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d74e8afc 2X<operator>
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4perlop - Perl operators and precedence
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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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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
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
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
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
d74e8afc 191X<unary operator> X<operator, unary>
a0d0e21e 192
5f05dabc 193Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower
a0d0e21e 194precedence version of this.
d74e8afc 195X<!>
a0d0e21e 196
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197Unary "-" performs arithmetic negation if the operand is numeric,
198including any string that looks like a number. If the operand is
199an identifier, a string consisting of a minus sign concatenated
200with the identifier is returned. Otherwise, if the string starts
201with a plus or minus, a string starting with the opposite sign is
202returned. One effect of these rules is that -bareword is equivalent
8705167b 203to the string "-bareword". If, however, the string begins with a
353c6505 204non-alphabetic character (excluding "+" or "-"), Perl will attempt to convert
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205the string to a numeric and the arithmetic negation is performed. If the
206string cannot be cleanly converted to a numeric, Perl will give the warning
207B<Argument "the string" isn't numeric in negation (-) at ...>.
d74e8afc 208X<-> X<negation, arithmetic>
a0d0e21e 209
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210Unary "~" performs bitwise negation, i.e., 1's complement. For
212L<Bitwise String Operators>.) Note that the width of the result is
213platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64
214bits wide on a 64-bit platform, so if you are expecting a certain bit
d042e63d 215width, remember to use the & operator to mask off the excess bits.
d74e8afc 216X<~> X<negation, binary>
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217
218Unary "+" has no effect whatsoever, even on strings. It is useful
219syntactically for separating a function name from a parenthesized expression
220that would otherwise be interpreted as the complete list of function
5ba421f6 221arguments. (See examples above under L<Terms and List Operators (Leftward)>.)
d74e8afc 222X<+>
a0d0e21e 223
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224Unary "\" creates a reference to whatever follows it. See L<perlreftut>
225and L<perlref>. Do not confuse this behavior with the behavior of
226backslash within a string, although both forms do convey the notion
227of protecting the next thing from interpolation.
d74e8afc 228X<\> X<reference> X<backslash>
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229
d74e8afc 231X<binding> X<operator, binding> X<=~> X<!~>
a0d0e21e 232
c07a80fd 233Binary "=~" binds a scalar expression to a pattern match. Certain operations
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234search or modify the string \$_ by default. This operator makes that kind
235of operation work on some other string. The right argument is a search
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236pattern, substitution, or transliteration. The left argument is what is
237supposed to be searched, substituted, or transliterated instead of the default
f8bab1e9 238\$_. When used in scalar context, the return value generally indicates the
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239success of the operation. The exceptions are substitution (s///)
240and transliteration (y///) with the C</r> (non-destructive) option,
241which cause the B<r>eturn value to be the result of the substitution.
242Behavior in list context depends on the particular operator.
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243See L</"Regexp Quote-Like Operators"> for details and L<perlretut> for
244examples using these operators.
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245
246If the right argument is an expression rather than a search pattern,
2c268ad5 247substitution, or transliteration, it is interpreted as a search pattern at run
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248time. Note that this means that its contents will be interpolated twice, so
249
250 '\\' =~ q'\\';
251
252is not ok, as the regex engine will end up trying to compile the
253pattern C<\>, which it will consider a syntax error.
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254
255Binary "!~" is just like "=~" except the return value is negated in
256the logical sense.
257
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258Binary "!~" with a non-destructive substitution (s///r) or transliteration
259(y///r) is a syntax error.
4f4d7508 260
d74e8afc 262X<operator, multiplicative>
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263
264Binary "*" multiplies two numbers.
d74e8afc 265X<*>
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266
267Binary "/" divides two numbers.
d74e8afc 268X</> X<slash>
a0d0e21e 269
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270Binary "%" is the modulo operator, which computes the division
271remainder of its first argument with respect to its second argument.
272Given integer
54310121 273operands C<\$a> and C<\$b>: If C<\$b> is positive, then C<\$a % \$b> is
f7918450 274C<\$a> minus the largest multiple of C<\$b> less than or equal to
54310121 275C<\$a>. If C<\$b> is negative, then C<\$a % \$b> is C<\$a> minus the
276smallest multiple of C<\$b> that is not less than C<\$a> (i.e. the
89b4f0ad 277result will be less than or equal to zero). If the operands
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278C<\$a> and C<\$b> are floating point values and the absolute value of
279C<\$b> (that is C<abs(\$b)>) is less than C<(UV_MAX + 1)>, only
280the integer portion of C<\$a> and C<\$b> will be used in the operation
281(Note: here C<UV_MAX> means the maximum of the unsigned integer type).
282If the absolute value of the right operand (C<abs(\$b)>) is greater than
283or equal to C<(UV_MAX + 1)>, "%" computes the floating-point remainder
284C<\$r> in the equation C<(\$r = \$a - \$i*\$b)> where C<\$i> is a certain
f7918450 285integer that makes C<\$r> have the same sign as the right operand
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286C<\$b> (B<not> as the left operand C<\$a> like C function C<fmod()>)
287and the absolute value less than that of C<\$b>.
0412d526 288Note that when C<use integer> is in scope, "%" gives you direct access
f7918450 289to the modulo operator as implemented by your C compiler. This
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290operator is not as well defined for negative operands, but it will
291execute faster.
f7918450 292X<%> X<remainder> X<modulo> X<mod>
55d729e4 293
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294Binary "x" is the repetition operator. In scalar context or if the left
295operand is not enclosed in parentheses, it returns a string consisting
296of the left operand repeated the number of times specified by the right
297operand. In list context, if the left operand is enclosed in
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298parentheses or is a list formed by C<qw/STRING/>, it repeats the list.
299If the right operand is zero or negative, it returns an empty string
300or an empty list, depending on the context.
d74e8afc 301X<x>
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302
303 print '-' x 80; # print row of dashes
304
305 print "\t" x (\$tab/8), ' ' x (\$tab%8); # tab over
306
307 @ones = (1) x 80; # a list of 80 1's
308 @ones = (5) x @ones; # set all elements to 5
309
310
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313
314Binary "+" returns the sum of two numbers.
d74e8afc 315X<+>
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316
317Binary "-" returns the difference of two numbers.
d74e8afc 318X<->
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319
320Binary "." concatenates two strings.
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321X<string, concatenation> X<concatenation>
322X<cat> X<concat> X<concatenate> X<.>
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323
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325X<shift operator> X<operator, shift> X<<< << >>>
326X<<< >> >>> X<right shift> X<left shift> X<bitwise shift>
327X<shl> X<shr> X<shift, right> X<shift, left>
a0d0e21e 328
55497cff 329Binary "<<" returns the value of its left argument shifted left by the
330number of bits specified by the right argument. Arguments should be
a0d0e21e 332
55497cff 333Binary ">>" returns the value of its left argument shifted right by
334the number of bits specified by the right argument. Arguments should
a0d0e21e 336
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337Note that both "<<" and ">>" in Perl are implemented directly using
338"<<" and ">>" in C. If C<use integer> (see L<Integer Arithmetic>) is
339in force then signed C integers are used, else unsigned C integers are
340used. Either way, the implementation isn't going to generate results
341larger than the size of the integer type Perl was built with (32 bits
342or 64 bits).
343
344The result of overflowing the range of the integers is undefined
345because it is undefined also in C. In other words, using 32-bit
346integers, C<< 1 << 32 >> is undefined. Shifting by a negative number
347of bits is also undefined.
348
d74e8afc 350X<operator, named unary>
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351
352The various named unary operators are treated as functions with one
568e6d8b 353argument, with optional parentheses.
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354
355If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
356is followed by a left parenthesis as the next token, the operator and
357arguments within parentheses are taken to be of highest precedence,
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358just like a normal function call. For example,
359because named unary operators are higher precedence than ||:
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360
361 chdir \$foo || die; # (chdir \$foo) || die
362 chdir(\$foo) || die; # (chdir \$foo) || die
363 chdir (\$foo) || die; # (chdir \$foo) || die
364 chdir +(\$foo) || die; # (chdir \$foo) || die
365
3981b0eb 366but, because * is higher precedence than named operators:
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367
368 chdir \$foo * 20; # chdir (\$foo * 20)
369 chdir(\$foo) * 20; # (chdir \$foo) * 20
370 chdir (\$foo) * 20; # (chdir \$foo) * 20
371 chdir +(\$foo) * 20; # chdir (\$foo * 20)
372
373 rand 10 * 20; # rand (10 * 20)
374 rand(10) * 20; # (rand 10) * 20
375 rand (10) * 20; # (rand 10) * 20
376 rand +(10) * 20; # rand (10 * 20)
377
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378Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are
379treated like named unary operators, but they don't follow this functional
380parenthesis rule. That means, for example, that C<-f(\$file).".bak"> is
381equivalent to C<-f "\$file.bak">.
d74e8afc 382X<-X> X<filetest> X<operator, filetest>
568e6d8b 383
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385
d74e8afc 387X<relational operator> X<operator, relational>
a0d0e21e 388
35f2feb0 389Binary "<" returns true if the left argument is numerically less than
a0d0e21e 390the right argument.
d74e8afc 391X<< < >>
a0d0e21e 392
35f2feb0 393Binary ">" returns true if the left argument is numerically greater
a0d0e21e 394than the right argument.
d74e8afc 395X<< > >>
a0d0e21e 396
35f2feb0 397Binary "<=" returns true if the left argument is numerically less than
a0d0e21e 398or equal to the right argument.
d74e8afc 399X<< <= >>
a0d0e21e 400
35f2feb0 401Binary ">=" returns true if the left argument is numerically greater
a0d0e21e 402than or equal to the right argument.
d74e8afc 403X<< >= >>
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404
405Binary "lt" returns true if the left argument is stringwise less than
406the right argument.
d74e8afc 407X<< lt >>
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408
409Binary "gt" returns true if the left argument is stringwise greater
410than the right argument.
d74e8afc 411X<< gt >>
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412
413Binary "le" returns true if the left argument is stringwise less than
414or equal to the right argument.
d74e8afc 415X<< le >>
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416
417Binary "ge" returns true if the left argument is stringwise greater
418than or equal to the right argument.
d74e8afc 419X<< ge >>
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420
d74e8afc 422X<equality> X<equal> X<equals> X<operator, equality>
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423
424Binary "==" returns true if the left argument is numerically equal to
425the right argument.
d74e8afc 426X<==>
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427
428Binary "!=" returns true if the left argument is numerically not equal
429to the right argument.
d74e8afc 430X<!=>
a0d0e21e 431
35f2feb0 432Binary "<=>" returns -1, 0, or 1 depending on whether the left
6ee5d4e7 433argument is numerically less than, equal to, or greater than the right
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435values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
436"<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
437returns true, as does NaN != anything else. If your platform doesn't
438support NaNs then NaN is just a string with numeric value 0.
d74e8afc 439X<< <=> >> X<spaceship>
7d3a9d88 440
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441 perl -le '\$a = "NaN"; print "No NaN support here" if \$a == \$a'
442 perl -le '\$a = "NaN"; print "NaN support here" if \$a != \$a'
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443
444Binary "eq" returns true if the left argument is stringwise equal to
445the right argument.
d74e8afc 446X<eq>
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447
448Binary "ne" returns true if the left argument is stringwise not equal
449to the right argument.
d74e8afc 450X<ne>
a0d0e21e 451
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452Binary "cmp" returns -1, 0, or 1 depending on whether the left
453argument is stringwise less than, equal to, or greater than the right
454argument.
d74e8afc 455X<cmp>
a0d0e21e 456
0d863452 457Binary "~~" does a smart match between its arguments. Smart matching
0f7107a0 458is described in L<perlsyn/"Smart matching in detail">.
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459X<~~>
460
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461"lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
462by the current locale if C<use locale> is in effect. See L<perllocale>.
463
d74e8afc 465X<operator, bitwise, and> X<bitwise and> X<&>
a0d0e21e 466
2cdc098b 467Binary "&" returns its operands ANDed together bit by bit.
a0d0e21e 469
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470Note that "&" has lower priority than relational operators, so for example
471the brackets are essential in a test like
472
473 print "Even\n" if (\$x & 1) == 0;
474
a0d0e21e 475=head2 Bitwise Or and Exclusive Or
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476X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor>
477X<bitwise xor> X<^>
a0d0e21e 478
2cdc098b 479Binary "|" returns its operands ORed together bit by bit.
a0d0e21e 481
2cdc098b 482Binary "^" returns its operands XORed together bit by bit.
a0d0e21e 484
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485Note that "|" and "^" have lower priority than relational operators, so
486for example the brackets are essential in a test like
487
488 print "false\n" if (8 | 2) != 10;
489
d74e8afc 491X<&&> X<logical and> X<operator, logical, and>
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492
493Binary "&&" performs a short-circuit logical AND operation. That is,
494if the left operand is false, the right operand is not even evaluated.
495Scalar or list context propagates down to the right operand if it
496is evaluated.
497
d74e8afc 499X<||> X<operator, logical, or>
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500
501Binary "||" performs a short-circuit logical OR operation. That is,
502if the left operand is true, the right operand is not even evaluated.
503Scalar or list context propagates down to the right operand if it
504is evaluated.
505
d74e8afc 507X<//> X<operator, logical, defined-or>
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508
509Although it has no direct equivalent in C, Perl's C<//> operator is related
89d205f2 510to its C-style or. In fact, it's exactly the same as C<||>, except that it
c963b151 511tests the left hand side's definedness instead of its truth. Thus, C<\$a // \$b>
89d205f2 512is similar to C<defined(\$a) || \$b> (except that it returns the value of C<\$a>
d90d5a38 513rather than the value of C<defined(\$a)>) and yields the same result as
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514C<defined(\$a) ? \$a : \$b> (except that the ternary-operator form can be
515used as a lvalue, while C<\$a // \$b> cannot). This is very useful for
516providing default values for variables. If you actually want to test if
517at least one of C<\$a> and C<\$b> is defined, use C<defined(\$a // \$b)>.
c963b151 518
d042e63d
MS
519The C<||>, C<//> and C<&&> operators return the last value evaluated
520(unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably
521portable way to find out the home directory might be:
a0d0e21e 522
c963b151
BD
523 \$home = \$ENV{'HOME'} // \$ENV{'LOGDIR'} //
524 (getpwuid(\$<))[7] // die "You're homeless!\n";
a0d0e21e 525
5a964f20
TC
526In particular, this means that you shouldn't use this
527for selecting between two aggregates for assignment:
528
529 @a = @b || @c; # this is wrong
530 @a = scalar(@b) || @c; # really meant this
531 @a = @b ? @b : @c; # this works fine, though
532
f23102e2
RGS
533As more readable alternatives to C<&&> and C<||> when used for
534control flow, Perl provides the C<and> and C<or> operators (see below).
535The short-circuit behavior is identical. The precedence of "and"
c963b151 536and "or" is much lower, however, so that you can safely use them after a
5a964f20 537list operator without the need for parentheses:
a0d0e21e
LW
538
540 or gripe(), next LINE;
541
542With the C-style operators that would have been written like this:
543
545 || (gripe(), next LINE);
546
eeb6a2c9 547Using "or" for assignment is unlikely to do what you want; see below.
5a964f20
TC
548
d74e8afc 550X<operator, range> X<range> X<..> X<...>
a0d0e21e
LW
551
552Binary ".." is the range operator, which is really two different
fb53bbb2 553operators depending on the context. In list context, it returns a
54ae734e 554list of values counting (up by ones) from the left value to the right
2cdbc966 555value. If the left value is greater than the right value then it
fb53bbb2 556returns the empty list. The range operator is useful for writing
54ae734e 557C<foreach (1..10)> loops and for doing slice operations on arrays. In
2cdbc966
JD
558the current implementation, no temporary array is created when the
559range operator is used as the expression in C<foreach> loops, but older
560versions of Perl might burn a lot of memory when you write something
561like this:
a0d0e21e
LW
562
563 for (1 .. 1_000_000) {
564 # code
54310121 565 }
a0d0e21e 566
8f0f46f8 567The range operator also works on strings, using the magical
568auto-increment, see below.
54ae734e 569
5a964f20 570In scalar context, ".." returns a boolean value. The operator is
8f0f46f8 571bistable, like a flip-flop, and emulates the line-range (comma)
572operator of B<sed>, B<awk>, and various editors. Each ".." operator
573maintains its own boolean state, even across calls to a subroutine
574that contains it. It is false as long as its left operand is false.
a0d0e21e
LW
575Once the left operand is true, the range operator stays true until the
576right operand is true, I<AFTER> which the range operator becomes false
8f0f46f8 577again. It doesn't become false till the next time the range operator
578is evaluated. It can test the right operand and become false on the
579same evaluation it became true (as in B<awk>), but it still returns
580true once. If you don't want it to test the right operand until the
581next evaluation, as in B<sed>, just use three dots ("...") instead of
19799a22
GS
582two. In all other regards, "..." behaves just like ".." does.
583
584The right operand is not evaluated while the operator is in the
585"false" state, and the left operand is not evaluated while the
586operator is in the "true" state. The precedence is a little lower
587than || and &&. The value returned is either the empty string for
8f0f46f8 588false, or a sequence number (beginning with 1) for true. The sequence
589number is reset for each range encountered. The final sequence number
590in a range has the string "E0" appended to it, which doesn't affect
591its numeric value, but gives you something to search for if you want
592to exclude the endpoint. You can exclude the beginning point by
593waiting for the sequence number to be greater than 1.
df5f8116
CW
594
595If either operand of scalar ".." is a constant expression,
596that operand is considered true if it is equal (C<==>) to the current
597input line number (the C<\$.> variable).
598
599To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>,
600but that is only an issue if you use a floating point expression; when
601implicitly using C<\$.> as described in the previous paragraph, the
602comparison is C<int(EXPR) == int(\$.)> which is only an issue when C<\$.>
603is set to a floating point value and you are not reading from a file.
604Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what
605you want in scalar context because each of the operands are evaluated
606using their integer representation.
607
608Examples:
a0d0e21e
LW
609
610As a scalar operator:
611
df5f8116 612 if (101 .. 200) { print; } # print 2nd hundred lines, short for
950b09ed 613 # if (\$. == 101 .. \$. == 200) { print; }
9f10b797
RGS
614
615 next LINE if (1 .. /^\$/); # skip header lines, short for
f343f960 616 # next LINE if (\$. == 1 .. /^\$/);
9f10b797
RGS
617 # (typically in a loop labeled LINE)
618
619 s/^/> / if (/^\$/ .. eof()); # quote body
a0d0e21e 620
5a964f20
TC
621 # parse mail messages
622 while (<>) {
623 \$in_header = 1 .. /^\$/;
df5f8116
CW
624 \$in_body = /^\$/ .. eof;
f343f960 626 # do something
df5f8116 627 } else { # in body
f343f960 628 # do something else
df5f8116 629 }
5a964f20 630 } continue {
df5f8116 631 close ARGV if eof; # reset \$. each file
5a964f20
TC
632 }
633
acf31ca5
SF
634Here's a simple example to illustrate the difference between
635the two range operators:
636
637 @lines = (" - Foo",
638 "01 - Bar",
639 "1 - Baz",
640 " - Quux");
641
9f10b797
RGS
642 foreach (@lines) {
643 if (/0/ .. /1/) {
acf31ca5
SF
644 print "\$_\n";
645 }
646 }
647
9f10b797
RGS
648This program will print only the line containing "Bar". If
649the range operator is changed to C<...>, it will also print the
acf31ca5
SF
650"Baz" line.
651
652And now some examples as a list operator:
a0d0e21e
LW
653
654 for (101 .. 200) { print; } # print \$_ 100 times
3e3baf6d 655 @foo = @foo[0 .. \$#foo]; # an expensive no-op
a0d0e21e
LW
656 @foo = @foo[\$#foo-4 .. \$#foo]; # slice last 5 items
657
5a964f20 658The range operator (in list context) makes use of the magical
5f05dabc 659auto-increment algorithm if the operands are strings. You
a0d0e21e
LW
660can say
661
662 @alphabet = ('A' .. 'Z');
663
54ae734e 664to get all normal letters of the English alphabet, or
a0d0e21e
LW
665
666 \$hexdigit = (0 .. 9, 'a' .. 'f')[\$num & 15];
667
668to get a hexadecimal digit, or
669
670 @z2 = ('01' .. '31'); print \$z2[\$mday];
671
ea4f5703
YST
672to get dates with leading zeros.
673
674If the final value specified is not in the sequence that the magical
675increment would produce, the sequence goes until the next value would
676be longer than the final value specified.
677
678If the initial value specified isn't part of a magical increment
679sequence (that is, a non-empty string matching "/^[a-zA-Z]*[0-9]*\z/"),
680only the initial value will be returned. So the following will only
681return an alpha:
682
683 use charnames 'greek';
684 my @greek_small = ("\N{alpha}" .. "\N{omega}");
685
686To get lower-case greek letters, use this instead:
687
950b09ed
KW
688 my @greek_small = map { chr } ( ord("\N{alpha}") ..
689 ord("\N{omega}") );
a0d0e21e 690
df5f8116
CW
691Because each operand is evaluated in integer form, C<2.18 .. 3.14> will
692return two elements in list context.
693
694 @list = (2.18 .. 3.14); # same as @list = (2 .. 3);
695
d74e8afc 697X<operator, conditional> X<operator, ternary> X<ternary> X<?:>
a0d0e21e
LW
698
699Ternary "?:" is the conditional operator, just as in C. It works much
700like an if-then-else. If the argument before the ? is true, the
701argument before the : is returned, otherwise the argument after the :
cb1a09d0
702is returned. For example:
703
54310121 704 printf "I have %d dog%s.\n", \$n,
cb1a09d0
705 (\$n == 1) ? '' : "s";
706
707Scalar or list context propagates downward into the 2nd
54310121 708or 3rd argument, whichever is selected.
cb1a09d0
709
710 \$a = \$ok ? \$b : \$c; # get a scalar
711 @a = \$ok ? @b : @c; # get an array
712 \$a = \$ok ? @b : @c; # oops, that's just a count!
713
714The operator may be assigned to if both the 2nd and 3rd arguments are
715legal lvalues (meaning that you can assign to them):
a0d0e21e
LW
716
717 (\$a_or_b ? \$a : \$b) = \$c;
718
5a964f20
TC
719Because this operator produces an assignable result, using assignments
720without parentheses will get you in trouble. For example, this:
721
722 \$a % 2 ? \$a += 10 : \$a += 2
723
724Really means this:
725
726 ((\$a % 2) ? (\$a += 10) : \$a) += 2
727
728Rather than this:
729
730 (\$a % 2) ? (\$a += 10) : (\$a += 2)
731
19799a22
GS
732That should probably be written more simply as:
733
734 \$a += (\$a % 2) ? 10 : 2;
735
d74e8afc 737X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=>
5ac3b81c 738X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=>
d74e8afc 739X<%=> X<^=> X<x=>
a0d0e21e
LW
740
741"=" is the ordinary assignment operator.
742
743Assignment operators work as in C. That is,
744
745 \$a += 2;
746
747is equivalent to
748
749 \$a = \$a + 2;
750
751although without duplicating any side effects that dereferencing the lvalue
54310121 752might trigger, such as from tie(). Other assignment operators work similarly.
753The following are recognized:
a0d0e21e
LW
754
755 **= += *= &= <<= &&=
9f10b797
RGS
756 -= /= |= >>= ||=
757 .= %= ^= //=
758 x=
a0d0e21e 759
19799a22 760Although these are grouped by family, they all have the precedence
a0d0e21e
LW
761of assignment.
762
b350dd2f
GS
763Unlike in C, the scalar assignment operator produces a valid lvalue.
764Modifying an assignment is equivalent to doing the assignment and
765then modifying the variable that was assigned to. This is useful
766for modifying a copy of something, like this:
a0d0e21e
LW
767
768 (\$tmp = \$global) =~ tr [A-Z] [a-z];
769
770Likewise,
771
772 (\$a += 2) *= 3;
773
774is equivalent to
775
776 \$a += 2;
777 \$a *= 3;
778
b350dd2f
GS
779Similarly, a list assignment in list context produces the list of
780lvalues assigned to, and a list assignment in scalar context returns
781the number of elements produced by the expression on the right hand
782side of the assignment.
783
d74e8afc 785X<comma> X<operator, comma> X<,>
a0d0e21e 786
5a964f20 787Binary "," is the comma operator. In scalar context it evaluates
a0d0e21e
LW
788its left argument, throws that value away, then evaluates its right
789argument and returns that value. This is just like C's comma operator.
790
5a964f20 791In list context, it's just the list argument separator, and inserts
ed5c6d31
PJ
792both its arguments into the list. These arguments are also evaluated
793from left to right.
a0d0e21e 794
344f2c40
IG
795The C<< => >> operator is a synonym for the comma except that it causes
796its left operand to be interpreted as a string if it begins with a letter
797or underscore and is composed only of letters, digits and underscores.
798This includes operands that might otherwise be interpreted as operators,
799constants, single number v-strings or function calls. If in doubt about
800this behaviour, the left operand can be quoted explicitly.
801
802Otherwise, the C<< => >> operator behaves exactly as the comma operator
803or list argument separator, according to context.
804
805For example:
a44e5664
MS
806
807 use constant FOO => "something";
808
809 my %h = ( FOO => 23 );
810
811is equivalent to:
812
813 my %h = ("FOO", 23);
814
815It is I<NOT>:
816
817 my %h = ("something", 23);
818
719b43e8
RGS
819The C<< => >> operator is helpful in documenting the correspondence
820between keys and values in hashes, and other paired elements in lists.
748a9306 821
a44e5664
MS
822 %hash = ( \$key => \$value );
824
678ae90b
RGS
be25f609 827
e8163f9b 828The yada yada operator (noted C<...>) is a placeholder for code. Perl
829parses it without error, but when you try to execute a yada yada, it
830throws an exception with the text C<Unimplemented>:
831
832 sub unimplemented { ... }
833
834 eval { unimplemented() };
835 if( \$@ eq 'Unimplemented' ) {
837 }
838
839You can only use the yada yada to stand in for a complete statement.
841
842 { ... }
843
844 sub foo { ... }
845
846 ...;
847
848 eval { ... };
849
850 sub foo {
851 my( \$self ) = shift;
852
853 ...;
854 }
855
856 do { my \$n; ...; print 'Hurrah!' };
857
858The yada yada cannot stand in for an expression that is part of a
859larger statement since the C<...> is also the three-dot version of the
860range operator (see L<Range Operators>). These examples of the yada
862
863 print ...;
864
865 open my(\$fh), '>', '/dev/passwd' or ...;
866
867 if( \$condition && ... ) { print "Hello\n" };
868
869There are some cases where Perl can't immediately tell the difference
870between an expression and a statement. For instance, the syntax for a
871block and an anonymous hash reference constructor look the same unless
872there's something in the braces that give Perl a hint. The yada yada
873is a syntax error if Perl doesn't guess that the C<{ ... }> is a
874block. In that case, it doesn't think the C<...> is the yada yada
875because it's expecting an expression instead of a statement:
876
877 my @transformed = map { ... } @input; # syntax error
878
879You can use a C<;> inside your block to denote that the C<{ ... }> is
880a block and not a hash reference constructor. Now the yada yada works:
881
882 my @transformed = map {; ... } @input; # ; disambiguates
883
884 my @transformed = map { ...; } @input; # ; disambiguates
be25f609 885
d74e8afc 887X<operator, list, rightward> X<list operator>
a0d0e21e
LW
888
889On the right side of a list operator, it has very low precedence,
890such that it controls all comma-separated expressions found there.
891The only operators with lower precedence are the logical operators
892"and", "or", and "not", which may be used to evaluate calls to list
893operators without the need for extra parentheses:
894
895 open HANDLE, "filename"
896 or die "Can't open: \$!\n";
897
5ba421f6 898See also discussion of list operators in L<Terms and List Operators (Leftward)>.
a0d0e21e
LW
899
d74e8afc 901X<operator, logical, not> X<not>
a0d0e21e
LW
902
903Unary "not" returns the logical negation of the expression to its right.
904It's the equivalent of "!" except for the very low precedence.
905
d74e8afc 907X<operator, logical, and> X<and>
a0d0e21e
LW
908
909Binary "and" returns the logical conjunction of the two surrounding
910expressions. It's equivalent to && except for the very low
5f05dabc 911precedence. This means that it short-circuits: i.e., the right
a0d0e21e
LW
912expression is evaluated only if the left expression is true.
913
c963b151 914=head2 Logical or, Defined or, and Exclusive Or
f23102e2 915X<operator, logical, or> X<operator, logical, xor>
d74e8afc 916X<operator, logical, defined or> X<operator, logical, exclusive or>
f23102e2 917X<or> X<xor>
a0d0e21e
LW
918
919Binary "or" returns the logical disjunction of the two surrounding
5a964f20
TC
920expressions. It's equivalent to || except for the very low precedence.
921This makes it useful for control flow
922
923 print FH \$data or die "Can't write to FH: \$!";
924
925This means that it short-circuits: i.e., the right expression is evaluated
926only if the left expression is false. Due to its precedence, you should
927probably avoid using this for assignment, only for control flow.
928
929 \$a = \$b or \$c; # bug: this is wrong
930 (\$a = \$b) or \$c; # really means this
931 \$a = \$b || \$c; # better written this way
932
19799a22 933However, when it's a list-context assignment and you're trying to use
5a964f20
TC
934"||" for control flow, you probably need "or" so that the assignment
935takes higher precedence.
936
937 @info = stat(\$file) || die; # oops, scalar sense of stat!
938 @info = stat(\$file) or die; # better, now @info gets its due
939
c963b151
BD
940Then again, you could always use parentheses.
941
a0d0e21e
LW
942Binary "xor" returns the exclusive-OR of the two surrounding expressions.
943It cannot short circuit, of course.
944
945=head2 C Operators Missing From Perl
d74e8afc
ITB
946X<operator, missing from perl> X<&> X<*>
947X<typecasting> X<(TYPE)>
a0d0e21e
LW
948
949Here is what C has that Perl doesn't:
950
951=over 8
952
953=item unary &
954
955Address-of operator. (But see the "\" operator for taking a reference.)
956
957=item unary *
958
54310121 959Dereference-address operator. (Perl's prefix dereferencing
a0d0e21e
LW
960operators are typed: \$, @, %, and &.)
961
962=item (TYPE)
963
19799a22 964Type-casting operator.
a0d0e21e
LW
965
966=back
967
5f05dabc 968=head2 Quote and Quote-like Operators
89d205f2 969X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m>
d74e8afc
ITB
970X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>>
971X<escape sequence> X<escape>
972
a0d0e21e
LW
973
974While we usually think of quotes as literal values, in Perl they
975function as operators, providing various kinds of interpolating and
976pattern matching capabilities. Perl provides customary quote characters
977for these behaviors, but also provides a way for you to choose your
978quote character for any of them. In the following table, a C<{}> represents
9f10b797 979any pair of delimiters you choose.
a0d0e21e 980
TP
981 Customary Generic Meaning Interpolates
982 '' q{} Literal no
983 "" qq{} Literal yes
af9219ee 984 `` qx{} Command yes*
2c268ad5 985 qw{} Word list no
af9219ee
MG
986 // m{} Pattern match yes*
987 qr{} Pattern yes*
988 s{}{} Substitution yes*
2c268ad5 989 tr{}{} Transliteration no (but see below)
7e3b091d 990 <<EOF here-doc yes*
a0d0e21e 991
af9219ee
MG
992 * unless the delimiter is ''.
993
87275199
GS
994Non-bracketing delimiters use the same character fore and aft, but the four
995sorts of brackets (round, angle, square, curly) will all nest, which means
9f10b797 996that
87275199 997
9f10b797 998 q{foo{bar}baz}
35f2feb0 999
9f10b797 1000is the same as
87275199
GS
1001
1002 'foo{bar}baz'
1003
1004Note, however, that this does not always work for quoting Perl code:
1005
1006 \$s = q{ if(\$a eq "}") ... }; # WRONG
1007
83df6a1d
JH
1008is a syntax error. The C<Text::Balanced> module (from CPAN, and
1009starting from Perl 5.8 part of the standard distribution) is able
1010to do this properly.
87275199 1011
19799a22 1012There can be whitespace between the operator and the quoting
fb73857a 1013characters, except when C<#> is being used as the quoting character.
19799a22
GS
1014C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
1015operator C<q> followed by a comment. Its argument will be taken
1016from the next line. This allows you to write:
fb73857a 1017
1018 s {foo} # Replace foo
1019 {bar} # with bar.
1020
904501ec
MG
1021The following escape sequences are available in constructs that interpolate
1022and in transliterations.
5691ca5f 1023X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N> X<\N{}>
04341565 1024X<\o{}>
5691ca5f 1025
2c4c1ff2
KW
1026 Sequence Note Description
1027 \t tab (HT, TAB)
1028 \n newline (NL)
1029 \r return (CR)
1030 \f form feed (FF)
1031 \b backspace (BS)
1032 \a alarm (bell) (BEL)
1033 \e escape (ESC)
1034 \x{263a} [1,8] hex char (example: SMILEY)
1035 \x1b [2,8] restricted range hex char (example: ESC)
fb121860 1036 \N{name} [3] named Unicode character or character sequence
2c4c1ff2
KW
1037 \N{U+263D} [4,8] Unicode character (example: FIRST QUARTER MOON)
1038 \c[ [5] control char (example: chr(27))
1039 \o{23072} [6,8] octal char (example: SMILEY)
1040 \033 [7,8] restricted range octal char (example: ESC)
5691ca5f
KW
1041
1042=over 4
1043
1044=item [1]
1045
2c4c1ff2
KW
1046The result is the character specified by the hexadecimal number between
1047the braces. See L</[8]> below for details on which character.
96448467
DG
1048
1049Only hexadecimal digits are valid between the braces. If an invalid
1050character is encountered, a warning will be issued and the invalid
1051character and all subsequent characters (valid or invalid) within the
1053
1054If there are no valid digits between the braces, the generated character is
1055the NULL character (C<\x{00}>). However, an explicit empty brace (C<\x{}>)
1056will not cause a warning.
40687185
KW
1057
1058=item [2]
1059
2c4c1ff2
KW
1060The result is the character specified by the hexadecimal number in the range
10610x00 to 0xFF. See L</[8]> below for details on which character.
96448467
DG
1062
1063Only hexadecimal digits are valid following C<\x>. When C<\x> is followed
2c4c1ff2 1064by fewer than two valid digits, any valid digits will be zero-padded. This
96448467 1065means that C<\x7> will be interpreted as C<\x07> and C<\x> alone will be
2c4c1ff2 1066interpreted as C<\x00>. Except at the end of a string, having fewer than
96448467
DG
1067two valid digits will result in a warning. Note that while the warning
1068says the illegal character is ignored, it is only ignored as part of the
1069escape and will still be used as the subsequent character in the string.
1070For example:
1071
1072 Original Result Warns?
1073 "\x7" "\x07" no
1074 "\x" "\x00" no
1075 "\x7q" "\x07q" yes
1076 "\xq" "\x00q" yes
1077
40687185
KW
1078=item [3]
1079
fb121860 1080The result is the Unicode character or character sequence given by I<name>.
2c4c1ff2 1081See L<charnames>.
40687185
KW
1082
1083=item [4]
1084
2c4c1ff2
KW
1085C<\N{U+I<hexadecimal number>}> means the Unicode character whose Unicode code
40687185
KW
1087
1088=item [5]
1089
5691ca5f
KW
1090The character following C<\c> is mapped to some other character as shown in the
1091table:
1092
1093 Sequence Value
1094 \c@ chr(0)
1095 \cA chr(1)
1096 \ca chr(1)
1097 \cB chr(2)
1098 \cb chr(2)
1099 ...
1100 \cZ chr(26)
1101 \cz chr(26)
1102 \c[ chr(27)
1103 \c] chr(29)
1104 \c^ chr(30)
1105 \c? chr(127)
1106
1107Also, C<\c\I<X>> yields C< chr(28) . "I<X>"> for any I<X>, but cannot come at the
1108end of a string, because the backslash would be parsed as escaping the end
1109quote.
1110
1111On ASCII platforms, the resulting characters from the list above are the
1112complete set of ASCII controls. This isn't the case on EBCDIC platforms; see
1113L<perlebcdic/OPERATOR DIFFERENCES> for the complete list of what these
1114sequences mean on both ASCII and EBCDIC platforms.
1115
1116Use of any other character following the "c" besides those listed above is
6ae9f32a
KW
1117discouraged, and may become deprecated or forbidden. What happens for those
1118other characters currently though, is that the value is derived by inverting
1119the 7th bit (0x40).
5691ca5f
KW
1120
1121To get platform independent controls, you can use C<\N{...}>.
1122
40687185
KW
1123=item [6]
1124
2c4c1ff2
KW
1125The result is the character specified by the octal number between the braces.
1126See L</[8]> below for details on which character.
04341565
DG
1127
1128If a character that isn't an octal digit is encountered, a warning is raised,
1129and the value is based on the octal digits before it, discarding it and all
1130following characters up to the closing brace. It is a fatal error if there are
1131no octal digits at all.
1132
1133=item [7]
1134
2c4c1ff2
KW
1135The result is the character specified by the three digit octal number in the
1136range 000 to 777 (but best to not use above 077, see next paragraph). See
1137L</[8]> below for details on which character.
1138
1139Some contexts allow 2 or even 1 digit, but any usage without exactly
40687185 1140three digits, the first being a zero, may give unintended results. (For
04341565
DG
1141example, see L<perlrebackslash/Octal escapes>.) Starting in Perl 5.14, you may
1142use C<\o{}> instead which avoids all these problems. Otherwise, it is best to
1143use this construct only for ordinals C<\077> and below, remembering to pad to
1144the left with zeros to make three digits. For larger ordinals, either use
d90d5a38 1145C<\o{}> , or convert to something else, such as to hex and use C<\x{}>
40687185 1147
40687185
KW
1148Having fewer than 3 digits may lead to a misleading warning message that says
1149that what follows is ignored. For example, C<"\128"> in the ASCII character set
1150is equivalent to the two characters C<"\n8">, but the warning C<Illegal octal
1151digit '8' ignored> will be thrown. To avoid this warning, make sure to pad
1152your octal number with C<0>s: C<"\0128">.
5691ca5f 1153
2c4c1ff2
KW
1154=item [8]
1155
1156Several of the constructs above specify a character by a number. That number
1157gives the character's position in the character set encoding (indexed from 0).
1158This is called synonymously its ordinal, code position, or code point). Perl
1159works on platforms that have a native encoding currently of either ASCII/Latin1
1160or EBCDIC, each of which allow specification of 256 characters. In general, if
1161the number is 255 (0xFF, 0377) or below, Perl interprets this in the platform's
1162native encoding. If the number is 256 (0x100, 0400) or above, Perl interprets
1163it as as a Unicode code point and the result is the corresponding Unicode
1164character. For example C<\x{50}> and C<\o{120}> both are the number 80 in
1165decimal, which is less than 256, so the number is interpreted in the native
1166character set encoding. In ASCII the character in the 80th position (indexed
1167from 0) is the letter "P", and in EBCDIC it is the ampersand symbol "&".
1168C<\x{100}> and C<\o{400}> are both 256 in decimal, so the number is interpreted
1169as a Unicode code point no matter what the native encoding is. The name of the
1170character in the 100th position (indexed by 0) in Unicode is
1171C<LATIN CAPITAL LETTER A WITH MACRON>.
1172
1173There are a couple of exceptions to the above rule. C<\N{U+I<hex number>}> is
1174always interpreted as a Unicode code point, so that C<\N{U+0050}> is "P" even
1175on EBCDIC platforms. And if L<C<S<use encoding>>|encoding> is in effect, the
1176number is considered to be in that encoding, and is translated from that into
1177the platform's native encoding if there is a corresponding native character;
1178otherwise to Unicode.
1179
5691ca5f 1180=back
4c77eaa2 1181
e526e8bb
KW
1182B<NOTE>: Unlike C and other languages, Perl has no C<\v> escape sequence for
1183the vertical tab (VT - ASCII 11), but you may use C<\ck> or C<\x0b>. (C<\v>
1184does have meaning in regular expression patterns in Perl, see L<perlre>.)
1185
1186The following escape sequences are available in constructs that interpolate,
904501ec 1187but not in transliterations.
d74e8afc 1188X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q>
904501ec 1189
a0d0e21e
LW
1190 \l lowercase next char
1191 \u uppercase next char
1192 \L lowercase till \E
1193 \U uppercase till \E
1d2dff63 1194 \Q quote non-word characters till \E
7e31b643 1195 \E end either case modification or quoted section
a0d0e21e 1196
95cc3e0c
JH
1197If C<use locale> is in effect, the case map used by C<\l>, C<\L>,
1198C<\u> and C<\U> is taken from the current locale. See L<perllocale>.
1199If Unicode (for example, C<\N{}> or wide hex characters of 0x100 or
1200beyond) is being used, the case map used by C<\l>, C<\L>, C<\u> and
e526e8bb 1201C<\U> is as defined by Unicode.
a034a98d 1202
5a964f20
TC
1203All systems use the virtual C<"\n"> to represent a line terminator,
1204called a "newline". There is no such thing as an unvarying, physical
19799a22 1205newline character. It is only an illusion that the operating system,
5a964f20
TC
1206device drivers, C libraries, and Perl all conspire to preserve. Not all
1207systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
1208on a Mac, these are reversed, and on systems without line terminator,
1209printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
1210you mean a "newline" for your system, but use the literal ASCII when you
1211need an exact character. For example, most networking protocols expect
2a380090 1212and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
5a964f20
TC
1213and although they often accept just C<"\012">, they seldom tolerate just
1214C<"\015">. If you get in the habit of using C<"\n"> for networking,
1215you may be burned some day.
d74e8afc
ITB
1216X<newline> X<line terminator> X<eol> X<end of line>
1217X<\n> X<\r> X<\r\n>
5a964f20 1218
904501ec
MG
1219For constructs that do interpolate, variables beginning with "C<\$>"
1220or "C<@>" are interpolated. Subscripted variables such as C<\$a[3]> or
A
1221C<< \$href->{key}[0] >> are also interpolated, as are array and hash slices.
1222But method calls such as C<< \$obj->meth >> are not.
af9219ee
MG
1223
1224Interpolating an array or slice interpolates the elements in order,
1225separated by the value of C<\$">, so is equivalent to interpolating
6deea57f
TS
1226C<join \$", @array>. "Punctuation" arrays such as C<@*> are only
1227interpolated if the name is enclosed in braces C<@{*}>, but special
1228arrays C<@_>, C<@+>, and C<@-> are interpolated, even without braces.
af9219ee 1229
bc7b91c6
EB
1230For double-quoted strings, the quoting from C<\Q> is applied after
1231interpolation and escapes are processed.
1232
1233 "abc\Qfoo\tbar\$s\Exyz"
1234
1235is equivalent to
1236
1237 "abc" . quotemeta("foo\tbar\$s") . "xyz"
1238
1239For the pattern of regex operators (C<qr//>, C<m//> and C<s///>),
1240the quoting from C<\Q> is applied after interpolation is processed,
1241but before escapes are processed. This allows the pattern to match
1242literally (except for C<\$> and C<@>). For example, the following matches:
1243
1244 '\s\t' =~ /\Q\s\t/
1245
1246Because C<\$> or C<@> trigger interpolation, you'll need to use something
1247like C</\Quser\E\@\Qhost/> to match them literally.
1d2dff63 1248
a0d0e21e
LW
1249Patterns are subject to an additional level of interpretation as a
1250regular expression. This is done as a second pass, after variables are
1251interpolated, so that regular expressions may be incorporated into the
1252pattern from the variables. If this is not what you want, use C<\Q> to
1253interpolate a variable literally.
1254
19799a22
GS
1255Apart from the behavior described above, Perl does not expand
1256multiple levels of interpolation. In particular, contrary to the
1257expectations of shell programmers, back-quotes do I<NOT> interpolate
1258within double quotes, nor do single quotes impede evaluation of
1259variables when used within double quotes.
a0d0e21e 1260
d74e8afc 1262X<operator, regexp>
cb1a09d0 1263
5f05dabc 1264Here are the quote-like operators that apply to pattern
cb1a09d0
1265matching and related activities.
1266
a0d0e21e
LW
1267=over 8
1268
87e95b7f 1269=item qr/STRING/msixpo
01c6f5f4 1270X<qr> X</i> X</m> X</o> X</s> X</x> X</p>
a0d0e21e 1271
87e95b7f
YO
1272This operator quotes (and possibly compiles) its I<STRING> as a regular
1273expression. I<STRING> is interpolated the same way as I<PATTERN>
1274in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
1275is done. Returns a Perl value which may be used instead of the
64c5a566 1276corresponding C</STRING/msixpo> expression. The returned value is a
85dd5c8b 1277normalized version of the original pattern. It magically differs from
64c5a566 1278a string containing the same characters: C<ref(qr/x/)> returns "Regexp",
85dd5c8b 1279even though dereferencing the result returns undef.
a0d0e21e 1280
87e95b7f
YO
1281For example,
1282
1283 \$rex = qr/my.STRING/is;
85dd5c8b 1284 print \$rex; # prints (?si-xm:my.STRING)
87e95b7f
YO
1285 s/\$rex/foo/;
1286
1287is equivalent to
1288
1289 s/my.STRING/foo/is;
1290
1291The result may be used as a subpattern in a match:
1292
1293 \$re = qr/\$pattern/;
1294 \$string =~ /foo\${re}bar/; # can be interpolated in other patterns
1295 \$string =~ \$re; # or used standalone
1296 \$string =~ /\$re/; # or this way
1297
1298Since Perl may compile the pattern at the moment of execution of qr()
1299operator, using qr() may have speed advantages in some situations,
1300notably if the result of qr() is used standalone:
1301
1302 sub match {
1303 my \$patterns = shift;
1304 my @compiled = map qr/\$_/i, @\$patterns;
1305 grep {
1306 my \$success = 0;
1307 foreach my \$pat (@compiled) {
1308 \$success = 1, last if /\$pat/;
1309 }
1310 \$success;
1311 } @_;
5a964f20
TC
1312 }
1313
87e95b7f
YO
1314Precompilation of the pattern into an internal representation at
1315the moment of qr() avoids a need to recompile the pattern every
1316time a match C</\$pat/> is attempted. (Perl has many other internal
1317optimizations, but none would be triggered in the above example if
1318we did not use qr() operator.)
1319
1320Options are:
1321
1322 m Treat string as multiple lines.
1323 s Treat string as single line. (Make . match a newline)
1324 i Do case-insensitive pattern matching.
1325 x Use extended regular expressions.
1326 p When matching preserve a copy of the matched string so
1327 that \${^PREMATCH}, \${^MATCH}, \${^POSTMATCH} will be defined.
1328 o Compile pattern only once.
1329
1330If a precompiled pattern is embedded in a larger pattern then the effect
1331of 'msixp' will be propagated appropriately. The effect of the 'o'
1332modifier has is not propagated, being restricted to those patterns
1333explicitly using it.
1334
1335See L<perlre> for additional information on valid syntax for STRING, and
1336for a detailed look at the semantics of regular expressions.
a0d0e21e 1337
87e95b7f 1338=item m/PATTERN/msixpogc
89d205f2
YO
1339X<m> X<operator, match>
1340X<regexp, options> X<regexp> X<regex, options> X<regex>
01c6f5f4 1341X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c>
a0d0e21e 1342
87e95b7f 1343=item /PATTERN/msixpogc
a0d0e21e 1344
5a964f20 1345Searches a string for a pattern match, and in scalar context returns
19799a22
GS
1346true if it succeeds, false if it fails. If no string is specified
1347via the C<=~> or C<!~> operator, the \$_ string is searched. (The
1348string specified with C<=~> need not be an lvalue--it may be the
1349result of an expression evaluation, but remember the C<=~> binds
1351discussion of additional considerations that apply when C<use locale>
1352is in effect.
a0d0e21e 1353
01c6f5f4
RGS
1354Options are as described in C<qr//>; in addition, the following match
1355process modifiers are available:
a0d0e21e 1356
950b09ed
KW
1357 g Match globally, i.e., find all occurrences.
1358 c Do not reset search position on a failed match when /g is in effect.
a0d0e21e
LW
1359
1360If "/" is the delimiter then the initial C<m> is optional. With the C<m>
ed02a3bf 1361you can use any pair of non-whitespace characters
19799a22
GS
1362as delimiters. This is particularly useful for matching path names
1363that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
7bac28a0 1364the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
19799a22 1365If "'" is the delimiter, no interpolation is performed on the PATTERN.
ed02a3bf
DN
1366When using a character valid in an identifier, whitespace is required
1367after the C<m>.
a0d0e21e
LW
1368
1369PATTERN may contain variables, which will be interpolated (and the
f70b4f9c 1370pattern recompiled) every time the pattern search is evaluated, except
1f247705
GS
1371for when the delimiter is a single quote. (Note that C<\$(>, C<\$)>, and
1372C<\$|> are not interpolated because they look like end-of-string tests.)
f70b4f9c
AB
1373If you want such a pattern to be compiled only once, add a C</o> after
1374the trailing delimiter. This avoids expensive run-time recompilations,
1375and is useful when the value you are interpolating won't change over
1376the life of the script. However, mentioning C</o> constitutes a promise
1377that you won't change the variables in the pattern. If you change them,
a0d0e21e 1379
e9d89077
DN
1380=item The empty pattern //
1381
5a964f20 1382If the PATTERN evaluates to the empty string, the last
d65afb4b
HS
1383I<successfully> matched regular expression is used instead. In this
1384case, only the C<g> and C<c> flags on the empty pattern is honoured -
1385the other flags are taken from the original pattern. If no match has
1386previously succeeded, this will (silently) act instead as a genuine
1387empty pattern (which will always match).
a0d0e21e 1388
89d205f2
YO
1389Note that it's possible to confuse Perl into thinking C<//> (the empty
1390regex) is really C<//> (the defined-or operator). Perl is usually pretty
1392C<\$a///> (is that C<(\$a) / (//)> or C<\$a // />?) and C<print \$fh //>
1393(C<print \$fh(//> or C<print(\$fh //>?). In all of these examples, Perl
1394will assume you meant defined-or. If you meant the empty regex, just
1395use parentheses or spaces to disambiguate, or even prefix the empty
c963b151
BD
1396regex with an C<m> (so C<//> becomes C<m//>).
1397
e9d89077
DN
1398=item Matching in list context
1399
19799a22 1400If the C</g> option is not used, C<m//> in list context returns a
a0d0e21e 1401list consisting of the subexpressions matched by the parentheses in the
f7e33566
GS
1402pattern, i.e., (C<\$1>, C<\$2>, C<\$3>...). (Note that here C<\$1> etc. are
1403also set, and that this differs from Perl 4's behavior.) When there are
1404no parentheses in the pattern, the return value is the list C<(1)> for
1405success. With or without parentheses, an empty list is returned upon
1406failure.
a0d0e21e
LW
1407
1408Examples:
1409
1410 open(TTY, '/dev/tty');
1411 <TTY> =~ /^y/i && foo(); # do foo if desired
1412
1413 if (/Version: *([0-9.]*)/) { \$version = \$1; }
1414
1415 next if m#^/usr/spool/uucp#;
1416
1417 # poor man's grep
1418 \$arg = shift;
1419 while (<>) {
1420 print if /\$arg/o; # compile only once
1421 }
1422
1423 if ((\$F1, \$F2, \$Etc) = (\$foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
1424
1425This last example splits \$foo into the first two words and the
5f05dabc 1426remainder of the line, and assigns those three fields to \$F1, \$F2, and
1427\$Etc. The conditional is true if any variables were assigned, i.e., if
a0d0e21e
LW
1428the pattern matched.
1429
19799a22 1430The C</g> modifier specifies global pattern matching--that is,
3dd93342 1431matching as many times as possible within the string. How it behaves
1432depends on the context. In list context, it returns a list of the
19799a22 1433substrings matched by any capturing parentheses in the regular
3dd93342 1434expression. If there are no parentheses, it returns a list of all
19799a22
GS
1435the matched strings, as if there were parentheses around the whole
1436pattern.
a0d0e21e 1437
7e86de3e 1438In scalar context, each execution of C<m//g> finds the next match,
19799a22 1439returning true if it matches, and false if there is no further match.
3dd93342 1440The position after the last match can be read or set using the C<pos()>
1441function; see L<perlfunc/pos>. A failed match normally resets the
7e86de3e 1442search position to the beginning of the string, but you can avoid that
3dd93342 1443by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
7e86de3e 1444string also resets the search position.
c90c0ff4 1445
e9d89077
DN
1446=item \G assertion
1447
c90c0ff4 1448You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
3dd93342 1449zero-width assertion that matches the exact position where the
1450previous C<m//g>, if any, left off. Without the C</g> modifier, the
1451C<\G> assertion still anchors at C<pos()> as it was at the start of
1452the operation (see L<perlfunc/pos>), but the match is of course only
1453attempted once. Using C<\G> without C</g> on a target string that has
1454not previously had a C</g> match applied to it is the same as using
1455the C<\A> assertion to match the beginning of the string. Note also
1456that, currently, C<\G> is only properly supported when anchored at the
1457very beginning of the pattern.
c90c0ff4 1458
1459Examples:
a0d0e21e
LW
1460
1461 # list context
1462 (\$one,\$five,\$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
1463
1464 # scalar context
5d43e42d 1465 \$/ = "";
19799a22
GS
1466 while (defined(\$paragraph = <>)) {
1467 while (\$paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
1468 \$sentences++;
a0d0e21e
LW
1469 }
1470 }
1471 print "\$sentences\n";
1472
c90c0ff4 1473 # using m//gc with \G
137443ea 1474 \$_ = "ppooqppqq";
44a8e56a 1475 while (\$i++ < 2) {
1476 print "1: '";
c90c0ff4 1477 print \$1 while /(o)/gc; print "', pos=", pos, "\n";
44a8e56a 1478 print "2: '";
c90c0ff4 1479 print \$1 if /\G(q)/gc; print "', pos=", pos, "\n";
44a8e56a 1480 print "3: '";
c90c0ff4 1481 print \$1 while /(p)/gc; print "', pos=", pos, "\n";
44a8e56a 1482 }
5d43e42d 1483 print "Final: '\$1', pos=",pos,"\n" if /\G(.)/;
44a8e56a 1484
1485The last example should print:
1486
1487 1: 'oo', pos=4
137443ea 1488 2: 'q', pos=5
44a8e56a 1489 3: 'pp', pos=7
1490 1: '', pos=7
137443ea 1491 2: 'q', pos=8
1492 3: '', pos=8
5d43e42d
DC
1493 Final: 'q', pos=8
1494
1495Notice that the final match matched C<q> instead of C<p>, which a match
1496without the C<\G> anchor would have done. Also note that the final match
ac036724 1497did not update C<pos>. C<pos> is only updated on a C</g> match. If the
5d43e42d
DC
1498final match did indeed match C<p>, it's a good bet that you're running an
1499older (pre-5.6.0) Perl.
44a8e56a 1500
c90c0ff4 1501A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
e7ea3e70 1502combine several regexps like this to process a string part-by-part,
c90c0ff4 1503doing different actions depending on which regexp matched. Each
1504regexp tries to match where the previous one leaves off.
e7ea3e70 1505
3fe9a6f1 1506 \$_ = <<'EOL';
950b09ed 1507 \$url = URI::URL->new( "http://example.com/" ); die if \$url eq "xXx";
3fe9a6f1 1508 EOL
1509 LOOP:
e7ea3e70 1510 {
950b09ed
KW
1511 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
1512 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
1513 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
1514 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
1515 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
1516 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
1517 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
1518 print ". That's all!\n";
e7ea3e70
IZ
1519 }
1520
1521Here is the output (split into several lines):
1522
51622cce 1523 line-noise lowercase line-noise UPPERCASE line-noise UPPERCASE
1524 line-noise lowercase line-noise lowercase line-noise lowercase
1525 lowercase line-noise lowercase lowercase line-noise lowercase
1526 lowercase line-noise MiXeD line-noise. That's all!
44a8e56a 1527
87e95b7f
YO
1528=item ?PATTERN?
1529X<?>
1530
1531This is just like the C</pattern/> search, except that it matches only
1532once between calls to the reset() operator. This is a useful
1533optimization when you want to see only the first occurrence of
1534something in each file of a set of files, for instance. Only C<??>
1535patterns local to the current package are reset.
1536
1537 while (<>) {
1538 if (?^\$?) {
1539 # blank line between header and body
1540 }
1541 } continue {
1542 reset if eof; # clear ?? status for next file
1543 }
1544
1545This usage is vaguely deprecated, which means it just might possibly
1546be removed in some distant future version of Perl, perhaps somewhere
1547around the year 2168.
1548
4f4d7508 1549=item s/PATTERN/REPLACEMENT/msixpogcer
87e95b7f 1550X<substitute> X<substitution> X<replace> X<regexp, replace>
4f4d7508 1551X<regexp, substitute> X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> X</e> X</r>
87e95b7f
YO
1552
1553Searches a string for a pattern, and if found, replaces that pattern
1554with the replacement text and returns the number of substitutions
1555made. Otherwise it returns false (specifically, the empty string).
1556
4f4d7508
DC
1557If the C</r> (non-destructive) option is used then it will perform the
1558substitution on a copy of the string and return the copy whether or not a
1559substitution occurred. The original string will always remain unchanged in
c8dbf8cd 1560this case. The copy will always be a plain string, even if the input is an
4f4d7508
DC
1561object or a tied variable.
1562
87e95b7f
YO
1563If no string is specified via the C<=~> or C<!~> operator, the C<\$_>
1564variable is searched and modified. (The string specified with C<=~> must
1565be scalar variable, an array element, a hash element, or an assignment
1566to one of those, i.e., an lvalue.)
1567
1568If the delimiter chosen is a single quote, no interpolation is
1569done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1570PATTERN contains a \$ that looks like a variable rather than an
1571end-of-string test, the variable will be interpolated into the pattern
1572at run-time. If you want the pattern compiled only once the first time
1573the variable is interpolated, use the C</o> option. If the pattern
1574evaluates to the empty string, the last successfully executed regular
1575expression is used instead. See L<perlre> for further explanation on these.
1576See L<perllocale> for discussion of additional considerations that apply
1577when C<use locale> is in effect.
1578
1579Options are as with m// with the addition of the following replacement
1580specific options:
1581
1582 e Evaluate the right side as an expression.
4f4d7508
DC
1583 ee Evaluate the right side as a string then eval the result.
1584 r Return substitution and leave the original string untouched.
87e95b7f 1585
ed02a3bf
DN
1586Any non-whitespace delimiter may replace the slashes. Add space after
1587the C<s> when using a character allowed in identifiers. If single quotes
1588are used, no interpretation is done on the replacement string (the C</e>
1589modifier overrides this, however). Unlike Perl 4, Perl 5 treats backticks
1590as normal delimiters; the replacement text is not evaluated as a command.
1591If the PATTERN is delimited by bracketing quotes, the REPLACEMENT has
1592its own pair of quotes, which may or may not be bracketing quotes, e.g.,
87e95b7f
YO
1593C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
1594replacement portion to be treated as a full-fledged Perl expression
1595and evaluated right then and there. It is, however, syntax checked at
1596compile-time. A second C<e> modifier will cause the replacement portion
1597to be C<eval>ed before being run as a Perl expression.
1598
1599Examples:
1600
1601 s/\bgreen\b/mauve/g; # don't change wintergreen
1602
1603 \$path =~ s|/usr/bin|/usr/local/bin|;
1604
1606
1607 (\$foo = \$bar) =~ s/this/that/; # copy first, then change
4f4d7508
DC
1608 (\$foo = "\$bar") =~ s/this/that/; # convert to string, copy, then change
1609 \$foo = \$bar =~ s/this/that/r; # Same as above using /r
1610 \$foo = \$bar =~ s/this/that/r
1611 =~ s/that/the other/r; # Chained substitutes using /r
1612 @foo = map { s/this/that/r } @bar # /r is very useful in maps
87e95b7f
YO
1613
1614 \$count = (\$paragraph =~ s/Mister\b/Mr./g); # get change-count
1615
1616 \$_ = 'abc123xyz';
1617 s/\d+/\$&*2/e; # yields 'abc246xyz'
1618 s/\d+/sprintf("%5d",\$&)/e; # yields 'abc 246xyz'
1619 s/\w/\$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1620
1621 s/%(.)/\$percent{\$1}/g; # change percent escapes; no /e
1622 s/%(.)/\$percent{\$1} || \$&/ge; # expr now, so /e
1623 s/^=(\w+)/pod(\$1)/ge; # use function call
1624
4f4d7508
DC
1625 \$_ = 'abc123xyz';
1626 \$a = s/abc/def/r; # \$a is 'def123xyz' and
1627 # \$_ remains 'abc123xyz'.
1628
87e95b7f
YO
1629 # expand variables in \$_, but dynamics only, using
1630 # symbolic dereferencing
1631 s/\\$(\w+)/\${\$1}/g;
1632
1633 # Add one to the value of any numbers in the string
1634 s/(\d+)/1 + \$1/eg;
1635
1636 # This will expand any embedded scalar variable
1637 # (including lexicals) in \$_ : First \$1 is interpolated
1638 # to the variable name, and then evaluated
1639 s/(\\$\w+)/\$1/eeg;
1640
1641 # Delete (most) C comments.
1642 \$program =~ s {
1643 /\* # Match the opening delimiter.
1644 .*? # Match a minimal number of characters.
1645 \*/ # Match the closing delimiter.
1646 } []gsx;
1647
1648 s/^\s*(.*?)\s*\$/\$1/; # trim whitespace in \$_, expensively
1649
1650 for (\$variable) { # trim whitespace in \$variable, cheap
1651 s/^\s+//;
1652 s/\s+\$//;
1653 }
1654
1655 s/([^ ]*) *([^ ]*)/\$2 \$1/; # reverse 1st two fields
1656
1657Note the use of \$ instead of \ in the last example. Unlike
1658B<sed>, we use the \<I<digit>> form in only the left hand side.
1659Anywhere else it's \$<I<digit>>.
1660
1661Occasionally, you can't use just a C</g> to get all the changes
1662to occur that you might want. Here are two common cases:
1663
1664 # put commas in the right places in an integer
1665 1 while s/(\d)(\d\d\d)(?!\d)/\$1,\$2/g;
1666
1667 # expand tabs to 8-column spacing
1668 1 while s/\t+/' ' x (length(\$&)*8 - length(\$`)%8)/e;
1669
1670=back
1671
1673X<operator, quote-like>
1674
01c6f5f4
RGS
1675=over 4
1676
a0d0e21e 1677=item q/STRING/
5d44bfff 1678X<q> X<quote, single> X<'> X<''>
a0d0e21e 1679
5d44bfff 1680=item 'STRING'
a0d0e21e 1681
19799a22 1682A single-quoted, literal string. A backslash represents a backslash
68dc0745 1683unless followed by the delimiter or another backslash, in which case
1684the delimiter or backslash is interpolated.
a0d0e21e
LW
1685
1686 \$foo = q!I said, "You said, 'She said it.'"!;
1687 \$bar = q('This is it.');
68dc0745 1688 \$baz = '\n'; # a two-character string
a0d0e21e
LW
1689
1690=item qq/STRING/
d74e8afc 1691X<qq> X<quote, double> X<"> X<"">
a0d0e21e
LW
1692
1693=item "STRING"
1694
1695A double-quoted, interpolated string.
1696
1697 \$_ .= qq
1698 (*** The previous line contains the naughty word "\$1".\n)
19799a22 1699 if /\b(tcl|java|python)\b/i; # :-)
68dc0745 1700 \$baz = "\n"; # a one-character string
a0d0e21e
LW
1701
1702=item qx/STRING/
d74e8afc 1703X<qx> X<`> X<``> X<backtick>
a0d0e21e
LW
1704
1705=item `STRING`
1706
43dd4d21
JH
1707A string which is (possibly) interpolated and then executed as a
1708system command with C</bin/sh> or its equivalent. Shell wildcards,
1709pipes, and redirections will be honored. The collected standard
1710output of the command is returned; standard error is unaffected. In
1711scalar context, it comes back as a single (potentially multi-line)
1712string, or undef if the command failed. In list context, returns a
1713list of lines (however you've defined lines with \$/ or
1714\$INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
5a964f20
TC
1715
1716Because backticks do not affect standard error, use shell file descriptor
1717syntax (assuming the shell supports this) if you care to address this.
1718To capture a command's STDERR and STDOUT together:
a0d0e21e 1719
5a964f20
TC
1720 \$output = `cmd 2>&1`;
1721
1722To capture a command's STDOUT but discard its STDERR:
1723
1724 \$output = `cmd 2>/dev/null`;
1725
1726To capture a command's STDERR but discard its STDOUT (ordering is
1727important here):
1728
1729 \$output = `cmd 2>&1 1>/dev/null`;
1730
1731To exchange a command's STDOUT and STDERR in order to capture the STDERR
1732but leave its STDOUT to come out the old STDERR:
1733
1734 \$output = `cmd 3>&1 1>&2 2>&3 3>&-`;
1735
1736To read both a command's STDOUT and its STDERR separately, it's easiest
2359510d
SD
1737to redirect them separately to files, and then read from those files
1738when the program is done:
5a964f20 1739
2359510d 1740 system("program args 1>program.stdout 2>program.stderr");
5a964f20 1741
30398227
SP
1742The STDIN filehandle used by the command is inherited from Perl's STDIN.
1743For example:
1744
1745 open BLAM, "blam" || die "Can't open: \$!";
1746 open STDIN, "<&BLAM";
1747 print `sort`;
1748
1749will print the sorted contents of the file "blam".
1750
5a964f20
TC
1751Using single-quote as a delimiter protects the command from Perl's
1752double-quote interpolation, passing it on to the shell instead:
1753
1754 \$perl_info = qx(ps \$\$); # that's Perl's \$\$
1755 \$shell_info = qx'ps \$\$'; # that's the new shell's \$\$
1756
19799a22 1757How that string gets evaluated is entirely subject to the command
5a964f20
TC
1758interpreter on your system. On most platforms, you will have to protect
1759shell metacharacters if you want them treated literally. This is in
1760practice difficult to do, as it's unclear how to escape which characters.
1761See L<perlsec> for a clean and safe example of a manual fork() and exec()
1762to emulate backticks safely.
a0d0e21e 1763
bb32b41a
GS
1764On some platforms (notably DOS-like ones), the shell may not be
1765capable of dealing with multiline commands, so putting newlines in
1766the string may not get you what you want. You may be able to evaluate
1767multiple commands in a single line by separating them with the command
1768separator character, if your shell supports that (e.g. C<;> on many Unix
1769shells; C<&> on the Windows NT C<cmd> shell).
1770
0f897271
GS
1771Beginning with v5.6.0, Perl will attempt to flush all files opened for
1772output before starting the child process, but this may not be supported
1773on some platforms (see L<perlport>). To be safe, you may need to set
1774C<\$|> (\$AUTOFLUSH in English) or call the C<autoflush()> method of
1775C<IO::Handle> on any open handles.
1776
bb32b41a
GS
1777Beware that some command shells may place restrictions on the length
1778of the command line. You must ensure your strings don't exceed this
1779limit after any necessary interpolations. See the platform-specific
1781
5a964f20
TC
1782Using this operator can lead to programs that are difficult to port,
1783because the shell commands called vary between systems, and may in
1784fact not be present at all. As one example, the C<type> command under
1785the POSIX shell is very different from the C<type> command under DOS.
1786That doesn't mean you should go out of your way to avoid backticks
1787when they're the right way to get something done. Perl was made to be
1788a glue language, and one of the things it glues together is commands.
1789Just understand what you're getting yourself into.
bb32b41a 1790
da87341d 1791See L</"I/O Operators"> for more discussion.
a0d0e21e 1792
945c54fd 1793=item qw/STRING/
d74e8afc 1794X<qw> X<quote, list> X<quote, words>
945c54fd
JH
1795
1796Evaluates to a list of the words extracted out of STRING, using embedded
1797whitespace as the word delimiters. It can be understood as being roughly
1798equivalent to:
1799
1800 split(' ', q/STRING/);
1801
efb1e162
CW
1802the differences being that it generates a real list at compile time, and
1803in scalar context it returns the last element in the list. So
945c54fd
JH
1804this expression:
1805
1806 qw(foo bar baz)
1807
1808is semantically equivalent to the list:
1809
1810 'foo', 'bar', 'baz'
1811
1812Some frequently seen examples:
1813
1814 use POSIX qw( setlocale localeconv )
1815 @EXPORT = qw( foo bar baz );
1816
1817A common mistake is to try to separate the words with comma or to
1818put comments into a multi-line C<qw>-string. For this reason, the
89d205f2 1819C<use warnings> pragma and the B<-w> switch (that is, the C<\$^W> variable)
945c54fd
JH
1820produces warnings if the STRING contains the "," or the "#" character.
1821
a0d0e21e 1822
8ff32507 1823=item tr/SEARCHLIST/REPLACEMENTLIST/cdsr
d74e8afc 1824X<tr> X<y> X<transliterate> X</c> X</d> X</s>
a0d0e21e 1825
8ff32507 1826=item y/SEARCHLIST/REPLACEMENTLIST/cdsr
a0d0e21e 1827
2c268ad5 1828Transliterates all occurrences of the characters found in the search list
a0d0e21e
LW
1829with the corresponding character in the replacement list. It returns
1830the number of characters replaced or deleted. If no string is
2c268ad5 1831specified via the =~ or !~ operator, the \$_ string is transliterated. (The
54310121 1832string specified with =~ must be a scalar variable, an array element, a
1833hash element, or an assignment to one of those, i.e., an lvalue.)
8ff32507
FC
1835If the C</r> (non-destructive) option is used then it will perform the
1836replacement on a copy of the string and return the copy whether or not it
1837was modified. The original string will always remain unchanged in
1838this case. The copy will always be a plain string, even if the input is an
1839object or a tied variable.
1840
89d205f2 1841A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
2c268ad5 1842does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
54310121 1843For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1844SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1845its own pair of quotes, which may or may not be bracketing quotes,
a0d0e21e 1847
cc255d5f 1848Note that C<tr> does B<not> do regular expression character classes
e0c83546 1849such as C<\d> or C<[:lower:]>. The C<tr> operator is not equivalent to
cc255d5f
JH
1850the tr(1) utility. If you want to map strings between lower/upper
1851cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider
1852using the C<s> operator if you need regular expressions.
1853
JH
1854Note also that the whole range idea is rather unportable between
1855character sets--and even within character sets they may cause results
1856you probably didn't expect. A sound principle is to use only ranges
1857that begin from and end at either alphabets of equal case (a-e, A-E),
1858or digits (0-4). Anything else is unsafe. If in doubt, spell out the
1859character sets in full.
1860
a0d0e21e
LW
1861Options:
1862
1863 c Complement the SEARCHLIST.
1864 d Delete found but unreplaced characters.
1865 s Squash duplicate replaced characters.
8ff32507
FC
1866 r Return the modified string and leave the original string
1867 untouched.
a0d0e21e 1868
19799a22
GS
1869If the C</c> modifier is specified, the SEARCHLIST character set
1870is complemented. If the C</d> modifier is specified, any characters
1872(Note that this is slightly more flexible than the behavior of some
1873B<tr> programs, which delete anything they find in the SEARCHLIST,
1874period.) If the C</s> modifier is specified, sequences of characters
1875that were transliterated to the same character are squashed down
1876to a single instance of the character.
a0d0e21e
LW
1877
1878If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1879exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1880than the SEARCHLIST, the final character is replicated till it is long
5a964f20 1881enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
a0d0e21e
LW
1882This latter is useful for counting characters in a class or for
1883squashing character sequences in a class.
1884
1885Examples:
1886
1887 \$ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1888
1889 \$cnt = tr/*/*/; # count the stars in \$_
1890
1891 \$cnt = \$sky =~ tr/*/*/; # count the stars in \$sky
1892
1893 \$cnt = tr/0-9//; # count the digits in \$_
1894
1895 tr/a-zA-Z//s; # bookkeeper -> bokeper
1896
1897 (\$HOST = \$host) =~ tr/a-z/A-Z/;
8ff32507
FC
1898 \$HOST = \$host =~ tr/a-z/A-Z/r; # same thing
1899
1900 \$HOST = \$host =~ tr/a-z/A-Z/r # chained with s///
1901 =~ s/:/ -p/r;
a0d0e21e
LW
1902
1903 tr/a-zA-Z/ /cs; # change non-alphas to single space
1904
8ff32507
FC
1905 @stripped = map tr/a-zA-Z/ /csr, @original;
1906 # /r with map
1907
a0d0e21e
LW
1908 tr [\200-\377]
1909 [\000-\177]; # delete 8th bit
1910
19799a22
GS
1911If multiple transliterations are given for a character, only the
1912first one is used:
748a9306
LW
1913
1914 tr/AAA/XYZ/
1915
2c268ad5 1916will transliterate any A to X.
748a9306 1917
19799a22 1918Because the transliteration table is built at compile time, neither
a0d0e21e 1919the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
19799a22
GS
1920interpolation. That means that if you want to use variables, you
1921must use an eval():
a0d0e21e
LW
1922
1923 eval "tr/\$oldlist/\$newlist/";
1924 die \$@ if \$@;
1925
1926 eval "tr/\$oldlist/\$newlist/, 1" or die \$@;
1927
7e3b091d 1928=item <<EOF
d74e8afc 1929X<here-doc> X<heredoc> X<here-document> X<<< << >>>
7e3b091d
DA
1930
1931A line-oriented form of quoting is based on the shell "here-document"
1932syntax. Following a C<< << >> you specify a string to terminate
1933the quoted material, and all lines following the current line down to
89d205f2
YO
1934the terminating string are the value of the item.
1935
1936The terminating string may be either an identifier (a word), or some
1937quoted text. An unquoted identifier works like double quotes.
1938There may not be a space between the C<< << >> and the identifier,
1939unless the identifier is explicitly quoted. (If you put a space it
1940will be treated as a null identifier, which is valid, and matches the
1941first empty line.) The terminating string must appear by itself
1942(unquoted and with no surrounding whitespace) on the terminating line.
1943
1944If the terminating string is quoted, the type of quotes used determine
1945the treatment of the text.
1946
1947=over 4
1948
1949=item Double Quotes
1950
1951Double quotes indicate that the text will be interpolated using exactly
1952the same rules as normal double quoted strings.
7e3b091d
DA
1953
1954 print <<EOF;
1955 The price is \$Price.
1956 EOF
1957
1958 print << "EOF"; # same as above
1959 The price is \$Price.
1960 EOF
1961
89d205f2
YO
1962
1963=item Single Quotes
1964
1965Single quotes indicate the text is to be treated literally with no
1966interpolation of its content. This is similar to single quoted
1967strings except that backslashes have no special meaning, with C<\\>
1968being treated as two backslashes and not one as they would in every
1969other quoting construct.
1970
1971This is the only form of quoting in perl where there is no need
1972to worry about escaping content, something that code generators
1973can and do make good use of.
1974
1975=item Backticks
1976
1977The content of the here doc is treated just as it would be if the
1978string were embedded in backticks. Thus the content is interpolated
1979as though it were double quoted and then executed via the shell, with
1980the results of the execution returned.
1981
1982 print << `EOC`; # execute command and get results
7e3b091d 1983 echo hi there
7e3b091d
DA
1984 EOC
1985
89d205f2
YO
1986=back
1987
1988It is possible to stack multiple here-docs in a row:
1989
7e3b091d
DA
1990 print <<"foo", <<"bar"; # you can stack them
1991 I said foo.
1992 foo
1993 I said bar.
1994 bar
1995
1996 myfunc(<< "THIS", 23, <<'THAT');
1997 Here's a line
1998 or two.
1999 THIS
2000 and here's another.
2001 THAT
2002
2003Just don't forget that you have to put a semicolon on the end
2004to finish the statement, as Perl doesn't know you're not going to
2005try to do this:
2006
2007 print <<ABC
2008 179231
2009 ABC
2010 + 20;
2011
872d7e53
TS
2012If you want to remove the line terminator from your here-docs,
2013use C<chomp()>.
2014
2015 chomp(\$string = <<'END');
2016 This is a string.
2017 END
2018
2019If you want your here-docs to be indented with the rest of the code,
2020you'll need to remove leading whitespace from each line manually:
7e3b091d
DA
2021
2022 (\$quote = <<'FINIS') =~ s/^\s+//gm;
89d205f2 2023 The Road goes ever on and on,
7e3b091d
DA
2024 down from the door where it began.
2025 FINIS
2026
2027If you use a here-doc within a delimited construct, such as in C<s///eg>,
2028the quoted material must come on the lines following the final delimiter.
2030
2031 s/this/<<E . 'that'
2032 the other
2033 E
2034 . 'more '/eg;
2035
2036you have to write
2037
89d205f2
YO
2038 s/this/<<E . 'that'
2039 . 'more '/eg;
2040 the other
2041 E
7e3b091d
DA
2042
2043If the terminating identifier is on the last line of the program, you
2044must be sure there is a newline after it; otherwise, Perl will give the
2045warning B<Can't find string terminator "END" anywhere before EOF...>.
2046
89d205f2 2047Additionally, the quoting rules for the end of string identifier are not
ac036724 2048related to Perl's quoting rules. C<q()>, C<qq()>, and the like are not
89d205f2
YO
2049supported in place of C<''> and C<"">, and the only interpolation is for
2050backslashing the quoting character:
7e3b091d
DA
2051
2052 print << "abc\"def";
2053 testing...
2054 abc"def
2055
2056Finally, quoted strings cannot span multiple lines. The general rule is
2057that the identifier must be a string literal. Stick with that, and you
2058should be safe.
2059
a0d0e21e
LW
2060=back
2061
75e14d17 2062=head2 Gory details of parsing quoted constructs
d74e8afc 2063X<quote, gory details>
75e14d17 2064
19799a22
GS
2065When presented with something that might have several different
2066interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
2067principle to pick the most probable interpretation. This strategy
2068is so successful that Perl programmers often do not suspect the
2069ambivalence of what they write. But from time to time, Perl's
2070notions differ substantially from what the author honestly meant.
2071
2072This section hopes to clarify how Perl handles quoted constructs.
2073Although the most common reason to learn this is to unravel labyrinthine
2074regular expressions, because the initial steps of parsing are the
2075same for all quoting operators, they are all discussed together.
2076
2077The most important Perl parsing rule is the first one discussed
2078below: when processing a quoted construct, Perl first finds the end
2079of that construct, then interprets its contents. If you understand
2080this rule, you may skip the rest of this section on the first
2082expectations much less frequently than this first one.
2083
2084Some passes discussed below are performed concurrently, but because
2085their results are the same, we consider them individually. For different
2086quoting constructs, Perl performs different numbers of passes, from
6deea57f 2087one to four, but these passes are always performed in the same order.
75e14d17 2088
13a2d996 2089=over 4
75e14d17
IZ
2090
2091=item Finding the end
2092
6deea57f
TS
2093The first pass is finding the end of the quoted construct, where
2094the information about the delimiters is used in parsing.
2095During this search, text between the starting and ending delimiters
2096is copied to a safe location. The text copied gets delimiter-independent.
2097
2098If the construct is a here-doc, the ending delimiter is a line
2099that has a terminating string as the content. Therefore C<<<EOF> is
2100terminated by C<EOF> immediately followed by C<"\n"> and starting
2101from the first column of the terminating line.
2102When searching for the terminating line of a here-doc, nothing
2103is skipped. In other words, lines after the here-doc syntax
2104are compared with the terminating string line by line.
2105
2106For the constructs except here-docs, single characters are used as starting
2107and ending delimiters. If the starting delimiter is an opening punctuation
2108(that is C<(>, C<[>, C<{>, or C<< < >>), the ending delimiter is the
2109corresponding closing punctuation (that is C<)>, C<]>, C<}>, or C<< > >>).
2110If the starting delimiter is an unpaired character like C</> or a closing
2111punctuation, the ending delimiter is same as the starting delimiter.
2112Therefore a C</> terminates a C<qq//> construct, while a C<]> terminates
2113C<qq[]> and C<qq]]> constructs.
2114
2115When searching for single-character delimiters, escaped delimiters
2116and C<\\> are skipped. For example, while searching for terminating C</>,
2117combinations of C<\\> and C<\/> are skipped. If the delimiters are
2118bracketing, nested pairs are also skipped. For example, while searching
2119for closing C<]> paired with the opening C<[>, combinations of C<\\>, C<\]>,
2120and C<\[> are all skipped, and nested C<[> and C<]> are skipped as well.
2121However, when backslashes are used as the delimiters (like C<qq\\> and
2122C<tr\\\>), nothing is skipped.
2123During the search for the end, backslashes that escape delimiters
2124are removed (exactly speaking, they are not copied to the safe location).
75e14d17 2125
19799a22
GS
2126For constructs with three-part delimiters (C<s///>, C<y///>, and
2127C<tr///>), the search is repeated once more.
6deea57f
TS
2128If the first delimiter is not an opening punctuation, three delimiters must
2129be same such as C<s!!!> and C<tr)))>, in which case the second delimiter
2130terminates the left part and starts the right part at once.
2131If the left part is delimited by bracketing punctuations (that is C<()>,
2132C<[]>, C<{}>, or C<< <> >>), the right part needs another pair of
2133delimiters such as C<s(){}> and C<tr[]//>. In these cases, whitespaces
2134and comments are allowed between both parts, though the comment must follow
2135at least one whitespace; otherwise a character expected as the start of
2136the comment may be regarded as the starting delimiter of the right part.
75e14d17 2137
19799a22
GS
2138During this search no attention is paid to the semantics of the construct.
2139Thus:
75e14d17
IZ
2140
2141 "\$hash{"\$foo/\$bar"}"
2142
2a94b7ce 2143or:
75e14d17 2144
89d205f2 2145 m/
2a94b7ce 2146 bar # NOT a comment, this slash / terminated m//!
75e14d17
IZ
2147 /x
2148
19799a22
GS
2149do not form legal quoted expressions. The quoted part ends on the
2150first C<"> and C</>, and the rest happens to be a syntax error.
2151Because the slash that terminated C<m//> was followed by a C<SPACE>,
2152the example above is not C<m//x>, but rather C<m//> with no C</x>
2153modifier. So the embedded C<#> is interpreted as a literal C<#>.
75e14d17 2154
89d205f2
YO
2155Also no attention is paid to C<\c\> (multichar control char syntax) during
2156this search. Thus the second C<\> in C<qq/\c\/> is interpreted as a part
2157of C<\/>, and the following C</> is not recognized as a delimiter.
0d594e51
TS
2158Instead, use C<\034> or C<\x1c> at the end of quoted constructs.
2159
75e14d17 2160=item Interpolation
d74e8afc 2161X<interpolation>
75e14d17 2162
19799a22 2163The next step is interpolation in the text obtained, which is now
89d205f2 2164delimiter-independent. There are multiple cases.
75e14d17 2165
13a2d996 2166=over 4
75e14d17 2167
89d205f2 2168=item C<<<'EOF'>
75e14d17
IZ
2169
2170No interpolation is performed.
6deea57f
TS
2171Note that the combination C<\\> is left intact, since escaped delimiters
2172are not available for here-docs.
75e14d17 2173
6deea57f 2174=item C<m''>, the pattern of C<s'''>
89d205f2 2175
6deea57f
TS
2176No interpolation is performed at this stage.
2177Any backslashed sequences including C<\\> are treated at the stage
2178to L</"parsing regular expressions">.
89d205f2 2179
6deea57f 2180=item C<''>, C<q//>, C<tr'''>, C<y'''>, the replacement of C<s'''>
75e14d17 2181
89d205f2 2182The only interpolation is removal of C<\> from pairs of C<\\>.
6deea57f
TS
2183Therefore C<-> in C<tr'''> and C<y'''> is treated literally
2184as a hyphen and no character range is available.
2185C<\1> in the replacement of C<s'''> does not work as C<\$1>.
89d205f2
YO
2186
2187=item C<tr///>, C<y///>
2188
6deea57f
TS
2189No variable interpolation occurs. String modifying combinations for
2190case and quoting such as C<\Q>, C<\U>, and C<\E> are not recognized.
2191The other escape sequences such as C<\200> and C<\t> and backslashed
2192characters such as C<\\> and C<\-> are converted to appropriate literals.
89d205f2
YO
2193The character C<-> is treated specially and therefore C<\-> is treated
2194as a literal C<->.
75e14d17 2195
89d205f2 2196=item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF">
75e14d17 2197
19799a22
GS
2198C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
2199converted to corresponding Perl constructs. Thus, C<"\$foo\Qbaz\$bar">
2200is converted to C<\$foo . (quotemeta("baz" . \$bar))> internally.
6deea57f
TS
2201The other escape sequences such as C<\200> and C<\t> and backslashed
2202characters such as C<\\> and C<\-> are replaced with appropriate
2203expansions.
2a94b7ce 2204
19799a22
GS
2205Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
2206is interpolated in the usual way. Something like C<"\Q\\E"> has
2207no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
2208result is the same as for C<"\\\\E">. As a general rule, backslashes
2209between C<\Q> and C<\E> may lead to counterintuitive results. So,
2210C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
2211as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
2a94b7ce
IZ
2212
2213 \$str = '\t';
2214 return "\Q\$str";
2215
2216may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
2217
19799a22 2218Interpolated scalars and arrays are converted internally to the C<join> and
92d29cee 2219C<.> catenation operations. Thus, C<"\$foo XXX '@arr'"> becomes:
75e14d17 2220
19799a22 2221 \$foo . " XXX '" . (join \$", @arr) . "'";
75e14d17 2222
19799a22 2223All operations above are performed simultaneously, left to right.
75e14d17 2224
19799a22
GS
2225Because the result of C<"\Q STRING \E"> has all metacharacters
2226quoted, there is no way to insert a literal C<\$> or C<@> inside a
2227C<\Q\E> pair. If protected by C<\>, C<\$> will be quoted to became
2228C<"\\\\$">; if not, it is interpreted as the start of an interpolated
2229scalar.
75e14d17 2230
19799a22 2231Note also that the interpolation code needs to make a decision on
89d205f2 2232where the interpolated scalar ends. For instance, whether
35f2feb0 2233C<< "a \$b -> {c}" >> really means:
75e14d17
IZ
2234
2235 "a " . \$b . " -> {c}";
2236
2a94b7ce 2237or:
75e14d17
IZ
2238
2239 "a " . \$b -> {c};
2240
19799a22
GS
2241Most of the time, the longest possible text that does not include
2242spaces between components and which contains matching braces or
2243brackets. because the outcome may be determined by voting based
2244on heuristic estimators, the result is not strictly predictable.
2245Fortunately, it's usually correct for ambiguous cases.
75e14d17 2246
6deea57f 2247=item the replacement of C<s///>
75e14d17 2248
19799a22 2249Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
6deea57f
TS
2250happens as with C<qq//> constructs.
2251
2252It is at this step that C<\1> is begrudgingly converted to C<\$1> in
2253the replacement text of C<s///>, in order to correct the incorrigible
2254I<sed> hackers who haven't picked up the saner idiom yet. A warning
2255is emitted if the C<use warnings> pragma or the B<-w> command-line flag
2256(that is, the C<\$^W> variable) was set.
2257
2258=item C<RE> in C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
2259
cc74c5bd
TS
2260Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\E>,
2261and interpolation happens (almost) as with C<qq//> constructs.
2262
5d03b57c
KW
2263Processing of C<\N{...}> is also done here, and compiled into an intermediate
2264form for the regex compiler. (This is because, as mentioned below, the regex
2265compilation may be done at execution time, and C<\N{...}> is a compile-time
2266construct.)
2267
cc74c5bd
TS
2268However any other combinations of C<\> followed by a character
2269are not substituted but only skipped, in order to parse them
2270as regular expressions at the following step.
6deea57f 2271As C<\c> is skipped at this step, C<@> of C<\c@> in RE is possibly
1749ea0d 2272treated as an array symbol (for example C<@foo>),
6deea57f 2273even though the same text in C<qq//> gives interpolation of C<\c@>.
6deea57f
TS
2274
2275Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
19799a22
GS
2276a C<#>-comment in a C<//x>-regular expression, no processing is
2277performed whatsoever. This is the first step at which the presence
2278of the C<//x> modifier is relevant.
2279
1749ea0d
TS
2280Interpolation in patterns has several quirks: C<\$|>, C<\$(>, C<\$)>, C<@+>
2281and C<@-> are not interpolated, and constructs C<\$var[SOMETHING]> are
2282voted (by several different estimators) to be either an array element
2283or C<\$var> followed by an RE alternative. This is where the notation
19799a22
GS
2284C<\${arr[\$bar]}> comes handy: C</\${arr[0-9]}/> is interpreted as
2285array element C<-9>, not as a regular expression from the variable
2286C<\$arr> followed by a digit, which would be the interpretation of
2287C</\$arr[0-9]/>. Since voting among different estimators may occur,
2288the result is not predictable.
2289
19799a22
GS
2290The lack of processing of C<\\> creates specific restrictions on
2291the post-processed text. If the delimiter is C</>, one cannot get
2292the combination C<\/> into the result of this step. C</> will
2293finish the regular expression, C<\/> will be stripped to C</> on
2294the previous step, and C<\\/> will be left as is. Because C</> is
2295equivalent to C<\/> inside a regular expression, this does not
2296matter unless the delimiter happens to be character special to the
2297RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
2298alphanumeric char, as in:
2a94b7ce
IZ
2299
2300 m m ^ a \s* b mmx;
2301
19799a22 2302In the RE above, which is intentionally obfuscated for illustration, the
6deea57f 2303delimiter is C<m>, the modifier is C<mx>, and after delimiter-removal the
89d205f2 2304RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
19799a22
GS
2305reason you're encouraged to restrict your delimiters to non-alphanumeric,
2306non-whitespace choices.
75e14d17
IZ
2307
2308=back
2309
19799a22 2310This step is the last one for all constructs except regular expressions,
75e14d17
IZ
2311which are processed further.
2312
6deea57f
TS
2313=item parsing regular expressions
2314X<regexp, parse>
75e14d17 2315
19799a22 2316Previous steps were performed during the compilation of Perl code,
ac036724 2317but this one happens at run time, although it may be optimized to
19799a22 2318be calculated at compile time if appropriate. After preprocessing
6deea57f 2319described above, and possibly after evaluation if concatenation,
19799a22
GS
2320joining, casing translation, or metaquoting are involved, the
2321resulting I<string> is passed to the RE engine for compilation.
2322
2323Whatever happens in the RE engine might be better discussed in L<perlre>,
2324but for the sake of continuity, we shall do so here.
2325
2326This is another step where the presence of the C<//x> modifier is
2327relevant. The RE engine scans the string from left to right and
2328converts it to a finite automaton.
2329
2330Backslashed characters are either replaced with corresponding
2331literal strings (as with C<\{>), or else they generate special nodes
2332in the finite automaton (as with C<\b>). Characters special to the
2333RE engine (such as C<|>) generate corresponding nodes or groups of
2334nodes. C<(?#...)> comments are ignored. All the rest is either
2335converted to literal strings to match, or else is ignored (as is
2336whitespace and C<#>-style comments if C<//x> is present).
2337
2338Parsing of the bracketed character class construct, C<[...]>, is
2339rather different than the rule used for the rest of the pattern.
2340The terminator of this construct is found using the same rules as
2341for finding the terminator of a C<{}>-delimited construct, the only
2342exception being that C<]> immediately following C<[> is treated as
2343though preceded by a backslash. Similarly, the terminator of
2344C<(?{...})> is found using the same rules as for finding the
2345terminator of a C<{}>-delimited construct.
2346
2347It is possible to inspect both the string given to RE engine and the
2348resulting finite automaton. See the arguments C<debug>/C<debugcolor>
2349in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
4a4eefd0 2350switch documented in L<perlrun/"Command Switches">.
75e14d17
IZ
2351
2352=item Optimization of regular expressions
d74e8afc 2353X<regexp, optimization>
75e14d17 2354
7522fed5 2355This step is listed for completeness only. Since it does not change
75e14d17 2356semantics, details of this step are not documented and are subject
19799a22
GS
2357to change without notice. This step is performed over the finite
2358automaton that was generated during the previous pass.
2a94b7ce 2359
19799a22
GS
2360It is at this stage that C<split()> silently optimizes C</^/> to
2361mean C</^/m>.
75e14d17
IZ
2362
2363=back
2364
d74e8afc
ITB
2366X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle>
2367X<< <> >> X<@ARGV>
a0d0e21e 2368
54310121 2369There are several I/O operators you should know about.
7b8d334a 2371A string enclosed by backticks (grave accents) first undergoes
19799a22
GS
2372double-quote interpolation. It is then interpreted as an external
2373command, and the output of that command is the value of the
e9c56f9b
JH
2374backtick string, like in a shell. In scalar context, a single string
2375consisting of all output is returned. In list context, a list of
2376values is returned, one per line of output. (You can set C<\$/> to use
2377a different line terminator.) The command is executed each time the
2378pseudo-literal is evaluated. The status value of the command is
2379returned in C<\$?> (see L<perlvar> for the interpretation of C<\$?>).
2380Unlike in B<csh>, no translation is done on the return data--newlines
2381remain newlines. Unlike in any of the shells, single quotes do not
2382hide variable names in the command from interpretation. To pass a
2383literal dollar-sign through to the shell you need to hide it with a
2384backslash. The generalized form of backticks is C<qx//>. (Because
2385backticks always undergo shell expansion as well, see L<perlsec> for
2386security concerns.)
d74e8afc 2387X<qx> X<`> X<``> X<backtick> X<glob>
19799a22
GS
2388
2389In scalar context, evaluating a filehandle in angle brackets yields
2390the next line from that file (the newline, if any, included), or
2391C<undef> at end-of-file or on error. When C<\$/> is set to C<undef>
2392(sometimes known as file-slurp mode) and the file is empty, it
2393returns C<''> the first time, followed by C<undef> subsequently.
2394
2395Ordinarily you must assign the returned value to a variable, but
2396there is one situation where an automatic assignment happens. If
2397and only if the input symbol is the only thing inside the conditional
2398of a C<while> statement (even if disguised as a C<for(;;)> loop),
2399the value is automatically assigned to the global variable \$_,
2400destroying whatever was there previously. (This may seem like an
2401odd thing to you, but you'll use the construct in almost every Perl
17b829fa 2402script you write.) The \$_ variable is not implicitly localized.
19799a22
GS
2403You'll have to put a C<local \$_;> before the loop if you want that
2404to happen.
2405
2406The following lines are equivalent:
a0d0e21e 2407
748a9306 2408 while (defined(\$_ = <STDIN>)) { print; }
7b8d334a 2409 while (\$_ = <STDIN>) { print; }
a0d0e21e
LW
2410 while (<STDIN>) { print; }
2411 for (;<STDIN>;) { print; }
748a9306 2412 print while defined(\$_ = <STDIN>);
7b8d334a 2413 print while (\$_ = <STDIN>);
a0d0e21e
LW
2414 print while <STDIN>;
2415
19799a22 2416This also behaves similarly, but avoids \$_ :
7b8d334a 2417
89d205f2 2418 while (my \$line = <STDIN>) { print \$line }
7b8d334a 2419
19799a22
GS
2420In these loop constructs, the assigned value (whether assignment
2421is automatic or explicit) is then tested to see whether it is
2422defined. The defined test avoids problems where line has a string
2423value that would be treated as false by Perl, for example a "" or
2424a "0" with no trailing newline. If you really mean for such values
2425to terminate the loop, they should be tested for explicitly:
7b8d334a
GS
2426
2427 while ((\$_ = <STDIN>) ne '0') { ... }
2428 while (<STDIN>) { last unless \$_; ... }
2429
5ef4d93e 2430In other boolean contexts, C<< <filehandle> >> without an
2431explicit C<defined> test or comparison elicits a warning if the
9f1b1f2d 2432C<use warnings> pragma or the B<-w>
19799a22 2433command-line switch (the C<\$^W> variable) is in effect.
7b8d334a 2434
5f05dabc 2435The filehandles STDIN, STDOUT, and STDERR are predefined. (The
19799a22
GS
2436filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
2437in packages, where they would be interpreted as local identifiers
2438rather than global.) Additional filehandles may be created with
2439the open() function, amongst others. See L<perlopentut> and
2440L<perlfunc/open> for details on this.
d74e8afc 2441X<stdin> X<stdout> X<sterr>
a0d0e21e 2442
35f2feb0 2443If a <FILEHANDLE> is used in a context that is looking for
19799a22
GS
2444a list, a list comprising all input lines is returned, one line per
2445list element. It's easy to grow to a rather large data space this
2446way, so use with care.
a0d0e21e 2447
35f2feb0 2448<FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
35f2feb0
GS
2451The null filehandle <> is special: it can be used to emulate the
2452behavior of B<sed> and B<awk>. Input from <> comes either from
a0d0e21e 2453standard input, or from each file listed on the command line. Here's
35f2feb0 2454how it works: the first time <> is evaluated, the @ARGV array is
5a964f20 2455checked, and if it is empty, C<\$ARGV[0]> is set to "-", which when opened
a0d0e21e
LW
2456gives you standard input. The @ARGV array is then processed as a list
2457of filenames. The loop
2458
2459 while (<>) {
2460 ... # code for each line
2461 }
2462
2463is equivalent to the following Perl-like pseudo code:
2464
3e3baf6d 2465 unshift(@ARGV, '-') unless @ARGV;
a0d0e21e
LW
2466 while (\$ARGV = shift) {
2467 open(ARGV, \$ARGV);
2468 while (<ARGV>) {
2469 ... # code for each line
2470 }
2471 }
2472
19799a22
GS
2473except that it isn't so cumbersome to say, and will actually work.
2474It really does shift the @ARGV array and put the current filename
2475into the \$ARGV variable. It also uses filehandle I<ARGV>
ac036724 2476internally. <> is just a synonym for <ARGV>, which
19799a22 2477is magical. (The pseudo code above doesn't work because it treats
35f2feb0 2478<ARGV> as non-magical.)
a0d0e21e 2479
48ab5743
ML
2480Since the null filehandle uses the two argument form of L<perlfunc/open>
2481it interprets special characters, so if you have a script like this:
2482
2483 while (<>) {
2484 print;
2485 }
2486
2487and call it with C<perl dangerous.pl 'rm -rfv *|'>, it actually opens a
2488pipe, executes the C<rm> command and reads C<rm>'s output from that pipe.
2489If you want all items in C<@ARGV> to be interpreted as file names, you
2490can use the module C<ARGV::readonly> from CPAN.
2491
35f2feb0 2492You can modify @ARGV before the first <> as long as the array ends up
a0d0e21e 2493containing the list of filenames you really want. Line numbers (C<\$.>)
19799a22
GS
2494continue as though the input were one big happy file. See the example
2495in L<perlfunc/eof> for how to reset line numbers on each file.
5a964f20 2496
89d205f2 2497If you want to set @ARGV to your own list of files, go right ahead.
5a964f20
TC
2498This sets @ARGV to all plain text files if no @ARGV was given:
2499
2500 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
a0d0e21e 2501
5a964f20
TC
2502You can even set them to pipe commands. For example, this automatically
2503filters compressed arguments through B<gzip>:
2504
2505 @ARGV = map { /\.(gz|Z)\$/ ? "gzip -dc < \$_ |" : \$_ } @ARGV;
2506
2507If you want to pass switches into your script, you can use one of the
a0d0e21e
LW
2508Getopts modules or put a loop on the front like this:
2509
2510 while (\$_ = \$ARGV[0], /^-/) {
2511 shift;
2512 last if /^--\$/;
2513 if (/^-D(.*)/) { \$debug = \$1 }
2514 if (/^-v/) { \$verbose++ }
5a964f20 2515 # ... # other switches
a0d0e21e 2516 }
5a964f20 2517
a0d0e21e 2518 while (<>) {
5a964f20 2519 # ... # code for each line
a0d0e21e
LW
2520 }
2521
89d205f2
YO
2522The <> symbol will return C<undef> for end-of-file only once.
2523If you call it again after this, it will assume you are processing another
19799a22 2524@ARGV list, and if you haven't set @ARGV, will read input from STDIN.
a0d0e21e 2525
b159ebd3 2526If what the angle brackets contain is a simple scalar variable (e.g.,
35f2feb0 2527<\$foo>), then that variable contains the name of the
19799a22
GS
2528filehandle to input from, or its typeglob, or a reference to the
2529same. For example:
cb1a09d0
2530
2531 \$fh = \*STDIN;
2532 \$line = <\$fh>;
a0d0e21e 2533
5a964f20
TC
2534If what's within the angle brackets is neither a filehandle nor a simple
2535scalar variable containing a filehandle name, typeglob, or typeglob
2536reference, it is interpreted as a filename pattern to be globbed, and
2537either a list of filenames or the next filename in the list is returned,
19799a22 2538depending on context. This distinction is determined on syntactic
35f2feb0
GS
2539grounds alone. That means C<< <\$x> >> is always a readline() from
2540an indirect handle, but C<< <\$hash{key}> >> is always a glob().
5a964f20 2541That's because \$x is a simple scalar variable, but C<\$hash{key}> is
ef191992
YST
2542not--it's a hash element. Even C<< <\$x > >> (note the extra space)
2543is treated as C<glob("\$x ")>, not C<readline(\$x)>.
5a964f20
TC
2544
2545One level of double-quote interpretation is done first, but you can't
35f2feb0 2546say C<< <\$foo> >> because that's an indirect filehandle as explained
5a964f20
TC
2547in the previous paragraph. (In older versions of Perl, programmers
2548would insert curly brackets to force interpretation as a filename glob:
35f2feb0 2549C<< <\${foo}> >>. These days, it's considered cleaner to call the
5a964f20 2550internal function directly as C<glob(\$foo)>, which is probably the right
19799a22 2551way to have done it in the first place.) For example:
a0d0e21e
LW
2552
2553 while (<*.c>) {
2554 chmod 0644, \$_;
2555 }
2556
3a4b19e4 2557is roughly equivalent to:
a0d0e21e
LW
2558
2559 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
2560 while (<FOO>) {
5b3eff12 2561 chomp;
a0d0e21e
LW
2562 chmod 0644, \$_;
2563 }
2564
3a4b19e4
GS
2565except that the globbing is actually done internally using the standard
2566C<File::Glob> extension. Of course, the shortest way to do the above is:
a0d0e21e
LW
2567
2568 chmod 0644, <*.c>;
2569
19799a22
GS
2570A (file)glob evaluates its (embedded) argument only when it is
2571starting a new list. All values must be read before it will start
2572over. In list context, this isn't important because you automatically
2573get them all anyway. However, in scalar context the operator returns
069e01df 2574the next value each time it's called, or C<undef> when the list has
19799a22
GS
2575run out. As with filehandle reads, an automatic C<defined> is
2576generated when the glob occurs in the test part of a C<while>,
2577because legal glob returns (e.g. a file called F<0>) would otherwise
2578terminate the loop. Again, C<undef> is returned only once. So if
2579you're expecting a single value from a glob, it is much better to
2580say
4633a7c4
LW
2581
2582 (\$file) = <blurch*>;
2583
2584than
2585
2586 \$file = <blurch*>;
2587
2588because the latter will alternate between returning a filename and
19799a22 2589returning false.
4633a7c4 2590
b159ebd3 2591If you're trying to do variable interpolation, it's definitely better
4633a7c4 2592to use the glob() function, because the older notation can cause people
e37d713d 2593to become confused with the indirect filehandle notation.
4633a7c4
LW
2594
2595 @files = glob("\$dir/*.[ch]");
2596 @files = glob(\$files[\$i]);
2597
d74e8afc 2599X<constant folding> X<folding>
a0d0e21e
LW
2600
2601Like C, Perl does a certain amount of expression evaluation at
19799a22 2602compile time whenever it determines that all arguments to an
a0d0e21e
LW
2603operator are static and have no side effects. In particular, string
2604concatenation happens at compile time between literals that don't do
19799a22 2605variable substitution. Backslash interpolation also happens at
a0d0e21e
LW
2606compile time. You can say
2607
2608 'Now is the time for all' . "\n" .
2609 'good men to come to.'
2610
54310121 2611and this all reduces to one string internally. Likewise, if
a0d0e21e
LW
2612you say
2613
2614 foreach \$file (@filenames) {
5a964f20 2615 if (-s \$file > 5 + 100 * 2**16) { }
54310121 2616 }
a0d0e21e 2617
19799a22
GS
2618the compiler will precompute the number which that expression
2619represents so that the interpreter won't have to.
a0d0e21e 2620
d74e8afc 2622X<no-op> X<nop>
fd1abbef
DN
2623
2624Perl doesn't officially have a no-op operator, but the bare constants
2625C<0> and C<1> are special-cased to not produce a warning in a void
2626context, so you can for example safely do
2627
2628 1 while foo();
2629
d74e8afc 2631X<operator, bitwise, string>
TP
2632
2633Bitstrings of any size may be manipulated by the bitwise operators
2634(C<~ | & ^>).
2635
19799a22
GS
2636If the operands to a binary bitwise op are strings of different
2637sizes, B<|> and B<^> ops act as though the shorter operand had
2638additional zero bits on the right, while the B<&> op acts as though
2639the longer operand were truncated to the length of the shorter.
2640The granularity for such extension or truncation is one or more
2641bytes.
89d205f2 2643 # ASCII-based examples
TP
2644 print "j p \n" ^ " a h"; # prints "JAPH\n"
2645 print "JA" | " ph\n"; # prints "japh\n"
2646 print "japh\nJunk" & '_____'; # prints "JAPH\n";
2647 print 'p N\$' ^ " E<H\n"; # prints "Perl\n";
2648
19799a22 2649If you are intending to manipulate bitstrings, be certain that
2c268ad5 2650you're supplying bitstrings: If an operand is a number, that will imply
19799a22 2651a B<numeric> bitwise operation. You may explicitly show which type of
TP
2652operation you intend by using C<""> or C<0+>, as in the examples below.
2653
4358a253
SS
2654 \$foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
2655 \$foo = '150' | 105; # yields 255
TP
2656 \$foo = 150 | '105'; # yields 255
2657 \$foo = '150' | '105'; # yields string '155' (under ASCII)
2658
2659 \$baz = 0+\$foo & 0+\$bar; # both ops explicitly numeric
2660 \$biz = "\$foo" ^ "\$bar"; # both ops explicitly stringy
a0d0e21e 2661
1ae175c8
GS
2662See L<perlfunc/vec> for information on how to manipulate individual bits
2663in a bit vector.
2664
d74e8afc 2666X<integer>
a0d0e21e 2667
19799a22 2668By default, Perl assumes that it must do most of its arithmetic in
a0d0e21e
LW
2669floating point. But by saying
2670
2671 use integer;
2672
3eab78e3
CW
2673you may tell the compiler to use integer operations
2674(see L<integer> for a detailed explanation) from here to the end of
2675the enclosing BLOCK. An inner BLOCK may countermand this by saying
a0d0e21e
LW
2676
2677 no integer;
2678
19799a22 2679which lasts until the end of that BLOCK. Note that this doesn't
3eab78e3
CW
2680mean everything is an integer, merely that Perl will use integer
2681operations for arithmetic, comparison, and bitwise operators. For
2682example, even under C<use integer>, if you take the C<sqrt(2)>, you'll
2683still get C<1.4142135623731> or so.
19799a22
GS
2684
2685Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
13a2d996 2687L<Bitwise String Operators>.) However, C<use integer> still has meaning for
19799a22
GS
2688them. By default, their results are interpreted as unsigned integers, but
2689if C<use integer> is in effect, their results are interpreted
2690as signed integers. For example, C<~0> usually evaluates to a large
0be96356 2691integral value. However, C<use integer; ~0> is C<-1> on two's-complement
19799a22 2692machines.
68dc0745 2693
d74e8afc 2695X<floating-point> X<floating point> X<float> X<real>
68dc0745 2696
2697While C<use integer> provides integer-only arithmetic, there is no
19799a22
GS
2698analogous mechanism to provide automatic rounding or truncation to a
2699certain number of decimal places. For rounding to a certain number
2700of digits, sprintf() or printf() is usually the easiest route.
2701See L<perlfaq4>.
68dc0745 2702
5a964f20
TC
2703Floating-point numbers are only approximations to what a mathematician
2704would call real numbers. There are infinitely more reals than floats,
2705so some corners must be cut. For example:
2706
2707 printf "%.20g\n", 123456789123456789;
2708 # produces 123456789123456784
2709
8548cb57
RGS
2710Testing for exact floating-point equality or inequality is not a
2711good idea. Here's a (relatively expensive) work-around to compare
5a964f20
TC
2712whether two floating-point numbers are equal to a particular number of
2713decimal places. See Knuth, volume II, for a more robust treatment of
2714this topic.
2715
2716 sub fp_equal {
2717 my (\$X, \$Y, \$POINTS) = @_;
2718 my (\$tX, \$tY);
2719 \$tX = sprintf("%.\${POINTS}g", \$X);
2720 \$tY = sprintf("%.\${POINTS}g", \$Y);
2721 return \$tX eq \$tY;
2722 }
2723
68dc0745 2724The POSIX module (part of the standard perl distribution) implements
19799a22
GS
2725ceil(), floor(), and other mathematical and trigonometric functions.
2726The Math::Complex module (part of the standard perl distribution)
2727defines mathematical functions that work on both the reals and the
2728imaginary numbers. Math::Complex not as efficient as POSIX, but
68dc0745 2729POSIX can't work with complex numbers.
2730
2731Rounding in financial applications can have serious implications, and
2732the rounding method used should be specified precisely. In these
2733cases, it probably pays not to trust whichever system rounding is
2734being used by Perl, but to instead implement the rounding function you
2735need yourself.
5a964f20
TC
2736
d74e8afc 2738X<number, arbitrary precision>
5a964f20
TC
2739
2740The standard Math::BigInt and Math::BigFloat modules provide
19799a22 2741variable-precision arithmetic and overloaded operators, although
cd5c4fce 2742they're currently pretty slow. At the cost of some space and
19799a22
GS
2743considerable speed, they avoid the normal pitfalls associated with
2744limited-precision representations.
5a964f20
TC
2745
2746 use Math::BigInt;
2747 \$x = Math::BigInt->new('123456789123456789');
2748 print \$x * \$x;
2749
2750 # prints +15241578780673678515622620750190521
19799a22 2751
cd5c4fce
T
2752There are several modules that let you calculate with (bound only by
2753memory and cpu-time) unlimited or fixed precision. There are also
2754some non-standard modules that provide faster implementations via
2755external C libraries.
2756
2757Here is a short, but incomplete summary:
2758
950b09ed
KW
2759 Math::Fraction big, unlimited fractions like 9973 / 12967
2760 Math::String treat string sequences like numbers
2761 Math::FixedPrecision calculate with a fixed precision
2762 Math::Currency for currency calculations
2763 Bit::Vector manipulate bit vectors fast (uses C)
2764 Math::BigIntFast Bit::Vector wrapper for big numbers