3 perlop - Perl operators and precedence
7 Perl operators have the following associativity and precedence,
8 listed from highest precedence to lowest. Operators borrowed from
9 C keep the same precedence relationship with each other, even where
10 C's precedence is slightly screwy. (This makes learning Perl easier
11 for C folks.) With very few exceptions, these all operate on scalar
12 values only, not array values.
14 left terms and list operators (leftward)
18 right ! ~ \ and unary + and -
23 nonassoc named unary operators
24 nonassoc < > <= >= lt gt le ge
25 nonassoc == != <=> eq ne cmp
34 nonassoc list operators (rightward)
39 In the following sections, these operators are covered in precedence order.
41 Many operators can be overloaded for objects. See L<overload>.
45 =head2 Terms and List Operators (Leftward)
47 A TERM has the highest precedence in Perl. They include variables,
48 quote and quote-like operators, any expression in parentheses,
49 and any function whose arguments are parenthesized. Actually, there
50 aren't really functions in this sense, just list operators and unary
51 operators behaving as functions because you put parentheses around
52 the arguments. These are all documented in L<perlfunc>.
54 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
55 is followed by a left parenthesis as the next token, the operator and
56 arguments within parentheses are taken to be of highest precedence,
57 just like a normal function call.
59 In the absence of parentheses, the precedence of list operators such as
60 C<print>, C<sort>, or C<chmod> is either very high or very low depending on
61 whether you are looking at the left side or the right side of the operator.
64 @ary = (1, 3, sort 4, 2);
65 print @ary; # prints 1324
67 the commas on the right of the sort are evaluated before the sort,
68 but the commas on the left are evaluated after. In other words,
69 list operators tend to gobble up all arguments that follow, and
70 then act like a simple TERM with regard to the preceding expression.
71 Be careful with parentheses:
73 # These evaluate exit before doing the print:
74 print($foo, exit); # Obviously not what you want.
75 print $foo, exit; # Nor is this.
77 # These do the print before evaluating exit:
78 (print $foo), exit; # This is what you want.
79 print($foo), exit; # Or this.
80 print ($foo), exit; # Or even this.
84 print ($foo & 255) + 1, "\n";
86 probably doesn't do what you expect at first glance. See
87 L<Named Unary Operators> for more discussion of this.
89 Also parsed as terms are the C<do {}> and C<eval {}> constructs, as
90 well as subroutine and method calls, and the anonymous
91 constructors C<[]> and C<{}>.
93 See also L<Quote and Quote-like Operators> toward the end of this section,
94 as well as L<"I/O Operators">.
96 =head2 The Arrow Operator
98 "C<< -> >>" is an infix dereference operator, just as it is in C
99 and C++. If the right side is either a C<[...]>, C<{...}>, or a
100 C<(...)> subscript, then the left side must be either a hard or
101 symbolic reference to an array, a hash, or a subroutine respectively.
102 (Or technically speaking, a location capable of holding a hard
103 reference, if it's an array or hash reference being used for
104 assignment.) See L<perlreftut> and L<perlref>.
106 Otherwise, the right side is a method name or a simple scalar
107 variable containing either the method name or a subroutine reference,
108 and the left side must be either an object (a blessed reference)
109 or a class name (that is, a package name). See L<perlobj>.
111 =head2 Auto-increment and Auto-decrement
113 "++" and "--" work as in C. That is, if placed before a variable, they
114 increment or decrement the variable before returning the value, and if
115 placed after, increment or decrement the variable after returning the value.
117 The auto-increment operator has a little extra builtin magic to it. If
118 you increment a variable that is numeric, or that has ever been used in
119 a numeric context, you get a normal increment. If, however, the
120 variable has been used in only string contexts since it was set, and
121 has a value that is not the empty string and matches the pattern
122 C</^[a-zA-Z]*[0-9]*\z/>, the increment is done as a string, preserving each
123 character within its range, with carry:
125 print ++($foo = '99'); # prints '100'
126 print ++($foo = 'a0'); # prints 'a1'
127 print ++($foo = 'Az'); # prints 'Ba'
128 print ++($foo = 'zz'); # prints 'aaa'
130 C<undef> is always treated as numeric, and in particular is changed
131 to C<0> before incrementing (so that a post-increment of an undef value
132 will return C<0> rather than C<undef>).
134 The auto-decrement operator is not magical.
136 =head2 Exponentiation
138 Binary "**" is the exponentiation operator. It binds even more
139 tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is
140 implemented using C's pow(3) function, which actually works on doubles
143 =head2 Symbolic Unary Operators
145 Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower
146 precedence version of this.
148 Unary "-" performs arithmetic negation if the operand is numeric. If
149 the operand is an identifier, a string consisting of a minus sign
150 concatenated with the identifier is returned. Otherwise, if the string
151 starts with a plus or minus, a string starting with the opposite sign
152 is returned. One effect of these rules is that C<-bareword> is equivalent
155 Unary "~" performs bitwise negation, i.e., 1's complement. For
156 example, C<0666 & ~027> is 0640. (See also L<Integer Arithmetic> and
157 L<Bitwise String Operators>.) Note that the width of the result is
158 platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64
159 bits wide on a 64-bit platform, so if you are expecting a certain bit
160 width, remember use the & operator to mask off the excess bits.
162 Unary "+" has no effect whatsoever, even on strings. It is useful
163 syntactically for separating a function name from a parenthesized expression
164 that would otherwise be interpreted as the complete list of function
165 arguments. (See examples above under L<Terms and List Operators (Leftward)>.)
167 Unary "\" creates a reference to whatever follows it. See L<perlreftut>
168 and L<perlref>. Do not confuse this behavior with the behavior of
169 backslash within a string, although both forms do convey the notion
170 of protecting the next thing from interpolation.
172 =head2 Binding Operators
174 Binary "=~" binds a scalar expression to a pattern match. Certain operations
175 search or modify the string $_ by default. This operator makes that kind
176 of operation work on some other string. The right argument is a search
177 pattern, substitution, or transliteration. The left argument is what is
178 supposed to be searched, substituted, or transliterated instead of the default
179 $_. When used in scalar context, the return value generally indicates the
180 success of the operation. Behavior in list context depends on the particular
181 operator. See L</"Regexp Quote-Like Operators"> for details.
183 If the right argument is an expression rather than a search pattern,
184 substitution, or transliteration, it is interpreted as a search pattern at run
187 Binary "!~" is just like "=~" except the return value is negated in
190 =head2 Multiplicative Operators
192 Binary "*" multiplies two numbers.
194 Binary "/" divides two numbers.
196 Binary "%" computes the modulus of two numbers. Given integer
197 operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
198 C<$a> minus the largest multiple of C<$b> that is not greater than
199 C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
200 smallest multiple of C<$b> that is not less than C<$a> (i.e. the
201 result will be less than or equal to zero).
202 Note that when C<use integer> is in scope, "%" gives you direct access
203 to the modulus operator as implemented by your C compiler. This
204 operator is not as well defined for negative operands, but it will
207 Binary "x" is the repetition operator. In scalar context or if the left
208 operand is not enclosed in parentheses, it returns a string consisting
209 of the left operand repeated the number of times specified by the right
210 operand. In list context, if the left operand is enclosed in
211 parentheses, it repeats the list.
213 print '-' x 80; # print row of dashes
215 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
217 @ones = (1) x 80; # a list of 80 1's
218 @ones = (5) x @ones; # set all elements to 5
221 =head2 Additive Operators
223 Binary "+" returns the sum of two numbers.
225 Binary "-" returns the difference of two numbers.
227 Binary "." concatenates two strings.
229 =head2 Shift Operators
231 Binary "<<" returns the value of its left argument shifted left by the
232 number of bits specified by the right argument. Arguments should be
233 integers. (See also L<Integer Arithmetic>.)
235 Binary ">>" returns the value of its left argument shifted right by
236 the number of bits specified by the right argument. Arguments should
237 be integers. (See also L<Integer Arithmetic>.)
239 Note that both "<<" and ">>" in Perl are implemented directly using
240 "<<" and ">>" in C. If C<use integer> (see L<Integer Arithmetic>) is
241 in force then signed C integers are used, else unsigned C integers are
242 used. Either way, the implementation isn't going to generate results
243 larger than the size of the integer type Perl was built with (32 bits
246 The result of overflowing the range of the integers is undefined
247 because it is undefined also in C. In other words, using 32-bit
248 integers, C<< 1 << 32 >> is undefined. Shifting by a negative number
249 of bits is also undefined.
251 =head2 Named Unary Operators
253 The various named unary operators are treated as functions with one
254 argument, with optional parentheses. These include the filetest
255 operators, like C<-f>, C<-M>, etc. See L<perlfunc>.
257 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
258 is followed by a left parenthesis as the next token, the operator and
259 arguments within parentheses are taken to be of highest precedence,
260 just like a normal function call. For example,
261 because named unary operators are higher precedence than ||:
263 chdir $foo || die; # (chdir $foo) || die
264 chdir($foo) || die; # (chdir $foo) || die
265 chdir ($foo) || die; # (chdir $foo) || die
266 chdir +($foo) || die; # (chdir $foo) || die
268 but, because * is higher precedence than named operators:
270 chdir $foo * 20; # chdir ($foo * 20)
271 chdir($foo) * 20; # (chdir $foo) * 20
272 chdir ($foo) * 20; # (chdir $foo) * 20
273 chdir +($foo) * 20; # chdir ($foo * 20)
275 rand 10 * 20; # rand (10 * 20)
276 rand(10) * 20; # (rand 10) * 20
277 rand (10) * 20; # (rand 10) * 20
278 rand +(10) * 20; # rand (10 * 20)
280 See also L<"Terms and List Operators (Leftward)">.
282 =head2 Relational Operators
284 Binary "<" returns true if the left argument is numerically less than
287 Binary ">" returns true if the left argument is numerically greater
288 than the right argument.
290 Binary "<=" returns true if the left argument is numerically less than
291 or equal to the right argument.
293 Binary ">=" returns true if the left argument is numerically greater
294 than or equal to the right argument.
296 Binary "lt" returns true if the left argument is stringwise less than
299 Binary "gt" returns true if the left argument is stringwise greater
300 than the right argument.
302 Binary "le" returns true if the left argument is stringwise less than
303 or equal to the right argument.
305 Binary "ge" returns true if the left argument is stringwise greater
306 than or equal to the right argument.
308 =head2 Equality Operators
310 Binary "==" returns true if the left argument is numerically equal to
313 Binary "!=" returns true if the left argument is numerically not equal
314 to the right argument.
316 Binary "<=>" returns -1, 0, or 1 depending on whether the left
317 argument is numerically less than, equal to, or greater than the right
318 argument. If your platform supports NaNs (not-a-numbers) as numeric
319 values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
320 "<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
321 returns true, as does NaN != anything else. If your platform doesn't
322 support NaNs then NaN is just a string with numeric value 0.
324 perl -le '$a = NaN; print "No NaN support here" if $a == $a'
325 perl -le '$a = NaN; print "NaN support here" if $a != $a'
327 Binary "eq" returns true if the left argument is stringwise equal to
330 Binary "ne" returns true if the left argument is stringwise not equal
331 to the right argument.
333 Binary "cmp" returns -1, 0, or 1 depending on whether the left
334 argument is stringwise less than, equal to, or greater than the right
337 "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
338 by the current locale if C<use locale> is in effect. See L<perllocale>.
342 Binary "&" returns its operands ANDed together bit by bit.
343 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
345 Note that "&" has lower priority than relational operators, so for example
346 the brackets are essential in a test like
348 print "Even\n" if ($x & 1) == 0;
350 =head2 Bitwise Or and Exclusive Or
352 Binary "|" returns its operands ORed together bit by bit.
353 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
355 Binary "^" returns its operands XORed together bit by bit.
356 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
358 Note that "|" and "^" have lower priority than relational operators, so
359 for example the brackets are essential in a test like
361 print "false\n" if (8 | 2) != 10;
363 =head2 C-style Logical And
365 Binary "&&" performs a short-circuit logical AND operation. That is,
366 if the left operand is false, the right operand is not even evaluated.
367 Scalar or list context propagates down to the right operand if it
370 =head2 C-style Logical Or
372 Binary "||" performs a short-circuit logical OR operation. That is,
373 if the left operand is true, the right operand is not even evaluated.
374 Scalar or list context propagates down to the right operand if it
377 =head2 C-style Logical Defined-Or
379 Although it has no direct equivalent in C, Perl's C<//> operator is related
380 to its C-style or. In fact, it's exactly the same as C<||>, except that it
381 tests the left hand side's definedness instead of its truth. Thus, C<$a // $b>
382 is similar to C<defined($a) || $b> (except that it returns the value of C<$a>
383 rather than the value of C<defined($a)>) and is exactly equivalent to
384 C<defined($a) ? $a : $b>. This is very useful for providing default values
385 for variables. If you actually want to test if at least one of C<$a> and C<$b> is
386 defined, use C<defined($a // $b)>.
388 The C<||>, C<//> and C<&&> operators differ from C's in that, rather than returning
389 0 or 1, they return the last value evaluated. Thus, a reasonably portable
390 way to find out the home directory might be:
392 $home = $ENV{'HOME'} // $ENV{'LOGDIR'} //
393 (getpwuid($<))[7] // die "You're homeless!\n";
395 In particular, this means that you shouldn't use this
396 for selecting between two aggregates for assignment:
398 @a = @b || @c; # this is wrong
399 @a = scalar(@b) || @c; # really meant this
400 @a = @b ? @b : @c; # this works fine, though
402 As more readable alternatives to C<&&>, C<//> and C<||> when used for
403 control flow, Perl provides C<and>, C<err> and C<or> operators (see below).
404 The short-circuit behavior is identical. The precedence of "and", "err"
405 and "or" is much lower, however, so that you can safely use them after a
406 list operator without the need for parentheses:
408 unlink "alpha", "beta", "gamma"
409 or gripe(), next LINE;
411 With the C-style operators that would have been written like this:
413 unlink("alpha", "beta", "gamma")
414 || (gripe(), next LINE);
416 Using "or" for assignment is unlikely to do what you want; see below.
418 =head2 Range Operators
420 Binary ".." is the range operator, which is really two different
421 operators depending on the context. In list context, it returns a
422 list of values counting (up by ones) from the left value to the right
423 value. If the left value is greater than the right value then it
424 returns the empty list. The range operator is useful for writing
425 C<foreach (1..10)> loops and for doing slice operations on arrays. In
426 the current implementation, no temporary array is created when the
427 range operator is used as the expression in C<foreach> loops, but older
428 versions of Perl might burn a lot of memory when you write something
431 for (1 .. 1_000_000) {
435 The range operator also works on strings, using the magical auto-increment,
438 In scalar context, ".." returns a boolean value. The operator is
439 bistable, like a flip-flop, and emulates the line-range (comma) operator
440 of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
441 own boolean state. It is false as long as its left operand is false.
442 Once the left operand is true, the range operator stays true until the
443 right operand is true, I<AFTER> which the range operator becomes false
444 again. It doesn't become false till the next time the range operator is
445 evaluated. It can test the right operand and become false on the same
446 evaluation it became true (as in B<awk>), but it still returns true once.
447 If you don't want it to test the right operand till the next
448 evaluation, as in B<sed>, just use three dots ("...") instead of
449 two. In all other regards, "..." behaves just like ".." does.
451 The right operand is not evaluated while the operator is in the
452 "false" state, and the left operand is not evaluated while the
453 operator is in the "true" state. The precedence is a little lower
454 than || and &&. The value returned is either the empty string for
455 false, or a sequence number (beginning with 1) for true. The
456 sequence number is reset for each range encountered. The final
457 sequence number in a range has the string "E0" appended to it, which
458 doesn't affect its numeric value, but gives you something to search
459 for if you want to exclude the endpoint. You can exclude the
460 beginning point by waiting for the sequence number to be greater
463 If either operand of scalar ".." is a constant expression,
464 that operand is considered true if it is equal (C<==>) to the current
465 input line number (the C<$.> variable).
467 To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>,
468 but that is only an issue if you use a floating point expression; when
469 implicitly using C<$.> as described in the previous paragraph, the
470 comparison is C<int(EXPR) == int($.)> which is only an issue when C<$.>
471 is set to a floating point value and you are not reading from a file.
472 Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what
473 you want in scalar context because each of the operands are evaluated
474 using their integer representation.
478 As a scalar operator:
480 if (101 .. 200) { print; } # print 2nd hundred lines, short for
481 # if ($. == 101 .. $. == 200) ...
482 next line if (1 .. /^$/); # skip header lines, short for
483 # ... if ($. == 1 .. /^$/);
484 s/^/> / if (/^$/ .. eof()); # quote body
486 # parse mail messages
488 $in_header = 1 .. /^$/;
489 $in_body = /^$/ .. eof;
496 close ARGV if eof; # reset $. each file
501 for (101 .. 200) { print; } # print $_ 100 times
502 @foo = @foo[0 .. $#foo]; # an expensive no-op
503 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
505 The range operator (in list context) makes use of the magical
506 auto-increment algorithm if the operands are strings. You
509 @alphabet = ('A' .. 'Z');
511 to get all normal letters of the English alphabet, or
513 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
515 to get a hexadecimal digit, or
517 @z2 = ('01' .. '31'); print $z2[$mday];
519 to get dates with leading zeros. If the final value specified is not
520 in the sequence that the magical increment would produce, the sequence
521 goes until the next value would be longer than the final value
524 Because each operand is evaluated in integer form, C<2.18 .. 3.14> will
525 return two elements in list context.
527 @list = (2.18 .. 3.14); # same as @list = (2 .. 3);
529 =head2 Conditional Operator
531 Ternary "?:" is the conditional operator, just as in C. It works much
532 like an if-then-else. If the argument before the ? is true, the
533 argument before the : is returned, otherwise the argument after the :
534 is returned. For example:
536 printf "I have %d dog%s.\n", $n,
537 ($n == 1) ? '' : "s";
539 Scalar or list context propagates downward into the 2nd
540 or 3rd argument, whichever is selected.
542 $a = $ok ? $b : $c; # get a scalar
543 @a = $ok ? @b : @c; # get an array
544 $a = $ok ? @b : @c; # oops, that's just a count!
546 The operator may be assigned to if both the 2nd and 3rd arguments are
547 legal lvalues (meaning that you can assign to them):
549 ($a_or_b ? $a : $b) = $c;
551 Because this operator produces an assignable result, using assignments
552 without parentheses will get you in trouble. For example, this:
554 $a % 2 ? $a += 10 : $a += 2
558 (($a % 2) ? ($a += 10) : $a) += 2
562 ($a % 2) ? ($a += 10) : ($a += 2)
564 That should probably be written more simply as:
566 $a += ($a % 2) ? 10 : 2;
568 =head2 Assignment Operators
570 "=" is the ordinary assignment operator.
572 Assignment operators work as in C. That is,
580 although without duplicating any side effects that dereferencing the lvalue
581 might trigger, such as from tie(). Other assignment operators work similarly.
582 The following are recognized:
589 Although these are grouped by family, they all have the precedence
592 Unlike in C, the scalar assignment operator produces a valid lvalue.
593 Modifying an assignment is equivalent to doing the assignment and
594 then modifying the variable that was assigned to. This is useful
595 for modifying a copy of something, like this:
597 ($tmp = $global) =~ tr [A-Z] [a-z];
608 Similarly, a list assignment in list context produces the list of
609 lvalues assigned to, and a list assignment in scalar context returns
610 the number of elements produced by the expression on the right hand
611 side of the assignment.
613 =head2 Comma Operator
615 Binary "," is the comma operator. In scalar context it evaluates
616 its left argument, throws that value away, then evaluates its right
617 argument and returns that value. This is just like C's comma operator.
619 In list context, it's just the list argument separator, and inserts
620 both its arguments into the list.
622 The => digraph is mostly just a synonym for the comma operator. It's useful for
623 documenting arguments that come in pairs. As of release 5.001, it also forces
624 any word to the left of it to be interpreted as a string.
626 =head2 List Operators (Rightward)
628 On the right side of a list operator, it has very low precedence,
629 such that it controls all comma-separated expressions found there.
630 The only operators with lower precedence are the logical operators
631 "and", "or", and "not", which may be used to evaluate calls to list
632 operators without the need for extra parentheses:
634 open HANDLE, "filename"
635 or die "Can't open: $!\n";
637 See also discussion of list operators in L<Terms and List Operators (Leftward)>.
641 Unary "not" returns the logical negation of the expression to its right.
642 It's the equivalent of "!" except for the very low precedence.
646 Binary "and" returns the logical conjunction of the two surrounding
647 expressions. It's equivalent to && except for the very low
648 precedence. This means that it short-circuits: i.e., the right
649 expression is evaluated only if the left expression is true.
651 =head2 Logical or, Defined or, and Exclusive Or
653 Binary "or" returns the logical disjunction of the two surrounding
654 expressions. It's equivalent to || except for the very low precedence.
655 This makes it useful for control flow
657 print FH $data or die "Can't write to FH: $!";
659 This means that it short-circuits: i.e., the right expression is evaluated
660 only if the left expression is false. Due to its precedence, you should
661 probably avoid using this for assignment, only for control flow.
663 $a = $b or $c; # bug: this is wrong
664 ($a = $b) or $c; # really means this
665 $a = $b || $c; # better written this way
667 However, when it's a list-context assignment and you're trying to use
668 "||" for control flow, you probably need "or" so that the assignment
669 takes higher precedence.
671 @info = stat($file) || die; # oops, scalar sense of stat!
672 @info = stat($file) or die; # better, now @info gets its due
674 Then again, you could always use parentheses.
676 Binary "err" is equivalent to C<//>--it's just like binary "or", except it tests
677 its left argument's definedness instead of its truth. There are two ways to
678 remember "err": either because many functions return C<undef> on an B<err>or,
679 or as a sort of correction: C<$a=($b err 'default')>
681 Binary "xor" returns the exclusive-OR of the two surrounding expressions.
682 It cannot short circuit, of course.
684 =head2 C Operators Missing From Perl
686 Here is what C has that Perl doesn't:
692 Address-of operator. (But see the "\" operator for taking a reference.)
696 Dereference-address operator. (Perl's prefix dereferencing
697 operators are typed: $, @, %, and &.)
701 Type-casting operator.
705 =head2 Quote and Quote-like Operators
707 While we usually think of quotes as literal values, in Perl they
708 function as operators, providing various kinds of interpolating and
709 pattern matching capabilities. Perl provides customary quote characters
710 for these behaviors, but also provides a way for you to choose your
711 quote character for any of them. In the following table, a C<{}> represents
712 any pair of delimiters you choose.
714 Customary Generic Meaning Interpolates
719 // m{} Pattern match yes*
721 s{}{} Substitution yes*
722 tr{}{} Transliteration no (but see below)
725 * unless the delimiter is ''.
727 Non-bracketing delimiters use the same character fore and aft, but the four
728 sorts of brackets (round, angle, square, curly) will all nest, which means
737 Note, however, that this does not always work for quoting Perl code:
739 $s = q{ if($a eq "}") ... }; # WRONG
741 is a syntax error. The C<Text::Balanced> module (from CPAN, and
742 starting from Perl 5.8 part of the standard distribution) is able
745 There can be whitespace between the operator and the quoting
746 characters, except when C<#> is being used as the quoting character.
747 C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
748 operator C<q> followed by a comment. Its argument will be taken
749 from the next line. This allows you to write:
751 s {foo} # Replace foo
754 The following escape sequences are available in constructs that interpolate
755 and in transliterations.
762 \a alarm (bell) (BEL)
764 \033 octal char (ESC)
766 \x{263a} wide hex char (SMILEY)
767 \c[ control char (ESC)
768 \N{name} named Unicode character
770 The following escape sequences are available in constructs that interpolate
771 but not in transliterations.
773 \l lowercase next char
774 \u uppercase next char
777 \E end case modification
778 \Q quote non-word characters till \E
780 If C<use locale> is in effect, the case map used by C<\l>, C<\L>,
781 C<\u> and C<\U> is taken from the current locale. See L<perllocale>.
782 If Unicode (for example, C<\N{}> or wide hex characters of 0x100 or
783 beyond) is being used, the case map used by C<\l>, C<\L>, C<\u> and
784 C<\U> is as defined by Unicode. For documentation of C<\N{name}>,
787 All systems use the virtual C<"\n"> to represent a line terminator,
788 called a "newline". There is no such thing as an unvarying, physical
789 newline character. It is only an illusion that the operating system,
790 device drivers, C libraries, and Perl all conspire to preserve. Not all
791 systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
792 on a Mac, these are reversed, and on systems without line terminator,
793 printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
794 you mean a "newline" for your system, but use the literal ASCII when you
795 need an exact character. For example, most networking protocols expect
796 and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
797 and although they often accept just C<"\012">, they seldom tolerate just
798 C<"\015">. If you get in the habit of using C<"\n"> for networking,
799 you may be burned some day.
801 For constructs that do interpolate, variables beginning with "C<$>"
802 or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
803 C<< $href->{key}[0] >> are also interpolated, as are array and hash slices.
804 But method calls such as C<< $obj->meth >> are not.
806 Interpolating an array or slice interpolates the elements in order,
807 separated by the value of C<$">, so is equivalent to interpolating
808 C<join $", @array>. "Punctuation" arrays such as C<@+> are only
809 interpolated if the name is enclosed in braces C<@{+}>.
811 You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
812 An unescaped C<$> or C<@> interpolates the corresponding variable,
813 while escaping will cause the literal string C<\$> to be inserted.
814 You'll need to write something like C<m/\Quser\E\@\Qhost/>.
816 Patterns are subject to an additional level of interpretation as a
817 regular expression. This is done as a second pass, after variables are
818 interpolated, so that regular expressions may be incorporated into the
819 pattern from the variables. If this is not what you want, use C<\Q> to
820 interpolate a variable literally.
822 Apart from the behavior described above, Perl does not expand
823 multiple levels of interpolation. In particular, contrary to the
824 expectations of shell programmers, back-quotes do I<NOT> interpolate
825 within double quotes, nor do single quotes impede evaluation of
826 variables when used within double quotes.
828 =head2 Regexp Quote-Like Operators
830 Here are the quote-like operators that apply to pattern
831 matching and related activities.
837 This is just like the C</pattern/> search, except that it matches only
838 once between calls to the reset() operator. This is a useful
839 optimization when you want to see only the first occurrence of
840 something in each file of a set of files, for instance. Only C<??>
841 patterns local to the current package are reset.
845 # blank line between header and body
848 reset if eof; # clear ?? status for next file
851 This usage is vaguely deprecated, which means it just might possibly
852 be removed in some distant future version of Perl, perhaps somewhere
853 around the year 2168.
855 =item m/PATTERN/cgimosx
857 =item /PATTERN/cgimosx
859 Searches a string for a pattern match, and in scalar context returns
860 true if it succeeds, false if it fails. If no string is specified
861 via the C<=~> or C<!~> operator, the $_ string is searched. (The
862 string specified with C<=~> need not be an lvalue--it may be the
863 result of an expression evaluation, but remember the C<=~> binds
864 rather tightly.) See also L<perlre>. See L<perllocale> for
865 discussion of additional considerations that apply when C<use locale>
870 c Do not reset search position on a failed match when /g is in effect.
871 g Match globally, i.e., find all occurrences.
872 i Do case-insensitive pattern matching.
873 m Treat string as multiple lines.
874 o Compile pattern only once.
875 s Treat string as single line.
876 x Use extended regular expressions.
878 If "/" is the delimiter then the initial C<m> is optional. With the C<m>
879 you can use any pair of non-alphanumeric, non-whitespace characters
880 as delimiters. This is particularly useful for matching path names
881 that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
882 the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
883 If "'" is the delimiter, no interpolation is performed on the PATTERN.
885 PATTERN may contain variables, which will be interpolated (and the
886 pattern recompiled) every time the pattern search is evaluated, except
887 for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
888 C<$|> are not interpolated because they look like end-of-string tests.)
889 If you want such a pattern to be compiled only once, add a C</o> after
890 the trailing delimiter. This avoids expensive run-time recompilations,
891 and is useful when the value you are interpolating won't change over
892 the life of the script. However, mentioning C</o> constitutes a promise
893 that you won't change the variables in the pattern. If you change them,
894 Perl won't even notice. See also L<"qr/STRING/imosx">.
896 If the PATTERN evaluates to the empty string, the last
897 I<successfully> matched regular expression is used instead. In this
898 case, only the C<g> and C<c> flags on the empty pattern is honoured -
899 the other flags are taken from the original pattern. If no match has
900 previously succeeded, this will (silently) act instead as a genuine
901 empty pattern (which will always match).
903 Note that it's possible to confuse Perl into thinking C<//> (the empty
904 regex) is really C<//> (the defined-or operator). Perl is usually pretty
905 good about this, but some pathological cases might trigger this, such as
906 C<$a///> (is that C<($a) / (//)> or C<$a // />?) and C<print $fh //>
907 (C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl
908 will assume you meant defined-or. If you meant the empty regex, just
909 use parentheses or spaces to disambiguate, or even prefix the empty
910 regex with an C<m> (so C<//> becomes C<m//>).
912 If the C</g> option is not used, C<m//> in list context returns a
913 list consisting of the subexpressions matched by the parentheses in the
914 pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are
915 also set, and that this differs from Perl 4's behavior.) When there are
916 no parentheses in the pattern, the return value is the list C<(1)> for
917 success. With or without parentheses, an empty list is returned upon
922 open(TTY, '/dev/tty');
923 <TTY> =~ /^y/i && foo(); # do foo if desired
925 if (/Version: *([0-9.]*)/) { $version = $1; }
927 next if m#^/usr/spool/uucp#;
932 print if /$arg/o; # compile only once
935 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
937 This last example splits $foo into the first two words and the
938 remainder of the line, and assigns those three fields to $F1, $F2, and
939 $Etc. The conditional is true if any variables were assigned, i.e., if
942 The C</g> modifier specifies global pattern matching--that is,
943 matching as many times as possible within the string. How it behaves
944 depends on the context. In list context, it returns a list of the
945 substrings matched by any capturing parentheses in the regular
946 expression. If there are no parentheses, it returns a list of all
947 the matched strings, as if there were parentheses around the whole
950 In scalar context, each execution of C<m//g> finds the next match,
951 returning true if it matches, and false if there is no further match.
952 The position after the last match can be read or set using the pos()
953 function; see L<perlfunc/pos>. A failed match normally resets the
954 search position to the beginning of the string, but you can avoid that
955 by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
956 string also resets the search position.
958 You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
959 zero-width assertion that matches the exact position where the previous
960 C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion
961 still anchors at pos(), but the match is of course only attempted once.
962 Using C<\G> without C</g> on a target string that has not previously had a
963 C</g> match applied to it is the same as using the C<\A> assertion to match
964 the beginning of the string. Note also that, currently, C<\G> is only
965 properly supported when anchored at the very beginning of the pattern.
970 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
974 while (defined($paragraph = <>)) {
975 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
979 print "$sentences\n";
981 # using m//gc with \G
985 print $1 while /(o)/gc; print "', pos=", pos, "\n";
987 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
989 print $1 while /(p)/gc; print "', pos=", pos, "\n";
991 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
993 The last example should print:
1003 Notice that the final match matched C<q> instead of C<p>, which a match
1004 without the C<\G> anchor would have done. Also note that the final match
1005 did not update C<pos> -- C<pos> is only updated on a C</g> match. If the
1006 final match did indeed match C<p>, it's a good bet that you're running an
1007 older (pre-5.6.0) Perl.
1009 A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
1010 combine several regexps like this to process a string part-by-part,
1011 doing different actions depending on which regexp matched. Each
1012 regexp tries to match where the previous one leaves off.
1015 $url = new URI::URL "http://www/"; die if $url eq "xXx";
1019 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
1020 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
1021 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
1022 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
1023 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
1024 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
1025 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
1026 print ". That's all!\n";
1029 Here is the output (split into several lines):
1031 line-noise lowercase line-noise lowercase UPPERCASE line-noise
1032 UPPERCASE line-noise lowercase line-noise lowercase line-noise
1033 lowercase lowercase line-noise lowercase lowercase line-noise
1034 MiXeD line-noise. That's all!
1040 A single-quoted, literal string. A backslash represents a backslash
1041 unless followed by the delimiter or another backslash, in which case
1042 the delimiter or backslash is interpolated.
1044 $foo = q!I said, "You said, 'She said it.'"!;
1045 $bar = q('This is it.');
1046 $baz = '\n'; # a two-character string
1052 A double-quoted, interpolated string.
1055 (*** The previous line contains the naughty word "$1".\n)
1056 if /\b(tcl|java|python)\b/i; # :-)
1057 $baz = "\n"; # a one-character string
1059 =item qr/STRING/imosx
1061 This operator quotes (and possibly compiles) its I<STRING> as a regular
1062 expression. I<STRING> is interpolated the same way as I<PATTERN>
1063 in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
1064 is done. Returns a Perl value which may be used instead of the
1065 corresponding C</STRING/imosx> expression.
1069 $rex = qr/my.STRING/is;
1076 The result may be used as a subpattern in a match:
1079 $string =~ /foo${re}bar/; # can be interpolated in other patterns
1080 $string =~ $re; # or used standalone
1081 $string =~ /$re/; # or this way
1083 Since Perl may compile the pattern at the moment of execution of qr()
1084 operator, using qr() may have speed advantages in some situations,
1085 notably if the result of qr() is used standalone:
1088 my $patterns = shift;
1089 my @compiled = map qr/$_/i, @$patterns;
1092 foreach my $pat (@compiled) {
1093 $success = 1, last if /$pat/;
1099 Precompilation of the pattern into an internal representation at
1100 the moment of qr() avoids a need to recompile the pattern every
1101 time a match C</$pat/> is attempted. (Perl has many other internal
1102 optimizations, but none would be triggered in the above example if
1103 we did not use qr() operator.)
1107 i Do case-insensitive pattern matching.
1108 m Treat string as multiple lines.
1109 o Compile pattern only once.
1110 s Treat string as single line.
1111 x Use extended regular expressions.
1113 See L<perlre> for additional information on valid syntax for STRING, and
1114 for a detailed look at the semantics of regular expressions.
1120 A string which is (possibly) interpolated and then executed as a
1121 system command with C</bin/sh> or its equivalent. Shell wildcards,
1122 pipes, and redirections will be honored. The collected standard
1123 output of the command is returned; standard error is unaffected. In
1124 scalar context, it comes back as a single (potentially multi-line)
1125 string, or undef if the command failed. In list context, returns a
1126 list of lines (however you've defined lines with $/ or
1127 $INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
1129 Because backticks do not affect standard error, use shell file descriptor
1130 syntax (assuming the shell supports this) if you care to address this.
1131 To capture a command's STDERR and STDOUT together:
1133 $output = `cmd 2>&1`;
1135 To capture a command's STDOUT but discard its STDERR:
1137 $output = `cmd 2>/dev/null`;
1139 To capture a command's STDERR but discard its STDOUT (ordering is
1142 $output = `cmd 2>&1 1>/dev/null`;
1144 To exchange a command's STDOUT and STDERR in order to capture the STDERR
1145 but leave its STDOUT to come out the old STDERR:
1147 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
1149 To read both a command's STDOUT and its STDERR separately, it's easiest
1150 and safest to redirect them separately to files, and then read from those
1151 files when the program is done:
1153 system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr");
1155 Using single-quote as a delimiter protects the command from Perl's
1156 double-quote interpolation, passing it on to the shell instead:
1158 $perl_info = qx(ps $$); # that's Perl's $$
1159 $shell_info = qx'ps $$'; # that's the new shell's $$
1161 How that string gets evaluated is entirely subject to the command
1162 interpreter on your system. On most platforms, you will have to protect
1163 shell metacharacters if you want them treated literally. This is in
1164 practice difficult to do, as it's unclear how to escape which characters.
1165 See L<perlsec> for a clean and safe example of a manual fork() and exec()
1166 to emulate backticks safely.
1168 On some platforms (notably DOS-like ones), the shell may not be
1169 capable of dealing with multiline commands, so putting newlines in
1170 the string may not get you what you want. You may be able to evaluate
1171 multiple commands in a single line by separating them with the command
1172 separator character, if your shell supports that (e.g. C<;> on many Unix
1173 shells; C<&> on the Windows NT C<cmd> shell).
1175 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1176 output before starting the child process, but this may not be supported
1177 on some platforms (see L<perlport>). To be safe, you may need to set
1178 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1179 C<IO::Handle> on any open handles.
1181 Beware that some command shells may place restrictions on the length
1182 of the command line. You must ensure your strings don't exceed this
1183 limit after any necessary interpolations. See the platform-specific
1184 release notes for more details about your particular environment.
1186 Using this operator can lead to programs that are difficult to port,
1187 because the shell commands called vary between systems, and may in
1188 fact not be present at all. As one example, the C<type> command under
1189 the POSIX shell is very different from the C<type> command under DOS.
1190 That doesn't mean you should go out of your way to avoid backticks
1191 when they're the right way to get something done. Perl was made to be
1192 a glue language, and one of the things it glues together is commands.
1193 Just understand what you're getting yourself into.
1195 See L<"I/O Operators"> for more discussion.
1199 Evaluates to a list of the words extracted out of STRING, using embedded
1200 whitespace as the word delimiters. It can be understood as being roughly
1203 split(' ', q/STRING/);
1205 the differences being that it generates a real list at compile time, and
1206 in scalar context it returns the last element in the list. So
1211 is semantically equivalent to the list:
1215 Some frequently seen examples:
1217 use POSIX qw( setlocale localeconv )
1218 @EXPORT = qw( foo bar baz );
1220 A common mistake is to try to separate the words with comma or to
1221 put comments into a multi-line C<qw>-string. For this reason, the
1222 C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
1223 produces warnings if the STRING contains the "," or the "#" character.
1225 =item s/PATTERN/REPLACEMENT/egimosx
1227 Searches a string for a pattern, and if found, replaces that pattern
1228 with the replacement text and returns the number of substitutions
1229 made. Otherwise it returns false (specifically, the empty string).
1231 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
1232 variable is searched and modified. (The string specified with C<=~> must
1233 be scalar variable, an array element, a hash element, or an assignment
1234 to one of those, i.e., an lvalue.)
1236 If the delimiter chosen is a single quote, no interpolation is
1237 done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1238 PATTERN contains a $ that looks like a variable rather than an
1239 end-of-string test, the variable will be interpolated into the pattern
1240 at run-time. If you want the pattern compiled only once the first time
1241 the variable is interpolated, use the C</o> option. If the pattern
1242 evaluates to the empty string, the last successfully executed regular
1243 expression is used instead. See L<perlre> for further explanation on these.
1244 See L<perllocale> for discussion of additional considerations that apply
1245 when C<use locale> is in effect.
1249 e Evaluate the right side as an expression.
1250 g Replace globally, i.e., all occurrences.
1251 i Do case-insensitive pattern matching.
1252 m Treat string as multiple lines.
1253 o Compile pattern only once.
1254 s Treat string as single line.
1255 x Use extended regular expressions.
1257 Any non-alphanumeric, non-whitespace delimiter may replace the
1258 slashes. If single quotes are used, no interpretation is done on the
1259 replacement string (the C</e> modifier overrides this, however). Unlike
1260 Perl 4, Perl 5 treats backticks as normal delimiters; the replacement
1261 text is not evaluated as a command. If the
1262 PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
1263 pair of quotes, which may or may not be bracketing quotes, e.g.,
1264 C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
1265 replacement portion to be treated as a full-fledged Perl expression
1266 and evaluated right then and there. It is, however, syntax checked at
1267 compile-time. A second C<e> modifier will cause the replacement portion
1268 to be C<eval>ed before being run as a Perl expression.
1272 s/\bgreen\b/mauve/g; # don't change wintergreen
1274 $path =~ s|/usr/bin|/usr/local/bin|;
1276 s/Login: $foo/Login: $bar/; # run-time pattern
1278 ($foo = $bar) =~ s/this/that/; # copy first, then change
1280 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
1283 s/\d+/$&*2/e; # yields 'abc246xyz'
1284 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
1285 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1287 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
1288 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
1289 s/^=(\w+)/&pod($1)/ge; # use function call
1291 # expand variables in $_, but dynamics only, using
1292 # symbolic dereferencing
1295 # Add one to the value of any numbers in the string
1298 # This will expand any embedded scalar variable
1299 # (including lexicals) in $_ : First $1 is interpolated
1300 # to the variable name, and then evaluated
1303 # Delete (most) C comments.
1305 /\* # Match the opening delimiter.
1306 .*? # Match a minimal number of characters.
1307 \*/ # Match the closing delimiter.
1310 s/^\s*(.*?)\s*$/$1/; # trim white space in $_, expensively
1312 for ($variable) { # trim white space in $variable, cheap
1317 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
1319 Note the use of $ instead of \ in the last example. Unlike
1320 B<sed>, we use the \<I<digit>> form in only the left hand side.
1321 Anywhere else it's $<I<digit>>.
1323 Occasionally, you can't use just a C</g> to get all the changes
1324 to occur that you might want. Here are two common cases:
1326 # put commas in the right places in an integer
1327 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
1329 # expand tabs to 8-column spacing
1330 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
1332 =item tr/SEARCHLIST/REPLACEMENTLIST/cds
1334 =item y/SEARCHLIST/REPLACEMENTLIST/cds
1336 Transliterates all occurrences of the characters found in the search list
1337 with the corresponding character in the replacement list. It returns
1338 the number of characters replaced or deleted. If no string is
1339 specified via the =~ or !~ operator, the $_ string is transliterated. (The
1340 string specified with =~ must be a scalar variable, an array element, a
1341 hash element, or an assignment to one of those, i.e., an lvalue.)
1343 A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
1344 does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
1345 For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1346 SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1347 its own pair of quotes, which may or may not be bracketing quotes,
1348 e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
1350 Note that C<tr> does B<not> do regular expression character classes
1351 such as C<\d> or C<[:lower:]>. The <tr> operator is not equivalent to
1352 the tr(1) utility. If you want to map strings between lower/upper
1353 cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider
1354 using the C<s> operator if you need regular expressions.
1356 Note also that the whole range idea is rather unportable between
1357 character sets--and even within character sets they may cause results
1358 you probably didn't expect. A sound principle is to use only ranges
1359 that begin from and end at either alphabets of equal case (a-e, A-E),
1360 or digits (0-4). Anything else is unsafe. If in doubt, spell out the
1361 character sets in full.
1365 c Complement the SEARCHLIST.
1366 d Delete found but unreplaced characters.
1367 s Squash duplicate replaced characters.
1369 If the C</c> modifier is specified, the SEARCHLIST character set
1370 is complemented. If the C</d> modifier is specified, any characters
1371 specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
1372 (Note that this is slightly more flexible than the behavior of some
1373 B<tr> programs, which delete anything they find in the SEARCHLIST,
1374 period.) If the C</s> modifier is specified, sequences of characters
1375 that were transliterated to the same character are squashed down
1376 to a single instance of the character.
1378 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1379 exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1380 than the SEARCHLIST, the final character is replicated till it is long
1381 enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
1382 This latter is useful for counting characters in a class or for
1383 squashing character sequences in a class.
1387 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1389 $cnt = tr/*/*/; # count the stars in $_
1391 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1393 $cnt = tr/0-9//; # count the digits in $_
1395 tr/a-zA-Z//s; # bookkeeper -> bokeper
1397 ($HOST = $host) =~ tr/a-z/A-Z/;
1399 tr/a-zA-Z/ /cs; # change non-alphas to single space
1402 [\000-\177]; # delete 8th bit
1404 If multiple transliterations are given for a character, only the
1409 will transliterate any A to X.
1411 Because the transliteration table is built at compile time, neither
1412 the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
1413 interpolation. That means that if you want to use variables, you
1416 eval "tr/$oldlist/$newlist/";
1419 eval "tr/$oldlist/$newlist/, 1" or die $@;
1423 A line-oriented form of quoting is based on the shell "here-document"
1424 syntax. Following a C<< << >> you specify a string to terminate
1425 the quoted material, and all lines following the current line down to
1426 the terminating string are the value of the item. The terminating
1427 string may be either an identifier (a word), or some quoted text. If
1428 quoted, the type of quotes you use determines the treatment of the
1429 text, just as in regular quoting. An unquoted identifier works like
1430 double quotes. There must be no space between the C<< << >> and
1431 the identifier, unless the identifier is quoted. (If you put a space it
1432 will be treated as a null identifier, which is valid, and matches the first
1433 empty line.) The terminating string must appear by itself (unquoted and
1434 with no surrounding whitespace) on the terminating line.
1437 The price is $Price.
1440 print << "EOF"; # same as above
1441 The price is $Price.
1444 print << `EOC`; # execute commands
1449 print <<"foo", <<"bar"; # you can stack them
1455 myfunc(<< "THIS", 23, <<'THAT');
1462 Just don't forget that you have to put a semicolon on the end
1463 to finish the statement, as Perl doesn't know you're not going to
1471 If you want your here-docs to be indented with the
1472 rest of the code, you'll need to remove leading whitespace
1473 from each line manually:
1475 ($quote = <<'FINIS') =~ s/^\s+//gm;
1476 The Road goes ever on and on,
1477 down from the door where it began.
1480 If you use a here-doc within a delimited construct, such as in C<s///eg>,
1481 the quoted material must come on the lines following the final delimiter.
1496 If the terminating identifier is on the last line of the program, you
1497 must be sure there is a newline after it; otherwise, Perl will give the
1498 warning B<Can't find string terminator "END" anywhere before EOF...>.
1500 Additionally, the quoting rules for the identifier are not related to
1501 Perl's quoting rules -- C<q()>, C<qq()>, and the like are not supported
1502 in place of C<''> and C<"">, and the only interpolation is for backslashing
1503 the quoting character:
1505 print << "abc\"def";
1509 Finally, quoted strings cannot span multiple lines. The general rule is
1510 that the identifier must be a string literal. Stick with that, and you
1515 =head2 Gory details of parsing quoted constructs
1517 When presented with something that might have several different
1518 interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
1519 principle to pick the most probable interpretation. This strategy
1520 is so successful that Perl programmers often do not suspect the
1521 ambivalence of what they write. But from time to time, Perl's
1522 notions differ substantially from what the author honestly meant.
1524 This section hopes to clarify how Perl handles quoted constructs.
1525 Although the most common reason to learn this is to unravel labyrinthine
1526 regular expressions, because the initial steps of parsing are the
1527 same for all quoting operators, they are all discussed together.
1529 The most important Perl parsing rule is the first one discussed
1530 below: when processing a quoted construct, Perl first finds the end
1531 of that construct, then interprets its contents. If you understand
1532 this rule, you may skip the rest of this section on the first
1533 reading. The other rules are likely to contradict the user's
1534 expectations much less frequently than this first one.
1536 Some passes discussed below are performed concurrently, but because
1537 their results are the same, we consider them individually. For different
1538 quoting constructs, Perl performs different numbers of passes, from
1539 one to five, but these passes are always performed in the same order.
1543 =item Finding the end
1545 The first pass is finding the end of the quoted construct, whether
1546 it be a multicharacter delimiter C<"\nEOF\n"> in the C<<<EOF>
1547 construct, a C</> that terminates a C<qq//> construct, a C<]> which
1548 terminates C<qq[]> construct, or a C<< > >> which terminates a
1549 fileglob started with C<< < >>.
1551 When searching for single-character non-pairing delimiters, such
1552 as C</>, combinations of C<\\> and C<\/> are skipped. However,
1553 when searching for single-character pairing delimiter like C<[>,
1554 combinations of C<\\>, C<\]>, and C<\[> are all skipped, and nested
1555 C<[>, C<]> are skipped as well. When searching for multicharacter
1556 delimiters, nothing is skipped.
1558 For constructs with three-part delimiters (C<s///>, C<y///>, and
1559 C<tr///>), the search is repeated once more.
1561 During this search no attention is paid to the semantics of the construct.
1564 "$hash{"$foo/$bar"}"
1569 bar # NOT a comment, this slash / terminated m//!
1572 do not form legal quoted expressions. The quoted part ends on the
1573 first C<"> and C</>, and the rest happens to be a syntax error.
1574 Because the slash that terminated C<m//> was followed by a C<SPACE>,
1575 the example above is not C<m//x>, but rather C<m//> with no C</x>
1576 modifier. So the embedded C<#> is interpreted as a literal C<#>.
1578 =item Removal of backslashes before delimiters
1580 During the second pass, text between the starting and ending
1581 delimiters is copied to a safe location, and the C<\> is removed
1582 from combinations consisting of C<\> and delimiter--or delimiters,
1583 meaning both starting and ending delimiters will should these differ.
1584 This removal does not happen for multi-character delimiters.
1585 Note that the combination C<\\> is left intact, just as it was.
1587 Starting from this step no information about the delimiters is
1592 The next step is interpolation in the text obtained, which is now
1593 delimiter-independent. There are four different cases.
1597 =item C<<<'EOF'>, C<m''>, C<s'''>, C<tr///>, C<y///>
1599 No interpolation is performed.
1603 The only interpolation is removal of C<\> from pairs C<\\>.
1605 =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>
1607 C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
1608 converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
1609 is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
1610 The other combinations are replaced with appropriate expansions.
1612 Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
1613 is interpolated in the usual way. Something like C<"\Q\\E"> has
1614 no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
1615 result is the same as for C<"\\\\E">. As a general rule, backslashes
1616 between C<\Q> and C<\E> may lead to counterintuitive results. So,
1617 C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
1618 as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
1623 may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
1625 Interpolated scalars and arrays are converted internally to the C<join> and
1626 C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
1628 $foo . " XXX '" . (join $", @arr) . "'";
1630 All operations above are performed simultaneously, left to right.
1632 Because the result of C<"\Q STRING \E"> has all metacharacters
1633 quoted, there is no way to insert a literal C<$> or C<@> inside a
1634 C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
1635 C<"\\\$">; if not, it is interpreted as the start of an interpolated
1638 Note also that the interpolation code needs to make a decision on
1639 where the interpolated scalar ends. For instance, whether
1640 C<< "a $b -> {c}" >> really means:
1642 "a " . $b . " -> {c}";
1648 Most of the time, the longest possible text that does not include
1649 spaces between components and which contains matching braces or
1650 brackets. because the outcome may be determined by voting based
1651 on heuristic estimators, the result is not strictly predictable.
1652 Fortunately, it's usually correct for ambiguous cases.
1654 =item C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
1656 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
1657 happens (almost) as with C<qq//> constructs, but the substitution
1658 of C<\> followed by RE-special chars (including C<\>) is not
1659 performed. Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
1660 a C<#>-comment in a C<//x>-regular expression, no processing is
1661 performed whatsoever. This is the first step at which the presence
1662 of the C<//x> modifier is relevant.
1664 Interpolation has several quirks: C<$|>, C<$(>, and C<$)> are not
1665 interpolated, and constructs C<$var[SOMETHING]> are voted (by several
1666 different estimators) to be either an array element or C<$var>
1667 followed by an RE alternative. This is where the notation
1668 C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
1669 array element C<-9>, not as a regular expression from the variable
1670 C<$arr> followed by a digit, which would be the interpretation of
1671 C</$arr[0-9]/>. Since voting among different estimators may occur,
1672 the result is not predictable.
1674 It is at this step that C<\1> is begrudgingly converted to C<$1> in
1675 the replacement text of C<s///> to correct the incorrigible
1676 I<sed> hackers who haven't picked up the saner idiom yet. A warning
1677 is emitted if the C<use warnings> pragma or the B<-w> command-line flag
1678 (that is, the C<$^W> variable) was set.
1680 The lack of processing of C<\\> creates specific restrictions on
1681 the post-processed text. If the delimiter is C</>, one cannot get
1682 the combination C<\/> into the result of this step. C</> will
1683 finish the regular expression, C<\/> will be stripped to C</> on
1684 the previous step, and C<\\/> will be left as is. Because C</> is
1685 equivalent to C<\/> inside a regular expression, this does not
1686 matter unless the delimiter happens to be character special to the
1687 RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
1688 alphanumeric char, as in:
1692 In the RE above, which is intentionally obfuscated for illustration, the
1693 delimiter is C<m>, the modifier is C<mx>, and after backslash-removal the
1694 RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
1695 reason you're encouraged to restrict your delimiters to non-alphanumeric,
1696 non-whitespace choices.
1700 This step is the last one for all constructs except regular expressions,
1701 which are processed further.
1703 =item Interpolation of regular expressions
1705 Previous steps were performed during the compilation of Perl code,
1706 but this one happens at run time--although it may be optimized to
1707 be calculated at compile time if appropriate. After preprocessing
1708 described above, and possibly after evaluation if catenation,
1709 joining, casing translation, or metaquoting are involved, the
1710 resulting I<string> is passed to the RE engine for compilation.
1712 Whatever happens in the RE engine might be better discussed in L<perlre>,
1713 but for the sake of continuity, we shall do so here.
1715 This is another step where the presence of the C<//x> modifier is
1716 relevant. The RE engine scans the string from left to right and
1717 converts it to a finite automaton.
1719 Backslashed characters are either replaced with corresponding
1720 literal strings (as with C<\{>), or else they generate special nodes
1721 in the finite automaton (as with C<\b>). Characters special to the
1722 RE engine (such as C<|>) generate corresponding nodes or groups of
1723 nodes. C<(?#...)> comments are ignored. All the rest is either
1724 converted to literal strings to match, or else is ignored (as is
1725 whitespace and C<#>-style comments if C<//x> is present).
1727 Parsing of the bracketed character class construct, C<[...]>, is
1728 rather different than the rule used for the rest of the pattern.
1729 The terminator of this construct is found using the same rules as
1730 for finding the terminator of a C<{}>-delimited construct, the only
1731 exception being that C<]> immediately following C<[> is treated as
1732 though preceded by a backslash. Similarly, the terminator of
1733 C<(?{...})> is found using the same rules as for finding the
1734 terminator of a C<{}>-delimited construct.
1736 It is possible to inspect both the string given to RE engine and the
1737 resulting finite automaton. See the arguments C<debug>/C<debugcolor>
1738 in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
1739 switch documented in L<perlrun/"Command Switches">.
1741 =item Optimization of regular expressions
1743 This step is listed for completeness only. Since it does not change
1744 semantics, details of this step are not documented and are subject
1745 to change without notice. This step is performed over the finite
1746 automaton that was generated during the previous pass.
1748 It is at this stage that C<split()> silently optimizes C</^/> to
1753 =head2 I/O Operators
1755 There are several I/O operators you should know about.
1757 A string enclosed by backticks (grave accents) first undergoes
1758 double-quote interpolation. It is then interpreted as an external
1759 command, and the output of that command is the value of the
1760 backtick string, like in a shell. In scalar context, a single string
1761 consisting of all output is returned. In list context, a list of
1762 values is returned, one per line of output. (You can set C<$/> to use
1763 a different line terminator.) The command is executed each time the
1764 pseudo-literal is evaluated. The status value of the command is
1765 returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
1766 Unlike in B<csh>, no translation is done on the return data--newlines
1767 remain newlines. Unlike in any of the shells, single quotes do not
1768 hide variable names in the command from interpretation. To pass a
1769 literal dollar-sign through to the shell you need to hide it with a
1770 backslash. The generalized form of backticks is C<qx//>. (Because
1771 backticks always undergo shell expansion as well, see L<perlsec> for
1774 In scalar context, evaluating a filehandle in angle brackets yields
1775 the next line from that file (the newline, if any, included), or
1776 C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
1777 (sometimes known as file-slurp mode) and the file is empty, it
1778 returns C<''> the first time, followed by C<undef> subsequently.
1780 Ordinarily you must assign the returned value to a variable, but
1781 there is one situation where an automatic assignment happens. If
1782 and only if the input symbol is the only thing inside the conditional
1783 of a C<while> statement (even if disguised as a C<for(;;)> loop),
1784 the value is automatically assigned to the global variable $_,
1785 destroying whatever was there previously. (This may seem like an
1786 odd thing to you, but you'll use the construct in almost every Perl
1787 script you write.) The $_ variable is not implicitly localized.
1788 You'll have to put a C<local $_;> before the loop if you want that
1791 The following lines are equivalent:
1793 while (defined($_ = <STDIN>)) { print; }
1794 while ($_ = <STDIN>) { print; }
1795 while (<STDIN>) { print; }
1796 for (;<STDIN>;) { print; }
1797 print while defined($_ = <STDIN>);
1798 print while ($_ = <STDIN>);
1799 print while <STDIN>;
1801 This also behaves similarly, but avoids $_ :
1803 while (my $line = <STDIN>) { print $line }
1805 In these loop constructs, the assigned value (whether assignment
1806 is automatic or explicit) is then tested to see whether it is
1807 defined. The defined test avoids problems where line has a string
1808 value that would be treated as false by Perl, for example a "" or
1809 a "0" with no trailing newline. If you really mean for such values
1810 to terminate the loop, they should be tested for explicitly:
1812 while (($_ = <STDIN>) ne '0') { ... }
1813 while (<STDIN>) { last unless $_; ... }
1815 In other boolean contexts, C<< <I<filehandle>> >> without an
1816 explicit C<defined> test or comparison elicit a warning if the
1817 C<use warnings> pragma or the B<-w>
1818 command-line switch (the C<$^W> variable) is in effect.
1820 The filehandles STDIN, STDOUT, and STDERR are predefined. (The
1821 filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
1822 in packages, where they would be interpreted as local identifiers
1823 rather than global.) Additional filehandles may be created with
1824 the open() function, amongst others. See L<perlopentut> and
1825 L<perlfunc/open> for details on this.
1827 If a <FILEHANDLE> is used in a context that is looking for
1828 a list, a list comprising all input lines is returned, one line per
1829 list element. It's easy to grow to a rather large data space this
1830 way, so use with care.
1832 <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
1833 See L<perlfunc/readline>.
1835 The null filehandle <> is special: it can be used to emulate the
1836 behavior of B<sed> and B<awk>. Input from <> comes either from
1837 standard input, or from each file listed on the command line. Here's
1838 how it works: the first time <> is evaluated, the @ARGV array is
1839 checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
1840 gives you standard input. The @ARGV array is then processed as a list
1841 of filenames. The loop
1844 ... # code for each line
1847 is equivalent to the following Perl-like pseudo code:
1849 unshift(@ARGV, '-') unless @ARGV;
1850 while ($ARGV = shift) {
1853 ... # code for each line
1857 except that it isn't so cumbersome to say, and will actually work.
1858 It really does shift the @ARGV array and put the current filename
1859 into the $ARGV variable. It also uses filehandle I<ARGV>
1860 internally--<> is just a synonym for <ARGV>, which
1861 is magical. (The pseudo code above doesn't work because it treats
1862 <ARGV> as non-magical.)
1864 You can modify @ARGV before the first <> as long as the array ends up
1865 containing the list of filenames you really want. Line numbers (C<$.>)
1866 continue as though the input were one big happy file. See the example
1867 in L<perlfunc/eof> for how to reset line numbers on each file.
1869 If you want to set @ARGV to your own list of files, go right ahead.
1870 This sets @ARGV to all plain text files if no @ARGV was given:
1872 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
1874 You can even set them to pipe commands. For example, this automatically
1875 filters compressed arguments through B<gzip>:
1877 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
1879 If you want to pass switches into your script, you can use one of the
1880 Getopts modules or put a loop on the front like this:
1882 while ($_ = $ARGV[0], /^-/) {
1885 if (/^-D(.*)/) { $debug = $1 }
1886 if (/^-v/) { $verbose++ }
1887 # ... # other switches
1891 # ... # code for each line
1894 The <> symbol will return C<undef> for end-of-file only once.
1895 If you call it again after this, it will assume you are processing another
1896 @ARGV list, and if you haven't set @ARGV, will read input from STDIN.
1898 If what the angle brackets contain is a simple scalar variable (e.g.,
1899 <$foo>), then that variable contains the name of the
1900 filehandle to input from, or its typeglob, or a reference to the
1906 If what's within the angle brackets is neither a filehandle nor a simple
1907 scalar variable containing a filehandle name, typeglob, or typeglob
1908 reference, it is interpreted as a filename pattern to be globbed, and
1909 either a list of filenames or the next filename in the list is returned,
1910 depending on context. This distinction is determined on syntactic
1911 grounds alone. That means C<< <$x> >> is always a readline() from
1912 an indirect handle, but C<< <$hash{key}> >> is always a glob().
1913 That's because $x is a simple scalar variable, but C<$hash{key}> is
1914 not--it's a hash element.
1916 One level of double-quote interpretation is done first, but you can't
1917 say C<< <$foo> >> because that's an indirect filehandle as explained
1918 in the previous paragraph. (In older versions of Perl, programmers
1919 would insert curly brackets to force interpretation as a filename glob:
1920 C<< <${foo}> >>. These days, it's considered cleaner to call the
1921 internal function directly as C<glob($foo)>, which is probably the right
1922 way to have done it in the first place.) For example:
1928 is roughly equivalent to:
1930 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
1936 except that the globbing is actually done internally using the standard
1937 C<File::Glob> extension. Of course, the shortest way to do the above is:
1941 A (file)glob evaluates its (embedded) argument only when it is
1942 starting a new list. All values must be read before it will start
1943 over. In list context, this isn't important because you automatically
1944 get them all anyway. However, in scalar context the operator returns
1945 the next value each time it's called, or C<undef> when the list has
1946 run out. As with filehandle reads, an automatic C<defined> is
1947 generated when the glob occurs in the test part of a C<while>,
1948 because legal glob returns (e.g. a file called F<0>) would otherwise
1949 terminate the loop. Again, C<undef> is returned only once. So if
1950 you're expecting a single value from a glob, it is much better to
1953 ($file) = <blurch*>;
1959 because the latter will alternate between returning a filename and
1962 If you're trying to do variable interpolation, it's definitely better
1963 to use the glob() function, because the older notation can cause people
1964 to become confused with the indirect filehandle notation.
1966 @files = glob("$dir/*.[ch]");
1967 @files = glob($files[$i]);
1969 =head2 Constant Folding
1971 Like C, Perl does a certain amount of expression evaluation at
1972 compile time whenever it determines that all arguments to an
1973 operator are static and have no side effects. In particular, string
1974 concatenation happens at compile time between literals that don't do
1975 variable substitution. Backslash interpolation also happens at
1976 compile time. You can say
1978 'Now is the time for all' . "\n" .
1979 'good men to come to.'
1981 and this all reduces to one string internally. Likewise, if
1984 foreach $file (@filenames) {
1985 if (-s $file > 5 + 100 * 2**16) { }
1988 the compiler will precompute the number which that expression
1989 represents so that the interpreter won't have to.
1991 =head2 Bitwise String Operators
1993 Bitstrings of any size may be manipulated by the bitwise operators
1996 If the operands to a binary bitwise op are strings of different
1997 sizes, B<|> and B<^> ops act as though the shorter operand had
1998 additional zero bits on the right, while the B<&> op acts as though
1999 the longer operand were truncated to the length of the shorter.
2000 The granularity for such extension or truncation is one or more
2003 # ASCII-based examples
2004 print "j p \n" ^ " a h"; # prints "JAPH\n"
2005 print "JA" | " ph\n"; # prints "japh\n"
2006 print "japh\nJunk" & '_____'; # prints "JAPH\n";
2007 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
2009 If you are intending to manipulate bitstrings, be certain that
2010 you're supplying bitstrings: If an operand is a number, that will imply
2011 a B<numeric> bitwise operation. You may explicitly show which type of
2012 operation you intend by using C<""> or C<0+>, as in the examples below.
2014 $foo = 150 | 105 ; # yields 255 (0x96 | 0x69 is 0xFF)
2015 $foo = '150' | 105 ; # yields 255
2016 $foo = 150 | '105'; # yields 255
2017 $foo = '150' | '105'; # yields string '155' (under ASCII)
2019 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
2020 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
2022 See L<perlfunc/vec> for information on how to manipulate individual bits
2025 =head2 Integer Arithmetic
2027 By default, Perl assumes that it must do most of its arithmetic in
2028 floating point. But by saying
2032 you may tell the compiler that it's okay to use integer operations
2033 (if it feels like it) from here to the end of the enclosing BLOCK.
2034 An inner BLOCK may countermand this by saying
2038 which lasts until the end of that BLOCK. Note that this doesn't
2039 mean everything is only an integer, merely that Perl may use integer
2040 operations if it is so inclined. For example, even under C<use
2041 integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731>
2044 Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
2045 and ">>") always produce integral results. (But see also
2046 L<Bitwise String Operators>.) However, C<use integer> still has meaning for
2047 them. By default, their results are interpreted as unsigned integers, but
2048 if C<use integer> is in effect, their results are interpreted
2049 as signed integers. For example, C<~0> usually evaluates to a large
2050 integral value. However, C<use integer; ~0> is C<-1> on twos-complement
2053 =head2 Floating-point Arithmetic
2055 While C<use integer> provides integer-only arithmetic, there is no
2056 analogous mechanism to provide automatic rounding or truncation to a
2057 certain number of decimal places. For rounding to a certain number
2058 of digits, sprintf() or printf() is usually the easiest route.
2061 Floating-point numbers are only approximations to what a mathematician
2062 would call real numbers. There are infinitely more reals than floats,
2063 so some corners must be cut. For example:
2065 printf "%.20g\n", 123456789123456789;
2066 # produces 123456789123456784
2068 Testing for exact equality of floating-point equality or inequality is
2069 not a good idea. Here's a (relatively expensive) work-around to compare
2070 whether two floating-point numbers are equal to a particular number of
2071 decimal places. See Knuth, volume II, for a more robust treatment of
2075 my ($X, $Y, $POINTS) = @_;
2077 $tX = sprintf("%.${POINTS}g", $X);
2078 $tY = sprintf("%.${POINTS}g", $Y);
2082 The POSIX module (part of the standard perl distribution) implements
2083 ceil(), floor(), and other mathematical and trigonometric functions.
2084 The Math::Complex module (part of the standard perl distribution)
2085 defines mathematical functions that work on both the reals and the
2086 imaginary numbers. Math::Complex not as efficient as POSIX, but
2087 POSIX can't work with complex numbers.
2089 Rounding in financial applications can have serious implications, and
2090 the rounding method used should be specified precisely. In these
2091 cases, it probably pays not to trust whichever system rounding is
2092 being used by Perl, but to instead implement the rounding function you
2095 =head2 Bigger Numbers
2097 The standard Math::BigInt and Math::BigFloat modules provide
2098 variable-precision arithmetic and overloaded operators, although
2099 they're currently pretty slow. At the cost of some space and
2100 considerable speed, they avoid the normal pitfalls associated with
2101 limited-precision representations.
2104 $x = Math::BigInt->new('123456789123456789');
2107 # prints +15241578780673678515622620750190521
2109 There are several modules that let you calculate with (bound only by
2110 memory and cpu-time) unlimited or fixed precision. There are also
2111 some non-standard modules that provide faster implementations via
2112 external C libraries.
2114 Here is a short, but incomplete summary:
2116 Math::Fraction big, unlimited fractions like 9973 / 12967
2117 Math::String treat string sequences like numbers
2118 Math::FixedPrecision calculate with a fixed precision
2119 Math::Currency for currency calculations
2120 Bit::Vector manipulate bit vectors fast (uses C)
2121 Math::BigIntFast Bit::Vector wrapper for big numbers
2122 Math::Pari provides access to the Pari C library
2123 Math::BigInteger uses an external C library
2124 Math::Cephes uses external Cephes C library (no big numbers)
2125 Math::Cephes::Fraction fractions via the Cephes library
2126 Math::GMP another one using an external C library