4 perlop - Perl operators and precedence
8 =head2 Operator Precedence and Associativity
9 X<operator, precedence> X<precedence> X<associativity>
11 Operator precedence and associativity work in Perl more or less like
12 they do in mathematics.
14 I<Operator precedence> means some operators are evaluated before
15 others. For example, in C<2 + 4 * 5>, the multiplication has higher
16 precedence so C<4 * 5> is evaluated first yielding C<2 + 20 ==
17 22> and not C<6 * 5 == 30>.
19 I<Operator associativity> defines what happens if a sequence of the
20 same operators is used one after another: whether the evaluator will
21 evaluate the left operations first or the right. For example, in C<8
22 - 4 - 2>, subtraction is left associative so Perl evaluates the
23 expression left to right. C<8 - 4> is evaluated first making the
24 expression C<4 - 2 == 2> and not C<8 - 2 == 6>.
26 Perl operators have the following associativity and precedence,
27 listed from highest precedence to lowest. Operators borrowed from
28 C keep the same precedence relationship with each other, even where
29 C's precedence is slightly screwy. (This makes learning Perl easier
30 for C folks.) With very few exceptions, these all operate on scalar
31 values only, not array values.
33 left terms and list operators (leftward)
37 right ! ~ \ and unary + and -
42 nonassoc named unary operators
43 nonassoc < > <= >= lt gt le ge
44 nonassoc == != <=> eq ne cmp ~~
53 nonassoc list operators (rightward)
58 In the following sections, these operators are covered in precedence order.
60 Many operators can be overloaded for objects. See L<overload>.
62 =head2 Terms and List Operators (Leftward)
63 X<list operator> X<operator, list> X<term>
65 A TERM has the highest precedence in Perl. They include variables,
66 quote and quote-like operators, any expression in parentheses,
67 and any function whose arguments are parenthesized. Actually, there
68 aren't really functions in this sense, just list operators and unary
69 operators behaving as functions because you put parentheses around
70 the arguments. These are all documented in L<perlfunc>.
72 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
73 is followed by a left parenthesis as the next token, the operator and
74 arguments within parentheses are taken to be of highest precedence,
75 just like a normal function call.
77 In the absence of parentheses, the precedence of list operators such as
78 C<print>, C<sort>, or C<chmod> is either very high or very low depending on
79 whether you are looking at the left side or the right side of the operator.
82 @ary = (1, 3, sort 4, 2);
83 print @ary; # prints 1324
85 the commas on the right of the sort are evaluated before the sort,
86 but the commas on the left are evaluated after. In other words,
87 list operators tend to gobble up all arguments that follow, and
88 then act like a simple TERM with regard to the preceding expression.
89 Be careful with parentheses:
91 # These evaluate exit before doing the print:
92 print($foo, exit); # Obviously not what you want.
93 print $foo, exit; # Nor is this.
95 # These do the print before evaluating exit:
96 (print $foo), exit; # This is what you want.
97 print($foo), exit; # Or this.
98 print ($foo), exit; # Or even this.
102 print ($foo & 255) + 1, "\n";
104 probably doesn't do what you expect at first glance. The parentheses
105 enclose the argument list for C<print> which is evaluated (printing
106 the result of C<$foo & 255>). Then one is added to the return value
107 of C<print> (usually 1). The result is something like this:
109 1 + 1, "\n"; # Obviously not what you meant.
111 To do what you meant properly, you must write:
113 print(($foo & 255) + 1, "\n");
115 See L<Named Unary Operators> for more discussion of this.
117 Also parsed as terms are the C<do {}> and C<eval {}> constructs, as
118 well as subroutine and method calls, and the anonymous
119 constructors C<[]> and C<{}>.
121 See also L<Quote and Quote-like Operators> toward the end of this section,
122 as well as L</"I/O Operators">.
124 =head2 The Arrow Operator
125 X<arrow> X<dereference> X<< -> >>
127 "C<< -> >>" is an infix dereference operator, just as it is in C
128 and C++. If the right side is either a C<[...]>, C<{...}>, or a
129 C<(...)> subscript, then the left side must be either a hard or
130 symbolic reference to an array, a hash, or a subroutine respectively.
131 (Or technically speaking, a location capable of holding a hard
132 reference, if it's an array or hash reference being used for
133 assignment.) See L<perlreftut> and L<perlref>.
135 Otherwise, the right side is a method name or a simple scalar
136 variable containing either the method name or a subroutine reference,
137 and the left side must be either an object (a blessed reference)
138 or a class name (that is, a package name). See L<perlobj>.
140 =head2 Auto-increment and Auto-decrement
141 X<increment> X<auto-increment> X<++> X<decrement> X<auto-decrement> X<-->
143 "++" and "--" work as in C. That is, if placed before a variable,
144 they increment or decrement the variable by one before returning the
145 value, and if placed after, increment or decrement after returning the
149 print $i++; # prints 0
150 print ++$j; # prints 1
152 Note that just as in C, Perl doesn't define B<when> the variable is
153 incremented or decremented. You just know it will be done sometime
154 before or after the value is returned. This also means that modifying
155 a variable twice in the same statement will lead to undefined behaviour.
156 Avoid statements like:
161 Perl will not guarantee what the result of the above statements is.
163 The auto-increment operator has a little extra builtin magic to it. If
164 you increment a variable that is numeric, or that has ever been used in
165 a numeric context, you get a normal increment. If, however, the
166 variable has been used in only string contexts since it was set, and
167 has a value that is not the empty string and matches the pattern
168 C</^[a-zA-Z]*[0-9]*\z/>, the increment is done as a string, preserving each
169 character within its range, with carry:
171 print ++($foo = '99'); # prints '100'
172 print ++($foo = 'a0'); # prints 'a1'
173 print ++($foo = 'Az'); # prints 'Ba'
174 print ++($foo = 'zz'); # prints 'aaa'
176 C<undef> is always treated as numeric, and in particular is changed
177 to C<0> before incrementing (so that a post-increment of an undef value
178 will return C<0> rather than C<undef>).
180 The auto-decrement operator is not magical.
182 =head2 Exponentiation
183 X<**> X<exponentiation> X<power>
185 Binary "**" is the exponentiation operator. It binds even more
186 tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is
187 implemented using C's pow(3) function, which actually works on doubles
190 =head2 Symbolic Unary Operators
191 X<unary operator> X<operator, unary>
193 Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower
194 precedence version of this.
197 Unary "-" performs arithmetic negation if the operand is numeric,
198 including any string that looks like a number. If the operand is
199 an identifier, a string consisting of a minus sign concatenated
200 with the identifier is returned. Otherwise, if the string starts
201 with a plus or minus, a string starting with the opposite sign is
202 returned. One effect of these rules is that -bareword is equivalent
203 to the string "-bareword". If, however, the string begins with a
204 non-alphabetic character (excluding "+" or "-"), Perl will attempt to convert
205 the string to a numeric and the arithmetic negation is performed. If the
206 string cannot be cleanly converted to a numeric, Perl will give the warning
207 B<Argument "the string" isn't numeric in negation (-) at ...>.
208 X<-> X<negation, arithmetic>
210 Unary "~" performs bitwise negation, i.e., 1's complement. For
211 example, C<0666 & ~027> is 0640. (See also L<Integer Arithmetic> and
212 L<Bitwise String Operators>.) Note that the width of the result is
213 platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64
214 bits wide on a 64-bit platform, so if you are expecting a certain bit
215 width, remember to use the & operator to mask off the excess bits.
216 X<~> X<negation, binary>
218 Unary "+" has no effect whatsoever, even on strings. It is useful
219 syntactically for separating a function name from a parenthesized expression
220 that would otherwise be interpreted as the complete list of function
221 arguments. (See examples above under L<Terms and List Operators (Leftward)>.)
224 Unary "\" creates a reference to whatever follows it. See L<perlreftut>
225 and L<perlref>. Do not confuse this behavior with the behavior of
226 backslash within a string, although both forms do convey the notion
227 of protecting the next thing from interpolation.
228 X<\> X<reference> X<backslash>
230 =head2 Binding Operators
231 X<binding> X<operator, binding> X<=~> X<!~>
233 Binary "=~" binds a scalar expression to a pattern match. Certain operations
234 search or modify the string $_ by default. This operator makes that kind
235 of operation work on some other string. The right argument is a search
236 pattern, substitution, or transliteration. The left argument is what is
237 supposed to be searched, substituted, or transliterated instead of the default
238 $_. When used in scalar context, the return value generally indicates the
239 success of the operation. The exceptions are substitution (s///)
240 and transliteration (y///) with the C</r> (non-destructive) option,
241 which cause the B<r>eturn value to be the result of the substitution.
242 Behavior in list context depends on the particular operator.
243 See L</"Regexp Quote-Like Operators"> for details and L<perlretut> for
244 examples using these operators.
246 If the right argument is an expression rather than a search pattern,
247 substitution, or transliteration, it is interpreted as a search pattern at run
248 time. Note that this means that its contents will be interpolated twice, so
252 is not ok, as the regex engine will end up trying to compile the
253 pattern C<\>, which it will consider a syntax error.
255 Binary "!~" is just like "=~" except the return value is negated in
258 Binary "!~" with a non-destructive substitution (s///r) or transliteration
259 (y///r) is a syntax error.
261 =head2 Multiplicative Operators
262 X<operator, multiplicative>
264 Binary "*" multiplies two numbers.
267 Binary "/" divides two numbers.
270 Binary "%" is the modulo operator, which computes the division
271 remainder of its first argument with respect to its second argument.
273 operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
274 C<$a> minus the largest multiple of C<$b> less than or equal to
275 C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
276 smallest multiple of C<$b> that is not less than C<$a> (i.e. the
277 result will be less than or equal to zero). If the operands
278 C<$a> and C<$b> are floating point values and the absolute value of
279 C<$b> (that is C<abs($b)>) is less than C<(UV_MAX + 1)>, only
280 the integer portion of C<$a> and C<$b> will be used in the operation
281 (Note: here C<UV_MAX> means the maximum of the unsigned integer type).
282 If the absolute value of the right operand (C<abs($b)>) is greater than
283 or equal to C<(UV_MAX + 1)>, "%" computes the floating-point remainder
284 C<$r> in the equation C<($r = $a - $i*$b)> where C<$i> is a certain
285 integer that makes C<$r> have the same sign as the right operand
286 C<$b> (B<not> as the left operand C<$a> like C function C<fmod()>)
287 and the absolute value less than that of C<$b>.
288 Note that when C<use integer> is in scope, "%" gives you direct access
289 to the modulo operator as implemented by your C compiler. This
290 operator is not as well defined for negative operands, but it will
292 X<%> X<remainder> X<modulo> X<mod>
294 Binary "x" is the repetition operator. In scalar context or if the left
295 operand is not enclosed in parentheses, it returns a string consisting
296 of the left operand repeated the number of times specified by the right
297 operand. In list context, if the left operand is enclosed in
298 parentheses or is a list formed by C<qw/STRING/>, it repeats the list.
299 If the right operand is zero or negative, it returns an empty string
300 or an empty list, depending on the context.
303 print '-' x 80; # print row of dashes
305 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
307 @ones = (1) x 80; # a list of 80 1's
308 @ones = (5) x @ones; # set all elements to 5
311 =head2 Additive Operators
312 X<operator, additive>
314 Binary "+" returns the sum of two numbers.
317 Binary "-" returns the difference of two numbers.
320 Binary "." concatenates two strings.
321 X<string, concatenation> X<concatenation>
322 X<cat> X<concat> X<concatenate> X<.>
324 =head2 Shift Operators
325 X<shift operator> X<operator, shift> X<<< << >>>
326 X<<< >> >>> X<right shift> X<left shift> X<bitwise shift>
327 X<shl> X<shr> X<shift, right> X<shift, left>
329 Binary "<<" returns the value of its left argument shifted left by the
330 number of bits specified by the right argument. Arguments should be
331 integers. (See also L<Integer Arithmetic>.)
333 Binary ">>" returns the value of its left argument shifted right by
334 the number of bits specified by the right argument. Arguments should
335 be integers. (See also L<Integer Arithmetic>.)
337 Note that both "<<" and ">>" in Perl are implemented directly using
338 "<<" and ">>" in C. If C<use integer> (see L<Integer Arithmetic>) is
339 in force then signed C integers are used, else unsigned C integers are
340 used. Either way, the implementation isn't going to generate results
341 larger than the size of the integer type Perl was built with (32 bits
344 The result of overflowing the range of the integers is undefined
345 because it is undefined also in C. In other words, using 32-bit
346 integers, C<< 1 << 32 >> is undefined. Shifting by a negative number
347 of bits is also undefined.
349 =head2 Named Unary Operators
350 X<operator, named unary>
352 The various named unary operators are treated as functions with one
353 argument, with optional parentheses.
355 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
356 is followed by a left parenthesis as the next token, the operator and
357 arguments within parentheses are taken to be of highest precedence,
358 just like a normal function call. For example,
359 because named unary operators are higher precedence than ||:
361 chdir $foo || die; # (chdir $foo) || die
362 chdir($foo) || die; # (chdir $foo) || die
363 chdir ($foo) || die; # (chdir $foo) || die
364 chdir +($foo) || die; # (chdir $foo) || die
366 but, because * is higher precedence than named operators:
368 chdir $foo * 20; # chdir ($foo * 20)
369 chdir($foo) * 20; # (chdir $foo) * 20
370 chdir ($foo) * 20; # (chdir $foo) * 20
371 chdir +($foo) * 20; # chdir ($foo * 20)
373 rand 10 * 20; # rand (10 * 20)
374 rand(10) * 20; # (rand 10) * 20
375 rand (10) * 20; # (rand 10) * 20
376 rand +(10) * 20; # rand (10 * 20)
378 Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are
379 treated like named unary operators, but they don't follow this functional
380 parenthesis rule. That means, for example, that C<-f($file).".bak"> is
381 equivalent to C<-f "$file.bak">.
382 X<-X> X<filetest> X<operator, filetest>
384 See also L<"Terms and List Operators (Leftward)">.
386 =head2 Relational Operators
387 X<relational operator> X<operator, relational>
389 Binary "<" returns true if the left argument is numerically less than
393 Binary ">" returns true if the left argument is numerically greater
394 than the right argument.
397 Binary "<=" returns true if the left argument is numerically less than
398 or equal to the right argument.
401 Binary ">=" returns true if the left argument is numerically greater
402 than or equal to the right argument.
405 Binary "lt" returns true if the left argument is stringwise less than
409 Binary "gt" returns true if the left argument is stringwise greater
410 than the right argument.
413 Binary "le" returns true if the left argument is stringwise less than
414 or equal to the right argument.
417 Binary "ge" returns true if the left argument is stringwise greater
418 than or equal to the right argument.
421 =head2 Equality Operators
422 X<equality> X<equal> X<equals> X<operator, equality>
424 Binary "==" returns true if the left argument is numerically equal to
428 Binary "!=" returns true if the left argument is numerically not equal
429 to the right argument.
432 Binary "<=>" returns -1, 0, or 1 depending on whether the left
433 argument is numerically less than, equal to, or greater than the right
434 argument. If your platform supports NaNs (not-a-numbers) as numeric
435 values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
436 "<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
437 returns true, as does NaN != anything else. If your platform doesn't
438 support NaNs then NaN is just a string with numeric value 0.
439 X<< <=> >> X<spaceship>
441 perl -le '$a = "NaN"; print "No NaN support here" if $a == $a'
442 perl -le '$a = "NaN"; print "NaN support here" if $a != $a'
444 Binary "eq" returns true if the left argument is stringwise equal to
448 Binary "ne" returns true if the left argument is stringwise not equal
449 to the right argument.
452 Binary "cmp" returns -1, 0, or 1 depending on whether the left
453 argument is stringwise less than, equal to, or greater than the right
457 Binary "~~" does a smart match between its arguments. Smart matching
458 is described in L<perlsyn/"Smart matching in detail">.
461 "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
462 by the current locale if C<use locale> is in effect. See L<perllocale>.
465 X<operator, bitwise, and> X<bitwise and> X<&>
467 Binary "&" returns its operands ANDed together bit by bit.
468 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
470 Note that "&" has lower priority than relational operators, so for example
471 the brackets are essential in a test like
473 print "Even\n" if ($x & 1) == 0;
475 =head2 Bitwise Or and Exclusive Or
476 X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor>
479 Binary "|" returns its operands ORed together bit by bit.
480 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
482 Binary "^" returns its operands XORed together bit by bit.
483 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
485 Note that "|" and "^" have lower priority than relational operators, so
486 for example the brackets are essential in a test like
488 print "false\n" if (8 | 2) != 10;
490 =head2 C-style Logical And
491 X<&&> X<logical and> X<operator, logical, and>
493 Binary "&&" performs a short-circuit logical AND operation. That is,
494 if the left operand is false, the right operand is not even evaluated.
495 Scalar or list context propagates down to the right operand if it
498 =head2 C-style Logical Or
499 X<||> X<operator, logical, or>
501 Binary "||" performs a short-circuit logical OR operation. That is,
502 if the left operand is true, the right operand is not even evaluated.
503 Scalar or list context propagates down to the right operand if it
506 =head2 C-style Logical Defined-Or
507 X<//> X<operator, logical, defined-or>
509 Although it has no direct equivalent in C, Perl's C<//> operator is related
510 to its C-style or. In fact, it's exactly the same as C<||>, except that it
511 tests the left hand side's definedness instead of its truth. Thus, C<$a // $b>
512 is similar to C<defined($a) || $b> (except that it returns the value of C<$a>
513 rather than the value of C<defined($a)>) and is exactly equivalent to
514 C<defined($a) ? $a : $b>. This is very useful for providing default values
515 for variables. If you actually want to test if at least one of C<$a> and
516 C<$b> is defined, use C<defined($a // $b)>.
518 The C<||>, C<//> and C<&&> operators return the last value evaluated
519 (unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably
520 portable way to find out the home directory might be:
522 $home = $ENV{'HOME'} // $ENV{'LOGDIR'} //
523 (getpwuid($<))[7] // die "You're homeless!\n";
525 In particular, this means that you shouldn't use this
526 for selecting between two aggregates for assignment:
528 @a = @b || @c; # this is wrong
529 @a = scalar(@b) || @c; # really meant this
530 @a = @b ? @b : @c; # this works fine, though
532 As more readable alternatives to C<&&> and C<||> when used for
533 control flow, Perl provides the C<and> and C<or> operators (see below).
534 The short-circuit behavior is identical. The precedence of "and"
535 and "or" is much lower, however, so that you can safely use them after a
536 list operator without the need for parentheses:
538 unlink "alpha", "beta", "gamma"
539 or gripe(), next LINE;
541 With the C-style operators that would have been written like this:
543 unlink("alpha", "beta", "gamma")
544 || (gripe(), next LINE);
546 Using "or" for assignment is unlikely to do what you want; see below.
548 =head2 Range Operators
549 X<operator, range> X<range> X<..> X<...>
551 Binary ".." is the range operator, which is really two different
552 operators depending on the context. In list context, it returns a
553 list of values counting (up by ones) from the left value to the right
554 value. If the left value is greater than the right value then it
555 returns the empty list. The range operator is useful for writing
556 C<foreach (1..10)> loops and for doing slice operations on arrays. In
557 the current implementation, no temporary array is created when the
558 range operator is used as the expression in C<foreach> loops, but older
559 versions of Perl might burn a lot of memory when you write something
562 for (1 .. 1_000_000) {
566 The range operator also works on strings, using the magical
567 auto-increment, see below.
569 In scalar context, ".." returns a boolean value. The operator is
570 bistable, like a flip-flop, and emulates the line-range (comma)
571 operator of B<sed>, B<awk>, and various editors. Each ".." operator
572 maintains its own boolean state, even across calls to a subroutine
573 that contains it. It is false as long as its left operand is false.
574 Once the left operand is true, the range operator stays true until the
575 right operand is true, I<AFTER> which the range operator becomes false
576 again. It doesn't become false till the next time the range operator
577 is evaluated. It can test the right operand and become false on the
578 same evaluation it became true (as in B<awk>), but it still returns
579 true once. If you don't want it to test the right operand until the
580 next evaluation, as in B<sed>, just use three dots ("...") instead of
581 two. In all other regards, "..." behaves just like ".." does.
583 The right operand is not evaluated while the operator is in the
584 "false" state, and the left operand is not evaluated while the
585 operator is in the "true" state. The precedence is a little lower
586 than || and &&. The value returned is either the empty string for
587 false, or a sequence number (beginning with 1) for true. The sequence
588 number is reset for each range encountered. The final sequence number
589 in a range has the string "E0" appended to it, which doesn't affect
590 its numeric value, but gives you something to search for if you want
591 to exclude the endpoint. You can exclude the beginning point by
592 waiting for the sequence number to be greater than 1.
594 If either operand of scalar ".." is a constant expression,
595 that operand is considered true if it is equal (C<==>) to the current
596 input line number (the C<$.> variable).
598 To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>,
599 but that is only an issue if you use a floating point expression; when
600 implicitly using C<$.> as described in the previous paragraph, the
601 comparison is C<int(EXPR) == int($.)> which is only an issue when C<$.>
602 is set to a floating point value and you are not reading from a file.
603 Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what
604 you want in scalar context because each of the operands are evaluated
605 using their integer representation.
609 As a scalar operator:
611 if (101 .. 200) { print; } # print 2nd hundred lines, short for
612 # if ($. == 101 .. $. == 200) { print; }
614 next LINE if (1 .. /^$/); # skip header lines, short for
615 # next LINE if ($. == 1 .. /^$/);
616 # (typically in a loop labeled LINE)
618 s/^/> / if (/^$/ .. eof()); # quote body
620 # parse mail messages
622 $in_header = 1 .. /^$/;
623 $in_body = /^$/ .. eof;
630 close ARGV if eof; # reset $. each file
633 Here's a simple example to illustrate the difference between
634 the two range operators:
647 This program will print only the line containing "Bar". If
648 the range operator is changed to C<...>, it will also print the
651 And now some examples as a list operator:
653 for (101 .. 200) { print; } # print $_ 100 times
654 @foo = @foo[0 .. $#foo]; # an expensive no-op
655 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
657 The range operator (in list context) makes use of the magical
658 auto-increment algorithm if the operands are strings. You
661 @alphabet = ('A' .. 'Z');
663 to get all normal letters of the English alphabet, or
665 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
667 to get a hexadecimal digit, or
669 @z2 = ('01' .. '31'); print $z2[$mday];
671 to get dates with leading zeros.
673 If the final value specified is not in the sequence that the magical
674 increment would produce, the sequence goes until the next value would
675 be longer than the final value specified.
677 If the initial value specified isn't part of a magical increment
678 sequence (that is, a non-empty string matching "/^[a-zA-Z]*[0-9]*\z/"),
679 only the initial value will be returned. So the following will only
682 use charnames 'greek';
683 my @greek_small = ("\N{alpha}" .. "\N{omega}");
685 To get lower-case greek letters, use this instead:
687 my @greek_small = map { chr } ( ord("\N{alpha}") ..
690 Because each operand is evaluated in integer form, C<2.18 .. 3.14> will
691 return two elements in list context.
693 @list = (2.18 .. 3.14); # same as @list = (2 .. 3);
695 =head2 Conditional Operator
696 X<operator, conditional> X<operator, ternary> X<ternary> X<?:>
698 Ternary "?:" is the conditional operator, just as in C. It works much
699 like an if-then-else. If the argument before the ? is true, the
700 argument before the : is returned, otherwise the argument after the :
701 is returned. For example:
703 printf "I have %d dog%s.\n", $n,
704 ($n == 1) ? '' : "s";
706 Scalar or list context propagates downward into the 2nd
707 or 3rd argument, whichever is selected.
709 $a = $ok ? $b : $c; # get a scalar
710 @a = $ok ? @b : @c; # get an array
711 $a = $ok ? @b : @c; # oops, that's just a count!
713 The operator may be assigned to if both the 2nd and 3rd arguments are
714 legal lvalues (meaning that you can assign to them):
716 ($a_or_b ? $a : $b) = $c;
718 Because this operator produces an assignable result, using assignments
719 without parentheses will get you in trouble. For example, this:
721 $a % 2 ? $a += 10 : $a += 2
725 (($a % 2) ? ($a += 10) : $a) += 2
729 ($a % 2) ? ($a += 10) : ($a += 2)
731 That should probably be written more simply as:
733 $a += ($a % 2) ? 10 : 2;
735 =head2 Assignment Operators
736 X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=>
737 X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=>
740 "=" is the ordinary assignment operator.
742 Assignment operators work as in C. That is,
750 although without duplicating any side effects that dereferencing the lvalue
751 might trigger, such as from tie(). Other assignment operators work similarly.
752 The following are recognized:
759 Although these are grouped by family, they all have the precedence
762 Unlike in C, the scalar assignment operator produces a valid lvalue.
763 Modifying an assignment is equivalent to doing the assignment and
764 then modifying the variable that was assigned to. This is useful
765 for modifying a copy of something, like this:
767 ($tmp = $global) =~ tr [A-Z] [a-z];
778 Similarly, a list assignment in list context produces the list of
779 lvalues assigned to, and a list assignment in scalar context returns
780 the number of elements produced by the expression on the right hand
781 side of the assignment.
783 =head2 Comma Operator
784 X<comma> X<operator, comma> X<,>
786 Binary "," is the comma operator. In scalar context it evaluates
787 its left argument, throws that value away, then evaluates its right
788 argument and returns that value. This is just like C's comma operator.
790 In list context, it's just the list argument separator, and inserts
791 both its arguments into the list. These arguments are also evaluated
794 The C<< => >> operator is a synonym for the comma except that it causes
795 its left operand to be interpreted as a string if it begins with a letter
796 or underscore and is composed only of letters, digits and underscores.
797 This includes operands that might otherwise be interpreted as operators,
798 constants, single number v-strings or function calls. If in doubt about
799 this behaviour, the left operand can be quoted explicitly.
801 Otherwise, the C<< => >> operator behaves exactly as the comma operator
802 or list argument separator, according to context.
806 use constant FOO => "something";
808 my %h = ( FOO => 23 );
816 my %h = ("something", 23);
818 The C<< => >> operator is helpful in documenting the correspondence
819 between keys and values in hashes, and other paired elements in lists.
821 %hash = ( $key => $value );
822 login( $username => $password );
824 =head2 Yada Yada Operator
825 X<...> X<... operator> X<yada yada operator>
827 The yada yada operator (noted C<...>) is a placeholder for code. Perl
828 parses it without error, but when you try to execute a yada yada, it
829 throws an exception with the text C<Unimplemented>:
831 sub unimplemented { ... }
833 eval { unimplemented() };
834 if( $@ eq 'Unimplemented' ) {
835 print "I found the yada yada!\n";
838 You can only use the yada yada to stand in for a complete statement.
839 These examples of the yada yada work:
855 do { my $n; ...; print 'Hurrah!' };
857 The yada yada cannot stand in for an expression that is part of a
858 larger statement since the C<...> is also the three-dot version of the
859 range operator (see L<Range Operators>). These examples of the yada
860 yada are still syntax errors:
864 open my($fh), '>', '/dev/passwd' or ...;
866 if( $condition && ... ) { print "Hello\n" };
868 There are some cases where Perl can't immediately tell the difference
869 between an expression and a statement. For instance, the syntax for a
870 block and an anonymous hash reference constructor look the same unless
871 there's something in the braces that give Perl a hint. The yada yada
872 is a syntax error if Perl doesn't guess that the C<{ ... }> is a
873 block. In that case, it doesn't think the C<...> is the yada yada
874 because it's expecting an expression instead of a statement:
876 my @transformed = map { ... } @input; # syntax error
878 You can use a C<;> inside your block to denote that the C<{ ... }> is
879 a block and not a hash reference constructor. Now the yada yada works:
881 my @transformed = map {; ... } @input; # ; disambiguates
883 my @transformed = map { ...; } @input; # ; disambiguates
885 =head2 List Operators (Rightward)
886 X<operator, list, rightward> X<list operator>
888 On the right side of a list operator, it has very low precedence,
889 such that it controls all comma-separated expressions found there.
890 The only operators with lower precedence are the logical operators
891 "and", "or", and "not", which may be used to evaluate calls to list
892 operators without the need for extra parentheses:
894 open HANDLE, "filename"
895 or die "Can't open: $!\n";
897 See also discussion of list operators in L<Terms and List Operators (Leftward)>.
900 X<operator, logical, not> X<not>
902 Unary "not" returns the logical negation of the expression to its right.
903 It's the equivalent of "!" except for the very low precedence.
906 X<operator, logical, and> X<and>
908 Binary "and" returns the logical conjunction of the two surrounding
909 expressions. It's equivalent to && except for the very low
910 precedence. This means that it short-circuits: i.e., the right
911 expression is evaluated only if the left expression is true.
913 =head2 Logical or, Defined or, and Exclusive Or
914 X<operator, logical, or> X<operator, logical, xor>
915 X<operator, logical, defined or> X<operator, logical, exclusive or>
918 Binary "or" returns the logical disjunction of the two surrounding
919 expressions. It's equivalent to || except for the very low precedence.
920 This makes it useful for control flow
922 print FH $data or die "Can't write to FH: $!";
924 This means that it short-circuits: i.e., the right expression is evaluated
925 only if the left expression is false. Due to its precedence, you should
926 probably avoid using this for assignment, only for control flow.
928 $a = $b or $c; # bug: this is wrong
929 ($a = $b) or $c; # really means this
930 $a = $b || $c; # better written this way
932 However, when it's a list-context assignment and you're trying to use
933 "||" for control flow, you probably need "or" so that the assignment
934 takes higher precedence.
936 @info = stat($file) || die; # oops, scalar sense of stat!
937 @info = stat($file) or die; # better, now @info gets its due
939 Then again, you could always use parentheses.
941 Binary "xor" returns the exclusive-OR of the two surrounding expressions.
942 It cannot short circuit, of course.
944 =head2 C Operators Missing From Perl
945 X<operator, missing from perl> X<&> X<*>
946 X<typecasting> X<(TYPE)>
948 Here is what C has that Perl doesn't:
954 Address-of operator. (But see the "\" operator for taking a reference.)
958 Dereference-address operator. (Perl's prefix dereferencing
959 operators are typed: $, @, %, and &.)
963 Type-casting operator.
967 =head2 Quote and Quote-like Operators
968 X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m>
969 X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>>
970 X<escape sequence> X<escape>
973 While we usually think of quotes as literal values, in Perl they
974 function as operators, providing various kinds of interpolating and
975 pattern matching capabilities. Perl provides customary quote characters
976 for these behaviors, but also provides a way for you to choose your
977 quote character for any of them. In the following table, a C<{}> represents
978 any pair of delimiters you choose.
980 Customary Generic Meaning Interpolates
985 // m{} Pattern match yes*
987 s{}{} Substitution yes*
988 tr{}{} Transliteration no (but see below)
991 * unless the delimiter is ''.
993 Non-bracketing delimiters use the same character fore and aft, but the four
994 sorts of brackets (round, angle, square, curly) will all nest, which means
1003 Note, however, that this does not always work for quoting Perl code:
1005 $s = q{ if($a eq "}") ... }; # WRONG
1007 is a syntax error. The C<Text::Balanced> module (from CPAN, and
1008 starting from Perl 5.8 part of the standard distribution) is able
1009 to do this properly.
1011 There can be whitespace between the operator and the quoting
1012 characters, except when C<#> is being used as the quoting character.
1013 C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
1014 operator C<q> followed by a comment. Its argument will be taken
1015 from the next line. This allows you to write:
1017 s {foo} # Replace foo
1020 The following escape sequences are available in constructs that interpolate
1021 and in transliterations.
1022 X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N> X<\N{}>
1025 Sequence Note Description
1031 \a alarm (bell) (BEL)
1033 \x{263a} [1,8] hex char (example: SMILEY)
1034 \x1b [2,8] restricted range hex char (example: ESC)
1035 \N{name} [3] named Unicode character or character sequence
1036 \N{U+263D} [4,8] Unicode character (example: FIRST QUARTER MOON)
1037 \c[ [5] control char (example: chr(27))
1038 \o{23072} [6,8] octal char (example: SMILEY)
1039 \033 [7,8] restricted range octal char (example: ESC)
1045 The result is the character specified by the hexadecimal number between
1046 the braces. See L</[8]> below for details on which character.
1048 Only hexadecimal digits are valid between the braces. If an invalid
1049 character is encountered, a warning will be issued and the invalid
1050 character and all subsequent characters (valid or invalid) within the
1051 braces will be discarded.
1053 If there are no valid digits between the braces, the generated character is
1054 the NULL character (C<\x{00}>). However, an explicit empty brace (C<\x{}>)
1055 will not cause a warning.
1059 The result is the character specified by the hexadecimal number in the range
1060 0x00 to 0xFF. See L</[8]> below for details on which character.
1062 Only hexadecimal digits are valid following C<\x>. When C<\x> is followed
1063 by fewer than two valid digits, any valid digits will be zero-padded. This
1064 means that C<\x7> will be interpreted as C<\x07> and C<\x> alone will be
1065 interpreted as C<\x00>. Except at the end of a string, having fewer than
1066 two valid digits will result in a warning. Note that while the warning
1067 says the illegal character is ignored, it is only ignored as part of the
1068 escape and will still be used as the subsequent character in the string.
1071 Original Result Warns?
1079 The result is the Unicode character or character sequence given by I<name>.
1084 C<\N{U+I<hexadecimal number>}> means the Unicode character whose Unicode code
1085 point is I<hexadecimal number>.
1089 The character following C<\c> is mapped to some other character as shown in the
1106 Also, C<\c\I<X>> yields C< chr(28) . "I<X>"> for any I<X>, but cannot come at the
1107 end of a string, because the backslash would be parsed as escaping the end
1110 On ASCII platforms, the resulting characters from the list above are the
1111 complete set of ASCII controls. This isn't the case on EBCDIC platforms; see
1112 L<perlebcdic/OPERATOR DIFFERENCES> for the complete list of what these
1113 sequences mean on both ASCII and EBCDIC platforms.
1115 Use of any other character following the "c" besides those listed above is
1116 discouraged, and may become deprecated or forbidden. What happens for those
1117 other characters currently though, is that the value is derived by inverting
1120 To get platform independent controls, you can use C<\N{...}>.
1124 The result is the character specified by the octal number between the braces.
1125 See L</[8]> below for details on which character.
1127 If a character that isn't an octal digit is encountered, a warning is raised,
1128 and the value is based on the octal digits before it, discarding it and all
1129 following characters up to the closing brace. It is a fatal error if there are
1130 no octal digits at all.
1134 The result is the character specified by the three digit octal number in the
1135 range 000 to 777 (but best to not use above 077, see next paragraph). See
1136 L</[8]> below for details on which character.
1138 Some contexts allow 2 or even 1 digit, but any usage without exactly
1139 three digits, the first being a zero, may give unintended results. (For
1140 example, see L<perlrebackslash/Octal escapes>.) Starting in Perl 5.14, you may
1141 use C<\o{}> instead which avoids all these problems. Otherwise, it is best to
1142 use this construct only for ordinals C<\077> and below, remembering to pad to
1143 the left with zeros to make three digits. For larger ordinals, either use
1144 C<\o{}> , or convert to someething else, such as to hex and use C<\x{}>
1147 Having fewer than 3 digits may lead to a misleading warning message that says
1148 that what follows is ignored. For example, C<"\128"> in the ASCII character set
1149 is equivalent to the two characters C<"\n8">, but the warning C<Illegal octal
1150 digit '8' ignored> will be thrown. To avoid this warning, make sure to pad
1151 your octal number with C<0>s: C<"\0128">.
1155 Several of the constructs above specify a character by a number. That number
1156 gives the character's position in the character set encoding (indexed from 0).
1157 This is called synonymously its ordinal, code position, or code point). Perl
1158 works on platforms that have a native encoding currently of either ASCII/Latin1
1159 or EBCDIC, each of which allow specification of 256 characters. In general, if
1160 the number is 255 (0xFF, 0377) or below, Perl interprets this in the platform's
1161 native encoding. If the number is 256 (0x100, 0400) or above, Perl interprets
1162 it as as a Unicode code point and the result is the corresponding Unicode
1163 character. For example C<\x{50}> and C<\o{120}> both are the number 80 in
1164 decimal, which is less than 256, so the number is interpreted in the native
1165 character set encoding. In ASCII the character in the 80th position (indexed
1166 from 0) is the letter "P", and in EBCDIC it is the ampersand symbol "&".
1167 C<\x{100}> and C<\o{400}> are both 256 in decimal, so the number is interpreted
1168 as a Unicode code point no matter what the native encoding is. The name of the
1169 character in the 100th position (indexed by 0) in Unicode is
1170 C<LATIN CAPITAL LETTER A WITH MACRON>.
1172 There are a couple of exceptions to the above rule. C<\N{U+I<hex number>}> is
1173 always interpreted as a Unicode code point, so that C<\N{U+0050}> is "P" even
1174 on EBCDIC platforms. And if L<C<S<use encoding>>|encoding> is in effect, the
1175 number is considered to be in that encoding, and is translated from that into
1176 the platform's native encoding if there is a corresponding native character;
1177 otherwise to Unicode.
1181 B<NOTE>: Unlike C and other languages, Perl has no C<\v> escape sequence for
1182 the vertical tab (VT - ASCII 11), but you may use C<\ck> or C<\x0b>. (C<\v>
1183 does have meaning in regular expression patterns in Perl, see L<perlre>.)
1185 The following escape sequences are available in constructs that interpolate,
1186 but not in transliterations.
1187 X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q>
1189 \l lowercase next char
1190 \u uppercase next char
1191 \L lowercase till \E
1192 \U uppercase till \E
1193 \Q quote non-word characters till \E
1194 \E end either case modification or quoted section
1196 If C<use locale> is in effect, the case map used by C<\l>, C<\L>,
1197 C<\u> and C<\U> is taken from the current locale. See L<perllocale>.
1198 If Unicode (for example, C<\N{}> or wide hex characters of 0x100 or
1199 beyond) is being used, the case map used by C<\l>, C<\L>, C<\u> and
1200 C<\U> is as defined by Unicode.
1202 All systems use the virtual C<"\n"> to represent a line terminator,
1203 called a "newline". There is no such thing as an unvarying, physical
1204 newline character. It is only an illusion that the operating system,
1205 device drivers, C libraries, and Perl all conspire to preserve. Not all
1206 systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
1207 on a Mac, these are reversed, and on systems without line terminator,
1208 printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
1209 you mean a "newline" for your system, but use the literal ASCII when you
1210 need an exact character. For example, most networking protocols expect
1211 and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
1212 and although they often accept just C<"\012">, they seldom tolerate just
1213 C<"\015">. If you get in the habit of using C<"\n"> for networking,
1214 you may be burned some day.
1215 X<newline> X<line terminator> X<eol> X<end of line>
1218 For constructs that do interpolate, variables beginning with "C<$>"
1219 or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
1220 C<< $href->{key}[0] >> are also interpolated, as are array and hash slices.
1221 But method calls such as C<< $obj->meth >> are not.
1223 Interpolating an array or slice interpolates the elements in order,
1224 separated by the value of C<$">, so is equivalent to interpolating
1225 C<join $", @array>. "Punctuation" arrays such as C<@*> are only
1226 interpolated if the name is enclosed in braces C<@{*}>, but special
1227 arrays C<@_>, C<@+>, and C<@-> are interpolated, even without braces.
1229 For double-quoted strings, the quoting from C<\Q> is applied after
1230 interpolation and escapes are processed.
1232 "abc\Qfoo\tbar$s\Exyz"
1236 "abc" . quotemeta("foo\tbar$s") . "xyz"
1238 For the pattern of regex operators (C<qr//>, C<m//> and C<s///>),
1239 the quoting from C<\Q> is applied after interpolation is processed,
1240 but before escapes are processed. This allows the pattern to match
1241 literally (except for C<$> and C<@>). For example, the following matches:
1245 Because C<$> or C<@> trigger interpolation, you'll need to use something
1246 like C</\Quser\E\@\Qhost/> to match them literally.
1248 Patterns are subject to an additional level of interpretation as a
1249 regular expression. This is done as a second pass, after variables are
1250 interpolated, so that regular expressions may be incorporated into the
1251 pattern from the variables. If this is not what you want, use C<\Q> to
1252 interpolate a variable literally.
1254 Apart from the behavior described above, Perl does not expand
1255 multiple levels of interpolation. In particular, contrary to the
1256 expectations of shell programmers, back-quotes do I<NOT> interpolate
1257 within double quotes, nor do single quotes impede evaluation of
1258 variables when used within double quotes.
1260 =head2 Regexp Quote-Like Operators
1263 Here are the quote-like operators that apply to pattern
1264 matching and related activities.
1268 =item qr/STRING/msixpo
1269 X<qr> X</i> X</m> X</o> X</s> X</x> X</p>
1271 This operator quotes (and possibly compiles) its I<STRING> as a regular
1272 expression. I<STRING> is interpolated the same way as I<PATTERN>
1273 in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
1274 is done. Returns a Perl value which may be used instead of the
1275 corresponding C</STRING/msixpo> expression. The returned value is a
1276 normalized version of the original pattern. It magically differs from
1277 a string containing the same characters: C<ref(qr/x/)> returns "Regexp",
1278 even though dereferencing the result returns undef.
1282 $rex = qr/my.STRING/is;
1283 print $rex; # prints (?si-xm:my.STRING)
1290 The result may be used as a subpattern in a match:
1293 $string =~ /foo${re}bar/; # can be interpolated in other patterns
1294 $string =~ $re; # or used standalone
1295 $string =~ /$re/; # or this way
1297 Since Perl may compile the pattern at the moment of execution of qr()
1298 operator, using qr() may have speed advantages in some situations,
1299 notably if the result of qr() is used standalone:
1302 my $patterns = shift;
1303 my @compiled = map qr/$_/i, @$patterns;
1306 foreach my $pat (@compiled) {
1307 $success = 1, last if /$pat/;
1313 Precompilation of the pattern into an internal representation at
1314 the moment of qr() avoids a need to recompile the pattern every
1315 time a match C</$pat/> is attempted. (Perl has many other internal
1316 optimizations, but none would be triggered in the above example if
1317 we did not use qr() operator.)
1321 m Treat string as multiple lines.
1322 s Treat string as single line. (Make . match a newline)
1323 i Do case-insensitive pattern matching.
1324 x Use extended regular expressions.
1325 p When matching preserve a copy of the matched string so
1326 that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be defined.
1327 o Compile pattern only once.
1329 If a precompiled pattern is embedded in a larger pattern then the effect
1330 of 'msixp' will be propagated appropriately. The effect of the 'o'
1331 modifier has is not propagated, being restricted to those patterns
1332 explicitly using it.
1334 See L<perlre> for additional information on valid syntax for STRING, and
1335 for a detailed look at the semantics of regular expressions.
1337 =item m/PATTERN/msixpogc
1338 X<m> X<operator, match>
1339 X<regexp, options> X<regexp> X<regex, options> X<regex>
1340 X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c>
1342 =item /PATTERN/msixpogc
1344 Searches a string for a pattern match, and in scalar context returns
1345 true if it succeeds, false if it fails. If no string is specified
1346 via the C<=~> or C<!~> operator, the $_ string is searched. (The
1347 string specified with C<=~> need not be an lvalue--it may be the
1348 result of an expression evaluation, but remember the C<=~> binds
1349 rather tightly.) See also L<perlre>. See L<perllocale> for
1350 discussion of additional considerations that apply when C<use locale>
1353 Options are as described in C<qr//>; in addition, the following match
1354 process modifiers are available:
1356 g Match globally, i.e., find all occurrences.
1357 c Do not reset search position on a failed match when /g is in effect.
1359 If "/" is the delimiter then the initial C<m> is optional. With the C<m>
1360 you can use any pair of non-whitespace characters
1361 as delimiters. This is particularly useful for matching path names
1362 that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
1363 the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
1364 If "'" is the delimiter, no interpolation is performed on the PATTERN.
1365 When using a character valid in an identifier, whitespace is required
1368 PATTERN may contain variables, which will be interpolated (and the
1369 pattern recompiled) every time the pattern search is evaluated, except
1370 for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
1371 C<$|> are not interpolated because they look like end-of-string tests.)
1372 If you want such a pattern to be compiled only once, add a C</o> after
1373 the trailing delimiter. This avoids expensive run-time recompilations,
1374 and is useful when the value you are interpolating won't change over
1375 the life of the script. However, mentioning C</o> constitutes a promise
1376 that you won't change the variables in the pattern. If you change them,
1377 Perl won't even notice. See also L<"qr/STRING/msixpo">.
1379 =item The empty pattern //
1381 If the PATTERN evaluates to the empty string, the last
1382 I<successfully> matched regular expression is used instead. In this
1383 case, only the C<g> and C<c> flags on the empty pattern is honoured -
1384 the other flags are taken from the original pattern. If no match has
1385 previously succeeded, this will (silently) act instead as a genuine
1386 empty pattern (which will always match).
1388 Note that it's possible to confuse Perl into thinking C<//> (the empty
1389 regex) is really C<//> (the defined-or operator). Perl is usually pretty
1390 good about this, but some pathological cases might trigger this, such as
1391 C<$a///> (is that C<($a) / (//)> or C<$a // />?) and C<print $fh //>
1392 (C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl
1393 will assume you meant defined-or. If you meant the empty regex, just
1394 use parentheses or spaces to disambiguate, or even prefix the empty
1395 regex with an C<m> (so C<//> becomes C<m//>).
1397 =item Matching in list context
1399 If the C</g> option is not used, C<m//> in list context returns a
1400 list consisting of the subexpressions matched by the parentheses in the
1401 pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are
1402 also set, and that this differs from Perl 4's behavior.) When there are
1403 no parentheses in the pattern, the return value is the list C<(1)> for
1404 success. With or without parentheses, an empty list is returned upon
1409 open(TTY, '/dev/tty');
1410 <TTY> =~ /^y/i && foo(); # do foo if desired
1412 if (/Version: *([0-9.]*)/) { $version = $1; }
1414 next if m#^/usr/spool/uucp#;
1419 print if /$arg/o; # compile only once
1422 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
1424 This last example splits $foo into the first two words and the
1425 remainder of the line, and assigns those three fields to $F1, $F2, and
1426 $Etc. The conditional is true if any variables were assigned, i.e., if
1427 the pattern matched.
1429 The C</g> modifier specifies global pattern matching--that is,
1430 matching as many times as possible within the string. How it behaves
1431 depends on the context. In list context, it returns a list of the
1432 substrings matched by any capturing parentheses in the regular
1433 expression. If there are no parentheses, it returns a list of all
1434 the matched strings, as if there were parentheses around the whole
1437 In scalar context, each execution of C<m//g> finds the next match,
1438 returning true if it matches, and false if there is no further match.
1439 The position after the last match can be read or set using the C<pos()>
1440 function; see L<perlfunc/pos>. A failed match normally resets the
1441 search position to the beginning of the string, but you can avoid that
1442 by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
1443 string also resets the search position.
1447 You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
1448 zero-width assertion that matches the exact position where the
1449 previous C<m//g>, if any, left off. Without the C</g> modifier, the
1450 C<\G> assertion still anchors at C<pos()> as it was at the start of
1451 the operation (see L<perlfunc/pos>), but the match is of course only
1452 attempted once. Using C<\G> without C</g> on a target string that has
1453 not previously had a C</g> match applied to it is the same as using
1454 the C<\A> assertion to match the beginning of the string. Note also
1455 that, currently, C<\G> is only properly supported when anchored at the
1456 very beginning of the pattern.
1461 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
1465 while (defined($paragraph = <>)) {
1466 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
1470 print "$sentences\n";
1472 # using m//gc with \G
1476 print $1 while /(o)/gc; print "', pos=", pos, "\n";
1478 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
1480 print $1 while /(p)/gc; print "', pos=", pos, "\n";
1482 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
1484 The last example should print:
1494 Notice that the final match matched C<q> instead of C<p>, which a match
1495 without the C<\G> anchor would have done. Also note that the final match
1496 did not update C<pos>. C<pos> is only updated on a C</g> match. If the
1497 final match did indeed match C<p>, it's a good bet that you're running an
1498 older (pre-5.6.0) Perl.
1500 A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
1501 combine several regexps like this to process a string part-by-part,
1502 doing different actions depending on which regexp matched. Each
1503 regexp tries to match where the previous one leaves off.
1506 $url = URI::URL->new( "http://example.com/" ); die if $url eq "xXx";
1510 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
1511 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
1512 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
1513 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
1514 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
1515 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
1516 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
1517 print ". That's all!\n";
1520 Here is the output (split into several lines):
1522 line-noise lowercase line-noise UPPERCASE line-noise UPPERCASE
1523 line-noise lowercase line-noise lowercase line-noise lowercase
1524 lowercase line-noise lowercase lowercase line-noise lowercase
1525 lowercase line-noise MiXeD line-noise. That's all!
1530 This is just like the C</pattern/> search, except that it matches only
1531 once between calls to the reset() operator. This is a useful
1532 optimization when you want to see only the first occurrence of
1533 something in each file of a set of files, for instance. Only C<??>
1534 patterns local to the current package are reset.
1538 # blank line between header and body
1541 reset if eof; # clear ?? status for next file
1544 This usage is vaguely deprecated, which means it just might possibly
1545 be removed in some distant future version of Perl, perhaps somewhere
1546 around the year 2168.
1548 =item s/PATTERN/REPLACEMENT/msixpogcer
1549 X<substitute> X<substitution> X<replace> X<regexp, replace>
1550 X<regexp, substitute> X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> X</e> X</r>
1552 Searches a string for a pattern, and if found, replaces that pattern
1553 with the replacement text and returns the number of substitutions
1554 made. Otherwise it returns false (specifically, the empty string).
1556 If the C</r> (non-destructive) option is used then it will perform the
1557 substitution on a copy of the string and return the copy whether or not a
1558 substitution occurred. The original string will always remain unchanged in
1559 this case. The copy will always be a plain string, even if the input is an
1560 object or a tied variable.
1562 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
1563 variable is searched and modified. (The string specified with C<=~> must
1564 be scalar variable, an array element, a hash element, or an assignment
1565 to one of those, i.e., an lvalue.)
1567 If the delimiter chosen is a single quote, no interpolation is
1568 done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1569 PATTERN contains a $ that looks like a variable rather than an
1570 end-of-string test, the variable will be interpolated into the pattern
1571 at run-time. If you want the pattern compiled only once the first time
1572 the variable is interpolated, use the C</o> option. If the pattern
1573 evaluates to the empty string, the last successfully executed regular
1574 expression is used instead. See L<perlre> for further explanation on these.
1575 See L<perllocale> for discussion of additional considerations that apply
1576 when C<use locale> is in effect.
1578 Options are as with m// with the addition of the following replacement
1581 e Evaluate the right side as an expression.
1582 ee Evaluate the right side as a string then eval the result.
1583 r Return substitution and leave the original string untouched.
1585 Any non-whitespace delimiter may replace the slashes. Add space after
1586 the C<s> when using a character allowed in identifiers. If single quotes
1587 are used, no interpretation is done on the replacement string (the C</e>
1588 modifier overrides this, however). Unlike Perl 4, Perl 5 treats backticks
1589 as normal delimiters; the replacement text is not evaluated as a command.
1590 If the PATTERN is delimited by bracketing quotes, the REPLACEMENT has
1591 its own pair of quotes, which may or may not be bracketing quotes, e.g.,
1592 C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
1593 replacement portion to be treated as a full-fledged Perl expression
1594 and evaluated right then and there. It is, however, syntax checked at
1595 compile-time. A second C<e> modifier will cause the replacement portion
1596 to be C<eval>ed before being run as a Perl expression.
1600 s/\bgreen\b/mauve/g; # don't change wintergreen
1602 $path =~ s|/usr/bin|/usr/local/bin|;
1604 s/Login: $foo/Login: $bar/; # run-time pattern
1606 ($foo = $bar) =~ s/this/that/; # copy first, then change
1607 ($foo = "$bar") =~ s/this/that/; # convert to string, copy, then change
1608 $foo = $bar =~ s/this/that/r; # Same as above using /r
1609 $foo = $bar =~ s/this/that/r
1610 =~ s/that/the other/r; # Chained substitutes using /r
1611 @foo = map { s/this/that/r } @bar # /r is very useful in maps
1613 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
1616 s/\d+/$&*2/e; # yields 'abc246xyz'
1617 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
1618 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1620 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
1621 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
1622 s/^=(\w+)/pod($1)/ge; # use function call
1625 $a = s/abc/def/r; # $a is 'def123xyz' and
1626 # $_ remains 'abc123xyz'.
1628 # expand variables in $_, but dynamics only, using
1629 # symbolic dereferencing
1632 # Add one to the value of any numbers in the string
1635 # This will expand any embedded scalar variable
1636 # (including lexicals) in $_ : First $1 is interpolated
1637 # to the variable name, and then evaluated
1640 # Delete (most) C comments.
1642 /\* # Match the opening delimiter.
1643 .*? # Match a minimal number of characters.
1644 \*/ # Match the closing delimiter.
1647 s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_, expensively
1649 for ($variable) { # trim whitespace in $variable, cheap
1654 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
1656 Note the use of $ instead of \ in the last example. Unlike
1657 B<sed>, we use the \<I<digit>> form in only the left hand side.
1658 Anywhere else it's $<I<digit>>.
1660 Occasionally, you can't use just a C</g> to get all the changes
1661 to occur that you might want. Here are two common cases:
1663 # put commas in the right places in an integer
1664 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
1666 # expand tabs to 8-column spacing
1667 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
1671 =head2 Quote-Like Operators
1672 X<operator, quote-like>
1677 X<q> X<quote, single> X<'> X<''>
1681 A single-quoted, literal string. A backslash represents a backslash
1682 unless followed by the delimiter or another backslash, in which case
1683 the delimiter or backslash is interpolated.
1685 $foo = q!I said, "You said, 'She said it.'"!;
1686 $bar = q('This is it.');
1687 $baz = '\n'; # a two-character string
1690 X<qq> X<quote, double> X<"> X<"">
1694 A double-quoted, interpolated string.
1697 (*** The previous line contains the naughty word "$1".\n)
1698 if /\b(tcl|java|python)\b/i; # :-)
1699 $baz = "\n"; # a one-character string
1702 X<qx> X<`> X<``> X<backtick>
1706 A string which is (possibly) interpolated and then executed as a
1707 system command with C</bin/sh> or its equivalent. Shell wildcards,
1708 pipes, and redirections will be honored. The collected standard
1709 output of the command is returned; standard error is unaffected. In
1710 scalar context, it comes back as a single (potentially multi-line)
1711 string, or undef if the command failed. In list context, returns a
1712 list of lines (however you've defined lines with $/ or
1713 $INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
1715 Because backticks do not affect standard error, use shell file descriptor
1716 syntax (assuming the shell supports this) if you care to address this.
1717 To capture a command's STDERR and STDOUT together:
1719 $output = `cmd 2>&1`;
1721 To capture a command's STDOUT but discard its STDERR:
1723 $output = `cmd 2>/dev/null`;
1725 To capture a command's STDERR but discard its STDOUT (ordering is
1728 $output = `cmd 2>&1 1>/dev/null`;
1730 To exchange a command's STDOUT and STDERR in order to capture the STDERR
1731 but leave its STDOUT to come out the old STDERR:
1733 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
1735 To read both a command's STDOUT and its STDERR separately, it's easiest
1736 to redirect them separately to files, and then read from those files
1737 when the program is done:
1739 system("program args 1>program.stdout 2>program.stderr");
1741 The STDIN filehandle used by the command is inherited from Perl's STDIN.
1744 open BLAM, "blam" || die "Can't open: $!";
1745 open STDIN, "<&BLAM";
1748 will print the sorted contents of the file "blam".
1750 Using single-quote as a delimiter protects the command from Perl's
1751 double-quote interpolation, passing it on to the shell instead:
1753 $perl_info = qx(ps $$); # that's Perl's $$
1754 $shell_info = qx'ps $$'; # that's the new shell's $$
1756 How that string gets evaluated is entirely subject to the command
1757 interpreter on your system. On most platforms, you will have to protect
1758 shell metacharacters if you want them treated literally. This is in
1759 practice difficult to do, as it's unclear how to escape which characters.
1760 See L<perlsec> for a clean and safe example of a manual fork() and exec()
1761 to emulate backticks safely.
1763 On some platforms (notably DOS-like ones), the shell may not be
1764 capable of dealing with multiline commands, so putting newlines in
1765 the string may not get you what you want. You may be able to evaluate
1766 multiple commands in a single line by separating them with the command
1767 separator character, if your shell supports that (e.g. C<;> on many Unix
1768 shells; C<&> on the Windows NT C<cmd> shell).
1770 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1771 output before starting the child process, but this may not be supported
1772 on some platforms (see L<perlport>). To be safe, you may need to set
1773 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1774 C<IO::Handle> on any open handles.
1776 Beware that some command shells may place restrictions on the length
1777 of the command line. You must ensure your strings don't exceed this
1778 limit after any necessary interpolations. See the platform-specific
1779 release notes for more details about your particular environment.
1781 Using this operator can lead to programs that are difficult to port,
1782 because the shell commands called vary between systems, and may in
1783 fact not be present at all. As one example, the C<type> command under
1784 the POSIX shell is very different from the C<type> command under DOS.
1785 That doesn't mean you should go out of your way to avoid backticks
1786 when they're the right way to get something done. Perl was made to be
1787 a glue language, and one of the things it glues together is commands.
1788 Just understand what you're getting yourself into.
1790 See L</"I/O Operators"> for more discussion.
1793 X<qw> X<quote, list> X<quote, words>
1795 Evaluates to a list of the words extracted out of STRING, using embedded
1796 whitespace as the word delimiters. It can be understood as being roughly
1799 split(' ', q/STRING/);
1801 the differences being that it generates a real list at compile time, and
1802 in scalar context it returns the last element in the list. So
1807 is semantically equivalent to the list:
1811 Some frequently seen examples:
1813 use POSIX qw( setlocale localeconv )
1814 @EXPORT = qw( foo bar baz );
1816 A common mistake is to try to separate the words with comma or to
1817 put comments into a multi-line C<qw>-string. For this reason, the
1818 C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
1819 produces warnings if the STRING contains the "," or the "#" character.
1822 =item tr/SEARCHLIST/REPLACEMENTLIST/cdsr
1823 X<tr> X<y> X<transliterate> X</c> X</d> X</s>
1825 =item y/SEARCHLIST/REPLACEMENTLIST/cdsr
1827 Transliterates all occurrences of the characters found in the search list
1828 with the corresponding character in the replacement list. It returns
1829 the number of characters replaced or deleted. If no string is
1830 specified via the =~ or !~ operator, the $_ string is transliterated. (The
1831 string specified with =~ must be a scalar variable, an array element, a
1832 hash element, or an assignment to one of those, i.e., an lvalue.)
1834 If the C</r> (non-destructive) option is used then it will perform the
1835 replacement on a copy of the string and return the copy whether or not it
1836 was modified. The original string will always remain unchanged in
1837 this case. The copy will always be a plain string, even if the input is an
1838 object or a tied variable.
1840 A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
1841 does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
1842 For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1843 SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1844 its own pair of quotes, which may or may not be bracketing quotes,
1845 e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
1847 Note that C<tr> does B<not> do regular expression character classes
1848 such as C<\d> or C<[:lower:]>. The C<tr> operator is not equivalent to
1849 the tr(1) utility. If you want to map strings between lower/upper
1850 cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider
1851 using the C<s> operator if you need regular expressions.
1853 Note also that the whole range idea is rather unportable between
1854 character sets--and even within character sets they may cause results
1855 you probably didn't expect. A sound principle is to use only ranges
1856 that begin from and end at either alphabets of equal case (a-e, A-E),
1857 or digits (0-4). Anything else is unsafe. If in doubt, spell out the
1858 character sets in full.
1862 c Complement the SEARCHLIST.
1863 d Delete found but unreplaced characters.
1864 s Squash duplicate replaced characters.
1865 r Return the modified string and leave the original string
1868 If the C</c> modifier is specified, the SEARCHLIST character set
1869 is complemented. If the C</d> modifier is specified, any characters
1870 specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
1871 (Note that this is slightly more flexible than the behavior of some
1872 B<tr> programs, which delete anything they find in the SEARCHLIST,
1873 period.) If the C</s> modifier is specified, sequences of characters
1874 that were transliterated to the same character are squashed down
1875 to a single instance of the character.
1877 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1878 exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1879 than the SEARCHLIST, the final character is replicated till it is long
1880 enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
1881 This latter is useful for counting characters in a class or for
1882 squashing character sequences in a class.
1886 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1888 $cnt = tr/*/*/; # count the stars in $_
1890 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1892 $cnt = tr/0-9//; # count the digits in $_
1894 tr/a-zA-Z//s; # bookkeeper -> bokeper
1896 ($HOST = $host) =~ tr/a-z/A-Z/;
1897 $HOST = $host =~ tr/a-z/A-Z/r; # same thing
1899 $HOST = $host =~ tr/a-z/A-Z/r # chained with s///
1902 tr/a-zA-Z/ /cs; # change non-alphas to single space
1904 @stripped = map tr/a-zA-Z/ /csr, @original;
1908 [\000-\177]; # delete 8th bit
1910 If multiple transliterations are given for a character, only the
1915 will transliterate any A to X.
1917 Because the transliteration table is built at compile time, neither
1918 the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
1919 interpolation. That means that if you want to use variables, you
1922 eval "tr/$oldlist/$newlist/";
1925 eval "tr/$oldlist/$newlist/, 1" or die $@;
1928 X<here-doc> X<heredoc> X<here-document> X<<< << >>>
1930 A line-oriented form of quoting is based on the shell "here-document"
1931 syntax. Following a C<< << >> you specify a string to terminate
1932 the quoted material, and all lines following the current line down to
1933 the terminating string are the value of the item.
1935 The terminating string may be either an identifier (a word), or some
1936 quoted text. An unquoted identifier works like double quotes.
1937 There may not be a space between the C<< << >> and the identifier,
1938 unless the identifier is explicitly quoted. (If you put a space it
1939 will be treated as a null identifier, which is valid, and matches the
1940 first empty line.) The terminating string must appear by itself
1941 (unquoted and with no surrounding whitespace) on the terminating line.
1943 If the terminating string is quoted, the type of quotes used determine
1944 the treatment of the text.
1950 Double quotes indicate that the text will be interpolated using exactly
1951 the same rules as normal double quoted strings.
1954 The price is $Price.
1957 print << "EOF"; # same as above
1958 The price is $Price.
1964 Single quotes indicate the text is to be treated literally with no
1965 interpolation of its content. This is similar to single quoted
1966 strings except that backslashes have no special meaning, with C<\\>
1967 being treated as two backslashes and not one as they would in every
1968 other quoting construct.
1970 This is the only form of quoting in perl where there is no need
1971 to worry about escaping content, something that code generators
1972 can and do make good use of.
1976 The content of the here doc is treated just as it would be if the
1977 string were embedded in backticks. Thus the content is interpolated
1978 as though it were double quoted and then executed via the shell, with
1979 the results of the execution returned.
1981 print << `EOC`; # execute command and get results
1987 It is possible to stack multiple here-docs in a row:
1989 print <<"foo", <<"bar"; # you can stack them
1995 myfunc(<< "THIS", 23, <<'THAT');
2002 Just don't forget that you have to put a semicolon on the end
2003 to finish the statement, as Perl doesn't know you're not going to
2011 If you want to remove the line terminator from your here-docs,
2014 chomp($string = <<'END');
2018 If you want your here-docs to be indented with the rest of the code,
2019 you'll need to remove leading whitespace from each line manually:
2021 ($quote = <<'FINIS') =~ s/^\s+//gm;
2022 The Road goes ever on and on,
2023 down from the door where it began.
2026 If you use a here-doc within a delimited construct, such as in C<s///eg>,
2027 the quoted material must come on the lines following the final delimiter.
2042 If the terminating identifier is on the last line of the program, you
2043 must be sure there is a newline after it; otherwise, Perl will give the
2044 warning B<Can't find string terminator "END" anywhere before EOF...>.
2046 Additionally, the quoting rules for the end of string identifier are not
2047 related to Perl's quoting rules. C<q()>, C<qq()>, and the like are not
2048 supported in place of C<''> and C<"">, and the only interpolation is for
2049 backslashing the quoting character:
2051 print << "abc\"def";
2055 Finally, quoted strings cannot span multiple lines. The general rule is
2056 that the identifier must be a string literal. Stick with that, and you
2061 =head2 Gory details of parsing quoted constructs
2062 X<quote, gory details>
2064 When presented with something that might have several different
2065 interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
2066 principle to pick the most probable interpretation. This strategy
2067 is so successful that Perl programmers often do not suspect the
2068 ambivalence of what they write. But from time to time, Perl's
2069 notions differ substantially from what the author honestly meant.
2071 This section hopes to clarify how Perl handles quoted constructs.
2072 Although the most common reason to learn this is to unravel labyrinthine
2073 regular expressions, because the initial steps of parsing are the
2074 same for all quoting operators, they are all discussed together.
2076 The most important Perl parsing rule is the first one discussed
2077 below: when processing a quoted construct, Perl first finds the end
2078 of that construct, then interprets its contents. If you understand
2079 this rule, you may skip the rest of this section on the first
2080 reading. The other rules are likely to contradict the user's
2081 expectations much less frequently than this first one.
2083 Some passes discussed below are performed concurrently, but because
2084 their results are the same, we consider them individually. For different
2085 quoting constructs, Perl performs different numbers of passes, from
2086 one to four, but these passes are always performed in the same order.
2090 =item Finding the end
2092 The first pass is finding the end of the quoted construct, where
2093 the information about the delimiters is used in parsing.
2094 During this search, text between the starting and ending delimiters
2095 is copied to a safe location. The text copied gets delimiter-independent.
2097 If the construct is a here-doc, the ending delimiter is a line
2098 that has a terminating string as the content. Therefore C<<<EOF> is
2099 terminated by C<EOF> immediately followed by C<"\n"> and starting
2100 from the first column of the terminating line.
2101 When searching for the terminating line of a here-doc, nothing
2102 is skipped. In other words, lines after the here-doc syntax
2103 are compared with the terminating string line by line.
2105 For the constructs except here-docs, single characters are used as starting
2106 and ending delimiters. If the starting delimiter is an opening punctuation
2107 (that is C<(>, C<[>, C<{>, or C<< < >>), the ending delimiter is the
2108 corresponding closing punctuation (that is C<)>, C<]>, C<}>, or C<< > >>).
2109 If the starting delimiter is an unpaired character like C</> or a closing
2110 punctuation, the ending delimiter is same as the starting delimiter.
2111 Therefore a C</> terminates a C<qq//> construct, while a C<]> terminates
2112 C<qq[]> and C<qq]]> constructs.
2114 When searching for single-character delimiters, escaped delimiters
2115 and C<\\> are skipped. For example, while searching for terminating C</>,
2116 combinations of C<\\> and C<\/> are skipped. If the delimiters are
2117 bracketing, nested pairs are also skipped. For example, while searching
2118 for closing C<]> paired with the opening C<[>, combinations of C<\\>, C<\]>,
2119 and C<\[> are all skipped, and nested C<[> and C<]> are skipped as well.
2120 However, when backslashes are used as the delimiters (like C<qq\\> and
2121 C<tr\\\>), nothing is skipped.
2122 During the search for the end, backslashes that escape delimiters
2123 are removed (exactly speaking, they are not copied to the safe location).
2125 For constructs with three-part delimiters (C<s///>, C<y///>, and
2126 C<tr///>), the search is repeated once more.
2127 If the first delimiter is not an opening punctuation, three delimiters must
2128 be same such as C<s!!!> and C<tr)))>, in which case the second delimiter
2129 terminates the left part and starts the right part at once.
2130 If the left part is delimited by bracketing punctuations (that is C<()>,
2131 C<[]>, C<{}>, or C<< <> >>), the right part needs another pair of
2132 delimiters such as C<s(){}> and C<tr[]//>. In these cases, whitespaces
2133 and comments are allowed between both parts, though the comment must follow
2134 at least one whitespace; otherwise a character expected as the start of
2135 the comment may be regarded as the starting delimiter of the right part.
2137 During this search no attention is paid to the semantics of the construct.
2140 "$hash{"$foo/$bar"}"
2145 bar # NOT a comment, this slash / terminated m//!
2148 do not form legal quoted expressions. The quoted part ends on the
2149 first C<"> and C</>, and the rest happens to be a syntax error.
2150 Because the slash that terminated C<m//> was followed by a C<SPACE>,
2151 the example above is not C<m//x>, but rather C<m//> with no C</x>
2152 modifier. So the embedded C<#> is interpreted as a literal C<#>.
2154 Also no attention is paid to C<\c\> (multichar control char syntax) during
2155 this search. Thus the second C<\> in C<qq/\c\/> is interpreted as a part
2156 of C<\/>, and the following C</> is not recognized as a delimiter.
2157 Instead, use C<\034> or C<\x1c> at the end of quoted constructs.
2162 The next step is interpolation in the text obtained, which is now
2163 delimiter-independent. There are multiple cases.
2169 No interpolation is performed.
2170 Note that the combination C<\\> is left intact, since escaped delimiters
2171 are not available for here-docs.
2173 =item C<m''>, the pattern of C<s'''>
2175 No interpolation is performed at this stage.
2176 Any backslashed sequences including C<\\> are treated at the stage
2177 to L</"parsing regular expressions">.
2179 =item C<''>, C<q//>, C<tr'''>, C<y'''>, the replacement of C<s'''>
2181 The only interpolation is removal of C<\> from pairs of C<\\>.
2182 Therefore C<-> in C<tr'''> and C<y'''> is treated literally
2183 as a hyphen and no character range is available.
2184 C<\1> in the replacement of C<s'''> does not work as C<$1>.
2186 =item C<tr///>, C<y///>
2188 No variable interpolation occurs. String modifying combinations for
2189 case and quoting such as C<\Q>, C<\U>, and C<\E> are not recognized.
2190 The other escape sequences such as C<\200> and C<\t> and backslashed
2191 characters such as C<\\> and C<\-> are converted to appropriate literals.
2192 The character C<-> is treated specially and therefore C<\-> is treated
2195 =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF">
2197 C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
2198 converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
2199 is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
2200 The other escape sequences such as C<\200> and C<\t> and backslashed
2201 characters such as C<\\> and C<\-> are replaced with appropriate
2204 Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
2205 is interpolated in the usual way. Something like C<"\Q\\E"> has
2206 no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
2207 result is the same as for C<"\\\\E">. As a general rule, backslashes
2208 between C<\Q> and C<\E> may lead to counterintuitive results. So,
2209 C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
2210 as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
2215 may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
2217 Interpolated scalars and arrays are converted internally to the C<join> and
2218 C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
2220 $foo . " XXX '" . (join $", @arr) . "'";
2222 All operations above are performed simultaneously, left to right.
2224 Because the result of C<"\Q STRING \E"> has all metacharacters
2225 quoted, there is no way to insert a literal C<$> or C<@> inside a
2226 C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
2227 C<"\\\$">; if not, it is interpreted as the start of an interpolated
2230 Note also that the interpolation code needs to make a decision on
2231 where the interpolated scalar ends. For instance, whether
2232 C<< "a $b -> {c}" >> really means:
2234 "a " . $b . " -> {c}";
2240 Most of the time, the longest possible text that does not include
2241 spaces between components and which contains matching braces or
2242 brackets. because the outcome may be determined by voting based
2243 on heuristic estimators, the result is not strictly predictable.
2244 Fortunately, it's usually correct for ambiguous cases.
2246 =item the replacement of C<s///>
2248 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
2249 happens as with C<qq//> constructs.
2251 It is at this step that C<\1> is begrudgingly converted to C<$1> in
2252 the replacement text of C<s///>, in order to correct the incorrigible
2253 I<sed> hackers who haven't picked up the saner idiom yet. A warning
2254 is emitted if the C<use warnings> pragma or the B<-w> command-line flag
2255 (that is, the C<$^W> variable) was set.
2257 =item C<RE> in C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
2259 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\E>,
2260 and interpolation happens (almost) as with C<qq//> constructs.
2262 Processing of C<\N{...}> is also done here, and compiled into an intermediate
2263 form for the regex compiler. (This is because, as mentioned below, the regex
2264 compilation may be done at execution time, and C<\N{...}> is a compile-time
2267 However any other combinations of C<\> followed by a character
2268 are not substituted but only skipped, in order to parse them
2269 as regular expressions at the following step.
2270 As C<\c> is skipped at this step, C<@> of C<\c@> in RE is possibly
2271 treated as an array symbol (for example C<@foo>),
2272 even though the same text in C<qq//> gives interpolation of C<\c@>.
2274 Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
2275 a C<#>-comment in a C<//x>-regular expression, no processing is
2276 performed whatsoever. This is the first step at which the presence
2277 of the C<//x> modifier is relevant.
2279 Interpolation in patterns has several quirks: C<$|>, C<$(>, C<$)>, C<@+>
2280 and C<@-> are not interpolated, and constructs C<$var[SOMETHING]> are
2281 voted (by several different estimators) to be either an array element
2282 or C<$var> followed by an RE alternative. This is where the notation
2283 C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
2284 array element C<-9>, not as a regular expression from the variable
2285 C<$arr> followed by a digit, which would be the interpretation of
2286 C</$arr[0-9]/>. Since voting among different estimators may occur,
2287 the result is not predictable.
2289 The lack of processing of C<\\> creates specific restrictions on
2290 the post-processed text. If the delimiter is C</>, one cannot get
2291 the combination C<\/> into the result of this step. C</> will
2292 finish the regular expression, C<\/> will be stripped to C</> on
2293 the previous step, and C<\\/> will be left as is. Because C</> is
2294 equivalent to C<\/> inside a regular expression, this does not
2295 matter unless the delimiter happens to be character special to the
2296 RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
2297 alphanumeric char, as in:
2301 In the RE above, which is intentionally obfuscated for illustration, the
2302 delimiter is C<m>, the modifier is C<mx>, and after delimiter-removal the
2303 RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
2304 reason you're encouraged to restrict your delimiters to non-alphanumeric,
2305 non-whitespace choices.
2309 This step is the last one for all constructs except regular expressions,
2310 which are processed further.
2312 =item parsing regular expressions
2315 Previous steps were performed during the compilation of Perl code,
2316 but this one happens at run time, although it may be optimized to
2317 be calculated at compile time if appropriate. After preprocessing
2318 described above, and possibly after evaluation if concatenation,
2319 joining, casing translation, or metaquoting are involved, the
2320 resulting I<string> is passed to the RE engine for compilation.
2322 Whatever happens in the RE engine might be better discussed in L<perlre>,
2323 but for the sake of continuity, we shall do so here.
2325 This is another step where the presence of the C<//x> modifier is
2326 relevant. The RE engine scans the string from left to right and
2327 converts it to a finite automaton.
2329 Backslashed characters are either replaced with corresponding
2330 literal strings (as with C<\{>), or else they generate special nodes
2331 in the finite automaton (as with C<\b>). Characters special to the
2332 RE engine (such as C<|>) generate corresponding nodes or groups of
2333 nodes. C<(?#...)> comments are ignored. All the rest is either
2334 converted to literal strings to match, or else is ignored (as is
2335 whitespace and C<#>-style comments if C<//x> is present).
2337 Parsing of the bracketed character class construct, C<[...]>, is
2338 rather different than the rule used for the rest of the pattern.
2339 The terminator of this construct is found using the same rules as
2340 for finding the terminator of a C<{}>-delimited construct, the only
2341 exception being that C<]> immediately following C<[> is treated as
2342 though preceded by a backslash. Similarly, the terminator of
2343 C<(?{...})> is found using the same rules as for finding the
2344 terminator of a C<{}>-delimited construct.
2346 It is possible to inspect both the string given to RE engine and the
2347 resulting finite automaton. See the arguments C<debug>/C<debugcolor>
2348 in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
2349 switch documented in L<perlrun/"Command Switches">.
2351 =item Optimization of regular expressions
2352 X<regexp, optimization>
2354 This step is listed for completeness only. Since it does not change
2355 semantics, details of this step are not documented and are subject
2356 to change without notice. This step is performed over the finite
2357 automaton that was generated during the previous pass.
2359 It is at this stage that C<split()> silently optimizes C</^/> to
2364 =head2 I/O Operators
2365 X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle>
2368 There are several I/O operators you should know about.
2370 A string enclosed by backticks (grave accents) first undergoes
2371 double-quote interpolation. It is then interpreted as an external
2372 command, and the output of that command is the value of the
2373 backtick string, like in a shell. In scalar context, a single string
2374 consisting of all output is returned. In list context, a list of
2375 values is returned, one per line of output. (You can set C<$/> to use
2376 a different line terminator.) The command is executed each time the
2377 pseudo-literal is evaluated. The status value of the command is
2378 returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
2379 Unlike in B<csh>, no translation is done on the return data--newlines
2380 remain newlines. Unlike in any of the shells, single quotes do not
2381 hide variable names in the command from interpretation. To pass a
2382 literal dollar-sign through to the shell you need to hide it with a
2383 backslash. The generalized form of backticks is C<qx//>. (Because
2384 backticks always undergo shell expansion as well, see L<perlsec> for
2386 X<qx> X<`> X<``> X<backtick> X<glob>
2388 In scalar context, evaluating a filehandle in angle brackets yields
2389 the next line from that file (the newline, if any, included), or
2390 C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
2391 (sometimes known as file-slurp mode) and the file is empty, it
2392 returns C<''> the first time, followed by C<undef> subsequently.
2394 Ordinarily you must assign the returned value to a variable, but
2395 there is one situation where an automatic assignment happens. If
2396 and only if the input symbol is the only thing inside the conditional
2397 of a C<while> statement (even if disguised as a C<for(;;)> loop),
2398 the value is automatically assigned to the global variable $_,
2399 destroying whatever was there previously. (This may seem like an
2400 odd thing to you, but you'll use the construct in almost every Perl
2401 script you write.) The $_ variable is not implicitly localized.
2402 You'll have to put a C<local $_;> before the loop if you want that
2405 The following lines are equivalent:
2407 while (defined($_ = <STDIN>)) { print; }
2408 while ($_ = <STDIN>) { print; }
2409 while (<STDIN>) { print; }
2410 for (;<STDIN>;) { print; }
2411 print while defined($_ = <STDIN>);
2412 print while ($_ = <STDIN>);
2413 print while <STDIN>;
2415 This also behaves similarly, but avoids $_ :
2417 while (my $line = <STDIN>) { print $line }
2419 In these loop constructs, the assigned value (whether assignment
2420 is automatic or explicit) is then tested to see whether it is
2421 defined. The defined test avoids problems where line has a string
2422 value that would be treated as false by Perl, for example a "" or
2423 a "0" with no trailing newline. If you really mean for such values
2424 to terminate the loop, they should be tested for explicitly:
2426 while (($_ = <STDIN>) ne '0') { ... }
2427 while (<STDIN>) { last unless $_; ... }
2429 In other boolean contexts, C<< <filehandle> >> without an
2430 explicit C<defined> test or comparison elicits a warning if the
2431 C<use warnings> pragma or the B<-w>
2432 command-line switch (the C<$^W> variable) is in effect.
2434 The filehandles STDIN, STDOUT, and STDERR are predefined. (The
2435 filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
2436 in packages, where they would be interpreted as local identifiers
2437 rather than global.) Additional filehandles may be created with
2438 the open() function, amongst others. See L<perlopentut> and
2439 L<perlfunc/open> for details on this.
2440 X<stdin> X<stdout> X<sterr>
2442 If a <FILEHANDLE> is used in a context that is looking for
2443 a list, a list comprising all input lines is returned, one line per
2444 list element. It's easy to grow to a rather large data space this
2445 way, so use with care.
2447 <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
2448 See L<perlfunc/readline>.
2450 The null filehandle <> is special: it can be used to emulate the
2451 behavior of B<sed> and B<awk>. Input from <> comes either from
2452 standard input, or from each file listed on the command line. Here's
2453 how it works: the first time <> is evaluated, the @ARGV array is
2454 checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
2455 gives you standard input. The @ARGV array is then processed as a list
2456 of filenames. The loop
2459 ... # code for each line
2462 is equivalent to the following Perl-like pseudo code:
2464 unshift(@ARGV, '-') unless @ARGV;
2465 while ($ARGV = shift) {
2468 ... # code for each line
2472 except that it isn't so cumbersome to say, and will actually work.
2473 It really does shift the @ARGV array and put the current filename
2474 into the $ARGV variable. It also uses filehandle I<ARGV>
2475 internally. <> is just a synonym for <ARGV>, which
2476 is magical. (The pseudo code above doesn't work because it treats
2477 <ARGV> as non-magical.)
2479 Since the null filehandle uses the two argument form of L<perlfunc/open>
2480 it interprets special characters, so if you have a script like this:
2486 and call it with C<perl dangerous.pl 'rm -rfv *|'>, it actually opens a
2487 pipe, executes the C<rm> command and reads C<rm>'s output from that pipe.
2488 If you want all items in C<@ARGV> to be interpreted as file names, you
2489 can use the module C<ARGV::readonly> from CPAN.
2491 You can modify @ARGV before the first <> as long as the array ends up
2492 containing the list of filenames you really want. Line numbers (C<$.>)
2493 continue as though the input were one big happy file. See the example
2494 in L<perlfunc/eof> for how to reset line numbers on each file.
2496 If you want to set @ARGV to your own list of files, go right ahead.
2497 This sets @ARGV to all plain text files if no @ARGV was given:
2499 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
2501 You can even set them to pipe commands. For example, this automatically
2502 filters compressed arguments through B<gzip>:
2504 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
2506 If you want to pass switches into your script, you can use one of the
2507 Getopts modules or put a loop on the front like this:
2509 while ($_ = $ARGV[0], /^-/) {
2512 if (/^-D(.*)/) { $debug = $1 }
2513 if (/^-v/) { $verbose++ }
2514 # ... # other switches
2518 # ... # code for each line
2521 The <> symbol will return C<undef> for end-of-file only once.
2522 If you call it again after this, it will assume you are processing another
2523 @ARGV list, and if you haven't set @ARGV, will read input from STDIN.
2525 If what the angle brackets contain is a simple scalar variable (e.g.,
2526 <$foo>), then that variable contains the name of the
2527 filehandle to input from, or its typeglob, or a reference to the
2533 If what's within the angle brackets is neither a filehandle nor a simple
2534 scalar variable containing a filehandle name, typeglob, or typeglob
2535 reference, it is interpreted as a filename pattern to be globbed, and
2536 either a list of filenames or the next filename in the list is returned,
2537 depending on context. This distinction is determined on syntactic
2538 grounds alone. That means C<< <$x> >> is always a readline() from
2539 an indirect handle, but C<< <$hash{key}> >> is always a glob().
2540 That's because $x is a simple scalar variable, but C<$hash{key}> is
2541 not--it's a hash element. Even C<< <$x > >> (note the extra space)
2542 is treated as C<glob("$x ")>, not C<readline($x)>.
2544 One level of double-quote interpretation is done first, but you can't
2545 say C<< <$foo> >> because that's an indirect filehandle as explained
2546 in the previous paragraph. (In older versions of Perl, programmers
2547 would insert curly brackets to force interpretation as a filename glob:
2548 C<< <${foo}> >>. These days, it's considered cleaner to call the
2549 internal function directly as C<glob($foo)>, which is probably the right
2550 way to have done it in the first place.) For example:
2556 is roughly equivalent to:
2558 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
2564 except that the globbing is actually done internally using the standard
2565 C<File::Glob> extension. Of course, the shortest way to do the above is:
2569 A (file)glob evaluates its (embedded) argument only when it is
2570 starting a new list. All values must be read before it will start
2571 over. In list context, this isn't important because you automatically
2572 get them all anyway. However, in scalar context the operator returns
2573 the next value each time it's called, or C<undef> when the list has
2574 run out. As with filehandle reads, an automatic C<defined> is
2575 generated when the glob occurs in the test part of a C<while>,
2576 because legal glob returns (e.g. a file called F<0>) would otherwise
2577 terminate the loop. Again, C<undef> is returned only once. So if
2578 you're expecting a single value from a glob, it is much better to
2581 ($file) = <blurch*>;
2587 because the latter will alternate between returning a filename and
2590 If you're trying to do variable interpolation, it's definitely better
2591 to use the glob() function, because the older notation can cause people
2592 to become confused with the indirect filehandle notation.
2594 @files = glob("$dir/*.[ch]");
2595 @files = glob($files[$i]);
2597 =head2 Constant Folding
2598 X<constant folding> X<folding>
2600 Like C, Perl does a certain amount of expression evaluation at
2601 compile time whenever it determines that all arguments to an
2602 operator are static and have no side effects. In particular, string
2603 concatenation happens at compile time between literals that don't do
2604 variable substitution. Backslash interpolation also happens at
2605 compile time. You can say
2607 'Now is the time for all' . "\n" .
2608 'good men to come to.'
2610 and this all reduces to one string internally. Likewise, if
2613 foreach $file (@filenames) {
2614 if (-s $file > 5 + 100 * 2**16) { }
2617 the compiler will precompute the number which that expression
2618 represents so that the interpreter won't have to.
2623 Perl doesn't officially have a no-op operator, but the bare constants
2624 C<0> and C<1> are special-cased to not produce a warning in a void
2625 context, so you can for example safely do
2629 =head2 Bitwise String Operators
2630 X<operator, bitwise, string>
2632 Bitstrings of any size may be manipulated by the bitwise operators
2635 If the operands to a binary bitwise op are strings of different
2636 sizes, B<|> and B<^> ops act as though the shorter operand had
2637 additional zero bits on the right, while the B<&> op acts as though
2638 the longer operand were truncated to the length of the shorter.
2639 The granularity for such extension or truncation is one or more
2642 # ASCII-based examples
2643 print "j p \n" ^ " a h"; # prints "JAPH\n"
2644 print "JA" | " ph\n"; # prints "japh\n"
2645 print "japh\nJunk" & '_____'; # prints "JAPH\n";
2646 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
2648 If you are intending to manipulate bitstrings, be certain that
2649 you're supplying bitstrings: If an operand is a number, that will imply
2650 a B<numeric> bitwise operation. You may explicitly show which type of
2651 operation you intend by using C<""> or C<0+>, as in the examples below.
2653 $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
2654 $foo = '150' | 105; # yields 255
2655 $foo = 150 | '105'; # yields 255
2656 $foo = '150' | '105'; # yields string '155' (under ASCII)
2658 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
2659 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
2661 See L<perlfunc/vec> for information on how to manipulate individual bits
2664 =head2 Integer Arithmetic
2667 By default, Perl assumes that it must do most of its arithmetic in
2668 floating point. But by saying
2672 you may tell the compiler to use integer operations
2673 (see L<integer> for a detailed explanation) from here to the end of
2674 the enclosing BLOCK. An inner BLOCK may countermand this by saying
2678 which lasts until the end of that BLOCK. Note that this doesn't
2679 mean everything is an integer, merely that Perl will use integer
2680 operations for arithmetic, comparison, and bitwise operators. For
2681 example, even under C<use integer>, if you take the C<sqrt(2)>, you'll
2682 still get C<1.4142135623731> or so.
2684 Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
2685 and ">>") always produce integral results. (But see also
2686 L<Bitwise String Operators>.) However, C<use integer> still has meaning for
2687 them. By default, their results are interpreted as unsigned integers, but
2688 if C<use integer> is in effect, their results are interpreted
2689 as signed integers. For example, C<~0> usually evaluates to a large
2690 integral value. However, C<use integer; ~0> is C<-1> on two's-complement
2693 =head2 Floating-point Arithmetic
2694 X<floating-point> X<floating point> X<float> X<real>
2696 While C<use integer> provides integer-only arithmetic, there is no
2697 analogous mechanism to provide automatic rounding or truncation to a
2698 certain number of decimal places. For rounding to a certain number
2699 of digits, sprintf() or printf() is usually the easiest route.
2702 Floating-point numbers are only approximations to what a mathematician
2703 would call real numbers. There are infinitely more reals than floats,
2704 so some corners must be cut. For example:
2706 printf "%.20g\n", 123456789123456789;
2707 # produces 123456789123456784
2709 Testing for exact floating-point equality or inequality is not a
2710 good idea. Here's a (relatively expensive) work-around to compare
2711 whether two floating-point numbers are equal to a particular number of
2712 decimal places. See Knuth, volume II, for a more robust treatment of
2716 my ($X, $Y, $POINTS) = @_;
2718 $tX = sprintf("%.${POINTS}g", $X);
2719 $tY = sprintf("%.${POINTS}g", $Y);
2723 The POSIX module (part of the standard perl distribution) implements
2724 ceil(), floor(), and other mathematical and trigonometric functions.
2725 The Math::Complex module (part of the standard perl distribution)
2726 defines mathematical functions that work on both the reals and the
2727 imaginary numbers. Math::Complex not as efficient as POSIX, but
2728 POSIX can't work with complex numbers.
2730 Rounding in financial applications can have serious implications, and
2731 the rounding method used should be specified precisely. In these
2732 cases, it probably pays not to trust whichever system rounding is
2733 being used by Perl, but to instead implement the rounding function you
2736 =head2 Bigger Numbers
2737 X<number, arbitrary precision>
2739 The standard Math::BigInt and Math::BigFloat modules provide
2740 variable-precision arithmetic and overloaded operators, although
2741 they're currently pretty slow. At the cost of some space and
2742 considerable speed, they avoid the normal pitfalls associated with
2743 limited-precision representations.
2746 $x = Math::BigInt->new('123456789123456789');
2749 # prints +15241578780673678515622620750190521
2751 There are several modules that let you calculate with (bound only by
2752 memory and cpu-time) unlimited or fixed precision. There are also
2753 some non-standard modules that provide faster implementations via
2754 external C libraries.
2756 Here is a short, but incomplete summary:
2758 Math::Fraction big, unlimited fractions like 9973 / 12967
2759 Math::String treat string sequences like numbers
2760 Math::FixedPrecision calculate with a fixed precision
2761 Math::Currency for currency calculations
2762 Bit::Vector manipulate bit vectors fast (uses C)
2763 Math::BigIntFast Bit::Vector wrapper for big numbers
2764 Math::Pari provides access to the Pari C library
2765 Math::BigInteger uses an external C library
2766 Math::Cephes uses external Cephes C library (no big numbers)
2767 Math::Cephes::Fraction fractions via the Cephes library
2768 Math::GMP another one using an external C library