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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
b033823e 154before or after the value is returned. This also means that modifying
c543c01b 155a variable twice in the same statement will lead to undefined behavior.
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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
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171 print ++(\$foo = "99"); # prints "100"
172 print ++(\$foo = "a0"); # prints "a1"
173 print ++(\$foo = "Az"); # prints "Ba"
174 print ++(\$foo = "zz"); # prints "aaa"
a0d0e21e 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
1ca345ed 193Unary "!" performs logical negation, that is, "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
1ca345ed 210Unary "~" performs bitwise negation, that is, 1's complement. For
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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
f113cf86 215width, remember to use the "&" operator to mask off the excess bits.
d74e8afc 216X<~> X<negation, binary>
a0d0e21e 217
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218When complementing strings, if all characters have ordinal values under
219256, then their complements will, also. But if they do not, all
220characters will be in either 32- or 64-bit complements, depending on your
221architecture. So for example, C<~"\x{3B1}"> is C<"\x{FFFF_FC4E}"> on
22232-bit machines and C<"\x{FFFF_FFFF_FFFF_FC4E}"> on 64-bit machines.
223
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224Unary "+" has no effect whatsoever, even on strings. It is useful
225syntactically for separating a function name from a parenthesized expression
226that would otherwise be interpreted as the complete list of function
5ba421f6 227arguments. (See examples above under L<Terms and List Operators (Leftward)>.)
d74e8afc 228X<+>
a0d0e21e 229
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230Unary "\" creates a reference to whatever follows it. See L<perlreftut>
231and L<perlref>. Do not confuse this behavior with the behavior of
232backslash within a string, although both forms do convey the notion
233of protecting the next thing from interpolation.
d74e8afc 234X<\> X<reference> X<backslash>
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235
d74e8afc 237X<binding> X<operator, binding> X<=~> X<!~>
a0d0e21e 238
c07a80fd 239Binary "=~" binds a scalar expression to a pattern match. Certain operations
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240search or modify the string \$_ by default. This operator makes that kind
241of operation work on some other string. The right argument is a search
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242pattern, substitution, or transliteration. The left argument is what is
243supposed to be searched, substituted, or transliterated instead of the default
f8bab1e9 244\$_. When used in scalar context, the return value generally indicates the
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245success of the operation. The exceptions are substitution (s///)
246and transliteration (y///) with the C</r> (non-destructive) option,
247which cause the B<r>eturn value to be the result of the substitution.
248Behavior in list context depends on the particular operator.
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249See L</"Regexp Quote-Like Operators"> for details and L<perlretut> for
250examples using these operators.
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251
252If the right argument is an expression rather than a search pattern,
2c268ad5 253substitution, or transliteration, it is interpreted as a search pattern at run
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254time. Note that this means that its contents will be interpolated twice, so
255
1ca345ed 256 '\\' =~ q'\\';
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257
258is not ok, as the regex engine will end up trying to compile the
259pattern C<\>, which it will consider a syntax error.
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260
261Binary "!~" is just like "=~" except the return value is negated in
262the logical sense.
263
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264Binary "!~" with a non-destructive substitution (s///r) or transliteration
265(y///r) is a syntax error.
4f4d7508 266
d74e8afc 268X<operator, multiplicative>
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269
270Binary "*" multiplies two numbers.
d74e8afc 271X<*>
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272
273Binary "/" divides two numbers.
d74e8afc 274X</> X<slash>
a0d0e21e 275
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276Binary "%" is the modulo operator, which computes the division
277remainder of its first argument with respect to its second argument.
278Given integer
54310121 279operands C<\$a> and C<\$b>: If C<\$b> is positive, then C<\$a % \$b> is
f7918450 280C<\$a> minus the largest multiple of C<\$b> less than or equal to
54310121 281C<\$a>. If C<\$b> is negative, then C<\$a % \$b> is C<\$a> minus the
1ca345ed 282smallest multiple of C<\$b> that is not less than C<\$a> (that is, the
89b4f0ad 283result will be less than or equal to zero). If the operands
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284C<\$a> and C<\$b> are floating point values and the absolute value of
285C<\$b> (that is C<abs(\$b)>) is less than C<(UV_MAX + 1)>, only
286the integer portion of C<\$a> and C<\$b> will be used in the operation
287(Note: here C<UV_MAX> means the maximum of the unsigned integer type).
288If the absolute value of the right operand (C<abs(\$b)>) is greater than
289or equal to C<(UV_MAX + 1)>, "%" computes the floating-point remainder
290C<\$r> in the equation C<(\$r = \$a - \$i*\$b)> where C<\$i> is a certain
f7918450 291integer that makes C<\$r> have the same sign as the right operand
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292C<\$b> (B<not> as the left operand C<\$a> like C function C<fmod()>)
293and the absolute value less than that of C<\$b>.
0412d526 294Note that when C<use integer> is in scope, "%" gives you direct access
f7918450 295to the modulo operator as implemented by your C compiler. This
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296operator is not as well defined for negative operands, but it will
297execute faster.
f7918450 298X<%> X<remainder> X<modulo> X<mod>
55d729e4 299
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300Binary "x" is the repetition operator. In scalar context or if the left
301operand is not enclosed in parentheses, it returns a string consisting
302of the left operand repeated the number of times specified by the right
303operand. In list context, if the left operand is enclosed in
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304parentheses or is a list formed by C<qw/STRING/>, it repeats the list.
305If the right operand is zero or negative, it returns an empty string
306or an empty list, depending on the context.
d74e8afc 307X<x>
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308
309 print '-' x 80; # print row of dashes
310
311 print "\t" x (\$tab/8), ' ' x (\$tab%8); # tab over
312
313 @ones = (1) x 80; # a list of 80 1's
314 @ones = (5) x @ones; # set all elements to 5
315
316
a0d0e21e 319
1ca345ed 320Binary C<+> returns the sum of two numbers.
d74e8afc 321X<+>
a0d0e21e 322
1ca345ed 323Binary C<-> returns the difference of two numbers.
d74e8afc 324X<->
a0d0e21e 325
1ca345ed 326Binary C<.> concatenates two strings.
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327X<string, concatenation> X<concatenation>
328X<cat> X<concat> X<concatenate> X<.>
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329
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331X<shift operator> X<operator, shift> X<<< << >>>
332X<<< >> >>> X<right shift> X<left shift> X<bitwise shift>
333X<shl> X<shr> X<shift, right> X<shift, left>
a0d0e21e 334
1ca345ed 335Binary C<<< << >>> returns the value of its left argument shifted left by the
55497cff 336number of bits specified by the right argument. Arguments should be
a0d0e21e 338
1ca345ed 339Binary C<<< >> >>> returns the value of its left argument shifted right by
55497cff 340the number of bits specified by the right argument. Arguments should
a0d0e21e 342
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343Note that both C<<< << >>> and C<<< >> >>> in Perl are implemented directly using
344C<<< << >>> and C<<< >> >>> in C. If C<use integer> (see L<Integer Arithmetic>) is
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345in force then signed C integers are used, else unsigned C integers are
346used. Either way, the implementation isn't going to generate results
347larger than the size of the integer type Perl was built with (32 bits
348or 64 bits).
349
350The result of overflowing the range of the integers is undefined
351because it is undefined also in C. In other words, using 32-bit
352integers, C<< 1 << 32 >> is undefined. Shifting by a negative number
353of bits is also undefined.
354
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355If you get tired of being subject to your platform's native integers,
356the C<use bigint> pragma neatly sidesteps the issue altogether:
357
358 print 20 << 20; # 20971520
359 print 20 << 40; # 5120 on 32-bit machines,
360 # 21990232555520 on 64-bit machines
361 use bigint;
362 print 20 << 100; # 25353012004564588029934064107520
363
d74e8afc 365X<operator, named unary>
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366
367The various named unary operators are treated as functions with one
568e6d8b 368argument, with optional parentheses.
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369
370If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
371is followed by a left parenthesis as the next token, the operator and
372arguments within parentheses are taken to be of highest precedence,
3981b0eb 373just like a normal function call. For example,
1ca345ed 374because named unary operators are higher precedence than C<||>:
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375
376 chdir \$foo || die; # (chdir \$foo) || die
377 chdir(\$foo) || die; # (chdir \$foo) || die
378 chdir (\$foo) || die; # (chdir \$foo) || die
379 chdir +(\$foo) || die; # (chdir \$foo) || die
380
3981b0eb 381but, because * is higher precedence than named operators:
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382
383 chdir \$foo * 20; # chdir (\$foo * 20)
384 chdir(\$foo) * 20; # (chdir \$foo) * 20
385 chdir (\$foo) * 20; # (chdir \$foo) * 20
386 chdir +(\$foo) * 20; # chdir (\$foo * 20)
387
388 rand 10 * 20; # rand (10 * 20)
389 rand(10) * 20; # (rand 10) * 20
390 rand (10) * 20; # (rand 10) * 20
391 rand +(10) * 20; # rand (10 * 20)
392
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393Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are
394treated like named unary operators, but they don't follow this functional
395parenthesis rule. That means, for example, that C<-f(\$file).".bak"> is
396equivalent to C<-f "\$file.bak">.
d74e8afc 397X<-X> X<filetest> X<operator, filetest>
568e6d8b 398
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400
d74e8afc 402X<relational operator> X<operator, relational>
a0d0e21e 403
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404Perl operators that return true or false generally return values
405that can be safely used as numbers. For example, the relational
406operators in this section and the equality operators in the next
407one return C<1> for true and a special version of the defined empty
408string, C<"">, which counts as a zero but is exempt from warnings
409about improper numeric conversions, just as C<"0 but true"> is.
410
35f2feb0 411Binary "<" returns true if the left argument is numerically less than
a0d0e21e 412the right argument.
d74e8afc 413X<< < >>
a0d0e21e 414
35f2feb0 415Binary ">" returns true if the left argument is numerically greater
a0d0e21e 416than the right argument.
d74e8afc 417X<< > >>
a0d0e21e 418
35f2feb0 419Binary "<=" returns true if the left argument is numerically less than
a0d0e21e 420or equal to the right argument.
d74e8afc 421X<< <= >>
a0d0e21e 422
35f2feb0 423Binary ">=" returns true if the left argument is numerically greater
a0d0e21e 424than or equal to the right argument.
d74e8afc 425X<< >= >>
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426
427Binary "lt" returns true if the left argument is stringwise less than
428the right argument.
d74e8afc 429X<< lt >>
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430
431Binary "gt" returns true if the left argument is stringwise greater
432than the right argument.
d74e8afc 433X<< gt >>
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434
435Binary "le" returns true if the left argument is stringwise less than
436or equal to the right argument.
d74e8afc 437X<< le >>
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438
439Binary "ge" returns true if the left argument is stringwise greater
440than or equal to the right argument.
d74e8afc 441X<< ge >>
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442
d74e8afc 444X<equality> X<equal> X<equals> X<operator, equality>
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445
446Binary "==" returns true if the left argument is numerically equal to
447the right argument.
d74e8afc 448X<==>
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449
450Binary "!=" returns true if the left argument is numerically not equal
451to the right argument.
d74e8afc 452X<!=>
a0d0e21e 453
35f2feb0 454Binary "<=>" returns -1, 0, or 1 depending on whether the left
6ee5d4e7 455argument is numerically less than, equal to, or greater than the right
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457values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
458"<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
459returns true, as does NaN != anything else. If your platform doesn't
460support NaNs then NaN is just a string with numeric value 0.
d74e8afc 461X<< <=> >> X<spaceship>
7d3a9d88 462
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463 \$ perl -le '\$a = "NaN"; print "No NaN support here" if \$a == \$a'
464 \$ perl -le '\$a = "NaN"; print "NaN support here" if \$a != \$a'
465
466(Note that the L<bigint>, L<bigrat>, and L<bignum> pragmas all
467support "NaN".)
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468
469Binary "eq" returns true if the left argument is stringwise equal to
470the right argument.
d74e8afc 471X<eq>
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472
473Binary "ne" returns true if the left argument is stringwise not equal
474to the right argument.
d74e8afc 475X<ne>
a0d0e21e 476
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477Binary "cmp" returns -1, 0, or 1 depending on whether the left
478argument is stringwise less than, equal to, or greater than the right
479argument.
d74e8afc 480X<cmp>
a0d0e21e 481
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482Binary "~~" does a smartmatch between its arguments. Smart matching
483is described in the next section.
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484X<~~>
485
a034a98d 486"lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
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487by the current locale if a legacy C<use locale> (but not
488C<use locale ':not_characters'>) is in effect. See
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489L<perllocale>. Do not mix these with Unicode, only with legacy binary
490encodings. The standard L<Unicode::Collate> and
491L<Unicode::Collate::Locale> modules offer much more powerful solutions to
492collation issues.
493
495
496First available in Perl 5.10.1 (the 5.10.0 version behaved differently),
497binary C<~~> does a "smartmatch" between its arguments. This is mostly
498used implicitly in the C<when> construct described in L<perlsyn>, although
499not all C<when> clauses call the smartmatch operator. Unique among all of
500Perl's operators, the smartmatch operator can recurse.
501
502It is also unique in that all other Perl operators impose a context
503(usually string or numeric context) on their operands, autoconverting
504those operands to those imposed contexts. In contrast, smartmatch
505I<infers> contexts from the actual types of its operands and uses that
506type information to select a suitable comparison mechanism.
507
508The C<~~> operator compares its operands "polymorphically", determining how
509to compare them according to their actual types (numeric, string, array,
510hash, etc.) Like the equality operators with which it shares the same
511precedence, C<~~> returns 1 for true and C<""> for false. It is often best
512read aloud as "in", "inside of", or "is contained in", because the left
513operand is often looked for I<inside> the right operand. That makes the
40bec8a5 514order of the operands to the smartmatch operand often opposite that of
1ca345ed
TC
515the regular match operator. In other words, the "smaller" thing is usually
516placed in the left operand and the larger one in the right.
517
518The behavior of a smartmatch depends on what type of things its arguments
519are, as determined by the following table. The first row of the table
520whose types apply determines the smartmatch behavior. Because what
521actually happens is mostly determined by the type of the second operand,
522the table is sorted on the right operand instead of on the left.
523
524 Left Right Description and pseudocode
525 ===============================================================
526 Any undef check whether Any is undefined
527 like: !defined Any
528
530
531 Right operand is an ARRAY:
532
533 Left Right Description and pseudocode
534 ===============================================================
535 ARRAY1 ARRAY2 recurse on paired elements of ARRAY1 and ARRAY2[2]
536 like: (ARRAY1[0] ~~ ARRAY2[0])
537 && (ARRAY1[1] ~~ ARRAY2[1]) && ...
538 HASH ARRAY any ARRAY elements exist as HASH keys
539 like: grep { exists HASH->{\$_} } ARRAY
540 Regexp ARRAY any ARRAY elements pattern match Regexp
541 like: grep { /Regexp/ } ARRAY
542 undef ARRAY undef in ARRAY
543 like: grep { !defined } ARRAY
40bec8a5 544 Any ARRAY smartmatch each ARRAY element[3]
1ca345ed
TC
545 like: grep { Any ~~ \$_ } ARRAY
546
547 Right operand is a HASH:
548
549 Left Right Description and pseudocode
550 ===============================================================
551 HASH1 HASH2 all same keys in both HASHes
552 like: keys HASH1 ==
553 grep { exists HASH2->{\$_} } keys HASH1
554 ARRAY HASH any ARRAY elements exist as HASH keys
555 like: grep { exists HASH->{\$_} } ARRAY
556 Regexp HASH any HASH keys pattern match Regexp
557 like: grep { /Regexp/ } keys HASH
558 undef HASH always false (undef can't be a key)
559 like: 0 == 1
560 Any HASH HASH key existence
561 like: exists HASH->{Any}
562
563 Right operand is CODE:
564
565 Left Right Description and pseudocode
566 ===============================================================
567 ARRAY CODE sub returns true on all ARRAY elements[1]
568 like: !grep { !CODE->(\$_) } ARRAY
569 HASH CODE sub returns true on all HASH keys[1]
570 like: !grep { !CODE->(\$_) } keys HASH
571 Any CODE sub passed Any returns true
572 like: CODE->(Any)
573
574Right operand is a Regexp:
575
576 Left Right Description and pseudocode
577 ===============================================================
578 ARRAY Regexp any ARRAY elements match Regexp
579 like: grep { /Regexp/ } ARRAY
580 HASH Regexp any HASH keys match Regexp
581 like: grep { /Regexp/ } keys HASH
582 Any Regexp pattern match
583 like: Any =~ /Regexp/
584
585 Other:
586
587 Left Right Description and pseudocode
588 ===============================================================
590 or fall back to...
591
592 Any Num numeric equality
593 like: Any == Num
594 Num nummy[4] numeric equality
595 like: Num == nummy
596 undef Any check whether undefined
597 like: !defined(Any)
598 Any Any string equality
599 like: Any eq Any
600
601
602Notes:
603
604=over
605
606=item 1.
607Empty hashes or arrays match.
608
609=item 2.
40bec8a5 610That is, each element smartmatches the element of the same index in the other array.[3]
1ca345ed
TC
611
612=item 3.
613If a circular reference is found, fall back to referential equality.
614
615=item 4.
616Either an actual number, or a string that looks like one.
617
618=back
619
620The smartmatch implicitly dereferences any non-blessed hash or array
621reference, so the C<I<HASH>> and C<I<ARRAY>> entries apply in those cases.
622For blessed references, the C<I<Object>> entries apply. Smartmatches
623involving hashes only consider hash keys, never hash values.
624
625The "like" code entry is not always an exact rendition. For example, the
40bec8a5 626smartmatch operator short-circuits whenever possible, but C<grep> does
1ca345ed
TC
627not. Also, C<grep> in scalar context returns the number of matches, but
628C<~~> returns only true or false.
629
630Unlike most operators, the smartmatch operator knows to treat C<undef>
631specially:
632
633 use v5.10.1;
634 @array = (1, 2, 3, undef, 4, 5);
635 say "some elements undefined" if undef ~~ @array;
636
637Each operand is considered in a modified scalar context, the modification
638being that array and hash variables are passed by reference to the
639operator, which implicitly dereferences them. Both elements
640of each pair are the same:
641
642 use v5.10.1;
643
644 my %hash = (red => 1, blue => 2, green => 3,
645 orange => 4, yellow => 5, purple => 6,
646 black => 7, grey => 8, white => 9);
647
648 my @array = qw(red blue green);
649
650 say "some array elements in hash keys" if @array ~~ %hash;
651 say "some array elements in hash keys" if \@array ~~ \%hash;
652
653 say "red in array" if "red" ~~ @array;
654 say "red in array" if "red" ~~ \@array;
655
656 say "some keys end in e" if /e\$/ ~~ %hash;
657 say "some keys end in e" if /e\$/ ~~ \%hash;
658
40bec8a5
TC
659Two arrays smartmatch if each element in the first array smartmatches
660(that is, is "in") the corresponding element in the second array,
661recursively.
1ca345ed
TC
662
663 use v5.10.1;
664 my @little = qw(red blue green);
665 my @bigger = ("red", "blue", [ "orange", "green" ] );
666 if (@little ~~ @bigger) { # true!
667 say "little is contained in bigger";
668 }
669
670Because the smartmatch operator recurses on nested arrays, this
671will still report that "red" is in the array.
672
673 use v5.10.1;
674 my @array = qw(red blue green);
675 my \$nested_array = [[[[[[[ @array ]]]]]]];
676 say "red in array" if "red" ~~ \$nested_array;
677
678If two arrays smartmatch each other, then they are deep
679copies of each others' values, as this example reports:
680
681 use v5.12.0;
682 my @a = (0, 1, 2, [3, [4, 5], 6], 7);
683 my @b = (0, 1, 2, [3, [4, 5], 6], 7);
684
685 if (@a ~~ @b && @b ~~ @a) {
686 say "a and b are deep copies of each other";
687 }
688 elsif (@a ~~ @b) {
689 say "a smartmatches in b";
690 }
691 elsif (@b ~~ @a) {
692 say "b smartmatches in a";
693 }
694 else {
695 say "a and b don't smartmatch each other at all";
696 }
697
698
699If you were to set C<\$b[3] = 4>, then instead of reporting that "a and b
700are deep copies of each other", it now reports that "b smartmatches in a".
701That because the corresponding position in C<@a> contains an array that
702(eventually) has a 4 in it.
703
704Smartmatching one hash against another reports whether both contain the
705same keys, no more and no less. This could be used to see whether two
706records have the same field names, without caring what values those fields
707might have. For example:
708
709 use v5.10.1;
710 sub make_dogtag {
711 state \$REQUIRED_FIELDS = { name=>1, rank=>1, serial_num=>1 };
712
713 my (\$class, \$init_fields) = @_;
714
715 die "Must supply (only) name, rank, and serial number"
716 unless \$init_fields ~~ \$REQUIRED_FIELDS;
717
718 ...
719 }
720
722
723 use v5.10.1;
724 sub make_dogtag {
725 state \$REQUIRED_FIELDS = { name=>1, rank=>1, serial_num=>1 };
726
727 my (\$class, \$init_fields) = @_;
728
729 die "Must supply (at least) name, rank, and serial number"
730 unless [keys %{\$init_fields}] ~~ \$REQUIRED_FIELDS;
731
732 ...
733 }
734
735The smartmatch operator is most often used as the implicit operator of a
736C<when> clause. See the section on "Switch Statements" in L<perlsyn>.
737
739
40bec8a5
TC
740To avoid relying on an object's underlying representation, if the
741smartmatch's right operand is an object that doesn't overload C<~~>,
742it raises the exception "C<Smartmatching a non-overloaded object
743breaks encapsulation>". That's because one has no business digging
744around to see whether something is "in" an object. These are all
745illegal on objects without a C<~~> overload:
1ca345ed
TC
746
747 %hash ~~ \$object
748 42 ~~ \$object
749 "fred" ~~ \$object
750
751However, you can change the way an object is smartmatched by overloading
752the C<~~> operator. This is allowed to extend the usual smartmatch semantics.
754
755Using an object as the left operand is allowed, although not very useful.
758ignored. A left operand that is a non-overloaded object falls back on a
759string or numeric comparison of whatever the C<ref> operator returns. That
760means that
761
762 \$object ~~ X
763
764does I<not> invoke the overload method with C<I<X>> as an argument.
765Instead the above table is consulted as normal, and based on the type of
766C<I<X>>, overloading may or may not be invoked. For simple strings or
767numbers, in becomes equivalent to this:
768
769 \$object ~~ \$number ref(\$object) == \$number
770 \$object ~~ \$string ref(\$object) eq \$string
771
772For example, this reports that the handle smells IOish
773(but please don't really do this!):
774
775 use IO::Handle;
776 my \$fh = IO::Handle->new();
777 if (\$fh ~~ /\bIO\b/) {
778 say "handle smells IOish";
779 }
780
781That's because it treats C<\$fh> as a string like
782C<"IO::Handle=GLOB(0x8039e0)">, then pattern matches against that.
a034a98d 783
d74e8afc 785X<operator, bitwise, and> X<bitwise and> X<&>
a0d0e21e 786
2cdc098b 787Binary "&" returns its operands ANDed together bit by bit.
a0d0e21e 789
2cdc098b 790Note that "&" has lower priority than relational operators, so for example
1ca345ed 791the parentheses are essential in a test like
2cdc098b 792
1ca345ed 793 print "Even\n" if (\$x & 1) == 0;
2cdc098b 794
a0d0e21e 795=head2 Bitwise Or and Exclusive Or
d74e8afc
ITB
796X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor>
797X<bitwise xor> X<^>
a0d0e21e 798
2cdc098b 799Binary "|" returns its operands ORed together bit by bit.
a0d0e21e 801
2cdc098b 802Binary "^" returns its operands XORed together bit by bit.
a0d0e21e 804
2cdc098b
MG
805Note that "|" and "^" have lower priority than relational operators, so
806for example the brackets are essential in a test like
807
1ca345ed 808 print "false\n" if (8 | 2) != 10;
2cdc098b 809
d74e8afc 811X<&&> X<logical and> X<operator, logical, and>
a0d0e21e
LW
812
813Binary "&&" performs a short-circuit logical AND operation. That is,
814if the left operand is false, the right operand is not even evaluated.
815Scalar or list context propagates down to the right operand if it
816is evaluated.
817
d74e8afc 819X<||> X<operator, logical, or>
a0d0e21e
LW
820
821Binary "||" performs a short-circuit logical OR operation. That is,
822if the left operand is true, the right operand is not even evaluated.
823Scalar or list context propagates down to the right operand if it
824is evaluated.
825
d74e8afc 827X<//> X<operator, logical, defined-or>
c963b151
BD
828
829Although it has no direct equivalent in C, Perl's C<//> operator is related
89d205f2 830to its C-style or. In fact, it's exactly the same as C<||>, except that it
95bee9ba
A
831tests the left hand side's definedness instead of its truth. Thus,
832C<< EXPR1 // EXPR2 >> returns the value of C<< EXPR1 >> if it's defined,
833otherwise, the value of C<< EXPR2 >> is returned. (C<< EXPR1 >> is evaluated
834in scalar context, C<< EXPR2 >> in the context of C<< // >> itself). Usually,
835this is the same result as C<< defined(EXPR1) ? EXPR1 : EXPR2 >> (except that
836the ternary-operator form can be used as a lvalue, while C<< EXPR1 // EXPR2 >>
837cannot). This is very useful for
bdc7923b
RGS
838providing default values for variables. If you actually want to test if
839at least one of C<\$a> and C<\$b> is defined, use C<defined(\$a // \$b)>.
c963b151 840
d042e63d
MS
841The C<||>, C<//> and C<&&> operators return the last value evaluated
842(unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably
843portable way to find out the home directory might be:
a0d0e21e 844
c543c01b
TC
845 \$home = \$ENV{HOME}
846 // \$ENV{LOGDIR}
847 // (getpwuid(\$<))[7]
848 // die "You're homeless!\n";
a0d0e21e 849
5a964f20
TC
850In particular, this means that you shouldn't use this
851for selecting between two aggregates for assignment:
852
853 @a = @b || @c; # this is wrong
854 @a = scalar(@b) || @c; # really meant this
855 @a = @b ? @b : @c; # this works fine, though
856
1ca345ed 857As alternatives to C<&&> and C<||> when used for
f23102e2
RGS
858control flow, Perl provides the C<and> and C<or> operators (see below).
859The short-circuit behavior is identical. The precedence of "and"
c963b151 860and "or" is much lower, however, so that you can safely use them after a
5a964f20 861list operator without the need for parentheses:
a0d0e21e
LW
862
864 or gripe(), next LINE;
865
866With the C-style operators that would have been written like this:
867
869 || (gripe(), next LINE);
870
1ca345ed
TC
871It would be even more readable to write that this way:
872
874 gripe();
875 next LINE;
876 }
877
eeb6a2c9 878Using "or" for assignment is unlikely to do what you want; see below.
5a964f20
TC
879
d74e8afc 881X<operator, range> X<range> X<..> X<...>
a0d0e21e
LW
882
883Binary ".." is the range operator, which is really two different
fb53bbb2 884operators depending on the context. In list context, it returns a
54ae734e 885list of values counting (up by ones) from the left value to the right
2cdbc966 886value. If the left value is greater than the right value then it
fb53bbb2 887returns the empty list. The range operator is useful for writing
54ae734e 888C<foreach (1..10)> loops and for doing slice operations on arrays. In
2cdbc966
JD
889the current implementation, no temporary array is created when the
890range operator is used as the expression in C<foreach> loops, but older
891versions of Perl might burn a lot of memory when you write something
892like this:
a0d0e21e
LW
893
894 for (1 .. 1_000_000) {
895 # code
54310121 896 }
a0d0e21e 897
8f0f46f8 898The range operator also works on strings, using the magical
899auto-increment, see below.
54ae734e 900
5a964f20 901In scalar context, ".." returns a boolean value. The operator is
8f0f46f8 902bistable, like a flip-flop, and emulates the line-range (comma)
903operator of B<sed>, B<awk>, and various editors. Each ".." operator
904maintains its own boolean state, even across calls to a subroutine
905that contains it. It is false as long as its left operand is false.
a0d0e21e
LW
906Once the left operand is true, the range operator stays true until the
907right operand is true, I<AFTER> which the range operator becomes false
8f0f46f8 908again. It doesn't become false till the next time the range operator
909is evaluated. It can test the right operand and become false on the
910same evaluation it became true (as in B<awk>), but it still returns
911true once. If you don't want it to test the right operand until the
912next evaluation, as in B<sed>, just use three dots ("...") instead of
19799a22
GS
913two. In all other regards, "..." behaves just like ".." does.
914
915The right operand is not evaluated while the operator is in the
916"false" state, and the left operand is not evaluated while the
917operator is in the "true" state. The precedence is a little lower
918than || and &&. The value returned is either the empty string for
8f0f46f8 919false, or a sequence number (beginning with 1) for true. The sequence
920number is reset for each range encountered. The final sequence number
921in a range has the string "E0" appended to it, which doesn't affect
922its numeric value, but gives you something to search for if you want
923to exclude the endpoint. You can exclude the beginning point by
924waiting for the sequence number to be greater than 1.
df5f8116
CW
925
926If either operand of scalar ".." is a constant expression,
927that operand is considered true if it is equal (C<==>) to the current
928input line number (the C<\$.> variable).
929
930To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>,
931but that is only an issue if you use a floating point expression; when
932implicitly using C<\$.> as described in the previous paragraph, the
933comparison is C<int(EXPR) == int(\$.)> which is only an issue when C<\$.>
934is set to a floating point value and you are not reading from a file.
935Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what
936you want in scalar context because each of the operands are evaluated
937using their integer representation.
938
939Examples:
a0d0e21e
LW
940
941As a scalar operator:
942
df5f8116 943 if (101 .. 200) { print; } # print 2nd hundred lines, short for
950b09ed 944 # if (\$. == 101 .. \$. == 200) { print; }
9f10b797
RGS
945
946 next LINE if (1 .. /^\$/); # skip header lines, short for
f343f960 947 # next LINE if (\$. == 1 .. /^\$/);
9f10b797
RGS
948 # (typically in a loop labeled LINE)
949
950 s/^/> / if (/^\$/ .. eof()); # quote body
a0d0e21e 951
5a964f20
TC
952 # parse mail messages
953 while (<>) {
954 \$in_header = 1 .. /^\$/;
df5f8116
CW
955 \$in_body = /^\$/ .. eof;
f343f960 957 # do something
df5f8116 958 } else { # in body
f343f960 959 # do something else
df5f8116 960 }
5a964f20 961 } continue {
df5f8116 962 close ARGV if eof; # reset \$. each file
5a964f20
TC
963 }
964
acf31ca5
SF
965Here's a simple example to illustrate the difference between
966the two range operators:
967
968 @lines = (" - Foo",
969 "01 - Bar",
970 "1 - Baz",
971 " - Quux");
972
9f10b797
RGS
973 foreach (@lines) {
974 if (/0/ .. /1/) {
acf31ca5
SF
975 print "\$_\n";
976 }
977 }
978
9f10b797
RGS
979This program will print only the line containing "Bar". If
980the range operator is changed to C<...>, it will also print the
acf31ca5
SF
981"Baz" line.
982
983And now some examples as a list operator:
a0d0e21e 984
1ca345ed
TC
985 for (101 .. 200) { print } # print \$_ 100 times
986 @foo = @foo[0 .. \$#foo]; # an expensive no-op
987 @foo = @foo[\$#foo-4 .. \$#foo]; # slice last 5 items
a0d0e21e 988
5a964f20 989The range operator (in list context) makes use of the magical
5f05dabc 990auto-increment algorithm if the operands are strings. You
a0d0e21e
LW
991can say
992
c543c01b 993 @alphabet = ("A" .. "Z");
a0d0e21e 994
54ae734e 995to get all normal letters of the English alphabet, or
a0d0e21e 996
c543c01b 997 \$hexdigit = (0 .. 9, "a" .. "f")[\$num & 15];
a0d0e21e
LW
998
999to get a hexadecimal digit, or
1000
1ca345ed
TC
1001 @z2 = ("01" .. "31");
1002 print \$z2[\$mday];
a0d0e21e 1003
ea4f5703
YST
1004to get dates with leading zeros.
1005
1006If the final value specified is not in the sequence that the magical
1007increment would produce, the sequence goes until the next value would
1008be longer than the final value specified.
1009
1010If the initial value specified isn't part of a magical increment
c543c01b 1011sequence (that is, a non-empty string matching C</^[a-zA-Z]*[0-9]*\z/>),
ea4f5703
YST
1012only the initial value will be returned. So the following will only
1013return an alpha:
1014
c543c01b 1015 use charnames "greek";
ea4f5703
YST
1016 my @greek_small = ("\N{alpha}" .. "\N{omega}");
1017
c543c01b
TC
1018To get the 25 traditional lowercase Greek letters, including both sigmas,
ea4f5703 1020
c543c01b 1021 use charnames "greek";
1ca345ed
TC
1022 my @greek_small = map { chr } ( ord("\N{alpha}")
1023 ..
1024 ord("\N{omega}")
1025 );
c543c01b
TC
1026
1027However, because there are I<many> other lowercase Greek characters than
1028just those, to match lowercase Greek characters in a regular expression,
1029you would use the pattern C</(?:(?=\p{Greek})\p{Lower})+/>.
a0d0e21e 1030
df5f8116
CW
1031Because each operand is evaluated in integer form, C<2.18 .. 3.14> will
1032return two elements in list context.
1033
1034 @list = (2.18 .. 3.14); # same as @list = (2 .. 3);
1035
d74e8afc 1037X<operator, conditional> X<operator, ternary> X<ternary> X<?:>
a0d0e21e
LW
1038
1039Ternary "?:" is the conditional operator, just as in C. It works much
1040like an if-then-else. If the argument before the ? is true, the
1041argument before the : is returned, otherwise the argument after the :
cb1a09d0
1042is returned. For example:
1043
54310121 1044 printf "I have %d dog%s.\n", \$n,
c543c01b 1045 (\$n == 1) ? "" : "s";
cb1a09d0
1046
1047Scalar or list context propagates downward into the 2nd
54310121 1048or 3rd argument, whichever is selected.
cb1a09d0
1049
1050 \$a = \$ok ? \$b : \$c; # get a scalar
1051 @a = \$ok ? @b : @c; # get an array
1052 \$a = \$ok ? @b : @c; # oops, that's just a count!
1053
1054The operator may be assigned to if both the 2nd and 3rd arguments are
1055legal lvalues (meaning that you can assign to them):
a0d0e21e
LW
1056
1057 (\$a_or_b ? \$a : \$b) = \$c;
1058
5a964f20
TC
1059Because this operator produces an assignable result, using assignments
1060without parentheses will get you in trouble. For example, this:
1061
1062 \$a % 2 ? \$a += 10 : \$a += 2
1063
1064Really means this:
1065
1066 ((\$a % 2) ? (\$a += 10) : \$a) += 2
1067
1068Rather than this:
1069
1070 (\$a % 2) ? (\$a += 10) : (\$a += 2)
1071
19799a22
GS
1072That should probably be written more simply as:
1073
1074 \$a += (\$a % 2) ? 10 : 2;
1075
d74e8afc 1077X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=>
5ac3b81c 1078X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=>
d74e8afc 1079X<%=> X<^=> X<x=>
a0d0e21e
LW
1080
1081"=" is the ordinary assignment operator.
1082
1083Assignment operators work as in C. That is,
1084
1085 \$a += 2;
1086
1087is equivalent to
1088
1089 \$a = \$a + 2;
1090
1091although without duplicating any side effects that dereferencing the lvalue
54310121 1092might trigger, such as from tie(). Other assignment operators work similarly.
1093The following are recognized:
a0d0e21e
LW
1094
1095 **= += *= &= <<= &&=
9f10b797
RGS
1096 -= /= |= >>= ||=
1097 .= %= ^= //=
1098 x=
a0d0e21e 1099
19799a22 1100Although these are grouped by family, they all have the precedence
a0d0e21e
LW
1101of assignment.
1102
b350dd2f
GS
1103Unlike in C, the scalar assignment operator produces a valid lvalue.
1104Modifying an assignment is equivalent to doing the assignment and
1105then modifying the variable that was assigned to. This is useful
1106for modifying a copy of something, like this:
a0d0e21e 1107
1ca345ed
TC
1108 (\$tmp = \$global) =~ tr/13579/24680/;
1109
1110Although as of 5.14, that can be also be accomplished this way:
1111
1112 use v5.14;
1113 \$tmp = (\$global =~ tr/13579/24680/r);
a0d0e21e
LW
1114
1115Likewise,
1116
1117 (\$a += 2) *= 3;
1118
1119is equivalent to
1120
1121 \$a += 2;
1122 \$a *= 3;
1123
b350dd2f
GS
1124Similarly, a list assignment in list context produces the list of
1125lvalues assigned to, and a list assignment in scalar context returns
1126the number of elements produced by the expression on the right hand
1127side of the assignment.
1128
d74e8afc 1130X<comma> X<operator, comma> X<,>
a0d0e21e 1131
5a964f20 1132Binary "," is the comma operator. In scalar context it evaluates
a0d0e21e
LW
1133its left argument, throws that value away, then evaluates its right
1134argument and returns that value. This is just like C's comma operator.
1135
5a964f20 1136In list context, it's just the list argument separator, and inserts
ed5c6d31
PJ
1137both its arguments into the list. These arguments are also evaluated
1138from left to right.
a0d0e21e 1139
4e1988c6
FC
1140The C<< => >> operator is a synonym for the comma except that it causes a
1141word on its left to be interpreted as a string if it begins with a letter
344f2c40
IG
1142or underscore and is composed only of letters, digits and underscores.
1143This includes operands that might otherwise be interpreted as operators,
1144constants, single number v-strings or function calls. If in doubt about
c543c01b 1145this behavior, the left operand can be quoted explicitly.
344f2c40
IG
1146
1147Otherwise, the C<< => >> operator behaves exactly as the comma operator
1148or list argument separator, according to context.
1149
1150For example:
a44e5664
MS
1151
1152 use constant FOO => "something";
1153
1154 my %h = ( FOO => 23 );
1155
1156is equivalent to:
1157
1158 my %h = ("FOO", 23);
1159
1160It is I<NOT>:
1161
1162 my %h = ("something", 23);
1163
719b43e8
RGS
1164The C<< => >> operator is helpful in documenting the correspondence
1165between keys and values in hashes, and other paired elements in lists.
748a9306 1166
a12b8f3c
FC
1167 %hash = ( \$key => \$value );
a44e5664 1169
4e1988c6
FC
1170The special quoting behavior ignores precedence, and hence may apply to
1171I<part> of the left operand:
1172
1173 print time.shift => "bbb";
1174
1175That example prints something like "1314363215shiftbbb", because the
1176C<< => >> implicitly quotes the C<shift> immediately on its left, ignoring
1177the fact that C<time.shift> is the entire left operand.
1178
d74e8afc 1180X<operator, list, rightward> X<list operator>
a0d0e21e 1181
c543c01b 1182On the right side of a list operator, the comma has very low precedence,
a0d0e21e
LW
1183such that it controls all comma-separated expressions found there.
1184The only operators with lower precedence are the logical operators
1185"and", "or", and "not", which may be used to evaluate calls to list
1ca345ed
TC
1186operators without the need for parentheses:
1187
1188 open HANDLE, "< :utf8", "filename" or die "Can't open: \$!\n";
1189
1190However, some people find that code harder to read than writing
1191it with parentheses:
1192
1193 open(HANDLE, "< :utf8", "filename") or die "Can't open: \$!\n";
1194
1195in which case you might as well just use the more customary "||" operator:
a0d0e21e 1196
1ca345ed 1197 open(HANDLE, "< :utf8", "filename") || die "Can't open: \$!\n";
a0d0e21e 1198
5ba421f6 1199See also discussion of list operators in L<Terms and List Operators (Leftward)>.
a0d0e21e
LW
1200
d74e8afc 1202X<operator, logical, not> X<not>
a0d0e21e
LW
1203
1204Unary "not" returns the logical negation of the expression to its right.
1205It's the equivalent of "!" except for the very low precedence.
1206
d74e8afc 1208X<operator, logical, and> X<and>
a0d0e21e
LW
1209
1210Binary "and" returns the logical conjunction of the two surrounding
c543c01b
TC
1211expressions. It's equivalent to C<&&> except for the very low
1212precedence. This means that it short-circuits: the right
a0d0e21e
LW
1213expression is evaluated only if the left expression is true.
1214
59ab9d6e 1215=head2 Logical or and Exclusive Or
f23102e2 1216X<operator, logical, or> X<operator, logical, xor>
59ab9d6e 1217X<operator, logical, exclusive or>
f23102e2 1218X<or> X<xor>
a0d0e21e
LW
1219
1220Binary "or" returns the logical disjunction of the two surrounding
c543c01b
TC
1221expressions. It's equivalent to C<||> except for the very low precedence.
1222This makes it useful for control flow:
5a964f20
TC
1223
1224 print FH \$data or die "Can't write to FH: \$!";
1225
c543c01b
TC
1226This means that it short-circuits: the right expression is evaluated
1227only if the left expression is false. Due to its precedence, you must
1228be careful to avoid using it as replacement for the C<||> operator.
1229It usually works out better for flow control than in assignments:
5a964f20
TC
1230
1231 \$a = \$b or \$c; # bug: this is wrong
1232 (\$a = \$b) or \$c; # really means this
1233 \$a = \$b || \$c; # better written this way
1234
19799a22 1235However, when it's a list-context assignment and you're trying to use
c543c01b 1236C<||> for control flow, you probably need "or" so that the assignment
5a964f20
TC
1237takes higher precedence.
1238
1239 @info = stat(\$file) || die; # oops, scalar sense of stat!
1240 @info = stat(\$file) or die; # better, now @info gets its due
1241
c963b151
BD
1242Then again, you could always use parentheses.
1243
1ca345ed 1244Binary C<xor> returns the exclusive-OR of the two surrounding expressions.
c543c01b 1245It cannot short-circuit (of course).
a0d0e21e 1246
59ab9d6e
MB
1247There is no low precedence operator for defined-OR.
1248
a0d0e21e 1249=head2 C Operators Missing From Perl
d74e8afc
ITB
1250X<operator, missing from perl> X<&> X<*>
1251X<typecasting> X<(TYPE)>
a0d0e21e
LW
1252
1253Here is what C has that Perl doesn't:
1254
1255=over 8
1256
1257=item unary &
1258
1259Address-of operator. (But see the "\" operator for taking a reference.)
1260
1261=item unary *
1262
54310121 1263Dereference-address operator. (Perl's prefix dereferencing
a0d0e21e
LW
1264operators are typed: \$, @, %, and &.)
1265
1266=item (TYPE)
1267
19799a22 1268Type-casting operator.
a0d0e21e
LW
1269
1270=back
1271
5f05dabc 1272=head2 Quote and Quote-like Operators
89d205f2 1273X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m>
d74e8afc
ITB
1274X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>>
1275X<escape sequence> X<escape>
1276
a0d0e21e
LW
1277While we usually think of quotes as literal values, in Perl they
1278function as operators, providing various kinds of interpolating and
1279pattern matching capabilities. Perl provides customary quote characters
1280for these behaviors, but also provides a way for you to choose your
1281quote character for any of them. In the following table, a C<{}> represents
9f10b797 1282any pair of delimiters you choose.
a0d0e21e 1283
TP
1284 Customary Generic Meaning Interpolates
1285 '' q{} Literal no
1286 "" qq{} Literal yes
af9219ee 1287 `` qx{} Command yes*
2c268ad5 1288 qw{} Word list no
af9219ee
MG
1289 // m{} Pattern match yes*
1290 qr{} Pattern yes*
1291 s{}{} Substitution yes*
2c268ad5 1292 tr{}{} Transliteration no (but see below)
c543c01b 1293 y{}{} Transliteration no (but see below)
7e3b091d 1294 <<EOF here-doc yes*
a0d0e21e 1295
af9219ee
MG
1296 * unless the delimiter is ''.
1297
87275199 1298Non-bracketing delimiters use the same character fore and aft, but the four
c543c01b 1299sorts of ASCII brackets (round, angle, square, curly) all nest, which means
9f10b797 1300that
87275199 1301
c543c01b 1302 q{foo{bar}baz}
35f2feb0 1303
9f10b797 1304is the same as
87275199 1305
c543c01b 1306 'foo{bar}baz'
87275199
GS
1307
1308Note, however, that this does not always work for quoting Perl code:
1309
c543c01b 1310 \$s = q{ if(\$a eq "}") ... }; # WRONG
87275199 1311
c543c01b
TC
1312is a syntax error. The C<Text::Balanced> module (standard as of v5.8,
1313and from CPAN before then) is able to do this properly.
87275199 1314
19799a22 1315There can be whitespace between the operator and the quoting
fb73857a 1316characters, except when C<#> is being used as the quoting character.
19799a22
GS
1317C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
1318operator C<q> followed by a comment. Its argument will be taken
1319from the next line. This allows you to write:
fb73857a 1320
1321 s {foo} # Replace foo
1322 {bar} # with bar.
1323
c543c01b
TC
1324The following escape sequences are available in constructs that interpolate,
1325and in transliterations:
5691ca5f 1326X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N> X<\N{}>
04341565 1327X<\o{}>
5691ca5f 1328
2c4c1ff2
KW
1329 Sequence Note Description
1330 \t tab (HT, TAB)
1331 \n newline (NL)
1332 \r return (CR)
1333 \f form feed (FF)
1334 \b backspace (BS)
1335 \a alarm (bell) (BEL)
1336 \e escape (ESC)
c543c01b 1337 \x{263A} [1,8] hex char (example: SMILEY)
2c4c1ff2 1338 \x1b [2,8] restricted range hex char (example: ESC)
fb121860 1339 \N{name} [3] named Unicode character or character sequence
2c4c1ff2
KW
1340 \N{U+263D} [4,8] Unicode character (example: FIRST QUARTER MOON)
1341 \c[ [5] control char (example: chr(27))
1342 \o{23072} [6,8] octal char (example: SMILEY)
1343 \033 [7,8] restricted range octal char (example: ESC)
5691ca5f
KW
1344
1345=over 4
1346
1347=item [1]
1348
2c4c1ff2
KW
1349The result is the character specified by the hexadecimal number between
1350the braces. See L</[8]> below for details on which character.
96448467
DG
1351
1352Only hexadecimal digits are valid between the braces. If an invalid
1353character is encountered, a warning will be issued and the invalid
1354character and all subsequent characters (valid or invalid) within the
1356
1357If there are no valid digits between the braces, the generated character is
1358the NULL character (C<\x{00}>). However, an explicit empty brace (C<\x{}>)
c543c01b 1359will not cause a warning (currently).
40687185
KW
1360
1361=item [2]
1362
2c4c1ff2
KW
1363The result is the character specified by the hexadecimal number in the range
13640x00 to 0xFF. See L</[8]> below for details on which character.
96448467
DG
1365
1366Only hexadecimal digits are valid following C<\x>. When C<\x> is followed
2c4c1ff2 1367by fewer than two valid digits, any valid digits will be zero-padded. This
c543c01b 1368means that C<\x7> will be interpreted as C<\x07>, and a lone <\x> will be
2c4c1ff2 1369interpreted as C<\x00>. Except at the end of a string, having fewer than
c543c01b 1370two valid digits will result in a warning. Note that although the warning
96448467
DG
1371says the illegal character is ignored, it is only ignored as part of the
1372escape and will still be used as the subsequent character in the string.
1373For example:
1374
1375 Original Result Warns?
1376 "\x7" "\x07" no
1377 "\x" "\x00" no
1378 "\x7q" "\x07q" yes
1379 "\xq" "\x00q" yes
1380
40687185
KW
1381=item [3]
1382
fb121860 1383The result is the Unicode character or character sequence given by I<name>.
2c4c1ff2 1384See L<charnames>.
40687185
KW
1385
1386=item [4]
1387
2c4c1ff2
KW
1388C<\N{U+I<hexadecimal number>}> means the Unicode character whose Unicode code
40687185
KW
1390
1391=item [5]
1392
5691ca5f
KW
1393The character following C<\c> is mapped to some other character as shown in the
1394table:
1395
1396 Sequence Value
1397 \c@ chr(0)
1398 \cA chr(1)
1399 \ca chr(1)
1400 \cB chr(2)
1401 \cb chr(2)
1402 ...
1403 \cZ chr(26)
1404 \cz chr(26)
1405 \c[ chr(27)
1406 \c] chr(29)
1407 \c^ chr(30)
1408 \c? chr(127)
1409
d813941f
KW
1410In other words, it's the character whose code point has had 64 xor'd with
1411its uppercase. C<\c?> is DELETE because C<ord("@") ^ 64> is 127, and
1412C<\c@> is NULL because the ord of "@" is 64, so xor'ing 64 itself produces 0.
1413
5691ca5f
KW
1414Also, C<\c\I<X>> yields C< chr(28) . "I<X>"> for any I<X>, but cannot come at the
1415end of a string, because the backslash would be parsed as escaping the end
1416quote.
1417
1418On ASCII platforms, the resulting characters from the list above are the
1419complete set of ASCII controls. This isn't the case on EBCDIC platforms; see
1420L<perlebcdic/OPERATOR DIFFERENCES> for the complete list of what these
1421sequences mean on both ASCII and EBCDIC platforms.
1422
1423Use of any other character following the "c" besides those listed above is
17a3df4c 1424discouraged, and some are deprecated with the intention of removing
9fef6a0d 1425those in a later Perl version. What happens for any of these
d813941f
KW
1426other characters currently though, is that the value is derived by xor'ing
1427with the seventh bit, which is 64.
5691ca5f
KW
1428
1429To get platform independent controls, you can use C<\N{...}>.
1430
40687185
KW
1431=item [6]
1432
2c4c1ff2
KW
1433The result is the character specified by the octal number between the braces.
1434See L</[8]> below for details on which character.
04341565
DG
1435
1436If a character that isn't an octal digit is encountered, a warning is raised,
1437and the value is based on the octal digits before it, discarding it and all
1438following characters up to the closing brace. It is a fatal error if there are
1439no octal digits at all.
1440
1441=item [7]
1442
c543c01b 1443The result is the character specified by the three-digit octal number in the
2c4c1ff2
KW
1444range 000 to 777 (but best to not use above 077, see next paragraph). See
1445L</[8]> below for details on which character.
1446
1447Some contexts allow 2 or even 1 digit, but any usage without exactly
40687185 1448three digits, the first being a zero, may give unintended results. (For
5db3e519
FC
1449example, in a regular expression it may be confused with a backreference;
1450see L<perlrebackslash/Octal escapes>.) Starting in Perl 5.14, you may
c543c01b 1451use C<\o{}> instead, which avoids all these problems. Otherwise, it is best to
04341565
DG
1452use this construct only for ordinals C<\077> and below, remembering to pad to
1453the left with zeros to make three digits. For larger ordinals, either use
9fef6a0d 1454C<\o{}>, or convert to something else, such as to hex and use C<\x{}>
40687185 1456
40687185
KW
1457Having fewer than 3 digits may lead to a misleading warning message that says
1458that what follows is ignored. For example, C<"\128"> in the ASCII character set
1459is equivalent to the two characters C<"\n8">, but the warning C<Illegal octal
5db3e519
FC
1460digit '8' ignored> will be thrown. If C<"\n8"> is what you want, you can
5691ca5f 1462
2c4c1ff2
KW
1463=item [8]
1464
c543c01b 1465Several constructs above specify a character by a number. That number
2c4c1ff2 1466gives the character's position in the character set encoding (indexed from 0).
c543c01b 1467This is called synonymously its ordinal, code position, or code point. Perl
2c4c1ff2
KW
1468works on platforms that have a native encoding currently of either ASCII/Latin1
1469or EBCDIC, each of which allow specification of 256 characters. In general, if
1470the number is 255 (0xFF, 0377) or below, Perl interprets this in the platform's
1471native encoding. If the number is 256 (0x100, 0400) or above, Perl interprets
c543c01b 1472it as a Unicode code point and the result is the corresponding Unicode
2c4c1ff2
KW
1473character. For example C<\x{50}> and C<\o{120}> both are the number 80 in
1474decimal, which is less than 256, so the number is interpreted in the native
1475character set encoding. In ASCII the character in the 80th position (indexed
1476from 0) is the letter "P", and in EBCDIC it is the ampersand symbol "&".
1477C<\x{100}> and C<\o{400}> are both 256 in decimal, so the number is interpreted
1478as a Unicode code point no matter what the native encoding is. The name of the
9fef6a0d 1479character in the 256th position (indexed by 0) in Unicode is
2c4c1ff2
KW
1480C<LATIN CAPITAL LETTER A WITH MACRON>.
1481
9fef6a0d 1482There are a couple of exceptions to the above rule. S<C<\N{U+I<hex number>}>> is
2c4c1ff2
KW
1483always interpreted as a Unicode code point, so that C<\N{U+0050}> is "P" even
1484on EBCDIC platforms. And if L<C<S<use encoding>>|encoding> is in effect, the
1485number is considered to be in that encoding, and is translated from that into
1486the platform's native encoding if there is a corresponding native character;
1487otherwise to Unicode.
1488
5691ca5f 1489=back
4c77eaa2 1490
e526e8bb
KW
1491B<NOTE>: Unlike C and other languages, Perl has no C<\v> escape sequence for
1492the vertical tab (VT - ASCII 11), but you may use C<\ck> or C<\x0b>. (C<\v>
1493does have meaning in regular expression patterns in Perl, see L<perlre>.)
1494
1495The following escape sequences are available in constructs that interpolate,
904501ec 1496but not in transliterations.
628253b8 1497X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q> X<\F>
904501ec 1498
c543c01b
TC
1499 \l lowercase next character only
1500 \u titlecase (not uppercase!) next character only
e4d34742
EB
1501 \L lowercase all characters till \E or end of string
1502 \U uppercase all characters till \E or end of string
628253b8 1503 \F foldcase all characters till \E or end of string
736fe711
KW
1504 \Q quote (disable) pattern metacharacters till \E or
1505 end of string
7e31b643 1506 \E end either case modification or quoted section
c543c01b
TC
1507 (whichever was last seen)
1508
736fe711
KW
1509See L<perlfunc/quotemeta> for the exact definition of characters that
1510are quoted by C<\Q>.
1511
628253b8 1512C<\L>, C<\U>, C<\F>, and C<\Q> can stack, in which case you need one
c543c01b
TC
1513C<\E> for each. For example:
1514
9fef6a0d
KW
1515 say"This \Qquoting \ubusiness \Uhere isn't quite\E done yet,\E is it?";
1516 This quoting\ Business\ HERE\ ISN\'T\ QUITE\ done\ yet\, is it?
a0d0e21e 1517
66cbab2c
KW
1518If C<use locale> is in effect (but not C<use locale ':not_characters'>),
1519the case map used by C<\l>, C<\L>,
c543c01b 1520C<\u>, and C<\U> is taken from the current locale. See L<perllocale>.
b6538e4f 1521If Unicode (for example, C<\N{}> or code points of 0x100 or
c543c01b
TC
1522beyond) is being used, the case map used by C<\l>, C<\L>, C<\u>, and
1523C<\U> is as defined by Unicode. That means that case-mapping
1524a single character can sometimes produce several characters.
628253b8 1525Under C<use locale>, C<\F> produces the same results as C<\L>.
a034a98d 1526
5a964f20
TC
1527All systems use the virtual C<"\n"> to represent a line terminator,
1528called a "newline". There is no such thing as an unvarying, physical
19799a22 1529newline character. It is only an illusion that the operating system,
5a964f20
TC
1530device drivers, C libraries, and Perl all conspire to preserve. Not all
1531systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
c543c01b
TC
1532on the ancient Macs (pre-MacOS X) of yesteryear, these used to be reversed,
1533and on systems without line terminator,
1534printing C<"\n"> might emit no actual data. In general, use C<"\n"> when
5a964f20
TC
1535you mean a "newline" for your system, but use the literal ASCII when you
1536need an exact character. For example, most networking protocols expect
2a380090 1537and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
5a964f20
TC
1538and although they often accept just C<"\012">, they seldom tolerate just
1539C<"\015">. If you get in the habit of using C<"\n"> for networking,
1540you may be burned some day.
d74e8afc
ITB
1541X<newline> X<line terminator> X<eol> X<end of line>
1542X<\n> X<\r> X<\r\n>
5a964f20 1543
904501ec
MG
1544For constructs that do interpolate, variables beginning with "C<\$>"
1545or "C<@>" are interpolated. Subscripted variables such as C<\$a[3]> or
A
1546C<< \$href->{key}[0] >> are also interpolated, as are array and hash slices.
1547But method calls such as C<< \$obj->meth >> are not.
af9219ee
MG
1548
1549Interpolating an array or slice interpolates the elements in order,
1550separated by the value of C<\$">, so is equivalent to interpolating
c543c01b
TC
1551C<join \$", @array>. "Punctuation" arrays such as C<@*> are usually
1552interpolated only if the name is enclosed in braces C<@{*}>, but the
1553arrays C<@_>, C<@+>, and C<@-> are interpolated even without braces.
af9219ee 1554
bc7b91c6
EB
1555For double-quoted strings, the quoting from C<\Q> is applied after
1556interpolation and escapes are processed.
1557
1558 "abc\Qfoo\tbar\$s\Exyz"
1559
1560is equivalent to
1561
1562 "abc" . quotemeta("foo\tbar\$s") . "xyz"
1563
1564For the pattern of regex operators (C<qr//>, C<m//> and C<s///>),
1565the quoting from C<\Q> is applied after interpolation is processed,
1566but before escapes are processed. This allows the pattern to match
1567literally (except for C<\$> and C<@>). For example, the following matches:
1568
1569 '\s\t' =~ /\Q\s\t/
1570
1571Because C<\$> or C<@> trigger interpolation, you'll need to use something
1572like C</\Quser\E\@\Qhost/> to match them literally.
1d2dff63 1573
a0d0e21e
LW
1574Patterns are subject to an additional level of interpretation as a
1575regular expression. This is done as a second pass, after variables are
1576interpolated, so that regular expressions may be incorporated into the
1577pattern from the variables. If this is not what you want, use C<\Q> to
1578interpolate a variable literally.
1579
19799a22
GS
1580Apart from the behavior described above, Perl does not expand
1581multiple levels of interpolation. In particular, contrary to the
1582expectations of shell programmers, back-quotes do I<NOT> interpolate
1583within double quotes, nor do single quotes impede evaluation of
1584variables when used within double quotes.
a0d0e21e 1585
d74e8afc 1587X<operator, regexp>
cb1a09d0 1588
5f05dabc 1589Here are the quote-like operators that apply to pattern
cb1a09d0
1590matching and related activities.
1591
a0d0e21e
LW
1592=over 8
1593
b6fa137b 1594=item qr/STRING/msixpodual
01c6f5f4 1595X<qr> X</i> X</m> X</o> X</s> X</x> X</p>
a0d0e21e 1596
87e95b7f
YO
1597This operator quotes (and possibly compiles) its I<STRING> as a regular
1598expression. I<STRING> is interpolated the same way as I<PATTERN>
1599in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
1600is done. Returns a Perl value which may be used instead of the
f6050459 1601corresponding C</STRING/msixpodual> expression. The returned value is a
85dd5c8b 1602normalized version of the original pattern. It magically differs from
1c8ee595
CO
1603a string containing the same characters: C<ref(qr/x/)> returns "Regexp";
1604however, dereferencing it is not well defined (you currently get the
1605normalized version of the original pattern, but this may change).
1606
a0d0e21e 1607
87e95b7f
YO
1608For example,
1609
1610 \$rex = qr/my.STRING/is;
85dd5c8b 1611 print \$rex; # prints (?si-xm:my.STRING)
87e95b7f
YO
1612 s/\$rex/foo/;
1613
1614is equivalent to
1615
1616 s/my.STRING/foo/is;
1617
1618The result may be used as a subpattern in a match:
1619
1620 \$re = qr/\$pattern/;
1621 \$string =~ /foo\${re}bar/; # can be interpolated in other patterns
1622 \$string =~ \$re; # or used standalone
1623 \$string =~ /\$re/; # or this way
1624
f6050459 1625Since Perl may compile the pattern at the moment of execution of the qr()
87e95b7f
YO
1626operator, using qr() may have speed advantages in some situations,
1627notably if the result of qr() is used standalone:
1628
1629 sub match {
1630 my \$patterns = shift;
1631 my @compiled = map qr/\$_/i, @\$patterns;
1632 grep {
1633 my \$success = 0;
1634 foreach my \$pat (@compiled) {
1635 \$success = 1, last if /\$pat/;
1636 }
1637 \$success;
1638 } @_;
5a964f20
TC
1639 }
1640
87e95b7f
YO
1641Precompilation of the pattern into an internal representation at
1642the moment of qr() avoids a need to recompile the pattern every
1643time a match C</\$pat/> is attempted. (Perl has many other internal
1644optimizations, but none would be triggered in the above example if
1645we did not use qr() operator.)
1646
765fa144 1647Options (specified by the following modifiers) are:
87e95b7f
YO
1648
1649 m Treat string as multiple lines.
1650 s Treat string as single line. (Make . match a newline)
1651 i Do case-insensitive pattern matching.
1652 x Use extended regular expressions.
1653 p When matching preserve a copy of the matched string so
1654 that \${^PREMATCH}, \${^MATCH}, \${^POSTMATCH} will be defined.
1655 o Compile pattern only once.
b5c53fdc
KW
1656 a ASCII-restrict: Use ASCII for \d, \s, \w; specifying two a's
1657 further restricts /i matching so that no ASCII character will
1658 match a non-ASCII one
18509dec
KW
1659 l Use the locale
1660 u Use Unicode rules
b6fa137b 1661 d Use Unicode or native charset, as in 5.12 and earlier
87e95b7f
YO
1662
1663If a precompiled pattern is embedded in a larger pattern then the effect
c543c01b 1664of "msixpluad" will be propagated appropriately. The effect the "o"
87e95b7f
YO
1665modifier has is not propagated, being restricted to those patterns
1666explicitly using it.
1667
b6fa137b 1668The last four modifiers listed above, added in Perl 5.14,
18509dec
KW
1669control the character set semantics, but C</a> is the only one you are likely
1670to want to specify explicitly; the other three are selected
1671automatically by various pragmas.
da392a17 1672
87e95b7f 1673See L<perlre> for additional information on valid syntax for STRING, and
5e2aa8f5 1674for a detailed look at the semantics of regular expressions. In
1ca345ed
TC
1675particular, all modifiers except the largely obsolete C</o> are further
1676explained in L<perlre/Modifiers>. C</o> is described in the next section.
a0d0e21e 1677
b6fa137b 1678=item m/PATTERN/msixpodualgc
89d205f2
YO
1679X<m> X<operator, match>
1680X<regexp, options> X<regexp> X<regex, options> X<regex>
01c6f5f4 1681X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c>
a0d0e21e 1682
b6fa137b 1683=item /PATTERN/msixpodualgc
a0d0e21e 1684
5a964f20 1685Searches a string for a pattern match, and in scalar context returns
19799a22
GS
1686true if it succeeds, false if it fails. If no string is specified
1687via the C<=~> or C<!~> operator, the \$_ string is searched. (The
1688string specified with C<=~> need not be an lvalue--it may be the
1689result of an expression evaluation, but remember the C<=~> binds
a0d0e21e 1691
f6050459 1692Options are as described in C<qr//> above; in addition, the following match
01c6f5f4 1693process modifiers are available:
a0d0e21e 1694
950b09ed
KW
1695 g Match globally, i.e., find all occurrences.
1696 c Do not reset search position on a failed match when /g is in effect.
a0d0e21e 1697
725a61d7 1698If "/" is the delimiter then the initial C<m> is optional. With the C<m>
c543c01b 1699you can use any pair of non-whitespace (ASCII) characters
725a61d7
Z
1700as delimiters. This is particularly useful for matching path names
1701that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
1702the delimiter, then a match-only-once rule applies,
1703described in C<m?PATTERN?> below.
19799a22 1704If "'" is the delimiter, no interpolation is performed on the PATTERN.
ed02a3bf
DN
1705When using a character valid in an identifier, whitespace is required
1706after the C<m>.
a0d0e21e 1707
532c9e80
KW
1708PATTERN may contain variables, which will be interpolated
1709every time the pattern search is evaluated, except
1f247705
GS
1710for when the delimiter is a single quote. (Note that C<\$(>, C<\$)>, and
1711C<\$|> are not interpolated because they look like end-of-string tests.)
532c9e80
KW
1712Perl will not recompile the pattern unless an interpolated
1713variable that it contains changes. You can force Perl to skip the
1714test and never recompile by adding a C</o> (which stands for "once")
1715after the trailing delimiter.
1716Once upon a time, Perl would recompile regular expressions
1717unnecessarily, and this modifier was useful to tell it not to do so, in the
1718interests of speed. But now, the only reasons to use C</o> are either:
1719
1720=over
1721
1722=item 1
1723
1724The variables are thousands of characters long and you know that they
1725don't change, and you need to wring out the last little bit of speed by
1726having Perl skip testing for that. (There is a maintenance penalty for
1727doing this, as mentioning C</o> constitutes a promise that you won't
18509dec 1728change the variables in the pattern. If you do change them, Perl won't
532c9e80
KW
1729even notice.)
1730
1731=item 2
1732
1733you want the pattern to use the initial values of the variables
1734regardless of whether they change or not. (But there are saner ways
1735of accomplishing this than using C</o>.)
1736
1737=back
a0d0e21e 1738
18509dec
KW
1739The bottom line is that using C</o> is almost never a good idea.
1740
e9d89077
DN
1741=item The empty pattern //
1742
5a964f20 1743If the PATTERN evaluates to the empty string, the last
d65afb4b 1744I<successfully> matched regular expression is used instead. In this
c543c01b 1745case, only the C<g> and C<c> flags on the empty pattern are honored;
d65afb4b
HS
1746the other flags are taken from the original pattern. If no match has
1747previously succeeded, this will (silently) act instead as a genuine
1748empty pattern (which will always match).
a0d0e21e 1749
89d205f2
YO
1750Note that it's possible to confuse Perl into thinking C<//> (the empty
1751regex) is really C<//> (the defined-or operator). Perl is usually pretty
1753C<\$a///> (is that C<(\$a) / (//)> or C<\$a // />?) and C<print \$fh //>
1754(C<print \$fh(//> or C<print(\$fh //>?). In all of these examples, Perl
1755will assume you meant defined-or. If you meant the empty regex, just
1756use parentheses or spaces to disambiguate, or even prefix the empty
c963b151
BD
1757regex with an C<m> (so C<//> becomes C<m//>).
1758
e9d89077
DN
1759=item Matching in list context
1760
19799a22 1761If the C</g> option is not used, C<m//> in list context returns a
a0d0e21e 1762list consisting of the subexpressions matched by the parentheses in the
1ca345ed 1763pattern, that is, (C<\$1>, C<\$2>, C<\$3>...). (Note that here C<\$1> etc. are
f7e33566
GS
1764also set, and that this differs from Perl 4's behavior.) When there are
1765no parentheses in the pattern, the return value is the list C<(1)> for
1766success. With or without parentheses, an empty list is returned upon
1767failure.
a0d0e21e
LW
1768
1769Examples:
1770
58bafa60 1771 open(TTY, "+</dev/tty")
c543c01b
TC
1772 || die "can't access /dev/tty: \$!";
1773
a0d0e21e
LW
1774 <TTY> =~ /^y/i && foo(); # do foo if desired
1775
1776 if (/Version: *([0-9.]*)/) { \$version = \$1; }
1777
1778 next if m#^/usr/spool/uucp#;
1779
1780 # poor man's grep
1781 \$arg = shift;
1782 while (<>) {
c543c01b 1783 print if /\$arg/o; # compile only once (no longer needed!)
a0d0e21e
LW
1784 }
1785
1786 if ((\$F1, \$F2, \$Etc) = (\$foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
1787
1788This last example splits \$foo into the first two words and the
5f05dabc 1789remainder of the line, and assigns those three fields to \$F1, \$F2, and
c543c01b
TC
1790\$Etc. The conditional is true if any variables were assigned; that is,
1791if the pattern matched.
a0d0e21e 1792
19799a22 1793The C</g> modifier specifies global pattern matching--that is,
3dd93342 1794matching as many times as possible within the string. How it behaves
1795depends on the context. In list context, it returns a list of the
19799a22 1796substrings matched by any capturing parentheses in the regular
3dd93342 1797expression. If there are no parentheses, it returns a list of all
19799a22
GS
1798the matched strings, as if there were parentheses around the whole
1799pattern.
a0d0e21e 1800
7e86de3e 1801In scalar context, each execution of C<m//g> finds the next match,
19799a22 1802returning true if it matches, and false if there is no further match.
3dd93342 1803The position after the last match can be read or set using the C<pos()>
1804function; see L<perlfunc/pos>. A failed match normally resets the
7e86de3e 1805search position to the beginning of the string, but you can avoid that
1ca345ed 1806by adding the C</c> modifier (for example, C<m//gc>). Modifying the target
7e86de3e 1807string also resets the search position.
c90c0ff4 1808
e9d89077
DN
1809=item \G assertion
1810
c90c0ff4 1811You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
3dd93342 1812zero-width assertion that matches the exact position where the
1813previous C<m//g>, if any, left off. Without the C</g> modifier, the
1814C<\G> assertion still anchors at C<pos()> as it was at the start of
1815the operation (see L<perlfunc/pos>), but the match is of course only
1816attempted once. Using C<\G> without C</g> on a target string that has
1817not previously had a C</g> match applied to it is the same as using
1818the C<\A> assertion to match the beginning of the string. Note also
1819that, currently, C<\G> is only properly supported when anchored at the
1820very beginning of the pattern.
c90c0ff4 1821
1822Examples:
a0d0e21e
LW
1823
1824 # list context
1825 (\$one,\$five,\$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
1826
1827 # scalar context
c543c01b
TC
1828 local \$/ = "";
1829 while (\$paragraph = <>) {
1830 while (\$paragraph =~ /\p{Ll}['")]*[.!?]+['")]*\s/g) {
19799a22 1831 \$sentences++;
a0d0e21e
LW
1832 }
1833 }
c543c01b
TC
1834 say \$sentences;
1835
1836Here's another way to check for sentences in a paragraph:
1837
1838 my \$sentence_rx = qr{
1839 (?: (?<= ^ ) | (?<= \s ) ) # after start-of-string or whitespace
1840 \p{Lu} # capital letter
1841 .*? # a bunch of anything
1842 (?<= \S ) # that ends in non-whitespace
1843 (?<! \b [DMS]r ) # but isn't a common abbreviation
1844 (?<! \b Mrs )
1845 (?<! \b Sra )
1846 (?<! \b St )
1847 [.?!] # followed by a sentence ender
1848 (?= \$ | \s ) # in front of end-of-string or whitespace
1849 }sx;
1850 local \$/ = "";
1851 while (my \$paragraph = <>) {
1852 say "NEW PARAGRAPH";
1853 my \$count = 0;
1854 while (\$paragraph =~ /(\$sentence_rx)/g) {
1855 printf "\tgot sentence %d: <%s>\n", ++\$count, \$1;
1856 }
1857 }
1858
1859Here's how to use C<m//gc> with C<\G>:
a0d0e21e 1860
137443ea 1861 \$_ = "ppooqppqq";
44a8e56a 1862 while (\$i++ < 2) {
1863 print "1: '";
c90c0ff4 1864 print \$1 while /(o)/gc; print "', pos=", pos, "\n";
44a8e56a 1865 print "2: '";
c90c0ff4 1866 print \$1 if /\G(q)/gc; print "', pos=", pos, "\n";
44a8e56a 1867 print "3: '";
c90c0ff4 1868 print \$1 while /(p)/gc; print "', pos=", pos, "\n";
44a8e56a 1869 }
5d43e42d 1870 print "Final: '\$1', pos=",pos,"\n" if /\G(.)/;
44a8e56a 1871
1872The last example should print:
1873
1874 1: 'oo', pos=4
137443ea 1875 2: 'q', pos=5
44a8e56a 1876 3: 'pp', pos=7
1877 1: '', pos=7
137443ea 1878 2: 'q', pos=8
1879 3: '', pos=8
5d43e42d
DC
1880 Final: 'q', pos=8
1881
1882Notice that the final match matched C<q> instead of C<p>, which a match
1883without the C<\G> anchor would have done. Also note that the final match
ac036724 1884did not update C<pos>. C<pos> is only updated on a C</g> match. If the
c543c01b
TC
1885final match did indeed match C<p>, it's a good bet that you're running a
1886very old (pre-5.6.0) version of Perl.
44a8e56a 1887
c90c0ff4 1888A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
e7ea3e70 1889combine several regexps like this to process a string part-by-part,
c90c0ff4 1890doing different actions depending on which regexp matched. Each
1891regexp tries to match where the previous one leaves off.
e7ea3e70 1892
3fe9a6f1 1893 \$_ = <<'EOL';
950b09ed 1894 \$url = URI::URL->new( "http://example.com/" ); die if \$url eq "xXx";
3fe9a6f1 1895 EOL
c543c01b
TC
1896
1897 LOOP: {
950b09ed 1898 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
c543c01b
TC
1899 print(" lowercase"), redo LOOP if /\G\p{Ll}+\b[,.;]?\s*/gc;
1900 print(" UPPERCASE"), redo LOOP if /\G\p{Lu}+\b[,.;]?\s*/gc;
1901 print(" Capitalized"), redo LOOP if /\G\p{Lu}\p{Ll}+\b[,.;]?\s*/gc;
1902 print(" MiXeD"), redo LOOP if /\G\pL+\b[,.;]?\s*/gc;
1903 print(" alphanumeric"), redo LOOP if /\G[\p{Alpha}\pN]+\b[,.;]?\s*/gc;
1904 print(" line-noise"), redo LOOP if /\G\W+/gc;
950b09ed 1905 print ". That's all!\n";
c543c01b 1906 }
e7ea3e70
IZ
1907
1908Here is the output (split into several lines):
1909
c543c01b
TC
1910 line-noise lowercase line-noise UPPERCASE line-noise UPPERCASE
1911 line-noise lowercase line-noise lowercase line-noise lowercase
1912 lowercase line-noise lowercase lowercase line-noise lowercase
1913 lowercase line-noise MiXeD line-noise. That's all!
44a8e56a 1914
c543c01b 1915=item m?PATTERN?msixpodualgc
725a61d7 1916X<?> X<operator, match-once>
87e95b7f 1917
c543c01b 1918=item ?PATTERN?msixpodualgc
55d389e7 1919
725a61d7
Z
1920This is just like the C<m/PATTERN/> search, except that it matches
1921only once between calls to the reset() operator. This is a useful
87e95b7f 1922optimization when you want to see only the first occurrence of
ceb131e8 1923something in each file of a set of files, for instance. Only C<m??>
87e95b7f
YO
1924patterns local to the current package are reset.
1925
1926 while (<>) {
ceb131e8 1927 if (m?^\$?) {
87e95b7f
YO
1928 # blank line between header and body
1929 }
1930 } continue {
725a61d7 1931 reset if eof; # clear m?? status for next file
87e95b7f
YO
1932 }
1933
c543c01b
TC
1934Another example switched the first "latin1" encoding it finds
1935to "utf8" in a pod file:
1936
1937 s//utf8/ if m? ^ =encoding \h+ \K latin1 ?x;
1938
1939The match-once behavior is controlled by the match delimiter being
725a61d7
Z
1940C<?>; with any other delimiter this is the normal C<m//> operator.
1941
1942For historical reasons, the leading C<m> in C<m?PATTERN?> is optional,
1943but the resulting C<?PATTERN?> syntax is deprecated, will warn on
c543c01b
TC
1944usage and might be removed from a future stable release of Perl (without
1945further notice!).
87e95b7f 1946
b6fa137b 1947=item s/PATTERN/REPLACEMENT/msixpodualgcer
87e95b7f 1948X<substitute> X<substitution> X<replace> X<regexp, replace>
4f4d7508 1949X<regexp, substitute> X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> X</e> X</r>
87e95b7f
YO
1950
1951Searches a string for a pattern, and if found, replaces that pattern
1952with the replacement text and returns the number of substitutions
1953made. Otherwise it returns false (specifically, the empty string).
1954
c543c01b 1955If the C</r> (non-destructive) option is used then it runs the
679563bb
KW
1956substitution on a copy of the string and instead of returning the
1957number of substitutions, it returns the copy whether or not a
c543c01b
TC
1958substitution occurred. The original string is never changed when
1959C</r> is used. The copy will always be a plain string, even if the
1960input is an object or a tied variable.
4f4d7508 1961
87e95b7f 1962If no string is specified via the C<=~> or C<!~> operator, the C<\$_>
c543c01b
TC
1963variable is searched and modified. Unless the C</r> option is used,
1964the string specified must be a scalar variable, an array element, a
1965hash element, or an assignment to one of those; that is, some sort of
1966scalar lvalue.
87e95b7f
YO
1967
1968If the delimiter chosen is a single quote, no interpolation is
1969done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1970PATTERN contains a \$ that looks like a variable rather than an
1971end-of-string test, the variable will be interpolated into the pattern
1972at run-time. If you want the pattern compiled only once the first time
1973the variable is interpolated, use the C</o> option. If the pattern
1974evaluates to the empty string, the last successfully executed regular
1975expression is used instead. See L<perlre> for further explanation on these.
87e95b7f
YO
1976
1977Options are as with m// with the addition of the following replacement
1978specific options:
1979
1980 e Evaluate the right side as an expression.
4f4d7508
DC
1981 ee Evaluate the right side as a string then eval the result.
1982 r Return substitution and leave the original string untouched.
87e95b7f 1983
ed02a3bf
DN
1984Any non-whitespace delimiter may replace the slashes. Add space after
1985the C<s> when using a character allowed in identifiers. If single quotes
1986are used, no interpretation is done on the replacement string (the C</e>
1987modifier overrides this, however). Unlike Perl 4, Perl 5 treats backticks
1988as normal delimiters; the replacement text is not evaluated as a command.
1989If the PATTERN is delimited by bracketing quotes, the REPLACEMENT has
1ca345ed 1990its own pair of quotes, which may or may not be bracketing quotes, for example,
87e95b7f
YO
1991C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
1992replacement portion to be treated as a full-fledged Perl expression
1993and evaluated right then and there. It is, however, syntax checked at
1994compile-time. A second C<e> modifier will cause the replacement portion
1995to be C<eval>ed before being run as a Perl expression.
1996
1997Examples:
1998
1999 s/\bgreen\b/mauve/g; # don't change wintergreen
2000
2001 \$path =~ s|/usr/bin|/usr/local/bin|;
2002
2004
2005 (\$foo = \$bar) =~ s/this/that/; # copy first, then change
4f4d7508
DC
2006 (\$foo = "\$bar") =~ s/this/that/; # convert to string, copy, then change
2007 \$foo = \$bar =~ s/this/that/r; # Same as above using /r
2008 \$foo = \$bar =~ s/this/that/r
2009 =~ s/that/the other/r; # Chained substitutes using /r
2010 @foo = map { s/this/that/r } @bar # /r is very useful in maps
87e95b7f
YO
2011
2012 \$count = (\$paragraph =~ s/Mister\b/Mr./g); # get change-count
2013
2014 \$_ = 'abc123xyz';
2015 s/\d+/\$&*2/e; # yields 'abc246xyz'
2016 s/\d+/sprintf("%5d",\$&)/e; # yields 'abc 246xyz'
2017 s/\w/\$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
2018
2019 s/%(.)/\$percent{\$1}/g; # change percent escapes; no /e
2020 s/%(.)/\$percent{\$1} || \$&/ge; # expr now, so /e
2021 s/^=(\w+)/pod(\$1)/ge; # use function call
2022
4f4d7508
DC
2023 \$_ = 'abc123xyz';
2024 \$a = s/abc/def/r; # \$a is 'def123xyz' and
2025 # \$_ remains 'abc123xyz'.
2026
87e95b7f
YO
2027 # expand variables in \$_, but dynamics only, using
2028 # symbolic dereferencing
2029 s/\\$(\w+)/\${\$1}/g;
2030
2031 # Add one to the value of any numbers in the string
2032 s/(\d+)/1 + \$1/eg;
2033
c543c01b
TC
2034 # Titlecase words in the last 30 characters only
2035 substr(\$str, -30) =~ s/\b(\p{Alpha}+)\b/\u\L\$1/g;
2036
87e95b7f
YO
2037 # This will expand any embedded scalar variable
2038 # (including lexicals) in \$_ : First \$1 is interpolated
2039 # to the variable name, and then evaluated
2040 s/(\\$\w+)/\$1/eeg;
2041
2042 # Delete (most) C comments.
2043 \$program =~ s {
2044 /\* # Match the opening delimiter.
2045 .*? # Match a minimal number of characters.
2046 \*/ # Match the closing delimiter.
2047 } []gsx;
2048
2049 s/^\s*(.*?)\s*\$/\$1/; # trim whitespace in \$_, expensively
2050
2051 for (\$variable) { # trim whitespace in \$variable, cheap
2052 s/^\s+//;
2053 s/\s+\$//;
2054 }
2055
2056 s/([^ ]*) *([^ ]*)/\$2 \$1/; # reverse 1st two fields
2057
2058Note the use of \$ instead of \ in the last example. Unlike
2059B<sed>, we use the \<I<digit>> form in only the left hand side.
2060Anywhere else it's \$<I<digit>>.
2061
2062Occasionally, you can't use just a C</g> to get all the changes
2063to occur that you might want. Here are two common cases:
2064
2065 # put commas in the right places in an integer
2066 1 while s/(\d)(\d\d\d)(?!\d)/\$1,\$2/g;
2067
2068 # expand tabs to 8-column spacing
2069 1 while s/\t+/' ' x (length(\$&)*8 - length(\$`)%8)/e;
2070
1c424184
FC
2071C<s///le> is treated as a substitution followed by the C<le> operator, not
2072the C</le> flags. This may change in a future version of Perl. It
2073produces a warning if warnings are enabled. To disambiguate, use a space
2074or change the order of the flags:
2075
2076 s/foo/bar/ le 5; # "le" infix operator
2077 s/foo/bar/el; # "e" and "l" flags
2078
87e95b7f
YO
2079=back
2080
2082X<operator, quote-like>
2083
01c6f5f4
RGS
2084=over 4
2085
a0d0e21e 2086=item q/STRING/
5d44bfff 2087X<q> X<quote, single> X<'> X<''>
a0d0e21e 2088
5d44bfff 2089=item 'STRING'
a0d0e21e 2090
19799a22 2091A single-quoted, literal string. A backslash represents a backslash
68dc0745 2092unless followed by the delimiter or another backslash, in which case
2093the delimiter or backslash is interpolated.
a0d0e21e
LW
2094
2095 \$foo = q!I said, "You said, 'She said it.'"!;
2096 \$bar = q('This is it.');
68dc0745 2097 \$baz = '\n'; # a two-character string
a0d0e21e
LW
2098
2099=item qq/STRING/
d74e8afc 2100X<qq> X<quote, double> X<"> X<"">
a0d0e21e
LW
2101
2102=item "STRING"
2103
2104A double-quoted, interpolated string.
2105
2106 \$_ .= qq
2107 (*** The previous line contains the naughty word "\$1".\n)
19799a22 2108 if /\b(tcl|java|python)\b/i; # :-)
68dc0745 2109 \$baz = "\n"; # a one-character string
a0d0e21e
LW
2110
2111=item qx/STRING/
d74e8afc 2112X<qx> X<`> X<``> X<backtick>
a0d0e21e
LW
2113
2114=item `STRING`
2115
43dd4d21
JH
2116A string which is (possibly) interpolated and then executed as a
2117system command with C</bin/sh> or its equivalent. Shell wildcards,
2118pipes, and redirections will be honored. The collected standard
2119output of the command is returned; standard error is unaffected. In
2120scalar context, it comes back as a single (potentially multi-line)
2121string, or undef if the command failed. In list context, returns a
2122list of lines (however you've defined lines with \$/ or
2123\$INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
5a964f20
TC
2124
2125Because backticks do not affect standard error, use shell file descriptor
2126syntax (assuming the shell supports this) if you care to address this.
2127To capture a command's STDERR and STDOUT together:
a0d0e21e 2128
5a964f20
TC
2129 \$output = `cmd 2>&1`;
2130
2131To capture a command's STDOUT but discard its STDERR:
2132
2133 \$output = `cmd 2>/dev/null`;
2134
2135To capture a command's STDERR but discard its STDOUT (ordering is
2136important here):
2137
2138 \$output = `cmd 2>&1 1>/dev/null`;
2139
2140To exchange a command's STDOUT and STDERR in order to capture the STDERR
2141but leave its STDOUT to come out the old STDERR:
2142
2143 \$output = `cmd 3>&1 1>&2 2>&3 3>&-`;
2144
2145To read both a command's STDOUT and its STDERR separately, it's easiest
2359510d
SD
2146to redirect them separately to files, and then read from those files
2147when the program is done:
5a964f20 2148
2359510d 2149 system("program args 1>program.stdout 2>program.stderr");
5a964f20 2150
30398227
SP
2151The STDIN filehandle used by the command is inherited from Perl's STDIN.
2152For example:
2153
c543c01b
TC
2154 open(SPLAT, "stuff") || die "can't open stuff: \$!";
2155 open(STDIN, "<&SPLAT") || die "can't dupe SPLAT: \$!";
40bbb707 2156 print STDOUT `sort`;
30398227 2157
40bbb707 2158will print the sorted contents of the file named F<"stuff">.
30398227 2159
5a964f20
TC
2160Using single-quote as a delimiter protects the command from Perl's
2161double-quote interpolation, passing it on to the shell instead:
2162
2163 \$perl_info = qx(ps \$\$); # that's Perl's \$\$
2164 \$shell_info = qx'ps \$\$'; # that's the new shell's \$\$
2165
19799a22 2166How that string gets evaluated is entirely subject to the command
5a964f20
TC
2167interpreter on your system. On most platforms, you will have to protect
2168shell metacharacters if you want them treated literally. This is in
2169practice difficult to do, as it's unclear how to escape which characters.
2170See L<perlsec> for a clean and safe example of a manual fork() and exec()
2171to emulate backticks safely.
a0d0e21e 2172
bb32b41a
GS
2173On some platforms (notably DOS-like ones), the shell may not be
2174capable of dealing with multiline commands, so putting newlines in
2175the string may not get you what you want. You may be able to evaluate
2176multiple commands in a single line by separating them with the command
1ca345ed
TC
2177separator character, if your shell supports that (for example, C<;> on
2178many Unix shells and C<&> on the Windows NT C<cmd> shell).
bb32b41a 2179
0f897271
GS
2180Beginning with v5.6.0, Perl will attempt to flush all files opened for
2181output before starting the child process, but this may not be supported
2182on some platforms (see L<perlport>). To be safe, you may need to set
2183C<\$|> (\$AUTOFLUSH in English) or call the C<autoflush()> method of
2184C<IO::Handle> on any open handles.
2185
bb32b41a
GS
2186Beware that some command shells may place restrictions on the length
2187of the command line. You must ensure your strings don't exceed this
2188limit after any necessary interpolations. See the platform-specific
2190
5a964f20
TC
2191Using this operator can lead to programs that are difficult to port,
2192because the shell commands called vary between systems, and may in
2193fact not be present at all. As one example, the C<type> command under
2194the POSIX shell is very different from the C<type> command under DOS.
2195That doesn't mean you should go out of your way to avoid backticks
2196when they're the right way to get something done. Perl was made to be
2197a glue language, and one of the things it glues together is commands.
2198Just understand what you're getting yourself into.
bb32b41a 2199
da87341d 2200See L</"I/O Operators"> for more discussion.
a0d0e21e 2201
945c54fd 2202=item qw/STRING/
d74e8afc 2203X<qw> X<quote, list> X<quote, words>
945c54fd
JH
2204
2205Evaluates to a list of the words extracted out of STRING, using embedded
2206whitespace as the word delimiters. It can be understood as being roughly
2207equivalent to:
2208
c543c01b 2209 split(" ", q/STRING/);
945c54fd 2210
efb1e162
CW
2211the differences being that it generates a real list at compile time, and
2212in scalar context it returns the last element in the list. So
945c54fd
JH
2213this expression:
2214
2215 qw(foo bar baz)
2216
2217is semantically equivalent to the list:
2218
c543c01b 2219 "foo", "bar", "baz"
945c54fd
JH
2220
2221Some frequently seen examples:
2222
2223 use POSIX qw( setlocale localeconv )
2224 @EXPORT = qw( foo bar baz );
2225
2226A common mistake is to try to separate the words with comma or to
2227put comments into a multi-line C<qw>-string. For this reason, the
89d205f2 2228C<use warnings> pragma and the B<-w> switch (that is, the C<\$^W> variable)
945c54fd
JH
2229produces warnings if the STRING contains the "," or the "#" character.
2230
8ff32507 2231=item tr/SEARCHLIST/REPLACEMENTLIST/cdsr
d74e8afc 2232X<tr> X<y> X<transliterate> X</c> X</d> X</s>
a0d0e21e 2233
8ff32507 2234=item y/SEARCHLIST/REPLACEMENTLIST/cdsr
a0d0e21e 2235
2c268ad5 2236Transliterates all occurrences of the characters found in the search list
a0d0e21e
LW
2237with the corresponding character in the replacement list. It returns
2238the number of characters replaced or deleted. If no string is
c543c01b
TC
2239specified via the C<=~> or C<!~> operator, the \$_ string is transliterated.
2240
2241If the C</r> (non-destructive) option is present, a new copy of the string
2242is made and its characters transliterated, and this copy is returned no
2243matter whether it was modified or not: the original string is always
2244left unchanged. The new copy is always a plain string, even if the input
2245string is an object or a tied variable.
c543c01b
TC
2247Unless the C</r> option is used, the string specified with C<=~> must be a
2248scalar variable, an array element, a hash element, or an assignment to one
2249of those; in other words, an lvalue.
8ff32507 2250
89d205f2 2251A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
2c268ad5 2252does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
54310121 2253For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
2254SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
c543c01b
TC
2255its own pair of quotes, which may or may not be bracketing quotes;
2256for example, C<tr[aeiouy][yuoiea]> or C<tr(+\-*/)/ABCD/>.
2257
2258Note that C<tr> does B<not> do regular expression character classes such as
2259C<\d> or C<\pL>. The C<tr> operator is not equivalent to the tr(1)
2260utility. If you want to map strings between lower/upper cases, see
2261L<perlfunc/lc> and L<perlfunc/uc>, and in general consider using the C<s>
2262operator if you need regular expressions. The C<\U>, C<\u>, C<\L>, and
2263C<\l> string-interpolation escapes on the right side of a substitution
2264operator will perform correct case-mappings, but C<tr[a-z][A-Z]> will not
2265(except sometimes on legacy 7-bit data).
cc255d5f 2266
JH
2267Note also that the whole range idea is rather unportable between
2268character sets--and even within character sets they may cause results
2269you probably didn't expect. A sound principle is to use only ranges
2270that begin from and end at either alphabets of equal case (a-e, A-E),
2271or digits (0-4). Anything else is unsafe. If in doubt, spell out the
2272character sets in full.
2273
a0d0e21e
LW
2274Options:
2275
2276 c Complement the SEARCHLIST.
2277 d Delete found but unreplaced characters.
2278 s Squash duplicate replaced characters.
8ff32507
FC
2279 r Return the modified string and leave the original string
2280 untouched.
a0d0e21e 2281
19799a22
GS
2282If the C</c> modifier is specified, the SEARCHLIST character set
2283is complemented. If the C</d> modifier is specified, any characters
2285(Note that this is slightly more flexible than the behavior of some
2286B<tr> programs, which delete anything they find in the SEARCHLIST,
2287period.) If the C</s> modifier is specified, sequences of characters
2288that were transliterated to the same character are squashed down
2289to a single instance of the character.
a0d0e21e
LW
2290
2291If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
2292exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
2293than the SEARCHLIST, the final character is replicated till it is long
5a964f20 2294enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
a0d0e21e
LW
2295This latter is useful for counting characters in a class or for
2296squashing character sequences in a class.
2297
2298Examples:
2299
c543c01b 2300 \$ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case ASCII
a0d0e21e
LW
2301
2302 \$cnt = tr/*/*/; # count the stars in \$_
2303
2304 \$cnt = \$sky =~ tr/*/*/; # count the stars in \$sky
2305
2306 \$cnt = tr/0-9//; # count the digits in \$_
2307
2308 tr/a-zA-Z//s; # bookkeeper -> bokeper
2309
2310 (\$HOST = \$host) =~ tr/a-z/A-Z/;
c543c01b 2311 \$HOST = \$host =~ tr/a-z/A-Z/r; # same thing
8ff32507 2312
c543c01b 2313 \$HOST = \$host =~ tr/a-z/A-Z/r # chained with s///r
8ff32507 2314 =~ s/:/ -p/r;
a0d0e21e
LW
2315
2316 tr/a-zA-Z/ /cs; # change non-alphas to single space
2317
8ff32507
FC
2318 @stripped = map tr/a-zA-Z/ /csr, @original;
2319 # /r with map
2320
a0d0e21e 2321 tr [\200-\377]
c543c01b 2322 [\000-\177]; # wickedly delete 8th bit
a0d0e21e 2323
19799a22
GS
2324If multiple transliterations are given for a character, only the
2325first one is used:
748a9306
LW
2326
2327 tr/AAA/XYZ/
2328
2c268ad5 2329will transliterate any A to X.
748a9306 2330
19799a22 2331Because the transliteration table is built at compile time, neither
a0d0e21e 2332the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
19799a22
GS
2333interpolation. That means that if you want to use variables, you
2334must use an eval():
a0d0e21e
LW
2335
2336 eval "tr/\$oldlist/\$newlist/";
2337 die \$@ if \$@;
2338
2339 eval "tr/\$oldlist/\$newlist/, 1" or die \$@;
2340
7e3b091d 2341=item <<EOF
d74e8afc 2342X<here-doc> X<heredoc> X<here-document> X<<< << >>>
7e3b091d
DA
2343
2344A line-oriented form of quoting is based on the shell "here-document"
2345syntax. Following a C<< << >> you specify a string to terminate
2346the quoted material, and all lines following the current line down to
89d205f2
YO
2347the terminating string are the value of the item.
2348
2349The terminating string may be either an identifier (a word), or some
2350quoted text. An unquoted identifier works like double quotes.
2351There may not be a space between the C<< << >> and the identifier,
2352unless the identifier is explicitly quoted. (If you put a space it
2353will be treated as a null identifier, which is valid, and matches the
2354first empty line.) The terminating string must appear by itself
2355(unquoted and with no surrounding whitespace) on the terminating line.
2356
2357If the terminating string is quoted, the type of quotes used determine
2358the treatment of the text.
2359
2360=over 4
2361
2362=item Double Quotes
2363
2364Double quotes indicate that the text will be interpolated using exactly
2365the same rules as normal double quoted strings.
7e3b091d
DA
2366
2367 print <<EOF;
2368 The price is \$Price.
2369 EOF
2370
2371 print << "EOF"; # same as above
2372 The price is \$Price.
2373 EOF
2374
89d205f2
YO
2375
2376=item Single Quotes
2377
2378Single quotes indicate the text is to be treated literally with no
2379interpolation of its content. This is similar to single quoted
2380strings except that backslashes have no special meaning, with C<\\>
2381being treated as two backslashes and not one as they would in every
2382other quoting construct.
2383
c543c01b
TC
2384Just as in the shell, a backslashed bareword following the C<<< << >>>
2385means the same thing as a single-quoted string does:
2386
2387 \$cost = <<'VISTA'; # hasta la ...
2388 That'll be \$10 please, ma'am.
2389 VISTA
2390
2391 \$cost = <<\VISTA; # Same thing!
2392 That'll be \$10 please, ma'am.
2393 VISTA
2394
89d205f2
YO
2395This is the only form of quoting in perl where there is no need
2396to worry about escaping content, something that code generators
2397can and do make good use of.
2398
2399=item Backticks
2400
2401The content of the here doc is treated just as it would be if the
2402string were embedded in backticks. Thus the content is interpolated
2403as though it were double quoted and then executed via the shell, with
2404the results of the execution returned.
2405
2406 print << `EOC`; # execute command and get results
7e3b091d 2407 echo hi there
7e3b091d
DA
2408 EOC
2409
89d205f2
YO
2410=back
2411
2412It is possible to stack multiple here-docs in a row:
2413
7e3b091d
DA
2414 print <<"foo", <<"bar"; # you can stack them
2415 I said foo.
2416 foo
2417 I said bar.
2418 bar
2419
2420 myfunc(<< "THIS", 23, <<'THAT');
2421 Here's a line
2422 or two.
2423 THIS
2424 and here's another.
2425 THAT
2426
2427Just don't forget that you have to put a semicolon on the end
2428to finish the statement, as Perl doesn't know you're not going to
2429try to do this:
2430
2431 print <<ABC
2432 179231
2433 ABC
2434 + 20;
2435
872d7e53
TS
2436If you want to remove the line terminator from your here-docs,
2437use C<chomp()>.
2438
2439 chomp(\$string = <<'END');
2440 This is a string.
2441 END
2442
2443If you want your here-docs to be indented with the rest of the code,
2444you'll need to remove leading whitespace from each line manually:
7e3b091d
DA
2445
2446 (\$quote = <<'FINIS') =~ s/^\s+//gm;
89d205f2 2447 The Road goes ever on and on,
7e3b091d
DA
2448 down from the door where it began.
2449 FINIS
2450
2451If you use a here-doc within a delimited construct, such as in C<s///eg>,
2452the quoted material must come on the lines following the final delimiter.
2454
2455 s/this/<<E . 'that'
2456 the other
2457 E
2458 . 'more '/eg;
2459
2460you have to write
2461
89d205f2
YO
2462 s/this/<<E . 'that'
2463 . 'more '/eg;
2464 the other
2465 E
7e3b091d
DA
2466
2467If the terminating identifier is on the last line of the program, you
2468must be sure there is a newline after it; otherwise, Perl will give the
2469warning B<Can't find string terminator "END" anywhere before EOF...>.
2470
c543c01b
TC
2471Additionally, quoting rules for the end-of-string identifier are
2472unrelated to Perl's quoting rules. C<q()>, C<qq()>, and the like are not
89d205f2
YO
2473supported in place of C<''> and C<"">, and the only interpolation is for
2474backslashing the quoting character:
7e3b091d
DA
2475
2476 print << "abc\"def";
2477 testing...
2478 abc"def
2479
2480Finally, quoted strings cannot span multiple lines. The general rule is
2481that the identifier must be a string literal. Stick with that, and you
2482should be safe.
2483
a0d0e21e
LW
2484=back
2485
75e14d17 2486=head2 Gory details of parsing quoted constructs
d74e8afc 2487X<quote, gory details>
75e14d17 2488
19799a22
GS
2489When presented with something that might have several different
2490interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
2491principle to pick the most probable interpretation. This strategy
2492is so successful that Perl programmers often do not suspect the
2493ambivalence of what they write. But from time to time, Perl's
2494notions differ substantially from what the author honestly meant.
2495
2496This section hopes to clarify how Perl handles quoted constructs.
2497Although the most common reason to learn this is to unravel labyrinthine
2498regular expressions, because the initial steps of parsing are the
2499same for all quoting operators, they are all discussed together.
2500
2501The most important Perl parsing rule is the first one discussed
2502below: when processing a quoted construct, Perl first finds the end
2503of that construct, then interprets its contents. If you understand
2504this rule, you may skip the rest of this section on the first
2506expectations much less frequently than this first one.
2507
2508Some passes discussed below are performed concurrently, but because
2509their results are the same, we consider them individually. For different
2510quoting constructs, Perl performs different numbers of passes, from
6deea57f 2511one to four, but these passes are always performed in the same order.
75e14d17 2512
13a2d996 2513=over 4
75e14d17
IZ
2514
2515=item Finding the end
2516
6deea57f
TS
2517The first pass is finding the end of the quoted construct, where
2518the information about the delimiters is used in parsing.
2519During this search, text between the starting and ending delimiters
2520is copied to a safe location. The text copied gets delimiter-independent.
2521
2522If the construct is a here-doc, the ending delimiter is a line
2523that has a terminating string as the content. Therefore C<<<EOF> is
2524terminated by C<EOF> immediately followed by C<"\n"> and starting
2525from the first column of the terminating line.
2526When searching for the terminating line of a here-doc, nothing
2527is skipped. In other words, lines after the here-doc syntax
2528are compared with the terminating string line by line.
2529
2530For the constructs except here-docs, single characters are used as starting
2531and ending delimiters. If the starting delimiter is an opening punctuation
2532(that is C<(>, C<[>, C<{>, or C<< < >>), the ending delimiter is the
2533corresponding closing punctuation (that is C<)>, C<]>, C<}>, or C<< > >>).
2534If the starting delimiter is an unpaired character like C</> or a closing
2535punctuation, the ending delimiter is same as the starting delimiter.
2536Therefore a C</> terminates a C<qq//> construct, while a C<]> terminates
2537C<qq[]> and C<qq]]> constructs.
2538
2539When searching for single-character delimiters, escaped delimiters
1ca345ed 2540and C<\\> are skipped. For example, while searching for terminating C</>,
6deea57f
TS
2541combinations of C<\\> and C<\/> are skipped. If the delimiters are
2542bracketing, nested pairs are also skipped. For example, while searching
2543for closing C<]> paired with the opening C<[>, combinations of C<\\>, C<\]>,
2544and C<\[> are all skipped, and nested C<[> and C<]> are skipped as well.
2545However, when backslashes are used as the delimiters (like C<qq\\> and
2546C<tr\\\>), nothing is skipped.
32581033
A
2547During the search for the end, backslashes that escape delimiters or
2548backslashes are removed (exactly speaking, they are not copied to the
2549safe location).
75e14d17 2550
19799a22
GS
2551For constructs with three-part delimiters (C<s///>, C<y///>, and
2552C<tr///>), the search is repeated once more.
6deea57f
TS
2553If the first delimiter is not an opening punctuation, three delimiters must
2554be same such as C<s!!!> and C<tr)))>, in which case the second delimiter
2555terminates the left part and starts the right part at once.
b6538e4f 2556If the left part is delimited by bracketing punctuation (that is C<()>,
6deea57f 2557C<[]>, C<{}>, or C<< <> >>), the right part needs another pair of
b6538e4f 2558delimiters such as C<s(){}> and C<tr[]//>. In these cases, whitespace
6deea57f 2559and comments are allowed between both parts, though the comment must follow
b6538e4f
TC
2560at least one whitespace character; otherwise a character expected as the
2561start of the comment may be regarded as the starting delimiter of the right part.
75e14d17 2562
19799a22
GS
2563During this search no attention is paid to the semantics of the construct.
2564Thus:
75e14d17
IZ
2565
2566 "\$hash{"\$foo/\$bar"}"
2567
2a94b7ce 2568or:
75e14d17 2569
89d205f2 2570 m/
2a94b7ce 2571 bar # NOT a comment, this slash / terminated m//!
75e14d17
IZ
2572 /x
2573
19799a22
GS
2574do not form legal quoted expressions. The quoted part ends on the
2575first C<"> and C</>, and the rest happens to be a syntax error.
2576Because the slash that terminated C<m//> was followed by a C<SPACE>,
2577the example above is not C<m//x>, but rather C<m//> with no C</x>
2578modifier. So the embedded C<#> is interpreted as a literal C<#>.
75e14d17 2579
89d205f2
YO
2580Also no attention is paid to C<\c\> (multichar control char syntax) during
2581this search. Thus the second C<\> in C<qq/\c\/> is interpreted as a part
2582of C<\/>, and the following C</> is not recognized as a delimiter.
0d594e51
TS
2583Instead, use C<\034> or C<\x1c> at the end of quoted constructs.
2584
75e14d17 2585=item Interpolation
d74e8afc 2586X<interpolation>
75e14d17 2587
19799a22 2588The next step is interpolation in the text obtained, which is now
89d205f2 2589delimiter-independent. There are multiple cases.
75e14d17 2590
13a2d996 2591=over 4
75e14d17 2592
89d205f2 2593=item C<<<'EOF'>
75e14d17
IZ
2594
2595No interpolation is performed.
6deea57f
TS
2596Note that the combination C<\\> is left intact, since escaped delimiters
2597are not available for here-docs.
75e14d17 2598
6deea57f 2599=item C<m''>, the pattern of C<s'''>
89d205f2 2600
6deea57f
TS
2601No interpolation is performed at this stage.
2602Any backslashed sequences including C<\\> are treated at the stage
2603to L</"parsing regular expressions">.
89d205f2 2604
6deea57f 2605=item C<''>, C<q//>, C<tr'''>, C<y'''>, the replacement of C<s'''>
75e14d17 2606
89d205f2 2607The only interpolation is removal of C<\> from pairs of C<\\>.
6deea57f
TS
2608Therefore C<-> in C<tr'''> and C<y'''> is treated literally
2609as a hyphen and no character range is available.
2610C<\1> in the replacement of C<s'''> does not work as C<\$1>.
89d205f2
YO
2611
2612=item C<tr///>, C<y///>
2613
6deea57f
TS
2614No variable interpolation occurs. String modifying combinations for
2615case and quoting such as C<\Q>, C<\U>, and C<\E> are not recognized.
2616The other escape sequences such as C<\200> and C<\t> and backslashed
2617characters such as C<\\> and C<\-> are converted to appropriate literals.
89d205f2
YO
2618The character C<-> is treated specially and therefore C<\-> is treated
2619as a literal C<->.
75e14d17 2620
89d205f2 2621=item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF">
75e14d17 2622
628253b8 2623C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F> (possibly paired with C<\E>) are
19799a22
GS
2624converted to corresponding Perl constructs. Thus, C<"\$foo\Qbaz\$bar">
2625is converted to C<\$foo . (quotemeta("baz" . \$bar))> internally.
6deea57f
TS
2626The other escape sequences such as C<\200> and C<\t> and backslashed
2627characters such as C<\\> and C<\-> are replaced with appropriate
2628expansions.
2a94b7ce 2629
19799a22
GS
2630Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
2631is interpolated in the usual way. Something like C<"\Q\\E"> has
2632no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
2633result is the same as for C<"\\\\E">. As a general rule, backslashes
2634between C<\Q> and C<\E> may lead to counterintuitive results. So,
2635C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
2636as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
2a94b7ce
IZ
2637
2638 \$str = '\t';
2639 return "\Q\$str";
2640
2641may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
2642
19799a22 2643Interpolated scalars and arrays are converted internally to the C<join> and
92d29cee 2644C<.> catenation operations. Thus, C<"\$foo XXX '@arr'"> becomes:
75e14d17 2645
19799a22 2646 \$foo . " XXX '" . (join \$", @arr) . "'";
75e14d17 2647
19799a22 2648All operations above are performed simultaneously, left to right.
75e14d17 2649
19799a22
GS
2650Because the result of C<"\Q STRING \E"> has all metacharacters
2651quoted, there is no way to insert a literal C<\$> or C<@> inside a
2652C<\Q\E> pair. If protected by C<\>, C<\$> will be quoted to became
2653C<"\\\\$">; if not, it is interpreted as the start of an interpolated
2654scalar.
75e14d17 2655
19799a22 2656Note also that the interpolation code needs to make a decision on
89d205f2 2657where the interpolated scalar ends. For instance, whether
35f2feb0 2658C<< "a \$b -> {c}" >> really means:
75e14d17
IZ
2659
2660 "a " . \$b . " -> {c}";
2661
2a94b7ce 2662or:
75e14d17
IZ
2663
2664 "a " . \$b -> {c};
2665
19799a22
GS
2666Most of the time, the longest possible text that does not include
2667spaces between components and which contains matching braces or
2668brackets. because the outcome may be determined by voting based
2669on heuristic estimators, the result is not strictly predictable.
2670Fortunately, it's usually correct for ambiguous cases.
75e14d17 2671
6deea57f 2672=item the replacement of C<s///>
75e14d17 2673
628253b8 2674Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F> and interpolation
6deea57f
TS
2675happens as with C<qq//> constructs.
2676
2677It is at this step that C<\1> is begrudgingly converted to C<\$1> in
2678the replacement text of C<s///>, in order to correct the incorrigible
2679I<sed> hackers who haven't picked up the saner idiom yet. A warning
2680is emitted if the C<use warnings> pragma or the B<-w> command-line flag
2681(that is, the C<\$^W> variable) was set.
2682
2683=item C<RE> in C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
2684
628253b8 2685Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F>, C<\E>,
cc74c5bd
TS
2686and interpolation happens (almost) as with C<qq//> constructs.
2687
5d03b57c
KW
2688Processing of C<\N{...}> is also done here, and compiled into an intermediate
2689form for the regex compiler. (This is because, as mentioned below, the regex
2690compilation may be done at execution time, and C<\N{...}> is a compile-time
2691construct.)
2692
cc74c5bd
TS
2693However any other combinations of C<\> followed by a character
2694are not substituted but only skipped, in order to parse them
2695as regular expressions at the following step.
6deea57f 2696As C<\c> is skipped at this step, C<@> of C<\c@> in RE is possibly
1749ea0d 2697treated as an array symbol (for example C<@foo>),
6deea57f 2698even though the same text in C<qq//> gives interpolation of C<\c@>.
6deea57f
TS
2699
2700Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
19799a22
GS
2701a C<#>-comment in a C<//x>-regular expression, no processing is
2702performed whatsoever. This is the first step at which the presence
2703of the C<//x> modifier is relevant.
2704
1749ea0d
TS
2705Interpolation in patterns has several quirks: C<\$|>, C<\$(>, C<\$)>, C<@+>
2706and C<@-> are not interpolated, and constructs C<\$var[SOMETHING]> are
2707voted (by several different estimators) to be either an array element
2708or C<\$var> followed by an RE alternative. This is where the notation
19799a22
GS
2709C<\${arr[\$bar]}> comes handy: C</\${arr[0-9]}/> is interpreted as
2710array element C<-9>, not as a regular expression from the variable
2711C<\$arr> followed by a digit, which would be the interpretation of
2712C</\$arr[0-9]/>. Since voting among different estimators may occur,
2713the result is not predictable.
2714
19799a22
GS
2715The lack of processing of C<\\> creates specific restrictions on
2716the post-processed text. If the delimiter is C</>, one cannot get
2717the combination C<\/> into the result of this step. C</> will
2718finish the regular expression, C<\/> will be stripped to C</> on
2719the previous step, and C<\\/> will be left as is. Because C</> is
2720equivalent to C<\/> inside a regular expression, this does not
2721matter unless the delimiter happens to be character special to the
2722RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
2723alphanumeric char, as in:
2a94b7ce
IZ
2724
2725 m m ^ a \s* b mmx;
2726
19799a22 2727In the RE above, which is intentionally obfuscated for illustration, the
6deea57f 2728delimiter is C<m>, the modifier is C<mx>, and after delimiter-removal the
89d205f2 2729RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
19799a22
GS
2730reason you're encouraged to restrict your delimiters to non-alphanumeric,
2731non-whitespace choices.
75e14d17
IZ
2732
2733=back
2734
19799a22 2735This step is the last one for all constructs except regular expressions,
75e14d17
IZ
2736which are processed further.
2737
6deea57f
TS
2738=item parsing regular expressions
2739X<regexp, parse>
75e14d17 2740
19799a22 2741Previous steps were performed during the compilation of Perl code,
ac036724 2742but this one happens at run time, although it may be optimized to
19799a22 2743be calculated at compile time if appropriate. After preprocessing
6deea57f 2744described above, and possibly after evaluation if concatenation,
19799a22
GS
2745joining, casing translation, or metaquoting are involved, the
2746resulting I<string> is passed to the RE engine for compilation.
2747
2748Whatever happens in the RE engine might be better discussed in L<perlre>,
2749but for the sake of continuity, we shall do so here.
2750
2751This is another step where the presence of the C<//x> modifier is
2752relevant. The RE engine scans the string from left to right and
2753converts it to a finite automaton.
2754
2755Backslashed characters are either replaced with corresponding
2756literal strings (as with C<\{>), or else they generate special nodes
2757in the finite automaton (as with C<\b>). Characters special to the
2758RE engine (such as C<|>) generate corresponding nodes or groups of
2759nodes. C<(?#...)> comments are ignored. All the rest is either
2760converted to literal strings to match, or else is ignored (as is
2761whitespace and C<#>-style comments if C<//x> is present).
2762
2763Parsing of the bracketed character class construct, C<[...]>, is
2764rather different than the rule used for the rest of the pattern.
2765The terminator of this construct is found using the same rules as
2766for finding the terminator of a C<{}>-delimited construct, the only
2767exception being that C<]> immediately following C<[> is treated as
2768though preceded by a backslash. Similarly, the terminator of
2769C<(?{...})> is found using the same rules as for finding the
2770terminator of a C<{}>-delimited construct.
2771
2772It is possible to inspect both the string given to RE engine and the
2773resulting finite automaton. See the arguments C<debug>/C<debugcolor>
2774in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
4a4eefd0 2775switch documented in L<perlrun/"Command Switches">.
75e14d17
IZ
2776
2777=item Optimization of regular expressions
d74e8afc 2778X<regexp, optimization>
75e14d17 2779
7522fed5 2780This step is listed for completeness only. Since it does not change
75e14d17 2781semantics, details of this step are not documented and are subject
19799a22
GS
2782to change without notice. This step is performed over the finite
2783automaton that was generated during the previous pass.
2a94b7ce 2784
19799a22
GS
2785It is at this stage that C<split()> silently optimizes C</^/> to
2786mean C</^/m>.
75e14d17
IZ
2787
2788=back
2789
d74e8afc
ITB
2791X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle>
2792X<< <> >> X<@ARGV>
a0d0e21e 2793
54310121 2794There are several I/O operators you should know about.
7b8d334a 2796A string enclosed by backticks (grave accents) first undergoes
19799a22
GS
2797double-quote interpolation. It is then interpreted as an external
2798command, and the output of that command is the value of the
e9c56f9b
JH
2799backtick string, like in a shell. In scalar context, a single string
2800consisting of all output is returned. In list context, a list of
2801values is returned, one per line of output. (You can set C<\$/> to use
2802a different line terminator.) The command is executed each time the
2803pseudo-literal is evaluated. The status value of the command is
2804returned in C<\$?> (see L<perlvar> for the interpretation of C<\$?>).
2805Unlike in B<csh>, no translation is done on the return data--newlines
2806remain newlines. Unlike in any of the shells, single quotes do not
2807hide variable names in the command from interpretation. To pass a
2808literal dollar-sign through to the shell you need to hide it with a
2809backslash. The generalized form of backticks is C<qx//>. (Because
2810backticks always undergo shell expansion as well, see L<perlsec> for
2811security concerns.)
d74e8afc 2812X<qx> X<`> X<``> X<backtick> X<glob>
19799a22
GS
2813
2814In scalar context, evaluating a filehandle in angle brackets yields
2815the next line from that file (the newline, if any, included), or
2816C<undef> at end-of-file or on error. When C<\$/> is set to C<undef>
2817(sometimes known as file-slurp mode) and the file is empty, it
2818returns C<''> the first time, followed by C<undef> subsequently.
2819
2820Ordinarily you must assign the returned value to a variable, but
2821there is one situation where an automatic assignment happens. If
2822and only if the input symbol is the only thing inside the conditional
2823of a C<while> statement (even if disguised as a C<for(;;)> loop),
2824the value is automatically assigned to the global variable \$_,
2825destroying whatever was there previously. (This may seem like an
2826odd thing to you, but you'll use the construct in almost every Perl
17b829fa 2827script you write.) The \$_ variable is not implicitly localized.
19799a22
GS
2828You'll have to put a C<local \$_;> before the loop if you want that
2829to happen.
2830
2831The following lines are equivalent:
a0d0e21e 2832
748a9306 2833 while (defined(\$_ = <STDIN>)) { print; }
7b8d334a 2834 while (\$_ = <STDIN>) { print; }
a0d0e21e
LW
2835 while (<STDIN>) { print; }
2836 for (;<STDIN>;) { print; }
748a9306 2837 print while defined(\$_ = <STDIN>);
7b8d334a 2838 print while (\$_ = <STDIN>);
a0d0e21e
LW
2839 print while <STDIN>;
2840
1ca345ed
TC
2841This also behaves similarly, but assigns to a lexical variable
7b8d334a 2843
89d205f2 2844 while (my \$line = <STDIN>) { print \$line }
7b8d334a 2845
19799a22
GS
2846In these loop constructs, the assigned value (whether assignment
2847is automatic or explicit) is then tested to see whether it is
1ca345ed
TC
2848defined. The defined test avoids problems where the line has a string
2849value that would be treated as false by Perl; for example a "" or
19799a22
GS
2850a "0" with no trailing newline. If you really mean for such values
2851to terminate the loop, they should be tested for explicitly:
7b8d334a
GS
2852
2853 while ((\$_ = <STDIN>) ne '0') { ... }
2854 while (<STDIN>) { last unless \$_; ... }
2855
1ca345ed 2856In other boolean contexts, C<< <FILEHANDLE> >> without an
5ef4d93e 2857explicit C<defined> test or comparison elicits a warning if the
9f1b1f2d 2858C<use warnings> pragma or the B<-w>
19799a22 2859command-line switch (the C<\$^W> variable) is in effect.
7b8d334a 2860
5f05dabc 2861The filehandles STDIN, STDOUT, and STDERR are predefined. (The
19799a22
GS
2862filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
2863in packages, where they would be interpreted as local identifiers
2864rather than global.) Additional filehandles may be created with
2865the open() function, amongst others. See L<perlopentut> and
2866L<perlfunc/open> for details on this.
d74e8afc 2867X<stdin> X<stdout> X<sterr>
a0d0e21e 2868
35f2feb0 2869If a <FILEHANDLE> is used in a context that is looking for
19799a22
GS
2870a list, a list comprising all input lines is returned, one line per
2871list element. It's easy to grow to a rather large data space this
2872way, so use with care.
a0d0e21e 2873
35f2feb0 2874<FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
35f2feb0 2877The null filehandle <> is special: it can be used to emulate the
1ca345ed
TC
2878behavior of B<sed> and B<awk>, and any other Unix filter program
2879that takes a list of filenames, doing the same to each line
2880of input from all of them. Input from <> comes either from
a0d0e21e 2881standard input, or from each file listed on the command line. Here's
35f2feb0 2882how it works: the first time <> is evaluated, the @ARGV array is
5a964f20 2883checked, and if it is empty, C<\$ARGV[0]> is set to "-", which when opened
a0d0e21e
LW
2884gives you standard input. The @ARGV array is then processed as a list
2885of filenames. The loop
2886
2887 while (<>) {
2888 ... # code for each line
2889 }
2890
2891is equivalent to the following Perl-like pseudo code:
2892
3e3baf6d 2893 unshift(@ARGV, '-') unless @ARGV;
a0d0e21e
LW
2894 while (\$ARGV = shift) {
2895 open(ARGV, \$ARGV);
2896 while (<ARGV>) {
2897 ... # code for each line
2898 }
2899 }
2900
19799a22
GS
2901except that it isn't so cumbersome to say, and will actually work.
2902It really does shift the @ARGV array and put the current filename
2903into the \$ARGV variable. It also uses filehandle I<ARGV>
ac036724 2904internally. <> is just a synonym for <ARGV>, which
19799a22 2905is magical. (The pseudo code above doesn't work because it treats
35f2feb0 2906<ARGV> as non-magical.)
a0d0e21e 2907
48ab5743
ML
2908Since the null filehandle uses the two argument form of L<perlfunc/open>
2909it interprets special characters, so if you have a script like this:
2910
2911 while (<>) {
2912 print;
2913 }
2914
2915and call it with C<perl dangerous.pl 'rm -rfv *|'>, it actually opens a
2916pipe, executes the C<rm> command and reads C<rm>'s output from that pipe.
2917If you want all items in C<@ARGV> to be interpreted as file names, you
2918can use the module C<ARGV::readonly> from CPAN.
2919
35f2feb0 2920You can modify @ARGV before the first <> as long as the array ends up
a0d0e21e 2921containing the list of filenames you really want. Line numbers (C<\$.>)
19799a22
GS
2922continue as though the input were one big happy file. See the example
2923in L<perlfunc/eof> for how to reset line numbers on each file.
5a964f20 2924
89d205f2 2925If you want to set @ARGV to your own list of files, go right ahead.
5a964f20
TC
2926This sets @ARGV to all plain text files if no @ARGV was given:
2927
2928 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
a0d0e21e 2929
5a964f20
TC
2930You can even set them to pipe commands. For example, this automatically
2931filters compressed arguments through B<gzip>:
2932
2933 @ARGV = map { /\.(gz|Z)\$/ ? "gzip -dc < \$_ |" : \$_ } @ARGV;
2934
2935If you want to pass switches into your script, you can use one of the
a0d0e21e
LW
2936Getopts modules or put a loop on the front like this:
2937
2938 while (\$_ = \$ARGV[0], /^-/) {
2939 shift;
2940 last if /^--\$/;
2941 if (/^-D(.*)/) { \$debug = \$1 }
2942 if (/^-v/) { \$verbose++ }
5a964f20 2943 # ... # other switches
a0d0e21e 2944 }
5a964f20 2945
a0d0e21e 2946 while (<>) {
5a964f20 2947 # ... # code for each line
a0d0e21e
LW
2948 }
2949
89d205f2
YO
2950The <> symbol will return C<undef> for end-of-file only once.
2951If you call it again after this, it will assume you are processing another
19799a22 2952@ARGV list, and if you haven't set @ARGV, will read input from STDIN.
a0d0e21e 2953
1ca345ed 2954If what the angle brackets contain is a simple scalar variable (for example,
35f2feb0 2955<\$foo>), then that variable contains the name of the
19799a22
GS
2956filehandle to input from, or its typeglob, or a reference to the
2957same. For example:
cb1a09d0
2958
2959 \$fh = \*STDIN;
2960 \$line = <\$fh>;
a0d0e21e 2961
5a964f20
TC
2962If what's within the angle brackets is neither a filehandle nor a simple
2963scalar variable containing a filehandle name, typeglob, or typeglob
2964reference, it is interpreted as a filename pattern to be globbed, and
2965either a list of filenames or the next filename in the list is returned,
19799a22 2966depending on context. This distinction is determined on syntactic
35f2feb0
GS
2967grounds alone. That means C<< <\$x> >> is always a readline() from
2968an indirect handle, but C<< <\$hash{key}> >> is always a glob().
5a964f20 2969That's because \$x is a simple scalar variable, but C<\$hash{key}> is
ef191992
YST
2970not--it's a hash element. Even C<< <\$x > >> (note the extra space)
2971is treated as C<glob("\$x ")>, not C<readline(\$x)>.
5a964f20
TC
2972
2973One level of double-quote interpretation is done first, but you can't
35f2feb0 2974say C<< <\$foo> >> because that's an indirect filehandle as explained
5a964f20
TC
2975in the previous paragraph. (In older versions of Perl, programmers
2976would insert curly brackets to force interpretation as a filename glob:
35f2feb0 2977C<< <\${foo}> >>. These days, it's considered cleaner to call the
5a964f20 2978internal function directly as C<glob(\$foo)>, which is probably the right
19799a22 2979way to have done it in the first place.) For example:
a0d0e21e
LW
2980
2981 while (<*.c>) {
2982 chmod 0644, \$_;
2983 }
2984
3a4b19e4 2985is roughly equivalent to:
a0d0e21e
LW
2986
2987 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
2988 while (<FOO>) {
5b3eff12 2989 chomp;
a0d0e21e
LW
2990 chmod 0644, \$_;
2991 }
2992
3a4b19e4
GS
2993except that the globbing is actually done internally using the standard
2994C<File::Glob> extension. Of course, the shortest way to do the above is:
a0d0e21e
LW
2995
2996 chmod 0644, <*.c>;
2997
19799a22
GS
2998A (file)glob evaluates its (embedded) argument only when it is
2999starting a new list. All values must be read before it will start
3000over. In list context, this isn't important because you automatically
3001get them all anyway. However, in scalar context the operator returns
069e01df 3002the next value each time it's called, or C<undef> when the list has
19799a22
GS
3003run out. As with filehandle reads, an automatic C<defined> is
3004generated when the glob occurs in the test part of a C<while>,
1ca345ed
TC
3005because legal glob returns (for example,
3006a file called F<0>) would otherwise
19799a22
GS
3007terminate the loop. Again, C<undef> is returned only once. So if
3008you're expecting a single value from a glob, it is much better to
3009say
4633a7c4
LW
3010
3011 (\$file) = <blurch*>;
3012
3013than
3014
3015 \$file = <blurch*>;
3016
3017because the latter will alternate between returning a filename and
19799a22 3018returning false.
4633a7c4 3019
b159ebd3 3020If you're trying to do variable interpolation, it's definitely better
4633a7c4 3021to use the glob() function, because the older notation can cause people
e37d713d 3022to become confused with the indirect filehandle notation.
4633a7c4
LW
3023
3024 @files = glob("\$dir/*.[ch]");
3025 @files = glob(\$files[\$i]);
3026
d74e8afc 3028X<constant folding> X<folding>
a0d0e21e
LW
3029
3030Like C, Perl does a certain amount of expression evaluation at
19799a22 3031compile time whenever it determines that all arguments to an
a0d0e21e
LW
3032operator are static and have no side effects. In particular, string
3033concatenation happens at compile time between literals that don't do
19799a22 3034variable substitution. Backslash interpolation also happens at
a0d0e21e
LW
3035compile time. You can say
3036
1ca345ed
TC
3037 'Now is the time for all'
3038 . "\n"
3039 . 'good men to come to.'
a0d0e21e 3040
54310121 3041and this all reduces to one string internally. Likewise, if
a0d0e21e
LW
3042you say
3043
3044 foreach \$file (@filenames) {
5a964f20 3045 if (-s \$file > 5 + 100 * 2**16) { }
54310121 3046 }
a0d0e21e 3047
1ca345ed 3048the compiler precomputes the number which that expression
19799a22 3049represents so that the interpreter won't have to.
a0d0e21e 3050
d74e8afc 3052X<no-op> X<nop>
fd1abbef
DN
3053
3054Perl doesn't officially have a no-op operator, but the bare constants
1ca345ed 3055C<0> and C<1> are special-cased not to produce a warning in void
fd1abbef
DN
3056context, so you can for example safely do
3057
3058 1 while foo();
3059
d74e8afc 3061X<operator, bitwise, string>
TP
3062
3063Bitstrings of any size may be manipulated by the bitwise operators
3064(C<~ | & ^>).
3065
19799a22
GS
3066If the operands to a binary bitwise op are strings of different
3067sizes, B<|> and B<^> ops act as though the shorter operand had
3068additional zero bits on the right, while the B<&> op acts as though
3069the longer operand were truncated to the length of the shorter.
3070The granularity for such extension or truncation is one or more
3071bytes.
89d205f2 3073 # ASCII-based examples
TP
3074 print "j p \n" ^ " a h"; # prints "JAPH\n"
3075 print "JA" | " ph\n"; # prints "japh\n"
3076 print "japh\nJunk" & '_____'; # prints "JAPH\n";
3077 print 'p N\$' ^ " E<H\n"; # prints "Perl\n";
3078
19799a22 3079If you are intending to manipulate bitstrings, be certain that
2c268ad5 3080you're supplying bitstrings: If an operand is a number, that will imply
19799a22 3081a B<numeric> bitwise operation. You may explicitly show which type of
TP
3082operation you intend by using C<""> or C<0+>, as in the examples below.
3083
4358a253
SS
3084 \$foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
3085 \$foo = '150' | 105; # yields 255
TP
3086 \$foo = 150 | '105'; # yields 255
3087 \$foo = '150' | '105'; # yields string '155' (under ASCII)
3088
3089 \$baz = 0+\$foo & 0+\$bar; # both ops explicitly numeric
3090 \$biz = "\$foo" ^ "\$bar"; # both ops explicitly stringy
a0d0e21e 3091
1ae175c8
GS
3092See L<perlfunc/vec> for information on how to manipulate individual bits
3093in a bit vector.
3094
d74e8afc 3096X<integer>
a0d0e21e 3097
19799a22 3098By default, Perl assumes that it must do most of its arithmetic in
a0d0e21e
LW
3099floating point. But by saying
3100
3101 use integer;
3102
3eab78e3
CW
3103you may tell the compiler to use integer operations
3104(see L<integer> for a detailed explanation) from here to the end of
3105the enclosing BLOCK. An inner BLOCK may countermand this by saying
a0d0e21e
LW
3106
3107 no integer;
3108
19799a22 3109which lasts until the end of that BLOCK. Note that this doesn't
3eab78e3
CW
3110mean everything is an integer, merely that Perl will use integer
3111operations for arithmetic, comparison, and bitwise operators. For
3112example, even under C<use integer>, if you take the C<sqrt(2)>, you'll
3113still get C<1.4142135623731> or so.
19799a22
GS
3114
3115Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
13a2d996 3117L<Bitwise String Operators>.) However, C<use integer> still has meaning for
19799a22
GS
3118them. By default, their results are interpreted as unsigned integers, but
3119if C<use integer> is in effect, their results are interpreted
3120as signed integers. For example, C<~0> usually evaluates to a large
0be96356 3121integral value. However, C<use integer; ~0> is C<-1> on two's-complement
19799a22 3122machines.
68dc0745 3123
06ce2fa3 3125
d74e8afc 3126X<floating-point> X<floating point> X<float> X<real>
68dc0745 3127
3128While C<use integer> provides integer-only arithmetic, there is no
19799a22
GS
3129analogous mechanism to provide automatic rounding or truncation to a
3130certain number of decimal places. For rounding to a certain number
3131of digits, sprintf() or printf() is usually the easiest route.
3132See L<perlfaq4>.
68dc0745 3133
5a964f20
TC
3134Floating-point numbers are only approximations to what a mathematician
3135would call real numbers. There are infinitely more reals than floats,
3136so some corners must be cut. For example:
3137
3138 printf "%.20g\n", 123456789123456789;
3139 # produces 123456789123456784
3140
8548cb57
RGS
3141Testing for exact floating-point equality or inequality is not a
3142good idea. Here's a (relatively expensive) work-around to compare
5a964f20
TC
3143whether two floating-point numbers are equal to a particular number of
3144decimal places. See Knuth, volume II, for a more robust treatment of
3145this topic.
3146
3147 sub fp_equal {
3148 my (\$X, \$Y, \$POINTS) = @_;
3149 my (\$tX, \$tY);
3150 \$tX = sprintf("%.\${POINTS}g", \$X);
3151 \$tY = sprintf("%.\${POINTS}g", \$Y);
3152 return \$tX eq \$tY;
3153 }
3154
68dc0745 3155The POSIX module (part of the standard perl distribution) implements
19799a22
GS
3156ceil(), floor(), and other mathematical and trigonometric functions.
3157The Math::Complex module (part of the standard perl distribution)
3158defines mathematical functions that work on both the reals and the
3159imaginary numbers. Math::Complex not as efficient as POSIX, but
68dc0745 3160POSIX can't work with complex numbers.
3161
3162Rounding in financial applications can have serious implications, and
3163the rounding method used should be specified precisely. In these
3164cases, it probably pays not to trust whichever system rounding is
3165being used by Perl, but to instead implement the rounding function you
3166need yourself.
5a964f20
TC
3167
d74e8afc 3169X<number, arbitrary precision>
5a964f20 3170
c543c01b
TC
3171The standard C<Math::BigInt>, C<Math::BigRat>, and C<Math::BigFloat> modules,
3172along with the C<bigint>, C<bigrat>, and C<bitfloat> pragmas, provide
19799a22 3173variable-precision arithmetic and overloaded operators, although
cd5c4fce 3174they're currently pretty slow. At the cost of some space and
19799a22
GS
3175considerable speed, they avoid the normal pitfalls associated with
3176limited-precision representations.
5a964f20 3177
c543c01b
TC
3178 use 5.010;
3179 use bigint; # easy interface to Math::BigInt
3180 \$x = 123456789123456789;
3181 say \$x * \$x;
3182 +15241578780673678515622620750190521
3183
3184Or with rationals:
3185
3186 use 5.010;
3187 use bigrat;
3188 \$a = 3/22;
3189 \$b = 4/6;
3190 say "a/b is ", \$a/\$b;
3191 say "a*b is ", \$a*\$b;
3192 a/b is 9/44
3193 a*b is 1/11
3194
3195Several modules let you calculate with (bound only by memory and CPU time)
3196unlimited or fixed precision. There are also some non-standard modules that
3197provide faster implementations via external C libraries.
cd5c4fce
T
3198
3199Here is a short, but incomplete summary:
3200
950b09ed
KW
3201 Math::Fraction big, unlimited fractions like 9973 / 12967
3202 Math::String treat string sequences like numbers
3203 Math::FixedPrecision calculate with a fixed precision
3204 Math::Currency for currency calculations
3205 Bit::Vector manipulate bit vectors fast (uses C)
3206 Math::BigIntFast Bit::Vector wrapper for big numbers