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]*$/>, the increment is done as a string, preserving each
123 character within its range, with carry:
125 print ++($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
304 Binary "eq" returns true if the left argument is stringwise equal to
307 Binary "ne" returns true if the left argument is stringwise not equal
308 to the right argument.
310 Binary "cmp" returns -1, 0, or 1 depending on whether the left argument is stringwise
311 less than, equal to, or greater than the right argument.
313 "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
314 by the current locale if C<use locale> is in effect. See L<perllocale>.
318 Binary "&" returns its operators ANDed together bit by bit.
319 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
321 =head2 Bitwise Or and Exclusive Or
323 Binary "|" returns its operators ORed together bit by bit.
324 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
326 Binary "^" returns its operators XORed together bit by bit.
327 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
329 =head2 C-style Logical And
331 Binary "&&" performs a short-circuit logical AND operation. That is,
332 if the left operand is false, the right operand is not even evaluated.
333 Scalar or list context propagates down to the right operand if it
336 =head2 C-style Logical Or
338 Binary "||" performs a short-circuit logical OR operation. That is,
339 if the left operand is true, the right operand is not even evaluated.
340 Scalar or list context propagates down to the right operand if it
343 The C<||> and C<&&> operators differ from C's in that, rather than returning
344 0 or 1, they return the last value evaluated. Thus, a reasonably portable
345 way to find out the home directory (assuming it's not "0") might be:
347 $home = $ENV{'HOME'} || $ENV{'LOGDIR'} ||
348 (getpwuid($<))[7] || die "You're homeless!\n";
350 In particular, this means that you shouldn't use this
351 for selecting between two aggregates for assignment:
353 @a = @b || @c; # this is wrong
354 @a = scalar(@b) || @c; # really meant this
355 @a = @b ? @b : @c; # this works fine, though
357 As more readable alternatives to C<&&> and C<||> when used for
358 control flow, Perl provides C<and> and C<or> operators (see below).
359 The short-circuit behavior is identical. The precedence of "and" and
360 "or" is much lower, however, so that you can safely use them after a
361 list operator without the need for parentheses:
363 unlink "alpha", "beta", "gamma"
364 or gripe(), next LINE;
366 With the C-style operators that would have been written like this:
368 unlink("alpha", "beta", "gamma")
369 || (gripe(), next LINE);
371 Using "or" for assignment is unlikely to do what you want; see below.
373 =head2 Range Operators
375 Binary ".." is the range operator, which is really two different
376 operators depending on the context. In list context, it returns an
377 array of values counting (up by ones) from the left value to the right
378 value. If the left value is greater than the right value then it
379 returns the empty array. The range operator is useful for writing
380 C<foreach (1..10)> loops and for doing slice operations on arrays. In
381 the current implementation, no temporary array is created when the
382 range operator is used as the expression in C<foreach> loops, but older
383 versions of Perl might burn a lot of memory when you write something
386 for (1 .. 1_000_000) {
390 In scalar context, ".." returns a boolean value. The operator is
391 bistable, like a flip-flop, and emulates the line-range (comma) operator
392 of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
393 own boolean state. It is false as long as its left operand is false.
394 Once the left operand is true, the range operator stays true until the
395 right operand is true, I<AFTER> which the range operator becomes false
396 again. It doesn't become false till the next time the range operator is
397 evaluated. It can test the right operand and become false on the same
398 evaluation it became true (as in B<awk>), but it still returns true once.
399 If you don't want it to test the right operand till the next
400 evaluation, as in B<sed>, just use three dots ("...") instead of
401 two. In all other regards, "..." behaves just like ".." does.
403 The right operand is not evaluated while the operator is in the
404 "false" state, and the left operand is not evaluated while the
405 operator is in the "true" state. The precedence is a little lower
406 than || and &&. The value returned is either the empty string for
407 false, or a sequence number (beginning with 1) for true. The
408 sequence number is reset for each range encountered. The final
409 sequence number in a range has the string "E0" appended to it, which
410 doesn't affect its numeric value, but gives you something to search
411 for if you want to exclude the endpoint. You can exclude the
412 beginning point by waiting for the sequence number to be greater
413 than 1. If either operand of scalar ".." is a constant expression,
414 that operand is implicitly compared to the C<$.> variable, the
415 current line number. Examples:
417 As a scalar operator:
419 if (101 .. 200) { print; } # print 2nd hundred lines
420 next line if (1 .. /^$/); # skip header lines
421 s/^/> / if (/^$/ .. eof()); # quote body
423 # parse mail messages
425 $in_header = 1 .. /^$/;
426 $in_body = /^$/ .. eof();
427 # do something based on those
429 close ARGV if eof; # reset $. each file
434 for (101 .. 200) { print; } # print $_ 100 times
435 @foo = @foo[0 .. $#foo]; # an expensive no-op
436 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
438 The range operator (in list context) makes use of the magical
439 auto-increment algorithm if the operands are strings. You
442 @alphabet = ('A' .. 'Z');
444 to get all normal letters of the alphabet, or
446 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
448 to get a hexadecimal digit, or
450 @z2 = ('01' .. '31'); print $z2[$mday];
452 to get dates with leading zeros. If the final value specified is not
453 in the sequence that the magical increment would produce, the sequence
454 goes until the next value would be longer than the final value
457 =head2 Conditional Operator
459 Ternary "?:" is the conditional operator, just as in C. It works much
460 like an if-then-else. If the argument before the ? is true, the
461 argument before the : is returned, otherwise the argument after the :
462 is returned. For example:
464 printf "I have %d dog%s.\n", $n,
465 ($n == 1) ? '' : "s";
467 Scalar or list context propagates downward into the 2nd
468 or 3rd argument, whichever is selected.
470 $a = $ok ? $b : $c; # get a scalar
471 @a = $ok ? @b : @c; # get an array
472 $a = $ok ? @b : @c; # oops, that's just a count!
474 The operator may be assigned to if both the 2nd and 3rd arguments are
475 legal lvalues (meaning that you can assign to them):
477 ($a_or_b ? $a : $b) = $c;
479 Because this operator produces an assignable result, using assignments
480 without parentheses will get you in trouble. For example, this:
482 $a % 2 ? $a += 10 : $a += 2
486 (($a % 2) ? ($a += 10) : $a) += 2
490 ($a % 2) ? ($a += 10) : ($a += 2)
492 That should probably be written more simply as:
494 $a += ($a % 2) ? 10 : 2;
496 =head2 Assignment Operators
498 "=" is the ordinary assignment operator.
500 Assignment operators work as in C. That is,
508 although without duplicating any side effects that dereferencing the lvalue
509 might trigger, such as from tie(). Other assignment operators work similarly.
510 The following are recognized:
517 Although these are grouped by family, they all have the precedence
520 Unlike in C, the scalar assignment operator produces a valid lvalue.
521 Modifying an assignment is equivalent to doing the assignment and
522 then modifying the variable that was assigned to. This is useful
523 for modifying a copy of something, like this:
525 ($tmp = $global) =~ tr [A-Z] [a-z];
536 Similarly, a list assignment in list context produces the list of
537 lvalues assigned to, and a list assignment in scalar context returns
538 the number of elements produced by the expression on the right hand
539 side of the assignment.
541 =head2 Comma Operator
543 Binary "," is the comma operator. In scalar context it evaluates
544 its left argument, throws that value away, then evaluates its right
545 argument and returns that value. This is just like C's comma operator.
547 In list context, it's just the list argument separator, and inserts
548 both its arguments into the list.
550 The => digraph is mostly just a synonym for the comma operator. It's useful for
551 documenting arguments that come in pairs. As of release 5.001, it also forces
552 any word to the left of it to be interpreted as a string.
554 =head2 List Operators (Rightward)
556 On the right side of a list operator, it has very low precedence,
557 such that it controls all comma-separated expressions found there.
558 The only operators with lower precedence are the logical operators
559 "and", "or", and "not", which may be used to evaluate calls to list
560 operators without the need for extra parentheses:
562 open HANDLE, "filename"
563 or die "Can't open: $!\n";
565 See also discussion of list operators in L<Terms and List Operators (Leftward)>.
569 Unary "not" returns the logical negation of the expression to its right.
570 It's the equivalent of "!" except for the very low precedence.
574 Binary "and" returns the logical conjunction of the two surrounding
575 expressions. It's equivalent to && except for the very low
576 precedence. This means that it short-circuits: i.e., the right
577 expression is evaluated only if the left expression is true.
579 =head2 Logical or and Exclusive Or
581 Binary "or" returns the logical disjunction of the two surrounding
582 expressions. It's equivalent to || except for the very low precedence.
583 This makes it useful for control flow
585 print FH $data or die "Can't write to FH: $!";
587 This means that it short-circuits: i.e., the right expression is evaluated
588 only if the left expression is false. Due to its precedence, you should
589 probably avoid using this for assignment, only for control flow.
591 $a = $b or $c; # bug: this is wrong
592 ($a = $b) or $c; # really means this
593 $a = $b || $c; # better written this way
595 However, when it's a list-context assignment and you're trying to use
596 "||" for control flow, you probably need "or" so that the assignment
597 takes higher precedence.
599 @info = stat($file) || die; # oops, scalar sense of stat!
600 @info = stat($file) or die; # better, now @info gets its due
602 Then again, you could always use parentheses.
604 Binary "xor" returns the exclusive-OR of the two surrounding expressions.
605 It cannot short circuit, of course.
607 =head2 C Operators Missing From Perl
609 Here is what C has that Perl doesn't:
615 Address-of operator. (But see the "\" operator for taking a reference.)
619 Dereference-address operator. (Perl's prefix dereferencing
620 operators are typed: $, @, %, and &.)
624 Type-casting operator.
628 =head2 Quote and Quote-like Operators
630 While we usually think of quotes as literal values, in Perl they
631 function as operators, providing various kinds of interpolating and
632 pattern matching capabilities. Perl provides customary quote characters
633 for these behaviors, but also provides a way for you to choose your
634 quote character for any of them. In the following table, a C<{}> represents
635 any pair of delimiters you choose.
637 Customary Generic Meaning Interpolates
640 `` qx{} Command yes (unless '' is delimiter)
642 // m{} Pattern match yes (unless '' is delimiter)
643 qr{} Pattern yes (unless '' is delimiter)
644 s{}{} Substitution yes (unless '' is delimiter)
645 tr{}{} Transliteration no (but see below)
647 Non-bracketing delimiters use the same character fore and aft, but the four
648 sorts of brackets (round, angle, square, curly) will all nest, which means
657 Note, however, that this does not always work for quoting Perl code:
659 $s = q{ if($a eq "}") ... }; # WRONG
661 is a syntax error. The C<Text::Balanced> module on CPAN is able to do this
664 There can be whitespace between the operator and the quoting
665 characters, except when C<#> is being used as the quoting character.
666 C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
667 operator C<q> followed by a comment. Its argument will be taken
668 from the next line. This allows you to write:
670 s {foo} # Replace foo
673 For constructs that do interpolate, variables beginning with "C<$>"
674 or "C<@>" are interpolated, as are the following escape sequences. Within
675 a transliteration, the first eleven of these sequences may be used.
682 \a alarm (bell) (BEL)
684 \033 octal char (ESC)
686 \x{263a} wide hex char (SMILEY)
687 \c[ control char (ESC)
690 \l lowercase next char
691 \u uppercase next char
694 \E end case modification
695 \Q quote non-word characters till \E
697 If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u>
698 and C<\U> is taken from the current locale. See L<perllocale>. For
699 documentation of C<\N{name}>, see L<charnames>.
701 All systems use the virtual C<"\n"> to represent a line terminator,
702 called a "newline". There is no such thing as an unvarying, physical
703 newline character. It is only an illusion that the operating system,
704 device drivers, C libraries, and Perl all conspire to preserve. Not all
705 systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
706 on a Mac, these are reversed, and on systems without line terminator,
707 printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
708 you mean a "newline" for your system, but use the literal ASCII when you
709 need an exact character. For example, most networking protocols expect
710 and prefer a CR+LF (C<"\012\015"> or C<"\cJ\cM">) for line terminators,
711 and although they often accept just C<"\012">, they seldom tolerate just
712 C<"\015">. If you get in the habit of using C<"\n"> for networking,
713 you may be burned some day.
715 You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
716 An unescaped C<$> or C<@> interpolates the corresponding variable,
717 while escaping will cause the literal string C<\$> to be inserted.
718 You'll need to write something like C<m/\Quser\E\@\Qhost/>.
720 Patterns are subject to an additional level of interpretation as a
721 regular expression. This is done as a second pass, after variables are
722 interpolated, so that regular expressions may be incorporated into the
723 pattern from the variables. If this is not what you want, use C<\Q> to
724 interpolate a variable literally.
726 Apart from the behavior described above, Perl does not expand
727 multiple levels of interpolation. In particular, contrary to the
728 expectations of shell programmers, back-quotes do I<NOT> interpolate
729 within double quotes, nor do single quotes impede evaluation of
730 variables when used within double quotes.
732 =head2 Regexp Quote-Like Operators
734 Here are the quote-like operators that apply to pattern
735 matching and related activities.
741 This is just like the C</pattern/> search, except that it matches only
742 once between calls to the reset() operator. This is a useful
743 optimization when you want to see only the first occurrence of
744 something in each file of a set of files, for instance. Only C<??>
745 patterns local to the current package are reset.
749 # blank line between header and body
752 reset if eof; # clear ?? status for next file
755 This usage is vaguely depreciated, which means it just might possibly
756 be removed in some distant future version of Perl, perhaps somewhere
757 around the year 2168.
759 =item m/PATTERN/cgimosx
761 =item /PATTERN/cgimosx
763 Searches a string for a pattern match, and in scalar context returns
764 true if it succeeds, false if it fails. If no string is specified
765 via the C<=~> or C<!~> operator, the $_ string is searched. (The
766 string specified with C<=~> need not be an lvalue--it may be the
767 result of an expression evaluation, but remember the C<=~> binds
768 rather tightly.) See also L<perlre>. See L<perllocale> for
769 discussion of additional considerations that apply when C<use locale>
774 c Do not reset search position on a failed match when /g is in effect.
775 g Match globally, i.e., find all occurrences.
776 i Do case-insensitive pattern matching.
777 m Treat string as multiple lines.
778 o Compile pattern only once.
779 s Treat string as single line.
780 x Use extended regular expressions.
782 If "/" is the delimiter then the initial C<m> is optional. With the C<m>
783 you can use any pair of non-alphanumeric, non-whitespace characters
784 as delimiters. This is particularly useful for matching path names
785 that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
786 the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
787 If "'" is the delimiter, no interpolation is performed on the PATTERN.
789 PATTERN may contain variables, which will be interpolated (and the
790 pattern recompiled) every time the pattern search is evaluated, except
791 for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
792 C<$|> are not interpolated because they look like end-of-string tests.)
793 If you want such a pattern to be compiled only once, add a C</o> after
794 the trailing delimiter. This avoids expensive run-time recompilations,
795 and is useful when the value you are interpolating won't change over
796 the life of the script. However, mentioning C</o> constitutes a promise
797 that you won't change the variables in the pattern. If you change them,
798 Perl won't even notice. See also L<"qr//">.
800 If the PATTERN evaluates to the empty string, the last
801 I<successfully> matched regular expression is used instead.
803 If the C</g> option is not used, C<m//> in list context returns a
804 list consisting of the subexpressions matched by the parentheses in the
805 pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are
806 also set, and that this differs from Perl 4's behavior.) When there are
807 no parentheses in the pattern, the return value is the list C<(1)> for
808 success. With or without parentheses, an empty list is returned upon
813 open(TTY, '/dev/tty');
814 <TTY> =~ /^y/i && foo(); # do foo if desired
816 if (/Version: *([0-9.]*)/) { $version = $1; }
818 next if m#^/usr/spool/uucp#;
823 print if /$arg/o; # compile only once
826 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
828 This last example splits $foo into the first two words and the
829 remainder of the line, and assigns those three fields to $F1, $F2, and
830 $Etc. The conditional is true if any variables were assigned, i.e., if
833 The C</g> modifier specifies global pattern matching--that is,
834 matching as many times as possible within the string. How it behaves
835 depends on the context. In list context, it returns a list of the
836 substrings matched by any capturing parentheses in the regular
837 expression. If there are no parentheses, it returns a list of all
838 the matched strings, as if there were parentheses around the whole
841 In scalar context, each execution of C<m//g> finds the next match,
842 returning true if it matches, and false if there is no further match.
843 The position after the last match can be read or set using the pos()
844 function; see L<perlfunc/pos>. A failed match normally resets the
845 search position to the beginning of the string, but you can avoid that
846 by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
847 string also resets the search position.
849 You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
850 zero-width assertion that matches the exact position where the previous
851 C<m//g>, if any, left off. The C<\G> assertion is not supported without
852 the C</g> modifier. (Currently, without C</g>, C<\G> behaves just like
853 C<\A>, but that's accidental and may change in the future.)
858 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
861 $/ = ""; $* = 1; # $* deprecated in modern perls
862 while (defined($paragraph = <>)) {
863 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
867 print "$sentences\n";
869 # using m//gc with \G
873 print $1 while /(o)/gc; print "', pos=", pos, "\n";
875 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
877 print $1 while /(p)/gc; print "', pos=", pos, "\n";
880 The last example should print:
889 A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
890 combine several regexps like this to process a string part-by-part,
891 doing different actions depending on which regexp matched. Each
892 regexp tries to match where the previous one leaves off.
895 $url = new URI::URL "http://www/"; die if $url eq "xXx";
899 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
900 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
901 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
902 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
903 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
904 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
905 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
906 print ". That's all!\n";
909 Here is the output (split into several lines):
911 line-noise lowercase line-noise lowercase UPPERCASE line-noise
912 UPPERCASE line-noise lowercase line-noise lowercase line-noise
913 lowercase lowercase line-noise lowercase lowercase line-noise
914 MiXeD line-noise. That's all!
920 A single-quoted, literal string. A backslash represents a backslash
921 unless followed by the delimiter or another backslash, in which case
922 the delimiter or backslash is interpolated.
924 $foo = q!I said, "You said, 'She said it.'"!;
925 $bar = q('This is it.');
926 $baz = '\n'; # a two-character string
932 A double-quoted, interpolated string.
935 (*** The previous line contains the naughty word "$1".\n)
936 if /\b(tcl|java|python)\b/i; # :-)
937 $baz = "\n"; # a one-character string
939 =item qr/STRING/imosx
941 This operators quotes--and compiles--its I<STRING> as a regular
942 expression. I<STRING> is interpolated the same way as I<PATTERN>
943 in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
944 is done. Returns a Perl value which may be used instead of the
945 corresponding C</STRING/imosx> expression.
949 $rex = qr/my.STRING/is;
956 The result may be used as a subpattern in a match:
959 $string =~ /foo${re}bar/; # can be interpolated in other patterns
960 $string =~ $re; # or used standalone
961 $string =~ /$re/; # or this way
963 Since Perl may compile the pattern at the moment of execution of qr()
964 operator, using qr() may have speed advantages in some situations,
965 notably if the result of qr() is used standalone:
968 my $patterns = shift;
969 my @compiled = map qr/$_/i, @$patterns;
972 foreach my $pat (@compiled) {
973 $success = 1, last if /$pat/;
979 Precompilation of the pattern into an internal representation at
980 the moment of qr() avoids a need to recompile the pattern every
981 time a match C</$pat/> is attempted. (Perl has many other internal
982 optimizations, but none would be triggered in the above example if
983 we did not use qr() operator.)
987 i Do case-insensitive pattern matching.
988 m Treat string as multiple lines.
989 o Compile pattern only once.
990 s Treat string as single line.
991 x Use extended regular expressions.
993 See L<perlre> for additional information on valid syntax for STRING, and
994 for a detailed look at the semantics of regular expressions.
1000 A string which is (possibly) interpolated and then executed as a
1001 system command with C</bin/sh> or its equivalent. Shell wildcards,
1002 pipes, and redirections will be honored. The collected standard
1003 output of the command is returned; standard error is unaffected. In
1004 scalar context, it comes back as a single (potentially multi-line)
1005 string, or undef if the command failed. In list context, returns a
1006 list of lines (however you've defined lines with $/ or
1007 $INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
1009 Because backticks do not affect standard error, use shell file descriptor
1010 syntax (assuming the shell supports this) if you care to address this.
1011 To capture a command's STDERR and STDOUT together:
1013 $output = `cmd 2>&1`;
1015 To capture a command's STDOUT but discard its STDERR:
1017 $output = `cmd 2>/dev/null`;
1019 To capture a command's STDERR but discard its STDOUT (ordering is
1022 $output = `cmd 2>&1 1>/dev/null`;
1024 To exchange a command's STDOUT and STDERR in order to capture the STDERR
1025 but leave its STDOUT to come out the old STDERR:
1027 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
1029 To read both a command's STDOUT and its STDERR separately, it's easiest
1030 and safest to redirect them separately to files, and then read from those
1031 files when the program is done:
1033 system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr");
1035 Using single-quote as a delimiter protects the command from Perl's
1036 double-quote interpolation, passing it on to the shell instead:
1038 $perl_info = qx(ps $$); # that's Perl's $$
1039 $shell_info = qx'ps $$'; # that's the new shell's $$
1041 How that string gets evaluated is entirely subject to the command
1042 interpreter on your system. On most platforms, you will have to protect
1043 shell metacharacters if you want them treated literally. This is in
1044 practice difficult to do, as it's unclear how to escape which characters.
1045 See L<perlsec> for a clean and safe example of a manual fork() and exec()
1046 to emulate backticks safely.
1048 On some platforms (notably DOS-like ones), the shell may not be
1049 capable of dealing with multiline commands, so putting newlines in
1050 the string may not get you what you want. You may be able to evaluate
1051 multiple commands in a single line by separating them with the command
1052 separator character, if your shell supports that (e.g. C<;> on many Unix
1053 shells; C<&> on the Windows NT C<cmd> shell).
1055 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1056 output before starting the child process, but this may not be supported
1057 on some platforms (see L<perlport>). To be safe, you may need to set
1058 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1059 C<IO::Handle> on any open handles.
1061 Beware that some command shells may place restrictions on the length
1062 of the command line. You must ensure your strings don't exceed this
1063 limit after any necessary interpolations. See the platform-specific
1064 release notes for more details about your particular environment.
1066 Using this operator can lead to programs that are difficult to port,
1067 because the shell commands called vary between systems, and may in
1068 fact not be present at all. As one example, the C<type> command under
1069 the POSIX shell is very different from the C<type> command under DOS.
1070 That doesn't mean you should go out of your way to avoid backticks
1071 when they're the right way to get something done. Perl was made to be
1072 a glue language, and one of the things it glues together is commands.
1073 Just understand what you're getting yourself into.
1075 See L<"I/O Operators"> for more discussion.
1079 Evaluates to a list of the words extracted out of STRING, using embedded
1080 whitespace as the word delimiters. It can be understood as being roughly
1083 split(' ', q/STRING/);
1085 the difference being that it generates a real list at compile time. So
1090 is semantically equivalent to the list:
1094 Some frequently seen examples:
1096 use POSIX qw( setlocale localeconv )
1097 @EXPORT = qw( foo bar baz );
1099 A common mistake is to try to separate the words with comma or to
1100 put comments into a multi-line C<qw>-string. For this reason, the
1101 C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
1102 produces warnings if the STRING contains the "," or the "#" character.
1104 =item s/PATTERN/REPLACEMENT/egimosx
1106 Searches a string for a pattern, and if found, replaces that pattern
1107 with the replacement text and returns the number of substitutions
1108 made. Otherwise it returns false (specifically, the empty string).
1110 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
1111 variable is searched and modified. (The string specified with C<=~> must
1112 be scalar variable, an array element, a hash element, or an assignment
1113 to one of those, i.e., an lvalue.)
1115 If the delimiter chosen is a single quote, no interpolation is
1116 done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1117 PATTERN contains a $ that looks like a variable rather than an
1118 end-of-string test, the variable will be interpolated into the pattern
1119 at run-time. If you want the pattern compiled only once the first time
1120 the variable is interpolated, use the C</o> option. If the pattern
1121 evaluates to the empty string, the last successfully executed regular
1122 expression is used instead. See L<perlre> for further explanation on these.
1123 See L<perllocale> for discussion of additional considerations that apply
1124 when C<use locale> is in effect.
1128 e Evaluate the right side as an expression.
1129 g Replace globally, i.e., all occurrences.
1130 i Do case-insensitive pattern matching.
1131 m Treat string as multiple lines.
1132 o Compile pattern only once.
1133 s Treat string as single line.
1134 x Use extended regular expressions.
1136 Any non-alphanumeric, non-whitespace delimiter may replace the
1137 slashes. If single quotes are used, no interpretation is done on the
1138 replacement string (the C</e> modifier overrides this, however). Unlike
1139 Perl 4, Perl 5 treats backticks as normal delimiters; the replacement
1140 text is not evaluated as a command. If the
1141 PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
1142 pair of quotes, which may or may not be bracketing quotes, e.g.,
1143 C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
1144 replacement portion to be treated as a full-fledged Perl expression
1145 and evaluated right then and there. It is, however, syntax checked at
1146 compile-time. A second C<e> modifier will cause the replacement portion
1147 to be C<eval>ed before being run as a Perl expression.
1151 s/\bgreen\b/mauve/g; # don't change wintergreen
1153 $path =~ s|/usr/bin|/usr/local/bin|;
1155 s/Login: $foo/Login: $bar/; # run-time pattern
1157 ($foo = $bar) =~ s/this/that/; # copy first, then change
1159 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
1162 s/\d+/$&*2/e; # yields 'abc246xyz'
1163 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
1164 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1166 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
1167 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
1168 s/^=(\w+)/&pod($1)/ge; # use function call
1170 # expand variables in $_, but dynamics only, using
1171 # symbolic dereferencing
1174 # Add one to the value of any numbers in the string
1177 # This will expand any embedded scalar variable
1178 # (including lexicals) in $_ : First $1 is interpolated
1179 # to the variable name, and then evaluated
1182 # Delete (most) C comments.
1184 /\* # Match the opening delimiter.
1185 .*? # Match a minimal number of characters.
1186 \*/ # Match the closing delimiter.
1189 s/^\s*(.*?)\s*$/$1/; # trim white space in $_, expensively
1191 for ($variable) { # trim white space in $variable, cheap
1196 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
1198 Note the use of $ instead of \ in the last example. Unlike
1199 B<sed>, we use the \<I<digit>> form in only the left hand side.
1200 Anywhere else it's $<I<digit>>.
1202 Occasionally, you can't use just a C</g> to get all the changes
1203 to occur that you might want. Here are two common cases:
1205 # put commas in the right places in an integer
1206 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
1208 # expand tabs to 8-column spacing
1209 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
1211 =item tr/SEARCHLIST/REPLACEMENTLIST/cds
1213 =item y/SEARCHLIST/REPLACEMENTLIST/cds
1215 Transliterates all occurrences of the characters found in the search list
1216 with the corresponding character in the replacement list. It returns
1217 the number of characters replaced or deleted. If no string is
1218 specified via the =~ or !~ operator, the $_ string is transliterated. (The
1219 string specified with =~ must be a scalar variable, an array element, a
1220 hash element, or an assignment to one of those, i.e., an lvalue.)
1222 A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
1223 does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
1224 For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1225 SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1226 its own pair of quotes, which may or may not be bracketing quotes,
1227 e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
1229 Note that C<tr> does B<not> do regular expression character classes
1230 such as C<\d> or C<[:lower:]>. The <tr> operator is not equivalent to
1231 the tr(1) utility. If you want to map strings between lower/upper
1232 cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider
1233 using the C<s> operator if you need regular expressions.
1235 Note also that the whole range idea is rather unportable between
1236 character sets--and even within character sets they may cause results
1237 you probably didn't expect. A sound principle is to use only ranges
1238 that begin from and end at either alphabets of equal case (a-e, A-E),
1239 or digits (0-4). Anything else is unsafe. If in doubt, spell out the
1240 character sets in full.
1244 c Complement the SEARCHLIST.
1245 d Delete found but unreplaced characters.
1246 s Squash duplicate replaced characters.
1248 If the C</c> modifier is specified, the SEARCHLIST character set
1249 is complemented. If the C</d> modifier is specified, any characters
1250 specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
1251 (Note that this is slightly more flexible than the behavior of some
1252 B<tr> programs, which delete anything they find in the SEARCHLIST,
1253 period.) If the C</s> modifier is specified, sequences of characters
1254 that were transliterated to the same character are squashed down
1255 to a single instance of the character.
1257 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1258 exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1259 than the SEARCHLIST, the final character is replicated till it is long
1260 enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
1261 This latter is useful for counting characters in a class or for
1262 squashing character sequences in a class.
1266 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1268 $cnt = tr/*/*/; # count the stars in $_
1270 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1272 $cnt = tr/0-9//; # count the digits in $_
1274 tr/a-zA-Z//s; # bookkeeper -> bokeper
1276 ($HOST = $host) =~ tr/a-z/A-Z/;
1278 tr/a-zA-Z/ /cs; # change non-alphas to single space
1281 [\000-\177]; # delete 8th bit
1283 If multiple transliterations are given for a character, only the
1288 will transliterate any A to X.
1290 Because the transliteration table is built at compile time, neither
1291 the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
1292 interpolation. That means that if you want to use variables, you
1295 eval "tr/$oldlist/$newlist/";
1298 eval "tr/$oldlist/$newlist/, 1" or die $@;
1302 =head2 Gory details of parsing quoted constructs
1304 When presented with something that might have several different
1305 interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
1306 principle to pick the most probable interpretation. This strategy
1307 is so successful that Perl programmers often do not suspect the
1308 ambivalence of what they write. But from time to time, Perl's
1309 notions differ substantially from what the author honestly meant.
1311 This section hopes to clarify how Perl handles quoted constructs.
1312 Although the most common reason to learn this is to unravel labyrinthine
1313 regular expressions, because the initial steps of parsing are the
1314 same for all quoting operators, they are all discussed together.
1316 The most important Perl parsing rule is the first one discussed
1317 below: when processing a quoted construct, Perl first finds the end
1318 of that construct, then interprets its contents. If you understand
1319 this rule, you may skip the rest of this section on the first
1320 reading. The other rules are likely to contradict the user's
1321 expectations much less frequently than this first one.
1323 Some passes discussed below are performed concurrently, but because
1324 their results are the same, we consider them individually. For different
1325 quoting constructs, Perl performs different numbers of passes, from
1326 one to five, but these passes are always performed in the same order.
1330 =item Finding the end
1332 The first pass is finding the end of the quoted construct, whether
1333 it be a multicharacter delimiter C<"\nEOF\n"> in the C<<<EOF>
1334 construct, a C</> that terminates a C<qq//> construct, a C<]> which
1335 terminates C<qq[]> construct, or a C<< > >> which terminates a
1336 fileglob started with C<< < >>.
1338 When searching for single-character non-pairing delimiters, such
1339 as C</>, combinations of C<\\> and C<\/> are skipped. However,
1340 when searching for single-character pairing delimiter like C<[>,
1341 combinations of C<\\>, C<\]>, and C<\[> are all skipped, and nested
1342 C<[>, C<]> are skipped as well. When searching for multicharacter
1343 delimiters, nothing is skipped.
1345 For constructs with three-part delimiters (C<s///>, C<y///>, and
1346 C<tr///>), the search is repeated once more.
1348 During this search no attention is paid to the semantics of the construct.
1351 "$hash{"$foo/$bar"}"
1356 bar # NOT a comment, this slash / terminated m//!
1359 do not form legal quoted expressions. The quoted part ends on the
1360 first C<"> and C</>, and the rest happens to be a syntax error.
1361 Because the slash that terminated C<m//> was followed by a C<SPACE>,
1362 the example above is not C<m//x>, but rather C<m//> with no C</x>
1363 modifier. So the embedded C<#> is interpreted as a literal C<#>.
1365 =item Removal of backslashes before delimiters
1367 During the second pass, text between the starting and ending
1368 delimiters is copied to a safe location, and the C<\> is removed
1369 from combinations consisting of C<\> and delimiter--or delimiters,
1370 meaning both starting and ending delimiters will should these differ.
1371 This removal does not happen for multi-character delimiters.
1372 Note that the combination C<\\> is left intact, just as it was.
1374 Starting from this step no information about the delimiters is
1379 The next step is interpolation in the text obtained, which is now
1380 delimiter-independent. There are four different cases.
1384 =item C<<<'EOF'>, C<m''>, C<s'''>, C<tr///>, C<y///>
1386 No interpolation is performed.
1390 The only interpolation is removal of C<\> from pairs C<\\>.
1392 =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>
1394 C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
1395 converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
1396 is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
1397 The other combinations are replaced with appropriate expansions.
1399 Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
1400 is interpolated in the usual way. Something like C<"\Q\\E"> has
1401 no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
1402 result is the same as for C<"\\\\E">. As a general rule, backslashes
1403 between C<\Q> and C<\E> may lead to counterintuitive results. So,
1404 C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
1405 as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
1410 may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
1412 Interpolated scalars and arrays are converted internally to the C<join> and
1413 C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
1415 $foo . " XXX '" . (join $", @arr) . "'";
1417 All operations above are performed simultaneously, left to right.
1419 Because the result of C<"\Q STRING \E"> has all metacharacters
1420 quoted, there is no way to insert a literal C<$> or C<@> inside a
1421 C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
1422 C<"\\\$">; if not, it is interpreted as the start of an interpolated
1425 Note also that the interpolation code needs to make a decision on
1426 where the interpolated scalar ends. For instance, whether
1427 C<< "a $b -> {c}" >> really means:
1429 "a " . $b . " -> {c}";
1435 Most of the time, the longest possible text that does not include
1436 spaces between components and which contains matching braces or
1437 brackets. because the outcome may be determined by voting based
1438 on heuristic estimators, the result is not strictly predictable.
1439 Fortunately, it's usually correct for ambiguous cases.
1441 =item C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
1443 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
1444 happens (almost) as with C<qq//> constructs, but the substitution
1445 of C<\> followed by RE-special chars (including C<\>) is not
1446 performed. Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
1447 a C<#>-comment in a C<//x>-regular expression, no processing is
1448 performed whatsoever. This is the first step at which the presence
1449 of the C<//x> modifier is relevant.
1451 Interpolation has several quirks: C<$|>, C<$(>, and C<$)> are not
1452 interpolated, and constructs C<$var[SOMETHING]> are voted (by several
1453 different estimators) to be either an array element or C<$var>
1454 followed by an RE alternative. This is where the notation
1455 C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
1456 array element C<-9>, not as a regular expression from the variable
1457 C<$arr> followed by a digit, which would be the interpretation of
1458 C</$arr[0-9]/>. Since voting among different estimators may occur,
1459 the result is not predictable.
1461 It is at this step that C<\1> is begrudgingly converted to C<$1> in
1462 the replacement text of C<s///> to correct the incorrigible
1463 I<sed> hackers who haven't picked up the saner idiom yet. A warning
1464 is emitted if the C<use warnings> pragma or the B<-w> command-line flag
1465 (that is, the C<$^W> variable) was set.
1467 The lack of processing of C<\\> creates specific restrictions on
1468 the post-processed text. If the delimiter is C</>, one cannot get
1469 the combination C<\/> into the result of this step. C</> will
1470 finish the regular expression, C<\/> will be stripped to C</> on
1471 the previous step, and C<\\/> will be left as is. Because C</> is
1472 equivalent to C<\/> inside a regular expression, this does not
1473 matter unless the delimiter happens to be character special to the
1474 RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
1475 alphanumeric char, as in:
1479 In the RE above, which is intentionally obfuscated for illustration, the
1480 delimiter is C<m>, the modifier is C<mx>, and after backslash-removal the
1481 RE is the same as for C<m/ ^ a s* b /mx>). There's more than one
1482 reason you're encouraged to restrict your delimiters to non-alphanumeric,
1483 non-whitespace choices.
1487 This step is the last one for all constructs except regular expressions,
1488 which are processed further.
1490 =item Interpolation of regular expressions
1492 Previous steps were performed during the compilation of Perl code,
1493 but this one happens at run time--although it may be optimized to
1494 be calculated at compile time if appropriate. After preprocessing
1495 described above, and possibly after evaluation if catenation,
1496 joining, casing translation, or metaquoting are involved, the
1497 resulting I<string> is passed to the RE engine for compilation.
1499 Whatever happens in the RE engine might be better discussed in L<perlre>,
1500 but for the sake of continuity, we shall do so here.
1502 This is another step where the presence of the C<//x> modifier is
1503 relevant. The RE engine scans the string from left to right and
1504 converts it to a finite automaton.
1506 Backslashed characters are either replaced with corresponding
1507 literal strings (as with C<\{>), or else they generate special nodes
1508 in the finite automaton (as with C<\b>). Characters special to the
1509 RE engine (such as C<|>) generate corresponding nodes or groups of
1510 nodes. C<(?#...)> comments are ignored. All the rest is either
1511 converted to literal strings to match, or else is ignored (as is
1512 whitespace and C<#>-style comments if C<//x> is present).
1514 Parsing of the bracketed character class construct, C<[...]>, is
1515 rather different than the rule used for the rest of the pattern.
1516 The terminator of this construct is found using the same rules as
1517 for finding the terminator of a C<{}>-delimited construct, the only
1518 exception being that C<]> immediately following C<[> is treated as
1519 though preceded by a backslash. Similarly, the terminator of
1520 C<(?{...})> is found using the same rules as for finding the
1521 terminator of a C<{}>-delimited construct.
1523 It is possible to inspect both the string given to RE engine and the
1524 resulting finite automaton. See the arguments C<debug>/C<debugcolor>
1525 in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
1526 switch documented in L<perlrun/"Command Switches">.
1528 =item Optimization of regular expressions
1530 This step is listed for completeness only. Since it does not change
1531 semantics, details of this step are not documented and are subject
1532 to change without notice. This step is performed over the finite
1533 automaton that was generated during the previous pass.
1535 It is at this stage that C<split()> silently optimizes C</^/> to
1540 =head2 I/O Operators
1542 There are several I/O operators you should know about.
1544 A string enclosed by backticks (grave accents) first undergoes
1545 double-quote interpolation. It is then interpreted as an external
1546 command, and the output of that command is the value of the
1548 string consisting of all output is returned. In list context, a
1549 list of values is returned, one per line of output. (You can set
1550 C<$/> to use a different line terminator.) The command is executed
1551 each time the pseudo-literal is evaluated. The status value of the
1552 command is returned in C<$?> (see L<perlvar> for the interpretation
1553 of C<$?>). Unlike in B<csh>, no translation is done on the return
1554 data--newlines remain newlines. Unlike in any of the shells, single
1555 quotes do not hide variable names in the command from interpretation.
1556 To pass a literal dollar-sign through to the shell you need to hide
1557 it with a backslash. The generalized form of backticks is C<qx//>.
1558 (Because backticks always undergo shell expansion as well, see
1559 L<perlsec> for security concerns.)
1561 In scalar context, evaluating a filehandle in angle brackets yields
1562 the next line from that file (the newline, if any, included), or
1563 C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
1564 (sometimes known as file-slurp mode) and the file is empty, it
1565 returns C<''> the first time, followed by C<undef> subsequently.
1567 Ordinarily you must assign the returned value to a variable, but
1568 there is one situation where an automatic assignment happens. If
1569 and only if the input symbol is the only thing inside the conditional
1570 of a C<while> statement (even if disguised as a C<for(;;)> loop),
1571 the value is automatically assigned to the global variable $_,
1572 destroying whatever was there previously. (This may seem like an
1573 odd thing to you, but you'll use the construct in almost every Perl
1574 script you write.) The $_ variables is not implicitly localized.
1575 You'll have to put a C<local $_;> before the loop if you want that
1578 The following lines are equivalent:
1580 while (defined($_ = <STDIN>)) { print; }
1581 while ($_ = <STDIN>) { print; }
1582 while (<STDIN>) { print; }
1583 for (;<STDIN>;) { print; }
1584 print while defined($_ = <STDIN>);
1585 print while ($_ = <STDIN>);
1586 print while <STDIN>;
1588 This also behaves similarly, but avoids $_ :
1590 while (my $line = <STDIN>) { print $line }
1592 In these loop constructs, the assigned value (whether assignment
1593 is automatic or explicit) is then tested to see whether it is
1594 defined. The defined test avoids problems where line has a string
1595 value that would be treated as false by Perl, for example a "" or
1596 a "0" with no trailing newline. If you really mean for such values
1597 to terminate the loop, they should be tested for explicitly:
1599 while (($_ = <STDIN>) ne '0') { ... }
1600 while (<STDIN>) { last unless $_; ... }
1602 In other boolean contexts, C<< <I<filehandle>> >> without an
1603 explicit C<defined> test or comparison elicit a warning if the
1604 C<use warnings> pragma or the B<-w>
1605 command-line switch (the C<$^W> variable) is in effect.
1607 The filehandles STDIN, STDOUT, and STDERR are predefined. (The
1608 filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
1609 in packages, where they would be interpreted as local identifiers
1610 rather than global.) Additional filehandles may be created with
1611 the open() function, amongst others. See L<perlopentut> and
1612 L<perlfunc/open> for details on this.
1614 If a <FILEHANDLE> is used in a context that is looking for
1615 a list, a list comprising all input lines is returned, one line per
1616 list element. It's easy to grow to a rather large data space this
1617 way, so use with care.
1619 <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
1620 See L<perlfunc/readline>.
1622 The null filehandle <> is special: it can be used to emulate the
1623 behavior of B<sed> and B<awk>. Input from <> comes either from
1624 standard input, or from each file listed on the command line. Here's
1625 how it works: the first time <> is evaluated, the @ARGV array is
1626 checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
1627 gives you standard input. The @ARGV array is then processed as a list
1628 of filenames. The loop
1631 ... # code for each line
1634 is equivalent to the following Perl-like pseudo code:
1636 unshift(@ARGV, '-') unless @ARGV;
1637 while ($ARGV = shift) {
1640 ... # code for each line
1644 except that it isn't so cumbersome to say, and will actually work.
1645 It really does shift the @ARGV array and put the current filename
1646 into the $ARGV variable. It also uses filehandle I<ARGV>
1647 internally--<> is just a synonym for <ARGV>, which
1648 is magical. (The pseudo code above doesn't work because it treats
1649 <ARGV> as non-magical.)
1651 You can modify @ARGV before the first <> as long as the array ends up
1652 containing the list of filenames you really want. Line numbers (C<$.>)
1653 continue as though the input were one big happy file. See the example
1654 in L<perlfunc/eof> for how to reset line numbers on each file.
1656 If you want to set @ARGV to your own list of files, go right ahead.
1657 This sets @ARGV to all plain text files if no @ARGV was given:
1659 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
1661 You can even set them to pipe commands. For example, this automatically
1662 filters compressed arguments through B<gzip>:
1664 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
1666 If you want to pass switches into your script, you can use one of the
1667 Getopts modules or put a loop on the front like this:
1669 while ($_ = $ARGV[0], /^-/) {
1672 if (/^-D(.*)/) { $debug = $1 }
1673 if (/^-v/) { $verbose++ }
1674 # ... # other switches
1678 # ... # code for each line
1681 The <> symbol will return C<undef> for end-of-file only once.
1682 If you call it again after this, it will assume you are processing another
1683 @ARGV list, and if you haven't set @ARGV, will read input from STDIN.
1685 If angle brackets contain is a simple scalar variable (e.g.,
1686 <$foo>), then that variable contains the name of the
1687 filehandle to input from, or its typeglob, or a reference to the
1693 If what's within the angle brackets is neither a filehandle nor a simple
1694 scalar variable containing a filehandle name, typeglob, or typeglob
1695 reference, it is interpreted as a filename pattern to be globbed, and
1696 either a list of filenames or the next filename in the list is returned,
1697 depending on context. This distinction is determined on syntactic
1698 grounds alone. That means C<< <$x> >> is always a readline() from
1699 an indirect handle, but C<< <$hash{key}> >> is always a glob().
1700 That's because $x is a simple scalar variable, but C<$hash{key}> is
1701 not--it's a hash element.
1703 One level of double-quote interpretation is done first, but you can't
1704 say C<< <$foo> >> because that's an indirect filehandle as explained
1705 in the previous paragraph. (In older versions of Perl, programmers
1706 would insert curly brackets to force interpretation as a filename glob:
1707 C<< <${foo}> >>. These days, it's considered cleaner to call the
1708 internal function directly as C<glob($foo)>, which is probably the right
1709 way to have done it in the first place.) For example:
1715 is roughly equivalent to:
1717 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
1723 except that the globbing is actually done internally using the standard
1724 C<File::Glob> extension. Of course, the shortest way to do the above is:
1728 A (file)glob evaluates its (embedded) argument only when it is
1729 starting a new list. All values must be read before it will start
1730 over. In list context, this isn't important because you automatically
1731 get them all anyway. However, in scalar context the operator returns
1732 the next value each time it's called, or C<undef> when the list has
1733 run out. As with filehandle reads, an automatic C<defined> is
1734 generated when the glob occurs in the test part of a C<while>,
1735 because legal glob returns (e.g. a file called F<0>) would otherwise
1736 terminate the loop. Again, C<undef> is returned only once. So if
1737 you're expecting a single value from a glob, it is much better to
1740 ($file) = <blurch*>;
1746 because the latter will alternate between returning a filename and
1749 It you're trying to do variable interpolation, it's definitely better
1750 to use the glob() function, because the older notation can cause people
1751 to become confused with the indirect filehandle notation.
1753 @files = glob("$dir/*.[ch]");
1754 @files = glob($files[$i]);
1756 =head2 Constant Folding
1758 Like C, Perl does a certain amount of expression evaluation at
1759 compile time whenever it determines that all arguments to an
1760 operator are static and have no side effects. In particular, string
1761 concatenation happens at compile time between literals that don't do
1762 variable substitution. Backslash interpolation also happens at
1763 compile time. You can say
1765 'Now is the time for all' . "\n" .
1766 'good men to come to.'
1768 and this all reduces to one string internally. Likewise, if
1771 foreach $file (@filenames) {
1772 if (-s $file > 5 + 100 * 2**16) { }
1775 the compiler will precompute the number which that expression
1776 represents so that the interpreter won't have to.
1778 =head2 Bitwise String Operators
1780 Bitstrings of any size may be manipulated by the bitwise operators
1783 If the operands to a binary bitwise op are strings of different
1784 sizes, B<|> and B<^> ops act as though the shorter operand had
1785 additional zero bits on the right, while the B<&> op acts as though
1786 the longer operand were truncated to the length of the shorter.
1787 The granularity for such extension or truncation is one or more
1790 # ASCII-based examples
1791 print "j p \n" ^ " a h"; # prints "JAPH\n"
1792 print "JA" | " ph\n"; # prints "japh\n"
1793 print "japh\nJunk" & '_____'; # prints "JAPH\n";
1794 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
1796 If you are intending to manipulate bitstrings, be certain that
1797 you're supplying bitstrings: If an operand is a number, that will imply
1798 a B<numeric> bitwise operation. You may explicitly show which type of
1799 operation you intend by using C<""> or C<0+>, as in the examples below.
1801 $foo = 150 | 105 ; # yields 255 (0x96 | 0x69 is 0xFF)
1802 $foo = '150' | 105 ; # yields 255
1803 $foo = 150 | '105'; # yields 255
1804 $foo = '150' | '105'; # yields string '155' (under ASCII)
1806 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
1807 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
1809 See L<perlfunc/vec> for information on how to manipulate individual bits
1812 =head2 Integer Arithmetic
1814 By default, Perl assumes that it must do most of its arithmetic in
1815 floating point. But by saying
1819 you may tell the compiler that it's okay to use integer operations
1820 (if it feels like it) from here to the end of the enclosing BLOCK.
1821 An inner BLOCK may countermand this by saying
1825 which lasts until the end of that BLOCK. Note that this doesn't
1826 mean everything is only an integer, merely that Perl may use integer
1827 operations if it is so inclined. For example, even under C<use
1828 integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731>
1831 Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
1832 and ">>") always produce integral results. (But see also L<Bitwise
1833 String Operators>.) However, C<use integer> still has meaning for
1834 them. By default, their results are interpreted as unsigned integers, but
1835 if C<use integer> is in effect, their results are interpreted
1836 as signed integers. For example, C<~0> usually evaluates to a large
1837 integral value. However, C<use integer; ~0> is C<-1> on twos-complement
1840 =head2 Floating-point Arithmetic
1842 While C<use integer> provides integer-only arithmetic, there is no
1843 analogous mechanism to provide automatic rounding or truncation to a
1844 certain number of decimal places. For rounding to a certain number
1845 of digits, sprintf() or printf() is usually the easiest route.
1848 Floating-point numbers are only approximations to what a mathematician
1849 would call real numbers. There are infinitely more reals than floats,
1850 so some corners must be cut. For example:
1852 printf "%.20g\n", 123456789123456789;
1853 # produces 123456789123456784
1855 Testing for exact equality of floating-point equality or inequality is
1856 not a good idea. Here's a (relatively expensive) work-around to compare
1857 whether two floating-point numbers are equal to a particular number of
1858 decimal places. See Knuth, volume II, for a more robust treatment of
1862 my ($X, $Y, $POINTS) = @_;
1864 $tX = sprintf("%.${POINTS}g", $X);
1865 $tY = sprintf("%.${POINTS}g", $Y);
1869 The POSIX module (part of the standard perl distribution) implements
1870 ceil(), floor(), and other mathematical and trigonometric functions.
1871 The Math::Complex module (part of the standard perl distribution)
1872 defines mathematical functions that work on both the reals and the
1873 imaginary numbers. Math::Complex not as efficient as POSIX, but
1874 POSIX can't work with complex numbers.
1876 Rounding in financial applications can have serious implications, and
1877 the rounding method used should be specified precisely. In these
1878 cases, it probably pays not to trust whichever system rounding is
1879 being used by Perl, but to instead implement the rounding function you
1882 =head2 Bigger Numbers
1884 The standard Math::BigInt and Math::BigFloat modules provide
1885 variable-precision arithmetic and overloaded operators, although
1886 they're currently pretty slow. At the cost of some space and
1887 considerable speed, they avoid the normal pitfalls associated with
1888 limited-precision representations.
1891 $x = Math::BigInt->new('123456789123456789');
1894 # prints +15241578780673678515622620750190521
1896 The non-standard modules SSLeay::BN and Math::Pari provide
1897 equivalent functionality (and much more) with a substantial
1898 performance savings.