4 perlop - Perl operators and precedence
8 In Perl, the operator determines what operation is performed,
9 independent of the type of the operands. For example S<C<$x + $y>>
10 is always a numeric addition, and if C<$x> or C<$y> do not contain
11 numbers, an attempt is made to convert them to numbers first.
13 This is in contrast to many other dynamic languages, where the
14 operation is determined by the type of the first argument. It also
15 means that Perl has two versions of some operators, one for numeric
16 and one for string comparison. For example S<C<$x == $y>> compares
17 two numbers for equality, and S<C<$x eq $y>> compares two strings.
19 There are a few exceptions though: C<x> can be either string
20 repetition or list repetition, depending on the type of the left
21 operand, and C<&>, C<|>, C<^> and C<~> can be either string or numeric bit
24 =head2 Operator Precedence and Associativity
25 X<operator, precedence> X<precedence> X<associativity>
27 Operator precedence and associativity work in Perl more or less like
28 they do in mathematics.
30 I<Operator precedence> means some operators are evaluated before
31 others. For example, in S<C<2 + 4 * 5>>, the multiplication has higher
32 precedence so S<C<4 * 5>> is evaluated first yielding S<C<2 + 20 ==
33 22>> and not S<C<6 * 5 == 30>>.
35 I<Operator associativity> defines what happens if a sequence of the
36 same operators is used one after another: whether the evaluator will
37 evaluate the left operations first, or the right first. For example, in
38 S<C<8 - 4 - 2>>, subtraction is left associative so Perl evaluates the
39 expression left to right. S<C<8 - 4>> is evaluated first making the
40 expression S<C<4 - 2 == 2>> and not S<C<8 - 2 == 6>>.
42 Perl operators have the following associativity and precedence,
43 listed from highest precedence to lowest. Operators borrowed from
44 C keep the same precedence relationship with each other, even where
45 C's precedence is slightly screwy. (This makes learning Perl easier
46 for C folks.) With very few exceptions, these all operate on scalar
47 values only, not array values.
49 left terms and list operators (leftward)
53 right ! ~ \ and unary + and -
58 nonassoc named unary operators
59 nonassoc < > <= >= lt gt le ge
60 nonassoc == != <=> eq ne cmp ~~
67 right = += -= *= etc. goto last next redo dump
69 nonassoc list operators (rightward)
74 In the following sections, these operators are covered in detail, in the
75 same order in which they appear in the table above.
77 Many operators can be overloaded for objects. See L<overload>.
79 =head2 Terms and List Operators (Leftward)
80 X<list operator> X<operator, list> X<term>
82 A TERM has the highest precedence in Perl. They include variables,
83 quote and quote-like operators, any expression in parentheses,
84 and any function whose arguments are parenthesized. Actually, there
85 aren't really functions in this sense, just list operators and unary
86 operators behaving as functions because you put parentheses around
87 the arguments. These are all documented in L<perlfunc>.
89 If any list operator (C<print()>, etc.) or any unary operator (C<chdir()>, etc.)
90 is followed by a left parenthesis as the next token, the operator and
91 arguments within parentheses are taken to be of highest precedence,
92 just like a normal function call.
94 In the absence of parentheses, the precedence of list operators such as
95 C<print>, C<sort>, or C<chmod> is either very high or very low depending on
96 whether you are looking at the left side or the right side of the operator.
99 @ary = (1, 3, sort 4, 2);
100 print @ary; # prints 1324
102 the commas on the right of the C<sort> are evaluated before the C<sort>,
103 but the commas on the left are evaluated after. In other words,
104 list operators tend to gobble up all arguments that follow, and
105 then act like a simple TERM with regard to the preceding expression.
106 Be careful with parentheses:
108 # These evaluate exit before doing the print:
109 print($foo, exit); # Obviously not what you want.
110 print $foo, exit; # Nor is this.
112 # These do the print before evaluating exit:
113 (print $foo), exit; # This is what you want.
114 print($foo), exit; # Or this.
115 print ($foo), exit; # Or even this.
119 print ($foo & 255) + 1, "\n";
121 probably doesn't do what you expect at first glance. The parentheses
122 enclose the argument list for C<print> which is evaluated (printing
123 the result of S<C<$foo & 255>>). Then one is added to the return value
124 of C<print> (usually 1). The result is something like this:
126 1 + 1, "\n"; # Obviously not what you meant.
128 To do what you meant properly, you must write:
130 print(($foo & 255) + 1, "\n");
132 See L</Named Unary Operators> for more discussion of this.
134 Also parsed as terms are the S<C<do {}>> and S<C<eval {}>> constructs, as
135 well as subroutine and method calls, and the anonymous
136 constructors C<[]> and C<{}>.
138 See also L</Quote and Quote-like Operators> toward the end of this section,
139 as well as L</"I/O Operators">.
141 =head2 The Arrow Operator
142 X<arrow> X<dereference> X<< -> >>
144 "C<< -> >>" is an infix dereference operator, just as it is in C
145 and C++. If the right side is either a C<[...]>, C<{...}>, or a
146 C<(...)> subscript, then the left side must be either a hard or
147 symbolic reference to an array, a hash, or a subroutine respectively.
148 (Or technically speaking, a location capable of holding a hard
149 reference, if it's an array or hash reference being used for
150 assignment.) See L<perlreftut> and L<perlref>.
152 Otherwise, the right side is a method name or a simple scalar
153 variable containing either the method name or a subroutine reference,
154 and the left side must be either an object (a blessed reference)
155 or a class name (that is, a package name). See L<perlobj>.
157 The dereferencing cases (as opposed to method-calling cases) are
158 somewhat extended by the C<postderef> feature. For the
159 details of that feature, consult L<perlref/Postfix Dereference Syntax>.
161 =head2 Auto-increment and Auto-decrement
162 X<increment> X<auto-increment> X<++> X<decrement> X<auto-decrement> X<-->
164 C<"++"> and C<"--"> work as in C. That is, if placed before a variable,
165 they increment or decrement the variable by one before returning the
166 value, and if placed after, increment or decrement after returning the
170 print $i++; # prints 0
171 print ++$j; # prints 1
173 Note that just as in C, Perl doesn't define B<when> the variable is
174 incremented or decremented. You just know it will be done sometime
175 before or after the value is returned. This also means that modifying
176 a variable twice in the same statement will lead to undefined behavior.
177 Avoid statements like:
182 Perl will not guarantee what the result of the above statements is.
184 The auto-increment operator has a little extra builtin magic to it. If
185 you increment a variable that is numeric, or that has ever been used in
186 a numeric context, you get a normal increment. If, however, the
187 variable has been used in only string contexts since it was set, and
188 has a value that is not the empty string and matches the pattern
189 C</^[a-zA-Z]*[0-9]*\z/>, the increment is done as a string, preserving each
190 character within its range, with carry:
192 print ++($foo = "99"); # prints "100"
193 print ++($foo = "a0"); # prints "a1"
194 print ++($foo = "Az"); # prints "Ba"
195 print ++($foo = "zz"); # prints "aaa"
197 C<undef> is always treated as numeric, and in particular is changed
198 to C<0> before incrementing (so that a post-increment of an undef value
199 will return C<0> rather than C<undef>).
201 The auto-decrement operator is not magical.
203 =head2 Exponentiation
204 X<**> X<exponentiation> X<power>
206 Binary C<"**"> is the exponentiation operator. It binds even more
207 tightly than unary minus, so C<-2**4> is C<-(2**4)>, not C<(-2)**4>.
209 implemented using C's C<pow(3)> function, which actually works on doubles
212 Note that certain exponentiation expressions are ill-defined:
213 these include C<0**0>, C<1**Inf>, and C<Inf**0>. Do not expect
214 any particular results from these special cases, the results
215 are platform-dependent.
217 =head2 Symbolic Unary Operators
218 X<unary operator> X<operator, unary>
220 Unary C<"!"> performs logical negation, that is, "not". See also C<not> for a lower
221 precedence version of this.
224 Unary C<"-"> performs arithmetic negation if the operand is numeric,
225 including any string that looks like a number. If the operand is
226 an identifier, a string consisting of a minus sign concatenated
227 with the identifier is returned. Otherwise, if the string starts
228 with a plus or minus, a string starting with the opposite sign is
229 returned. One effect of these rules is that C<-bareword> is equivalent
230 to the string C<"-bareword">. If, however, the string begins with a
231 non-alphabetic character (excluding C<"+"> or C<"-">), Perl will attempt
233 the string to a numeric, and the arithmetic negation is performed. If the
234 string cannot be cleanly converted to a numeric, Perl will give the warning
235 B<Argument "the string" isn't numeric in negation (-) at ...>.
236 X<-> X<negation, arithmetic>
238 Unary C<"~"> performs bitwise negation, that is, 1's complement. For
239 example, S<C<0666 & ~027>> is 0640. (See also L</Integer Arithmetic> and
240 L</Bitwise String Operators>.) Note that the width of the result is
241 platform-dependent: C<~0> is 32 bits wide on a 32-bit platform, but 64
242 bits wide on a 64-bit platform, so if you are expecting a certain bit
243 width, remember to use the C<"&"> operator to mask off the excess bits.
244 X<~> X<negation, binary>
246 When complementing strings, if all characters have ordinal values under
247 256, then their complements will, also. But if they do not, all
248 characters will be in either 32- or 64-bit complements, depending on your
249 architecture. So for example, C<~"\x{3B1}"> is C<"\x{FFFF_FC4E}"> on
250 32-bit machines and C<"\x{FFFF_FFFF_FFFF_FC4E}"> on 64-bit machines.
252 If the experimental "bitwise" feature is enabled via S<C<use feature
253 'bitwise'>>, then unary C<"~"> always treats its argument as a number, and an
254 alternate form of the operator, C<"~.">, always treats its argument as a
255 string. So C<~0> and C<~"0"> will both give 2**32-1 on 32-bit platforms,
256 whereas C<~.0> and C<~."0"> will both yield C<"\xff">. This feature
257 produces a warning unless you use S<C<no warnings 'experimental::bitwise'>>.
259 Unary C<"+"> has no effect whatsoever, even on strings. It is useful
260 syntactically for separating a function name from a parenthesized expression
261 that would otherwise be interpreted as the complete list of function
262 arguments. (See examples above under L</Terms and List Operators (Leftward)>.)
265 Unary C<"\"> creates a reference to whatever follows it. See L<perlreftut>
266 and L<perlref>. Do not confuse this behavior with the behavior of
267 backslash within a string, although both forms do convey the notion
268 of protecting the next thing from interpolation.
269 X<\> X<reference> X<backslash>
271 =head2 Binding Operators
272 X<binding> X<operator, binding> X<=~> X<!~>
274 Binary C<"=~"> binds a scalar expression to a pattern match. Certain operations
275 search or modify the string C<$_> by default. This operator makes that kind
276 of operation work on some other string. The right argument is a search
277 pattern, substitution, or transliteration. The left argument is what is
278 supposed to be searched, substituted, or transliterated instead of the default
279 C<$_>. When used in scalar context, the return value generally indicates the
280 success of the operation. The exceptions are substitution (C<s///>)
281 and transliteration (C<y///>) with the C</r> (non-destructive) option,
282 which cause the B<r>eturn value to be the result of the substitution.
283 Behavior in list context depends on the particular operator.
284 See L</"Regexp Quote-Like Operators"> for details and L<perlretut> for
285 examples using these operators.
287 If the right argument is an expression rather than a search pattern,
288 substitution, or transliteration, it is interpreted as a search pattern at run
289 time. Note that this means that its
290 contents will be interpolated twice, so
294 is not ok, as the regex engine will end up trying to compile the
295 pattern C<\>, which it will consider a syntax error.
297 Binary C<"!~"> is just like C<"=~"> except the return value is negated in
300 Binary C<"!~"> with a non-destructive substitution (C<s///r>) or transliteration
301 (C<y///r>) is a syntax error.
303 =head2 Multiplicative Operators
304 X<operator, multiplicative>
306 Binary C<"*"> multiplies two numbers.
309 Binary C<"/"> divides two numbers.
312 Binary C<"%"> is the modulo operator, which computes the division
313 remainder of its first argument with respect to its second argument.
315 operands C<$m> and C<$n>: If C<$n> is positive, then S<C<$m % $n>> is
316 C<$m> minus the largest multiple of C<$n> less than or equal to
317 C<$m>. If C<$n> is negative, then S<C<$m % $n>> is C<$m> minus the
318 smallest multiple of C<$n> that is not less than C<$m> (that is, the
319 result will be less than or equal to zero). If the operands
320 C<$m> and C<$n> are floating point values and the absolute value of
321 C<$n> (that is C<abs($n)>) is less than S<C<(UV_MAX + 1)>>, only
322 the integer portion of C<$m> and C<$n> will be used in the operation
323 (Note: here C<UV_MAX> means the maximum of the unsigned integer type).
324 If the absolute value of the right operand (C<abs($n)>) is greater than
325 or equal to S<C<(UV_MAX + 1)>>, C<"%"> computes the floating-point remainder
326 C<$r> in the equation S<C<($r = $m - $i*$n)>> where C<$i> is a certain
327 integer that makes C<$r> have the same sign as the right operand
328 C<$n> (B<not> as the left operand C<$m> like C function C<fmod()>)
329 and the absolute value less than that of C<$n>.
330 Note that when S<C<use integer>> is in scope, C<"%"> gives you direct access
331 to the modulo operator as implemented by your C compiler. This
332 operator is not as well defined for negative operands, but it will
334 X<%> X<remainder> X<modulo> X<mod>
336 Binary C<"x"> is the repetition operator. In scalar context or if the left
337 operand is not enclosed in parentheses, it returns a string consisting
338 of the left operand repeated the number of times specified by the right
339 operand. In list context, if the left operand is enclosed in
340 parentheses or is a list formed by C<qw/I<STRING>/>, it repeats the list.
341 If the right operand is zero or negative (raising a warning on
342 negative), it returns an empty string
343 or an empty list, depending on the context.
346 print '-' x 80; # print row of dashes
348 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
350 @ones = (1) x 80; # a list of 80 1's
351 @ones = (5) x @ones; # set all elements to 5
354 =head2 Additive Operators
355 X<operator, additive>
357 Binary C<"+"> returns the sum of two numbers.
360 Binary C<"-"> returns the difference of two numbers.
363 Binary C<"."> concatenates two strings.
364 X<string, concatenation> X<concatenation>
365 X<cat> X<concat> X<concatenate> X<.>
367 =head2 Shift Operators
368 X<shift operator> X<operator, shift> X<<< << >>>
369 X<<< >> >>> X<right shift> X<left shift> X<bitwise shift>
370 X<shl> X<shr> X<shift, right> X<shift, left>
372 Binary C<<< "<<" >>> returns the value of its left argument shifted left by the
373 number of bits specified by the right argument. Arguments should be
374 integers. (See also L</Integer Arithmetic>.)
376 Binary C<<< ">>" >>> returns the value of its left argument shifted right by
377 the number of bits specified by the right argument. Arguments should
378 be integers. (See also L</Integer Arithmetic>.)
380 If S<C<use integer>> (see L</Integer Arithmetic>) is in force then
381 signed C integers are used (I<arithmetic shift>), otherwise unsigned C
382 integers are used (I<logical shift>), even for negative shiftees.
383 In arithmetic right shift the sign bit is replicated on the left,
384 in logical shift zero bits come in from the left.
386 Either way, the implementation isn't going to generate results larger
387 than the size of the integer type Perl was built with (32 bits or 64 bits).
389 Shifting by negative number of bits means the reverse shift: left
390 shift becomes right shift, right shift becomes left shift. This is
391 unlike in C, where negative shift is undefined.
393 Shifting by more bits than the size of the integers means most of the
394 time zero (all bits fall off), except that under S<C<use integer>>
395 right overshifting a negative shiftee results in -1. This is unlike
396 in C, where shifting by too many bits is undefined. A common C
397 behavior is "shift by modulo wordbits", so that for example
399 1 >> 64 == 1 >> (64 % 64) == 1 >> 0 == 1 # Common C behavior.
401 but that is completely accidental.
403 If you get tired of being subject to your platform's native integers,
404 the S<C<use bigint>> pragma neatly sidesteps the issue altogether:
406 print 20 << 20; # 20971520
407 print 20 << 40; # 5120 on 32-bit machines,
408 # 21990232555520 on 64-bit machines
410 print 20 << 100; # 25353012004564588029934064107520
412 =head2 Named Unary Operators
413 X<operator, named unary>
415 The various named unary operators are treated as functions with one
416 argument, with optional parentheses.
418 If any list operator (C<print()>, etc.) or any unary operator (C<chdir()>, etc.)
419 is followed by a left parenthesis as the next token, the operator and
420 arguments within parentheses are taken to be of highest precedence,
421 just like a normal function call. For example,
422 because named unary operators are higher precedence than C<||>:
424 chdir $foo || die; # (chdir $foo) || die
425 chdir($foo) || die; # (chdir $foo) || die
426 chdir ($foo) || die; # (chdir $foo) || die
427 chdir +($foo) || die; # (chdir $foo) || die
429 but, because C<"*"> is higher precedence than named operators:
431 chdir $foo * 20; # chdir ($foo * 20)
432 chdir($foo) * 20; # (chdir $foo) * 20
433 chdir ($foo) * 20; # (chdir $foo) * 20
434 chdir +($foo) * 20; # chdir ($foo * 20)
436 rand 10 * 20; # rand (10 * 20)
437 rand(10) * 20; # (rand 10) * 20
438 rand (10) * 20; # (rand 10) * 20
439 rand +(10) * 20; # rand (10 * 20)
441 Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are
442 treated like named unary operators, but they don't follow this functional
443 parenthesis rule. That means, for example, that C<-f($file).".bak"> is
444 equivalent to S<C<-f "$file.bak">>.
445 X<-X> X<filetest> X<operator, filetest>
447 See also L</"Terms and List Operators (Leftward)">.
449 =head2 Relational Operators
450 X<relational operator> X<operator, relational>
452 Perl operators that return true or false generally return values
453 that can be safely used as numbers. For example, the relational
454 operators in this section and the equality operators in the next
455 one return C<1> for true and a special version of the defined empty
456 string, C<"">, which counts as a zero but is exempt from warnings
457 about improper numeric conversions, just as S<C<"0 but true">> is.
459 Binary C<< "<" >> returns true if the left argument is numerically less than
463 Binary C<< ">" >> returns true if the left argument is numerically greater
464 than the right argument.
467 Binary C<< "<=" >> returns true if the left argument is numerically less than
468 or equal to the right argument.
471 Binary C<< ">=" >> returns true if the left argument is numerically greater
472 than or equal to the right argument.
475 Binary C<"lt"> returns true if the left argument is stringwise less than
479 Binary C<"gt"> returns true if the left argument is stringwise greater
480 than the right argument.
483 Binary C<"le"> returns true if the left argument is stringwise less than
484 or equal to the right argument.
487 Binary C<"ge"> returns true if the left argument is stringwise greater
488 than or equal to the right argument.
491 =head2 Equality Operators
492 X<equality> X<equal> X<equals> X<operator, equality>
494 Binary C<< "==" >> returns true if the left argument is numerically equal to
498 Binary C<< "!=" >> returns true if the left argument is numerically not equal
499 to the right argument.
502 Binary C<< "<=>" >> returns -1, 0, or 1 depending on whether the left
503 argument is numerically less than, equal to, or greater than the right
504 argument. If your platform supports C<NaN>'s (not-a-numbers) as numeric
505 values, using them with C<< "<=>" >> returns undef. C<NaN> is not
506 C<< "<" >>, C<< "==" >>, C<< ">" >>, C<< "<=" >> or C<< ">=" >> anything
507 (even C<NaN>), so those 5 return false. S<C<< NaN != NaN >>> returns
508 true, as does S<C<NaN !=> I<anything else>>. If your platform doesn't
509 support C<NaN>'s then C<NaN> is just a string with numeric value 0.
513 $ perl -le '$x = "NaN"; print "No NaN support here" if $x == $x'
514 $ perl -le '$x = "NaN"; print "NaN support here" if $x != $x'
516 (Note that the L<bigint>, L<bigrat>, and L<bignum> pragmas all
519 Binary C<"eq"> returns true if the left argument is stringwise equal to
523 Binary C<"ne"> returns true if the left argument is stringwise not equal
524 to the right argument.
527 Binary C<"cmp"> returns -1, 0, or 1 depending on whether the left
528 argument is stringwise less than, equal to, or greater than the right
532 Binary C<"~~"> does a smartmatch between its arguments. Smart matching
533 is described in the next section.
536 C<"lt">, C<"le">, C<"ge">, C<"gt"> and C<"cmp"> use the collation (sort)
537 order specified by the current C<LC_COLLATE> locale if a S<C<use
538 locale>> form that includes collation is in effect. See L<perllocale>.
539 Do not mix these with Unicode,
540 only use them with legacy 8-bit locale encodings.
541 The standard C<L<Unicode::Collate>> and
542 C<L<Unicode::Collate::Locale>> modules offer much more powerful
543 solutions to collation issues.
545 For case-insensitive comparisions, look at the L<perlfunc/fc> case-folding
546 function, available in Perl v5.16 or later:
548 if ( fc($x) eq fc($y) ) { ... }
550 =head2 Smartmatch Operator
552 First available in Perl 5.10.1 (the 5.10.0 version behaved differently),
553 binary C<~~> does a "smartmatch" between its arguments. This is mostly
554 used implicitly in the C<when> construct described in L<perlsyn>, although
555 not all C<when> clauses call the smartmatch operator. Unique among all of
556 Perl's operators, the smartmatch operator can recurse. The smartmatch
557 operator is L<experimental|perlpolicy/experimental> and its behavior is
560 It is also unique in that all other Perl operators impose a context
561 (usually string or numeric context) on their operands, autoconverting
562 those operands to those imposed contexts. In contrast, smartmatch
563 I<infers> contexts from the actual types of its operands and uses that
564 type information to select a suitable comparison mechanism.
566 The C<~~> operator compares its operands "polymorphically", determining how
567 to compare them according to their actual types (numeric, string, array,
568 hash, etc.) Like the equality operators with which it shares the same
569 precedence, C<~~> returns 1 for true and C<""> for false. It is often best
570 read aloud as "in", "inside of", or "is contained in", because the left
571 operand is often looked for I<inside> the right operand. That makes the
572 order of the operands to the smartmatch operand often opposite that of
573 the regular match operator. In other words, the "smaller" thing is usually
574 placed in the left operand and the larger one in the right.
576 The behavior of a smartmatch depends on what type of things its arguments
577 are, as determined by the following table. The first row of the table
578 whose types apply determines the smartmatch behavior. Because what
579 actually happens is mostly determined by the type of the second operand,
580 the table is sorted on the right operand instead of on the left.
582 Left Right Description and pseudocode
583 ===============================================================
584 Any undef check whether Any is undefined
587 Any Object invoke ~~ overloading on Object, or die
589 Right operand is an ARRAY:
591 Left Right Description and pseudocode
592 ===============================================================
593 ARRAY1 ARRAY2 recurse on paired elements of ARRAY1 and ARRAY2[2]
594 like: (ARRAY1[0] ~~ ARRAY2[0])
595 && (ARRAY1[1] ~~ ARRAY2[1]) && ...
596 HASH ARRAY any ARRAY elements exist as HASH keys
597 like: grep { exists HASH->{$_} } ARRAY
598 Regexp ARRAY any ARRAY elements pattern match Regexp
599 like: grep { /Regexp/ } ARRAY
600 undef ARRAY undef in ARRAY
601 like: grep { !defined } ARRAY
602 Any ARRAY smartmatch each ARRAY element[3]
603 like: grep { Any ~~ $_ } ARRAY
605 Right operand is a HASH:
607 Left Right Description and pseudocode
608 ===============================================================
609 HASH1 HASH2 all same keys in both HASHes
611 grep { exists HASH2->{$_} } keys HASH1
612 ARRAY HASH any ARRAY elements exist as HASH keys
613 like: grep { exists HASH->{$_} } ARRAY
614 Regexp HASH any HASH keys pattern match Regexp
615 like: grep { /Regexp/ } keys HASH
616 undef HASH always false (undef can't be a key)
618 Any HASH HASH key existence
619 like: exists HASH->{Any}
621 Right operand is CODE:
623 Left Right Description and pseudocode
624 ===============================================================
625 ARRAY CODE sub returns true on all ARRAY elements[1]
626 like: !grep { !CODE->($_) } ARRAY
627 HASH CODE sub returns true on all HASH keys[1]
628 like: !grep { !CODE->($_) } keys HASH
629 Any CODE sub passed Any returns true
632 Right operand is a Regexp:
634 Left Right Description and pseudocode
635 ===============================================================
636 ARRAY Regexp any ARRAY elements match Regexp
637 like: grep { /Regexp/ } ARRAY
638 HASH Regexp any HASH keys match Regexp
639 like: grep { /Regexp/ } keys HASH
640 Any Regexp pattern match
641 like: Any =~ /Regexp/
645 Left Right Description and pseudocode
646 ===============================================================
647 Object Any invoke ~~ overloading on Object,
650 Any Num numeric equality
652 Num nummy[4] numeric equality
654 undef Any check whether undefined
656 Any Any string equality
665 Empty hashes or arrays match.
668 That is, each element smartmatches the element of the same index in the other array.[3]
671 If a circular reference is found, fall back to referential equality.
674 Either an actual number, or a string that looks like one.
678 The smartmatch implicitly dereferences any non-blessed hash or array
679 reference, so the C<I<HASH>> and C<I<ARRAY>> entries apply in those cases.
680 For blessed references, the C<I<Object>> entries apply. Smartmatches
681 involving hashes only consider hash keys, never hash values.
683 The "like" code entry is not always an exact rendition. For example, the
684 smartmatch operator short-circuits whenever possible, but C<grep> does
685 not. Also, C<grep> in scalar context returns the number of matches, but
686 C<~~> returns only true or false.
688 Unlike most operators, the smartmatch operator knows to treat C<undef>
692 @array = (1, 2, 3, undef, 4, 5);
693 say "some elements undefined" if undef ~~ @array;
695 Each operand is considered in a modified scalar context, the modification
696 being that array and hash variables are passed by reference to the
697 operator, which implicitly dereferences them. Both elements
698 of each pair are the same:
702 my %hash = (red => 1, blue => 2, green => 3,
703 orange => 4, yellow => 5, purple => 6,
704 black => 7, grey => 8, white => 9);
706 my @array = qw(red blue green);
708 say "some array elements in hash keys" if @array ~~ %hash;
709 say "some array elements in hash keys" if \@array ~~ \%hash;
711 say "red in array" if "red" ~~ @array;
712 say "red in array" if "red" ~~ \@array;
714 say "some keys end in e" if /e$/ ~~ %hash;
715 say "some keys end in e" if /e$/ ~~ \%hash;
717 Two arrays smartmatch if each element in the first array smartmatches
718 (that is, is "in") the corresponding element in the second array,
722 my @little = qw(red blue green);
723 my @bigger = ("red", "blue", [ "orange", "green" ] );
724 if (@little ~~ @bigger) { # true!
725 say "little is contained in bigger";
728 Because the smartmatch operator recurses on nested arrays, this
729 will still report that "red" is in the array.
732 my @array = qw(red blue green);
733 my $nested_array = [[[[[[[ @array ]]]]]]];
734 say "red in array" if "red" ~~ $nested_array;
736 If two arrays smartmatch each other, then they are deep
737 copies of each others' values, as this example reports:
740 my @a = (0, 1, 2, [3, [4, 5], 6], 7);
741 my @b = (0, 1, 2, [3, [4, 5], 6], 7);
743 if (@a ~~ @b && @b ~~ @a) {
744 say "a and b are deep copies of each other";
747 say "a smartmatches in b";
750 say "b smartmatches in a";
753 say "a and b don't smartmatch each other at all";
757 If you were to set S<C<$b[3] = 4>>, then instead of reporting that "a and b
758 are deep copies of each other", it now reports that C<"b smartmatches in a">.
759 That's because the corresponding position in C<@a> contains an array that
760 (eventually) has a 4 in it.
762 Smartmatching one hash against another reports whether both contain the
763 same keys, no more and no less. This could be used to see whether two
764 records have the same field names, without caring what values those fields
765 might have. For example:
769 state $REQUIRED_FIELDS = { name=>1, rank=>1, serial_num=>1 };
771 my ($class, $init_fields) = @_;
773 die "Must supply (only) name, rank, and serial number"
774 unless $init_fields ~~ $REQUIRED_FIELDS;
779 However, this only does what you mean if C<$init_fields> is indeed a hash
780 reference. The condition C<$init_fields ~~ $REQUIRED_FIELDS> also allows the
781 strings C<"name">, C<"rank">, C<"serial_num"> as well as any array reference
782 that contains C<"name"> or C<"rank"> or C<"serial_num"> anywhere to pass
785 The smartmatch operator is most often used as the implicit operator of a
786 C<when> clause. See the section on "Switch Statements" in L<perlsyn>.
788 =head3 Smartmatching of Objects
790 To avoid relying on an object's underlying representation, if the
791 smartmatch's right operand is an object that doesn't overload C<~~>,
792 it raises the exception "C<Smartmatching a non-overloaded object
793 breaks encapsulation>". That's because one has no business digging
794 around to see whether something is "in" an object. These are all
795 illegal on objects without a C<~~> overload:
801 However, you can change the way an object is smartmatched by overloading
802 the C<~~> operator. This is allowed to
803 extend the usual smartmatch semantics.
804 For objects that do have an C<~~> overload, see L<overload>.
806 Using an object as the left operand is allowed, although not very useful.
807 Smartmatching rules take precedence over overloading, so even if the
808 object in the left operand has smartmatch overloading, this will be
809 ignored. A left operand that is a non-overloaded object falls back on a
810 string or numeric comparison of whatever the C<ref> operator returns. That
815 does I<not> invoke the overload method with C<I<X>> as an argument.
816 Instead the above table is consulted as normal, and based on the type of
817 C<I<X>>, overloading may or may not be invoked. For simple strings or
818 numbers, "in" becomes equivalent to this:
820 $object ~~ $number ref($object) == $number
821 $object ~~ $string ref($object) eq $string
823 For example, this reports that the handle smells IOish
824 (but please don't really do this!):
827 my $fh = IO::Handle->new();
828 if ($fh ~~ /\bIO\b/) {
829 say "handle smells IOish";
832 That's because it treats C<$fh> as a string like
833 C<"IO::Handle=GLOB(0x8039e0)">, then pattern matches against that.
836 X<operator, bitwise, and> X<bitwise and> X<&>
838 Binary C<"&"> returns its operands ANDed together bit by bit. Although no
839 warning is currently raised, the result is not well defined when this operation
840 is performed on operands that aren't either numbers (see
841 L</Integer Arithmetic>) nor bitstrings (see L</Bitwise String Operators>).
843 Note that C<"&"> has lower priority than relational operators, so for example
844 the parentheses are essential in a test like
846 print "Even\n" if ($x & 1) == 0;
848 If the experimental "bitwise" feature is enabled via S<C<use feature
849 'bitwise'>>, then this operator always treats its operand as numbers. This
850 feature produces a warning unless you also use C<S<no warnings
851 'experimental::bitwise'>>.
853 =head2 Bitwise Or and Exclusive Or
854 X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor>
857 Binary C<"|"> returns its operands ORed together bit by bit.
859 Binary C<"^"> returns its operands XORed together bit by bit.
861 Although no warning is currently raised, the results are not well
862 defined when these operations are performed on operands that aren't either
863 numbers (see L</Integer Arithmetic>) nor bitstrings (see L</Bitwise String
866 Note that C<"|"> and C<"^"> have lower priority than relational operators, so
867 for example the parentheses are essential in a test like
869 print "false\n" if (8 | 2) != 10;
871 If the experimental "bitwise" feature is enabled via S<C<use feature
872 'bitwise'>>, then this operator always treats its operand as numbers. This
873 feature produces a warning unless you also use S<C<no warnings
874 'experimental::bitwise'>>.
876 =head2 C-style Logical And
877 X<&&> X<logical and> X<operator, logical, and>
879 Binary C<"&&"> performs a short-circuit logical AND operation. That is,
880 if the left operand is false, the right operand is not even evaluated.
881 Scalar or list context propagates down to the right operand if it
884 =head2 C-style Logical Or
885 X<||> X<operator, logical, or>
887 Binary C<"||"> performs a short-circuit logical OR operation. That is,
888 if the left operand is true, the right operand is not even evaluated.
889 Scalar or list context propagates down to the right operand if it
892 =head2 Logical Defined-Or
893 X<//> X<operator, logical, defined-or>
895 Although it has no direct equivalent in C, Perl's C<//> operator is related
896 to its C-style "or". In fact, it's exactly the same as C<||>, except that it
897 tests the left hand side's definedness instead of its truth. Thus,
898 S<C<< EXPR1 // EXPR2 >>> returns the value of C<< EXPR1 >> if it's defined,
899 otherwise, the value of C<< EXPR2 >> is returned.
900 (C<< EXPR1 >> is evaluated in scalar context, C<< EXPR2 >>
901 in the context of C<< // >> itself). Usually,
902 this is the same result as S<C<< defined(EXPR1) ? EXPR1 : EXPR2 >>> (except that
903 the ternary-operator form can be used as a lvalue, while S<C<< EXPR1 // EXPR2 >>>
904 cannot). This is very useful for
905 providing default values for variables. If you actually want to test if
906 at least one of C<$x> and C<$y> is defined, use S<C<defined($x // $y)>>.
908 The C<||>, C<//> and C<&&> operators return the last value evaluated
909 (unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably
910 portable way to find out the home directory might be:
915 // die "You're homeless!\n";
917 In particular, this means that you shouldn't use this
918 for selecting between two aggregates for assignment:
920 @a = @b || @c; # This doesn't do the right thing
921 @a = scalar(@b) || @c; # because it really means this.
922 @a = @b ? @b : @c; # This works fine, though.
924 As alternatives to C<&&> and C<||> when used for
925 control flow, Perl provides the C<and> and C<or> operators (see below).
926 The short-circuit behavior is identical. The precedence of C<"and">
927 and C<"or"> is much lower, however, so that you can safely use them after a
928 list operator without the need for parentheses:
930 unlink "alpha", "beta", "gamma"
931 or gripe(), next LINE;
933 With the C-style operators that would have been written like this:
935 unlink("alpha", "beta", "gamma")
936 || (gripe(), next LINE);
938 It would be even more readable to write that this way:
940 unless(unlink("alpha", "beta", "gamma")) {
945 Using C<"or"> for assignment is unlikely to do what you want; see below.
947 =head2 Range Operators
948 X<operator, range> X<range> X<..> X<...>
950 Binary C<".."> is the range operator, which is really two different
951 operators depending on the context. In list context, it returns a
952 list of values counting (up by ones) from the left value to the right
953 value. If the left value is greater than the right value then it
954 returns the empty list. The range operator is useful for writing
955 S<C<foreach (1..10)>> loops and for doing slice operations on arrays. In
956 the current implementation, no temporary array is created when the
957 range operator is used as the expression in C<foreach> loops, but older
958 versions of Perl might burn a lot of memory when you write something
961 for (1 .. 1_000_000) {
965 The range operator also works on strings, using the magical
966 auto-increment, see below.
968 In scalar context, C<".."> returns a boolean value. The operator is
969 bistable, like a flip-flop, and emulates the line-range (comma)
970 operator of B<sed>, B<awk>, and various editors. Each C<".."> operator
971 maintains its own boolean state, even across calls to a subroutine
972 that contains it. It is false as long as its left operand is false.
973 Once the left operand is true, the range operator stays true until the
974 right operand is true, I<AFTER> which the range operator becomes false
975 again. It doesn't become false till the next time the range operator
976 is evaluated. It can test the right operand and become false on the
977 same evaluation it became true (as in B<awk>), but it still returns
978 true once. If you don't want it to test the right operand until the
979 next evaluation, as in B<sed>, just use three dots (C<"...">) instead of
980 two. In all other regards, C<"..."> behaves just like C<".."> does.
982 The right operand is not evaluated while the operator is in the
983 "false" state, and the left operand is not evaluated while the
984 operator is in the "true" state. The precedence is a little lower
985 than || and &&. The value returned is either the empty string for
986 false, or a sequence number (beginning with 1) for true. The sequence
987 number is reset for each range encountered. The final sequence number
988 in a range has the string C<"E0"> appended to it, which doesn't affect
989 its numeric value, but gives you something to search for if you want
990 to exclude the endpoint. You can exclude the beginning point by
991 waiting for the sequence number to be greater than 1.
993 If either operand of scalar C<".."> is a constant expression,
994 that operand is considered true if it is equal (C<==>) to the current
995 input line number (the C<$.> variable).
997 To be pedantic, the comparison is actually S<C<int(EXPR) == int(EXPR)>>,
998 but that is only an issue if you use a floating point expression; when
999 implicitly using C<$.> as described in the previous paragraph, the
1000 comparison is S<C<int(EXPR) == int($.)>> which is only an issue when C<$.>
1001 is set to a floating point value and you are not reading from a file.
1002 Furthermore, S<C<"span" .. "spat">> or S<C<2.18 .. 3.14>> will not do what
1003 you want in scalar context because each of the operands are evaluated
1004 using their integer representation.
1008 As a scalar operator:
1010 if (101 .. 200) { print; } # print 2nd hundred lines, short for
1011 # if ($. == 101 .. $. == 200) { print; }
1013 next LINE if (1 .. /^$/); # skip header lines, short for
1014 # next LINE if ($. == 1 .. /^$/);
1015 # (typically in a loop labeled LINE)
1017 s/^/> / if (/^$/ .. eof()); # quote body
1019 # parse mail messages
1021 $in_header = 1 .. /^$/;
1022 $in_body = /^$/ .. eof;
1029 close ARGV if eof; # reset $. each file
1032 Here's a simple example to illustrate the difference between
1033 the two range operators:
1046 This program will print only the line containing "Bar". If
1047 the range operator is changed to C<...>, it will also print the
1050 And now some examples as a list operator:
1052 for (101 .. 200) { print } # print $_ 100 times
1053 @foo = @foo[0 .. $#foo]; # an expensive no-op
1054 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
1056 The range operator (in list context) makes use of the magical
1057 auto-increment algorithm if the operands are strings. You
1060 @alphabet = ("A" .. "Z");
1062 to get all normal letters of the English alphabet, or
1064 $hexdigit = (0 .. 9, "a" .. "f")[$num & 15];
1066 to get a hexadecimal digit, or
1068 @z2 = ("01" .. "31");
1071 to get dates with leading zeros.
1073 If the final value specified is not in the sequence that the magical
1074 increment would produce, the sequence goes until the next value would
1075 be longer than the final value specified.
1077 If the initial value specified isn't part of a magical increment
1078 sequence (that is, a non-empty string matching C</^[a-zA-Z]*[0-9]*\z/>),
1079 only the initial value will be returned. So the following will only
1082 use charnames "greek";
1083 my @greek_small = ("\N{alpha}" .. "\N{omega}");
1085 To get the 25 traditional lowercase Greek letters, including both sigmas,
1086 you could use this instead:
1088 use charnames "greek";
1089 my @greek_small = map { chr } ( ord("\N{alpha}")
1094 However, because there are I<many> other lowercase Greek characters than
1095 just those, to match lowercase Greek characters in a regular expression,
1096 you could use the pattern C</(?:(?=\p{Greek})\p{Lower})+/> (or the
1097 L<experimental feature|perlrecharclass/Extended Bracketed Character
1098 Classes> C<S</(?[ \p{Greek} & \p{Lower} ])+/>>).
1100 Because each operand is evaluated in integer form, S<C<2.18 .. 3.14>> will
1101 return two elements in list context.
1103 @list = (2.18 .. 3.14); # same as @list = (2 .. 3);
1105 =head2 Conditional Operator
1106 X<operator, conditional> X<operator, ternary> X<ternary> X<?:>
1108 Ternary C<"?:"> is the conditional operator, just as in C. It works much
1109 like an if-then-else. If the argument before the C<?> is true, the
1110 argument before the C<:> is returned, otherwise the argument after the
1111 C<:> is returned. For example:
1113 printf "I have %d dog%s.\n", $n,
1114 ($n == 1) ? "" : "s";
1116 Scalar or list context propagates downward into the 2nd
1117 or 3rd argument, whichever is selected.
1119 $x = $ok ? $y : $z; # get a scalar
1120 @x = $ok ? @y : @z; # get an array
1121 $x = $ok ? @y : @z; # oops, that's just a count!
1123 The operator may be assigned to if both the 2nd and 3rd arguments are
1124 legal lvalues (meaning that you can assign to them):
1126 ($x_or_y ? $x : $y) = $z;
1128 Because this operator produces an assignable result, using assignments
1129 without parentheses will get you in trouble. For example, this:
1131 $x % 2 ? $x += 10 : $x += 2
1135 (($x % 2) ? ($x += 10) : $x) += 2
1139 ($x % 2) ? ($x += 10) : ($x += 2)
1141 That should probably be written more simply as:
1143 $x += ($x % 2) ? 10 : 2;
1145 =head2 Assignment Operators
1146 X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=>
1147 X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=>
1148 X<%=> X<^=> X<x=> X<&.=> X<|.=> X<^.=>
1150 C<"="> is the ordinary assignment operator.
1152 Assignment operators work as in C. That is,
1160 although without duplicating any side effects that dereferencing the lvalue
1161 might trigger, such as from C<tie()>. Other assignment operators work similarly.
1162 The following are recognized:
1164 **= += *= &= &.= <<= &&=
1165 -= /= |= |.= >>= ||=
1169 Although these are grouped by family, they all have the precedence
1170 of assignment. These combined assignment operators can only operate on
1171 scalars, whereas the ordinary assignment operator can assign to arrays,
1172 hashes, lists and even references. (See L<"Context"|perldata/Context>
1173 and L<perldata/List value constructors>, and L<perlref/Assigning to
1176 Unlike in C, the scalar assignment operator produces a valid lvalue.
1177 Modifying an assignment is equivalent to doing the assignment and
1178 then modifying the variable that was assigned to. This is useful
1179 for modifying a copy of something, like this:
1181 ($tmp = $global) =~ tr/13579/24680/;
1183 Although as of 5.14, that can be also be accomplished this way:
1186 $tmp = ($global =~ tr/13579/24680/r);
1197 Similarly, a list assignment in list context produces the list of
1198 lvalues assigned to, and a list assignment in scalar context returns
1199 the number of elements produced by the expression on the right hand
1200 side of the assignment.
1202 The three dotted bitwise assignment operators (C<&.=> C<|.=> C<^.=>) are new in
1203 Perl 5.22 and experimental. See L</Bitwise String Operators>.
1205 =head2 Comma Operator
1206 X<comma> X<operator, comma> X<,>
1208 Binary C<","> is the comma operator. In scalar context it evaluates
1209 its left argument, throws that value away, then evaluates its right
1210 argument and returns that value. This is just like C's comma operator.
1212 In list context, it's just the list argument separator, and inserts
1213 both its arguments into the list. These arguments are also evaluated
1216 The C<< => >> operator (sometimes pronounced "fat comma") is a synonym
1217 for the comma except that it causes a
1218 word on its left to be interpreted as a string if it begins with a letter
1219 or underscore and is composed only of letters, digits and underscores.
1220 This includes operands that might otherwise be interpreted as operators,
1221 constants, single number v-strings or function calls. If in doubt about
1222 this behavior, the left operand can be quoted explicitly.
1224 Otherwise, the C<< => >> operator behaves exactly as the comma operator
1225 or list argument separator, according to context.
1229 use constant FOO => "something";
1231 my %h = ( FOO => 23 );
1235 my %h = ("FOO", 23);
1239 my %h = ("something", 23);
1241 The C<< => >> operator is helpful in documenting the correspondence
1242 between keys and values in hashes, and other paired elements in lists.
1244 %hash = ( $key => $value );
1245 login( $username => $password );
1247 The special quoting behavior ignores precedence, and hence may apply to
1248 I<part> of the left operand:
1250 print time.shift => "bbb";
1252 That example prints something like C<"1314363215shiftbbb">, because the
1253 C<< => >> implicitly quotes the C<shift> immediately on its left, ignoring
1254 the fact that C<time.shift> is the entire left operand.
1256 =head2 List Operators (Rightward)
1257 X<operator, list, rightward> X<list operator>
1259 On the right side of a list operator, the comma has very low precedence,
1260 such that it controls all comma-separated expressions found there.
1261 The only operators with lower precedence are the logical operators
1262 C<"and">, C<"or">, and C<"not">, which may be used to evaluate calls to list
1263 operators without the need for parentheses:
1265 open HANDLE, "< :utf8", "filename" or die "Can't open: $!\n";
1267 However, some people find that code harder to read than writing
1268 it with parentheses:
1270 open(HANDLE, "< :utf8", "filename") or die "Can't open: $!\n";
1272 in which case you might as well just use the more customary C<"||"> operator:
1274 open(HANDLE, "< :utf8", "filename") || die "Can't open: $!\n";
1276 See also discussion of list operators in L</Terms and List Operators (Leftward)>.
1279 X<operator, logical, not> X<not>
1281 Unary C<"not"> returns the logical negation of the expression to its right.
1282 It's the equivalent of C<"!"> except for the very low precedence.
1285 X<operator, logical, and> X<and>
1287 Binary C<"and"> returns the logical conjunction of the two surrounding
1288 expressions. It's equivalent to C<&&> except for the very low
1289 precedence. This means that it short-circuits: the right
1290 expression is evaluated only if the left expression is true.
1292 =head2 Logical or and Exclusive Or
1293 X<operator, logical, or> X<operator, logical, xor>
1294 X<operator, logical, exclusive or>
1297 Binary C<"or"> returns the logical disjunction of the two surrounding
1298 expressions. It's equivalent to C<||> except for the very low precedence.
1299 This makes it useful for control flow:
1301 print FH $data or die "Can't write to FH: $!";
1303 This means that it short-circuits: the right expression is evaluated
1304 only if the left expression is false. Due to its precedence, you must
1305 be careful to avoid using it as replacement for the C<||> operator.
1306 It usually works out better for flow control than in assignments:
1308 $x = $y or $z; # bug: this is wrong
1309 ($x = $y) or $z; # really means this
1310 $x = $y || $z; # better written this way
1312 However, when it's a list-context assignment and you're trying to use
1313 C<||> for control flow, you probably need C<"or"> so that the assignment
1314 takes higher precedence.
1316 @info = stat($file) || die; # oops, scalar sense of stat!
1317 @info = stat($file) or die; # better, now @info gets its due
1319 Then again, you could always use parentheses.
1321 Binary C<"xor"> returns the exclusive-OR of the two surrounding expressions.
1322 It cannot short-circuit (of course).
1324 There is no low precedence operator for defined-OR.
1326 =head2 C Operators Missing From Perl
1327 X<operator, missing from perl> X<&> X<*>
1328 X<typecasting> X<(TYPE)>
1330 Here is what C has that Perl doesn't:
1336 Address-of operator. (But see the C<"\"> operator for taking a reference.)
1340 Dereference-address operator. (Perl's prefix dereferencing
1341 operators are typed: C<$>, C<@>, C<%>, and C<&>.)
1345 Type-casting operator.
1349 =head2 Quote and Quote-like Operators
1350 X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m>
1351 X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>>
1352 X<escape sequence> X<escape>
1354 While we usually think of quotes as literal values, in Perl they
1355 function as operators, providing various kinds of interpolating and
1356 pattern matching capabilities. Perl provides customary quote characters
1357 for these behaviors, but also provides a way for you to choose your
1358 quote character for any of them. In the following table, a C<{}> represents
1359 any pair of delimiters you choose.
1361 Customary Generic Meaning Interpolates
1364 `` qx{} Command yes*
1366 // m{} Pattern match yes*
1368 s{}{} Substitution yes*
1369 tr{}{} Transliteration no (but see below)
1370 y{}{} Transliteration no (but see below)
1373 * unless the delimiter is ''.
1375 Non-bracketing delimiters use the same character fore and aft, but the four
1376 sorts of ASCII brackets (round, angle, square, curly) all nest, which means
1385 Note, however, that this does not always work for quoting Perl code:
1387 $s = q{ if($x eq "}") ... }; # WRONG
1389 is a syntax error. The C<L<Text::Balanced>> module (standard as of v5.8,
1390 and from CPAN before then) is able to do this properly.
1392 There can (and in some cases, must) be whitespace between the operator
1394 characters, except when C<#> is being used as the quoting character.
1395 C<q#foo#> is parsed as the string C<foo>, while S<C<q #foo#>> is the
1396 operator C<q> followed by a comment. Its argument will be taken
1397 from the next line. This allows you to write:
1399 s {foo} # Replace foo
1402 The cases where whitespace must be used are when the quoting character
1403 is a word character (meaning it matches C</\w/>):
1405 q XfooX # Works: means the string 'foo'
1408 The following escape sequences are available in constructs that interpolate,
1409 and in transliterations:
1410 X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N> X<\N{}>
1413 Sequence Note Description
1419 \a alarm (bell) (BEL)
1421 \x{263A} [1,8] hex char (example: SMILEY)
1422 \x1b [2,8] restricted range hex char (example: ESC)
1423 \N{name} [3] named Unicode character or character sequence
1424 \N{U+263D} [4,8] Unicode character (example: FIRST QUARTER MOON)
1425 \c[ [5] control char (example: chr(27))
1426 \o{23072} [6,8] octal char (example: SMILEY)
1427 \033 [7,8] restricted range octal char (example: ESC)
1433 The result is the character specified by the hexadecimal number between
1434 the braces. See L</[8]> below for details on which character.
1436 Only hexadecimal digits are valid between the braces. If an invalid
1437 character is encountered, a warning will be issued and the invalid
1438 character and all subsequent characters (valid or invalid) within the
1439 braces will be discarded.
1441 If there are no valid digits between the braces, the generated character is
1442 the NULL character (C<\x{00}>). However, an explicit empty brace (C<\x{}>)
1443 will not cause a warning (currently).
1447 The result is the character specified by the hexadecimal number in the range
1448 0x00 to 0xFF. See L</[8]> below for details on which character.
1450 Only hexadecimal digits are valid following C<\x>. When C<\x> is followed
1451 by fewer than two valid digits, any valid digits will be zero-padded. This
1452 means that C<\x7> will be interpreted as C<\x07>, and a lone C<"\x"> will be
1453 interpreted as C<\x00>. Except at the end of a string, having fewer than
1454 two valid digits will result in a warning. Note that although the warning
1455 says the illegal character is ignored, it is only ignored as part of the
1456 escape and will still be used as the subsequent character in the string.
1459 Original Result Warns?
1467 The result is the Unicode character or character sequence given by I<name>.
1472 S<C<\N{U+I<hexadecimal number>}>> means the Unicode character whose Unicode code
1473 point is I<hexadecimal number>.
1477 The character following C<\c> is mapped to some other character as shown in the
1490 # See below for chr(28)
1494 \c? chr(127) # (on ASCII platforms; see below for link to
1495 # EBCDIC discussion)
1497 In other words, it's the character whose code point has had 64 xor'd with
1498 its uppercase. C<\c?> is DELETE on ASCII platforms because
1499 S<C<ord("?") ^ 64>> is 127, and
1500 C<\c@> is NULL because the ord of C<"@"> is 64, so xor'ing 64 itself produces 0.
1502 Also, C<\c\I<X>> yields S<C< chr(28) . "I<X>">> for any I<X>, but cannot come at the
1503 end of a string, because the backslash would be parsed as escaping the end
1506 On ASCII platforms, the resulting characters from the list above are the
1507 complete set of ASCII controls. This isn't the case on EBCDIC platforms; see
1508 L<perlebcdic/OPERATOR DIFFERENCES> for a full discussion of the
1509 differences between these for ASCII versus EBCDIC platforms.
1511 Use of any other character following the C<"c"> besides those listed above is
1512 discouraged, and as of Perl v5.20, the only characters actually allowed
1513 are the printable ASCII ones, minus the left brace C<"{">. What happens
1514 for any of the allowed other characters is that the value is derived by
1515 xor'ing with the seventh bit, which is 64, and a warning raised if
1516 enabled. Using the non-allowed characters generates a fatal error.
1518 To get platform independent controls, you can use C<\N{...}>.
1522 The result is the character specified by the octal number between the braces.
1523 See L</[8]> below for details on which character.
1525 If a character that isn't an octal digit is encountered, a warning is raised,
1526 and the value is based on the octal digits before it, discarding it and all
1527 following characters up to the closing brace. It is a fatal error if there are
1528 no octal digits at all.
1532 The result is the character specified by the three-digit octal number in the
1533 range 000 to 777 (but best to not use above 077, see next paragraph). See
1534 L</[8]> below for details on which character.
1536 Some contexts allow 2 or even 1 digit, but any usage without exactly
1537 three digits, the first being a zero, may give unintended results. (For
1538 example, in a regular expression it may be confused with a backreference;
1539 see L<perlrebackslash/Octal escapes>.) Starting in Perl 5.14, you may
1540 use C<\o{}> instead, which avoids all these problems. Otherwise, it is best to
1541 use this construct only for ordinals C<\077> and below, remembering to pad to
1542 the left with zeros to make three digits. For larger ordinals, either use
1543 C<\o{}>, or convert to something else, such as to hex and use C<\N{U+}>
1544 (which is portable between platforms with different character sets) or
1549 Several constructs above specify a character by a number. That number
1550 gives the character's position in the character set encoding (indexed from 0).
1551 This is called synonymously its ordinal, code position, or code point. Perl
1552 works on platforms that have a native encoding currently of either ASCII/Latin1
1553 or EBCDIC, each of which allow specification of 256 characters. In general, if
1554 the number is 255 (0xFF, 0377) or below, Perl interprets this in the platform's
1555 native encoding. If the number is 256 (0x100, 0400) or above, Perl interprets
1556 it as a Unicode code point and the result is the corresponding Unicode
1557 character. For example C<\x{50}> and C<\o{120}> both are the number 80 in
1558 decimal, which is less than 256, so the number is interpreted in the native
1559 character set encoding. In ASCII the character in the 80th position (indexed
1560 from 0) is the letter C<"P">, and in EBCDIC it is the ampersand symbol C<"&">.
1561 C<\x{100}> and C<\o{400}> are both 256 in decimal, so the number is interpreted
1562 as a Unicode code point no matter what the native encoding is. The name of the
1563 character in the 256th position (indexed by 0) in Unicode is
1564 C<LATIN CAPITAL LETTER A WITH MACRON>.
1566 There are a couple of exceptions to the above rule. S<C<\N{U+I<hex number>}>> is
1567 always interpreted as a Unicode code point, so that C<\N{U+0050}> is C<"P"> even
1568 on EBCDIC platforms. And if C<S<L<use encoding|encoding>>> is in effect, the
1569 number is considered to be in that encoding, and is translated from that into
1570 the platform's native encoding if there is a corresponding native character;
1571 otherwise to Unicode.
1575 B<NOTE>: Unlike C and other languages, Perl has no C<\v> escape sequence for
1576 the vertical tab (VT, which is 11 in both ASCII and EBCDIC), but you may
1577 use C<\N{VT}>, C<\ck>, C<\N{U+0b}>, or C<\x0b>. (C<\v>
1578 does have meaning in regular expression patterns in Perl, see L<perlre>.)
1580 The following escape sequences are available in constructs that interpolate,
1581 but not in transliterations.
1582 X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q> X<\F>
1584 \l lowercase next character only
1585 \u titlecase (not uppercase!) next character only
1586 \L lowercase all characters till \E or end of string
1587 \U uppercase all characters till \E or end of string
1588 \F foldcase all characters till \E or end of string
1589 \Q quote (disable) pattern metacharacters till \E or
1591 \E end either case modification or quoted section
1592 (whichever was last seen)
1594 See L<perlfunc/quotemeta> for the exact definition of characters that
1595 are quoted by C<\Q>.
1597 C<\L>, C<\U>, C<\F>, and C<\Q> can stack, in which case you need one
1598 C<\E> for each. For example:
1600 say"This \Qquoting \ubusiness \Uhere isn't quite\E done yet,\E is it?";
1601 This quoting\ Business\ HERE\ ISN\'T\ QUITE\ done\ yet\, is it?
1603 If a S<C<use locale>> form that includes C<LC_CTYPE> is in effect (see
1604 L<perllocale>), the case map used by C<\l>, C<\L>, C<\u>, and C<\U> is
1605 taken from the current locale. If Unicode (for example, C<\N{}> or code
1606 points of 0x100 or beyond) is being used, the case map used by C<\l>,
1607 C<\L>, C<\u>, and C<\U> is as defined by Unicode. That means that
1608 case-mapping a single character can sometimes produce a sequence of
1610 Under S<C<use locale>>, C<\F> produces the same results as C<\L>
1611 for all locales but a UTF-8 one, where it instead uses the Unicode
1614 All systems use the virtual C<"\n"> to represent a line terminator,
1615 called a "newline". There is no such thing as an unvarying, physical
1616 newline character. It is only an illusion that the operating system,
1617 device drivers, C libraries, and Perl all conspire to preserve. Not all
1618 systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
1619 on the ancient Macs (pre-MacOS X) of yesteryear, these used to be reversed,
1620 and on systems without a line terminator,
1621 printing C<"\n"> might emit no actual data. In general, use C<"\n"> when
1622 you mean a "newline" for your system, but use the literal ASCII when you
1623 need an exact character. For example, most networking protocols expect
1624 and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
1625 and although they often accept just C<"\012">, they seldom tolerate just
1626 C<"\015">. If you get in the habit of using C<"\n"> for networking,
1627 you may be burned some day.
1628 X<newline> X<line terminator> X<eol> X<end of line>
1631 For constructs that do interpolate, variables beginning with "C<$>"
1632 or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
1633 C<< $href->{key}[0] >> are also interpolated, as are array and hash slices.
1634 But method calls such as C<< $obj->meth >> are not.
1636 Interpolating an array or slice interpolates the elements in order,
1637 separated by the value of C<$">, so is equivalent to interpolating
1638 S<C<join $", @array>>. "Punctuation" arrays such as C<@*> are usually
1639 interpolated only if the name is enclosed in braces C<@{*}>, but the
1640 arrays C<@_>, C<@+>, and C<@-> are interpolated even without braces.
1642 For double-quoted strings, the quoting from C<\Q> is applied after
1643 interpolation and escapes are processed.
1645 "abc\Qfoo\tbar$s\Exyz"
1649 "abc" . quotemeta("foo\tbar$s") . "xyz"
1651 For the pattern of regex operators (C<qr//>, C<m//> and C<s///>),
1652 the quoting from C<\Q> is applied after interpolation is processed,
1653 but before escapes are processed. This allows the pattern to match
1654 literally (except for C<$> and C<@>). For example, the following matches:
1658 Because C<$> or C<@> trigger interpolation, you'll need to use something
1659 like C</\Quser\E\@\Qhost/> to match them literally.
1661 Patterns are subject to an additional level of interpretation as a
1662 regular expression. This is done as a second pass, after variables are
1663 interpolated, so that regular expressions may be incorporated into the
1664 pattern from the variables. If this is not what you want, use C<\Q> to
1665 interpolate a variable literally.
1667 Apart from the behavior described above, Perl does not expand
1668 multiple levels of interpolation. In particular, contrary to the
1669 expectations of shell programmers, back-quotes do I<NOT> interpolate
1670 within double quotes, nor do single quotes impede evaluation of
1671 variables when used within double quotes.
1673 =head2 Regexp Quote-Like Operators
1676 Here are the quote-like operators that apply to pattern
1677 matching and related activities.
1681 =item C<qr/I<STRING>/msixpodualn>
1682 X<qr> X</i> X</m> X</o> X</s> X</x> X</p>
1684 This operator quotes (and possibly compiles) its I<STRING> as a regular
1685 expression. I<STRING> is interpolated the same way as I<PATTERN>
1686 in C<m/I<PATTERN>/>. If C<"'"> is used as the delimiter, no variable
1687 interpolation is done. Returns a Perl value which may be used instead of the
1688 corresponding C</I<STRING>/msixpodualn> expression. The returned value is a
1689 normalized version of the original pattern. It magically differs from
1690 a string containing the same characters: C<ref(qr/x/)> returns "Regexp";
1691 however, dereferencing it is not well defined (you currently get the
1692 normalized version of the original pattern, but this may change).
1697 $rex = qr/my.STRING/is;
1698 print $rex; # prints (?si-xm:my.STRING)
1705 The result may be used as a subpattern in a match:
1708 $string =~ /foo${re}bar/; # can be interpolated in other
1710 $string =~ $re; # or used standalone
1711 $string =~ /$re/; # or this way
1713 Since Perl may compile the pattern at the moment of execution of the C<qr()>
1714 operator, using C<qr()> may have speed advantages in some situations,
1715 notably if the result of C<qr()> is used standalone:
1718 my $patterns = shift;
1719 my @compiled = map qr/$_/i, @$patterns;
1722 foreach my $pat (@compiled) {
1723 $success = 1, last if /$pat/;
1729 Precompilation of the pattern into an internal representation at
1730 the moment of C<qr()> avoids the need to recompile the pattern every
1731 time a match C</$pat/> is attempted. (Perl has many other internal
1732 optimizations, but none would be triggered in the above example if
1733 we did not use C<qr()> operator.)
1735 Options (specified by the following modifiers) are:
1737 m Treat string as multiple lines.
1738 s Treat string as single line. (Make . match a newline)
1739 i Do case-insensitive pattern matching.
1740 x Use extended regular expressions.
1741 p When matching preserve a copy of the matched string so
1742 that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be
1743 defined (ignored starting in v5.20) as these are always
1744 defined starting in that relese
1745 o Compile pattern only once.
1746 a ASCII-restrict: Use ASCII for \d, \s, \w; specifying two
1747 a's further restricts things to that that no ASCII
1748 character will match a non-ASCII one under /i.
1749 l Use the current run-time locale's rules.
1750 u Use Unicode rules.
1751 d Use Unicode or native charset, as in 5.12 and earlier.
1752 n Non-capture mode. Don't let () fill in $1, $2, etc...
1754 If a precompiled pattern is embedded in a larger pattern then the effect
1755 of C<"msixpluadn"> will be propagated appropriately. The effect that the
1756 C</o> modifier has is not propagated, being restricted to those patterns
1757 explicitly using it.
1759 The last four modifiers listed above, added in Perl 5.14,
1760 control the character set rules, but C</a> is the only one you are likely
1761 to want to specify explicitly; the other three are selected
1762 automatically by various pragmas.
1764 See L<perlre> for additional information on valid syntax for I<STRING>, and
1765 for a detailed look at the semantics of regular expressions. In
1766 particular, all modifiers except the largely obsolete C</o> are further
1767 explained in L<perlre/Modifiers>. C</o> is described in the next section.
1769 =item C<m/I<PATTERN>/msixpodualngc>
1770 X<m> X<operator, match>
1771 X<regexp, options> X<regexp> X<regex, options> X<regex>
1772 X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c>
1774 =item C</I<PATTERN>/msixpodualngc>
1776 Searches a string for a pattern match, and in scalar context returns
1777 true if it succeeds, false if it fails. If no string is specified
1778 via the C<=~> or C<!~> operator, the C<$_> string is searched. (The
1779 string specified with C<=~> need not be an lvalue--it may be the
1780 result of an expression evaluation, but remember the C<=~> binds
1781 rather tightly.) See also L<perlre>.
1783 Options are as described in C<qr//> above; in addition, the following match
1784 process modifiers are available:
1786 g Match globally, i.e., find all occurrences.
1787 c Do not reset search position on a failed match when /g is
1790 If C<"/"> is the delimiter then the initial C<m> is optional. With the C<m>
1791 you can use any pair of non-whitespace (ASCII) characters
1792 as delimiters. This is particularly useful for matching path names
1793 that contain C<"/">, to avoid LTS (leaning toothpick syndrome). If C<"?"> is
1794 the delimiter, then a match-only-once rule applies,
1795 described in C<m?I<PATTERN>?> below. If C<"'"> (single quote) is the delimiter,
1796 no variable interpolation is performed on the I<PATTERN>.
1797 When using a delimiter character valid in an identifier, whitespace is required
1800 I<PATTERN> may contain variables, which will be interpolated
1801 every time the pattern search is evaluated, except
1802 for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
1803 C<$|> are not interpolated because they look like end-of-string tests.)
1804 Perl will not recompile the pattern unless an interpolated
1805 variable that it contains changes. You can force Perl to skip the
1806 test and never recompile by adding a C</o> (which stands for "once")
1807 after the trailing delimiter.
1808 Once upon a time, Perl would recompile regular expressions
1809 unnecessarily, and this modifier was useful to tell it not to do so, in the
1810 interests of speed. But now, the only reasons to use C</o> are one of:
1816 The variables are thousands of characters long and you know that they
1817 don't change, and you need to wring out the last little bit of speed by
1818 having Perl skip testing for that. (There is a maintenance penalty for
1819 doing this, as mentioning C</o> constitutes a promise that you won't
1820 change the variables in the pattern. If you do change them, Perl won't
1825 you want the pattern to use the initial values of the variables
1826 regardless of whether they change or not. (But there are saner ways
1827 of accomplishing this than using C</o>.)
1831 If the pattern contains embedded code, such as
1834 $code = 'foo(?{ $x })';
1837 then perl will recompile each time, even though the pattern string hasn't
1838 changed, to ensure that the current value of C<$x> is seen each time.
1839 Use C</o> if you want to avoid this.
1843 The bottom line is that using C</o> is almost never a good idea.
1845 =item The empty pattern C<//>
1847 If the I<PATTERN> evaluates to the empty string, the last
1848 I<successfully> matched regular expression is used instead. In this
1849 case, only the C<g> and C<c> flags on the empty pattern are honored;
1850 the other flags are taken from the original pattern. If no match has
1851 previously succeeded, this will (silently) act instead as a genuine
1852 empty pattern (which will always match).
1854 Note that it's possible to confuse Perl into thinking C<//> (the empty
1855 regex) is really C<//> (the defined-or operator). Perl is usually pretty
1856 good about this, but some pathological cases might trigger this, such as
1857 C<$x///> (is that S<C<($x) / (//)>> or S<C<$x // />>?) and S<C<print $fh //>>
1858 (S<C<print $fh(//>> or S<C<print($fh //>>?). In all of these examples, Perl
1859 will assume you meant defined-or. If you meant the empty regex, just
1860 use parentheses or spaces to disambiguate, or even prefix the empty
1861 regex with an C<m> (so C<//> becomes C<m//>).
1863 =item Matching in list context
1865 If the C</g> option is not used, C<m//> in list context returns a
1866 list consisting of the subexpressions matched by the parentheses in the
1867 pattern, that is, (C<$1>, C<$2>, C<$3>...) (Note that here C<$1> etc. are
1868 also set). When there are no parentheses in the pattern, the return
1869 value is the list C<(1)> for success.
1870 With or without parentheses, an empty list is returned upon failure.
1874 open(TTY, "+</dev/tty")
1875 || die "can't access /dev/tty: $!";
1877 <TTY> =~ /^y/i && foo(); # do foo if desired
1879 if (/Version: *([0-9.]*)/) { $version = $1; }
1881 next if m#^/usr/spool/uucp#;
1886 print if /$arg/o; # compile only once (no longer needed!)
1889 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
1891 This last example splits C<$foo> into the first two words and the
1892 remainder of the line, and assigns those three fields to C<$F1>, C<$F2>, and
1893 C<$Etc>. The conditional is true if any variables were assigned; that is,
1894 if the pattern matched.
1896 The C</g> modifier specifies global pattern matching--that is,
1897 matching as many times as possible within the string. How it behaves
1898 depends on the context. In list context, it returns a list of the
1899 substrings matched by any capturing parentheses in the regular
1900 expression. If there are no parentheses, it returns a list of all
1901 the matched strings, as if there were parentheses around the whole
1904 In scalar context, each execution of C<m//g> finds the next match,
1905 returning true if it matches, and false if there is no further match.
1906 The position after the last match can be read or set using the C<pos()>
1907 function; see L<perlfunc/pos>. A failed match normally resets the
1908 search position to the beginning of the string, but you can avoid that
1909 by adding the C</c> modifier (for example, C<m//gc>). Modifying the target
1910 string also resets the search position.
1912 =item C<\G I<assertion>>
1914 You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
1915 zero-width assertion that matches the exact position where the
1916 previous C<m//g>, if any, left off. Without the C</g> modifier, the
1917 C<\G> assertion still anchors at C<pos()> as it was at the start of
1918 the operation (see L<perlfunc/pos>), but the match is of course only
1919 attempted once. Using C<\G> without C</g> on a target string that has
1920 not previously had a C</g> match applied to it is the same as using
1921 the C<\A> assertion to match the beginning of the string. Note also
1922 that, currently, C<\G> is only properly supported when anchored at the
1923 very beginning of the pattern.
1928 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
1932 while ($paragraph = <>) {
1933 while ($paragraph =~ /\p{Ll}['")]*[.!?]+['")]*\s/g) {
1939 Here's another way to check for sentences in a paragraph:
1941 my $sentence_rx = qr{
1942 (?: (?<= ^ ) | (?<= \s ) ) # after start-of-string or
1944 \p{Lu} # capital letter
1945 .*? # a bunch of anything
1946 (?<= \S ) # that ends in non-
1948 (?<! \b [DMS]r ) # but isn't a common abbr.
1952 [.?!] # followed by a sentence
1954 (?= $ | \s ) # in front of end-of-string
1958 while (my $paragraph = <>) {
1959 say "NEW PARAGRAPH";
1961 while ($paragraph =~ /($sentence_rx)/g) {
1962 printf "\tgot sentence %d: <%s>\n", ++$count, $1;
1966 Here's how to use C<m//gc> with C<\G>:
1971 print $1 while /(o)/gc; print "', pos=", pos, "\n";
1973 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
1975 print $1 while /(p)/gc; print "', pos=", pos, "\n";
1977 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
1979 The last example should print:
1989 Notice that the final match matched C<q> instead of C<p>, which a match
1990 without the C<\G> anchor would have done. Also note that the final match
1991 did not update C<pos>. C<pos> is only updated on a C</g> match. If the
1992 final match did indeed match C<p>, it's a good bet that you're running a
1993 very old (pre-5.6.0) version of Perl.
1995 A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
1996 combine several regexps like this to process a string part-by-part,
1997 doing different actions depending on which regexp matched. Each
1998 regexp tries to match where the previous one leaves off.
2001 $url = URI::URL->new( "http://example.com/" );
2002 die if $url eq "xXx";
2006 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
2007 print(" lowercase"), redo LOOP
2008 if /\G\p{Ll}+\b[,.;]?\s*/gc;
2009 print(" UPPERCASE"), redo LOOP
2010 if /\G\p{Lu}+\b[,.;]?\s*/gc;
2011 print(" Capitalized"), redo LOOP
2012 if /\G\p{Lu}\p{Ll}+\b[,.;]?\s*/gc;
2013 print(" MiXeD"), redo LOOP if /\G\pL+\b[,.;]?\s*/gc;
2014 print(" alphanumeric"), redo LOOP
2015 if /\G[\p{Alpha}\pN]+\b[,.;]?\s*/gc;
2016 print(" line-noise"), redo LOOP if /\G\W+/gc;
2017 print ". That's all!\n";
2020 Here is the output (split into several lines):
2022 line-noise lowercase line-noise UPPERCASE line-noise UPPERCASE
2023 line-noise lowercase line-noise lowercase line-noise lowercase
2024 lowercase line-noise lowercase lowercase line-noise lowercase
2025 lowercase line-noise MiXeD line-noise. That's all!
2027 =item C<m?I<PATTERN>?msixpodualngc>
2028 X<?> X<operator, match-once>
2030 =item C<?I<PATTERN>?msixpodualngc>
2032 This is just like the C<m/I<PATTERN>/> search, except that it matches
2033 only once between calls to the C<reset()> operator. This is a useful
2034 optimization when you want to see only the first occurrence of
2035 something in each file of a set of files, for instance. Only C<m??>
2036 patterns local to the current package are reset.
2040 # blank line between header and body
2043 reset if eof; # clear m?? status for next file
2046 Another example switched the first "latin1" encoding it finds
2047 to "utf8" in a pod file:
2049 s//utf8/ if m? ^ =encoding \h+ \K latin1 ?x;
2051 The match-once behavior is controlled by the match delimiter being
2052 C<?>; with any other delimiter this is the normal C<m//> operator.
2054 In the past, the leading C<m> in C<m?I<PATTERN>?> was optional, but omitting it
2055 would produce a deprecation warning. As of v5.22.0, omitting it produces a
2056 syntax error. If you encounter this construct in older code, you can just add
2059 =item C<s/I<PATTERN>/I<REPLACEMENT>/msixpodualngcer>
2060 X<substitute> X<substitution> X<replace> X<regexp, replace>
2061 X<regexp, substitute> X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> X</e> X</r>
2063 Searches a string for a pattern, and if found, replaces that pattern
2064 with the replacement text and returns the number of substitutions
2065 made. Otherwise it returns false (specifically, the empty string).
2067 If the C</r> (non-destructive) option is used then it runs the
2068 substitution on a copy of the string and instead of returning the
2069 number of substitutions, it returns the copy whether or not a
2070 substitution occurred. The original string is never changed when
2071 C</r> is used. The copy will always be a plain string, even if the
2072 input is an object or a tied variable.
2074 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
2075 variable is searched and modified. Unless the C</r> option is used,
2076 the string specified must be a scalar variable, an array element, a
2077 hash element, or an assignment to one of those; that is, some sort of
2080 If the delimiter chosen is a single quote, no variable interpolation is
2081 done on either the I<PATTERN> or the I<REPLACEMENT>. Otherwise, if the
2082 I<PATTERN> contains a C<$> that looks like a variable rather than an
2083 end-of-string test, the variable will be interpolated into the pattern
2084 at run-time. If you want the pattern compiled only once the first time
2085 the variable is interpolated, use the C</o> option. If the pattern
2086 evaluates to the empty string, the last successfully executed regular
2087 expression is used instead. See L<perlre> for further explanation on these.
2089 Options are as with C<m//> with the addition of the following replacement
2092 e Evaluate the right side as an expression.
2093 ee Evaluate the right side as a string then eval the
2095 r Return substitution and leave the original string
2098 Any non-whitespace delimiter may replace the slashes. Add space after
2099 the C<s> when using a character allowed in identifiers. If single quotes
2100 are used, no interpretation is done on the replacement string (the C</e>
2101 modifier overrides this, however). Note that Perl treats backticks
2102 as normal delimiters; the replacement text is not evaluated as a command.
2103 If the I<PATTERN> is delimited by bracketing quotes, the I<REPLACEMENT> has
2104 its own pair of quotes, which may or may not be bracketing quotes, for example,
2105 C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
2106 replacement portion to be treated as a full-fledged Perl expression
2107 and evaluated right then and there. It is, however, syntax checked at
2108 compile-time. A second C<e> modifier will cause the replacement portion
2109 to be C<eval>ed before being run as a Perl expression.
2113 s/\bgreen\b/mauve/g; # don't change wintergreen
2115 $path =~ s|/usr/bin|/usr/local/bin|;
2117 s/Login: $foo/Login: $bar/; # run-time pattern
2119 ($foo = $bar) =~ s/this/that/; # copy first, then
2121 ($foo = "$bar") =~ s/this/that/; # convert to string,
2123 $foo = $bar =~ s/this/that/r; # Same as above using /r
2124 $foo = $bar =~ s/this/that/r
2125 =~ s/that/the other/r; # Chained substitutes
2127 @foo = map { s/this/that/r } @bar # /r is very useful in
2130 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-cnt
2133 s/\d+/$&*2/e; # yields 'abc246xyz'
2134 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
2135 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
2137 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
2138 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
2139 s/^=(\w+)/pod($1)/ge; # use function call
2142 $x = s/abc/def/r; # $x is 'def123xyz' and
2143 # $_ remains 'abc123xyz'.
2145 # expand variables in $_, but dynamics only, using
2146 # symbolic dereferencing
2149 # Add one to the value of any numbers in the string
2152 # Titlecase words in the last 30 characters only
2153 substr($str, -30) =~ s/\b(\p{Alpha}+)\b/\u\L$1/g;
2155 # This will expand any embedded scalar variable
2156 # (including lexicals) in $_ : First $1 is interpolated
2157 # to the variable name, and then evaluated
2160 # Delete (most) C comments.
2162 /\* # Match the opening delimiter.
2163 .*? # Match a minimal number of characters.
2164 \*/ # Match the closing delimiter.
2167 s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_,
2170 for ($variable) { # trim whitespace in $variable,
2176 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
2178 Note the use of C<$> instead of C<\> in the last example. Unlike
2179 B<sed>, we use the \<I<digit>> form only in the left hand side.
2180 Anywhere else it's $<I<digit>>.
2182 Occasionally, you can't use just a C</g> to get all the changes
2183 to occur that you might want. Here are two common cases:
2185 # put commas in the right places in an integer
2186 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
2188 # expand tabs to 8-column spacing
2189 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
2193 =head2 Quote-Like Operators
2194 X<operator, quote-like>
2198 =item C<q/I<STRING>/>
2199 X<q> X<quote, single> X<'> X<''>
2201 =item C<'I<STRING>'>
2203 A single-quoted, literal string. A backslash represents a backslash
2204 unless followed by the delimiter or another backslash, in which case
2205 the delimiter or backslash is interpolated.
2207 $foo = q!I said, "You said, 'She said it.'"!;
2208 $bar = q('This is it.');
2209 $baz = '\n'; # a two-character string
2211 =item C<qq/I<STRING>/>
2212 X<qq> X<quote, double> X<"> X<"">
2216 A double-quoted, interpolated string.
2219 (*** The previous line contains the naughty word "$1".\n)
2220 if /\b(tcl|java|python)\b/i; # :-)
2221 $baz = "\n"; # a one-character string
2223 =item C<qx/I<STRING>/>
2224 X<qx> X<`> X<``> X<backtick>
2226 =item C<`I<STRING>`>
2228 A string which is (possibly) interpolated and then executed as a
2229 system command with F</bin/sh> or its equivalent. Shell wildcards,
2230 pipes, and redirections will be honored. The collected standard
2231 output of the command is returned; standard error is unaffected. In
2232 scalar context, it comes back as a single (potentially multi-line)
2233 string, or C<undef> if the command failed. In list context, returns a
2234 list of lines (however you've defined lines with C<$/> or
2235 C<$INPUT_RECORD_SEPARATOR>), or an empty list if the command failed.
2237 Because backticks do not affect standard error, use shell file descriptor
2238 syntax (assuming the shell supports this) if you care to address this.
2239 To capture a command's STDERR and STDOUT together:
2241 $output = `cmd 2>&1`;
2243 To capture a command's STDOUT but discard its STDERR:
2245 $output = `cmd 2>/dev/null`;
2247 To capture a command's STDERR but discard its STDOUT (ordering is
2250 $output = `cmd 2>&1 1>/dev/null`;
2252 To exchange a command's STDOUT and STDERR in order to capture the STDERR
2253 but leave its STDOUT to come out the old STDERR:
2255 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
2257 To read both a command's STDOUT and its STDERR separately, it's easiest
2258 to redirect them separately to files, and then read from those files
2259 when the program is done:
2261 system("program args 1>program.stdout 2>program.stderr");
2263 The STDIN filehandle used by the command is inherited from Perl's STDIN.
2266 open(SPLAT, "stuff") || die "can't open stuff: $!";
2267 open(STDIN, "<&SPLAT") || die "can't dupe SPLAT: $!";
2268 print STDOUT `sort`;
2270 will print the sorted contents of the file named F<"stuff">.
2272 Using single-quote as a delimiter protects the command from Perl's
2273 double-quote interpolation, passing it on to the shell instead:
2275 $perl_info = qx(ps $$); # that's Perl's $$
2276 $shell_info = qx'ps $$'; # that's the new shell's $$
2278 How that string gets evaluated is entirely subject to the command
2279 interpreter on your system. On most platforms, you will have to protect
2280 shell metacharacters if you want them treated literally. This is in
2281 practice difficult to do, as it's unclear how to escape which characters.
2282 See L<perlsec> for a clean and safe example of a manual C<fork()> and C<exec()>
2283 to emulate backticks safely.
2285 On some platforms (notably DOS-like ones), the shell may not be
2286 capable of dealing with multiline commands, so putting newlines in
2287 the string may not get you what you want. You may be able to evaluate
2288 multiple commands in a single line by separating them with the command
2289 separator character, if your shell supports that (for example, C<;> on
2290 many Unix shells and C<&> on the Windows NT C<cmd> shell).
2292 Perl will attempt to flush all files opened for
2293 output before starting the child process, but this may not be supported
2294 on some platforms (see L<perlport>). To be safe, you may need to set
2295 C<$|> (C<$AUTOFLUSH> in C<L<English>>) or call the C<autoflush()> method of
2296 C<L<IO::Handle>> on any open handles.
2298 Beware that some command shells may place restrictions on the length
2299 of the command line. You must ensure your strings don't exceed this
2300 limit after any necessary interpolations. See the platform-specific
2301 release notes for more details about your particular environment.
2303 Using this operator can lead to programs that are difficult to port,
2304 because the shell commands called vary between systems, and may in
2305 fact not be present at all. As one example, the C<type> command under
2306 the POSIX shell is very different from the C<type> command under DOS.
2307 That doesn't mean you should go out of your way to avoid backticks
2308 when they're the right way to get something done. Perl was made to be
2309 a glue language, and one of the things it glues together is commands.
2310 Just understand what you're getting yourself into.
2312 Like C<system>, backticks put the child process exit code in C<$?>.
2313 If you'd like to manually inspect failure, you can check all possible
2314 failure modes by inspecting C<$?> like this:
2317 print "failed to execute: $!\n";
2320 printf "child died with signal %d, %s coredump\n",
2321 ($? & 127), ($? & 128) ? 'with' : 'without';
2324 printf "child exited with value %d\n", $? >> 8;
2327 Use the L<open> pragma to control the I/O layers used when reading the
2328 output of the command, for example:
2330 use open IN => ":encoding(UTF-8)";
2331 my $x = `cmd-producing-utf-8`;
2333 See L</"I/O Operators"> for more discussion.
2335 =item C<qw/I<STRING>/>
2336 X<qw> X<quote, list> X<quote, words>
2338 Evaluates to a list of the words extracted out of I<STRING>, using embedded
2339 whitespace as the word delimiters. It can be understood as being roughly
2342 split(" ", q/STRING/);
2344 the differences being that it generates a real list at compile time, and
2345 in scalar context it returns the last element in the list. So
2350 is semantically equivalent to the list:
2354 Some frequently seen examples:
2356 use POSIX qw( setlocale localeconv )
2357 @EXPORT = qw( foo bar baz );
2359 A common mistake is to try to separate the words with commas or to
2360 put comments into a multi-line C<qw>-string. For this reason, the
2361 S<C<use warnings>> pragma and the B<-w> switch (that is, the C<$^W> variable)
2362 produces warnings if the I<STRING> contains the C<","> or the C<"#"> character.
2364 =item C<tr/I<SEARCHLIST>/I<REPLACEMENTLIST>/cdsr>
2365 X<tr> X<y> X<transliterate> X</c> X</d> X</s>
2367 =item C<y/I<SEARCHLIST>/I<REPLACEMENTLIST>/cdsr>
2369 Transliterates all occurrences of the characters found in the search list
2370 with the corresponding character in the replacement list. It returns
2371 the number of characters replaced or deleted. If no string is
2372 specified via the C<=~> or C<!~> operator, the C<$_> string is transliterated.
2374 If the C</r> (non-destructive) option is present, a new copy of the string
2375 is made and its characters transliterated, and this copy is returned no
2376 matter whether it was modified or not: the original string is always
2377 left unchanged. The new copy is always a plain string, even if the input
2378 string is an object or a tied variable.
2380 Unless the C</r> option is used, the string specified with C<=~> must be a
2381 scalar variable, an array element, a hash element, or an assignment to one
2382 of those; in other words, an lvalue.
2384 A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
2385 does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
2386 For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
2387 I<SEARCHLIST> is delimited by bracketing quotes, the I<REPLACEMENTLIST>
2388 must have its own pair of quotes, which may or may not be bracketing
2389 quotes; for example, C<tr[aeiouy][yuoiea]> or C<tr(+\-*/)/ABCD/>.
2391 Characters may be literals or any of the escape sequences accepted in
2392 double-quoted strings. But there is no variable interpolation, so C<"$">
2393 and C<"@"> are treated as literals. A hyphen at the beginning or end, or
2394 preceded by a backslash is considered a literal. Escape sequence
2395 details are in L<the table near the beginning of this section|/Quote and
2396 Quote-like Operators>.
2398 Note that C<tr> does B<not> do regular expression character classes such as
2399 C<\d> or C<\pL>. The C<tr> operator is not equivalent to the C<L<tr(1)>>
2400 utility. C<tr[a-z][A-Z]> will uppercase the 26 letters "a" through "z",
2401 but for case changing not confined to ASCII, use
2402 L<C<lc>|perlfunc/lc>, L<C<uc>|perlfunc/uc>,
2403 L<C<lcfirst>|perlfunc/lcfirst>, L<C<ucfirst>|perlfunc/ucfirst>
2404 (all documented in L<perlfunc>), or the
2405 L<substitution operator C<sE<sol>I<PATTERN>E<sol>I<REPLACEMENT>E<sol>>|/sE<sol>PATTERNE<sol>REPLACEMENTE<sol>msixpodualngcer>
2406 (with C<\U>, C<\u>, C<\L>, and C<\l> string-interpolation escapes in the
2407 I<REPLACEMENT> portion).
2409 Most ranges are unportable between character sets, but certain ones
2410 signal Perl to do special handling to make them portable. There are two
2411 classes of portable ranges. The first are any subsets of the ranges
2412 C<A-Z>, C<a-z>, and C<0-9>, when expressed as literal characters.
2416 capitalizes the letters C<"h">, C<"i">, C<"j">, and C<"k"> and nothing
2417 else, no matter what the platform's character set is. In contrast, all
2420 tr/\x68-\x6B/\x48-\x4B/
2424 do the same capitalizations as the previous example when run on ASCII
2425 platforms, but something completely different on EBCDIC ones.
2427 The second class of portable ranges is invoked when one or both of the
2428 range's end points are expressed as C<\N{...}>
2430 $string =~ tr/\N{U+20}-\N{U+7E}//d;
2432 removes from C<$string> all the platform's characters which are
2433 equivalent to any of Unicode U+0020, U+0021, ... U+007D, U+007E. This
2434 is a portable range, and has the same effect on every platform it is
2435 run on. It turns out that in this example, these are the ASCII
2436 printable characters. So after this is run, C<$string> has only
2437 controls and characters which have no ASCII equivalents.
2439 But, even for portable ranges, it is not generally obvious what is
2440 included without having to look things up. A sound principle is to use
2441 only ranges that begin from and end at either ASCII alphabetics of equal
2442 case (C<b-e>, C<B-E>), or digits (C<1-4>). Anything else is unclear
2443 (and unportable unless C<\N{...}> is used). If in doubt, spell out the
2444 character sets in full.
2448 c Complement the SEARCHLIST.
2449 d Delete found but unreplaced characters.
2450 s Squash duplicate replaced characters.
2451 r Return the modified string and leave the original string
2454 If the C</c> modifier is specified, the I<SEARCHLIST> character set
2455 is complemented. If the C</d> modifier is specified, any characters
2456 specified by I<SEARCHLIST> not found in I<REPLACEMENTLIST> are deleted.
2457 (Note that this is slightly more flexible than the behavior of some
2458 B<tr> programs, which delete anything they find in the I<SEARCHLIST>,
2459 period.) If the C</s> modifier is specified, sequences of characters
2460 that were transliterated to the same character are squashed down
2461 to a single instance of the character.
2463 If the C</d> modifier is used, the I<REPLACEMENTLIST> is always interpreted
2464 exactly as specified. Otherwise, if the I<REPLACEMENTLIST> is shorter
2465 than the I<SEARCHLIST>, the final character is replicated till it is long
2466 enough. If the I<REPLACEMENTLIST> is empty, the I<SEARCHLIST> is replicated.
2467 This latter is useful for counting characters in a class or for
2468 squashing character sequences in a class.
2472 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case ASCII
2474 $cnt = tr/*/*/; # count the stars in $_
2476 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
2478 $cnt = tr/0-9//; # count the digits in $_
2480 tr/a-zA-Z//s; # bookkeeper -> bokeper
2482 ($HOST = $host) =~ tr/a-z/A-Z/;
2483 $HOST = $host =~ tr/a-z/A-Z/r; # same thing
2485 $HOST = $host =~ tr/a-z/A-Z/r # chained with s///r
2488 tr/a-zA-Z/ /cs; # change non-alphas to single space
2490 @stripped = map tr/a-zA-Z/ /csr, @original;
2494 [\000-\177]; # wickedly delete 8th bit
2496 If multiple transliterations are given for a character, only the
2501 will transliterate any A to X.
2503 Because the transliteration table is built at compile time, neither
2504 the I<SEARCHLIST> nor the I<REPLACEMENTLIST> are subjected to double quote
2505 interpolation. That means that if you want to use variables, you
2506 must use an C<eval()>:
2508 eval "tr/$oldlist/$newlist/";
2511 eval "tr/$oldlist/$newlist/, 1" or die $@;
2513 =item C<< <<I<EOF> >>
2514 X<here-doc> X<heredoc> X<here-document> X<<< << >>>
2516 A line-oriented form of quoting is based on the shell "here-document"
2517 syntax. Following a C<< << >> you specify a string to terminate
2518 the quoted material, and all lines following the current line down to
2519 the terminating string are the value of the item.
2521 Prefixing the terminating string with a C<~> specifies that you
2522 want to use L</Indented Here-docs> (see below).
2524 The terminating string may be either an identifier (a word), or some
2525 quoted text. An unquoted identifier works like double quotes.
2526 There may not be a space between the C<< << >> and the identifier,
2527 unless the identifier is explicitly quoted. (If you put a space it
2528 will be treated as a null identifier, which is valid, and matches the
2529 first empty line.) The terminating string must appear by itself
2530 (unquoted and with no surrounding whitespace) on the terminating line.
2532 If the terminating string is quoted, the type of quotes used determine
2533 the treatment of the text.
2539 Double quotes indicate that the text will be interpolated using exactly
2540 the same rules as normal double quoted strings.
2543 The price is $Price.
2546 print << "EOF"; # same as above
2547 The price is $Price.
2553 Single quotes indicate the text is to be treated literally with no
2554 interpolation of its content. This is similar to single quoted
2555 strings except that backslashes have no special meaning, with C<\\>
2556 being treated as two backslashes and not one as they would in every
2557 other quoting construct.
2559 Just as in the shell, a backslashed bareword following the C<<< << >>>
2560 means the same thing as a single-quoted string does:
2562 $cost = <<'VISTA'; # hasta la ...
2563 That'll be $10 please, ma'am.
2566 $cost = <<\VISTA; # Same thing!
2567 That'll be $10 please, ma'am.
2570 This is the only form of quoting in perl where there is no need
2571 to worry about escaping content, something that code generators
2572 can and do make good use of.
2576 The content of the here doc is treated just as it would be if the
2577 string were embedded in backticks. Thus the content is interpolated
2578 as though it were double quoted and then executed via the shell, with
2579 the results of the execution returned.
2581 print << `EOC`; # execute command and get results
2589 =item Indented Here-docs
2591 The here-doc modifier C<~> allows you to indent your here-docs to make
2592 the code more readable:
2604 ...with no leading whitespace.
2606 The delimiter is used to determine the B<exact> whitespace to
2607 remove from the beginning of each line. All lines B<must> have
2608 at least the same starting whitespace (except lines only
2609 containing a newline) or perl will croak. Tabs and spaces can
2610 be mixed, but are matched exactly. One tab will not be equal to
2613 Additional beginning whitespace (beyond what preceded the
2614 delimiter) will be preserved:
2617 This text is not indented
2618 This text is indented with two spaces
2619 This text is indented with two tabs
2622 Finally, the modifier may be used with all of the forms
2630 And whitespace may be used between the C<~> and quoted delimiters:
2632 <<~ 'EOF'; # ... "EOF", `EOF`
2636 It is possible to stack multiple here-docs in a row:
2638 print <<"foo", <<"bar"; # you can stack them
2644 myfunc(<< "THIS", 23, <<'THAT');
2651 Just don't forget that you have to put a semicolon on the end
2652 to finish the statement, as Perl doesn't know you're not going to
2660 If you want to remove the line terminator from your here-docs,
2663 chomp($string = <<'END');
2667 If you want your here-docs to be indented with the rest of the code,
2668 you'll need to remove leading whitespace from each line manually:
2670 ($quote = <<'FINIS') =~ s/^\s+//gm;
2671 The Road goes ever on and on,
2672 down from the door where it began.
2675 If you use a here-doc within a delimited construct, such as in C<s///eg>,
2676 the quoted material must still come on the line following the
2677 C<<< <<FOO >>> marker, which means it may be inside the delimited
2685 It works this way as of Perl 5.18. Historically, it was inconsistent, and
2686 you would have to write
2693 outside of string evals.
2695 Additionally, quoting rules for the end-of-string identifier are
2696 unrelated to Perl's quoting rules. C<q()>, C<qq()>, and the like are not
2697 supported in place of C<''> and C<"">, and the only interpolation is for
2698 backslashing the quoting character:
2700 print << "abc\"def";
2704 Finally, quoted strings cannot span multiple lines. The general rule is
2705 that the identifier must be a string literal. Stick with that, and you
2710 =head2 Gory details of parsing quoted constructs
2711 X<quote, gory details>
2713 When presented with something that might have several different
2714 interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
2715 principle to pick the most probable interpretation. This strategy
2716 is so successful that Perl programmers often do not suspect the
2717 ambivalence of what they write. But from time to time, Perl's
2718 notions differ substantially from what the author honestly meant.
2720 This section hopes to clarify how Perl handles quoted constructs.
2721 Although the most common reason to learn this is to unravel labyrinthine
2722 regular expressions, because the initial steps of parsing are the
2723 same for all quoting operators, they are all discussed together.
2725 The most important Perl parsing rule is the first one discussed
2726 below: when processing a quoted construct, Perl first finds the end
2727 of that construct, then interprets its contents. If you understand
2728 this rule, you may skip the rest of this section on the first
2729 reading. The other rules are likely to contradict the user's
2730 expectations much less frequently than this first one.
2732 Some passes discussed below are performed concurrently, but because
2733 their results are the same, we consider them individually. For different
2734 quoting constructs, Perl performs different numbers of passes, from
2735 one to four, but these passes are always performed in the same order.
2739 =item Finding the end
2741 The first pass is finding the end of the quoted construct. This results
2742 in saving to a safe location a copy of the text (between the starting
2743 and ending delimiters), normalized as necessary to avoid needing to know
2744 what the original delimiters were.
2746 If the construct is a here-doc, the ending delimiter is a line
2747 that has a terminating string as the content. Therefore C<<<EOF> is
2748 terminated by C<EOF> immediately followed by C<"\n"> and starting
2749 from the first column of the terminating line.
2750 When searching for the terminating line of a here-doc, nothing
2751 is skipped. In other words, lines after the here-doc syntax
2752 are compared with the terminating string line by line.
2754 For the constructs except here-docs, single characters are used as starting
2755 and ending delimiters. If the starting delimiter is an opening punctuation
2756 (that is C<(>, C<[>, C<{>, or C<< < >>), the ending delimiter is the
2757 corresponding closing punctuation (that is C<)>, C<]>, C<}>, or C<< > >>).
2758 If the starting delimiter is an unpaired character like C</> or a closing
2759 punctuation, the ending delimiter is the same as the starting delimiter.
2760 Therefore a C</> terminates a C<qq//> construct, while a C<]> terminates
2761 both C<qq[]> and C<qq]]> constructs.
2763 When searching for single-character delimiters, escaped delimiters
2764 and C<\\> are skipped. For example, while searching for terminating C</>,
2765 combinations of C<\\> and C<\/> are skipped. If the delimiters are
2766 bracketing, nested pairs are also skipped. For example, while searching
2767 for a closing C<]> paired with the opening C<[>, combinations of C<\\>, C<\]>,
2768 and C<\[> are all skipped, and nested C<[> and C<]> are skipped as well.
2769 However, when backslashes are used as the delimiters (like C<qq\\> and
2770 C<tr\\\>), nothing is skipped.
2771 During the search for the end, backslashes that escape delimiters or
2772 other backslashes are removed (exactly speaking, they are not copied to the
2775 For constructs with three-part delimiters (C<s///>, C<y///>, and
2776 C<tr///>), the search is repeated once more.
2777 If the first delimiter is not an opening punctuation, the three delimiters must
2778 be the same, such as C<s!!!> and C<tr)))>,
2779 in which case the second delimiter
2780 terminates the left part and starts the right part at once.
2781 If the left part is delimited by bracketing punctuation (that is C<()>,
2782 C<[]>, C<{}>, or C<< <> >>), the right part needs another pair of
2783 delimiters such as C<s(){}> and C<tr[]//>. In these cases, whitespace
2784 and comments are allowed between the two parts, although the comment must follow
2785 at least one whitespace character; otherwise a character expected as the
2786 start of the comment may be regarded as the starting delimiter of the right part.
2788 During this search no attention is paid to the semantics of the construct.
2791 "$hash{"$foo/$bar"}"
2796 bar # NOT a comment, this slash / terminated m//!
2799 do not form legal quoted expressions. The quoted part ends on the
2800 first C<"> and C</>, and the rest happens to be a syntax error.
2801 Because the slash that terminated C<m//> was followed by a C<SPACE>,
2802 the example above is not C<m//x>, but rather C<m//> with no C</x>
2803 modifier. So the embedded C<#> is interpreted as a literal C<#>.
2805 Also no attention is paid to C<\c\> (multichar control char syntax) during
2806 this search. Thus the second C<\> in C<qq/\c\/> is interpreted as a part
2807 of C<\/>, and the following C</> is not recognized as a delimiter.
2808 Instead, use C<\034> or C<\x1c> at the end of quoted constructs.
2813 The next step is interpolation in the text obtained, which is now
2814 delimiter-independent. There are multiple cases.
2820 No interpolation is performed.
2821 Note that the combination C<\\> is left intact, since escaped delimiters
2822 are not available for here-docs.
2824 =item C<m''>, the pattern of C<s'''>
2826 No interpolation is performed at this stage.
2827 Any backslashed sequences including C<\\> are treated at the stage
2828 to L</"parsing regular expressions">.
2830 =item C<''>, C<q//>, C<tr'''>, C<y'''>, the replacement of C<s'''>
2832 The only interpolation is removal of C<\> from pairs of C<\\>.
2833 Therefore C<"-"> in C<tr'''> and C<y'''> is treated literally
2834 as a hyphen and no character range is available.
2835 C<\1> in the replacement of C<s'''> does not work as C<$1>.
2837 =item C<tr///>, C<y///>
2839 No variable interpolation occurs. String modifying combinations for
2840 case and quoting such as C<\Q>, C<\U>, and C<\E> are not recognized.
2841 The other escape sequences such as C<\200> and C<\t> and backslashed
2842 characters such as C<\\> and C<\-> are converted to appropriate literals.
2843 The character C<"-"> is treated specially and therefore C<\-> is treated
2844 as a literal C<"-">.
2846 =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF">
2848 C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F> (possibly paired with C<\E>) are
2849 converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
2850 is converted to S<C<$foo . (quotemeta("baz" . $bar))>> internally.
2851 The other escape sequences such as C<\200> and C<\t> and backslashed
2852 characters such as C<\\> and C<\-> are replaced with appropriate
2855 Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
2856 is interpolated in the usual way. Something like C<"\Q\\E"> has
2857 no C<\E> inside. Instead, it has C<\Q>, C<\\>, and C<E>, so the
2858 result is the same as for C<"\\\\E">. As a general rule, backslashes
2859 between C<\Q> and C<\E> may lead to counterintuitive results. So,
2860 C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
2861 as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
2866 may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
2868 Interpolated scalars and arrays are converted internally to the C<join> and
2869 C<"."> catenation operations. Thus, S<C<"$foo XXX '@arr'">> becomes:
2871 $foo . " XXX '" . (join $", @arr) . "'";
2873 All operations above are performed simultaneously, left to right.
2875 Because the result of S<C<"\Q I<STRING> \E">> has all metacharacters
2876 quoted, there is no way to insert a literal C<$> or C<@> inside a
2877 C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to become
2878 C<"\\\$">; if not, it is interpreted as the start of an interpolated
2881 Note also that the interpolation code needs to make a decision on
2882 where the interpolated scalar ends. For instance, whether
2883 S<C<< "a $x -> {c}" >>> really means:
2885 "a " . $x . " -> {c}";
2891 Most of the time, the longest possible text that does not include
2892 spaces between components and which contains matching braces or
2893 brackets. because the outcome may be determined by voting based
2894 on heuristic estimators, the result is not strictly predictable.
2895 Fortunately, it's usually correct for ambiguous cases.
2897 =item the replacement of C<s///>
2899 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F> and interpolation
2900 happens as with C<qq//> constructs.
2902 It is at this step that C<\1> is begrudgingly converted to C<$1> in
2903 the replacement text of C<s///>, in order to correct the incorrigible
2904 I<sed> hackers who haven't picked up the saner idiom yet. A warning
2905 is emitted if the S<C<use warnings>> pragma or the B<-w> command-line flag
2906 (that is, the C<$^W> variable) was set.
2908 =item C<RE> in C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
2910 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F>, C<\E>,
2911 and interpolation happens (almost) as with C<qq//> constructs.
2913 Processing of C<\N{...}> is also done here, and compiled into an intermediate
2914 form for the regex compiler. (This is because, as mentioned below, the regex
2915 compilation may be done at execution time, and C<\N{...}> is a compile-time
2918 However any other combinations of C<\> followed by a character
2919 are not substituted but only skipped, in order to parse them
2920 as regular expressions at the following step.
2921 As C<\c> is skipped at this step, C<@> of C<\c@> in RE is possibly
2922 treated as an array symbol (for example C<@foo>),
2923 even though the same text in C<qq//> gives interpolation of C<\c@>.
2925 Code blocks such as C<(?{BLOCK})> are handled by temporarily passing control
2926 back to the perl parser, in a similar way that an interpolated array
2927 subscript expression such as C<"foo$array[1+f("[xyz")]bar"> would be.
2929 Moreover, inside C<(?{BLOCK})>, S<C<(?# comment )>>, and
2930 a C<#>-comment in a C</x>-regular expression, no processing is
2931 performed whatsoever. This is the first step at which the presence
2932 of the C</x> modifier is relevant.
2934 Interpolation in patterns has several quirks: C<$|>, C<$(>, C<$)>, C<@+>
2935 and C<@-> are not interpolated, and constructs C<$var[SOMETHING]> are
2936 voted (by several different estimators) to be either an array element
2937 or C<$var> followed by an RE alternative. This is where the notation
2938 C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
2939 array element C<-9>, not as a regular expression from the variable
2940 C<$arr> followed by a digit, which would be the interpretation of
2941 C</$arr[0-9]/>. Since voting among different estimators may occur,
2942 the result is not predictable.
2944 The lack of processing of C<\\> creates specific restrictions on
2945 the post-processed text. If the delimiter is C</>, one cannot get
2946 the combination C<\/> into the result of this step. C</> will
2947 finish the regular expression, C<\/> will be stripped to C</> on
2948 the previous step, and C<\\/> will be left as is. Because C</> is
2949 equivalent to C<\/> inside a regular expression, this does not
2950 matter unless the delimiter happens to be character special to the
2951 RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
2952 alphanumeric char, as in:
2956 In the RE above, which is intentionally obfuscated for illustration, the
2957 delimiter is C<m>, the modifier is C<mx>, and after delimiter-removal the
2958 RE is the same as for S<C<m/ ^ a \s* b /mx>>. There's more than one
2959 reason you're encouraged to restrict your delimiters to non-alphanumeric,
2960 non-whitespace choices.
2964 This step is the last one for all constructs except regular expressions,
2965 which are processed further.
2967 =item parsing regular expressions
2970 Previous steps were performed during the compilation of Perl code,
2971 but this one happens at run time, although it may be optimized to
2972 be calculated at compile time if appropriate. After preprocessing
2973 described above, and possibly after evaluation if concatenation,
2974 joining, casing translation, or metaquoting are involved, the
2975 resulting I<string> is passed to the RE engine for compilation.
2977 Whatever happens in the RE engine might be better discussed in L<perlre>,
2978 but for the sake of continuity, we shall do so here.
2980 This is another step where the presence of the C</x> modifier is
2981 relevant. The RE engine scans the string from left to right and
2982 converts it into a finite automaton.
2984 Backslashed characters are either replaced with corresponding
2985 literal strings (as with C<\{>), or else they generate special nodes
2986 in the finite automaton (as with C<\b>). Characters special to the
2987 RE engine (such as C<|>) generate corresponding nodes or groups of
2988 nodes. C<(?#...)> comments are ignored. All the rest is either
2989 converted to literal strings to match, or else is ignored (as is
2990 whitespace and C<#>-style comments if C</x> is present).
2992 Parsing of the bracketed character class construct, C<[...]>, is
2993 rather different than the rule used for the rest of the pattern.
2994 The terminator of this construct is found using the same rules as
2995 for finding the terminator of a C<{}>-delimited construct, the only
2996 exception being that C<]> immediately following C<[> is treated as
2997 though preceded by a backslash.
2999 The terminator of runtime C<(?{...})> is found by temporarily switching
3000 control to the perl parser, which should stop at the point where the
3001 logically balancing terminating C<}> is found.
3003 It is possible to inspect both the string given to RE engine and the
3004 resulting finite automaton. See the arguments C<debug>/C<debugcolor>
3005 in the S<C<use L<re>>> pragma, as well as Perl's B<-Dr> command-line
3006 switch documented in L<perlrun/"Command Switches">.
3008 =item Optimization of regular expressions
3009 X<regexp, optimization>
3011 This step is listed for completeness only. Since it does not change
3012 semantics, details of this step are not documented and are subject
3013 to change without notice. This step is performed over the finite
3014 automaton that was generated during the previous pass.
3016 It is at this stage that C<split()> silently optimizes C</^/> to
3021 =head2 I/O Operators
3022 X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle>
3023 X<< <> >> X<< <<>> >> X<@ARGV>
3025 There are several I/O operators you should know about.
3027 A string enclosed by backticks (grave accents) first undergoes
3028 double-quote interpolation. It is then interpreted as an external
3029 command, and the output of that command is the value of the
3030 backtick string, like in a shell. In scalar context, a single string
3031 consisting of all output is returned. In list context, a list of
3032 values is returned, one per line of output. (You can set C<$/> to use
3033 a different line terminator.) The command is executed each time the
3034 pseudo-literal is evaluated. The status value of the command is
3035 returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
3036 Unlike in B<csh>, no translation is done on the return data--newlines
3037 remain newlines. Unlike in any of the shells, single quotes do not
3038 hide variable names in the command from interpretation. To pass a
3039 literal dollar-sign through to the shell you need to hide it with a
3040 backslash. The generalized form of backticks is C<qx//>. (Because
3041 backticks always undergo shell expansion as well, see L<perlsec> for
3043 X<qx> X<`> X<``> X<backtick> X<glob>
3045 In scalar context, evaluating a filehandle in angle brackets yields
3046 the next line from that file (the newline, if any, included), or
3047 C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
3048 (sometimes known as file-slurp mode) and the file is empty, it
3049 returns C<''> the first time, followed by C<undef> subsequently.
3051 Ordinarily you must assign the returned value to a variable, but
3052 there is one situation where an automatic assignment happens. If
3053 and only if the input symbol is the only thing inside the conditional
3054 of a C<while> statement (even if disguised as a C<for(;;)> loop),
3055 the value is automatically assigned to the global variable C<$_>,
3056 destroying whatever was there previously. (This may seem like an
3057 odd thing to you, but you'll use the construct in almost every Perl
3058 script you write.) The C<$_> variable is not implicitly localized.
3059 You'll have to put a S<C<local $_;>> before the loop if you want that
3062 The following lines are equivalent:
3064 while (defined($_ = <STDIN>)) { print; }
3065 while ($_ = <STDIN>) { print; }
3066 while (<STDIN>) { print; }
3067 for (;<STDIN>;) { print; }
3068 print while defined($_ = <STDIN>);
3069 print while ($_ = <STDIN>);
3070 print while <STDIN>;
3072 This also behaves similarly, but assigns to a lexical variable
3073 instead of to C<$_>:
3075 while (my $line = <STDIN>) { print $line }
3077 In these loop constructs, the assigned value (whether assignment
3078 is automatic or explicit) is then tested to see whether it is
3079 defined. The defined test avoids problems where the line has a string
3080 value that would be treated as false by Perl; for example a "" or
3081 a C<"0"> with no trailing newline. If you really mean for such values
3082 to terminate the loop, they should be tested for explicitly:
3084 while (($_ = <STDIN>) ne '0') { ... }
3085 while (<STDIN>) { last unless $_; ... }
3087 In other boolean contexts, C<< <I<FILEHANDLE>> >> without an
3088 explicit C<defined> test or comparison elicits a warning if the
3089 S<C<use warnings>> pragma or the B<-w>
3090 command-line switch (the C<$^W> variable) is in effect.
3092 The filehandles STDIN, STDOUT, and STDERR are predefined. (The
3093 filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
3094 in packages, where they would be interpreted as local identifiers
3095 rather than global.) Additional filehandles may be created with
3096 the C<open()> function, amongst others. See L<perlopentut> and
3097 L<perlfunc/open> for details on this.
3098 X<stdin> X<stdout> X<sterr>
3100 If a C<< <I<FILEHANDLE>> >> is used in a context that is looking for
3101 a list, a list comprising all input lines is returned, one line per
3102 list element. It's easy to grow to a rather large data space this
3103 way, so use with care.
3105 C<< <I<FILEHANDLE>> >> may also be spelled C<readline(*I<FILEHANDLE>)>.
3106 See L<perlfunc/readline>.
3108 The null filehandle C<< <> >> is special: it can be used to emulate the
3109 behavior of B<sed> and B<awk>, and any other Unix filter program
3110 that takes a list of filenames, doing the same to each line
3111 of input from all of them. Input from C<< <> >> comes either from
3112 standard input, or from each file listed on the command line. Here's
3113 how it works: the first time C<< <> >> is evaluated, the C<@ARGV> array is
3114 checked, and if it is empty, C<$ARGV[0]> is set to C<"-">, which when opened
3115 gives you standard input. The C<@ARGV> array is then processed as a list
3116 of filenames. The loop
3119 ... # code for each line
3122 is equivalent to the following Perl-like pseudo code:
3124 unshift(@ARGV, '-') unless @ARGV;
3125 while ($ARGV = shift) {
3128 ... # code for each line
3132 except that it isn't so cumbersome to say, and will actually work.
3133 It really does shift the C<@ARGV> array and put the current filename
3134 into the C<$ARGV> variable. It also uses filehandle I<ARGV>
3135 internally. C<< <> >> is just a synonym for C<< <ARGV> >>, which
3136 is magical. (The pseudo code above doesn't work because it treats
3137 C<< <ARGV> >> as non-magical.)
3139 Since the null filehandle uses the two argument form of L<perlfunc/open>
3140 it interprets special characters, so if you have a script like this:
3146 and call it with S<C<perl dangerous.pl 'rm -rfv *|'>>, it actually opens a
3147 pipe, executes the C<rm> command and reads C<rm>'s output from that pipe.
3148 If you want all items in C<@ARGV> to be interpreted as file names, you
3149 can use the module C<ARGV::readonly> from CPAN, or use the double bracket:
3155 Using double angle brackets inside of a while causes the open to use the
3156 three argument form (with the second argument being C<< < >>), so all
3157 arguments in C<ARGV> are treated as literal filenames (including C<"-">).
3158 (Note that for convenience, if you use C<< <<>> >> and if C<@ARGV> is
3159 empty, it will still read from the standard input.)
3161 You can modify C<@ARGV> before the first C<< <> >> as long as the array ends up
3162 containing the list of filenames you really want. Line numbers (C<$.>)
3163 continue as though the input were one big happy file. See the example
3164 in L<perlfunc/eof> for how to reset line numbers on each file.
3166 If you want to set C<@ARGV> to your own list of files, go right ahead.
3167 This sets C<@ARGV> to all plain text files if no C<@ARGV> was given:
3169 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
3171 You can even set them to pipe commands. For example, this automatically
3172 filters compressed arguments through B<gzip>:
3174 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
3176 If you want to pass switches into your script, you can use one of the
3177 C<Getopts> modules or put a loop on the front like this:
3179 while ($_ = $ARGV[0], /^-/) {
3182 if (/^-D(.*)/) { $debug = $1 }
3183 if (/^-v/) { $verbose++ }
3184 # ... # other switches
3188 # ... # code for each line
3191 The C<< <> >> symbol will return C<undef> for end-of-file only once.
3192 If you call it again after this, it will assume you are processing another
3193 C<@ARGV> list, and if you haven't set C<@ARGV>, will read input from STDIN.
3195 If what the angle brackets contain is a simple scalar variable (for example,
3196 C<$foo>), then that variable contains the name of the
3197 filehandle to input from, or its typeglob, or a reference to the
3203 If what's within the angle brackets is neither a filehandle nor a simple
3204 scalar variable containing a filehandle name, typeglob, or typeglob
3205 reference, it is interpreted as a filename pattern to be globbed, and
3206 either a list of filenames or the next filename in the list is returned,
3207 depending on context. This distinction is determined on syntactic
3208 grounds alone. That means C<< <$x> >> is always a C<readline()> from
3209 an indirect handle, but C<< <$hash{key}> >> is always a C<glob()>.
3210 That's because C<$x> is a simple scalar variable, but C<$hash{key}> is
3211 not--it's a hash element. Even C<< <$x > >> (note the extra space)
3212 is treated as C<glob("$x ")>, not C<readline($x)>.
3214 One level of double-quote interpretation is done first, but you can't
3215 say C<< <$foo> >> because that's an indirect filehandle as explained
3216 in the previous paragraph. (In older versions of Perl, programmers
3217 would insert curly brackets to force interpretation as a filename glob:
3218 C<< <${foo}> >>. These days, it's considered cleaner to call the
3219 internal function directly as C<glob($foo)>, which is probably the right
3220 way to have done it in the first place.) For example:
3226 is roughly equivalent to:
3228 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
3234 except that the globbing is actually done internally using the standard
3235 C<L<File::Glob>> extension. Of course, the shortest way to do the above is:
3239 A (file)glob evaluates its (embedded) argument only when it is
3240 starting a new list. All values must be read before it will start
3241 over. In list context, this isn't important because you automatically
3242 get them all anyway. However, in scalar context the operator returns
3243 the next value each time it's called, or C<undef> when the list has
3244 run out. As with filehandle reads, an automatic C<defined> is
3245 generated when the glob occurs in the test part of a C<while>,
3246 because legal glob returns (for example,
3247 a file called F<0>) would otherwise
3248 terminate the loop. Again, C<undef> is returned only once. So if
3249 you're expecting a single value from a glob, it is much better to
3252 ($file) = <blurch*>;
3258 because the latter will alternate between returning a filename and
3261 If you're trying to do variable interpolation, it's definitely better
3262 to use the C<glob()> function, because the older notation can cause people
3263 to become confused with the indirect filehandle notation.
3265 @files = glob("$dir/*.[ch]");
3266 @files = glob($files[$i]);
3268 =head2 Constant Folding
3269 X<constant folding> X<folding>
3271 Like C, Perl does a certain amount of expression evaluation at
3272 compile time whenever it determines that all arguments to an
3273 operator are static and have no side effects. In particular, string
3274 concatenation happens at compile time between literals that don't do
3275 variable substitution. Backslash interpolation also happens at
3276 compile time. You can say
3278 'Now is the time for all'
3280 . 'good men to come to.'
3282 and this all reduces to one string internally. Likewise, if
3285 foreach $file (@filenames) {
3286 if (-s $file > 5 + 100 * 2**16) { }
3289 the compiler precomputes the number which that expression
3290 represents so that the interpreter won't have to.
3295 Perl doesn't officially have a no-op operator, but the bare constants
3296 C<0> and C<1> are special-cased not to produce a warning in void
3297 context, so you can for example safely do
3301 =head2 Bitwise String Operators
3302 X<operator, bitwise, string> X<&.> X<|.> X<^.> X<~.>
3304 Bitstrings of any size may be manipulated by the bitwise operators
3307 If the operands to a binary bitwise op are strings of different
3308 sizes, B<|> and B<^> ops act as though the shorter operand had
3309 additional zero bits on the right, while the B<&> op acts as though
3310 the longer operand were truncated to the length of the shorter.
3311 The granularity for such extension or truncation is one or more
3314 # ASCII-based examples
3315 print "j p \n" ^ " a h"; # prints "JAPH\n"
3316 print "JA" | " ph\n"; # prints "japh\n"
3317 print "japh\nJunk" & '_____'; # prints "JAPH\n";
3318 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
3320 If you are intending to manipulate bitstrings, be certain that
3321 you're supplying bitstrings: If an operand is a number, that will imply
3322 a B<numeric> bitwise operation. You may explicitly show which type of
3323 operation you intend by using C<""> or C<0+>, as in the examples below.
3325 $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
3326 $foo = '150' | 105; # yields 255
3327 $foo = 150 | '105'; # yields 255
3328 $foo = '150' | '105'; # yields string '155' (under ASCII)
3330 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
3331 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
3333 This somewhat unpredictable behavior can be avoided with the experimental
3334 "bitwise" feature, new in Perl 5.22. You can enable it via S<C<use feature
3335 'bitwise'>>. By default, it will warn unless the C<"experimental::bitwise">
3336 warnings category has been disabled. (S<C<use experimental 'bitwise'>> will
3337 enable the feature and disable the warning.) Under this feature, the four
3338 standard bitwise operators (C<~ | & ^>) are always numeric. Adding a dot
3339 after each operator (C<~. |. &. ^.>) forces it to treat its operands as
3342 use experimental "bitwise";
3343 $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
3344 $foo = '150' | 105; # yields 255
3345 $foo = 150 | '105'; # yields 255
3346 $foo = '150' | '105'; # yields 255
3347 $foo = 150 |. 105; # yields string '155'
3348 $foo = '150' |. 105; # yields string '155'
3349 $foo = 150 |.'105'; # yields string '155'
3350 $foo = '150' |.'105'; # yields string '155'
3352 $baz = $foo & $bar; # both operands numeric
3353 $biz = $foo ^. $bar; # both operands stringy
3355 The assignment variants of these operators (C<&= |= ^= &.= |.= ^.=>)
3356 behave likewise under the feature.
3358 The behavior of these operators is problematic (and subject to change)
3359 if either or both of the strings are encoded in UTF-8 (see
3360 L<perlunicode/Byte and Character Semantics>.
3362 See L<perlfunc/vec> for information on how to manipulate individual bits
3365 =head2 Integer Arithmetic
3368 By default, Perl assumes that it must do most of its arithmetic in
3369 floating point. But by saying
3373 you may tell the compiler to use integer operations
3374 (see L<integer> for a detailed explanation) from here to the end of
3375 the enclosing BLOCK. An inner BLOCK may countermand this by saying
3379 which lasts until the end of that BLOCK. Note that this doesn't
3380 mean everything is an integer, merely that Perl will use integer
3381 operations for arithmetic, comparison, and bitwise operators. For
3382 example, even under S<C<use integer>>, if you take the C<sqrt(2)>, you'll
3383 still get C<1.4142135623731> or so.
3385 Used on numbers, the bitwise operators (C<&> C<|> C<^> C<~> C<< << >>
3386 C<< >> >>) always produce integral results. (But see also
3387 L</Bitwise String Operators>.) However, S<C<use integer>> still has meaning for
3388 them. By default, their results are interpreted as unsigned integers, but
3389 if S<C<use integer>> is in effect, their results are interpreted
3390 as signed integers. For example, C<~0> usually evaluates to a large
3391 integral value. However, S<C<use integer; ~0>> is C<-1> on two's-complement
3394 =head2 Floating-point Arithmetic
3396 X<floating-point> X<floating point> X<float> X<real>
3398 While S<C<use integer>> provides integer-only arithmetic, there is no
3399 analogous mechanism to provide automatic rounding or truncation to a
3400 certain number of decimal places. For rounding to a certain number
3401 of digits, C<sprintf()> or C<printf()> is usually the easiest route.
3404 Floating-point numbers are only approximations to what a mathematician
3405 would call real numbers. There are infinitely more reals than floats,
3406 so some corners must be cut. For example:
3408 printf "%.20g\n", 123456789123456789;
3409 # produces 123456789123456784
3411 Testing for exact floating-point equality or inequality is not a
3412 good idea. Here's a (relatively expensive) work-around to compare
3413 whether two floating-point numbers are equal to a particular number of
3414 decimal places. See Knuth, volume II, for a more robust treatment of
3418 my ($X, $Y, $POINTS) = @_;
3420 $tX = sprintf("%.${POINTS}g", $X);
3421 $tY = sprintf("%.${POINTS}g", $Y);
3425 The POSIX module (part of the standard perl distribution) implements
3426 C<ceil()>, C<floor()>, and other mathematical and trigonometric functions.
3427 The C<L<Math::Complex>> module (part of the standard perl distribution)
3428 defines mathematical functions that work on both the reals and the
3429 imaginary numbers. C<Math::Complex> is not as efficient as POSIX, but
3430 POSIX can't work with complex numbers.
3432 Rounding in financial applications can have serious implications, and
3433 the rounding method used should be specified precisely. In these
3434 cases, it probably pays not to trust whichever system rounding is
3435 being used by Perl, but to instead implement the rounding function you
3438 =head2 Bigger Numbers
3439 X<number, arbitrary precision>
3441 The standard C<L<Math::BigInt>>, C<L<Math::BigRat>>, and
3442 C<L<Math::BigFloat>> modules,
3443 along with the C<bignum>, C<bigint>, and C<bigrat> pragmas, provide
3444 variable-precision arithmetic and overloaded operators, although
3445 they're currently pretty slow. At the cost of some space and
3446 considerable speed, they avoid the normal pitfalls associated with
3447 limited-precision representations.
3450 use bigint; # easy interface to Math::BigInt
3451 $x = 123456789123456789;
3453 +15241578780673678515622620750190521
3461 say "x/y is ", $x/$y;
3462 say "x*y is ", $x*$y;
3466 Several modules let you calculate with unlimited or fixed precision
3467 (bound only by memory and CPU time). There
3468 are also some non-standard modules that
3469 provide faster implementations via external C libraries.
3471 Here is a short, but incomplete summary:
3473 Math::String treat string sequences like numbers
3474 Math::FixedPrecision calculate with a fixed precision
3475 Math::Currency for currency calculations
3476 Bit::Vector manipulate bit vectors fast (uses C)
3477 Math::BigIntFast Bit::Vector wrapper for big numbers
3478 Math::Pari provides access to the Pari C library
3479 Math::Cephes uses the external Cephes C library (no
3481 Math::Cephes::Fraction fractions via the Cephes library
3482 Math::GMP another one using an external C library
3483 Math::GMPz an alternative interface to libgmp's big ints
3484 Math::GMPq an interface to libgmp's fraction numbers
3485 Math::GMPf an interface to libgmp's floating point numbers