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 The auto-decrement operator is not magical.
132 =head2 Exponentiation
134 Binary "**" is the exponentiation operator. It binds even more
135 tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is
136 implemented using C's pow(3) function, which actually works on doubles
139 =head2 Symbolic Unary Operators
141 Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower
142 precedence version of this.
144 Unary "-" performs arithmetic negation if the operand is numeric. If
145 the operand is an identifier, a string consisting of a minus sign
146 concatenated with the identifier is returned. Otherwise, if the string
147 starts with a plus or minus, a string starting with the opposite sign
148 is returned. One effect of these rules is that C<-bareword> is equivalent
151 Unary "~" performs bitwise negation, i.e., 1's complement. For
152 example, C<0666 & ~027> is 0640. (See also L<Integer Arithmetic> and
153 L<Bitwise String Operators>.) Note that the width of the result is
154 platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64
155 bits wide on a 64-bit platform, so if you are expecting a certain bit
156 width, remember use the & operator to mask off the excess bits.
158 Unary "+" has no effect whatsoever, even on strings. It is useful
159 syntactically for separating a function name from a parenthesized expression
160 that would otherwise be interpreted as the complete list of function
161 arguments. (See examples above under L<Terms and List Operators (Leftward)>.)
163 Unary "\" creates a reference to whatever follows it. See L<perlreftut>
164 and L<perlref>. Do not confuse this behavior with the behavior of
165 backslash within a string, although both forms do convey the notion
166 of protecting the next thing from interpolation.
168 =head2 Binding Operators
170 Binary "=~" binds a scalar expression to a pattern match. Certain operations
171 search or modify the string $_ by default. This operator makes that kind
172 of operation work on some other string. The right argument is a search
173 pattern, substitution, or transliteration. The left argument is what is
174 supposed to be searched, substituted, or transliterated instead of the default
175 $_. When used in scalar context, the return value generally indicates the
176 success of the operation. Behavior in list context depends on the particular
177 operator. See L</"Regexp Quote-Like Operators"> for details.
179 If the right argument is an expression rather than a search pattern,
180 substitution, or transliteration, it is interpreted as a search pattern at run
181 time. This can be less efficient than an explicit search, because the
182 pattern must be compiled every time the expression is evaluated.
184 Binary "!~" is just like "=~" except the return value is negated in
187 =head2 Multiplicative Operators
189 Binary "*" multiplies two numbers.
191 Binary "/" divides two numbers.
193 Binary "%" computes the modulus of two numbers. Given integer
194 operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
195 C<$a> minus the largest multiple of C<$b> that is not greater than
196 C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
197 smallest multiple of C<$b> that is not less than C<$a> (i.e. the
198 result will be less than or equal to zero).
199 Note than when C<use integer> is in scope, "%" gives you direct access
200 to the modulus operator as implemented by your C compiler. This
201 operator is not as well defined for negative operands, but it will
204 Binary "x" is the repetition operator. In scalar context or if the left
205 operand is not enclosed in parentheses, it returns a string consisting
206 of the left operand repeated the number of times specified by the right
207 operand. In list context, if the left operand is enclosed in
208 parentheses, it repeats the list.
210 print '-' x 80; # print row of dashes
212 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
214 @ones = (1) x 80; # a list of 80 1's
215 @ones = (5) x @ones; # set all elements to 5
218 =head2 Additive Operators
220 Binary "+" returns the sum of two numbers.
222 Binary "-" returns the difference of two numbers.
224 Binary "." concatenates two strings.
226 =head2 Shift Operators
228 Binary "<<" returns the value of its left argument shifted left by the
229 number of bits specified by the right argument. Arguments should be
230 integers. (See also L<Integer Arithmetic>.)
232 Binary ">>" returns the value of its left argument shifted right by
233 the number of bits specified by the right argument. Arguments should
234 be integers. (See also L<Integer Arithmetic>.)
236 =head2 Named Unary Operators
238 The various named unary operators are treated as functions with one
239 argument, with optional parentheses. These include the filetest
240 operators, like C<-f>, C<-M>, etc. See L<perlfunc>.
242 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
243 is followed by a left parenthesis as the next token, the operator and
244 arguments within parentheses are taken to be of highest precedence,
245 just like a normal function call. Examples:
247 chdir $foo || die; # (chdir $foo) || die
248 chdir($foo) || die; # (chdir $foo) || die
249 chdir ($foo) || die; # (chdir $foo) || die
250 chdir +($foo) || die; # (chdir $foo) || die
252 but, because * is higher precedence than ||:
254 chdir $foo * 20; # chdir ($foo * 20)
255 chdir($foo) * 20; # (chdir $foo) * 20
256 chdir ($foo) * 20; # (chdir $foo) * 20
257 chdir +($foo) * 20; # chdir ($foo * 20)
259 rand 10 * 20; # rand (10 * 20)
260 rand(10) * 20; # (rand 10) * 20
261 rand (10) * 20; # (rand 10) * 20
262 rand +(10) * 20; # rand (10 * 20)
264 See also L<"Terms and List Operators (Leftward)">.
266 =head2 Relational Operators
268 Binary "<" returns true if the left argument is numerically less than
271 Binary ">" returns true if the left argument is numerically greater
272 than the right argument.
274 Binary "<=" returns true if the left argument is numerically less than
275 or equal to the right argument.
277 Binary ">=" returns true if the left argument is numerically greater
278 than or equal to the right argument.
280 Binary "lt" returns true if the left argument is stringwise less than
283 Binary "gt" returns true if the left argument is stringwise greater
284 than the right argument.
286 Binary "le" returns true if the left argument is stringwise less than
287 or equal to the right argument.
289 Binary "ge" returns true if the left argument is stringwise greater
290 than or equal to the right argument.
292 =head2 Equality Operators
294 Binary "==" returns true if the left argument is numerically equal to
297 Binary "!=" returns true if the left argument is numerically not equal
298 to the right argument.
300 Binary "<=>" returns -1, 0, or 1 depending on whether the left
301 argument is numerically less than, equal to, or greater than the right
302 argument. If your platform supports NaNs (not-a-numbers) as numeric
303 values, using them with "<=>" (or any other numeric comparison)
306 Binary "eq" returns true if the left argument is stringwise equal to
309 Binary "ne" returns true if the left argument is stringwise not equal
310 to the right argument.
312 Binary "cmp" returns -1, 0, or 1 depending on whether the left
313 argument is stringwise less than, equal to, or greater than the right
316 "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
317 by the current locale if C<use locale> is in effect. See L<perllocale>.
321 Binary "&" returns its operators ANDed together bit by bit.
322 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
324 =head2 Bitwise Or and Exclusive Or
326 Binary "|" returns its operators ORed together bit by bit.
327 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
329 Binary "^" returns its operators XORed together bit by bit.
330 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
332 =head2 C-style Logical And
334 Binary "&&" performs a short-circuit logical AND operation. That is,
335 if the left operand is false, the right operand is not even evaluated.
336 Scalar or list context propagates down to the right operand if it
339 =head2 C-style Logical Or
341 Binary "||" performs a short-circuit logical OR operation. That is,
342 if the left operand is true, the right operand is not even evaluated.
343 Scalar or list context propagates down to the right operand if it
346 The C<||> and C<&&> operators differ from C's in that, rather than returning
347 0 or 1, they return the last value evaluated. Thus, a reasonably portable
348 way to find out the home directory (assuming it's not "0") might be:
350 $home = $ENV{'HOME'} || $ENV{'LOGDIR'} ||
351 (getpwuid($<))[7] || die "You're homeless!\n";
353 In particular, this means that you shouldn't use this
354 for selecting between two aggregates for assignment:
356 @a = @b || @c; # this is wrong
357 @a = scalar(@b) || @c; # really meant this
358 @a = @b ? @b : @c; # this works fine, though
360 As more readable alternatives to C<&&> and C<||> when used for
361 control flow, Perl provides C<and> and C<or> operators (see below).
362 The short-circuit behavior is identical. The precedence of "and" and
363 "or" is much lower, however, so that you can safely use them after a
364 list operator without the need for parentheses:
366 unlink "alpha", "beta", "gamma"
367 or gripe(), next LINE;
369 With the C-style operators that would have been written like this:
371 unlink("alpha", "beta", "gamma")
372 || (gripe(), next LINE);
374 Using "or" for assignment is unlikely to do what you want; see below.
376 =head2 Range Operators
378 Binary ".." is the range operator, which is really two different
379 operators depending on the context. In list context, it returns an
380 array of values counting (up by ones) from the left value to the right
381 value. If the left value is greater than the right value then it
382 returns the empty array. The range operator is useful for writing
383 C<foreach (1..10)> loops and for doing slice operations on arrays. In
384 the current implementation, no temporary array is created when the
385 range operator is used as the expression in C<foreach> loops, but older
386 versions of Perl might burn a lot of memory when you write something
389 for (1 .. 1_000_000) {
393 In scalar context, ".." returns a boolean value. The operator is
394 bistable, like a flip-flop, and emulates the line-range (comma) operator
395 of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
396 own boolean state. It is false as long as its left operand is false.
397 Once the left operand is true, the range operator stays true until the
398 right operand is true, I<AFTER> which the range operator becomes false
399 again. It doesn't become false till the next time the range operator is
400 evaluated. It can test the right operand and become false on the same
401 evaluation it became true (as in B<awk>), but it still returns true once.
402 If you don't want it to test the right operand till the next
403 evaluation, as in B<sed>, just use three dots ("...") instead of
404 two. In all other regards, "..." behaves just like ".." does.
406 The right operand is not evaluated while the operator is in the
407 "false" state, and the left operand is not evaluated while the
408 operator is in the "true" state. The precedence is a little lower
409 than || and &&. The value returned is either the empty string for
410 false, or a sequence number (beginning with 1) for true. The
411 sequence number is reset for each range encountered. The final
412 sequence number in a range has the string "E0" appended to it, which
413 doesn't affect its numeric value, but gives you something to search
414 for if you want to exclude the endpoint. You can exclude the
415 beginning point by waiting for the sequence number to be greater
416 than 1. If either operand of scalar ".." is a constant expression,
417 that operand is implicitly compared to the C<$.> variable, the
418 current line number. Examples:
420 As a scalar operator:
422 if (101 .. 200) { print; } # print 2nd hundred lines
423 next line if (1 .. /^$/); # skip header lines
424 s/^/> / if (/^$/ .. eof()); # quote body
426 # parse mail messages
428 $in_header = 1 .. /^$/;
429 $in_body = /^$/ .. eof();
430 # do something based on those
432 close ARGV if eof; # reset $. each file
437 for (101 .. 200) { print; } # print $_ 100 times
438 @foo = @foo[0 .. $#foo]; # an expensive no-op
439 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
441 The range operator (in list context) makes use of the magical
442 auto-increment algorithm if the operands are strings. You
445 @alphabet = ('A' .. 'Z');
447 to get all normal letters of the alphabet, or
449 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
451 to get a hexadecimal digit, or
453 @z2 = ('01' .. '31'); print $z2[$mday];
455 to get dates with leading zeros. If the final value specified is not
456 in the sequence that the magical increment would produce, the sequence
457 goes until the next value would be longer than the final value
460 =head2 Conditional Operator
462 Ternary "?:" is the conditional operator, just as in C. It works much
463 like an if-then-else. If the argument before the ? is true, the
464 argument before the : is returned, otherwise the argument after the :
465 is returned. For example:
467 printf "I have %d dog%s.\n", $n,
468 ($n == 1) ? '' : "s";
470 Scalar or list context propagates downward into the 2nd
471 or 3rd argument, whichever is selected.
473 $a = $ok ? $b : $c; # get a scalar
474 @a = $ok ? @b : @c; # get an array
475 $a = $ok ? @b : @c; # oops, that's just a count!
477 The operator may be assigned to if both the 2nd and 3rd arguments are
478 legal lvalues (meaning that you can assign to them):
480 ($a_or_b ? $a : $b) = $c;
482 Because this operator produces an assignable result, using assignments
483 without parentheses will get you in trouble. For example, this:
485 $a % 2 ? $a += 10 : $a += 2
489 (($a % 2) ? ($a += 10) : $a) += 2
493 ($a % 2) ? ($a += 10) : ($a += 2)
495 That should probably be written more simply as:
497 $a += ($a % 2) ? 10 : 2;
499 =head2 Assignment Operators
501 "=" is the ordinary assignment operator.
503 Assignment operators work as in C. That is,
511 although without duplicating any side effects that dereferencing the lvalue
512 might trigger, such as from tie(). Other assignment operators work similarly.
513 The following are recognized:
520 Although these are grouped by family, they all have the precedence
523 Unlike in C, the scalar assignment operator produces a valid lvalue.
524 Modifying an assignment is equivalent to doing the assignment and
525 then modifying the variable that was assigned to. This is useful
526 for modifying a copy of something, like this:
528 ($tmp = $global) =~ tr [A-Z] [a-z];
539 Similarly, a list assignment in list context produces the list of
540 lvalues assigned to, and a list assignment in scalar context returns
541 the number of elements produced by the expression on the right hand
542 side of the assignment.
544 =head2 Comma Operator
546 Binary "," is the comma operator. In scalar context it evaluates
547 its left argument, throws that value away, then evaluates its right
548 argument and returns that value. This is just like C's comma operator.
550 In list context, it's just the list argument separator, and inserts
551 both its arguments into the list.
553 The => digraph is mostly just a synonym for the comma operator. It's useful for
554 documenting arguments that come in pairs. As of release 5.001, it also forces
555 any word to the left of it to be interpreted as a string.
557 =head2 List Operators (Rightward)
559 On the right side of a list operator, it has very low precedence,
560 such that it controls all comma-separated expressions found there.
561 The only operators with lower precedence are the logical operators
562 "and", "or", and "not", which may be used to evaluate calls to list
563 operators without the need for extra parentheses:
565 open HANDLE, "filename"
566 or die "Can't open: $!\n";
568 See also discussion of list operators in L<Terms and List Operators (Leftward)>.
572 Unary "not" returns the logical negation of the expression to its right.
573 It's the equivalent of "!" except for the very low precedence.
577 Binary "and" returns the logical conjunction of the two surrounding
578 expressions. It's equivalent to && except for the very low
579 precedence. This means that it short-circuits: i.e., the right
580 expression is evaluated only if the left expression is true.
582 =head2 Logical or and Exclusive Or
584 Binary "or" returns the logical disjunction of the two surrounding
585 expressions. It's equivalent to || except for the very low precedence.
586 This makes it useful for control flow
588 print FH $data or die "Can't write to FH: $!";
590 This means that it short-circuits: i.e., the right expression is evaluated
591 only if the left expression is false. Due to its precedence, you should
592 probably avoid using this for assignment, only for control flow.
594 $a = $b or $c; # bug: this is wrong
595 ($a = $b) or $c; # really means this
596 $a = $b || $c; # better written this way
598 However, when it's a list-context assignment and you're trying to use
599 "||" for control flow, you probably need "or" so that the assignment
600 takes higher precedence.
602 @info = stat($file) || die; # oops, scalar sense of stat!
603 @info = stat($file) or die; # better, now @info gets its due
605 Then again, you could always use parentheses.
607 Binary "xor" returns the exclusive-OR of the two surrounding expressions.
608 It cannot short circuit, of course.
610 =head2 C Operators Missing From Perl
612 Here is what C has that Perl doesn't:
618 Address-of operator. (But see the "\" operator for taking a reference.)
622 Dereference-address operator. (Perl's prefix dereferencing
623 operators are typed: $, @, %, and &.)
627 Type-casting operator.
631 =head2 Quote and Quote-like Operators
633 While we usually think of quotes as literal values, in Perl they
634 function as operators, providing various kinds of interpolating and
635 pattern matching capabilities. Perl provides customary quote characters
636 for these behaviors, but also provides a way for you to choose your
637 quote character for any of them. In the following table, a C<{}> represents
638 any pair of delimiters you choose.
640 Customary Generic Meaning Interpolates
643 `` qx{} Command yes (unless '' is delimiter)
645 // m{} Pattern match yes (unless '' is delimiter)
646 qr{} Pattern yes (unless '' is delimiter)
647 s{}{} Substitution yes (unless '' is delimiter)
648 tr{}{} Transliteration no (but see below)
650 Non-bracketing delimiters use the same character fore and aft, but the four
651 sorts of brackets (round, angle, square, curly) will all nest, which means
660 Note, however, that this does not always work for quoting Perl code:
662 $s = q{ if($a eq "}") ... }; # WRONG
664 is a syntax error. The C<Text::Balanced> module on CPAN is able to do this
667 There can be whitespace between the operator and the quoting
668 characters, except when C<#> is being used as the quoting character.
669 C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
670 operator C<q> followed by a comment. Its argument will be taken
671 from the next line. This allows you to write:
673 s {foo} # Replace foo
676 For constructs that do interpolate, variables beginning with "C<$>"
677 or "C<@>" are interpolated, as are the following escape sequences. Within
678 a transliteration, the first eleven of these sequences may be used.
685 \a alarm (bell) (BEL)
687 \033 octal char (ESC)
689 \x{263a} wide hex char (SMILEY)
690 \c[ control char (ESC)
693 \l lowercase next char
694 \u uppercase next char
697 \E end case modification
698 \Q quote non-word characters till \E
700 If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u>
701 and C<\U> is taken from the current locale. See L<perllocale>. For
702 documentation of C<\N{name}>, see L<charnames>.
704 All systems use the virtual C<"\n"> to represent a line terminator,
705 called a "newline". There is no such thing as an unvarying, physical
706 newline character. It is only an illusion that the operating system,
707 device drivers, C libraries, and Perl all conspire to preserve. Not all
708 systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
709 on a Mac, these are reversed, and on systems without line terminator,
710 printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
711 you mean a "newline" for your system, but use the literal ASCII when you
712 need an exact character. For example, most networking protocols expect
713 and prefer a CR+LF (C<"\012\015"> or C<"\cJ\cM">) for line terminators,
714 and although they often accept just C<"\012">, they seldom tolerate just
715 C<"\015">. If you get in the habit of using C<"\n"> for networking,
716 you may be burned some day.
718 You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
719 An unescaped C<$> or C<@> interpolates the corresponding variable,
720 while escaping will cause the literal string C<\$> to be inserted.
721 You'll need to write something like C<m/\Quser\E\@\Qhost/>.
723 Patterns are subject to an additional level of interpretation as a
724 regular expression. This is done as a second pass, after variables are
725 interpolated, so that regular expressions may be incorporated into the
726 pattern from the variables. If this is not what you want, use C<\Q> to
727 interpolate a variable literally.
729 Apart from the behavior described above, Perl does not expand
730 multiple levels of interpolation. In particular, contrary to the
731 expectations of shell programmers, back-quotes do I<NOT> interpolate
732 within double quotes, nor do single quotes impede evaluation of
733 variables when used within double quotes.
735 =head2 Regexp Quote-Like Operators
737 Here are the quote-like operators that apply to pattern
738 matching and related activities.
744 This is just like the C</pattern/> search, except that it matches only
745 once between calls to the reset() operator. This is a useful
746 optimization when you want to see only the first occurrence of
747 something in each file of a set of files, for instance. Only C<??>
748 patterns local to the current package are reset.
752 # blank line between header and body
755 reset if eof; # clear ?? status for next file
758 This usage is vaguely depreciated, which means it just might possibly
759 be removed in some distant future version of Perl, perhaps somewhere
760 around the year 2168.
762 =item m/PATTERN/cgimosx
764 =item /PATTERN/cgimosx
766 Searches a string for a pattern match, and in scalar context returns
767 true if it succeeds, false if it fails. If no string is specified
768 via the C<=~> or C<!~> operator, the $_ string is searched. (The
769 string specified with C<=~> need not be an lvalue--it may be the
770 result of an expression evaluation, but remember the C<=~> binds
771 rather tightly.) See also L<perlre>. See L<perllocale> for
772 discussion of additional considerations that apply when C<use locale>
777 c Do not reset search position on a failed match when /g is in effect.
778 g Match globally, i.e., find all occurrences.
779 i Do case-insensitive pattern matching.
780 m Treat string as multiple lines.
781 o Compile pattern only once.
782 s Treat string as single line.
783 x Use extended regular expressions.
785 If "/" is the delimiter then the initial C<m> is optional. With the C<m>
786 you can use any pair of non-alphanumeric, non-whitespace characters
787 as delimiters. This is particularly useful for matching path names
788 that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
789 the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
790 If "'" is the delimiter, no interpolation is performed on the PATTERN.
792 PATTERN may contain variables, which will be interpolated (and the
793 pattern recompiled) every time the pattern search is evaluated, except
794 for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
795 C<$|> are not interpolated because they look like end-of-string tests.)
796 If you want such a pattern to be compiled only once, add a C</o> after
797 the trailing delimiter. This avoids expensive run-time recompilations,
798 and is useful when the value you are interpolating won't change over
799 the life of the script. However, mentioning C</o> constitutes a promise
800 that you won't change the variables in the pattern. If you change them,
801 Perl won't even notice. See also L<"qr//">.
803 If the PATTERN evaluates to the empty string, the last
804 I<successfully> matched regular expression is used instead.
806 If the C</g> option is not used, C<m//> in list context returns a
807 list consisting of the subexpressions matched by the parentheses in the
808 pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are
809 also set, and that this differs from Perl 4's behavior.) When there are
810 no parentheses in the pattern, the return value is the list C<(1)> for
811 success. With or without parentheses, an empty list is returned upon
816 open(TTY, '/dev/tty');
817 <TTY> =~ /^y/i && foo(); # do foo if desired
819 if (/Version: *([0-9.]*)/) { $version = $1; }
821 next if m#^/usr/spool/uucp#;
826 print if /$arg/o; # compile only once
829 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
831 This last example splits $foo into the first two words and the
832 remainder of the line, and assigns those three fields to $F1, $F2, and
833 $Etc. The conditional is true if any variables were assigned, i.e., if
836 The C</g> modifier specifies global pattern matching--that is,
837 matching as many times as possible within the string. How it behaves
838 depends on the context. In list context, it returns a list of the
839 substrings matched by any capturing parentheses in the regular
840 expression. If there are no parentheses, it returns a list of all
841 the matched strings, as if there were parentheses around the whole
844 In scalar context, each execution of C<m//g> finds the next match,
845 returning true if it matches, and false if there is no further match.
846 The position after the last match can be read or set using the pos()
847 function; see L<perlfunc/pos>. A failed match normally resets the
848 search position to the beginning of the string, but you can avoid that
849 by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
850 string also resets the search position.
852 You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
853 zero-width assertion that matches the exact position where the previous
854 C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion
855 still anchors at pos(), but the match is of course only attempted once.
856 Using C<\G> without C</g> on a target string that has not previously had a
857 C</g> match applied to it is the same as using the C<\A> assertion to match
858 the beginning of the string.
863 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
867 while (defined($paragraph = <>)) {
868 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
872 print "$sentences\n";
874 # using m//gc with \G
878 print $1 while /(o)/gc; print "', pos=", pos, "\n";
880 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
882 print $1 while /(p)/gc; print "', pos=", pos, "\n";
884 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
886 The last example should print:
896 Notice that the final match matched C<q> instead of C<p>, which a match
897 without the C<\G> anchor would have done. Also note that the final match
898 did not update C<pos> -- C<pos> is only updated on a C</g> match. If the
899 final match did indeed match C<p>, it's a good bet that you're running an
900 older (pre-5.6.0) Perl.
902 A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
903 combine several regexps like this to process a string part-by-part,
904 doing different actions depending on which regexp matched. Each
905 regexp tries to match where the previous one leaves off.
908 $url = new URI::URL "http://www/"; die if $url eq "xXx";
912 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
913 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
914 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
915 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
916 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
917 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
918 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
919 print ". That's all!\n";
922 Here is the output (split into several lines):
924 line-noise lowercase line-noise lowercase UPPERCASE line-noise
925 UPPERCASE line-noise lowercase line-noise lowercase line-noise
926 lowercase lowercase line-noise lowercase lowercase line-noise
927 MiXeD line-noise. That's all!
933 A single-quoted, literal string. A backslash represents a backslash
934 unless followed by the delimiter or another backslash, in which case
935 the delimiter or backslash is interpolated.
937 $foo = q!I said, "You said, 'She said it.'"!;
938 $bar = q('This is it.');
939 $baz = '\n'; # a two-character string
945 A double-quoted, interpolated string.
948 (*** The previous line contains the naughty word "$1".\n)
949 if /\b(tcl|java|python)\b/i; # :-)
950 $baz = "\n"; # a one-character string
952 =item qr/STRING/imosx
954 This operators quotes--and compiles--its I<STRING> as a regular
955 expression. I<STRING> is interpolated the same way as I<PATTERN>
956 in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
957 is done. Returns a Perl value which may be used instead of the
958 corresponding C</STRING/imosx> expression.
962 $rex = qr/my.STRING/is;
969 The result may be used as a subpattern in a match:
972 $string =~ /foo${re}bar/; # can be interpolated in other patterns
973 $string =~ $re; # or used standalone
974 $string =~ /$re/; # or this way
976 Since Perl may compile the pattern at the moment of execution of qr()
977 operator, using qr() may have speed advantages in some situations,
978 notably if the result of qr() is used standalone:
981 my $patterns = shift;
982 my @compiled = map qr/$_/i, @$patterns;
985 foreach my $pat (@compiled) {
986 $success = 1, last if /$pat/;
992 Precompilation of the pattern into an internal representation at
993 the moment of qr() avoids a need to recompile the pattern every
994 time a match C</$pat/> is attempted. (Perl has many other internal
995 optimizations, but none would be triggered in the above example if
996 we did not use qr() operator.)
1000 i Do case-insensitive pattern matching.
1001 m Treat string as multiple lines.
1002 o Compile pattern only once.
1003 s Treat string as single line.
1004 x Use extended regular expressions.
1006 See L<perlre> for additional information on valid syntax for STRING, and
1007 for a detailed look at the semantics of regular expressions.
1013 A string which is (possibly) interpolated and then executed as a
1014 system command with C</bin/sh> or its equivalent. Shell wildcards,
1015 pipes, and redirections will be honored. The collected standard
1016 output of the command is returned; standard error is unaffected. In
1017 scalar context, it comes back as a single (potentially multi-line)
1018 string, or undef if the command failed. In list context, returns a
1019 list of lines (however you've defined lines with $/ or
1020 $INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
1022 Because backticks do not affect standard error, use shell file descriptor
1023 syntax (assuming the shell supports this) if you care to address this.
1024 To capture a command's STDERR and STDOUT together:
1026 $output = `cmd 2>&1`;
1028 To capture a command's STDOUT but discard its STDERR:
1030 $output = `cmd 2>/dev/null`;
1032 To capture a command's STDERR but discard its STDOUT (ordering is
1035 $output = `cmd 2>&1 1>/dev/null`;
1037 To exchange a command's STDOUT and STDERR in order to capture the STDERR
1038 but leave its STDOUT to come out the old STDERR:
1040 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
1042 To read both a command's STDOUT and its STDERR separately, it's easiest
1043 and safest to redirect them separately to files, and then read from those
1044 files when the program is done:
1046 system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr");
1048 Using single-quote as a delimiter protects the command from Perl's
1049 double-quote interpolation, passing it on to the shell instead:
1051 $perl_info = qx(ps $$); # that's Perl's $$
1052 $shell_info = qx'ps $$'; # that's the new shell's $$
1054 How that string gets evaluated is entirely subject to the command
1055 interpreter on your system. On most platforms, you will have to protect
1056 shell metacharacters if you want them treated literally. This is in
1057 practice difficult to do, as it's unclear how to escape which characters.
1058 See L<perlsec> for a clean and safe example of a manual fork() and exec()
1059 to emulate backticks safely.
1061 On some platforms (notably DOS-like ones), the shell may not be
1062 capable of dealing with multiline commands, so putting newlines in
1063 the string may not get you what you want. You may be able to evaluate
1064 multiple commands in a single line by separating them with the command
1065 separator character, if your shell supports that (e.g. C<;> on many Unix
1066 shells; C<&> on the Windows NT C<cmd> shell).
1068 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1069 output before starting the child process, but this may not be supported
1070 on some platforms (see L<perlport>). To be safe, you may need to set
1071 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1072 C<IO::Handle> on any open handles.
1074 Beware that some command shells may place restrictions on the length
1075 of the command line. You must ensure your strings don't exceed this
1076 limit after any necessary interpolations. See the platform-specific
1077 release notes for more details about your particular environment.
1079 Using this operator can lead to programs that are difficult to port,
1080 because the shell commands called vary between systems, and may in
1081 fact not be present at all. As one example, the C<type> command under
1082 the POSIX shell is very different from the C<type> command under DOS.
1083 That doesn't mean you should go out of your way to avoid backticks
1084 when they're the right way to get something done. Perl was made to be
1085 a glue language, and one of the things it glues together is commands.
1086 Just understand what you're getting yourself into.
1088 See L<"I/O Operators"> for more discussion.
1092 Evaluates to a list of the words extracted out of STRING, using embedded
1093 whitespace as the word delimiters. It can be understood as being roughly
1096 split(' ', q/STRING/);
1098 the difference being that it generates a real list at compile time. So
1103 is semantically equivalent to the list:
1107 Some frequently seen examples:
1109 use POSIX qw( setlocale localeconv )
1110 @EXPORT = qw( foo bar baz );
1112 A common mistake is to try to separate the words with comma or to
1113 put comments into a multi-line C<qw>-string. For this reason, the
1114 C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
1115 produces warnings if the STRING contains the "," or the "#" character.
1117 =item s/PATTERN/REPLACEMENT/egimosx
1119 Searches a string for a pattern, and if found, replaces that pattern
1120 with the replacement text and returns the number of substitutions
1121 made. Otherwise it returns false (specifically, the empty string).
1123 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
1124 variable is searched and modified. (The string specified with C<=~> must
1125 be scalar variable, an array element, a hash element, or an assignment
1126 to one of those, i.e., an lvalue.)
1128 If the delimiter chosen is a single quote, no interpolation is
1129 done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1130 PATTERN contains a $ that looks like a variable rather than an
1131 end-of-string test, the variable will be interpolated into the pattern
1132 at run-time. If you want the pattern compiled only once the first time
1133 the variable is interpolated, use the C</o> option. If the pattern
1134 evaluates to the empty string, the last successfully executed regular
1135 expression is used instead. See L<perlre> for further explanation on these.
1136 See L<perllocale> for discussion of additional considerations that apply
1137 when C<use locale> is in effect.
1141 e Evaluate the right side as an expression.
1142 g Replace globally, i.e., all occurrences.
1143 i Do case-insensitive pattern matching.
1144 m Treat string as multiple lines.
1145 o Compile pattern only once.
1146 s Treat string as single line.
1147 x Use extended regular expressions.
1149 Any non-alphanumeric, non-whitespace delimiter may replace the
1150 slashes. If single quotes are used, no interpretation is done on the
1151 replacement string (the C</e> modifier overrides this, however). Unlike
1152 Perl 4, Perl 5 treats backticks as normal delimiters; the replacement
1153 text is not evaluated as a command. If the
1154 PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
1155 pair of quotes, which may or may not be bracketing quotes, e.g.,
1156 C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
1157 replacement portion to be treated as a full-fledged Perl expression
1158 and evaluated right then and there. It is, however, syntax checked at
1159 compile-time. A second C<e> modifier will cause the replacement portion
1160 to be C<eval>ed before being run as a Perl expression.
1164 s/\bgreen\b/mauve/g; # don't change wintergreen
1166 $path =~ s|/usr/bin|/usr/local/bin|;
1168 s/Login: $foo/Login: $bar/; # run-time pattern
1170 ($foo = $bar) =~ s/this/that/; # copy first, then change
1172 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
1175 s/\d+/$&*2/e; # yields 'abc246xyz'
1176 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
1177 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1179 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
1180 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
1181 s/^=(\w+)/&pod($1)/ge; # use function call
1183 # expand variables in $_, but dynamics only, using
1184 # symbolic dereferencing
1187 # Add one to the value of any numbers in the string
1190 # This will expand any embedded scalar variable
1191 # (including lexicals) in $_ : First $1 is interpolated
1192 # to the variable name, and then evaluated
1195 # Delete (most) C comments.
1197 /\* # Match the opening delimiter.
1198 .*? # Match a minimal number of characters.
1199 \*/ # Match the closing delimiter.
1202 s/^\s*(.*?)\s*$/$1/; # trim white space in $_, expensively
1204 for ($variable) { # trim white space in $variable, cheap
1209 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
1211 Note the use of $ instead of \ in the last example. Unlike
1212 B<sed>, we use the \<I<digit>> form in only the left hand side.
1213 Anywhere else it's $<I<digit>>.
1215 Occasionally, you can't use just a C</g> to get all the changes
1216 to occur that you might want. Here are two common cases:
1218 # put commas in the right places in an integer
1219 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
1221 # expand tabs to 8-column spacing
1222 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
1224 =item tr/SEARCHLIST/REPLACEMENTLIST/cds
1226 =item y/SEARCHLIST/REPLACEMENTLIST/cds
1228 Transliterates all occurrences of the characters found in the search list
1229 with the corresponding character in the replacement list. It returns
1230 the number of characters replaced or deleted. If no string is
1231 specified via the =~ or !~ operator, the $_ string is transliterated. (The
1232 string specified with =~ must be a scalar variable, an array element, a
1233 hash element, or an assignment to one of those, i.e., an lvalue.)
1235 A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
1236 does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
1237 For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1238 SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1239 its own pair of quotes, which may or may not be bracketing quotes,
1240 e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
1242 Note that C<tr> does B<not> do regular expression character classes
1243 such as C<\d> or C<[:lower:]>. The <tr> operator is not equivalent to
1244 the tr(1) utility. If you want to map strings between lower/upper
1245 cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider
1246 using the C<s> operator if you need regular expressions.
1248 Note also that the whole range idea is rather unportable between
1249 character sets--and even within character sets they may cause results
1250 you probably didn't expect. A sound principle is to use only ranges
1251 that begin from and end at either alphabets of equal case (a-e, A-E),
1252 or digits (0-4). Anything else is unsafe. If in doubt, spell out the
1253 character sets in full.
1257 c Complement the SEARCHLIST.
1258 d Delete found but unreplaced characters.
1259 s Squash duplicate replaced characters.
1261 If the C</c> modifier is specified, the SEARCHLIST character set
1262 is complemented. If the C</d> modifier is specified, any characters
1263 specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
1264 (Note that this is slightly more flexible than the behavior of some
1265 B<tr> programs, which delete anything they find in the SEARCHLIST,
1266 period.) If the C</s> modifier is specified, sequences of characters
1267 that were transliterated to the same character are squashed down
1268 to a single instance of the character.
1270 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1271 exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1272 than the SEARCHLIST, the final character is replicated till it is long
1273 enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
1274 This latter is useful for counting characters in a class or for
1275 squashing character sequences in a class.
1279 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1281 $cnt = tr/*/*/; # count the stars in $_
1283 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1285 $cnt = tr/0-9//; # count the digits in $_
1287 tr/a-zA-Z//s; # bookkeeper -> bokeper
1289 ($HOST = $host) =~ tr/a-z/A-Z/;
1291 tr/a-zA-Z/ /cs; # change non-alphas to single space
1294 [\000-\177]; # delete 8th bit
1296 If multiple transliterations are given for a character, only the
1301 will transliterate any A to X.
1303 Because the transliteration table is built at compile time, neither
1304 the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
1305 interpolation. That means that if you want to use variables, you
1308 eval "tr/$oldlist/$newlist/";
1311 eval "tr/$oldlist/$newlist/, 1" or die $@;
1315 =head2 Gory details of parsing quoted constructs
1317 When presented with something that might have several different
1318 interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
1319 principle to pick the most probable interpretation. This strategy
1320 is so successful that Perl programmers often do not suspect the
1321 ambivalence of what they write. But from time to time, Perl's
1322 notions differ substantially from what the author honestly meant.
1324 This section hopes to clarify how Perl handles quoted constructs.
1325 Although the most common reason to learn this is to unravel labyrinthine
1326 regular expressions, because the initial steps of parsing are the
1327 same for all quoting operators, they are all discussed together.
1329 The most important Perl parsing rule is the first one discussed
1330 below: when processing a quoted construct, Perl first finds the end
1331 of that construct, then interprets its contents. If you understand
1332 this rule, you may skip the rest of this section on the first
1333 reading. The other rules are likely to contradict the user's
1334 expectations much less frequently than this first one.
1336 Some passes discussed below are performed concurrently, but because
1337 their results are the same, we consider them individually. For different
1338 quoting constructs, Perl performs different numbers of passes, from
1339 one to five, but these passes are always performed in the same order.
1343 =item Finding the end
1345 The first pass is finding the end of the quoted construct, whether
1346 it be a multicharacter delimiter C<"\nEOF\n"> in the C<<<EOF>
1347 construct, a C</> that terminates a C<qq//> construct, a C<]> which
1348 terminates C<qq[]> construct, or a C<< > >> which terminates a
1349 fileglob started with C<< < >>.
1351 When searching for single-character non-pairing delimiters, such
1352 as C</>, combinations of C<\\> and C<\/> are skipped. However,
1353 when searching for single-character pairing delimiter like C<[>,
1354 combinations of C<\\>, C<\]>, and C<\[> are all skipped, and nested
1355 C<[>, C<]> are skipped as well. When searching for multicharacter
1356 delimiters, nothing is skipped.
1358 For constructs with three-part delimiters (C<s///>, C<y///>, and
1359 C<tr///>), the search is repeated once more.
1361 During this search no attention is paid to the semantics of the construct.
1364 "$hash{"$foo/$bar"}"
1369 bar # NOT a comment, this slash / terminated m//!
1372 do not form legal quoted expressions. The quoted part ends on the
1373 first C<"> and C</>, and the rest happens to be a syntax error.
1374 Because the slash that terminated C<m//> was followed by a C<SPACE>,
1375 the example above is not C<m//x>, but rather C<m//> with no C</x>
1376 modifier. So the embedded C<#> is interpreted as a literal C<#>.
1378 =item Removal of backslashes before delimiters
1380 During the second pass, text between the starting and ending
1381 delimiters is copied to a safe location, and the C<\> is removed
1382 from combinations consisting of C<\> and delimiter--or delimiters,
1383 meaning both starting and ending delimiters will should these differ.
1384 This removal does not happen for multi-character delimiters.
1385 Note that the combination C<\\> is left intact, just as it was.
1387 Starting from this step no information about the delimiters is
1392 The next step is interpolation in the text obtained, which is now
1393 delimiter-independent. There are four different cases.
1397 =item C<<<'EOF'>, C<m''>, C<s'''>, C<tr///>, C<y///>
1399 No interpolation is performed.
1403 The only interpolation is removal of C<\> from pairs C<\\>.
1405 =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>
1407 C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
1408 converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
1409 is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
1410 The other combinations are replaced with appropriate expansions.
1412 Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
1413 is interpolated in the usual way. Something like C<"\Q\\E"> has
1414 no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
1415 result is the same as for C<"\\\\E">. As a general rule, backslashes
1416 between C<\Q> and C<\E> may lead to counterintuitive results. So,
1417 C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
1418 as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
1423 may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
1425 Interpolated scalars and arrays are converted internally to the C<join> and
1426 C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
1428 $foo . " XXX '" . (join $", @arr) . "'";
1430 All operations above are performed simultaneously, left to right.
1432 Because the result of C<"\Q STRING \E"> has all metacharacters
1433 quoted, there is no way to insert a literal C<$> or C<@> inside a
1434 C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
1435 C<"\\\$">; if not, it is interpreted as the start of an interpolated
1438 Note also that the interpolation code needs to make a decision on
1439 where the interpolated scalar ends. For instance, whether
1440 C<< "a $b -> {c}" >> really means:
1442 "a " . $b . " -> {c}";
1448 Most of the time, the longest possible text that does not include
1449 spaces between components and which contains matching braces or
1450 brackets. because the outcome may be determined by voting based
1451 on heuristic estimators, the result is not strictly predictable.
1452 Fortunately, it's usually correct for ambiguous cases.
1454 =item C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
1456 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
1457 happens (almost) as with C<qq//> constructs, but the substitution
1458 of C<\> followed by RE-special chars (including C<\>) is not
1459 performed. Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
1460 a C<#>-comment in a C<//x>-regular expression, no processing is
1461 performed whatsoever. This is the first step at which the presence
1462 of the C<//x> modifier is relevant.
1464 Interpolation has several quirks: C<$|>, C<$(>, and C<$)> are not
1465 interpolated, and constructs C<$var[SOMETHING]> are voted (by several
1466 different estimators) to be either an array element or C<$var>
1467 followed by an RE alternative. This is where the notation
1468 C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
1469 array element C<-9>, not as a regular expression from the variable
1470 C<$arr> followed by a digit, which would be the interpretation of
1471 C</$arr[0-9]/>. Since voting among different estimators may occur,
1472 the result is not predictable.
1474 It is at this step that C<\1> is begrudgingly converted to C<$1> in
1475 the replacement text of C<s///> to correct the incorrigible
1476 I<sed> hackers who haven't picked up the saner idiom yet. A warning
1477 is emitted if the C<use warnings> pragma or the B<-w> command-line flag
1478 (that is, the C<$^W> variable) was set.
1480 The lack of processing of C<\\> creates specific restrictions on
1481 the post-processed text. If the delimiter is C</>, one cannot get
1482 the combination C<\/> into the result of this step. C</> will
1483 finish the regular expression, C<\/> will be stripped to C</> on
1484 the previous step, and C<\\/> will be left as is. Because C</> is
1485 equivalent to C<\/> inside a regular expression, this does not
1486 matter unless the delimiter happens to be character special to the
1487 RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
1488 alphanumeric char, as in:
1492 In the RE above, which is intentionally obfuscated for illustration, the
1493 delimiter is C<m>, the modifier is C<mx>, and after backslash-removal the
1494 RE is the same as for C<m/ ^ a s* b /mx>). There's more than one
1495 reason you're encouraged to restrict your delimiters to non-alphanumeric,
1496 non-whitespace choices.
1500 This step is the last one for all constructs except regular expressions,
1501 which are processed further.
1503 =item Interpolation of regular expressions
1505 Previous steps were performed during the compilation of Perl code,
1506 but this one happens at run time--although it may be optimized to
1507 be calculated at compile time if appropriate. After preprocessing
1508 described above, and possibly after evaluation if catenation,
1509 joining, casing translation, or metaquoting are involved, the
1510 resulting I<string> is passed to the RE engine for compilation.
1512 Whatever happens in the RE engine might be better discussed in L<perlre>,
1513 but for the sake of continuity, we shall do so here.
1515 This is another step where the presence of the C<//x> modifier is
1516 relevant. The RE engine scans the string from left to right and
1517 converts it to a finite automaton.
1519 Backslashed characters are either replaced with corresponding
1520 literal strings (as with C<\{>), or else they generate special nodes
1521 in the finite automaton (as with C<\b>). Characters special to the
1522 RE engine (such as C<|>) generate corresponding nodes or groups of
1523 nodes. C<(?#...)> comments are ignored. All the rest is either
1524 converted to literal strings to match, or else is ignored (as is
1525 whitespace and C<#>-style comments if C<//x> is present).
1527 Parsing of the bracketed character class construct, C<[...]>, is
1528 rather different than the rule used for the rest of the pattern.
1529 The terminator of this construct is found using the same rules as
1530 for finding the terminator of a C<{}>-delimited construct, the only
1531 exception being that C<]> immediately following C<[> is treated as
1532 though preceded by a backslash. Similarly, the terminator of
1533 C<(?{...})> is found using the same rules as for finding the
1534 terminator of a C<{}>-delimited construct.
1536 It is possible to inspect both the string given to RE engine and the
1537 resulting finite automaton. See the arguments C<debug>/C<debugcolor>
1538 in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
1539 switch documented in L<perlrun/"Command Switches">.
1541 =item Optimization of regular expressions
1543 This step is listed for completeness only. Since it does not change
1544 semantics, details of this step are not documented and are subject
1545 to change without notice. This step is performed over the finite
1546 automaton that was generated during the previous pass.
1548 It is at this stage that C<split()> silently optimizes C</^/> to
1553 =head2 I/O Operators
1555 There are several I/O operators you should know about.
1557 A string enclosed by backticks (grave accents) first undergoes
1558 double-quote interpolation. It is then interpreted as an external
1559 command, and the output of that command is the value of the
1561 string consisting of all output is returned. In list context, a
1562 list of values is returned, one per line of output. (You can set
1563 C<$/> to use a different line terminator.) The command is executed
1564 each time the pseudo-literal is evaluated. The status value of the
1565 command is returned in C<$?> (see L<perlvar> for the interpretation
1566 of C<$?>). Unlike in B<csh>, no translation is done on the return
1567 data--newlines remain newlines. Unlike in any of the shells, single
1568 quotes do not hide variable names in the command from interpretation.
1569 To pass a literal dollar-sign through to the shell you need to hide
1570 it with a backslash. The generalized form of backticks is C<qx//>.
1571 (Because backticks always undergo shell expansion as well, see
1572 L<perlsec> for security concerns.)
1574 In scalar context, evaluating a filehandle in angle brackets yields
1575 the next line from that file (the newline, if any, included), or
1576 C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
1577 (sometimes known as file-slurp mode) and the file is empty, it
1578 returns C<''> the first time, followed by C<undef> subsequently.
1580 Ordinarily you must assign the returned value to a variable, but
1581 there is one situation where an automatic assignment happens. If
1582 and only if the input symbol is the only thing inside the conditional
1583 of a C<while> statement (even if disguised as a C<for(;;)> loop),
1584 the value is automatically assigned to the global variable $_,
1585 destroying whatever was there previously. (This may seem like an
1586 odd thing to you, but you'll use the construct in almost every Perl
1587 script you write.) The $_ variables is not implicitly localized.
1588 You'll have to put a C<local $_;> before the loop if you want that
1591 The following lines are equivalent:
1593 while (defined($_ = <STDIN>)) { print; }
1594 while ($_ = <STDIN>) { print; }
1595 while (<STDIN>) { print; }
1596 for (;<STDIN>;) { print; }
1597 print while defined($_ = <STDIN>);
1598 print while ($_ = <STDIN>);
1599 print while <STDIN>;
1601 This also behaves similarly, but avoids $_ :
1603 while (my $line = <STDIN>) { print $line }
1605 In these loop constructs, the assigned value (whether assignment
1606 is automatic or explicit) is then tested to see whether it is
1607 defined. The defined test avoids problems where line has a string
1608 value that would be treated as false by Perl, for example a "" or
1609 a "0" with no trailing newline. If you really mean for such values
1610 to terminate the loop, they should be tested for explicitly:
1612 while (($_ = <STDIN>) ne '0') { ... }
1613 while (<STDIN>) { last unless $_; ... }
1615 In other boolean contexts, C<< <I<filehandle>> >> without an
1616 explicit C<defined> test or comparison elicit a warning if the
1617 C<use warnings> pragma or the B<-w>
1618 command-line switch (the C<$^W> variable) is in effect.
1620 The filehandles STDIN, STDOUT, and STDERR are predefined. (The
1621 filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
1622 in packages, where they would be interpreted as local identifiers
1623 rather than global.) Additional filehandles may be created with
1624 the open() function, amongst others. See L<perlopentut> and
1625 L<perlfunc/open> for details on this.
1627 If a <FILEHANDLE> is used in a context that is looking for
1628 a list, a list comprising all input lines is returned, one line per
1629 list element. It's easy to grow to a rather large data space this
1630 way, so use with care.
1632 <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
1633 See L<perlfunc/readline>.
1635 The null filehandle <> is special: it can be used to emulate the
1636 behavior of B<sed> and B<awk>. Input from <> comes either from
1637 standard input, or from each file listed on the command line. Here's
1638 how it works: the first time <> is evaluated, the @ARGV array is
1639 checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
1640 gives you standard input. The @ARGV array is then processed as a list
1641 of filenames. The loop
1644 ... # code for each line
1647 is equivalent to the following Perl-like pseudo code:
1649 unshift(@ARGV, '-') unless @ARGV;
1650 while ($ARGV = shift) {
1653 ... # code for each line
1657 except that it isn't so cumbersome to say, and will actually work.
1658 It really does shift the @ARGV array and put the current filename
1659 into the $ARGV variable. It also uses filehandle I<ARGV>
1660 internally--<> is just a synonym for <ARGV>, which
1661 is magical. (The pseudo code above doesn't work because it treats
1662 <ARGV> as non-magical.)
1664 You can modify @ARGV before the first <> as long as the array ends up
1665 containing the list of filenames you really want. Line numbers (C<$.>)
1666 continue as though the input were one big happy file. See the example
1667 in L<perlfunc/eof> for how to reset line numbers on each file.
1669 If you want to set @ARGV to your own list of files, go right ahead.
1670 This sets @ARGV to all plain text files if no @ARGV was given:
1672 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
1674 You can even set them to pipe commands. For example, this automatically
1675 filters compressed arguments through B<gzip>:
1677 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
1679 If you want to pass switches into your script, you can use one of the
1680 Getopts modules or put a loop on the front like this:
1682 while ($_ = $ARGV[0], /^-/) {
1685 if (/^-D(.*)/) { $debug = $1 }
1686 if (/^-v/) { $verbose++ }
1687 # ... # other switches
1691 # ... # code for each line
1694 The <> symbol will return C<undef> for end-of-file only once.
1695 If you call it again after this, it will assume you are processing another
1696 @ARGV list, and if you haven't set @ARGV, will read input from STDIN.
1698 If angle brackets contain is a simple scalar variable (e.g.,
1699 <$foo>), then that variable contains the name of the
1700 filehandle to input from, or its typeglob, or a reference to the
1706 If what's within the angle brackets is neither a filehandle nor a simple
1707 scalar variable containing a filehandle name, typeglob, or typeglob
1708 reference, it is interpreted as a filename pattern to be globbed, and
1709 either a list of filenames or the next filename in the list is returned,
1710 depending on context. This distinction is determined on syntactic
1711 grounds alone. That means C<< <$x> >> is always a readline() from
1712 an indirect handle, but C<< <$hash{key}> >> is always a glob().
1713 That's because $x is a simple scalar variable, but C<$hash{key}> is
1714 not--it's a hash element.
1716 One level of double-quote interpretation is done first, but you can't
1717 say C<< <$foo> >> because that's an indirect filehandle as explained
1718 in the previous paragraph. (In older versions of Perl, programmers
1719 would insert curly brackets to force interpretation as a filename glob:
1720 C<< <${foo}> >>. These days, it's considered cleaner to call the
1721 internal function directly as C<glob($foo)>, which is probably the right
1722 way to have done it in the first place.) For example:
1728 is roughly equivalent to:
1730 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
1736 except that the globbing is actually done internally using the standard
1737 C<File::Glob> extension. Of course, the shortest way to do the above is:
1741 A (file)glob evaluates its (embedded) argument only when it is
1742 starting a new list. All values must be read before it will start
1743 over. In list context, this isn't important because you automatically
1744 get them all anyway. However, in scalar context the operator returns
1745 the next value each time it's called, or C<undef> when the list has
1746 run out. As with filehandle reads, an automatic C<defined> is
1747 generated when the glob occurs in the test part of a C<while>,
1748 because legal glob returns (e.g. a file called F<0>) would otherwise
1749 terminate the loop. Again, C<undef> is returned only once. So if
1750 you're expecting a single value from a glob, it is much better to
1753 ($file) = <blurch*>;
1759 because the latter will alternate between returning a filename and
1762 It you're trying to do variable interpolation, it's definitely better
1763 to use the glob() function, because the older notation can cause people
1764 to become confused with the indirect filehandle notation.
1766 @files = glob("$dir/*.[ch]");
1767 @files = glob($files[$i]);
1769 =head2 Constant Folding
1771 Like C, Perl does a certain amount of expression evaluation at
1772 compile time whenever it determines that all arguments to an
1773 operator are static and have no side effects. In particular, string
1774 concatenation happens at compile time between literals that don't do
1775 variable substitution. Backslash interpolation also happens at
1776 compile time. You can say
1778 'Now is the time for all' . "\n" .
1779 'good men to come to.'
1781 and this all reduces to one string internally. Likewise, if
1784 foreach $file (@filenames) {
1785 if (-s $file > 5 + 100 * 2**16) { }
1788 the compiler will precompute the number which that expression
1789 represents so that the interpreter won't have to.
1791 =head2 Bitwise String Operators
1793 Bitstrings of any size may be manipulated by the bitwise operators
1796 If the operands to a binary bitwise op are strings of different
1797 sizes, B<|> and B<^> ops act as though the shorter operand had
1798 additional zero bits on the right, while the B<&> op acts as though
1799 the longer operand were truncated to the length of the shorter.
1800 The granularity for such extension or truncation is one or more
1803 # ASCII-based examples
1804 print "j p \n" ^ " a h"; # prints "JAPH\n"
1805 print "JA" | " ph\n"; # prints "japh\n"
1806 print "japh\nJunk" & '_____'; # prints "JAPH\n";
1807 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
1809 If you are intending to manipulate bitstrings, be certain that
1810 you're supplying bitstrings: If an operand is a number, that will imply
1811 a B<numeric> bitwise operation. You may explicitly show which type of
1812 operation you intend by using C<""> or C<0+>, as in the examples below.
1814 $foo = 150 | 105 ; # yields 255 (0x96 | 0x69 is 0xFF)
1815 $foo = '150' | 105 ; # yields 255
1816 $foo = 150 | '105'; # yields 255
1817 $foo = '150' | '105'; # yields string '155' (under ASCII)
1819 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
1820 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
1822 See L<perlfunc/vec> for information on how to manipulate individual bits
1825 =head2 Integer Arithmetic
1827 By default, Perl assumes that it must do most of its arithmetic in
1828 floating point. But by saying
1832 you may tell the compiler that it's okay to use integer operations
1833 (if it feels like it) from here to the end of the enclosing BLOCK.
1834 An inner BLOCK may countermand this by saying
1838 which lasts until the end of that BLOCK. Note that this doesn't
1839 mean everything is only an integer, merely that Perl may use integer
1840 operations if it is so inclined. For example, even under C<use
1841 integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731>
1844 Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
1845 and ">>") always produce integral results. (But see also L<Bitwise
1846 String Operators>.) However, C<use integer> still has meaning for
1847 them. By default, their results are interpreted as unsigned integers, but
1848 if C<use integer> is in effect, their results are interpreted
1849 as signed integers. For example, C<~0> usually evaluates to a large
1850 integral value. However, C<use integer; ~0> is C<-1> on twos-complement
1853 =head2 Floating-point Arithmetic
1855 While C<use integer> provides integer-only arithmetic, there is no
1856 analogous mechanism to provide automatic rounding or truncation to a
1857 certain number of decimal places. For rounding to a certain number
1858 of digits, sprintf() or printf() is usually the easiest route.
1861 Floating-point numbers are only approximations to what a mathematician
1862 would call real numbers. There are infinitely more reals than floats,
1863 so some corners must be cut. For example:
1865 printf "%.20g\n", 123456789123456789;
1866 # produces 123456789123456784
1868 Testing for exact equality of floating-point equality or inequality is
1869 not a good idea. Here's a (relatively expensive) work-around to compare
1870 whether two floating-point numbers are equal to a particular number of
1871 decimal places. See Knuth, volume II, for a more robust treatment of
1875 my ($X, $Y, $POINTS) = @_;
1877 $tX = sprintf("%.${POINTS}g", $X);
1878 $tY = sprintf("%.${POINTS}g", $Y);
1882 The POSIX module (part of the standard perl distribution) implements
1883 ceil(), floor(), and other mathematical and trigonometric functions.
1884 The Math::Complex module (part of the standard perl distribution)
1885 defines mathematical functions that work on both the reals and the
1886 imaginary numbers. Math::Complex not as efficient as POSIX, but
1887 POSIX can't work with complex numbers.
1889 Rounding in financial applications can have serious implications, and
1890 the rounding method used should be specified precisely. In these
1891 cases, it probably pays not to trust whichever system rounding is
1892 being used by Perl, but to instead implement the rounding function you
1895 =head2 Bigger Numbers
1897 The standard Math::BigInt and Math::BigFloat modules provide
1898 variable-precision arithmetic and overloaded operators, although
1899 they're currently pretty slow. At the cost of some space and
1900 considerable speed, they avoid the normal pitfalls associated with
1901 limited-precision representations.
1904 $x = Math::BigInt->new('123456789123456789');
1907 # prints +15241578780673678515622620750190521
1909 The non-standard modules SSLeay::BN and Math::Pari provide
1910 equivalent functionality (and much more) with a substantial
1911 performance savings.