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($a)>) and yields the same result as
514 C<defined($a) ? $a : $b> (except that the ternary-operator form can be
515 used as a lvalue, while C<$a // $b> cannot). This is very useful for
516 providing default values for variables. If you actually want to test if
517 at least one of C<$a> and C<$b> is defined, use C<defined($a // $b)>.
519 The C<||>, C<//> and C<&&> operators return the last value evaluated
520 (unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably
521 portable way to find out the home directory might be:
523 $home = $ENV{'HOME'} // $ENV{'LOGDIR'} //
524 (getpwuid($<))[7] // die "You're homeless!\n";
526 In particular, this means that you shouldn't use this
527 for selecting between two aggregates for assignment:
529 @a = @b || @c; # this is wrong
530 @a = scalar(@b) || @c; # really meant this
531 @a = @b ? @b : @c; # this works fine, though
533 As more readable alternatives to C<&&> and C<||> when used for
534 control flow, Perl provides the C<and> and C<or> operators (see below).
535 The short-circuit behavior is identical. The precedence of "and"
536 and "or" is much lower, however, so that you can safely use them after a
537 list operator without the need for parentheses:
539 unlink "alpha", "beta", "gamma"
540 or gripe(), next LINE;
542 With the C-style operators that would have been written like this:
544 unlink("alpha", "beta", "gamma")
545 || (gripe(), next LINE);
547 Using "or" for assignment is unlikely to do what you want; see below.
549 =head2 Range Operators
550 X<operator, range> X<range> X<..> X<...>
552 Binary ".." is the range operator, which is really two different
553 operators depending on the context. In list context, it returns a
554 list of values counting (up by ones) from the left value to the right
555 value. If the left value is greater than the right value then it
556 returns the empty list. The range operator is useful for writing
557 C<foreach (1..10)> loops and for doing slice operations on arrays. In
558 the current implementation, no temporary array is created when the
559 range operator is used as the expression in C<foreach> loops, but older
560 versions of Perl might burn a lot of memory when you write something
563 for (1 .. 1_000_000) {
567 The range operator also works on strings, using the magical
568 auto-increment, see below.
570 In scalar context, ".." returns a boolean value. The operator is
571 bistable, like a flip-flop, and emulates the line-range (comma)
572 operator of B<sed>, B<awk>, and various editors. Each ".." operator
573 maintains its own boolean state, even across calls to a subroutine
574 that contains it. It is false as long as its left operand is false.
575 Once the left operand is true, the range operator stays true until the
576 right operand is true, I<AFTER> which the range operator becomes false
577 again. It doesn't become false till the next time the range operator
578 is evaluated. It can test the right operand and become false on the
579 same evaluation it became true (as in B<awk>), but it still returns
580 true once. If you don't want it to test the right operand until the
581 next evaluation, as in B<sed>, just use three dots ("...") instead of
582 two. In all other regards, "..." behaves just like ".." does.
584 The right operand is not evaluated while the operator is in the
585 "false" state, and the left operand is not evaluated while the
586 operator is in the "true" state. The precedence is a little lower
587 than || and &&. The value returned is either the empty string for
588 false, or a sequence number (beginning with 1) for true. The sequence
589 number is reset for each range encountered. The final sequence number
590 in a range has the string "E0" appended to it, which doesn't affect
591 its numeric value, but gives you something to search for if you want
592 to exclude the endpoint. You can exclude the beginning point by
593 waiting for the sequence number to be greater than 1.
595 If either operand of scalar ".." is a constant expression,
596 that operand is considered true if it is equal (C<==>) to the current
597 input line number (the C<$.> variable).
599 To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>,
600 but that is only an issue if you use a floating point expression; when
601 implicitly using C<$.> as described in the previous paragraph, the
602 comparison is C<int(EXPR) == int($.)> which is only an issue when C<$.>
603 is set to a floating point value and you are not reading from a file.
604 Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what
605 you want in scalar context because each of the operands are evaluated
606 using their integer representation.
610 As a scalar operator:
612 if (101 .. 200) { print; } # print 2nd hundred lines, short for
613 # if ($. == 101 .. $. == 200) { print; }
615 next LINE if (1 .. /^$/); # skip header lines, short for
616 # next LINE if ($. == 1 .. /^$/);
617 # (typically in a loop labeled LINE)
619 s/^/> / if (/^$/ .. eof()); # quote body
621 # parse mail messages
623 $in_header = 1 .. /^$/;
624 $in_body = /^$/ .. eof;
631 close ARGV if eof; # reset $. each file
634 Here's a simple example to illustrate the difference between
635 the two range operators:
648 This program will print only the line containing "Bar". If
649 the range operator is changed to C<...>, it will also print the
652 And now some examples as a list operator:
654 for (101 .. 200) { print; } # print $_ 100 times
655 @foo = @foo[0 .. $#foo]; # an expensive no-op
656 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
658 The range operator (in list context) makes use of the magical
659 auto-increment algorithm if the operands are strings. You
662 @alphabet = ('A' .. 'Z');
664 to get all normal letters of the English alphabet, or
666 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
668 to get a hexadecimal digit, or
670 @z2 = ('01' .. '31'); print $z2[$mday];
672 to get dates with leading zeros.
674 If the final value specified is not in the sequence that the magical
675 increment would produce, the sequence goes until the next value would
676 be longer than the final value specified.
678 If the initial value specified isn't part of a magical increment
679 sequence (that is, a non-empty string matching "/^[a-zA-Z]*[0-9]*\z/"),
680 only the initial value will be returned. So the following will only
683 use charnames 'greek';
684 my @greek_small = ("\N{alpha}" .. "\N{omega}");
686 To get lower-case greek letters, use this instead:
688 my @greek_small = map { chr } ( ord("\N{alpha}") ..
691 Because each operand is evaluated in integer form, C<2.18 .. 3.14> will
692 return two elements in list context.
694 @list = (2.18 .. 3.14); # same as @list = (2 .. 3);
696 =head2 Conditional Operator
697 X<operator, conditional> X<operator, ternary> X<ternary> X<?:>
699 Ternary "?:" is the conditional operator, just as in C. It works much
700 like an if-then-else. If the argument before the ? is true, the
701 argument before the : is returned, otherwise the argument after the :
702 is returned. For example:
704 printf "I have %d dog%s.\n", $n,
705 ($n == 1) ? '' : "s";
707 Scalar or list context propagates downward into the 2nd
708 or 3rd argument, whichever is selected.
710 $a = $ok ? $b : $c; # get a scalar
711 @a = $ok ? @b : @c; # get an array
712 $a = $ok ? @b : @c; # oops, that's just a count!
714 The operator may be assigned to if both the 2nd and 3rd arguments are
715 legal lvalues (meaning that you can assign to them):
717 ($a_or_b ? $a : $b) = $c;
719 Because this operator produces an assignable result, using assignments
720 without parentheses will get you in trouble. For example, this:
722 $a % 2 ? $a += 10 : $a += 2
726 (($a % 2) ? ($a += 10) : $a) += 2
730 ($a % 2) ? ($a += 10) : ($a += 2)
732 That should probably be written more simply as:
734 $a += ($a % 2) ? 10 : 2;
736 =head2 Assignment Operators
737 X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=>
738 X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=>
741 "=" is the ordinary assignment operator.
743 Assignment operators work as in C. That is,
751 although without duplicating any side effects that dereferencing the lvalue
752 might trigger, such as from tie(). Other assignment operators work similarly.
753 The following are recognized:
760 Although these are grouped by family, they all have the precedence
763 Unlike in C, the scalar assignment operator produces a valid lvalue.
764 Modifying an assignment is equivalent to doing the assignment and
765 then modifying the variable that was assigned to. This is useful
766 for modifying a copy of something, like this:
768 ($tmp = $global) =~ tr [A-Z] [a-z];
779 Similarly, a list assignment in list context produces the list of
780 lvalues assigned to, and a list assignment in scalar context returns
781 the number of elements produced by the expression on the right hand
782 side of the assignment.
784 =head2 Comma Operator
785 X<comma> X<operator, comma> X<,>
787 Binary "," is the comma operator. In scalar context it evaluates
788 its left argument, throws that value away, then evaluates its right
789 argument and returns that value. This is just like C's comma operator.
791 In list context, it's just the list argument separator, and inserts
792 both its arguments into the list. These arguments are also evaluated
795 The C<< => >> operator is a synonym for the comma except that it causes
796 its left operand to be interpreted as a string if it begins with a letter
797 or underscore and is composed only of letters, digits and underscores.
798 This includes operands that might otherwise be interpreted as operators,
799 constants, single number v-strings or function calls. If in doubt about
800 this behaviour, the left operand can be quoted explicitly.
802 Otherwise, the C<< => >> operator behaves exactly as the comma operator
803 or list argument separator, according to context.
807 use constant FOO => "something";
809 my %h = ( FOO => 23 );
817 my %h = ("something", 23);
819 The C<< => >> operator is helpful in documenting the correspondence
820 between keys and values in hashes, and other paired elements in lists.
822 %hash = ( $key => $value );
823 login( $username => $password );
825 =head2 Yada Yada Operator
826 X<...> X<... operator> X<yada yada operator>
828 The yada yada operator (noted C<...>) is a placeholder for code. Perl
829 parses it without error, but when you try to execute a yada yada, it
830 throws an exception with the text C<Unimplemented>:
832 sub unimplemented { ... }
834 eval { unimplemented() };
835 if( $@ eq 'Unimplemented' ) {
836 print "I found the yada yada!\n";
839 You can only use the yada yada to stand in for a complete statement.
840 These examples of the yada yada work:
856 do { my $n; ...; print 'Hurrah!' };
858 The yada yada cannot stand in for an expression that is part of a
859 larger statement since the C<...> is also the three-dot version of the
860 range operator (see L<Range Operators>). These examples of the yada
861 yada are still syntax errors:
865 open my($fh), '>', '/dev/passwd' or ...;
867 if( $condition && ... ) { print "Hello\n" };
869 There are some cases where Perl can't immediately tell the difference
870 between an expression and a statement. For instance, the syntax for a
871 block and an anonymous hash reference constructor look the same unless
872 there's something in the braces that give Perl a hint. The yada yada
873 is a syntax error if Perl doesn't guess that the C<{ ... }> is a
874 block. In that case, it doesn't think the C<...> is the yada yada
875 because it's expecting an expression instead of a statement:
877 my @transformed = map { ... } @input; # syntax error
879 You can use a C<;> inside your block to denote that the C<{ ... }> is
880 a block and not a hash reference constructor. Now the yada yada works:
882 my @transformed = map {; ... } @input; # ; disambiguates
884 my @transformed = map { ...; } @input; # ; disambiguates
886 =head2 List Operators (Rightward)
887 X<operator, list, rightward> X<list operator>
889 On the right side of a list operator, it has very low precedence,
890 such that it controls all comma-separated expressions found there.
891 The only operators with lower precedence are the logical operators
892 "and", "or", and "not", which may be used to evaluate calls to list
893 operators without the need for extra parentheses:
895 open HANDLE, "filename"
896 or die "Can't open: $!\n";
898 See also discussion of list operators in L<Terms and List Operators (Leftward)>.
901 X<operator, logical, not> X<not>
903 Unary "not" returns the logical negation of the expression to its right.
904 It's the equivalent of "!" except for the very low precedence.
907 X<operator, logical, and> X<and>
909 Binary "and" returns the logical conjunction of the two surrounding
910 expressions. It's equivalent to && except for the very low
911 precedence. This means that it short-circuits: i.e., the right
912 expression is evaluated only if the left expression is true.
914 =head2 Logical or, Defined or, and Exclusive Or
915 X<operator, logical, or> X<operator, logical, xor>
916 X<operator, logical, defined or> X<operator, logical, exclusive or>
919 Binary "or" returns the logical disjunction of the two surrounding
920 expressions. It's equivalent to || except for the very low precedence.
921 This makes it useful for control flow
923 print FH $data or die "Can't write to FH: $!";
925 This means that it short-circuits: i.e., the right expression is evaluated
926 only if the left expression is false. Due to its precedence, you should
927 probably avoid using this for assignment, only for control flow.
929 $a = $b or $c; # bug: this is wrong
930 ($a = $b) or $c; # really means this
931 $a = $b || $c; # better written this way
933 However, when it's a list-context assignment and you're trying to use
934 "||" for control flow, you probably need "or" so that the assignment
935 takes higher precedence.
937 @info = stat($file) || die; # oops, scalar sense of stat!
938 @info = stat($file) or die; # better, now @info gets its due
940 Then again, you could always use parentheses.
942 Binary "xor" returns the exclusive-OR of the two surrounding expressions.
943 It cannot short circuit, of course.
945 =head2 C Operators Missing From Perl
946 X<operator, missing from perl> X<&> X<*>
947 X<typecasting> X<(TYPE)>
949 Here is what C has that Perl doesn't:
955 Address-of operator. (But see the "\" operator for taking a reference.)
959 Dereference-address operator. (Perl's prefix dereferencing
960 operators are typed: $, @, %, and &.)
964 Type-casting operator.
968 =head2 Quote and Quote-like Operators
969 X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m>
970 X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>>
971 X<escape sequence> X<escape>
974 While we usually think of quotes as literal values, in Perl they
975 function as operators, providing various kinds of interpolating and
976 pattern matching capabilities. Perl provides customary quote characters
977 for these behaviors, but also provides a way for you to choose your
978 quote character for any of them. In the following table, a C<{}> represents
979 any pair of delimiters you choose.
981 Customary Generic Meaning Interpolates
986 // m{} Pattern match yes*
988 s{}{} Substitution yes*
989 tr{}{} Transliteration no (but see below)
992 * unless the delimiter is ''.
994 Non-bracketing delimiters use the same character fore and aft, but the four
995 sorts of brackets (round, angle, square, curly) will all nest, which means
1004 Note, however, that this does not always work for quoting Perl code:
1006 $s = q{ if($a eq "}") ... }; # WRONG
1008 is a syntax error. The C<Text::Balanced> module (from CPAN, and
1009 starting from Perl 5.8 part of the standard distribution) is able
1010 to do this properly.
1012 There can be whitespace between the operator and the quoting
1013 characters, except when C<#> is being used as the quoting character.
1014 C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
1015 operator C<q> followed by a comment. Its argument will be taken
1016 from the next line. This allows you to write:
1018 s {foo} # Replace foo
1021 The following escape sequences are available in constructs that interpolate
1022 and in transliterations.
1023 X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N> X<\N{}>
1026 Sequence Note Description
1032 \a alarm (bell) (BEL)
1034 \x{263a} [1,8] hex char (example: SMILEY)
1035 \x1b [2,8] restricted range hex char (example: ESC)
1036 \N{name} [3] named Unicode character or character sequence
1037 \N{U+263D} [4,8] Unicode character (example: FIRST QUARTER MOON)
1038 \c[ [5] control char (example: chr(27))
1039 \o{23072} [6,8] octal char (example: SMILEY)
1040 \033 [7,8] restricted range octal char (example: ESC)
1046 The result is the character specified by the hexadecimal number between
1047 the braces. See L</[8]> below for details on which character.
1049 Only hexadecimal digits are valid between the braces. If an invalid
1050 character is encountered, a warning will be issued and the invalid
1051 character and all subsequent characters (valid or invalid) within the
1052 braces will be discarded.
1054 If there are no valid digits between the braces, the generated character is
1055 the NULL character (C<\x{00}>). However, an explicit empty brace (C<\x{}>)
1056 will not cause a warning.
1060 The result is the character specified by the hexadecimal number in the range
1061 0x00 to 0xFF. See L</[8]> below for details on which character.
1063 Only hexadecimal digits are valid following C<\x>. When C<\x> is followed
1064 by fewer than two valid digits, any valid digits will be zero-padded. This
1065 means that C<\x7> will be interpreted as C<\x07> and C<\x> alone will be
1066 interpreted as C<\x00>. Except at the end of a string, having fewer than
1067 two valid digits will result in a warning. Note that while the warning
1068 says the illegal character is ignored, it is only ignored as part of the
1069 escape and will still be used as the subsequent character in the string.
1072 Original Result Warns?
1080 The result is the Unicode character or character sequence given by I<name>.
1085 C<\N{U+I<hexadecimal number>}> means the Unicode character whose Unicode code
1086 point is I<hexadecimal number>.
1090 The character following C<\c> is mapped to some other character as shown in the
1107 Also, C<\c\I<X>> yields C< chr(28) . "I<X>"> for any I<X>, but cannot come at the
1108 end of a string, because the backslash would be parsed as escaping the end
1111 On ASCII platforms, the resulting characters from the list above are the
1112 complete set of ASCII controls. This isn't the case on EBCDIC platforms; see
1113 L<perlebcdic/OPERATOR DIFFERENCES> for the complete list of what these
1114 sequences mean on both ASCII and EBCDIC platforms.
1116 Use of any other character following the "c" besides those listed above is
1117 discouraged, and some are deprecated with the intention of removing
1118 those in Perl 5.16. What happens for any of these
1119 other characters currently though, is that the value is derived by inverting
1122 To get platform independent controls, you can use C<\N{...}>.
1126 The result is the character specified by the octal number between the braces.
1127 See L</[8]> below for details on which character.
1129 If a character that isn't an octal digit is encountered, a warning is raised,
1130 and the value is based on the octal digits before it, discarding it and all
1131 following characters up to the closing brace. It is a fatal error if there are
1132 no octal digits at all.
1136 The result is the character specified by the three digit octal number in the
1137 range 000 to 777 (but best to not use above 077, see next paragraph). See
1138 L</[8]> below for details on which character.
1140 Some contexts allow 2 or even 1 digit, but any usage without exactly
1141 three digits, the first being a zero, may give unintended results. (For
1142 example, see L<perlrebackslash/Octal escapes>.) Starting in Perl 5.14, you may
1143 use C<\o{}> instead which avoids all these problems. Otherwise, it is best to
1144 use this construct only for ordinals C<\077> and below, remembering to pad to
1145 the left with zeros to make three digits. For larger ordinals, either use
1146 C<\o{}> , or convert to something else, such as to hex and use C<\x{}>
1149 Having fewer than 3 digits may lead to a misleading warning message that says
1150 that what follows is ignored. For example, C<"\128"> in the ASCII character set
1151 is equivalent to the two characters C<"\n8">, but the warning C<Illegal octal
1152 digit '8' ignored> will be thrown. To avoid this warning, make sure to pad
1153 your octal number with C<0>s: C<"\0128">.
1157 Several of the constructs above specify a character by a number. That number
1158 gives the character's position in the character set encoding (indexed from 0).
1159 This is called synonymously its ordinal, code position, or code point). Perl
1160 works on platforms that have a native encoding currently of either ASCII/Latin1
1161 or EBCDIC, each of which allow specification of 256 characters. In general, if
1162 the number is 255 (0xFF, 0377) or below, Perl interprets this in the platform's
1163 native encoding. If the number is 256 (0x100, 0400) or above, Perl interprets
1164 it as as a Unicode code point and the result is the corresponding Unicode
1165 character. For example C<\x{50}> and C<\o{120}> both are the number 80 in
1166 decimal, which is less than 256, so the number is interpreted in the native
1167 character set encoding. In ASCII the character in the 80th position (indexed
1168 from 0) is the letter "P", and in EBCDIC it is the ampersand symbol "&".
1169 C<\x{100}> and C<\o{400}> are both 256 in decimal, so the number is interpreted
1170 as a Unicode code point no matter what the native encoding is. The name of the
1171 character in the 100th position (indexed by 0) in Unicode is
1172 C<LATIN CAPITAL LETTER A WITH MACRON>.
1174 There are a couple of exceptions to the above rule. C<\N{U+I<hex number>}> is
1175 always interpreted as a Unicode code point, so that C<\N{U+0050}> is "P" even
1176 on EBCDIC platforms. And if L<C<S<use encoding>>|encoding> is in effect, the
1177 number is considered to be in that encoding, and is translated from that into
1178 the platform's native encoding if there is a corresponding native character;
1179 otherwise to Unicode.
1183 B<NOTE>: Unlike C and other languages, Perl has no C<\v> escape sequence for
1184 the vertical tab (VT - ASCII 11), but you may use C<\ck> or C<\x0b>. (C<\v>
1185 does have meaning in regular expression patterns in Perl, see L<perlre>.)
1187 The following escape sequences are available in constructs that interpolate,
1188 but not in transliterations.
1189 X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q>
1191 \l lowercase next char
1192 \u uppercase next char
1193 \L lowercase till \E
1194 \U uppercase till \E
1195 \Q quote non-word characters till \E
1196 \E end either case modification or quoted section
1198 If C<use locale> is in effect, the case map used by C<\l>, C<\L>,
1199 C<\u> and C<\U> is taken from the current locale. See L<perllocale>.
1200 If Unicode (for example, C<\N{}> or wide hex characters of 0x100 or
1201 beyond) is being used, the case map used by C<\l>, C<\L>, C<\u> and
1202 C<\U> is as defined by Unicode.
1204 All systems use the virtual C<"\n"> to represent a line terminator,
1205 called a "newline". There is no such thing as an unvarying, physical
1206 newline character. It is only an illusion that the operating system,
1207 device drivers, C libraries, and Perl all conspire to preserve. Not all
1208 systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
1209 on a Mac, these are reversed, and on systems without line terminator,
1210 printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
1211 you mean a "newline" for your system, but use the literal ASCII when you
1212 need an exact character. For example, most networking protocols expect
1213 and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
1214 and although they often accept just C<"\012">, they seldom tolerate just
1215 C<"\015">. If you get in the habit of using C<"\n"> for networking,
1216 you may be burned some day.
1217 X<newline> X<line terminator> X<eol> X<end of line>
1220 For constructs that do interpolate, variables beginning with "C<$>"
1221 or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
1222 C<< $href->{key}[0] >> are also interpolated, as are array and hash slices.
1223 But method calls such as C<< $obj->meth >> are not.
1225 Interpolating an array or slice interpolates the elements in order,
1226 separated by the value of C<$">, so is equivalent to interpolating
1227 C<join $", @array>. "Punctuation" arrays such as C<@*> are only
1228 interpolated if the name is enclosed in braces C<@{*}>, but special
1229 arrays C<@_>, C<@+>, and C<@-> are interpolated, even without braces.
1231 For double-quoted strings, the quoting from C<\Q> is applied after
1232 interpolation and escapes are processed.
1234 "abc\Qfoo\tbar$s\Exyz"
1238 "abc" . quotemeta("foo\tbar$s") . "xyz"
1240 For the pattern of regex operators (C<qr//>, C<m//> and C<s///>),
1241 the quoting from C<\Q> is applied after interpolation is processed,
1242 but before escapes are processed. This allows the pattern to match
1243 literally (except for C<$> and C<@>). For example, the following matches:
1247 Because C<$> or C<@> trigger interpolation, you'll need to use something
1248 like C</\Quser\E\@\Qhost/> to match them literally.
1250 Patterns are subject to an additional level of interpretation as a
1251 regular expression. This is done as a second pass, after variables are
1252 interpolated, so that regular expressions may be incorporated into the
1253 pattern from the variables. If this is not what you want, use C<\Q> to
1254 interpolate a variable literally.
1256 Apart from the behavior described above, Perl does not expand
1257 multiple levels of interpolation. In particular, contrary to the
1258 expectations of shell programmers, back-quotes do I<NOT> interpolate
1259 within double quotes, nor do single quotes impede evaluation of
1260 variables when used within double quotes.
1262 =head2 Regexp Quote-Like Operators
1265 Here are the quote-like operators that apply to pattern
1266 matching and related activities.
1270 =item qr/STRING/msixpo
1271 X<qr> X</i> X</m> X</o> X</s> X</x> X</p>
1273 This operator quotes (and possibly compiles) its I<STRING> as a regular
1274 expression. I<STRING> is interpolated the same way as I<PATTERN>
1275 in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
1276 is done. Returns a Perl value which may be used instead of the
1277 corresponding C</STRING/msixpo> expression. The returned value is a
1278 normalized version of the original pattern. It magically differs from
1279 a string containing the same characters: C<ref(qr/x/)> returns "Regexp",
1280 even though dereferencing the result returns undef.
1284 $rex = qr/my.STRING/is;
1285 print $rex; # prints (?si-xm:my.STRING)
1292 The result may be used as a subpattern in a match:
1295 $string =~ /foo${re}bar/; # can be interpolated in other patterns
1296 $string =~ $re; # or used standalone
1297 $string =~ /$re/; # or this way
1299 Since Perl may compile the pattern at the moment of execution of qr()
1300 operator, using qr() may have speed advantages in some situations,
1301 notably if the result of qr() is used standalone:
1304 my $patterns = shift;
1305 my @compiled = map qr/$_/i, @$patterns;
1308 foreach my $pat (@compiled) {
1309 $success = 1, last if /$pat/;
1315 Precompilation of the pattern into an internal representation at
1316 the moment of qr() avoids a need to recompile the pattern every
1317 time a match C</$pat/> is attempted. (Perl has many other internal
1318 optimizations, but none would be triggered in the above example if
1319 we did not use qr() operator.)
1321 Options (specified by the following modifiers) are:
1323 m Treat string as multiple lines.
1324 s Treat string as single line. (Make . match a newline)
1325 i Do case-insensitive pattern matching.
1326 x Use extended regular expressions.
1327 p When matching preserve a copy of the matched string so
1328 that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be defined.
1329 o Compile pattern only once.
1331 If a precompiled pattern is embedded in a larger pattern then the effect
1332 of 'msixp' will be propagated appropriately. The effect of the 'o'
1333 modifier has is not propagated, being restricted to those patterns
1334 explicitly using it.
1336 Several other modifiers to control the character set semantics were
1337 added for 5.14 that, unlike the ones listed above, may not be used
1338 after the final pattern delimiter, but only following a C<"(?"> inside
1339 the regular expression. (It is planned in 5.16 to make them usable in
1340 the suffix position.) These are B<C<"a">>, B<C<"d">>, B<C<"l">>, and
1341 B<C<"u">>. They are documented in L<perlre/Extended Patterns>.
1343 See L<perlre> for additional information on valid syntax for STRING, and
1344 for a detailed look at the semantics of regular expressions.
1346 =item m/PATTERN/msixpogc
1347 X<m> X<operator, match>
1348 X<regexp, options> X<regexp> X<regex, options> X<regex>
1349 X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c>
1351 =item /PATTERN/msixpogc
1353 Searches a string for a pattern match, and in scalar context returns
1354 true if it succeeds, false if it fails. If no string is specified
1355 via the C<=~> or C<!~> operator, the $_ string is searched. (The
1356 string specified with C<=~> need not be an lvalue--it may be the
1357 result of an expression evaluation, but remember the C<=~> binds
1358 rather tightly.) See also L<perlre>. See L<perllocale> for
1359 discussion of additional considerations that apply when C<use locale>
1362 Options are as described in C<qr//>; in addition, the following match
1363 process modifiers are available:
1365 g Match globally, i.e., find all occurrences.
1366 c Do not reset search position on a failed match when /g is in effect.
1368 If "/" is the delimiter then the initial C<m> is optional. With the C<m>
1369 you can use any pair of non-whitespace characters
1370 as delimiters. This is particularly useful for matching path names
1371 that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
1372 the delimiter, then a match-only-once rule applies,
1373 described in C<m?PATTERN?> below.
1374 If "'" is the delimiter, no interpolation is performed on the PATTERN.
1375 When using a character valid in an identifier, whitespace is required
1378 PATTERN may contain variables, which will be interpolated (and the
1379 pattern recompiled) every time the pattern search is evaluated, except
1380 for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
1381 C<$|> are not interpolated because they look like end-of-string tests.)
1382 If you want such a pattern to be compiled only once, add a C</o> after
1383 the trailing delimiter. This avoids expensive run-time recompilations,
1384 and is useful when the value you are interpolating won't change over
1385 the life of the script. However, mentioning C</o> constitutes a promise
1386 that you won't change the variables in the pattern. If you change them,
1387 Perl won't even notice. See also L<"qr/STRING/msixpo">.
1389 =item The empty pattern //
1391 If the PATTERN evaluates to the empty string, the last
1392 I<successfully> matched regular expression is used instead. In this
1393 case, only the C<g> and C<c> flags on the empty pattern is honoured -
1394 the other flags are taken from the original pattern. If no match has
1395 previously succeeded, this will (silently) act instead as a genuine
1396 empty pattern (which will always match).
1398 Note that it's possible to confuse Perl into thinking C<//> (the empty
1399 regex) is really C<//> (the defined-or operator). Perl is usually pretty
1400 good about this, but some pathological cases might trigger this, such as
1401 C<$a///> (is that C<($a) / (//)> or C<$a // />?) and C<print $fh //>
1402 (C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl
1403 will assume you meant defined-or. If you meant the empty regex, just
1404 use parentheses or spaces to disambiguate, or even prefix the empty
1405 regex with an C<m> (so C<//> becomes C<m//>).
1407 =item Matching in list context
1409 If the C</g> option is not used, C<m//> in list context returns a
1410 list consisting of the subexpressions matched by the parentheses in the
1411 pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are
1412 also set, and that this differs from Perl 4's behavior.) When there are
1413 no parentheses in the pattern, the return value is the list C<(1)> for
1414 success. With or without parentheses, an empty list is returned upon
1419 open(TTY, '/dev/tty');
1420 <TTY> =~ /^y/i && foo(); # do foo if desired
1422 if (/Version: *([0-9.]*)/) { $version = $1; }
1424 next if m#^/usr/spool/uucp#;
1429 print if /$arg/o; # compile only once
1432 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
1434 This last example splits $foo into the first two words and the
1435 remainder of the line, and assigns those three fields to $F1, $F2, and
1436 $Etc. The conditional is true if any variables were assigned, i.e., if
1437 the pattern matched.
1439 The C</g> modifier specifies global pattern matching--that is,
1440 matching as many times as possible within the string. How it behaves
1441 depends on the context. In list context, it returns a list of the
1442 substrings matched by any capturing parentheses in the regular
1443 expression. If there are no parentheses, it returns a list of all
1444 the matched strings, as if there were parentheses around the whole
1447 In scalar context, each execution of C<m//g> finds the next match,
1448 returning true if it matches, and false if there is no further match.
1449 The position after the last match can be read or set using the C<pos()>
1450 function; see L<perlfunc/pos>. A failed match normally resets the
1451 search position to the beginning of the string, but you can avoid that
1452 by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
1453 string also resets the search position.
1457 You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
1458 zero-width assertion that matches the exact position where the
1459 previous C<m//g>, if any, left off. Without the C</g> modifier, the
1460 C<\G> assertion still anchors at C<pos()> as it was at the start of
1461 the operation (see L<perlfunc/pos>), but the match is of course only
1462 attempted once. Using C<\G> without C</g> on a target string that has
1463 not previously had a C</g> match applied to it is the same as using
1464 the C<\A> assertion to match the beginning of the string. Note also
1465 that, currently, C<\G> is only properly supported when anchored at the
1466 very beginning of the pattern.
1471 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
1475 while (defined($paragraph = <>)) {
1476 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
1480 print "$sentences\n";
1482 # using m//gc with \G
1486 print $1 while /(o)/gc; print "', pos=", pos, "\n";
1488 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
1490 print $1 while /(p)/gc; print "', pos=", pos, "\n";
1492 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
1494 The last example should print:
1504 Notice that the final match matched C<q> instead of C<p>, which a match
1505 without the C<\G> anchor would have done. Also note that the final match
1506 did not update C<pos>. C<pos> is only updated on a C</g> match. If the
1507 final match did indeed match C<p>, it's a good bet that you're running an
1508 older (pre-5.6.0) Perl.
1510 A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
1511 combine several regexps like this to process a string part-by-part,
1512 doing different actions depending on which regexp matched. Each
1513 regexp tries to match where the previous one leaves off.
1516 $url = URI::URL->new( "http://example.com/" ); die if $url eq "xXx";
1520 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
1521 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
1522 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
1523 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
1524 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
1525 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
1526 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
1527 print ". That's all!\n";
1530 Here is the output (split into several lines):
1532 line-noise lowercase line-noise UPPERCASE line-noise UPPERCASE
1533 line-noise lowercase line-noise lowercase line-noise lowercase
1534 lowercase line-noise lowercase lowercase line-noise lowercase
1535 lowercase line-noise MiXeD line-noise. That's all!
1538 X<?> X<operator, match-once>
1542 This is just like the C<m/PATTERN/> search, except that it matches
1543 only once between calls to the reset() operator. This is a useful
1544 optimization when you want to see only the first occurrence of
1545 something in each file of a set of files, for instance. Only C<m??>
1546 patterns local to the current package are reset.
1550 # blank line between header and body
1553 reset if eof; # clear m?? status for next file
1556 The match-once behaviour is controlled by the match delimiter being
1557 C<?>; with any other delimiter this is the normal C<m//> operator.
1559 For historical reasons, the leading C<m> in C<m?PATTERN?> is optional,
1560 but the resulting C<?PATTERN?> syntax is deprecated, will warn on
1561 usage and may be removed from a future stable release of Perl without
1564 =item s/PATTERN/REPLACEMENT/msixpogcer
1565 X<substitute> X<substitution> X<replace> X<regexp, replace>
1566 X<regexp, substitute> X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> X</e> X</r>
1568 Searches a string for a pattern, and if found, replaces that pattern
1569 with the replacement text and returns the number of substitutions
1570 made. Otherwise it returns false (specifically, the empty string).
1572 If the C</r> (non-destructive) option is used then it will perform the
1573 substitution on a copy of the string and return the copy whether or not a
1574 substitution occurred. The original string will always remain unchanged in
1575 this case. The copy will always be a plain string, even if the input is an
1576 object or a tied variable.
1578 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
1579 variable is searched and modified. (The string specified with C<=~> must
1580 be scalar variable, an array element, a hash element, or an assignment
1581 to one of those, i.e., an lvalue.)
1583 If the delimiter chosen is a single quote, no interpolation is
1584 done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1585 PATTERN contains a $ that looks like a variable rather than an
1586 end-of-string test, the variable will be interpolated into the pattern
1587 at run-time. If you want the pattern compiled only once the first time
1588 the variable is interpolated, use the C</o> option. If the pattern
1589 evaluates to the empty string, the last successfully executed regular
1590 expression is used instead. See L<perlre> for further explanation on these.
1591 See L<perllocale> for discussion of additional considerations that apply
1592 when C<use locale> is in effect.
1594 Options are as with m// with the addition of the following replacement
1597 e Evaluate the right side as an expression.
1598 ee Evaluate the right side as a string then eval the result.
1599 r Return substitution and leave the original string untouched.
1601 Any non-whitespace delimiter may replace the slashes. Add space after
1602 the C<s> when using a character allowed in identifiers. If single quotes
1603 are used, no interpretation is done on the replacement string (the C</e>
1604 modifier overrides this, however). Unlike Perl 4, Perl 5 treats backticks
1605 as normal delimiters; the replacement text is not evaluated as a command.
1606 If the PATTERN is delimited by bracketing quotes, the REPLACEMENT has
1607 its own pair of quotes, which may or may not be bracketing quotes, e.g.,
1608 C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
1609 replacement portion to be treated as a full-fledged Perl expression
1610 and evaluated right then and there. It is, however, syntax checked at
1611 compile-time. A second C<e> modifier will cause the replacement portion
1612 to be C<eval>ed before being run as a Perl expression.
1616 s/\bgreen\b/mauve/g; # don't change wintergreen
1618 $path =~ s|/usr/bin|/usr/local/bin|;
1620 s/Login: $foo/Login: $bar/; # run-time pattern
1622 ($foo = $bar) =~ s/this/that/; # copy first, then change
1623 ($foo = "$bar") =~ s/this/that/; # convert to string, copy, then change
1624 $foo = $bar =~ s/this/that/r; # Same as above using /r
1625 $foo = $bar =~ s/this/that/r
1626 =~ s/that/the other/r; # Chained substitutes using /r
1627 @foo = map { s/this/that/r } @bar # /r is very useful in maps
1629 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
1632 s/\d+/$&*2/e; # yields 'abc246xyz'
1633 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
1634 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1636 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
1637 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
1638 s/^=(\w+)/pod($1)/ge; # use function call
1641 $a = s/abc/def/r; # $a is 'def123xyz' and
1642 # $_ remains 'abc123xyz'.
1644 # expand variables in $_, but dynamics only, using
1645 # symbolic dereferencing
1648 # Add one to the value of any numbers in the string
1651 # This will expand any embedded scalar variable
1652 # (including lexicals) in $_ : First $1 is interpolated
1653 # to the variable name, and then evaluated
1656 # Delete (most) C comments.
1658 /\* # Match the opening delimiter.
1659 .*? # Match a minimal number of characters.
1660 \*/ # Match the closing delimiter.
1663 s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_, expensively
1665 for ($variable) { # trim whitespace in $variable, cheap
1670 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
1672 Note the use of $ instead of \ in the last example. Unlike
1673 B<sed>, we use the \<I<digit>> form in only the left hand side.
1674 Anywhere else it's $<I<digit>>.
1676 Occasionally, you can't use just a C</g> to get all the changes
1677 to occur that you might want. Here are two common cases:
1679 # put commas in the right places in an integer
1680 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
1682 # expand tabs to 8-column spacing
1683 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
1687 =head2 Quote-Like Operators
1688 X<operator, quote-like>
1693 X<q> X<quote, single> X<'> X<''>
1697 A single-quoted, literal string. A backslash represents a backslash
1698 unless followed by the delimiter or another backslash, in which case
1699 the delimiter or backslash is interpolated.
1701 $foo = q!I said, "You said, 'She said it.'"!;
1702 $bar = q('This is it.');
1703 $baz = '\n'; # a two-character string
1706 X<qq> X<quote, double> X<"> X<"">
1710 A double-quoted, interpolated string.
1713 (*** The previous line contains the naughty word "$1".\n)
1714 if /\b(tcl|java|python)\b/i; # :-)
1715 $baz = "\n"; # a one-character string
1718 X<qx> X<`> X<``> X<backtick>
1722 A string which is (possibly) interpolated and then executed as a
1723 system command with C</bin/sh> or its equivalent. Shell wildcards,
1724 pipes, and redirections will be honored. The collected standard
1725 output of the command is returned; standard error is unaffected. In
1726 scalar context, it comes back as a single (potentially multi-line)
1727 string, or undef if the command failed. In list context, returns a
1728 list of lines (however you've defined lines with $/ or
1729 $INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
1731 Because backticks do not affect standard error, use shell file descriptor
1732 syntax (assuming the shell supports this) if you care to address this.
1733 To capture a command's STDERR and STDOUT together:
1735 $output = `cmd 2>&1`;
1737 To capture a command's STDOUT but discard its STDERR:
1739 $output = `cmd 2>/dev/null`;
1741 To capture a command's STDERR but discard its STDOUT (ordering is
1744 $output = `cmd 2>&1 1>/dev/null`;
1746 To exchange a command's STDOUT and STDERR in order to capture the STDERR
1747 but leave its STDOUT to come out the old STDERR:
1749 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
1751 To read both a command's STDOUT and its STDERR separately, it's easiest
1752 to redirect them separately to files, and then read from those files
1753 when the program is done:
1755 system("program args 1>program.stdout 2>program.stderr");
1757 The STDIN filehandle used by the command is inherited from Perl's STDIN.
1760 open BLAM, "blam" || die "Can't open: $!";
1761 open STDIN, "<&BLAM";
1764 will print the sorted contents of the file "blam".
1766 Using single-quote as a delimiter protects the command from Perl's
1767 double-quote interpolation, passing it on to the shell instead:
1769 $perl_info = qx(ps $$); # that's Perl's $$
1770 $shell_info = qx'ps $$'; # that's the new shell's $$
1772 How that string gets evaluated is entirely subject to the command
1773 interpreter on your system. On most platforms, you will have to protect
1774 shell metacharacters if you want them treated literally. This is in
1775 practice difficult to do, as it's unclear how to escape which characters.
1776 See L<perlsec> for a clean and safe example of a manual fork() and exec()
1777 to emulate backticks safely.
1779 On some platforms (notably DOS-like ones), the shell may not be
1780 capable of dealing with multiline commands, so putting newlines in
1781 the string may not get you what you want. You may be able to evaluate
1782 multiple commands in a single line by separating them with the command
1783 separator character, if your shell supports that (e.g. C<;> on many Unix
1784 shells; C<&> on the Windows NT C<cmd> shell).
1786 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1787 output before starting the child process, but this may not be supported
1788 on some platforms (see L<perlport>). To be safe, you may need to set
1789 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1790 C<IO::Handle> on any open handles.
1792 Beware that some command shells may place restrictions on the length
1793 of the command line. You must ensure your strings don't exceed this
1794 limit after any necessary interpolations. See the platform-specific
1795 release notes for more details about your particular environment.
1797 Using this operator can lead to programs that are difficult to port,
1798 because the shell commands called vary between systems, and may in
1799 fact not be present at all. As one example, the C<type> command under
1800 the POSIX shell is very different from the C<type> command under DOS.
1801 That doesn't mean you should go out of your way to avoid backticks
1802 when they're the right way to get something done. Perl was made to be
1803 a glue language, and one of the things it glues together is commands.
1804 Just understand what you're getting yourself into.
1806 See L</"I/O Operators"> for more discussion.
1809 X<qw> X<quote, list> X<quote, words>
1811 Evaluates to a list of the words extracted out of STRING, using embedded
1812 whitespace as the word delimiters. It can be understood as being roughly
1815 split(' ', q/STRING/);
1817 the differences being that it generates a real list at compile time, and
1818 in scalar context it returns the last element in the list. So
1823 is semantically equivalent to the list:
1827 Some frequently seen examples:
1829 use POSIX qw( setlocale localeconv )
1830 @EXPORT = qw( foo bar baz );
1832 A common mistake is to try to separate the words with comma or to
1833 put comments into a multi-line C<qw>-string. For this reason, the
1834 C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
1835 produces warnings if the STRING contains the "," or the "#" character.
1838 =item tr/SEARCHLIST/REPLACEMENTLIST/cdsr
1839 X<tr> X<y> X<transliterate> X</c> X</d> X</s>
1841 =item y/SEARCHLIST/REPLACEMENTLIST/cdsr
1843 Transliterates all occurrences of the characters found in the search list
1844 with the corresponding character in the replacement list. It returns
1845 the number of characters replaced or deleted. If no string is
1846 specified via the =~ or !~ operator, the $_ string is transliterated. (The
1847 string specified with =~ must be a scalar variable, an array element, a
1848 hash element, or an assignment to one of those, i.e., an lvalue.)
1850 If the C</r> (non-destructive) option is used then it will perform the
1851 replacement on a copy of the string and return the copy whether or not it
1852 was modified. The original string will always remain unchanged in
1853 this case. The copy will always be a plain string, even if the input is an
1854 object or a tied variable.
1856 A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
1857 does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
1858 For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1859 SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1860 its own pair of quotes, which may or may not be bracketing quotes,
1861 e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
1863 Note that C<tr> does B<not> do regular expression character classes
1864 such as C<\d> or C<[:lower:]>. The C<tr> operator is not equivalent to
1865 the tr(1) utility. If you want to map strings between lower/upper
1866 cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider
1867 using the C<s> operator if you need regular expressions.
1869 Note also that the whole range idea is rather unportable between
1870 character sets--and even within character sets they may cause results
1871 you probably didn't expect. A sound principle is to use only ranges
1872 that begin from and end at either alphabets of equal case (a-e, A-E),
1873 or digits (0-4). Anything else is unsafe. If in doubt, spell out the
1874 character sets in full.
1878 c Complement the SEARCHLIST.
1879 d Delete found but unreplaced characters.
1880 s Squash duplicate replaced characters.
1881 r Return the modified string and leave the original string
1884 If the C</c> modifier is specified, the SEARCHLIST character set
1885 is complemented. If the C</d> modifier is specified, any characters
1886 specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
1887 (Note that this is slightly more flexible than the behavior of some
1888 B<tr> programs, which delete anything they find in the SEARCHLIST,
1889 period.) If the C</s> modifier is specified, sequences of characters
1890 that were transliterated to the same character are squashed down
1891 to a single instance of the character.
1893 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1894 exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1895 than the SEARCHLIST, the final character is replicated till it is long
1896 enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
1897 This latter is useful for counting characters in a class or for
1898 squashing character sequences in a class.
1902 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1904 $cnt = tr/*/*/; # count the stars in $_
1906 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1908 $cnt = tr/0-9//; # count the digits in $_
1910 tr/a-zA-Z//s; # bookkeeper -> bokeper
1912 ($HOST = $host) =~ tr/a-z/A-Z/;
1913 $HOST = $host =~ tr/a-z/A-Z/r; # same thing
1915 $HOST = $host =~ tr/a-z/A-Z/r # chained with s///
1918 tr/a-zA-Z/ /cs; # change non-alphas to single space
1920 @stripped = map tr/a-zA-Z/ /csr, @original;
1924 [\000-\177]; # delete 8th bit
1926 If multiple transliterations are given for a character, only the
1931 will transliterate any A to X.
1933 Because the transliteration table is built at compile time, neither
1934 the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
1935 interpolation. That means that if you want to use variables, you
1938 eval "tr/$oldlist/$newlist/";
1941 eval "tr/$oldlist/$newlist/, 1" or die $@;
1944 X<here-doc> X<heredoc> X<here-document> X<<< << >>>
1946 A line-oriented form of quoting is based on the shell "here-document"
1947 syntax. Following a C<< << >> you specify a string to terminate
1948 the quoted material, and all lines following the current line down to
1949 the terminating string are the value of the item.
1951 The terminating string may be either an identifier (a word), or some
1952 quoted text. An unquoted identifier works like double quotes.
1953 There may not be a space between the C<< << >> and the identifier,
1954 unless the identifier is explicitly quoted. (If you put a space it
1955 will be treated as a null identifier, which is valid, and matches the
1956 first empty line.) The terminating string must appear by itself
1957 (unquoted and with no surrounding whitespace) on the terminating line.
1959 If the terminating string is quoted, the type of quotes used determine
1960 the treatment of the text.
1966 Double quotes indicate that the text will be interpolated using exactly
1967 the same rules as normal double quoted strings.
1970 The price is $Price.
1973 print << "EOF"; # same as above
1974 The price is $Price.
1980 Single quotes indicate the text is to be treated literally with no
1981 interpolation of its content. This is similar to single quoted
1982 strings except that backslashes have no special meaning, with C<\\>
1983 being treated as two backslashes and not one as they would in every
1984 other quoting construct.
1986 This is the only form of quoting in perl where there is no need
1987 to worry about escaping content, something that code generators
1988 can and do make good use of.
1992 The content of the here doc is treated just as it would be if the
1993 string were embedded in backticks. Thus the content is interpolated
1994 as though it were double quoted and then executed via the shell, with
1995 the results of the execution returned.
1997 print << `EOC`; # execute command and get results
2003 It is possible to stack multiple here-docs in a row:
2005 print <<"foo", <<"bar"; # you can stack them
2011 myfunc(<< "THIS", 23, <<'THAT');
2018 Just don't forget that you have to put a semicolon on the end
2019 to finish the statement, as Perl doesn't know you're not going to
2027 If you want to remove the line terminator from your here-docs,
2030 chomp($string = <<'END');
2034 If you want your here-docs to be indented with the rest of the code,
2035 you'll need to remove leading whitespace from each line manually:
2037 ($quote = <<'FINIS') =~ s/^\s+//gm;
2038 The Road goes ever on and on,
2039 down from the door where it began.
2042 If you use a here-doc within a delimited construct, such as in C<s///eg>,
2043 the quoted material must come on the lines following the final delimiter.
2058 If the terminating identifier is on the last line of the program, you
2059 must be sure there is a newline after it; otherwise, Perl will give the
2060 warning B<Can't find string terminator "END" anywhere before EOF...>.
2062 Additionally, the quoting rules for the end of string identifier are not
2063 related to Perl's quoting rules. C<q()>, C<qq()>, and the like are not
2064 supported in place of C<''> and C<"">, and the only interpolation is for
2065 backslashing the quoting character:
2067 print << "abc\"def";
2071 Finally, quoted strings cannot span multiple lines. The general rule is
2072 that the identifier must be a string literal. Stick with that, and you
2077 =head2 Gory details of parsing quoted constructs
2078 X<quote, gory details>
2080 When presented with something that might have several different
2081 interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
2082 principle to pick the most probable interpretation. This strategy
2083 is so successful that Perl programmers often do not suspect the
2084 ambivalence of what they write. But from time to time, Perl's
2085 notions differ substantially from what the author honestly meant.
2087 This section hopes to clarify how Perl handles quoted constructs.
2088 Although the most common reason to learn this is to unravel labyrinthine
2089 regular expressions, because the initial steps of parsing are the
2090 same for all quoting operators, they are all discussed together.
2092 The most important Perl parsing rule is the first one discussed
2093 below: when processing a quoted construct, Perl first finds the end
2094 of that construct, then interprets its contents. If you understand
2095 this rule, you may skip the rest of this section on the first
2096 reading. The other rules are likely to contradict the user's
2097 expectations much less frequently than this first one.
2099 Some passes discussed below are performed concurrently, but because
2100 their results are the same, we consider them individually. For different
2101 quoting constructs, Perl performs different numbers of passes, from
2102 one to four, but these passes are always performed in the same order.
2106 =item Finding the end
2108 The first pass is finding the end of the quoted construct, where
2109 the information about the delimiters is used in parsing.
2110 During this search, text between the starting and ending delimiters
2111 is copied to a safe location. The text copied gets delimiter-independent.
2113 If the construct is a here-doc, the ending delimiter is a line
2114 that has a terminating string as the content. Therefore C<<<EOF> is
2115 terminated by C<EOF> immediately followed by C<"\n"> and starting
2116 from the first column of the terminating line.
2117 When searching for the terminating line of a here-doc, nothing
2118 is skipped. In other words, lines after the here-doc syntax
2119 are compared with the terminating string line by line.
2121 For the constructs except here-docs, single characters are used as starting
2122 and ending delimiters. If the starting delimiter is an opening punctuation
2123 (that is C<(>, C<[>, C<{>, or C<< < >>), the ending delimiter is the
2124 corresponding closing punctuation (that is C<)>, C<]>, C<}>, or C<< > >>).
2125 If the starting delimiter is an unpaired character like C</> or a closing
2126 punctuation, the ending delimiter is same as the starting delimiter.
2127 Therefore a C</> terminates a C<qq//> construct, while a C<]> terminates
2128 C<qq[]> and C<qq]]> constructs.
2130 When searching for single-character delimiters, escaped delimiters
2131 and C<\\> are skipped. For example, while searching for terminating C</>,
2132 combinations of C<\\> and C<\/> are skipped. If the delimiters are
2133 bracketing, nested pairs are also skipped. For example, while searching
2134 for closing C<]> paired with the opening C<[>, combinations of C<\\>, C<\]>,
2135 and C<\[> are all skipped, and nested C<[> and C<]> are skipped as well.
2136 However, when backslashes are used as the delimiters (like C<qq\\> and
2137 C<tr\\\>), nothing is skipped.
2138 During the search for the end, backslashes that escape delimiters
2139 are removed (exactly speaking, they are not copied to the safe location).
2141 For constructs with three-part delimiters (C<s///>, C<y///>, and
2142 C<tr///>), the search is repeated once more.
2143 If the first delimiter is not an opening punctuation, three delimiters must
2144 be same such as C<s!!!> and C<tr)))>, in which case the second delimiter
2145 terminates the left part and starts the right part at once.
2146 If the left part is delimited by bracketing punctuations (that is C<()>,
2147 C<[]>, C<{}>, or C<< <> >>), the right part needs another pair of
2148 delimiters such as C<s(){}> and C<tr[]//>. In these cases, whitespaces
2149 and comments are allowed between both parts, though the comment must follow
2150 at least one whitespace; otherwise a character expected as the start of
2151 the comment may be regarded as the starting delimiter of the right part.
2153 During this search no attention is paid to the semantics of the construct.
2156 "$hash{"$foo/$bar"}"
2161 bar # NOT a comment, this slash / terminated m//!
2164 do not form legal quoted expressions. The quoted part ends on the
2165 first C<"> and C</>, and the rest happens to be a syntax error.
2166 Because the slash that terminated C<m//> was followed by a C<SPACE>,
2167 the example above is not C<m//x>, but rather C<m//> with no C</x>
2168 modifier. So the embedded C<#> is interpreted as a literal C<#>.
2170 Also no attention is paid to C<\c\> (multichar control char syntax) during
2171 this search. Thus the second C<\> in C<qq/\c\/> is interpreted as a part
2172 of C<\/>, and the following C</> is not recognized as a delimiter.
2173 Instead, use C<\034> or C<\x1c> at the end of quoted constructs.
2178 The next step is interpolation in the text obtained, which is now
2179 delimiter-independent. There are multiple cases.
2185 No interpolation is performed.
2186 Note that the combination C<\\> is left intact, since escaped delimiters
2187 are not available for here-docs.
2189 =item C<m''>, the pattern of C<s'''>
2191 No interpolation is performed at this stage.
2192 Any backslashed sequences including C<\\> are treated at the stage
2193 to L</"parsing regular expressions">.
2195 =item C<''>, C<q//>, C<tr'''>, C<y'''>, the replacement of C<s'''>
2197 The only interpolation is removal of C<\> from pairs of C<\\>.
2198 Therefore C<-> in C<tr'''> and C<y'''> is treated literally
2199 as a hyphen and no character range is available.
2200 C<\1> in the replacement of C<s'''> does not work as C<$1>.
2202 =item C<tr///>, C<y///>
2204 No variable interpolation occurs. String modifying combinations for
2205 case and quoting such as C<\Q>, C<\U>, and C<\E> are not recognized.
2206 The other escape sequences such as C<\200> and C<\t> and backslashed
2207 characters such as C<\\> and C<\-> are converted to appropriate literals.
2208 The character C<-> is treated specially and therefore C<\-> is treated
2211 =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF">
2213 C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
2214 converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
2215 is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
2216 The other escape sequences such as C<\200> and C<\t> and backslashed
2217 characters such as C<\\> and C<\-> are replaced with appropriate
2220 Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
2221 is interpolated in the usual way. Something like C<"\Q\\E"> has
2222 no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
2223 result is the same as for C<"\\\\E">. As a general rule, backslashes
2224 between C<\Q> and C<\E> may lead to counterintuitive results. So,
2225 C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
2226 as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
2231 may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
2233 Interpolated scalars and arrays are converted internally to the C<join> and
2234 C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
2236 $foo . " XXX '" . (join $", @arr) . "'";
2238 All operations above are performed simultaneously, left to right.
2240 Because the result of C<"\Q STRING \E"> has all metacharacters
2241 quoted, there is no way to insert a literal C<$> or C<@> inside a
2242 C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
2243 C<"\\\$">; if not, it is interpreted as the start of an interpolated
2246 Note also that the interpolation code needs to make a decision on
2247 where the interpolated scalar ends. For instance, whether
2248 C<< "a $b -> {c}" >> really means:
2250 "a " . $b . " -> {c}";
2256 Most of the time, the longest possible text that does not include
2257 spaces between components and which contains matching braces or
2258 brackets. because the outcome may be determined by voting based
2259 on heuristic estimators, the result is not strictly predictable.
2260 Fortunately, it's usually correct for ambiguous cases.
2262 =item the replacement of C<s///>
2264 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
2265 happens as with C<qq//> constructs.
2267 It is at this step that C<\1> is begrudgingly converted to C<$1> in
2268 the replacement text of C<s///>, in order to correct the incorrigible
2269 I<sed> hackers who haven't picked up the saner idiom yet. A warning
2270 is emitted if the C<use warnings> pragma or the B<-w> command-line flag
2271 (that is, the C<$^W> variable) was set.
2273 =item C<RE> in C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
2275 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\E>,
2276 and interpolation happens (almost) as with C<qq//> constructs.
2278 Processing of C<\N{...}> is also done here, and compiled into an intermediate
2279 form for the regex compiler. (This is because, as mentioned below, the regex
2280 compilation may be done at execution time, and C<\N{...}> is a compile-time
2283 However any other combinations of C<\> followed by a character
2284 are not substituted but only skipped, in order to parse them
2285 as regular expressions at the following step.
2286 As C<\c> is skipped at this step, C<@> of C<\c@> in RE is possibly
2287 treated as an array symbol (for example C<@foo>),
2288 even though the same text in C<qq//> gives interpolation of C<\c@>.
2290 Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
2291 a C<#>-comment in a C<//x>-regular expression, no processing is
2292 performed whatsoever. This is the first step at which the presence
2293 of the C<//x> modifier is relevant.
2295 Interpolation in patterns has several quirks: C<$|>, C<$(>, C<$)>, C<@+>
2296 and C<@-> are not interpolated, and constructs C<$var[SOMETHING]> are
2297 voted (by several different estimators) to be either an array element
2298 or C<$var> followed by an RE alternative. This is where the notation
2299 C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
2300 array element C<-9>, not as a regular expression from the variable
2301 C<$arr> followed by a digit, which would be the interpretation of
2302 C</$arr[0-9]/>. Since voting among different estimators may occur,
2303 the result is not predictable.
2305 The lack of processing of C<\\> creates specific restrictions on
2306 the post-processed text. If the delimiter is C</>, one cannot get
2307 the combination C<\/> into the result of this step. C</> will
2308 finish the regular expression, C<\/> will be stripped to C</> on
2309 the previous step, and C<\\/> will be left as is. Because C</> is
2310 equivalent to C<\/> inside a regular expression, this does not
2311 matter unless the delimiter happens to be character special to the
2312 RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
2313 alphanumeric char, as in:
2317 In the RE above, which is intentionally obfuscated for illustration, the
2318 delimiter is C<m>, the modifier is C<mx>, and after delimiter-removal the
2319 RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
2320 reason you're encouraged to restrict your delimiters to non-alphanumeric,
2321 non-whitespace choices.
2325 This step is the last one for all constructs except regular expressions,
2326 which are processed further.
2328 =item parsing regular expressions
2331 Previous steps were performed during the compilation of Perl code,
2332 but this one happens at run time, although it may be optimized to
2333 be calculated at compile time if appropriate. After preprocessing
2334 described above, and possibly after evaluation if concatenation,
2335 joining, casing translation, or metaquoting are involved, the
2336 resulting I<string> is passed to the RE engine for compilation.
2338 Whatever happens in the RE engine might be better discussed in L<perlre>,
2339 but for the sake of continuity, we shall do so here.
2341 This is another step where the presence of the C<//x> modifier is
2342 relevant. The RE engine scans the string from left to right and
2343 converts it to a finite automaton.
2345 Backslashed characters are either replaced with corresponding
2346 literal strings (as with C<\{>), or else they generate special nodes
2347 in the finite automaton (as with C<\b>). Characters special to the
2348 RE engine (such as C<|>) generate corresponding nodes or groups of
2349 nodes. C<(?#...)> comments are ignored. All the rest is either
2350 converted to literal strings to match, or else is ignored (as is
2351 whitespace and C<#>-style comments if C<//x> is present).
2353 Parsing of the bracketed character class construct, C<[...]>, is
2354 rather different than the rule used for the rest of the pattern.
2355 The terminator of this construct is found using the same rules as
2356 for finding the terminator of a C<{}>-delimited construct, the only
2357 exception being that C<]> immediately following C<[> is treated as
2358 though preceded by a backslash. Similarly, the terminator of
2359 C<(?{...})> is found using the same rules as for finding the
2360 terminator of a C<{}>-delimited construct.
2362 It is possible to inspect both the string given to RE engine and the
2363 resulting finite automaton. See the arguments C<debug>/C<debugcolor>
2364 in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
2365 switch documented in L<perlrun/"Command Switches">.
2367 =item Optimization of regular expressions
2368 X<regexp, optimization>
2370 This step is listed for completeness only. Since it does not change
2371 semantics, details of this step are not documented and are subject
2372 to change without notice. This step is performed over the finite
2373 automaton that was generated during the previous pass.
2375 It is at this stage that C<split()> silently optimizes C</^/> to
2380 =head2 I/O Operators
2381 X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle>
2384 There are several I/O operators you should know about.
2386 A string enclosed by backticks (grave accents) first undergoes
2387 double-quote interpolation. It is then interpreted as an external
2388 command, and the output of that command is the value of the
2389 backtick string, like in a shell. In scalar context, a single string
2390 consisting of all output is returned. In list context, a list of
2391 values is returned, one per line of output. (You can set C<$/> to use
2392 a different line terminator.) The command is executed each time the
2393 pseudo-literal is evaluated. The status value of the command is
2394 returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
2395 Unlike in B<csh>, no translation is done on the return data--newlines
2396 remain newlines. Unlike in any of the shells, single quotes do not
2397 hide variable names in the command from interpretation. To pass a
2398 literal dollar-sign through to the shell you need to hide it with a
2399 backslash. The generalized form of backticks is C<qx//>. (Because
2400 backticks always undergo shell expansion as well, see L<perlsec> for
2402 X<qx> X<`> X<``> X<backtick> X<glob>
2404 In scalar context, evaluating a filehandle in angle brackets yields
2405 the next line from that file (the newline, if any, included), or
2406 C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
2407 (sometimes known as file-slurp mode) and the file is empty, it
2408 returns C<''> the first time, followed by C<undef> subsequently.
2410 Ordinarily you must assign the returned value to a variable, but
2411 there is one situation where an automatic assignment happens. If
2412 and only if the input symbol is the only thing inside the conditional
2413 of a C<while> statement (even if disguised as a C<for(;;)> loop),
2414 the value is automatically assigned to the global variable $_,
2415 destroying whatever was there previously. (This may seem like an
2416 odd thing to you, but you'll use the construct in almost every Perl
2417 script you write.) The $_ variable is not implicitly localized.
2418 You'll have to put a C<local $_;> before the loop if you want that
2421 The following lines are equivalent:
2423 while (defined($_ = <STDIN>)) { print; }
2424 while ($_ = <STDIN>) { print; }
2425 while (<STDIN>) { print; }
2426 for (;<STDIN>;) { print; }
2427 print while defined($_ = <STDIN>);
2428 print while ($_ = <STDIN>);
2429 print while <STDIN>;
2431 This also behaves similarly, but avoids $_ :
2433 while (my $line = <STDIN>) { print $line }
2435 In these loop constructs, the assigned value (whether assignment
2436 is automatic or explicit) is then tested to see whether it is
2437 defined. The defined test avoids problems where line has a string
2438 value that would be treated as false by Perl, for example a "" or
2439 a "0" with no trailing newline. If you really mean for such values
2440 to terminate the loop, they should be tested for explicitly:
2442 while (($_ = <STDIN>) ne '0') { ... }
2443 while (<STDIN>) { last unless $_; ... }
2445 In other boolean contexts, C<< <filehandle> >> without an
2446 explicit C<defined> test or comparison elicits a warning if the
2447 C<use warnings> pragma or the B<-w>
2448 command-line switch (the C<$^W> variable) is in effect.
2450 The filehandles STDIN, STDOUT, and STDERR are predefined. (The
2451 filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
2452 in packages, where they would be interpreted as local identifiers
2453 rather than global.) Additional filehandles may be created with
2454 the open() function, amongst others. See L<perlopentut> and
2455 L<perlfunc/open> for details on this.
2456 X<stdin> X<stdout> X<sterr>
2458 If a <FILEHANDLE> is used in a context that is looking for
2459 a list, a list comprising all input lines is returned, one line per
2460 list element. It's easy to grow to a rather large data space this
2461 way, so use with care.
2463 <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
2464 See L<perlfunc/readline>.
2466 The null filehandle <> is special: it can be used to emulate the
2467 behavior of B<sed> and B<awk>. Input from <> comes either from
2468 standard input, or from each file listed on the command line. Here's
2469 how it works: the first time <> is evaluated, the @ARGV array is
2470 checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
2471 gives you standard input. The @ARGV array is then processed as a list
2472 of filenames. The loop
2475 ... # code for each line
2478 is equivalent to the following Perl-like pseudo code:
2480 unshift(@ARGV, '-') unless @ARGV;
2481 while ($ARGV = shift) {
2484 ... # code for each line
2488 except that it isn't so cumbersome to say, and will actually work.
2489 It really does shift the @ARGV array and put the current filename
2490 into the $ARGV variable. It also uses filehandle I<ARGV>
2491 internally. <> is just a synonym for <ARGV>, which
2492 is magical. (The pseudo code above doesn't work because it treats
2493 <ARGV> as non-magical.)
2495 Since the null filehandle uses the two argument form of L<perlfunc/open>
2496 it interprets special characters, so if you have a script like this:
2502 and call it with C<perl dangerous.pl 'rm -rfv *|'>, it actually opens a
2503 pipe, executes the C<rm> command and reads C<rm>'s output from that pipe.
2504 If you want all items in C<@ARGV> to be interpreted as file names, you
2505 can use the module C<ARGV::readonly> from CPAN.
2507 You can modify @ARGV before the first <> as long as the array ends up
2508 containing the list of filenames you really want. Line numbers (C<$.>)
2509 continue as though the input were one big happy file. See the example
2510 in L<perlfunc/eof> for how to reset line numbers on each file.
2512 If you want to set @ARGV to your own list of files, go right ahead.
2513 This sets @ARGV to all plain text files if no @ARGV was given:
2515 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
2517 You can even set them to pipe commands. For example, this automatically
2518 filters compressed arguments through B<gzip>:
2520 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
2522 If you want to pass switches into your script, you can use one of the
2523 Getopts modules or put a loop on the front like this:
2525 while ($_ = $ARGV[0], /^-/) {
2528 if (/^-D(.*)/) { $debug = $1 }
2529 if (/^-v/) { $verbose++ }
2530 # ... # other switches
2534 # ... # code for each line
2537 The <> symbol will return C<undef> for end-of-file only once.
2538 If you call it again after this, it will assume you are processing another
2539 @ARGV list, and if you haven't set @ARGV, will read input from STDIN.
2541 If what the angle brackets contain is a simple scalar variable (e.g.,
2542 <$foo>), then that variable contains the name of the
2543 filehandle to input from, or its typeglob, or a reference to the
2549 If what's within the angle brackets is neither a filehandle nor a simple
2550 scalar variable containing a filehandle name, typeglob, or typeglob
2551 reference, it is interpreted as a filename pattern to be globbed, and
2552 either a list of filenames or the next filename in the list is returned,
2553 depending on context. This distinction is determined on syntactic
2554 grounds alone. That means C<< <$x> >> is always a readline() from
2555 an indirect handle, but C<< <$hash{key}> >> is always a glob().
2556 That's because $x is a simple scalar variable, but C<$hash{key}> is
2557 not--it's a hash element. Even C<< <$x > >> (note the extra space)
2558 is treated as C<glob("$x ")>, not C<readline($x)>.
2560 One level of double-quote interpretation is done first, but you can't
2561 say C<< <$foo> >> because that's an indirect filehandle as explained
2562 in the previous paragraph. (In older versions of Perl, programmers
2563 would insert curly brackets to force interpretation as a filename glob:
2564 C<< <${foo}> >>. These days, it's considered cleaner to call the
2565 internal function directly as C<glob($foo)>, which is probably the right
2566 way to have done it in the first place.) For example:
2572 is roughly equivalent to:
2574 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
2580 except that the globbing is actually done internally using the standard
2581 C<File::Glob> extension. Of course, the shortest way to do the above is:
2585 A (file)glob evaluates its (embedded) argument only when it is
2586 starting a new list. All values must be read before it will start
2587 over. In list context, this isn't important because you automatically
2588 get them all anyway. However, in scalar context the operator returns
2589 the next value each time it's called, or C<undef> when the list has
2590 run out. As with filehandle reads, an automatic C<defined> is
2591 generated when the glob occurs in the test part of a C<while>,
2592 because legal glob returns (e.g. a file called F<0>) would otherwise
2593 terminate the loop. Again, C<undef> is returned only once. So if
2594 you're expecting a single value from a glob, it is much better to
2597 ($file) = <blurch*>;
2603 because the latter will alternate between returning a filename and
2606 If you're trying to do variable interpolation, it's definitely better
2607 to use the glob() function, because the older notation can cause people
2608 to become confused with the indirect filehandle notation.
2610 @files = glob("$dir/*.[ch]");
2611 @files = glob($files[$i]);
2613 =head2 Constant Folding
2614 X<constant folding> X<folding>
2616 Like C, Perl does a certain amount of expression evaluation at
2617 compile time whenever it determines that all arguments to an
2618 operator are static and have no side effects. In particular, string
2619 concatenation happens at compile time between literals that don't do
2620 variable substitution. Backslash interpolation also happens at
2621 compile time. You can say
2623 'Now is the time for all' . "\n" .
2624 'good men to come to.'
2626 and this all reduces to one string internally. Likewise, if
2629 foreach $file (@filenames) {
2630 if (-s $file > 5 + 100 * 2**16) { }
2633 the compiler will precompute the number which that expression
2634 represents so that the interpreter won't have to.
2639 Perl doesn't officially have a no-op operator, but the bare constants
2640 C<0> and C<1> are special-cased to not produce a warning in a void
2641 context, so you can for example safely do
2645 =head2 Bitwise String Operators
2646 X<operator, bitwise, string>
2648 Bitstrings of any size may be manipulated by the bitwise operators
2651 If the operands to a binary bitwise op are strings of different
2652 sizes, B<|> and B<^> ops act as though the shorter operand had
2653 additional zero bits on the right, while the B<&> op acts as though
2654 the longer operand were truncated to the length of the shorter.
2655 The granularity for such extension or truncation is one or more
2658 # ASCII-based examples
2659 print "j p \n" ^ " a h"; # prints "JAPH\n"
2660 print "JA" | " ph\n"; # prints "japh\n"
2661 print "japh\nJunk" & '_____'; # prints "JAPH\n";
2662 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
2664 If you are intending to manipulate bitstrings, be certain that
2665 you're supplying bitstrings: If an operand is a number, that will imply
2666 a B<numeric> bitwise operation. You may explicitly show which type of
2667 operation you intend by using C<""> or C<0+>, as in the examples below.
2669 $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
2670 $foo = '150' | 105; # yields 255
2671 $foo = 150 | '105'; # yields 255
2672 $foo = '150' | '105'; # yields string '155' (under ASCII)
2674 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
2675 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
2677 See L<perlfunc/vec> for information on how to manipulate individual bits
2680 =head2 Integer Arithmetic
2683 By default, Perl assumes that it must do most of its arithmetic in
2684 floating point. But by saying
2688 you may tell the compiler to use integer operations
2689 (see L<integer> for a detailed explanation) from here to the end of
2690 the enclosing BLOCK. An inner BLOCK may countermand this by saying
2694 which lasts until the end of that BLOCK. Note that this doesn't
2695 mean everything is an integer, merely that Perl will use integer
2696 operations for arithmetic, comparison, and bitwise operators. For
2697 example, even under C<use integer>, if you take the C<sqrt(2)>, you'll
2698 still get C<1.4142135623731> or so.
2700 Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
2701 and ">>") always produce integral results. (But see also
2702 L<Bitwise String Operators>.) However, C<use integer> still has meaning for
2703 them. By default, their results are interpreted as unsigned integers, but
2704 if C<use integer> is in effect, their results are interpreted
2705 as signed integers. For example, C<~0> usually evaluates to a large
2706 integral value. However, C<use integer; ~0> is C<-1> on two's-complement
2709 =head2 Floating-point Arithmetic
2710 X<floating-point> X<floating point> X<float> X<real>
2712 While C<use integer> provides integer-only arithmetic, there is no
2713 analogous mechanism to provide automatic rounding or truncation to a
2714 certain number of decimal places. For rounding to a certain number
2715 of digits, sprintf() or printf() is usually the easiest route.
2718 Floating-point numbers are only approximations to what a mathematician
2719 would call real numbers. There are infinitely more reals than floats,
2720 so some corners must be cut. For example:
2722 printf "%.20g\n", 123456789123456789;
2723 # produces 123456789123456784
2725 Testing for exact floating-point equality or inequality is not a
2726 good idea. Here's a (relatively expensive) work-around to compare
2727 whether two floating-point numbers are equal to a particular number of
2728 decimal places. See Knuth, volume II, for a more robust treatment of
2732 my ($X, $Y, $POINTS) = @_;
2734 $tX = sprintf("%.${POINTS}g", $X);
2735 $tY = sprintf("%.${POINTS}g", $Y);
2739 The POSIX module (part of the standard perl distribution) implements
2740 ceil(), floor(), and other mathematical and trigonometric functions.
2741 The Math::Complex module (part of the standard perl distribution)
2742 defines mathematical functions that work on both the reals and the
2743 imaginary numbers. Math::Complex not as efficient as POSIX, but
2744 POSIX can't work with complex numbers.
2746 Rounding in financial applications can have serious implications, and
2747 the rounding method used should be specified precisely. In these
2748 cases, it probably pays not to trust whichever system rounding is
2749 being used by Perl, but to instead implement the rounding function you
2752 =head2 Bigger Numbers
2753 X<number, arbitrary precision>
2755 The standard Math::BigInt and Math::BigFloat modules provide
2756 variable-precision arithmetic and overloaded operators, although
2757 they're currently pretty slow. At the cost of some space and
2758 considerable speed, they avoid the normal pitfalls associated with
2759 limited-precision representations.
2762 $x = Math::BigInt->new('123456789123456789');
2765 # prints +15241578780673678515622620750190521
2767 There are several modules that let you calculate with (bound only by
2768 memory and cpu-time) unlimited or fixed precision. There are also
2769 some non-standard modules that provide faster implementations via
2770 external C libraries.
2772 Here is a short, but incomplete summary:
2774 Math::Fraction big, unlimited fractions like 9973 / 12967
2775 Math::String treat string sequences like numbers
2776 Math::FixedPrecision calculate with a fixed precision
2777 Math::Currency for currency calculations
2778 Bit::Vector manipulate bit vectors fast (uses C)
2779 Math::BigIntFast Bit::Vector wrapper for big numbers
2780 Math::Pari provides access to the Pari C library
2781 Math::BigInteger uses an external C library
2782 Math::Cephes uses external Cephes C library (no big numbers)
2783 Math::Cephes::Fraction fractions via the Cephes library
2784 Math::GMP another one using an external C library