operator Precedence and Associativity
d74e8afc 9X<operator, precedence> X<precedence> X<associativity>
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10
11Operator precedence and associativity work in Perl more or less like
12they do in mathematics.
13
14I<Operator precedence> means some operators are evaluated before
15others. For example, in C<2 + 4 * 5>, the multiplication has higher
16precedence so C<4 * 5> is evaluated first yielding C<2 + 20 ==
1722> and not C<6 * 5 == 30>.
18
19I<Operator associativity> defines what happens if a sequence of the
20same operators is used one after another: whether the evaluator will
21evaluate the left operations first or the right. For example, in C<8
22- 4 - 2>, subtraction is left associative so Perl evaluates the
23expression left to right. C<8 - 4> is evaluated first making the
24expression C<4 - 2 == 2> and not C<8 - 2 == 6>.
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25
26Perl operators have the following associativity and precedence,
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27listed from highest precedence to lowest. Operators borrowed from
28C keep the same precedence relationship with each other, even where
29C's precedence is slightly screwy. (This makes learning Perl easier
30for C folks.) With very few exceptions, these all operate on scalar
31values only, not array values.
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32
33 left terms and list operators (leftward)
34 left ->
35 nonassoc ++ --
36 right **
37 right ! ~ \ and unary + and -
54310121 38 left =~ !~
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39 left * / % x
40 left + - .
41 left << >>
42 nonassoc named unary operators
43 nonassoc < > <= >= lt gt le ge
0d863452 44 nonassoc == != <=> eq ne cmp ~~
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45 left &
46 left | ^
47 left &&
c963b151 48 left || //
137443ea 49 nonassoc .. ...
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50 right ?:
51 right = += -= *= etc.
52 left , =>
53 nonassoc list operators (rightward)
a5f75d66 54 right not
a0d0e21e 55 left and
f23102e2 56 left or xor
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57
58In the following sections, these operators are covered in precedence order.
59
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60Many operators can be overloaded for objects. See L<overload>.
61
a0d0e21e 62=head2 Terms and List Operators (Leftward)
d74e8afc 63X<list operator> X<operator, list> X<term>
a0d0e21e 64
62c18ce2 65A TERM has the highest precedence in Perl. They include variables,
5f05dabc 66quote and quote-like operators, any expression in parentheses,
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67and any function whose arguments are parenthesized. Actually, there
68aren't really functions in this sense, just list operators and unary
69operators behaving as functions because you put parentheses around
70the arguments. These are all documented in L<perlfunc>.
71
72If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
73is followed by a left parenthesis as the next token, the operator and
74arguments within parentheses are taken to be of highest precedence,
75just like a normal function call.
76
77In the absence of parentheses, the precedence of list operators such as
78C<print>, C<sort>, or C<chmod> is either very high or very low depending on
54310121 79whether you are looking at the left side or the right side of the operator.
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80For example, in
81
82 @ary = (1, 3, sort 4, 2);
83 print @ary; # prints 1324
84
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85the commas on the right of the sort are evaluated before the sort,
86but the commas on the left are evaluated after. In other words,
87list operators tend to gobble up all arguments that follow, and
a0d0e21e 88then act like a simple TERM with regard to the preceding expression.
19799a22 89Be careful with parentheses:
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90
91 # These evaluate exit before doing the print:
92 print($foo, exit); # Obviously not what you want.
93 print $foo, exit; # Nor is this.
94
95 # These do the print before evaluating exit:
96 (print $foo), exit; # This is what you want.
97 print($foo), exit; # Or this.
98 print ($foo), exit; # Or even this.
99
100Also note that
101
102 print ($foo & 255) + 1, "\n";
103
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104probably doesn't do what you expect at first glance. The parentheses
105enclose the argument list for C<print> which is evaluated (printing
106the result of C<$foo & 255>). Then one is added to the return value
107of C<print> (usually 1). The result is something like this:
108
109 1 + 1, "\n"; # Obviously not what you meant.
110
111To do what you meant properly, you must write:
112
113 print(($foo & 255) + 1, "\n");
114
115See L<Named Unary Operators> for more discussion of this.
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116
117Also parsed as terms are the C<do {}> and C<eval {}> constructs, as
54310121 118well as subroutine and method calls, and the anonymous
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119constructors C<[]> and C<{}>.
120
2ae324a7 121See also L<Quote and Quote-like Operators> toward the end of this section,
da87341d 122as well as L</"I/O Operators">.
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123
124=head2 The Arrow Operator
d74e8afc 125X<arrow> X<dereference> X<< -> >>
a0d0e21e 126
35f2feb0 127"C<< -> >>" is an infix dereference operator, just as it is in C
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128and C++. If the right side is either a C<[...]>, C<{...}>, or a
129C<(...)> subscript, then the left side must be either a hard or
130symbolic reference to an array, a hash, or a subroutine respectively.
131(Or technically speaking, a location capable of holding a hard
132reference, if it's an array or hash reference being used for
133assignment.) See L<perlreftut> and L<perlref>.
a0d0e21e 134
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135Otherwise, the right side is a method name or a simple scalar
136variable containing either the method name or a subroutine reference,
137and the left side must be either an object (a blessed reference)
138or a class name (that is, a package name). See L<perlobj>.
a0d0e21e 139
5f05dabc 140=head2 Auto-increment and Auto-decrement
d74e8afc 141X<increment> X<auto-increment> X<++> X<decrement> X<auto-decrement> X<-->
a0d0e21e 142
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143"++" and "--" work as in C. That is, if placed before a variable,
144they increment or decrement the variable by one before returning the
145value, and if placed after, increment or decrement after returning the
146value.
147
148 $i = 0; $j = 0;
149 print $i++; # prints 0
150 print ++$j; # prints 1
a0d0e21e 151
b033823e 152Note that just as in C, Perl doesn't define B<when> the variable is
89d205f2 153incremented or decremented. You just know it will be done sometime
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154before or after the value is returned. This also means that modifying
155a variable twice in the same statement will lead to undefined behaviour.
156Avoid statements like:
157
158 $i = $i ++;
159 print ++ $i + $i ++;
160
161Perl will not guarantee what the result of the above statements is.
162
54310121 163The auto-increment operator has a little extra builtin magic to it. If
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164you increment a variable that is numeric, or that has ever been used in
165a numeric context, you get a normal increment. If, however, the
5f05dabc 166variable has been used in only string contexts since it was set, and
5a964f20 167has a value that is not the empty string and matches the pattern
9c0670e1 168C</^[a-zA-Z]*[0-9]*\z/>, the increment is done as a string, preserving each
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169character within its range, with carry:
170
171 print ++($foo = '99'); # prints '100'
172 print ++($foo = 'a0'); # prints 'a1'
173 print ++($foo = 'Az'); # prints 'Ba'
174 print ++($foo = 'zz'); # prints 'aaa'
175
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176C<undef> is always treated as numeric, and in particular is changed
177to C<0> before incrementing (so that a post-increment of an undef value
178will return C<0> rather than C<undef>).
179
5f05dabc 180The auto-decrement operator is not magical.
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181
182=head2 Exponentiation
d74e8afc 183X<**> X<exponentiation> X<power>
a0d0e21e 184
19799a22 185Binary "**" is the exponentiation operator. It binds even more
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186tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is
187implemented using C's pow(3) function, which actually works on doubles
188internally.)
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189
190=head2 Symbolic Unary Operators
d74e8afc 191X<unary operator> X<operator, unary>
a0d0e21e 192
5f05dabc 193Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower
a0d0e21e 194precedence version of this.
d74e8afc 195X<!>
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196
197Unary "-" performs arithmetic negation if the operand is numeric. If
198the operand is an identifier, a string consisting of a minus sign
199concatenated with the identifier is returned. Otherwise, if the string
200starts with a plus or minus, a string starting with the opposite sign
bff5667c 201is returned. One effect of these rules is that -bareword is equivalent
8705167b 202to the string "-bareword". If, however, the string begins with a
353c6505 203non-alphabetic character (excluding "+" or "-"), Perl will attempt to convert
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204the string to a numeric and the arithmetic negation is performed. If the
205string cannot be cleanly converted to a numeric, Perl will give the warning
206B<Argument "the string" isn't numeric in negation (-) at ...>.
d74e8afc 207X<-> X<negation, arithmetic>
a0d0e21e 208
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209Unary "~" performs bitwise negation, i.e., 1's complement. For
210example, C<0666 & ~027> is 0640. (See also L<Integer Arithmetic> and
211L<Bitwise String Operators>.) Note that the width of the result is
212platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64
213bits wide on a 64-bit platform, so if you are expecting a certain bit
d042e63d 214width, remember to use the & operator to mask off the excess bits.
d74e8afc 215X<~> X<negation, binary>
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216
217Unary "+" has no effect whatsoever, even on strings. It is useful
218syntactically for separating a function name from a parenthesized expression
219that would otherwise be interpreted as the complete list of function
5ba421f6 220arguments. (See examples above under L<Terms and List Operators (Leftward)>.)
d74e8afc 221X<+>
a0d0e21e 222
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223Unary "\" creates a reference to whatever follows it. See L<perlreftut>
224and L<perlref>. Do not confuse this behavior with the behavior of
225backslash within a string, although both forms do convey the notion
226of protecting the next thing from interpolation.
d74e8afc 227X<\> X<reference> X<backslash>
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228
229=head2 Binding Operators
d74e8afc 230X<binding> X<operator, binding> X<=~> X<!~>
a0d0e21e 231
c07a80fd 232Binary "=~" binds a scalar expression to a pattern match. Certain operations
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233search or modify the string $_ by default. This operator makes that kind
234of operation work on some other string. The right argument is a search
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235pattern, substitution, or transliteration. The left argument is what is
236supposed to be searched, substituted, or transliterated instead of the default
f8bab1e9 237$_. When used in scalar context, the return value generally indicates the
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238success of the operation. The exception is substitution with the C</r>
239(non-destructive) option, which causes the return value to be the result of
240the substition. Behavior in list context depends on the particular operator.
241See L</"Regexp Quote-Like Operators"> for details and L<perlretut> for
242examples using these operators.
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243
244If the right argument is an expression rather than a search pattern,
2c268ad5 245substitution, or transliteration, it is interpreted as a search pattern at run
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246time. Note that this means that its contents will be interpolated twice, so
247
248 '\\' =~ q'\\';
249
250is not ok, as the regex engine will end up trying to compile the
251pattern C<\>, which it will consider a syntax error.
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252
253Binary "!~" is just like "=~" except the return value is negated in
254the logical sense.
255
000c65fc 256Binary "!~" with a non-destructive substitution (s///r) is a syntax error.
4f4d7508 257
a0d0e21e 258=head2 Multiplicative Operators
d74e8afc 259X<operator, multiplicative>
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260
261Binary "*" multiplies two numbers.
d74e8afc 262X<*>
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263
264Binary "/" divides two numbers.
d74e8afc 265X</> X<slash>
a0d0e21e 266
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267Binary "%" is the modulo operator, which computes the division
268remainder of its first argument with respect to its second argument.
269Given integer
54310121 270operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
f7918450 271C<$a> minus the largest multiple of C<$b> less than or equal to
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272C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
273smallest multiple of C<$b> that is not less than C<$a> (i.e. the
89b4f0ad 274result will be less than or equal to zero). If the operands
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275C<$a> and C<$b> are floating point values and the absolute value of
276C<$b> (that is C<abs($b)>) is less than C<(UV_MAX + 1)>, only
277the integer portion of C<$a> and C<$b> will be used in the operation
278(Note: here C<UV_MAX> means the maximum of the unsigned integer type).
279If the absolute value of the right operand (C<abs($b)>) is greater than
280or equal to C<(UV_MAX + 1)>, "%" computes the floating-point remainder
281C<$r> in the equation C<($r = $a - $i*$b)> where C<$i> is a certain
f7918450 282integer that makes C<$r> have the same sign as the right operand
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283C<$b> (B<not> as the left operand C<$a> like C function C<fmod()>)
284and the absolute value less than that of C<$b>.
0412d526 285Note that when C<use integer> is in scope, "%" gives you direct access
f7918450 286to the modulo operator as implemented by your C compiler. This
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287operator is not as well defined for negative operands, but it will
288execute faster.
f7918450 289X<%> X<remainder> X<modulo> X<mod>
55d729e4 290
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291Binary "x" is the repetition operator. In scalar context or if the left
292operand is not enclosed in parentheses, it returns a string consisting
293of the left operand repeated the number of times specified by the right
294operand. In list context, if the left operand is enclosed in
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295parentheses or is a list formed by C<qw/STRING/>, it repeats the list.
296If the right operand is zero or negative, it returns an empty string
297or an empty list, depending on the context.
d74e8afc 298X<x>
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299
300 print '-' x 80; # print row of dashes
301
302 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
303
304 @ones = (1) x 80; # a list of 80 1's
305 @ones = (5) x @ones; # set all elements to 5
306
307
308=head2 Additive Operators
d74e8afc 309X<operator, additive>
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310
311Binary "+" returns the sum of two numbers.
d74e8afc 312X<+>
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313
314Binary "-" returns the difference of two numbers.
d74e8afc 315X<->
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316
317Binary "." concatenates two strings.
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318X<string, concatenation> X<concatenation>
319X<cat> X<concat> X<concatenate> X<.>
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320
321=head2 Shift Operators
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322X<shift operator> X<operator, shift> X<<< << >>>
323X<<< >> >>> X<right shift> X<left shift> X<bitwise shift>
324X<shl> X<shr> X<shift, right> X<shift, left>
a0d0e21e 325
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326Binary "<<" returns the value of its left argument shifted left by the
327number of bits specified by the right argument. Arguments should be
982ce180 328integers. (See also L<Integer Arithmetic>.)
a0d0e21e 329
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330Binary ">>" returns the value of its left argument shifted right by
331the number of bits specified by the right argument. Arguments should
982ce180 332be integers. (See also L<Integer Arithmetic>.)
a0d0e21e 333
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334Note that both "<<" and ">>" in Perl are implemented directly using
335"<<" and ">>" in C. If C<use integer> (see L<Integer Arithmetic>) is
336in force then signed C integers are used, else unsigned C integers are
337used. Either way, the implementation isn't going to generate results
338larger than the size of the integer type Perl was built with (32 bits
339or 64 bits).
340
341The result of overflowing the range of the integers is undefined
342because it is undefined also in C. In other words, using 32-bit
343integers, C<< 1 << 32 >> is undefined. Shifting by a negative number
344of bits is also undefined.
345
a0d0e21e 346=head2 Named Unary Operators
d74e8afc 347X<operator, named unary>
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348
349The various named unary operators are treated as functions with one
568e6d8b 350argument, with optional parentheses.
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351
352If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
353is followed by a left parenthesis as the next token, the operator and
354arguments within parentheses are taken to be of highest precedence,
3981b0eb 355just like a normal function call. For example,
356because named unary operators are higher precedence than ||:
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357
358 chdir $foo || die; # (chdir $foo) || die
359 chdir($foo) || die; # (chdir $foo) || die
360 chdir ($foo) || die; # (chdir $foo) || die
361 chdir +($foo) || die; # (chdir $foo) || die
362
3981b0eb 363but, because * is higher precedence than named operators:
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364
365 chdir $foo * 20; # chdir ($foo * 20)
366 chdir($foo) * 20; # (chdir $foo) * 20
367 chdir ($foo) * 20; # (chdir $foo) * 20
368 chdir +($foo) * 20; # chdir ($foo * 20)
369
370 rand 10 * 20; # rand (10 * 20)
371 rand(10) * 20; # (rand 10) * 20
372 rand (10) * 20; # (rand 10) * 20
373 rand +(10) * 20; # rand (10 * 20)
374
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375Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are
376treated like named unary operators, but they don't follow this functional
377parenthesis rule. That means, for example, that C<-f($file).".bak"> is
378equivalent to C<-f "$file.bak">.
d74e8afc 379X<-X> X<filetest> X<operator, filetest>
568e6d8b 380
5ba421f6 381See also L<"Terms and List Operators (Leftward)">.
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382
383=head2 Relational Operators
d74e8afc 384X<relational operator> X<operator, relational>
a0d0e21e 385
35f2feb0 386Binary "<" returns true if the left argument is numerically less than
a0d0e21e 387the right argument.
d74e8afc 388X<< < >>
a0d0e21e 389
35f2feb0 390Binary ">" returns true if the left argument is numerically greater
a0d0e21e 391than the right argument.
d74e8afc 392X<< > >>
a0d0e21e 393
35f2feb0 394Binary "<=" returns true if the left argument is numerically less than
a0d0e21e 395or equal to the right argument.
d74e8afc 396X<< <= >>
a0d0e21e 397
35f2feb0 398Binary ">=" returns true if the left argument is numerically greater
a0d0e21e 399than or equal to the right argument.
d74e8afc 400X<< >= >>
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401
402Binary "lt" returns true if the left argument is stringwise less than
403the right argument.
d74e8afc 404X<< lt >>
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405
406Binary "gt" returns true if the left argument is stringwise greater
407than the right argument.
d74e8afc 408X<< gt >>
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409
410Binary "le" returns true if the left argument is stringwise less than
411or equal to the right argument.
d74e8afc 412X<< le >>
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413
414Binary "ge" returns true if the left argument is stringwise greater
415than or equal to the right argument.
d74e8afc 416X<< ge >>
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417
418=head2 Equality Operators
d74e8afc 419X<equality> X<equal> X<equals> X<operator, equality>
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420
421Binary "==" returns true if the left argument is numerically equal to
422the right argument.
d74e8afc 423X<==>
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424
425Binary "!=" returns true if the left argument is numerically not equal
426to the right argument.
d74e8afc 427X<!=>
a0d0e21e 428
35f2feb0 429Binary "<=>" returns -1, 0, or 1 depending on whether the left
6ee5d4e7 430argument is numerically less than, equal to, or greater than the right
d4ad863d 431argument. If your platform supports NaNs (not-a-numbers) as numeric
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432values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
433"<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
434returns true, as does NaN != anything else. If your platform doesn't
435support NaNs then NaN is just a string with numeric value 0.
d74e8afc 436X<< <=> >> X<spaceship>
7d3a9d88 437
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438 perl -le '$a = "NaN"; print "No NaN support here" if $a == $a'
439 perl -le '$a = "NaN"; print "NaN support here" if $a != $a'
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440
441Binary "eq" returns true if the left argument is stringwise equal to
442the right argument.
d74e8afc 443X<eq>
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444
445Binary "ne" returns true if the left argument is stringwise not equal
446to the right argument.
d74e8afc 447X<ne>
a0d0e21e 448
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449Binary "cmp" returns -1, 0, or 1 depending on whether the left
450argument is stringwise less than, equal to, or greater than the right
451argument.
d74e8afc 452X<cmp>
a0d0e21e 453
0d863452 454Binary "~~" does a smart match between its arguments. Smart matching
0f7107a0 455is described in L<perlsyn/"Smart matching in detail">.
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456X<~~>
457
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458"lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
459by the current locale if C<use locale> is in effect. See L<perllocale>.
460
a0d0e21e 461=head2 Bitwise And
d74e8afc 462X<operator, bitwise, and> X<bitwise and> X<&>
a0d0e21e 463
2cdc098b 464Binary "&" returns its operands ANDed together bit by bit.
2c268ad5 465(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
a0d0e21e 466
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467Note that "&" has lower priority than relational operators, so for example
468the brackets are essential in a test like
469
470 print "Even\n" if ($x & 1) == 0;
471
a0d0e21e 472=head2 Bitwise Or and Exclusive Or
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473X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor>
474X<bitwise xor> X<^>
a0d0e21e 475
2cdc098b 476Binary "|" returns its operands ORed together bit by bit.
2c268ad5 477(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
a0d0e21e 478
2cdc098b 479Binary "^" returns its operands XORed together bit by bit.
2c268ad5 480(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
a0d0e21e 481
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482Note that "|" and "^" have lower priority than relational operators, so
483for example the brackets are essential in a test like
484
485 print "false\n" if (8 | 2) != 10;
486
a0d0e21e 487=head2 C-style Logical And
d74e8afc 488X<&&> X<logical and> X<operator, logical, and>
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489
490Binary "&&" performs a short-circuit logical AND operation. That is,
491if the left operand is false, the right operand is not even evaluated.
492Scalar or list context propagates down to the right operand if it
493is evaluated.
494
495=head2 C-style Logical Or
d74e8afc 496X<||> X<operator, logical, or>
a0d0e21e
LW
497
498Binary "||" performs a short-circuit logical OR operation. That is,
499if the left operand is true, the right operand is not even evaluated.
500Scalar or list context propagates down to the right operand if it
501is evaluated.
502
c963b151 503=head2 C-style Logical Defined-Or
d74e8afc 504X<//> X<operator, logical, defined-or>
c963b151
BD
505
506Although it has no direct equivalent in C, Perl's C<//> operator is related
89d205f2 507to its C-style or. In fact, it's exactly the same as C<||>, except that it
c963b151 508tests the left hand side's definedness instead of its truth. Thus, C<$a // $b>
89d205f2
YO
509is similar to C<defined($a) || $b> (except that it returns the value of C<$a>
510rather than the value of C<defined($a)>) and is exactly equivalent to
c963b151 511C<defined($a) ? $a : $b>. This is very useful for providing default values
89d205f2 512for variables. If you actually want to test if at least one of C<$a> and
d042e63d 513C<$b> is defined, use C<defined($a // $b)>.
c963b151 514
d042e63d
MS
515The C<||>, C<//> and C<&&> operators return the last value evaluated
516(unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably
517portable way to find out the home directory might be:
a0d0e21e 518
c963b151
BD
519 $home = $ENV{'HOME'} // $ENV{'LOGDIR'} //
520 (getpwuid($<))[7] // die "You're homeless!\n";
a0d0e21e 521
5a964f20
TC
522In particular, this means that you shouldn't use this
523for selecting between two aggregates for assignment:
524
525 @a = @b || @c; # this is wrong
526 @a = scalar(@b) || @c; # really meant this
527 @a = @b ? @b : @c; # this works fine, though
528
f23102e2
RGS
529As more readable alternatives to C<&&> and C<||> when used for
530control flow, Perl provides the C<and> and C<or> operators (see below).
531The short-circuit behavior is identical. The precedence of "and"
c963b151 532and "or" is much lower, however, so that you can safely use them after a
5a964f20 533list operator without the need for parentheses:
a0d0e21e
LW
534
535 unlink "alpha", "beta", "gamma"
536 or gripe(), next LINE;
537
538With the C-style operators that would have been written like this:
539
540 unlink("alpha", "beta", "gamma")
541 || (gripe(), next LINE);
542
eeb6a2c9 543Using "or" for assignment is unlikely to do what you want; see below.
5a964f20
TC
544
545=head2 Range Operators
d74e8afc 546X<operator, range> X<range> X<..> X<...>
a0d0e21e
LW
547
548Binary ".." is the range operator, which is really two different
fb53bbb2 549operators depending on the context. In list context, it returns a
54ae734e 550list of values counting (up by ones) from the left value to the right
2cdbc966 551value. If the left value is greater than the right value then it
fb53bbb2 552returns the empty list. The range operator is useful for writing
54ae734e 553C<foreach (1..10)> loops and for doing slice operations on arrays. In
2cdbc966
JD
554the current implementation, no temporary array is created when the
555range operator is used as the expression in C<foreach> loops, but older
556versions of Perl might burn a lot of memory when you write something
557like this:
a0d0e21e
LW
558
559 for (1 .. 1_000_000) {
560 # code
54310121 561 }
a0d0e21e 562
8f0f46f8 563The range operator also works on strings, using the magical
564auto-increment, see below.
54ae734e 565
5a964f20 566In scalar context, ".." returns a boolean value. The operator is
8f0f46f8 567bistable, like a flip-flop, and emulates the line-range (comma)
568operator of B<sed>, B<awk>, and various editors. Each ".." operator
569maintains its own boolean state, even across calls to a subroutine
570that contains it. It is false as long as its left operand is false.
a0d0e21e
LW
571Once the left operand is true, the range operator stays true until the
572right operand is true, I<AFTER> which the range operator becomes false
8f0f46f8 573again. It doesn't become false till the next time the range operator
574is evaluated. It can test the right operand and become false on the
575same evaluation it became true (as in B<awk>), but it still returns
576true once. If you don't want it to test the right operand until the
577next evaluation, as in B<sed>, just use three dots ("...") instead of
19799a22
GS
578two. In all other regards, "..." behaves just like ".." does.
579
580The right operand is not evaluated while the operator is in the
581"false" state, and the left operand is not evaluated while the
582operator is in the "true" state. The precedence is a little lower
583than || and &&. The value returned is either the empty string for
8f0f46f8 584false, or a sequence number (beginning with 1) for true. The sequence
585number is reset for each range encountered. The final sequence number
586in a range has the string "E0" appended to it, which doesn't affect
587its numeric value, but gives you something to search for if you want
588to exclude the endpoint. You can exclude the beginning point by
589waiting for the sequence number to be greater than 1.
df5f8116
CW
590
591If either operand of scalar ".." is a constant expression,
592that operand is considered true if it is equal (C<==>) to the current
593input line number (the C<$.> variable).
594
595To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>,
596but that is only an issue if you use a floating point expression; when
597implicitly using C<$.> as described in the previous paragraph, the
598comparison is C<int(EXPR) == int($.)> which is only an issue when C<$.>
599is set to a floating point value and you are not reading from a file.
600Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what
601you want in scalar context because each of the operands are evaluated
602using their integer representation.
603
604Examples:
a0d0e21e
LW
605
606As a scalar operator:
607
df5f8116 608 if (101 .. 200) { print; } # print 2nd hundred lines, short for
950b09ed 609 # if ($. == 101 .. $. == 200) { print; }
9f10b797
RGS
610
611 next LINE if (1 .. /^$/); # skip header lines, short for
f343f960 612 # next LINE if ($. == 1 .. /^$/);
9f10b797
RGS
613 # (typically in a loop labeled LINE)
614
615 s/^/> / if (/^$/ .. eof()); # quote body
a0d0e21e 616
5a964f20
TC
617 # parse mail messages
618 while (<>) {
619 $in_header = 1 .. /^$/;
df5f8116
CW
620 $in_body = /^$/ .. eof;
621 if ($in_header) {
f343f960 622 # do something
df5f8116 623 } else { # in body
f343f960 624 # do something else
df5f8116 625 }
5a964f20 626 } continue {
df5f8116 627 close ARGV if eof; # reset $. each file
5a964f20
TC
628 }
629
acf31ca5
SF
630Here's a simple example to illustrate the difference between
631the two range operators:
632
633 @lines = (" - Foo",
634 "01 - Bar",
635 "1 - Baz",
636 " - Quux");
637
9f10b797
RGS
638 foreach (@lines) {
639 if (/0/ .. /1/) {
acf31ca5
SF
640 print "$_\n";
641 }
642 }
643
9f10b797
RGS
644This program will print only the line containing "Bar". If
645the range operator is changed to C<...>, it will also print the
acf31ca5
SF
646"Baz" line.
647
648And now some examples as a list operator:
a0d0e21e
LW
649
650 for (101 .. 200) { print; } # print $_ 100 times
3e3baf6d 651 @foo = @foo[0 .. $#foo]; # an expensive no-op
a0d0e21e
LW
652 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
653
5a964f20 654The range operator (in list context) makes use of the magical
5f05dabc 655auto-increment algorithm if the operands are strings. You
a0d0e21e
LW
656can say
657
658 @alphabet = ('A' .. 'Z');
659
54ae734e 660to get all normal letters of the English alphabet, or
a0d0e21e
LW
661
662 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
663
664to get a hexadecimal digit, or
665
666 @z2 = ('01' .. '31'); print $z2[$mday];
667
ea4f5703
YST
668to get dates with leading zeros.
669
670If the final value specified is not in the sequence that the magical
671increment would produce, the sequence goes until the next value would
672be longer than the final value specified.
673
674If the initial value specified isn't part of a magical increment
675sequence (that is, a non-empty string matching "/^[a-zA-Z]*[0-9]*\z/"),
676only the initial value will be returned. So the following will only
677return an alpha:
678
679 use charnames 'greek';
680 my @greek_small = ("\N{alpha}" .. "\N{omega}");
681
682To get lower-case greek letters, use this instead:
683
950b09ed
KW
684 my @greek_small = map { chr } ( ord("\N{alpha}") ..
685 ord("\N{omega}") );
a0d0e21e 686
df5f8116
CW
687Because each operand is evaluated in integer form, C<2.18 .. 3.14> will
688return two elements in list context.
689
690 @list = (2.18 .. 3.14); # same as @list = (2 .. 3);
691
a0d0e21e 692=head2 Conditional Operator
d74e8afc 693X<operator, conditional> X<operator, ternary> X<ternary> X<?:>
a0d0e21e
LW
694
695Ternary "?:" is the conditional operator, just as in C. It works much
696like an if-then-else. If the argument before the ? is true, the
697argument before the : is returned, otherwise the argument after the :
cb1a09d0
AD
698is returned. For example:
699
54310121 700 printf "I have %d dog%s.\n", $n,
cb1a09d0
AD
701 ($n == 1) ? '' : "s";
702
703Scalar or list context propagates downward into the 2nd
54310121 704or 3rd argument, whichever is selected.
cb1a09d0
AD
705
706 $a = $ok ? $b : $c; # get a scalar
707 @a = $ok ? @b : @c; # get an array
708 $a = $ok ? @b : @c; # oops, that's just a count!
709
710The operator may be assigned to if both the 2nd and 3rd arguments are
711legal lvalues (meaning that you can assign to them):
a0d0e21e
LW
712
713 ($a_or_b ? $a : $b) = $c;
714
5a964f20
TC
715Because this operator produces an assignable result, using assignments
716without parentheses will get you in trouble. For example, this:
717
718 $a % 2 ? $a += 10 : $a += 2
719
720Really means this:
721
722 (($a % 2) ? ($a += 10) : $a) += 2
723
724Rather than this:
725
726 ($a % 2) ? ($a += 10) : ($a += 2)
727
19799a22
GS
728That should probably be written more simply as:
729
730 $a += ($a % 2) ? 10 : 2;
731
4633a7c4 732=head2 Assignment Operators
d74e8afc 733X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=>
5ac3b81c 734X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=>
d74e8afc 735X<%=> X<^=> X<x=>
a0d0e21e
LW
736
737"=" is the ordinary assignment operator.
738
739Assignment operators work as in C. That is,
740
741 $a += 2;
742
743is equivalent to
744
745 $a = $a + 2;
746
747although without duplicating any side effects that dereferencing the lvalue
54310121
PP
748might trigger, such as from tie(). Other assignment operators work similarly.
749The following are recognized:
a0d0e21e
LW
750
751 **= += *= &= <<= &&=
9f10b797
RGS
752 -= /= |= >>= ||=
753 .= %= ^= //=
754 x=
a0d0e21e 755
19799a22 756Although these are grouped by family, they all have the precedence
a0d0e21e
LW
757of assignment.
758
b350dd2f
GS
759Unlike in C, the scalar assignment operator produces a valid lvalue.
760Modifying an assignment is equivalent to doing the assignment and
761then modifying the variable that was assigned to. This is useful
762for modifying a copy of something, like this:
a0d0e21e
LW
763
764 ($tmp = $global) =~ tr [A-Z] [a-z];
765
766Likewise,
767
768 ($a += 2) *= 3;
769
770is equivalent to
771
772 $a += 2;
773 $a *= 3;
774
b350dd2f
GS
775Similarly, a list assignment in list context produces the list of
776lvalues assigned to, and a list assignment in scalar context returns
777the number of elements produced by the expression on the right hand
778side of the assignment.
779
748a9306 780=head2 Comma Operator
d74e8afc 781X<comma> X<operator, comma> X<,>
a0d0e21e 782
5a964f20 783Binary "," is the comma operator. In scalar context it evaluates
a0d0e21e
LW
784its left argument, throws that value away, then evaluates its right
785argument and returns that value. This is just like C's comma operator.
786
5a964f20 787In list context, it's just the list argument separator, and inserts
ed5c6d31
PJ
788both its arguments into the list. These arguments are also evaluated
789from left to right.
a0d0e21e 790
344f2c40
IG
791The C<< => >> operator is a synonym for the comma except that it causes
792its left operand to be interpreted as a string if it begins with a letter
793or underscore and is composed only of letters, digits and underscores.
794This includes operands that might otherwise be interpreted as operators,
795constants, single number v-strings or function calls. If in doubt about
796this behaviour, the left operand can be quoted explicitly.
797
798Otherwise, the C<< => >> operator behaves exactly as the comma operator
799or list argument separator, according to context.
800
801For example:
a44e5664
MS
802
803 use constant FOO => "something";
804
805 my %h = ( FOO => 23 );
806
807is equivalent to:
808
809 my %h = ("FOO", 23);
810
811It is I<NOT>:
812
813 my %h = ("something", 23);
814
719b43e8
RGS
815The C<< => >> operator is helpful in documenting the correspondence
816between keys and values in hashes, and other paired elements in lists.
748a9306 817
a44e5664
MS
818 %hash = ( $key => $value );
819 login( $username => $password );
820
678ae90b
RGS
821=head2 Yada Yada Operator
822X<...> X<... operator> X<yada yada operator>
be25f609 823
e8163f9b 824The yada yada operator (noted C<...>) is a placeholder for code. Perl
825parses it without error, but when you try to execute a yada yada, it
826throws an exception with the text C<Unimplemented>:
827
828 sub unimplemented { ... }
829
830 eval { unimplemented() };
831 if( $@ eq 'Unimplemented' ) {
832 print "I found the yada yada!\n";
833 }
834
835You can only use the yada yada to stand in for a complete statement.
836These examples of the yada yada work:
837
838 { ... }
839
840 sub foo { ... }
841
842 ...;
843
844 eval { ... };
845
846 sub foo {
847 my( $self ) = shift;
848
849 ...;
850 }
851
852 do { my $n; ...; print 'Hurrah!' };
853
854The yada yada cannot stand in for an expression that is part of a
855larger statement since the C<...> is also the three-dot version of the
856range operator (see L<Range Operators>). These examples of the yada
857yada are still syntax errors:
858
859 print ...;
860
861 open my($fh), '>', '/dev/passwd' or ...;
862
863 if( $condition && ... ) { print "Hello\n" };
864
865There are some cases where Perl can't immediately tell the difference
866between an expression and a statement. For instance, the syntax for a
867block and an anonymous hash reference constructor look the same unless
868there's something in the braces that give Perl a hint. The yada yada
869is a syntax error if Perl doesn't guess that the C<{ ... }> is a
870block. In that case, it doesn't think the C<...> is the yada yada
871because it's expecting an expression instead of a statement:
872
873 my @transformed = map { ... } @input; # syntax error
874
875You can use a C<;> inside your block to denote that the C<{ ... }> is
876a block and not a hash reference constructor. Now the yada yada works:
877
878 my @transformed = map {; ... } @input; # ; disambiguates
879
880 my @transformed = map { ...; } @input; # ; disambiguates
be25f609 881
a0d0e21e 882=head2 List Operators (Rightward)
d74e8afc 883X<operator, list, rightward> X<list operator>
a0d0e21e
LW
884
885On the right side of a list operator, it has very low precedence,
886such that it controls all comma-separated expressions found there.
887The only operators with lower precedence are the logical operators
888"and", "or", and "not", which may be used to evaluate calls to list
889operators without the need for extra parentheses:
890
891 open HANDLE, "filename"
892 or die "Can't open: $!\n";
893
5ba421f6 894See also discussion of list operators in L<Terms and List Operators (Leftward)>.
a0d0e21e
LW
895
896=head2 Logical Not
d74e8afc 897X<operator, logical, not> X<not>
a0d0e21e
LW
898
899Unary "not" returns the logical negation of the expression to its right.
900It's the equivalent of "!" except for the very low precedence.
901
902=head2 Logical And
d74e8afc 903X<operator, logical, and> X<and>
a0d0e21e
LW
904
905Binary "and" returns the logical conjunction of the two surrounding
906expressions. It's equivalent to && except for the very low
5f05dabc 907precedence. This means that it short-circuits: i.e., the right
a0d0e21e
LW
908expression is evaluated only if the left expression is true.
909
c963b151 910=head2 Logical or, Defined or, and Exclusive Or
f23102e2 911X<operator, logical, or> X<operator, logical, xor>
d74e8afc 912X<operator, logical, defined or> X<operator, logical, exclusive or>
f23102e2 913X<or> X<xor>
a0d0e21e
LW
914
915Binary "or" returns the logical disjunction of the two surrounding
5a964f20
TC
916expressions. It's equivalent to || except for the very low precedence.
917This makes it useful for control flow
918
919 print FH $data or die "Can't write to FH: $!";
920
921This means that it short-circuits: i.e., the right expression is evaluated
922only if the left expression is false. Due to its precedence, you should
923probably avoid using this for assignment, only for control flow.
924
925 $a = $b or $c; # bug: this is wrong
926 ($a = $b) or $c; # really means this
927 $a = $b || $c; # better written this way
928
19799a22 929However, when it's a list-context assignment and you're trying to use
5a964f20
TC
930"||" for control flow, you probably need "or" so that the assignment
931takes higher precedence.
932
933 @info = stat($file) || die; # oops, scalar sense of stat!
934 @info = stat($file) or die; # better, now @info gets its due
935
c963b151
BD
936Then again, you could always use parentheses.
937
a0d0e21e
LW
938Binary "xor" returns the exclusive-OR of the two surrounding expressions.
939It cannot short circuit, of course.
940
941=head2 C Operators Missing From Perl
d74e8afc
ITB
942X<operator, missing from perl> X<&> X<*>
943X<typecasting> X<(TYPE)>
a0d0e21e
LW
944
945Here is what C has that Perl doesn't:
946
947=over 8
948
949=item unary &
950
951Address-of operator. (But see the "\" operator for taking a reference.)
952
953=item unary *
954
54310121 955Dereference-address operator. (Perl's prefix dereferencing
a0d0e21e
LW
956operators are typed: $, @, %, and &.)
957
958=item (TYPE)
959
19799a22 960Type-casting operator.
a0d0e21e
LW
961
962=back
963
5f05dabc 964=head2 Quote and Quote-like Operators
89d205f2 965X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m>
d74e8afc
ITB
966X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>>
967X<escape sequence> X<escape>
968
a0d0e21e
LW
969
970While we usually think of quotes as literal values, in Perl they
971function as operators, providing various kinds of interpolating and
972pattern matching capabilities. Perl provides customary quote characters
973for these behaviors, but also provides a way for you to choose your
974quote character for any of them. In the following table, a C<{}> represents
9f10b797 975any pair of delimiters you choose.
a0d0e21e 976
2c268ad5
TP
977 Customary Generic Meaning Interpolates
978 '' q{} Literal no
979 "" qq{} Literal yes
af9219ee 980 `` qx{} Command yes*
2c268ad5 981 qw{} Word list no
af9219ee
MG
982 // m{} Pattern match yes*
983 qr{} Pattern yes*
984 s{}{} Substitution yes*
2c268ad5 985 tr{}{} Transliteration no (but see below)
7e3b091d 986 <<EOF here-doc yes*
a0d0e21e 987
af9219ee
MG
988 * unless the delimiter is ''.
989
87275199
GS
990Non-bracketing delimiters use the same character fore and aft, but the four
991sorts of brackets (round, angle, square, curly) will all nest, which means
9f10b797 992that
87275199 993
9f10b797 994 q{foo{bar}baz}
35f2feb0 995
9f10b797 996is the same as
87275199
GS
997
998 'foo{bar}baz'
999
1000Note, however, that this does not always work for quoting Perl code:
1001
1002 $s = q{ if($a eq "}") ... }; # WRONG
1003
83df6a1d
JH
1004is a syntax error. The C<Text::Balanced> module (from CPAN, and
1005starting from Perl 5.8 part of the standard distribution) is able
1006to do this properly.
87275199 1007
19799a22 1008There can be whitespace between the operator and the quoting
fb73857a 1009characters, except when C<#> is being used as the quoting character.
19799a22
GS
1010C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
1011operator C<q> followed by a comment. Its argument will be taken
1012from the next line. This allows you to write:
fb73857a
PP
1013
1014 s {foo} # Replace foo
1015 {bar} # with bar.
1016
904501ec
MG
1017The following escape sequences are available in constructs that interpolate
1018and in transliterations.
5691ca5f 1019X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N> X<\N{}>
04341565 1020X<\o{}>
5691ca5f 1021
2c4c1ff2
KW
1022 Sequence Note Description
1023 \t tab (HT, TAB)
1024 \n newline (NL)
1025 \r return (CR)
1026 \f form feed (FF)
1027 \b backspace (BS)
1028 \a alarm (bell) (BEL)
1029 \e escape (ESC)
1030 \x{263a} [1,8] hex char (example: SMILEY)
1031 \x1b [2,8] restricted range hex char (example: ESC)
1032 \N{name} [3] named Unicode character
1033 \N{U+263D} [4,8] Unicode character (example: FIRST QUARTER MOON)
1034 \c[ [5] control char (example: chr(27))
1035 \o{23072} [6,8] octal char (example: SMILEY)
1036 \033 [7,8] restricted range octal char (example: ESC)
5691ca5f
KW
1037
1038=over 4
1039
1040=item [1]
1041
2c4c1ff2
KW
1042The result is the character specified by the hexadecimal number between
1043the braces. See L</[8]> below for details on which character.
96448467
DG
1044
1045Only hexadecimal digits are valid between the braces. If an invalid
1046character is encountered, a warning will be issued and the invalid
1047character and all subsequent characters (valid or invalid) within the
1048braces will be discarded.
1049
1050If there are no valid digits between the braces, the generated character is
1051the NULL character (C<\x{00}>). However, an explicit empty brace (C<\x{}>)
1052will not cause a warning.
40687185
KW
1053
1054=item [2]
1055
2c4c1ff2
KW
1056The result is the character specified by the hexadecimal number in the range
10570x00 to 0xFF. See L</[8]> below for details on which character.
96448467
DG
1058
1059Only hexadecimal digits are valid following C<\x>. When C<\x> is followed
2c4c1ff2 1060by fewer than two valid digits, any valid digits will be zero-padded. This
96448467 1061means that C<\x7> will be interpreted as C<\x07> and C<\x> alone will be
2c4c1ff2 1062interpreted as C<\x00>. Except at the end of a string, having fewer than
96448467
DG
1063two valid digits will result in a warning. Note that while the warning
1064says the illegal character is ignored, it is only ignored as part of the
1065escape and will still be used as the subsequent character in the string.
1066For example:
1067
1068 Original Result Warns?
1069 "\x7" "\x07" no
1070 "\x" "\x00" no
1071 "\x7q" "\x07q" yes
1072 "\xq" "\x00q" yes
1073
40687185
KW
1074=item [3]
1075
2c4c1ff2
KW
1076The result is the Unicode character given by I<name>.
1077See L<charnames>.
40687185
KW
1078
1079=item [4]
1080
2c4c1ff2
KW
1081C<\N{U+I<hexadecimal number>}> means the Unicode character whose Unicode code
1082point is I<hexadecimal number>.
40687185
KW
1083
1084=item [5]
1085
5691ca5f
KW
1086The character following C<\c> is mapped to some other character as shown in the
1087table:
1088
1089 Sequence Value
1090 \c@ chr(0)
1091 \cA chr(1)
1092 \ca chr(1)
1093 \cB chr(2)
1094 \cb chr(2)
1095 ...
1096 \cZ chr(26)
1097 \cz chr(26)
1098 \c[ chr(27)
1099 \c] chr(29)
1100 \c^ chr(30)
1101 \c? chr(127)
1102
1103Also, C<\c\I<X>> yields C< chr(28) . "I<X>"> for any I<X>, but cannot come at the
1104end of a string, because the backslash would be parsed as escaping the end
1105quote.
1106
1107On ASCII platforms, the resulting characters from the list above are the
1108complete set of ASCII controls. This isn't the case on EBCDIC platforms; see
1109L<perlebcdic/OPERATOR DIFFERENCES> for the complete list of what these
1110sequences mean on both ASCII and EBCDIC platforms.
1111
1112Use of any other character following the "c" besides those listed above is
6ae9f32a
KW
1113discouraged, and may become deprecated or forbidden. What happens for those
1114other characters currently though, is that the value is derived by inverting
1115the 7th bit (0x40).
5691ca5f
KW
1116
1117To get platform independent controls, you can use C<\N{...}>.
1118
40687185
KW
1119=item [6]
1120
2c4c1ff2
KW
1121The result is the character specified by the octal number between the braces.
1122See L</[8]> below for details on which character.
04341565
DG
1123
1124If a character that isn't an octal digit is encountered, a warning is raised,
1125and the value is based on the octal digits before it, discarding it and all
1126following characters up to the closing brace. It is a fatal error if there are
1127no octal digits at all.
1128
1129=item [7]
1130
2c4c1ff2
KW
1131The result is the character specified by the three digit octal number in the
1132range 000 to 777 (but best to not use above 077, see next paragraph). See
1133L</[8]> below for details on which character.
1134
1135Some contexts allow 2 or even 1 digit, but any usage without exactly
40687185 1136three digits, the first being a zero, may give unintended results. (For
04341565
DG
1137example, see L<perlrebackslash/Octal escapes>.) Starting in Perl 5.14, you may
1138use C<\o{}> instead which avoids all these problems. Otherwise, it is best to
1139use this construct only for ordinals C<\077> and below, remembering to pad to
1140the left with zeros to make three digits. For larger ordinals, either use
1141C<\o{}> , or convert to someething else, such as to hex and use C<\x{}>
1142instead.
40687185
KW
1143
1144A backslash followed by a non-octal digit in a bracketed character class
1145(C<[\8]> or C<[\9]>) will be interpreted as a NULL character and the digit.
2c4c1ff2 1146
40687185
KW
1147Having fewer than 3 digits may lead to a misleading warning message that says
1148that what follows is ignored. For example, C<"\128"> in the ASCII character set
1149is equivalent to the two characters C<"\n8">, but the warning C<Illegal octal
1150digit '8' ignored> will be thrown. To avoid this warning, make sure to pad
1151your octal number with C<0>s: C<"\0128">.
5691ca5f 1152
2c4c1ff2
KW
1153=item [8]
1154
1155Several of the constructs above specify a character by a number. That number
1156gives the character's position in the character set encoding (indexed from 0).
1157This is called synonymously its ordinal, code position, or code point). Perl
1158works on platforms that have a native encoding currently of either ASCII/Latin1
1159or EBCDIC, each of which allow specification of 256 characters. In general, if
1160the number is 255 (0xFF, 0377) or below, Perl interprets this in the platform's
1161native encoding. If the number is 256 (0x100, 0400) or above, Perl interprets
1162it as as a Unicode code point and the result is the corresponding Unicode
1163character. For example C<\x{50}> and C<\o{120}> both are the number 80 in
1164decimal, which is less than 256, so the number is interpreted in the native
1165character set encoding. In ASCII the character in the 80th position (indexed
1166from 0) is the letter "P", and in EBCDIC it is the ampersand symbol "&".
1167C<\x{100}> and C<\o{400}> are both 256 in decimal, so the number is interpreted
1168as a Unicode code point no matter what the native encoding is. The name of the
1169character in the 100th position (indexed by 0) in Unicode is
1170C<LATIN CAPITAL LETTER A WITH MACRON>.
1171
1172There are a couple of exceptions to the above rule. C<\N{U+I<hex number>}> is
1173always interpreted as a Unicode code point, so that C<\N{U+0050}> is "P" even
1174on EBCDIC platforms. And if L<C<S<use encoding>>|encoding> is in effect, the
1175number is considered to be in that encoding, and is translated from that into
1176the platform's native encoding if there is a corresponding native character;
1177otherwise to Unicode.
1178
5691ca5f 1179=back
4c77eaa2 1180
e526e8bb
KW
1181B<NOTE>: Unlike C and other languages, Perl has no C<\v> escape sequence for
1182the vertical tab (VT - ASCII 11), but you may use C<\ck> or C<\x0b>. (C<\v>
1183does have meaning in regular expression patterns in Perl, see L<perlre>.)
1184
1185The following escape sequences are available in constructs that interpolate,
904501ec 1186but not in transliterations.
d74e8afc 1187X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q>
904501ec 1188
a0d0e21e
LW
1189 \l lowercase next char
1190 \u uppercase next char
1191 \L lowercase till \E
1192 \U uppercase till \E
1193 \E end case modification
1d2dff63 1194 \Q quote non-word characters till \E
a0d0e21e 1195
95cc3e0c
JH
1196If C<use locale> is in effect, the case map used by C<\l>, C<\L>,
1197C<\u> and C<\U> is taken from the current locale. See L<perllocale>.
1198If Unicode (for example, C<\N{}> or wide hex characters of 0x100 or
1199beyond) is being used, the case map used by C<\l>, C<\L>, C<\u> and
e526e8bb 1200C<\U> is as defined by Unicode.
a034a98d 1201
5a964f20
TC
1202All systems use the virtual C<"\n"> to represent a line terminator,
1203called a "newline". There is no such thing as an unvarying, physical
19799a22 1204newline character. It is only an illusion that the operating system,
5a964f20
TC
1205device drivers, C libraries, and Perl all conspire to preserve. Not all
1206systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
1207on a Mac, these are reversed, and on systems without line terminator,
1208printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
1209you mean a "newline" for your system, but use the literal ASCII when you
1210need an exact character. For example, most networking protocols expect
2a380090 1211and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
5a964f20
TC
1212and although they often accept just C<"\012">, they seldom tolerate just
1213C<"\015">. If you get in the habit of using C<"\n"> for networking,
1214you may be burned some day.
d74e8afc
ITB
1215X<newline> X<line terminator> X<eol> X<end of line>
1216X<\n> X<\r> X<\r\n>
5a964f20 1217
904501ec
MG
1218For constructs that do interpolate, variables beginning with "C<$>"
1219or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
ad0f383a
A
1220C<< $href->{key}[0] >> are also interpolated, as are array and hash slices.
1221But method calls such as C<< $obj->meth >> are not.
af9219ee
MG
1222
1223Interpolating an array or slice interpolates the elements in order,
1224separated by the value of C<$">, so is equivalent to interpolating
6deea57f
ST
1225C<join $", @array>. "Punctuation" arrays such as C<@*> are only
1226interpolated if the name is enclosed in braces C<@{*}>, but special
1227arrays C<@_>, C<@+>, and C<@-> are interpolated, even without braces.
af9219ee 1228
89d205f2
YO
1229You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
1230An unescaped C<$> or C<@> interpolates the corresponding variable,
1d2dff63 1231while escaping will cause the literal string C<\$> to be inserted.
89d205f2 1232You'll need to write something like C<m/\Quser\E\@\Qhost/>.
1d2dff63 1233
a0d0e21e
LW
1234Patterns are subject to an additional level of interpretation as a
1235regular expression. This is done as a second pass, after variables are
1236interpolated, so that regular expressions may be incorporated into the
1237pattern from the variables. If this is not what you want, use C<\Q> to
1238interpolate a variable literally.
1239
19799a22
GS
1240Apart from the behavior described above, Perl does not expand
1241multiple levels of interpolation. In particular, contrary to the
1242expectations of shell programmers, back-quotes do I<NOT> interpolate
1243within double quotes, nor do single quotes impede evaluation of
1244variables when used within double quotes.
a0d0e21e 1245
5f05dabc 1246=head2 Regexp Quote-Like Operators
d74e8afc 1247X<operator, regexp>
cb1a09d0 1248
5f05dabc 1249Here are the quote-like operators that apply to pattern
cb1a09d0
AD
1250matching and related activities.
1251
a0d0e21e
LW
1252=over 8
1253
87e95b7f 1254=item qr/STRING/msixpo
01c6f5f4 1255X<qr> X</i> X</m> X</o> X</s> X</x> X</p>
a0d0e21e 1256
87e95b7f
YO
1257This operator quotes (and possibly compiles) its I<STRING> as a regular
1258expression. I<STRING> is interpolated the same way as I<PATTERN>
1259in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
1260is done. Returns a Perl value which may be used instead of the
64c5a566 1261corresponding C</STRING/msixpo> expression. The returned value is a
85dd5c8b 1262normalized version of the original pattern. It magically differs from
64c5a566 1263a string containing the same characters: C<ref(qr/x/)> returns "Regexp",
85dd5c8b 1264even though dereferencing the result returns undef.
a0d0e21e 1265
87e95b7f
YO
1266For example,
1267
1268 $rex = qr/my.STRING/is;
85dd5c8b 1269 print $rex; # prints (?si-xm:my.STRING)
87e95b7f
YO
1270 s/$rex/foo/;
1271
1272is equivalent to
1273
1274 s/my.STRING/foo/is;
1275
1276The result may be used as a subpattern in a match:
1277
1278 $re = qr/$pattern/;
1279 $string =~ /foo${re}bar/; # can be interpolated in other patterns
1280 $string =~ $re; # or used standalone
1281 $string =~ /$re/; # or this way
1282
1283Since Perl may compile the pattern at the moment of execution of qr()
1284operator, using qr() may have speed advantages in some situations,
1285notably if the result of qr() is used standalone:
1286
1287 sub match {
1288 my $patterns = shift;
1289 my @compiled = map qr/$_/i, @$patterns;
1290 grep {
1291 my $success = 0;
1292 foreach my $pat (@compiled) {
1293 $success = 1, last if /$pat/;
1294 }
1295 $success;
1296 } @_;
5a964f20
TC
1297 }
1298
87e95b7f
YO
1299Precompilation of the pattern into an internal representation at
1300the moment of qr() avoids a need to recompile the pattern every
1301time a match C</$pat/> is attempted. (Perl has many other internal
1302optimizations, but none would be triggered in the above example if
1303we did not use qr() operator.)
1304
1305Options are:
1306
1307 m Treat string as multiple lines.
1308 s Treat string as single line. (Make . match a newline)
1309 i Do case-insensitive pattern matching.
1310 x Use extended regular expressions.
1311 p When matching preserve a copy of the matched string so
1312 that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be defined.
1313 o Compile pattern only once.
1314
1315If a precompiled pattern is embedded in a larger pattern then the effect
1316of 'msixp' will be propagated appropriately. The effect of the 'o'
1317modifier has is not propagated, being restricted to those patterns
1318explicitly using it.
1319
1320See L<perlre> for additional information on valid syntax for STRING, and
1321for a detailed look at the semantics of regular expressions.
a0d0e21e 1322
87e95b7f 1323=item m/PATTERN/msixpogc
89d205f2
YO
1324X<m> X<operator, match>
1325X<regexp, options> X<regexp> X<regex, options> X<regex>
01c6f5f4 1326X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c>
a0d0e21e 1327
87e95b7f 1328=item /PATTERN/msixpogc
a0d0e21e 1329
5a964f20 1330Searches a string for a pattern match, and in scalar context returns
19799a22
GS
1331true if it succeeds, false if it fails. If no string is specified
1332via the C<=~> or C<!~> operator, the $_ string is searched. (The
1333string specified with C<=~> need not be an lvalue--it may be the
1334result of an expression evaluation, but remember the C<=~> binds
1335rather tightly.) See also L<perlre>. See L<perllocale> for
1336discussion of additional considerations that apply when C<use locale>
1337is in effect.
a0d0e21e 1338
01c6f5f4
RGS
1339Options are as described in C<qr//>; in addition, the following match
1340process modifiers are available:
a0d0e21e 1341
950b09ed
KW
1342 g Match globally, i.e., find all occurrences.
1343 c Do not reset search position on a failed match when /g is in effect.
a0d0e21e
LW
1344
1345If "/" is the delimiter then the initial C<m> is optional. With the C<m>
ed02a3bf 1346you can use any pair of non-whitespace characters
19799a22
GS
1347as delimiters. This is particularly useful for matching path names
1348that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
7bac28a0 1349the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
19799a22 1350If "'" is the delimiter, no interpolation is performed on the PATTERN.
ed02a3bf
DN
1351When using a character valid in an identifier, whitespace is required
1352after the C<m>.
a0d0e21e
LW
1353
1354PATTERN may contain variables, which will be interpolated (and the
f70b4f9c 1355pattern recompiled) every time the pattern search is evaluated, except
1f247705
GS
1356for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
1357C<$|> are not interpolated because they look like end-of-string tests.)
f70b4f9c
AB
1358If you want such a pattern to be compiled only once, add a C</o> after
1359the trailing delimiter. This avoids expensive run-time recompilations,
1360and is useful when the value you are interpolating won't change over
1361the life of the script. However, mentioning C</o> constitutes a promise
1362that you won't change the variables in the pattern. If you change them,
01c6f5f4 1363Perl won't even notice. See also L<"qr/STRING/msixpo">.
a0d0e21e 1364
e9d89077
DN
1365=item The empty pattern //
1366
5a964f20 1367If the PATTERN evaluates to the empty string, the last
d65afb4b
HS
1368I<successfully> matched regular expression is used instead. In this
1369case, only the C<g> and C<c> flags on the empty pattern is honoured -
1370the other flags are taken from the original pattern. If no match has
1371previously succeeded, this will (silently) act instead as a genuine
1372empty pattern (which will always match).
a0d0e21e 1373
89d205f2
YO
1374Note that it's possible to confuse Perl into thinking C<//> (the empty
1375regex) is really C<//> (the defined-or operator). Perl is usually pretty
1376good about this, but some pathological cases might trigger this, such as
1377C<$a///> (is that C<($a) / (//)> or C<$a // />?) and C<print $fh //>
1378(C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl
1379will assume you meant defined-or. If you meant the empty regex, just
1380use parentheses or spaces to disambiguate, or even prefix the empty
c963b151
BD
1381regex with an C<m> (so C<//> becomes C<m//>).
1382
e9d89077
DN
1383=item Matching in list context
1384
19799a22 1385If the C</g> option is not used, C<m//> in list context returns a
a0d0e21e 1386list consisting of the subexpressions matched by the parentheses in the
f7e33566
GS
1387pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are
1388also set, and that this differs from Perl 4's behavior.) When there are
1389no parentheses in the pattern, the return value is the list C<(1)> for
1390success. With or without parentheses, an empty list is returned upon
1391failure.
a0d0e21e
LW
1392
1393Examples:
1394
1395 open(TTY, '/dev/tty');
1396 <TTY> =~ /^y/i && foo(); # do foo if desired
1397
1398 if (/Version: *([0-9.]*)/) { $version = $1; }
1399
1400 next if m#^/usr/spool/uucp#;
1401
1402 # poor man's grep
1403 $arg = shift;
1404 while (<>) {
1405 print if /$arg/o; # compile only once
1406 }
1407
1408 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
1409
1410This last example splits $foo into the first two words and the
5f05dabc
PP
1411remainder of the line, and assigns those three fields to $F1, $F2, and
1412$Etc. The conditional is true if any variables were assigned, i.e., if
a0d0e21e
LW
1413the pattern matched.
1414
19799a22
GS
1415The C</g> modifier specifies global pattern matching--that is,
1416matching as many times as possible within the string. How it behaves
1417depends on the context. In list context, it returns a list of the
1418substrings matched by any capturing parentheses in the regular
1419expression. If there are no parentheses, it returns a list of all
1420the matched strings, as if there were parentheses around the whole
1421pattern.
a0d0e21e 1422
7e86de3e 1423In scalar context, each execution of C<m//g> finds the next match,
19799a22 1424returning true if it matches, and false if there is no further match.
7e86de3e
G
1425The position after the last match can be read or set using the pos()
1426function; see L<perlfunc/pos>. A failed match normally resets the
1427search position to the beginning of the string, but you can avoid that
1428by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
1429string also resets the search position.
c90c0ff4 1430
e9d89077
DN
1431=item \G assertion
1432
c90c0ff4
PP
1433You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
1434zero-width assertion that matches the exact position where the previous
5d43e42d
DC
1435C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion
1436still anchors at pos(), but the match is of course only attempted once.
1437Using C<\G> without C</g> on a target string that has not previously had a
1438C</g> match applied to it is the same as using the C<\A> assertion to match
fe4b3f22
RGS
1439the beginning of the string. Note also that, currently, C<\G> is only
1440properly supported when anchored at the very beginning of the pattern.
c90c0ff4
PP
1441
1442Examples:
a0d0e21e
LW
1443
1444 # list context
1445 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
1446
1447 # scalar context
5d43e42d 1448 $/ = "";
19799a22
GS
1449 while (defined($paragraph = <>)) {
1450 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
1451 $sentences++;
a0d0e21e
LW
1452 }
1453 }
1454 print "$sentences\n";
1455
c90c0ff4 1456 # using m//gc with \G
137443ea 1457 $_ = "ppooqppqq";
44a8e56a
PP
1458 while ($i++ < 2) {
1459 print "1: '";
c90c0ff4 1460 print $1 while /(o)/gc; print "', pos=", pos, "\n";
44a8e56a 1461 print "2: '";
c90c0ff4 1462 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
44a8e56a 1463 print "3: '";
c90c0ff4 1464 print $1 while /(p)/gc; print "', pos=", pos, "\n";
44a8e56a 1465 }
5d43e42d 1466 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
44a8e56a
PP
1467
1468The last example should print:
1469
1470 1: 'oo', pos=4
137443ea 1471 2: 'q', pos=5
44a8e56a
PP
1472 3: 'pp', pos=7
1473 1: '', pos=7
137443ea
PP
1474 2: 'q', pos=8
1475 3: '', pos=8
5d43e42d
DC
1476 Final: 'q', pos=8
1477
1478Notice that the final match matched C<q> instead of C<p>, which a match
1479without the C<\G> anchor would have done. Also note that the final match
ac036724 1480did not update C<pos>. C<pos> is only updated on a C</g> match. If the
5d43e42d
DC
1481final match did indeed match C<p>, it's a good bet that you're running an
1482older (pre-5.6.0) Perl.
44a8e56a 1483
c90c0ff4 1484A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
e7ea3e70 1485combine several regexps like this to process a string part-by-part,
c90c0ff4
PP
1486doing different actions depending on which regexp matched. Each
1487regexp tries to match where the previous one leaves off.
e7ea3e70 1488
3fe9a6f1 1489 $_ = <<'EOL';
950b09ed 1490 $url = URI::URL->new( "http://example.com/" ); die if $url eq "xXx";
3fe9a6f1
PP
1491 EOL
1492 LOOP:
e7ea3e70 1493 {
950b09ed
KW
1494 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
1495 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
1496 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
1497 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
1498 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
1499 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
1500 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
1501 print ". That's all!\n";
e7ea3e70
IZ
1502 }
1503
1504Here is the output (split into several lines):
1505
1506 line-noise lowercase line-noise lowercase UPPERCASE line-noise
1507 UPPERCASE line-noise lowercase line-noise lowercase line-noise
1508 lowercase lowercase line-noise lowercase lowercase line-noise
1509 MiXeD line-noise. That's all!
44a8e56a 1510
87e95b7f
YO
1511=item ?PATTERN?
1512X<?>
1513
1514This is just like the C</pattern/> search, except that it matches only
1515once between calls to the reset() operator. This is a useful
1516optimization when you want to see only the first occurrence of
1517something in each file of a set of files, for instance. Only C<??>
1518patterns local to the current package are reset.
1519
1520 while (<>) {
1521 if (?^$?) {
1522 # blank line between header and body
1523 }
1524 } continue {
1525 reset if eof; # clear ?? status for next file
1526 }
1527
1528This usage is vaguely deprecated, which means it just might possibly
1529be removed in some distant future version of Perl, perhaps somewhere
1530around the year 2168.
1531
4f4d7508 1532=item s/PATTERN/REPLACEMENT/msixpogcer
87e95b7f 1533X<substitute> X<substitution> X<replace> X<regexp, replace>
4f4d7508 1534X<regexp, substitute> X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> X</e> X</r>
87e95b7f
YO
1535
1536Searches a string for a pattern, and if found, replaces that pattern
1537with the replacement text and returns the number of substitutions
1538made. Otherwise it returns false (specifically, the empty string).
1539
4f4d7508
DC
1540If the C</r> (non-destructive) option is used then it will perform the
1541substitution on a copy of the string and return the copy whether or not a
1542substitution occurred. The original string will always remain unchanged in
c8dbf8cd 1543this case. The copy will always be a plain string, even if the input is an
4f4d7508
DC
1544object or a tied variable.
1545
87e95b7f
YO
1546If no string is specified via the C<=~> or C<!~> operator, the C<$_>
1547variable is searched and modified. (The string specified with C<=~> must
1548be scalar variable, an array element, a hash element, or an assignment
1549to one of those, i.e., an lvalue.)
1550
1551If the delimiter chosen is a single quote, no interpolation is
1552done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1553PATTERN contains a $ that looks like a variable rather than an
1554end-of-string test, the variable will be interpolated into the pattern
1555at run-time. If you want the pattern compiled only once the first time
1556the variable is interpolated, use the C</o> option. If the pattern
1557evaluates to the empty string, the last successfully executed regular
1558expression is used instead. See L<perlre> for further explanation on these.
1559See L<perllocale> for discussion of additional considerations that apply
1560when C<use locale> is in effect.
1561
1562Options are as with m// with the addition of the following replacement
1563specific options:
1564
1565 e Evaluate the right side as an expression.
4f4d7508
DC
1566 ee Evaluate the right side as a string then eval the result.
1567 r Return substitution and leave the original string untouched.
87e95b7f 1568
ed02a3bf
DN
1569Any non-whitespace delimiter may replace the slashes. Add space after
1570the C<s> when using a character allowed in identifiers. If single quotes
1571are used, no interpretation is done on the replacement string (the C</e>
1572modifier overrides this, however). Unlike Perl 4, Perl 5 treats backticks
1573as normal delimiters; the replacement text is not evaluated as a command.
1574If the PATTERN is delimited by bracketing quotes, the REPLACEMENT has
1575its own pair of quotes, which may or may not be bracketing quotes, e.g.,
87e95b7f
YO
1576C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
1577replacement portion to be treated as a full-fledged Perl expression
1578and evaluated right then and there. It is, however, syntax checked at
1579compile-time. A second C<e> modifier will cause the replacement portion
1580to be C<eval>ed before being run as a Perl expression.
1581
1582Examples:
1583
1584 s/\bgreen\b/mauve/g; # don't change wintergreen
1585
1586 $path =~ s|/usr/bin|/usr/local/bin|;
1587
1588 s/Login: $foo/Login: $bar/; # run-time pattern
1589
1590 ($foo = $bar) =~ s/this/that/; # copy first, then change
4f4d7508
DC
1591 ($foo = "$bar") =~ s/this/that/; # convert to string, copy, then change
1592 $foo = $bar =~ s/this/that/r; # Same as above using /r
1593 $foo = $bar =~ s/this/that/r
1594 =~ s/that/the other/r; # Chained substitutes using /r
1595 @foo = map { s/this/that/r } @bar # /r is very useful in maps
87e95b7f
YO
1596
1597 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
1598
1599 $_ = 'abc123xyz';
1600 s/\d+/$&*2/e; # yields 'abc246xyz'
1601 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
1602 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1603
1604 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
1605 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
1606 s/^=(\w+)/pod($1)/ge; # use function call
1607
4f4d7508
DC
1608 $_ = 'abc123xyz';
1609 $a = s/abc/def/r; # $a is 'def123xyz' and
1610 # $_ remains 'abc123xyz'.
1611
87e95b7f
YO
1612 # expand variables in $_, but dynamics only, using
1613 # symbolic dereferencing
1614 s/\$(\w+)/${$1}/g;
1615
1616 # Add one to the value of any numbers in the string
1617 s/(\d+)/1 + $1/eg;
1618
1619 # This will expand any embedded scalar variable
1620 # (including lexicals) in $_ : First $1 is interpolated
1621 # to the variable name, and then evaluated
1622 s/(\$\w+)/$1/eeg;
1623
1624 # Delete (most) C comments.
1625 $program =~ s {
1626 /\* # Match the opening delimiter.
1627 .*? # Match a minimal number of characters.
1628 \*/ # Match the closing delimiter.
1629 } []gsx;
1630
1631 s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_, expensively
1632
1633 for ($variable) { # trim whitespace in $variable, cheap
1634 s/^\s+//;
1635 s/\s+$//;
1636 }
1637
1638 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
1639
1640Note the use of $ instead of \ in the last example. Unlike
1641B<sed>, we use the \<I<digit>> form in only the left hand side.
1642Anywhere else it's $<I<digit>>.
1643
1644Occasionally, you can't use just a C</g> to get all the changes
1645to occur that you might want. Here are two common cases:
1646
1647 # put commas in the right places in an integer
1648 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
1649
1650 # expand tabs to 8-column spacing
1651 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
1652
1653=back
1654
1655=head2 Quote-Like Operators
1656X<operator, quote-like>
1657
01c6f5f4
RGS
1658=over 4
1659
a0d0e21e 1660=item q/STRING/
5d44bfff 1661X<q> X<quote, single> X<'> X<''>
a0d0e21e 1662
5d44bfff 1663=item 'STRING'
a0d0e21e 1664
19799a22 1665A single-quoted, literal string. A backslash represents a backslash
68dc0745
PP
1666unless followed by the delimiter or another backslash, in which case
1667the delimiter or backslash is interpolated.
a0d0e21e
LW
1668
1669 $foo = q!I said, "You said, 'She said it.'"!;
1670 $bar = q('This is it.');
68dc0745 1671 $baz = '\n'; # a two-character string
a0d0e21e
LW
1672
1673=item qq/STRING/
d74e8afc 1674X<qq> X<quote, double> X<"> X<"">
a0d0e21e
LW
1675
1676=item "STRING"
1677
1678A double-quoted, interpolated string.
1679
1680 $_ .= qq
1681 (*** The previous line contains the naughty word "$1".\n)
19799a22 1682 if /\b(tcl|java|python)\b/i; # :-)
68dc0745 1683 $baz = "\n"; # a one-character string
a0d0e21e
LW
1684
1685=item qx/STRING/
d74e8afc 1686X<qx> X<`> X<``> X<backtick>
a0d0e21e
LW
1687
1688=item `STRING`
1689
43dd4d21
JH
1690A string which is (possibly) interpolated and then executed as a
1691system command with C</bin/sh> or its equivalent. Shell wildcards,
1692pipes, and redirections will be honored. The collected standard
1693output of the command is returned; standard error is unaffected. In
1694scalar context, it comes back as a single (potentially multi-line)
1695string, or undef if the command failed. In list context, returns a
1696list of lines (however you've defined lines with $/ or
1697$INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
5a964f20
TC
1698
1699Because backticks do not affect standard error, use shell file descriptor
1700syntax (assuming the shell supports this) if you care to address this.
1701To capture a command's STDERR and STDOUT together:
a0d0e21e 1702
5a964f20
TC
1703 $output = `cmd 2>&1`;
1704
1705To capture a command's STDOUT but discard its STDERR:
1706
1707 $output = `cmd 2>/dev/null`;
1708
1709To capture a command's STDERR but discard its STDOUT (ordering is
1710important here):
1711
1712 $output = `cmd 2>&1 1>/dev/null`;
1713
1714To exchange a command's STDOUT and STDERR in order to capture the STDERR
1715but leave its STDOUT to come out the old STDERR:
1716
1717 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
1718
1719To read both a command's STDOUT and its STDERR separately, it's easiest
2359510d
SD
1720to redirect them separately to files, and then read from those files
1721when the program is done:
5a964f20 1722
2359510d 1723 system("program args 1>program.stdout 2>program.stderr");
5a964f20 1724
30398227
SP
1725The STDIN filehandle used by the command is inherited from Perl's STDIN.
1726For example:
1727
1728 open BLAM, "blam" || die "Can't open: $!";
1729 open STDIN, "<&BLAM";
1730 print `sort`;
1731
1732will print the sorted contents of the file "blam".
1733
5a964f20
TC
1734Using single-quote as a delimiter protects the command from Perl's
1735double-quote interpolation, passing it on to the shell instead:
1736
1737 $perl_info = qx(ps $$); # that's Perl's $$
1738 $shell_info = qx'ps $$'; # that's the new shell's $$
1739
19799a22 1740How that string gets evaluated is entirely subject to the command
5a964f20
TC
1741interpreter on your system. On most platforms, you will have to protect
1742shell metacharacters if you want them treated literally. This is in
1743practice difficult to do, as it's unclear how to escape which characters.
1744See L<perlsec> for a clean and safe example of a manual fork() and exec()
1745to emulate backticks safely.
a0d0e21e 1746
bb32b41a
GS
1747On some platforms (notably DOS-like ones), the shell may not be
1748capable of dealing with multiline commands, so putting newlines in
1749the string may not get you what you want. You may be able to evaluate
1750multiple commands in a single line by separating them with the command
1751separator character, if your shell supports that (e.g. C<;> on many Unix
1752shells; C<&> on the Windows NT C<cmd> shell).
1753
0f897271
GS
1754Beginning with v5.6.0, Perl will attempt to flush all files opened for
1755output before starting the child process, but this may not be supported
1756on some platforms (see L<perlport>). To be safe, you may need to set
1757C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1758C<IO::Handle> on any open handles.
1759
bb32b41a
GS
1760Beware that some command shells may place restrictions on the length
1761of the command line. You must ensure your strings don't exceed this
1762limit after any necessary interpolations. See the platform-specific
1763release notes for more details about your particular environment.
1764
5a964f20
TC
1765Using this operator can lead to programs that are difficult to port,
1766because the shell commands called vary between systems, and may in
1767fact not be present at all. As one example, the C<type> command under
1768the POSIX shell is very different from the C<type> command under DOS.
1769That doesn't mean you should go out of your way to avoid backticks
1770when they're the right way to get something done. Perl was made to be
1771a glue language, and one of the things it glues together is commands.
1772Just understand what you're getting yourself into.
bb32b41a 1773
da87341d 1774See L</"I/O Operators"> for more discussion.
a0d0e21e 1775
945c54fd 1776=item qw/STRING/
d74e8afc 1777X<qw> X<quote, list> X<quote, words>
945c54fd
JH
1778
1779Evaluates to a list of the words extracted out of STRING, using embedded
1780whitespace as the word delimiters. It can be understood as being roughly
1781equivalent to:
1782
1783 split(' ', q/STRING/);
1784
efb1e162
CW
1785the differences being that it generates a real list at compile time, and
1786in scalar context it returns the last element in the list. So
945c54fd
JH
1787this expression:
1788
1789 qw(foo bar baz)
1790
1791is semantically equivalent to the list:
1792
1793 'foo', 'bar', 'baz'
1794
1795Some frequently seen examples:
1796
1797 use POSIX qw( setlocale localeconv )
1798 @EXPORT = qw( foo bar baz );
1799
1800A common mistake is to try to separate the words with comma or to
1801put comments into a multi-line C<qw>-string. For this reason, the
89d205f2 1802C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
945c54fd
JH
1803produces warnings if the STRING contains the "," or the "#" character.
1804
a0d0e21e 1805
6940069f 1806=item tr/SEARCHLIST/REPLACEMENTLIST/cds
d74e8afc 1807X<tr> X<y> X<transliterate> X</c> X</d> X</s>
a0d0e21e 1808
6940069f 1809=item y/SEARCHLIST/REPLACEMENTLIST/cds
a0d0e21e 1810
2c268ad5 1811Transliterates all occurrences of the characters found in the search list
a0d0e21e
LW
1812with the corresponding character in the replacement list. It returns
1813the number of characters replaced or deleted. If no string is
2c268ad5 1814specified via the =~ or !~ operator, the $_ string is transliterated. (The
54310121
PP
1815string specified with =~ must be a scalar variable, an array element, a
1816hash element, or an assignment to one of those, i.e., an lvalue.)
8ada0baa 1817
89d205f2 1818A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
2c268ad5 1819does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
54310121
PP
1820For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1821SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1822its own pair of quotes, which may or may not be bracketing quotes,
2c268ad5 1823e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
a0d0e21e 1824
cc255d5f 1825Note that C<tr> does B<not> do regular expression character classes
e0c83546 1826such as C<\d> or C<[:lower:]>. The C<tr> operator is not equivalent to
cc255d5f
JH
1827the tr(1) utility. If you want to map strings between lower/upper
1828cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider
1829using the C<s> operator if you need regular expressions.
1830
8ada0baa
JH
1831Note also that the whole range idea is rather unportable between
1832character sets--and even within character sets they may cause results
1833you probably didn't expect. A sound principle is to use only ranges
1834that begin from and end at either alphabets of equal case (a-e, A-E),
1835or digits (0-4). Anything else is unsafe. If in doubt, spell out the
1836character sets in full.
1837
a0d0e21e
LW
1838Options:
1839
1840 c Complement the SEARCHLIST.
1841 d Delete found but unreplaced characters.
1842 s Squash duplicate replaced characters.
1843
19799a22
GS
1844If the C</c> modifier is specified, the SEARCHLIST character set
1845is complemented. If the C</d> modifier is specified, any characters
1846specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
1847(Note that this is slightly more flexible than the behavior of some
1848B<tr> programs, which delete anything they find in the SEARCHLIST,
1849period.) If the C</s> modifier is specified, sequences of characters
1850that were transliterated to the same character are squashed down
1851to a single instance of the character.
a0d0e21e
LW
1852
1853If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1854exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1855than the SEARCHLIST, the final character is replicated till it is long
5a964f20 1856enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
a0d0e21e
LW
1857This latter is useful for counting characters in a class or for
1858squashing character sequences in a class.
1859
1860Examples:
1861
1862 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1863
1864 $cnt = tr/*/*/; # count the stars in $_
1865
1866 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1867
1868 $cnt = tr/0-9//; # count the digits in $_
1869
1870 tr/a-zA-Z//s; # bookkeeper -> bokeper
1871
1872 ($HOST = $host) =~ tr/a-z/A-Z/;
1873
1874 tr/a-zA-Z/ /cs; # change non-alphas to single space
1875
1876 tr [\200-\377]
1877 [\000-\177]; # delete 8th bit
1878
19799a22
GS
1879If multiple transliterations are given for a character, only the
1880first one is used:
748a9306
LW
1881
1882 tr/AAA/XYZ/
1883
2c268ad5 1884will transliterate any A to X.
748a9306 1885
19799a22 1886Because the transliteration table is built at compile time, neither
a0d0e21e 1887the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
19799a22
GS
1888interpolation. That means that if you want to use variables, you
1889must use an eval():
a0d0e21e
LW
1890
1891 eval "tr/$oldlist/$newlist/";
1892 die $@ if $@;
1893
1894 eval "tr/$oldlist/$newlist/, 1" or die $@;
1895
7e3b091d 1896=item <<EOF
d74e8afc 1897X<here-doc> X<heredoc> X<here-document> X<<< << >>>
7e3b091d
DA
1898
1899A line-oriented form of quoting is based on the shell "here-document"
1900syntax. Following a C<< << >> you specify a string to terminate
1901the quoted material, and all lines following the current line down to
89d205f2
YO
1902the terminating string are the value of the item.
1903
1904The terminating string may be either an identifier (a word), or some
1905quoted text. An unquoted identifier works like double quotes.
1906There may not be a space between the C<< << >> and the identifier,
1907unless the identifier is explicitly quoted. (If you put a space it
1908will be treated as a null identifier, which is valid, and matches the
1909first empty line.) The terminating string must appear by itself
1910(unquoted and with no surrounding whitespace) on the terminating line.
1911
1912If the terminating string is quoted, the type of quotes used determine
1913the treatment of the text.
1914
1915=over 4
1916
1917=item Double Quotes
1918
1919Double quotes indicate that the text will be interpolated using exactly
1920the same rules as normal double quoted strings.
7e3b091d
DA
1921
1922 print <<EOF;
1923 The price is $Price.
1924 EOF
1925
1926 print << "EOF"; # same as above
1927 The price is $Price.
1928 EOF
1929
89d205f2
YO
1930
1931=item Single Quotes
1932
1933Single quotes indicate the text is to be treated literally with no
1934interpolation of its content. This is similar to single quoted
1935strings except that backslashes have no special meaning, with C<\\>
1936being treated as two backslashes and not one as they would in every
1937other quoting construct.
1938
1939This is the only form of quoting in perl where there is no need
1940to worry about escaping content, something that code generators
1941can and do make good use of.
1942
1943=item Backticks
1944
1945The content of the here doc is treated just as it would be if the
1946string were embedded in backticks. Thus the content is interpolated
1947as though it were double quoted and then executed via the shell, with
1948the results of the execution returned.
1949
1950 print << `EOC`; # execute command and get results
7e3b091d 1951 echo hi there
7e3b091d
DA
1952 EOC
1953
89d205f2
YO
1954=back
1955
1956It is possible to stack multiple here-docs in a row:
1957
7e3b091d
DA
1958 print <<"foo", <<"bar"; # you can stack them
1959 I said foo.
1960 foo
1961 I said bar.
1962 bar
1963
1964 myfunc(<< "THIS", 23, <<'THAT');
1965 Here's a line
1966 or two.
1967 THIS
1968 and here's another.
1969 THAT
1970
1971Just don't forget that you have to put a semicolon on the end
1972to finish the statement, as Perl doesn't know you're not going to
1973try to do this:
1974
1975 print <<ABC
1976 179231
1977 ABC
1978 + 20;
1979
872d7e53
ST
1980If you want to remove the line terminator from your here-docs,
1981use C<chomp()>.
1982
1983 chomp($string = <<'END');
1984 This is a string.
1985 END
1986
1987If you want your here-docs to be indented with the rest of the code,
1988you'll need to remove leading whitespace from each line manually:
7e3b091d
DA
1989
1990 ($quote = <<'FINIS') =~ s/^\s+//gm;
89d205f2 1991 The Road goes ever on and on,
7e3b091d
DA
1992 down from the door where it began.
1993 FINIS
1994
1995If you use a here-doc within a delimited construct, such as in C<s///eg>,
1996the quoted material must come on the lines following the final delimiter.
1997So instead of
1998
1999 s/this/<<E . 'that'
2000 the other
2001 E
2002 . 'more '/eg;
2003
2004you have to write
2005
89d205f2
YO
2006 s/this/<<E . 'that'
2007 . 'more '/eg;
2008 the other
2009 E
7e3b091d
DA
2010
2011If the terminating identifier is on the last line of the program, you
2012must be sure there is a newline after it; otherwise, Perl will give the
2013warning B<Can't find string terminator "END" anywhere before EOF...>.
2014
89d205f2 2015Additionally, the quoting rules for the end of string identifier are not
ac036724 2016related to Perl's quoting rules. C<q()>, C<qq()>, and the like are not
89d205f2
YO
2017supported in place of C<''> and C<"">, and the only interpolation is for
2018backslashing the quoting character:
7e3b091d
DA
2019
2020 print << "abc\"def";
2021 testing...
2022 abc"def
2023
2024Finally, quoted strings cannot span multiple lines. The general rule is
2025that the identifier must be a string literal. Stick with that, and you
2026should be safe.
2027
a0d0e21e
LW
2028=back
2029
75e14d17 2030=head2 Gory details of parsing quoted constructs
d74e8afc 2031X<quote, gory details>
75e14d17 2032
19799a22
GS
2033When presented with something that might have several different
2034interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
2035principle to pick the most probable interpretation. This strategy
2036is so successful that Perl programmers often do not suspect the
2037ambivalence of what they write. But from time to time, Perl's
2038notions differ substantially from what the author honestly meant.
2039
2040This section hopes to clarify how Perl handles quoted constructs.
2041Although the most common reason to learn this is to unravel labyrinthine
2042regular expressions, because the initial steps of parsing are the
2043same for all quoting operators, they are all discussed together.
2044
2045The most important Perl parsing rule is the first one discussed
2046below: when processing a quoted construct, Perl first finds the end
2047of that construct, then interprets its contents. If you understand
2048this rule, you may skip the rest of this section on the first
2049reading. The other rules are likely to contradict the user's
2050expectations much less frequently than this first one.
2051
2052Some passes discussed below are performed concurrently, but because
2053their results are the same, we consider them individually. For different
2054quoting constructs, Perl performs different numbers of passes, from
6deea57f 2055one to four, but these passes are always performed in the same order.
75e14d17 2056
13a2d996 2057=over 4
75e14d17
IZ
2058
2059=item Finding the end
2060
6deea57f
ST
2061The first pass is finding the end of the quoted construct, where
2062the information about the delimiters is used in parsing.
2063During this search, text between the starting and ending delimiters
2064is copied to a safe location. The text copied gets delimiter-independent.
2065
2066If the construct is a here-doc, the ending delimiter is a line
2067that has a terminating string as the content. Therefore C<<<EOF> is
2068terminated by C<EOF> immediately followed by C<"\n"> and starting
2069from the first column of the terminating line.
2070When searching for the terminating line of a here-doc, nothing
2071is skipped. In other words, lines after the here-doc syntax
2072are compared with the terminating string line by line.
2073
2074For the constructs except here-docs, single characters are used as starting
2075and ending delimiters. If the starting delimiter is an opening punctuation
2076(that is C<(>, C<[>, C<{>, or C<< < >>), the ending delimiter is the
2077corresponding closing punctuation (that is C<)>, C<]>, C<}>, or C<< > >>).
2078If the starting delimiter is an unpaired character like C</> or a closing
2079punctuation, the ending delimiter is same as the starting delimiter.
2080Therefore a C</> terminates a C<qq//> construct, while a C<]> terminates
2081C<qq[]> and C<qq]]> constructs.
2082
2083When searching for single-character delimiters, escaped delimiters
2084and C<\\> are skipped. For example, while searching for terminating C</>,
2085combinations of C<\\> and C<\/> are skipped. If the delimiters are
2086bracketing, nested pairs are also skipped. For example, while searching
2087for closing C<]> paired with the opening C<[>, combinations of C<\\>, C<\]>,
2088and C<\[> are all skipped, and nested C<[> and C<]> are skipped as well.
2089However, when backslashes are used as the delimiters (like C<qq\\> and
2090C<tr\\\>), nothing is skipped.
2091During the search for the end, backslashes that escape delimiters
2092are removed (exactly speaking, they are not copied to the safe location).
75e14d17 2093
19799a22
GS
2094For constructs with three-part delimiters (C<s///>, C<y///>, and
2095C<tr///>), the search is repeated once more.
6deea57f
ST
2096If the first delimiter is not an opening punctuation, three delimiters must
2097be same such as C<s!!!> and C<tr)))>, in which case the second delimiter
2098terminates the left part and starts the right part at once.
2099If the left part is delimited by bracketing punctuations (that is C<()>,
2100C<[]>, C<{}>, or C<< <> >>), the right part needs another pair of
2101delimiters such as C<s(){}> and C<tr[]//>. In these cases, whitespaces
2102and comments are allowed between both parts, though the comment must follow
2103at least one whitespace; otherwise a character expected as the start of
2104the comment may be regarded as the starting delimiter of the right part.
75e14d17 2105
19799a22
GS
2106During this search no attention is paid to the semantics of the construct.
2107Thus:
75e14d17
IZ
2108
2109 "$hash{"$foo/$bar"}"
2110
2a94b7ce 2111or:
75e14d17 2112
89d205f2 2113 m/
2a94b7ce 2114 bar # NOT a comment, this slash / terminated m//!
75e14d17
IZ
2115 /x
2116
19799a22
GS
2117do not form legal quoted expressions. The quoted part ends on the
2118first C<"> and C</>, and the rest happens to be a syntax error.
2119Because the slash that terminated C<m//> was followed by a C<SPACE>,
2120the example above is not C<m//x>, but rather C<m//> with no C</x>
2121modifier. So the embedded C<#> is interpreted as a literal C<#>.
75e14d17 2122
89d205f2
YO
2123Also no attention is paid to C<\c\> (multichar control char syntax) during
2124this search. Thus the second C<\> in C<qq/\c\/> is interpreted as a part
2125of C<\/>, and the following C</> is not recognized as a delimiter.
0d594e51
ST
2126Instead, use C<\034> or C<\x1c> at the end of quoted constructs.
2127
75e14d17 2128=item Interpolation
d74e8afc 2129X<interpolation>
75e14d17 2130
19799a22 2131The next step is interpolation in the text obtained, which is now
89d205f2 2132delimiter-independent. There are multiple cases.
75e14d17 2133
13a2d996 2134=over 4
75e14d17 2135
89d205f2 2136=item C<<<'EOF'>
75e14d17
IZ
2137
2138No interpolation is performed.
6deea57f
ST
2139Note that the combination C<\\> is left intact, since escaped delimiters
2140are not available for here-docs.
75e14d17 2141
6deea57f 2142=item C<m''>, the pattern of C<s'''>
89d205f2 2143
6deea57f
ST
2144No interpolation is performed at this stage.
2145Any backslashed sequences including C<\\> are treated at the stage
2146to L</"parsing regular expressions">.
89d205f2 2147
6deea57f 2148=item C<''>, C<q//>, C<tr'''>, C<y'''>, the replacement of C<s'''>
75e14d17 2149
89d205f2 2150The only interpolation is removal of C<\> from pairs of C<\\>.
6deea57f
ST
2151Therefore C<-> in C<tr'''> and C<y'''> is treated literally
2152as a hyphen and no character range is available.
2153C<\1> in the replacement of C<s'''> does not work as C<$1>.
89d205f2
YO
2154
2155=item C<tr///>, C<y///>
2156
6deea57f
ST
2157No variable interpolation occurs. String modifying combinations for
2158case and quoting such as C<\Q>, C<\U>, and C<\E> are not recognized.
2159The other escape sequences such as C<\200> and C<\t> and backslashed
2160characters such as C<\\> and C<\-> are converted to appropriate literals.
89d205f2
YO
2161The character C<-> is treated specially and therefore C<\-> is treated
2162as a literal C<->.
75e14d17 2163
89d205f2 2164=item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF">
75e14d17 2165
19799a22
GS
2166C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
2167converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
2168is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
6deea57f
ST
2169The other escape sequences such as C<\200> and C<\t> and backslashed
2170characters such as C<\\> and C<\-> are replaced with appropriate
2171expansions.
2a94b7ce 2172
19799a22
GS
2173Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
2174is interpolated in the usual way. Something like C<"\Q\\E"> has
2175no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
2176result is the same as for C<"\\\\E">. As a general rule, backslashes
2177between C<\Q> and C<\E> may lead to counterintuitive results. So,
2178C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
2179as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
2a94b7ce
IZ
2180
2181 $str = '\t';
2182 return "\Q$str";
2183
2184may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
2185
19799a22 2186Interpolated scalars and arrays are converted internally to the C<join> and
92d29cee 2187C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
75e14d17 2188
19799a22 2189 $foo . " XXX '" . (join $", @arr) . "'";
75e14d17 2190
19799a22 2191All operations above are performed simultaneously, left to right.
75e14d17 2192
19799a22
GS
2193Because the result of C<"\Q STRING \E"> has all metacharacters
2194quoted, there is no way to insert a literal C<$> or C<@> inside a
2195C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
2196C<"\\\$">; if not, it is interpreted as the start of an interpolated
2197scalar.
75e14d17 2198
19799a22 2199Note also that the interpolation code needs to make a decision on
89d205f2 2200where the interpolated scalar ends. For instance, whether
35f2feb0 2201C<< "a $b -> {c}" >> really means:
75e14d17
IZ
2202
2203 "a " . $b . " -> {c}";
2204
2a94b7ce 2205or:
75e14d17
IZ
2206
2207 "a " . $b -> {c};
2208
19799a22
GS
2209Most of the time, the longest possible text that does not include
2210spaces between components and which contains matching braces or
2211brackets. because the outcome may be determined by voting based
2212on heuristic estimators, the result is not strictly predictable.
2213Fortunately, it's usually correct for ambiguous cases.
75e14d17 2214
6deea57f 2215=item the replacement of C<s///>
75e14d17 2216
19799a22 2217Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
6deea57f
ST
2218happens as with C<qq//> constructs.
2219
2220It is at this step that C<\1> is begrudgingly converted to C<$1> in
2221the replacement text of C<s///>, in order to correct the incorrigible
2222I<sed> hackers who haven't picked up the saner idiom yet. A warning
2223is emitted if the C<use warnings> pragma or the B<-w> command-line flag
2224(that is, the C<$^W> variable) was set.
2225
2226=item C<RE> in C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
2227
cc74c5bd
ST
2228Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\E>,
2229and interpolation happens (almost) as with C<qq//> constructs.
2230
5d03b57c
KW
2231Processing of C<\N{...}> is also done here, and compiled into an intermediate
2232form for the regex compiler. (This is because, as mentioned below, the regex
2233compilation may be done at execution time, and C<\N{...}> is a compile-time
2234construct.)
2235
cc74c5bd
ST
2236However any other combinations of C<\> followed by a character
2237are not substituted but only skipped, in order to parse them
2238as regular expressions at the following step.
6deea57f 2239As C<\c> is skipped at this step, C<@> of C<\c@> in RE is possibly
1749ea0d 2240treated as an array symbol (for example C<@foo>),
6deea57f 2241even though the same text in C<qq//> gives interpolation of C<\c@>.
6deea57f
ST
2242
2243Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
19799a22
GS
2244a C<#>-comment in a C<//x>-regular expression, no processing is
2245performed whatsoever. This is the first step at which the presence
2246of the C<//x> modifier is relevant.
2247
1749ea0d
ST
2248Interpolation in patterns has several quirks: C<$|>, C<$(>, C<$)>, C<@+>
2249and C<@-> are not interpolated, and constructs C<$var[SOMETHING]> are
2250voted (by several different estimators) to be either an array element
2251or C<$var> followed by an RE alternative. This is where the notation
19799a22
GS
2252C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
2253array element C<-9>, not as a regular expression from the variable
2254C<$arr> followed by a digit, which would be the interpretation of
2255C</$arr[0-9]/>. Since voting among different estimators may occur,
2256the result is not predictable.
2257
19799a22
GS
2258The lack of processing of C<\\> creates specific restrictions on
2259the post-processed text. If the delimiter is C</>, one cannot get
2260the combination C<\/> into the result of this step. C</> will
2261finish the regular expression, C<\/> will be stripped to C</> on
2262the previous step, and C<\\/> will be left as is. Because C</> is
2263equivalent to C<\/> inside a regular expression, this does not
2264matter unless the delimiter happens to be character special to the
2265RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
2266alphanumeric char, as in:
2a94b7ce
IZ
2267
2268 m m ^ a \s* b mmx;
2269
19799a22 2270In the RE above, which is intentionally obfuscated for illustration, the
6deea57f 2271delimiter is C<m>, the modifier is C<mx>, and after delimiter-removal the
89d205f2 2272RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
19799a22
GS
2273reason you're encouraged to restrict your delimiters to non-alphanumeric,
2274non-whitespace choices.
75e14d17
IZ
2275
2276=back
2277
19799a22 2278This step is the last one for all constructs except regular expressions,
75e14d17
IZ
2279which are processed further.
2280
6deea57f
ST
2281=item parsing regular expressions
2282X<regexp, parse>
75e14d17 2283
19799a22 2284Previous steps were performed during the compilation of Perl code,
ac036724 2285but this one happens at run time, although it may be optimized to
19799a22 2286be calculated at compile time if appropriate. After preprocessing
6deea57f 2287described above, and possibly after evaluation if concatenation,
19799a22
GS
2288joining, casing translation, or metaquoting are involved, the
2289resulting I<string> is passed to the RE engine for compilation.
2290
2291Whatever happens in the RE engine might be better discussed in L<perlre>,
2292but for the sake of continuity, we shall do so here.
2293
2294This is another step where the presence of the C<//x> modifier is
2295relevant. The RE engine scans the string from left to right and
2296converts it to a finite automaton.
2297
2298Backslashed characters are either replaced with corresponding
2299literal strings (as with C<\{>), or else they generate special nodes
2300in the finite automaton (as with C<\b>). Characters special to the
2301RE engine (such as C<|>) generate corresponding nodes or groups of
2302nodes. C<(?#...)> comments are ignored. All the rest is either
2303converted to literal strings to match, or else is ignored (as is
2304whitespace and C<#>-style comments if C<//x> is present).
2305
2306Parsing of the bracketed character class construct, C<[...]>, is
2307rather different than the rule used for the rest of the pattern.
2308The terminator of this construct is found using the same rules as
2309for finding the terminator of a C<{}>-delimited construct, the only
2310exception being that C<]> immediately following C<[> is treated as
2311though preceded by a backslash. Similarly, the terminator of
2312C<(?{...})> is found using the same rules as for finding the
2313terminator of a C<{}>-delimited construct.
2314
2315It is possible to inspect both the string given to RE engine and the
2316resulting finite automaton. See the arguments C<debug>/C<debugcolor>
2317in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
4a4eefd0 2318switch documented in L<perlrun/"Command Switches">.
75e14d17
IZ
2319
2320=item Optimization of regular expressions
d74e8afc 2321X<regexp, optimization>
75e14d17 2322
7522fed5 2323This step is listed for completeness only. Since it does not change
75e14d17 2324semantics, details of this step are not documented and are subject
19799a22
GS
2325to change without notice. This step is performed over the finite
2326automaton that was generated during the previous pass.
2a94b7ce 2327
19799a22
GS
2328It is at this stage that C<split()> silently optimizes C</^/> to
2329mean C</^/m>.
75e14d17
IZ
2330
2331=back
2332
a0d0e21e 2333=head2 I/O Operators
d74e8afc
ITB
2334X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle>
2335X<< <> >> X<@ARGV>
a0d0e21e 2336
54310121 2337There are several I/O operators you should know about.
fbad3eb5 2338
7b8d334a 2339A string enclosed by backticks (grave accents) first undergoes
19799a22
GS
2340double-quote interpolation. It is then interpreted as an external
2341command, and the output of that command is the value of the
e9c56f9b
JH
2342backtick string, like in a shell. In scalar context, a single string
2343consisting of all output is returned. In list context, a list of
2344values is returned, one per line of output. (You can set C<$/> to use
2345a different line terminator.) The command is executed each time the
2346pseudo-literal is evaluated. The status value of the command is
2347returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
2348Unlike in B<csh>, no translation is done on the return data--newlines
2349remain newlines. Unlike in any of the shells, single quotes do not
2350hide variable names in the command from interpretation. To pass a
2351literal dollar-sign through to the shell you need to hide it with a
2352backslash. The generalized form of backticks is C<qx//>. (Because
2353backticks always undergo shell expansion as well, see L<perlsec> for
2354security concerns.)
d74e8afc 2355X<qx> X<`> X<``> X<backtick> X<glob>
19799a22
GS
2356
2357In scalar context, evaluating a filehandle in angle brackets yields
2358the next line from that file (the newline, if any, included), or
2359C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
2360(sometimes known as file-slurp mode) and the file is empty, it
2361returns C<''> the first time, followed by C<undef> subsequently.
2362
2363Ordinarily you must assign the returned value to a variable, but
2364there is one situation where an automatic assignment happens. If
2365and only if the input symbol is the only thing inside the conditional
2366of a C<while> statement (even if disguised as a C<for(;;)> loop),
2367the value is automatically assigned to the global variable $_,
2368destroying whatever was there previously. (This may seem like an
2369odd thing to you, but you'll use the construct in almost every Perl
17b829fa 2370script you write.) The $_ variable is not implicitly localized.
19799a22
GS
2371You'll have to put a C<local $_;> before the loop if you want that
2372to happen.
2373
2374The following lines are equivalent:
a0d0e21e 2375
748a9306 2376 while (defined($_ = <STDIN>)) { print; }
7b8d334a 2377 while ($_ = <STDIN>) { print; }
a0d0e21e
LW
2378 while (<STDIN>) { print; }
2379 for (;<STDIN>;) { print; }
748a9306 2380 print while defined($_ = <STDIN>);
7b8d334a 2381 print while ($_ = <STDIN>);
a0d0e21e
LW
2382 print while <STDIN>;
2383
19799a22 2384This also behaves similarly, but avoids $_ :
7b8d334a 2385
89d205f2 2386 while (my $line = <STDIN>) { print $line }
7b8d334a 2387
19799a22
GS
2388In these loop constructs, the assigned value (whether assignment
2389is automatic or explicit) is then tested to see whether it is
2390defined. The defined test avoids problems where line has a string
2391value that would be treated as false by Perl, for example a "" or
2392a "0" with no trailing newline. If you really mean for such values
2393to terminate the loop, they should be tested for explicitly:
7b8d334a
GS
2394
2395 while (($_ = <STDIN>) ne '0') { ... }
2396 while (<STDIN>) { last unless $_; ... }
2397
5ef4d93e 2398In other boolean contexts, C<< <filehandle> >> without an
2399explicit C<defined> test or comparison elicits a warning if the
9f1b1f2d 2400C<use warnings> pragma or the B<-w>
19799a22 2401command-line switch (the C<$^W> variable) is in effect.
7b8d334a 2402
5f05dabc 2403The filehandles STDIN, STDOUT, and STDERR are predefined. (The
19799a22
GS
2404filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
2405in packages, where they would be interpreted as local identifiers
2406rather than global.) Additional filehandles may be created with
2407the open() function, amongst others. See L<perlopentut> and
2408L<perlfunc/open> for details on this.
d74e8afc 2409X<stdin> X<stdout> X<sterr>
a0d0e21e 2410
35f2feb0 2411If a <FILEHANDLE> is used in a context that is looking for
19799a22
GS
2412a list, a list comprising all input lines is returned, one line per
2413list element. It's easy to grow to a rather large data space this
2414way, so use with care.
a0d0e21e 2415
35f2feb0 2416<FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
19799a22 2417See L<perlfunc/readline>.
fbad3eb5 2418
35f2feb0
GS
2419The null filehandle <> is special: it can be used to emulate the
2420behavior of B<sed> and B<awk>. Input from <> comes either from
a0d0e21e 2421standard input, or from each file listed on the command line. Here's
35f2feb0 2422how it works: the first time <> is evaluated, the @ARGV array is
5a964f20 2423checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
a0d0e21e
LW
2424gives you standard input. The @ARGV array is then processed as a list
2425of filenames. The loop
2426
2427 while (<>) {
2428 ... # code for each line
2429 }
2430
2431is equivalent to the following Perl-like pseudo code:
2432
3e3baf6d 2433 unshift(@ARGV, '-') unless @ARGV;
a0d0e21e
LW
2434 while ($ARGV = shift) {
2435 open(ARGV, $ARGV);
2436 while (<ARGV>) {
2437 ... # code for each line
2438 }
2439 }
2440
19799a22
GS
2441except that it isn't so cumbersome to say, and will actually work.
2442It really does shift the @ARGV array and put the current filename
2443into the $ARGV variable. It also uses filehandle I<ARGV>
ac036724 2444internally. <> is just a synonym for <ARGV>, which
19799a22 2445is magical. (The pseudo code above doesn't work because it treats
35f2feb0 2446<ARGV> as non-magical.)
a0d0e21e 2447
48ab5743
ML
2448Since the null filehandle uses the two argument form of L<perlfunc/open>
2449it interprets special characters, so if you have a script like this:
2450
2451 while (<>) {
2452 print;
2453 }
2454
2455and call it with C<perl dangerous.pl 'rm -rfv *|'>, it actually opens a
2456pipe, executes the C<rm> command and reads C<rm>'s output from that pipe.
2457If you want all items in C<@ARGV> to be interpreted as file names, you
2458can use the module C<ARGV::readonly> from CPAN.
2459
35f2feb0 2460You can modify @ARGV before the first <> as long as the array ends up
a0d0e21e 2461containing the list of filenames you really want. Line numbers (C<$.>)
19799a22
GS
2462continue as though the input were one big happy file. See the example
2463in L<perlfunc/eof> for how to reset line numbers on each file.
5a964f20 2464
89d205f2 2465If you want to set @ARGV to your own list of files, go right ahead.
5a964f20
TC
2466This sets @ARGV to all plain text files if no @ARGV was given:
2467
2468 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
a0d0e21e 2469
5a964f20
TC
2470You can even set them to pipe commands. For example, this automatically
2471filters compressed arguments through B<gzip>:
2472
2473 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
2474
2475If you want to pass switches into your script, you can use one of the
a0d0e21e
LW
2476Getopts modules or put a loop on the front like this:
2477
2478 while ($_ = $ARGV[0], /^-/) {
2479 shift;
2480 last if /^--$/;
2481 if (/^-D(.*)/) { $debug = $1 }
2482 if (/^-v/) { $verbose++ }
5a964f20 2483 # ... # other switches
a0d0e21e 2484 }
5a964f20 2485
a0d0e21e 2486 while (<>) {
5a964f20 2487 # ... # code for each line
a0d0e21e
LW
2488 }
2489
89d205f2
YO
2490The <> symbol will return C<undef> for end-of-file only once.
2491If you call it again after this, it will assume you are processing another
19799a22 2492@ARGV list, and if you haven't set @ARGV, will read input from STDIN.
a0d0e21e 2493
b159ebd3 2494If what the angle brackets contain is a simple scalar variable (e.g.,
35f2feb0 2495<$foo>), then that variable contains the name of the
19799a22
GS
2496filehandle to input from, or its typeglob, or a reference to the
2497same. For example:
cb1a09d0
AD
2498
2499 $fh = \*STDIN;
2500 $line = <$fh>;
a0d0e21e 2501
5a964f20
TC
2502If what's within the angle brackets is neither a filehandle nor a simple
2503scalar variable containing a filehandle name, typeglob, or typeglob
2504reference, it is interpreted as a filename pattern to be globbed, and
2505either a list of filenames or the next filename in the list is returned,
19799a22 2506depending on context. This distinction is determined on syntactic
35f2feb0
GS
2507grounds alone. That means C<< <$x> >> is always a readline() from
2508an indirect handle, but C<< <$hash{key}> >> is always a glob().
5a964f20 2509That's because $x is a simple scalar variable, but C<$hash{key}> is
ef191992
YST
2510not--it's a hash element. Even C<< <$x > >> (note the extra space)
2511is treated as C<glob("$x ")>, not C<readline($x)>.
5a964f20
TC
2512
2513One level of double-quote interpretation is done first, but you can't
35f2feb0 2514say C<< <$foo> >> because that's an indirect filehandle as explained
5a964f20
TC
2515in the previous paragraph. (In older versions of Perl, programmers
2516would insert curly brackets to force interpretation as a filename glob:
35f2feb0 2517C<< <${foo}> >>. These days, it's considered cleaner to call the
5a964f20 2518internal function directly as C<glob($foo)>, which is probably the right
19799a22 2519way to have done it in the first place.) For example:
a0d0e21e
LW
2520
2521 while (<*.c>) {
2522 chmod 0644, $_;
2523 }
2524
3a4b19e4 2525is roughly equivalent to:
a0d0e21e
LW
2526
2527 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
2528 while (<FOO>) {
5b3eff12 2529 chomp;
a0d0e21e
LW
2530 chmod 0644, $_;
2531 }
2532
3a4b19e4
GS
2533except that the globbing is actually done internally using the standard
2534C<File::Glob> extension. Of course, the shortest way to do the above is:
a0d0e21e
LW
2535
2536 chmod 0644, <*.c>;
2537
19799a22
GS
2538A (file)glob evaluates its (embedded) argument only when it is
2539starting a new list. All values must be read before it will start
2540over. In list context, this isn't important because you automatically
2541get them all anyway. However, in scalar context the operator returns
069e01df 2542the next value each time it's called, or C<undef> when the list has
19799a22
GS
2543run out. As with filehandle reads, an automatic C<defined> is
2544generated when the glob occurs in the test part of a C<while>,
2545because legal glob returns (e.g. a file called F<0>) would otherwise
2546terminate the loop. Again, C<undef> is returned only once. So if
2547you're expecting a single value from a glob, it is much better to
2548say
4633a7c4
LW
2549
2550 ($file) = <blurch*>;
2551
2552than
2553
2554 $file = <blurch*>;
2555
2556because the latter will alternate between returning a filename and
19799a22 2557returning false.
4633a7c4 2558
b159ebd3 2559If you're trying to do variable interpolation, it's definitely better
4633a7c4 2560to use the glob() function, because the older notation can cause people
e37d713d 2561to become confused with the indirect filehandle notation.
4633a7c4
LW
2562
2563 @files = glob("$dir/*.[ch]");
2564 @files = glob($files[$i]);
2565
a0d0e21e 2566=head2 Constant Folding
d74e8afc 2567X<constant folding> X<folding>
a0d0e21e
LW
2568
2569Like C, Perl does a certain amount of expression evaluation at
19799a22 2570compile time whenever it determines that all arguments to an
a0d0e21e
LW
2571operator are static and have no side effects. In particular, string
2572concatenation happens at compile time between literals that don't do
19799a22 2573variable substitution. Backslash interpolation also happens at
a0d0e21e
LW
2574compile time. You can say
2575
2576 'Now is the time for all' . "\n" .
2577 'good men to come to.'
2578
54310121 2579and this all reduces to one string internally. Likewise, if
a0d0e21e
LW
2580you say
2581
2582 foreach $file (@filenames) {
5a964f20 2583 if (-s $file > 5 + 100 * 2**16) { }
54310121 2584 }
a0d0e21e 2585
19799a22
GS
2586the compiler will precompute the number which that expression
2587represents so that the interpreter won't have to.
a0d0e21e 2588
fd1abbef 2589=head2 No-ops
d74e8afc 2590X<no-op> X<nop>
fd1abbef
DN
2591
2592Perl doesn't officially have a no-op operator, but the bare constants
2593C<0> and C<1> are special-cased to not produce a warning in a void
2594context, so you can for example safely do
2595
2596 1 while foo();
2597
2c268ad5 2598=head2 Bitwise String Operators
d74e8afc 2599X<operator, bitwise, string>
2c268ad5
TP
2600
2601Bitstrings of any size may be manipulated by the bitwise operators
2602(C<~ | & ^>).
2603
19799a22
GS
2604If the operands to a binary bitwise op are strings of different
2605sizes, B<|> and B<^> ops act as though the shorter operand had
2606additional zero bits on the right, while the B<&> op acts as though
2607the longer operand were truncated to the length of the shorter.
2608The granularity for such extension or truncation is one or more
2609bytes.
2c268ad5 2610
89d205f2 2611 # ASCII-based examples
2c268ad5
TP
2612 print "j p \n" ^ " a h"; # prints "JAPH\n"
2613 print "JA" | " ph\n"; # prints "japh\n"
2614 print "japh\nJunk" & '_____'; # prints "JAPH\n";
2615 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
2616
19799a22 2617If you are intending to manipulate bitstrings, be certain that
2c268ad5 2618you're supplying bitstrings: If an operand is a number, that will imply
19799a22 2619a B<numeric> bitwise operation. You may explicitly show which type of
2c268ad5
TP
2620operation you intend by using C<""> or C<0+>, as in the examples below.
2621
4358a253
SS
2622 $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
2623 $foo = '150' | 105; # yields 255
2c268ad5
TP
2624 $foo = 150 | '105'; # yields 255
2625 $foo = '150' | '105'; # yields string '155' (under ASCII)
2626
2627 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
2628 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
a0d0e21e 2629
1ae175c8
GS
2630See L<perlfunc/vec> for information on how to manipulate individual bits
2631in a bit vector.
2632
55497cff 2633=head2 Integer Arithmetic
d74e8afc 2634X<integer>
a0d0e21e 2635
19799a22 2636By default, Perl assumes that it must do most of its arithmetic in
a0d0e21e
LW
2637floating point. But by saying
2638
2639 use integer;
2640
2641you may tell the compiler that it's okay to use integer operations
19799a22
GS
2642(if it feels like it) from here to the end of the enclosing BLOCK.
2643An inner BLOCK may countermand this by saying
a0d0e21e
LW
2644
2645 no integer;
2646
19799a22
GS
2647which lasts until the end of that BLOCK. Note that this doesn't
2648mean everything is only an integer, merely that Perl may use integer
2649operations if it is so inclined. For example, even under C<use
2650integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731>
2651or so.
2652
2653Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
89d205f2 2654and ">>") always produce integral results. (But see also
13a2d996 2655L<Bitwise String Operators>.) However, C<use integer> still has meaning for
19799a22
GS
2656them. By default, their results are interpreted as unsigned integers, but
2657if C<use integer> is in effect, their results are interpreted
2658as signed integers. For example, C<~0> usually evaluates to a large
0be96356 2659integral value. However, C<use integer; ~0> is C<-1> on two's-complement
19799a22 2660machines.
68dc0745
PP
2661
2662=head2 Floating-point Arithmetic
d74e8afc 2663X<floating-point> X<floating point> X<float> X<real>
68dc0745
PP
2664
2665While C<use integer> provides integer-only arithmetic, there is no
19799a22
GS
2666analogous mechanism to provide automatic rounding or truncation to a
2667certain number of decimal places. For rounding to a certain number
2668of digits, sprintf() or printf() is usually the easiest route.
2669See L<perlfaq4>.
68dc0745 2670
5a964f20
TC
2671Floating-point numbers are only approximations to what a mathematician
2672would call real numbers. There are infinitely more reals than floats,
2673so some corners must be cut. For example:
2674
2675 printf "%.20g\n", 123456789123456789;
2676 # produces 123456789123456784
2677
8548cb57
RGS
2678Testing for exact floating-point equality or inequality is not a
2679good idea. Here's a (relatively expensive) work-around to compare
5a964f20
TC
2680whether two floating-point numbers are equal to a particular number of
2681decimal places. See Knuth, volume II, for a more robust treatment of
2682this topic.
2683
2684 sub fp_equal {
2685 my ($X, $Y, $POINTS) = @_;
2686 my ($tX, $tY);
2687 $tX = sprintf("%.${POINTS}g", $X);
2688 $tY = sprintf("%.${POINTS}g", $Y);
2689 return $tX eq $tY;
2690 }
2691
68dc0745 2692The POSIX module (part of the standard perl distribution) implements
19799a22
GS
2693ceil(), floor(), and other mathematical and trigonometric functions.
2694The Math::Complex module (part of the standard perl distribution)
2695defines mathematical functions that work on both the reals and the
2696imaginary numbers. Math::Complex not as efficient as POSIX, but
68dc0745
PP
2697POSIX can't work with complex numbers.
2698
2699Rounding in financial applications can have serious implications, and
2700the rounding method used should be specified precisely. In these
2701cases, it probably pays not to trust whichever system rounding is
2702being used by Perl, but to instead implement the rounding function you
2703need yourself.
5a964f20
TC
2704
2705=head2 Bigger Numbers
d74e8afc 2706X<number, arbitrary precision>
5a964f20
TC
2707
2708The standard Math::BigInt and Math::BigFloat modules provide
19799a22 2709variable-precision arithmetic and overloaded operators, although
cd5c4fce 2710they're currently pretty slow. At the cost of some space and
19799a22
GS
2711considerable speed, they avoid the normal pitfalls associated with
2712limited-precision representations.
5a964f20
TC
2713
2714 use Math::BigInt;
2715 $x = Math::BigInt->new('123456789123456789');
2716 print $x * $x;
2717
2718 # prints +15241578780673678515622620750190521
19799a22 2719
cd5c4fce
T
2720There are several modules that let you calculate with (bound only by
2721memory and cpu-time) unlimited or fixed precision. There are also
2722some non-standard modules that provide faster implementations via
2723external C libraries.
2724
2725Here is a short, but incomplete summary:
2726
950b09ed
KW
2727 Math::Fraction big, unlimited fractions like 9973 / 12967
2728 Math::String treat string sequences like numbers
2729 Math::FixedPrecision calculate with a fixed precision
2730 Math::Currency for currency calculations
2731 Bit::Vector manipulate bit vectors fast (uses C)
2732 Math::BigIntFast Bit::Vector wrapper for big numbers
2733 Math::Pari provides access to the Pari C library
2734 Math::BigInteger uses an external C library
2735 Math::Cephes uses external Cephes C library (no big numbers)
2736 Math::Cephes::Fraction fractions via the Cephes library
2737 Math::GMP another one using an external C library
cd5c4fce
T
2738
2739Choose wisely.
16070b82
GS
2740
2741=cut