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 $_. The return value indicates the success of the operation. If the
176 right argument is an expression rather than a search pattern,
177 substitution, or transliteration, it is interpreted as a search pattern at run
178 time. This can be less efficient than an explicit search, because the
179 pattern must be compiled every time the expression is evaluated.
181 Binary "!~" is just like "=~" except the return value is negated in
184 =head2 Multiplicative Operators
186 Binary "*" multiplies two numbers.
188 Binary "/" divides two numbers.
190 Binary "%" computes the modulus of two numbers. Given integer
191 operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
192 C<$a> minus the largest multiple of C<$b> that is not greater than
193 C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
194 smallest multiple of C<$b> that is not less than C<$a> (i.e. the
195 result will be less than or equal to zero).
196 Note than when C<use integer> is in scope, "%" give you direct access
197 to the modulus operator as implemented by your C compiler. This
198 operator is not as well defined for negative operands, but it will
201 Binary "x" is the repetition operator. In scalar context or if the left
202 operand is not enclosed in parentheses, it returns a string consisting
203 of the left operand repeated the number of times specified by the right
204 operand. In list context, if the left operand is enclosed in
205 parentheses, it repeats the list.
207 print '-' x 80; # print row of dashes
209 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
211 @ones = (1) x 80; # a list of 80 1's
212 @ones = (5) x @ones; # set all elements to 5
215 =head2 Additive Operators
217 Binary "+" returns the sum of two numbers.
219 Binary "-" returns the difference of two numbers.
221 Binary "." concatenates two strings.
223 =head2 Shift Operators
225 Binary "<<" returns the value of its left argument shifted left by the
226 number of bits specified by the right argument. Arguments should be
227 integers. (See also L<Integer Arithmetic>.)
229 Binary ">>" returns the value of its left argument shifted right by
230 the number of bits specified by the right argument. Arguments should
231 be integers. (See also L<Integer Arithmetic>.)
233 =head2 Named Unary Operators
235 The various named unary operators are treated as functions with one
236 argument, with optional parentheses. These include the filetest
237 operators, like C<-f>, C<-M>, etc. See L<perlfunc>.
239 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
240 is followed by a left parenthesis as the next token, the operator and
241 arguments within parentheses are taken to be of highest precedence,
242 just like a normal function call. Examples:
244 chdir $foo || die; # (chdir $foo) || die
245 chdir($foo) || die; # (chdir $foo) || die
246 chdir ($foo) || die; # (chdir $foo) || die
247 chdir +($foo) || die; # (chdir $foo) || die
249 but, because * is higher precedence than ||:
251 chdir $foo * 20; # chdir ($foo * 20)
252 chdir($foo) * 20; # (chdir $foo) * 20
253 chdir ($foo) * 20; # (chdir $foo) * 20
254 chdir +($foo) * 20; # chdir ($foo * 20)
256 rand 10 * 20; # rand (10 * 20)
257 rand(10) * 20; # (rand 10) * 20
258 rand (10) * 20; # (rand 10) * 20
259 rand +(10) * 20; # rand (10 * 20)
261 See also L<"Terms and List Operators (Leftward)">.
263 =head2 Relational Operators
265 Binary "<" returns true if the left argument is numerically less than
268 Binary ">" returns true if the left argument is numerically greater
269 than the right argument.
271 Binary "<=" returns true if the left argument is numerically less than
272 or equal to the right argument.
274 Binary ">=" returns true if the left argument is numerically greater
275 than or equal to the right argument.
277 Binary "lt" returns true if the left argument is stringwise less than
280 Binary "gt" returns true if the left argument is stringwise greater
281 than the right argument.
283 Binary "le" returns true if the left argument is stringwise less than
284 or equal to the right argument.
286 Binary "ge" returns true if the left argument is stringwise greater
287 than or equal to the right argument.
289 =head2 Equality Operators
291 Binary "==" returns true if the left argument is numerically equal to
294 Binary "!=" returns true if the left argument is numerically not equal
295 to the right argument.
297 Binary "<=>" returns -1, 0, or 1 depending on whether the left
298 argument is numerically less than, equal to, or greater than the right
301 Binary "eq" returns true if the left argument is stringwise equal to
304 Binary "ne" returns true if the left argument is stringwise not equal
305 to the right argument.
307 Binary "cmp" returns -1, 0, or 1 depending on whether the left argument is stringwise
308 less than, equal to, or greater than the right argument.
310 "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
311 by the current locale if C<use locale> is in effect. See L<perllocale>.
315 Binary "&" returns its operators ANDed together bit by bit.
316 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
318 =head2 Bitwise Or and Exclusive Or
320 Binary "|" returns its operators ORed together bit by bit.
321 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
323 Binary "^" returns its operators XORed together bit by bit.
324 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
326 =head2 C-style Logical And
328 Binary "&&" performs a short-circuit logical AND operation. That is,
329 if the left operand is false, the right operand is not even evaluated.
330 Scalar or list context propagates down to the right operand if it
333 =head2 C-style Logical Or
335 Binary "||" performs a short-circuit logical OR operation. That is,
336 if the left operand is true, the right operand is not even evaluated.
337 Scalar or list context propagates down to the right operand if it
340 The C<||> and C<&&> operators differ from C's in that, rather than returning
341 0 or 1, they return the last value evaluated. Thus, a reasonably portable
342 way to find out the home directory (assuming it's not "0") might be:
344 $home = $ENV{'HOME'} || $ENV{'LOGDIR'} ||
345 (getpwuid($<))[7] || die "You're homeless!\n";
347 In particular, this means that you shouldn't use this
348 for selecting between two aggregates for assignment:
350 @a = @b || @c; # this is wrong
351 @a = scalar(@b) || @c; # really meant this
352 @a = @b ? @b : @c; # this works fine, though
354 As more readable alternatives to C<&&> and C<||> when used for
355 control flow, Perl provides C<and> and C<or> operators (see below).
356 The short-circuit behavior is identical. The precedence of "and" and
357 "or" is much lower, however, so that you can safely use them after a
358 list operator without the need for parentheses:
360 unlink "alpha", "beta", "gamma"
361 or gripe(), next LINE;
363 With the C-style operators that would have been written like this:
365 unlink("alpha", "beta", "gamma")
366 || (gripe(), next LINE);
368 Using "or" for assignment is unlikely to do what you want; see below.
370 =head2 Range Operators
372 Binary ".." is the range operator, which is really two different
373 operators depending on the context. In list context, it returns an
374 array of values counting (up by ones) from the left value to the right
375 value. If the left value is greater than the right value then it
376 returns the empty array. The range operator is useful for writing
377 C<foreach (1..10)> loops and for doing slice operations on arrays. In
378 the current implementation, no temporary array is created when the
379 range operator is used as the expression in C<foreach> loops, but older
380 versions of Perl might burn a lot of memory when you write something
383 for (1 .. 1_000_000) {
387 In scalar context, ".." returns a boolean value. The operator is
388 bistable, like a flip-flop, and emulates the line-range (comma) operator
389 of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
390 own boolean state. It is false as long as its left operand is false.
391 Once the left operand is true, the range operator stays true until the
392 right operand is true, I<AFTER> which the range operator becomes false
393 again. It doesn't become false till the next time the range operator is
394 evaluated. It can test the right operand and become false on the same
395 evaluation it became true (as in B<awk>), but it still returns true once.
396 If you don't want it to test the right operand till the next
397 evaluation, as in B<sed>, just use three dots ("...") instead of
398 two. In all other regards, "..." behaves just like ".." does.
400 The right operand is not evaluated while the operator is in the
401 "false" state, and the left operand is not evaluated while the
402 operator is in the "true" state. The precedence is a little lower
403 than || and &&. The value returned is either the empty string for
404 false, or a sequence number (beginning with 1) for true. The
405 sequence number is reset for each range encountered. The final
406 sequence number in a range has the string "E0" appended to it, which
407 doesn't affect its numeric value, but gives you something to search
408 for if you want to exclude the endpoint. You can exclude the
409 beginning point by waiting for the sequence number to be greater
410 than 1. If either operand of scalar ".." is a constant expression,
411 that operand is implicitly compared to the C<$.> variable, the
412 current line number. Examples:
414 As a scalar operator:
416 if (101 .. 200) { print; } # print 2nd hundred lines
417 next line if (1 .. /^$/); # skip header lines
418 s/^/> / if (/^$/ .. eof()); # quote body
420 # parse mail messages
422 $in_header = 1 .. /^$/;
423 $in_body = /^$/ .. eof();
424 # do something based on those
426 close ARGV if eof; # reset $. each file
431 for (101 .. 200) { print; } # print $_ 100 times
432 @foo = @foo[0 .. $#foo]; # an expensive no-op
433 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
435 The range operator (in list context) makes use of the magical
436 auto-increment algorithm if the operands are strings. You
439 @alphabet = ('A' .. 'Z');
441 to get all normal letters of the alphabet, or
443 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
445 to get a hexadecimal digit, or
447 @z2 = ('01' .. '31'); print $z2[$mday];
449 to get dates with leading zeros. If the final value specified is not
450 in the sequence that the magical increment would produce, the sequence
451 goes until the next value would be longer than the final value
454 =head2 Conditional Operator
456 Ternary "?:" is the conditional operator, just as in C. It works much
457 like an if-then-else. If the argument before the ? is true, the
458 argument before the : is returned, otherwise the argument after the :
459 is returned. For example:
461 printf "I have %d dog%s.\n", $n,
462 ($n == 1) ? '' : "s";
464 Scalar or list context propagates downward into the 2nd
465 or 3rd argument, whichever is selected.
467 $a = $ok ? $b : $c; # get a scalar
468 @a = $ok ? @b : @c; # get an array
469 $a = $ok ? @b : @c; # oops, that's just a count!
471 The operator may be assigned to if both the 2nd and 3rd arguments are
472 legal lvalues (meaning that you can assign to them):
474 ($a_or_b ? $a : $b) = $c;
476 Because this operator produces an assignable result, using assignments
477 without parentheses will get you in trouble. For example, this:
479 $a % 2 ? $a += 10 : $a += 2
483 (($a % 2) ? ($a += 10) : $a) += 2
487 ($a % 2) ? ($a += 10) : ($a += 2)
489 That should probably be written more simply as:
491 $a += ($a % 2) ? 10 : 2;
493 =head2 Assignment Operators
495 "=" is the ordinary assignment operator.
497 Assignment operators work as in C. That is,
505 although without duplicating any side effects that dereferencing the lvalue
506 might trigger, such as from tie(). Other assignment operators work similarly.
507 The following are recognized:
514 Although these are grouped by family, they all have the precedence
517 Unlike in C, the scalar assignment operator produces a valid lvalue.
518 Modifying an assignment is equivalent to doing the assignment and
519 then modifying the variable that was assigned to. This is useful
520 for modifying a copy of something, like this:
522 ($tmp = $global) =~ tr [A-Z] [a-z];
533 Similarly, a list assignment in list context produces the list of
534 lvalues assigned to, and a list assignment in scalar context returns
535 the number of elements produced by the expression on the right hand
536 side of the assignment.
538 =head2 Comma Operator
540 Binary "," is the comma operator. In scalar context it evaluates
541 its left argument, throws that value away, then evaluates its right
542 argument and returns that value. This is just like C's comma operator.
544 In list context, it's just the list argument separator, and inserts
545 both its arguments into the list.
547 The => digraph is mostly just a synonym for the comma operator. It's useful for
548 documenting arguments that come in pairs. As of release 5.001, it also forces
549 any word to the left of it to be interpreted as a string.
551 =head2 List Operators (Rightward)
553 On the right side of a list operator, it has very low precedence,
554 such that it controls all comma-separated expressions found there.
555 The only operators with lower precedence are the logical operators
556 "and", "or", and "not", which may be used to evaluate calls to list
557 operators without the need for extra parentheses:
559 open HANDLE, "filename"
560 or die "Can't open: $!\n";
562 See also discussion of list operators in L<Terms and List Operators (Leftward)>.
566 Unary "not" returns the logical negation of the expression to its right.
567 It's the equivalent of "!" except for the very low precedence.
571 Binary "and" returns the logical conjunction of the two surrounding
572 expressions. It's equivalent to && except for the very low
573 precedence. This means that it short-circuits: i.e., the right
574 expression is evaluated only if the left expression is true.
576 =head2 Logical or and Exclusive Or
578 Binary "or" returns the logical disjunction of the two surrounding
579 expressions. It's equivalent to || except for the very low precedence.
580 This makes it useful for control flow
582 print FH $data or die "Can't write to FH: $!";
584 This means that it short-circuits: i.e., the right expression is evaluated
585 only if the left expression is false. Due to its precedence, you should
586 probably avoid using this for assignment, only for control flow.
588 $a = $b or $c; # bug: this is wrong
589 ($a = $b) or $c; # really means this
590 $a = $b || $c; # better written this way
592 However, when it's a list-context assignment and you're trying to use
593 "||" for control flow, you probably need "or" so that the assignment
594 takes higher precedence.
596 @info = stat($file) || die; # oops, scalar sense of stat!
597 @info = stat($file) or die; # better, now @info gets its due
599 Then again, you could always use parentheses.
601 Binary "xor" returns the exclusive-OR of the two surrounding expressions.
602 It cannot short circuit, of course.
604 =head2 C Operators Missing From Perl
606 Here is what C has that Perl doesn't:
612 Address-of operator. (But see the "\" operator for taking a reference.)
616 Dereference-address operator. (Perl's prefix dereferencing
617 operators are typed: $, @, %, and &.)
621 Type-casting operator.
625 =head2 Quote and Quote-like Operators
627 While we usually think of quotes as literal values, in Perl they
628 function as operators, providing various kinds of interpolating and
629 pattern matching capabilities. Perl provides customary quote characters
630 for these behaviors, but also provides a way for you to choose your
631 quote character for any of them. In the following table, a C<{}> represents
632 any pair of delimiters you choose.
634 Customary Generic Meaning Interpolates
637 `` qx{} Command yes (unless '' is delimiter)
639 // m{} Pattern match yes (unless '' is delimiter)
640 qr{} Pattern yes (unless '' is delimiter)
641 s{}{} Substitution yes (unless '' is delimiter)
642 tr{}{} Transliteration no (but see below)
644 Non-bracketing delimiters use the same character fore and aft, but the four
645 sorts of brackets (round, angle, square, curly) will all nest, which means
654 Note, however, that this does not always work for quoting Perl code:
656 $s = q{ if($a eq "}") ... }; # WRONG
658 is a syntax error. The C<Text::Balanced> module on CPAN is able to do this
661 There can be whitespace between the operator and the quoting
662 characters, except when C<#> is being used as the quoting character.
663 C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
664 operator C<q> followed by a comment. Its argument will be taken
665 from the next line. This allows you to write:
667 s {foo} # Replace foo
670 For constructs that do interpolate, variables beginning with "C<$>"
671 or "C<@>" are interpolated, as are the following escape sequences. Within
672 a transliteration, the first eleven of these sequences may be used.
679 \a alarm (bell) (BEL)
681 \033 octal char (ESC)
683 \x{263a} wide hex char (SMILEY)
684 \c[ control char (ESC)
687 \l lowercase next char
688 \u uppercase next char
691 \E end case modification
692 \Q quote non-word characters till \E
694 If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u>
695 and C<\U> is taken from the current locale. See L<perllocale>. For
696 documentation of C<\N{name}>, see L<charnames>.
698 All systems use the virtual C<"\n"> to represent a line terminator,
699 called a "newline". There is no such thing as an unvarying, physical
700 newline character. It is only an illusion that the operating system,
701 device drivers, C libraries, and Perl all conspire to preserve. Not all
702 systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
703 on a Mac, these are reversed, and on systems without line terminator,
704 printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
705 you mean a "newline" for your system, but use the literal ASCII when you
706 need an exact character. For example, most networking protocols expect
707 and prefer a CR+LF (C<"\012\015"> or C<"\cJ\cM">) for line terminators,
708 and although they often accept just C<"\012">, they seldom tolerate just
709 C<"\015">. If you get in the habit of using C<"\n"> for networking,
710 you may be burned some day.
712 You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
713 An unescaped C<$> or C<@> interpolates the corresponding variable,
714 while escaping will cause the literal string C<\$> to be inserted.
715 You'll need to write something like C<m/\Quser\E\@\Qhost/>.
717 Patterns are subject to an additional level of interpretation as a
718 regular expression. This is done as a second pass, after variables are
719 interpolated, so that regular expressions may be incorporated into the
720 pattern from the variables. If this is not what you want, use C<\Q> to
721 interpolate a variable literally.
723 Apart from the behavior described above, Perl does not expand
724 multiple levels of interpolation. In particular, contrary to the
725 expectations of shell programmers, back-quotes do I<NOT> interpolate
726 within double quotes, nor do single quotes impede evaluation of
727 variables when used within double quotes.
729 =head2 Regexp Quote-Like Operators
731 Here are the quote-like operators that apply to pattern
732 matching and related activities.
738 This is just like the C</pattern/> search, except that it matches only
739 once between calls to the reset() operator. This is a useful
740 optimization when you want to see only the first occurrence of
741 something in each file of a set of files, for instance. Only C<??>
742 patterns local to the current package are reset.
746 # blank line between header and body
749 reset if eof; # clear ?? status for next file
752 This usage is vaguely depreciated, which means it just might possibly
753 be removed in some distant future version of Perl, perhaps somewhere
754 around the year 2168.
756 =item m/PATTERN/cgimosx
758 =item /PATTERN/cgimosx
760 Searches a string for a pattern match, and in scalar context returns
761 true if it succeeds, false if it fails. If no string is specified
762 via the C<=~> or C<!~> operator, the $_ string is searched. (The
763 string specified with C<=~> need not be an lvalue--it may be the
764 result of an expression evaluation, but remember the C<=~> binds
765 rather tightly.) See also L<perlre>. See L<perllocale> for
766 discussion of additional considerations that apply when C<use locale>
771 c Do not reset search position on a failed match when /g is in effect.
772 g Match globally, i.e., find all occurrences.
773 i Do case-insensitive pattern matching.
774 m Treat string as multiple lines.
775 o Compile pattern only once.
776 s Treat string as single line.
777 x Use extended regular expressions.
779 If "/" is the delimiter then the initial C<m> is optional. With the C<m>
780 you can use any pair of non-alphanumeric, non-whitespace characters
781 as delimiters. This is particularly useful for matching path names
782 that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
783 the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
784 If "'" is the delimiter, no interpolation is performed on the PATTERN.
786 PATTERN may contain variables, which will be interpolated (and the
787 pattern recompiled) every time the pattern search is evaluated, except
788 for when the delimiter is a single quote. (Note that C<$)> and C<$|>
789 might not be interpolated because they look like end-of-string tests.)
790 If you want such a pattern to be compiled only once, add a C</o> after
791 the trailing delimiter. This avoids expensive run-time recompilations,
792 and is useful when the value you are interpolating won't change over
793 the life of the script. However, mentioning C</o> constitutes a promise
794 that you won't change the variables in the pattern. If you change them,
795 Perl won't even notice. See also L<"qr//">.
797 If the PATTERN evaluates to the empty string, the last
798 I<successfully> matched regular expression is used instead.
800 If the C</g> option is not used, C<m//> in list context returns a
801 list consisting of the subexpressions matched by the parentheses in the
802 pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are
803 also set, and that this differs from Perl 4's behavior.) When there are
804 no parentheses in the pattern, the return value is the list C<(1)> for
805 success. With or without parentheses, an empty list is returned upon
810 open(TTY, '/dev/tty');
811 <TTY> =~ /^y/i && foo(); # do foo if desired
813 if (/Version: *([0-9.]*)/) { $version = $1; }
815 next if m#^/usr/spool/uucp#;
820 print if /$arg/o; # compile only once
823 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
825 This last example splits $foo into the first two words and the
826 remainder of the line, and assigns those three fields to $F1, $F2, and
827 $Etc. The conditional is true if any variables were assigned, i.e., if
830 The C</g> modifier specifies global pattern matching--that is,
831 matching as many times as possible within the string. How it behaves
832 depends on the context. In list context, it returns a list of the
833 substrings matched by any capturing parentheses in the regular
834 expression. If there are no parentheses, it returns a list of all
835 the matched strings, as if there were parentheses around the whole
838 In scalar context, each execution of C<m//g> finds the next match,
839 returning true if it matches, and false if there is no further match.
840 The position after the last match can be read or set using the pos()
841 function; see L<perlfunc/pos>. A failed match normally resets the
842 search position to the beginning of the string, but you can avoid that
843 by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
844 string also resets the search position.
846 You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
847 zero-width assertion that matches the exact position where the previous
848 C<m//g>, if any, left off. The C<\G> assertion is not supported without
849 the C</g> modifier. (Currently, without C</g>, C<\G> behaves just like
850 C<\A>, but that's accidental and may change in the future.)
855 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
858 $/ = ""; $* = 1; # $* deprecated in modern perls
859 while (defined($paragraph = <>)) {
860 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
864 print "$sentences\n";
866 # using m//gc with \G
870 print $1 while /(o)/gc; print "', pos=", pos, "\n";
872 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
874 print $1 while /(p)/gc; print "', pos=", pos, "\n";
877 The last example should print:
886 A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
887 combine several regexps like this to process a string part-by-part,
888 doing different actions depending on which regexp matched. Each
889 regexp tries to match where the previous one leaves off.
892 $url = new URI::URL "http://www/"; die if $url eq "xXx";
896 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
897 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
898 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
899 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
900 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
901 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
902 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
903 print ". That's all!\n";
906 Here is the output (split into several lines):
908 line-noise lowercase line-noise lowercase UPPERCASE line-noise
909 UPPERCASE line-noise lowercase line-noise lowercase line-noise
910 lowercase lowercase line-noise lowercase lowercase line-noise
911 MiXeD line-noise. That's all!
917 A single-quoted, literal string. A backslash represents a backslash
918 unless followed by the delimiter or another backslash, in which case
919 the delimiter or backslash is interpolated.
921 $foo = q!I said, "You said, 'She said it.'"!;
922 $bar = q('This is it.');
923 $baz = '\n'; # a two-character string
929 A double-quoted, interpolated string.
932 (*** The previous line contains the naughty word "$1".\n)
933 if /\b(tcl|java|python)\b/i; # :-)
934 $baz = "\n"; # a one-character string
936 =item qr/STRING/imosx
938 This operators quotes--and compiles--its I<STRING> as a regular
939 expression. I<STRING> is interpolated the same way as I<PATTERN>
940 in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
941 is done. Returns a Perl value which may be used instead of the
942 corresponding C</STRING/imosx> expression.
946 $rex = qr/my.STRING/is;
953 The result may be used as a subpattern in a match:
956 $string =~ /foo${re}bar/; # can be interpolated in other patterns
957 $string =~ $re; # or used standalone
958 $string =~ /$re/; # or this way
960 Since Perl may compile the pattern at the moment of execution of qr()
961 operator, using qr() may have speed advantages in some situations,
962 notably if the result of qr() is used standalone:
965 my $patterns = shift;
966 my @compiled = map qr/$_/i, @$patterns;
969 foreach my $pat (@compiled) {
970 $success = 1, last if /$pat/;
976 Precompilation of the pattern into an internal representation at
977 the moment of qr() avoids a need to recompile the pattern every
978 time a match C</$pat/> is attempted. (Perl has many other internal
979 optimizations, but none would be triggered in the above example if
980 we did not use qr() operator.)
984 i Do case-insensitive pattern matching.
985 m Treat string as multiple lines.
986 o Compile pattern only once.
987 s Treat string as single line.
988 x Use extended regular expressions.
990 See L<perlre> for additional information on valid syntax for STRING, and
991 for a detailed look at the semantics of regular expressions.
997 A string which is (possibly) interpolated and then executed as a system
998 command with C</bin/sh> or its equivalent. Shell wildcards, pipes,
999 and redirections will be honored. The collected standard output of the
1000 command is returned; standard error is unaffected. In scalar context,
1001 it comes back as a single (potentially multi-line) string. In list
1002 context, returns a list of lines (however you've defined lines with $/
1003 or $INPUT_RECORD_SEPARATOR).
1005 Because backticks do not affect standard error, use shell file descriptor
1006 syntax (assuming the shell supports this) if you care to address this.
1007 To capture a command's STDERR and STDOUT together:
1009 $output = `cmd 2>&1`;
1011 To capture a command's STDOUT but discard its STDERR:
1013 $output = `cmd 2>/dev/null`;
1015 To capture a command's STDERR but discard its STDOUT (ordering is
1018 $output = `cmd 2>&1 1>/dev/null`;
1020 To exchange a command's STDOUT and STDERR in order to capture the STDERR
1021 but leave its STDOUT to come out the old STDERR:
1023 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
1025 To read both a command's STDOUT and its STDERR separately, it's easiest
1026 and safest to redirect them separately to files, and then read from those
1027 files when the program is done:
1029 system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr");
1031 Using single-quote as a delimiter protects the command from Perl's
1032 double-quote interpolation, passing it on to the shell instead:
1034 $perl_info = qx(ps $$); # that's Perl's $$
1035 $shell_info = qx'ps $$'; # that's the new shell's $$
1037 How that string gets evaluated is entirely subject to the command
1038 interpreter on your system. On most platforms, you will have to protect
1039 shell metacharacters if you want them treated literally. This is in
1040 practice difficult to do, as it's unclear how to escape which characters.
1041 See L<perlsec> for a clean and safe example of a manual fork() and exec()
1042 to emulate backticks safely.
1044 On some platforms (notably DOS-like ones), the shell may not be
1045 capable of dealing with multiline commands, so putting newlines in
1046 the string may not get you what you want. You may be able to evaluate
1047 multiple commands in a single line by separating them with the command
1048 separator character, if your shell supports that (e.g. C<;> on many Unix
1049 shells; C<&> on the Windows NT C<cmd> shell).
1051 Beware that some command shells may place restrictions on the length
1052 of the command line. You must ensure your strings don't exceed this
1053 limit after any necessary interpolations. See the platform-specific
1054 release notes for more details about your particular environment.
1056 Using this operator can lead to programs that are difficult to port,
1057 because the shell commands called vary between systems, and may in
1058 fact not be present at all. As one example, the C<type> command under
1059 the POSIX shell is very different from the C<type> command under DOS.
1060 That doesn't mean you should go out of your way to avoid backticks
1061 when they're the right way to get something done. Perl was made to be
1062 a glue language, and one of the things it glues together is commands.
1063 Just understand what you're getting yourself into.
1065 See L<"I/O Operators"> for more discussion.
1069 Evaluates to a list of the words extracted out of STRING, using embedded
1070 whitespace as the word delimiters. It can be understood as being roughly
1073 split(' ', q/STRING/);
1075 the difference being that it generates a real list at compile time. So
1080 is semantically equivalent to the list:
1084 Some frequently seen examples:
1086 use POSIX qw( setlocale localeconv )
1087 @EXPORT = qw( foo bar baz );
1089 A common mistake is to try to separate the words with comma or to
1090 put comments into a multi-line C<qw>-string. For this reason, the
1091 B<-w> switch (that is, the C<$^W> variable) produces warnings if
1092 the STRING contains the "," or the "#" character.
1094 =item s/PATTERN/REPLACEMENT/egimosx
1096 Searches a string for a pattern, and if found, replaces that pattern
1097 with the replacement text and returns the number of substitutions
1098 made. Otherwise it returns false (specifically, the empty string).
1100 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
1101 variable is searched and modified. (The string specified with C<=~> must
1102 be scalar variable, an array element, a hash element, or an assignment
1103 to one of those, i.e., an lvalue.)
1105 If the delimiter chosen is a single quote, no interpolation is
1106 done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1107 PATTERN contains a $ that looks like a variable rather than an
1108 end-of-string test, the variable will be interpolated into the pattern
1109 at run-time. If you want the pattern compiled only once the first time
1110 the variable is interpolated, use the C</o> option. If the pattern
1111 evaluates to the empty string, the last successfully executed regular
1112 expression is used instead. See L<perlre> for further explanation on these.
1113 See L<perllocale> for discussion of additional considerations that apply
1114 when C<use locale> is in effect.
1118 e Evaluate the right side as an expression.
1119 g Replace globally, i.e., all occurrences.
1120 i Do case-insensitive pattern matching.
1121 m Treat string as multiple lines.
1122 o Compile pattern only once.
1123 s Treat string as single line.
1124 x Use extended regular expressions.
1126 Any non-alphanumeric, non-whitespace delimiter may replace the
1127 slashes. If single quotes are used, no interpretation is done on the
1128 replacement string (the C</e> modifier overrides this, however). Unlike
1129 Perl 4, Perl 5 treats backticks as normal delimiters; the replacement
1130 text is not evaluated as a command. If the
1131 PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
1132 pair of quotes, which may or may not be bracketing quotes, e.g.,
1133 C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
1134 replacement portion to be interpreted as a full-fledged Perl expression
1135 and eval()ed right then and there. It is, however, syntax checked at
1140 s/\bgreen\b/mauve/g; # don't change wintergreen
1142 $path =~ s|/usr/bin|/usr/local/bin|;
1144 s/Login: $foo/Login: $bar/; # run-time pattern
1146 ($foo = $bar) =~ s/this/that/; # copy first, then change
1148 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
1151 s/\d+/$&*2/e; # yields 'abc246xyz'
1152 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
1153 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1155 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
1156 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
1157 s/^=(\w+)/&pod($1)/ge; # use function call
1159 # expand variables in $_, but dynamics only, using
1160 # symbolic dereferencing
1163 # /e's can even nest; this will expand
1164 # any embedded scalar variable (including lexicals) in $_
1167 # Delete (most) C comments.
1169 /\* # Match the opening delimiter.
1170 .*? # Match a minimal number of characters.
1171 \*/ # Match the closing delimiter.
1174 s/^\s*(.*?)\s*$/$1/; # trim white space in $_, expensively
1176 for ($variable) { # trim white space in $variable, cheap
1181 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
1183 Note the use of $ instead of \ in the last example. Unlike
1184 B<sed>, we use the \<I<digit>> form in only the left hand side.
1185 Anywhere else it's $<I<digit>>.
1187 Occasionally, you can't use just a C</g> to get all the changes
1188 to occur that you might want. Here are two common cases:
1190 # put commas in the right places in an integer
1191 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
1193 # expand tabs to 8-column spacing
1194 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
1196 =item tr/SEARCHLIST/REPLACEMENTLIST/cdsUC
1198 =item y/SEARCHLIST/REPLACEMENTLIST/cdsUC
1200 Transliterates all occurrences of the characters found in the search list
1201 with the corresponding character in the replacement list. It returns
1202 the number of characters replaced or deleted. If no string is
1203 specified via the =~ or !~ operator, the $_ string is transliterated. (The
1204 string specified with =~ must be a scalar variable, an array element, a
1205 hash element, or an assignment to one of those, i.e., an lvalue.)
1207 A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
1208 does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
1209 For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1210 SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1211 its own pair of quotes, which may or may not be bracketing quotes,
1212 e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
1214 Note also that the whole range idea is rather unportable between
1215 character sets--and even within character sets they may cause results
1216 you probably didn't expect. A sound principle is to use only ranges
1217 that begin from and end at either alphabets of equal case (a-e, A-E),
1218 or digits (0-4). Anything else is unsafe. If in doubt, spell out the
1219 character sets in full.
1223 c Complement the SEARCHLIST.
1224 d Delete found but unreplaced characters.
1225 s Squash duplicate replaced characters.
1226 U Translate to/from UTF-8.
1227 C Translate to/from 8-bit char (octet).
1229 If the C</c> modifier is specified, the SEARCHLIST character set
1230 is complemented. If the C</d> modifier is specified, any characters
1231 specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
1232 (Note that this is slightly more flexible than the behavior of some
1233 B<tr> programs, which delete anything they find in the SEARCHLIST,
1234 period.) If the C</s> modifier is specified, sequences of characters
1235 that were transliterated to the same character are squashed down
1236 to a single instance of the character.
1238 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1239 exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1240 than the SEARCHLIST, the final character is replicated till it is long
1241 enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
1242 This latter is useful for counting characters in a class or for
1243 squashing character sequences in a class.
1245 The first C</U> or C</C> modifier applies to the left side of the translation.
1246 The second one applies to the right side. If present, these modifiers override
1247 the current utf8 state.
1251 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1253 $cnt = tr/*/*/; # count the stars in $_
1255 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1257 $cnt = tr/0-9//; # count the digits in $_
1259 tr/a-zA-Z//s; # bookkeeper -> bokeper
1261 ($HOST = $host) =~ tr/a-z/A-Z/;
1263 tr/a-zA-Z/ /cs; # change non-alphas to single space
1266 [\000-\177]; # delete 8th bit
1268 tr/\0-\xFF//CU; # change Latin-1 to Unicode
1269 tr/\0-\x{FF}//UC; # change Unicode to Latin-1
1271 If multiple transliterations are given for a character, only the
1276 will transliterate any A to X.
1278 Because the transliteration table is built at compile time, neither
1279 the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
1280 interpolation. That means that if you want to use variables, you
1283 eval "tr/$oldlist/$newlist/";
1286 eval "tr/$oldlist/$newlist/, 1" or die $@;
1290 =head2 Gory details of parsing quoted constructs
1292 When presented with something that might have several different
1293 interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
1294 principle to pick the most probable interpretation. This strategy
1295 is so successful that Perl programmers often do not suspect the
1296 ambivalence of what they write. But from time to time, Perl's
1297 notions differ substantially from what the author honestly meant.
1299 This section hopes to clarify how Perl handles quoted constructs.
1300 Although the most common reason to learn this is to unravel labyrinthine
1301 regular expressions, because the initial steps of parsing are the
1302 same for all quoting operators, they are all discussed together.
1304 The most important Perl parsing rule is the first one discussed
1305 below: when processing a quoted construct, Perl first finds the end
1306 of that construct, then interprets its contents. If you understand
1307 this rule, you may skip the rest of this section on the first
1308 reading. The other rules are likely to contradict the user's
1309 expectations much less frequently than this first one.
1311 Some passes discussed below are performed concurrently, but because
1312 their results are the same, we consider them individually. For different
1313 quoting constructs, Perl performs different numbers of passes, from
1314 one to five, but these passes are always performed in the same order.
1318 =item Finding the end
1320 The first pass is finding the end of the quoted construct, whether
1321 it be a multicharacter delimiter C<"\nEOF\n"> in the C<<<EOF>
1322 construct, a C</> that terminates a C<qq//> construct, a C<]> which
1323 terminates C<qq[]> construct, or a C<< > >> which terminates a
1324 fileglob started with C<< < >>.
1326 When searching for single-character non-pairing delimiters, such
1327 as C</>, combinations of C<\\> and C<\/> are skipped. However,
1328 when searching for single-character pairing delimiter like C<[>,
1329 combinations of C<\\>, C<\]>, and C<\[> are all skipped, and nested
1330 C<[>, C<]> are skipped as well. When searching for multicharacter
1331 delimiters, nothing is skipped.
1333 For constructs with three-part delimiters (C<s///>, C<y///>, and
1334 C<tr///>), the search is repeated once more.
1336 During this search no attention is paid to the semantics of the construct.
1339 "$hash{"$foo/$bar"}"
1344 bar # NOT a comment, this slash / terminated m//!
1347 do not form legal quoted expressions. The quoted part ends on the
1348 first C<"> and C</>, and the rest happens to be a syntax error.
1349 Because the slash that terminated C<m//> was followed by a C<SPACE>,
1350 the example above is not C<m//x>, but rather C<m//> with no C</x>
1351 modifier. So the embedded C<#> is interpreted as a literal C<#>.
1353 =item Removal of backslashes before delimiters
1355 During the second pass, text between the starting and ending
1356 delimiters is copied to a safe location, and the C<\> is removed
1357 from combinations consisting of C<\> and delimiter--or delimiters,
1358 meaning both starting and ending delimiters will should these differ.
1359 This removal does not happen for multi-character delimiters.
1360 Note that the combination C<\\> is left intact, just as it was.
1362 Starting from this step no information about the delimiters is
1367 The next step is interpolation in the text obtained, which is now
1368 delimiter-independent. There are four different cases.
1372 =item C<<<'EOF'>, C<m''>, C<s'''>, C<tr///>, C<y///>
1374 No interpolation is performed.
1378 The only interpolation is removal of C<\> from pairs C<\\>.
1380 =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>
1382 C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
1383 converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
1384 is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
1385 The other combinations are replaced with appropriate expansions.
1387 Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
1388 is interpolated in the usual way. Something like C<"\Q\\E"> has
1389 no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
1390 result is the same as for C<"\\\\E">. As a general rule, backslashes
1391 between C<\Q> and C<\E> may lead to counterintuitive results. So,
1392 C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
1393 as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
1398 may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
1400 Interpolated scalars and arrays are converted internally to the C<join> and
1401 C<.> catentation operations. Thus, C<"$foo XXX '@arr'"> becomes:
1403 $foo . " XXX '" . (join $", @arr) . "'";
1405 All operations above are performed simultaneously, left to right.
1407 Because the result of C<"\Q STRING \E"> has all metacharacters
1408 quoted, there is no way to insert a literal C<$> or C<@> inside a
1409 C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
1410 C<"\\\$">; if not, it is interpreted as the start of an interpolated
1413 Note also that the interpolation code needs to make a decision on
1414 where the interpolated scalar ends. For instance, whether
1415 C<< "a $b -> {c}" >> really means:
1417 "a " . $b . " -> {c}";
1423 Most of the time, the longest possible text that does not include
1424 spaces between components and which contains matching braces or
1425 brackets. because the outcome may be determined by voting based
1426 on heuristic estimators, the result is not strictly predictable.
1427 Fortunately, it's usually correct for ambiguous cases.
1429 =item C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
1431 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
1432 happens (almost) as with C<qq//> constructs, but the substitution
1433 of C<\> followed by RE-special chars (including C<\>) is not
1434 performed. Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
1435 a C<#>-comment in a C<//x>-regular expression, no processing is
1436 performed whatsoever. This is the first step at which the presence
1437 of the C<//x> modifier is relevant.
1439 Interpolation has several quirks: C<$|>, C<$(>, and C<$)> are not
1440 interpolated, and constructs C<$var[SOMETHING]> are voted (by several
1441 different estimators) to be either an array element or C<$var>
1442 followed by an RE alternative. This is where the notation
1443 C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
1444 array element C<-9>, not as a regular expression from the variable
1445 C<$arr> followed by a digit, which would be the interpretation of
1446 C</$arr[0-9]/>. Since voting among different estimators may occur,
1447 the result is not predictable.
1449 It is at this step that C<\1> is begrudgingly converted to C<$1> in
1450 the replacement text of C<s///> to correct the incorrigible
1451 I<sed> hackers who haven't picked up the saner idiom yet. A warning
1452 is emitted if the B<-w> command-line flag (that is, the C<$^W> variable)
1455 The lack of processing of C<\\> creates specific restrictions on
1456 the post-processed text. If the delimiter is C</>, one cannot get
1457 the combination C<\/> into the result of this step. C</> will
1458 finish the regular expression, C<\/> will be stripped to C</> on
1459 the previous step, and C<\\/> will be left as is. Because C</> is
1460 equivalent to C<\/> inside a regular expression, this does not
1461 matter unless the delimiter happens to be character special to the
1462 RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
1463 alphanumeric char, as in:
1467 In the RE above, which is intentionally obfuscated for illustration, the
1468 delimiter is C<m>, the modifier is C<mx>, and after backslash-removal the
1469 RE is the same as for C<m/ ^ a s* b /mx>). There's more than one
1470 reason you're encouraged to restrict your delimiters to non-alphanumeric,
1471 non-whitespace choices.
1475 This step is the last one for all constructs except regular expressions,
1476 which are processed further.
1478 =item Interpolation of regular expressions
1480 Previous steps were performed during the compilation of Perl code,
1481 but this one happens at run time--although it may be optimized to
1482 be calculated at compile time if appropriate. After preprocessing
1483 described above, and possibly after evaluation if catenation,
1484 joining, casing translation, or metaquoting are involved, the
1485 resulting I<string> is passed to the RE engine for compilation.
1487 Whatever happens in the RE engine might be better discussed in L<perlre>,
1488 but for the sake of continuity, we shall do so here.
1490 This is another step where the presence of the C<//x> modifier is
1491 relevant. The RE engine scans the string from left to right and
1492 converts it to a finite automaton.
1494 Backslashed characters are either replaced with corresponding
1495 literal strings (as with C<\{>), or else they generate special nodes
1496 in the finite automaton (as with C<\b>). Characters special to the
1497 RE engine (such as C<|>) generate corresponding nodes or groups of
1498 nodes. C<(?#...)> comments are ignored. All the rest is either
1499 converted to literal strings to match, or else is ignored (as is
1500 whitespace and C<#>-style comments if C<//x> is present).
1502 Parsing of the bracketed character class construct, C<[...]>, is
1503 rather different than the rule used for the rest of the pattern.
1504 The terminator of this construct is found using the same rules as
1505 for finding the terminator of a C<{}>-delimited construct, the only
1506 exception being that C<]> immediately following C<[> is treated as
1507 though preceded by a backslash. Similarly, the terminator of
1508 C<(?{...})> is found using the same rules as for finding the
1509 terminator of a C<{}>-delimited construct.
1511 It is possible to inspect both the string given to RE engine and the
1512 resulting finite automaton. See the arguments C<debug>/C<debugcolor>
1513 in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
1514 switch documented in L<perlrun/Switches>.
1516 =item Optimization of regular expressions
1518 This step is listed for completeness only. Since it does not change
1519 semantics, details of this step are not documented and are subject
1520 to change without notice. This step is performed over the finite
1521 automaton that was generated during the previous pass.
1523 It is at this stage that C<split()> silently optimizes C</^/> to
1528 =head2 I/O Operators
1530 There are several I/O operators you should know about.
1532 A string enclosed by backticks (grave accents) first undergoes
1533 double-quote interpolation. It is then interpreted as an external
1534 command, and the output of that command is the value of the
1536 string consisting of all output is returned. In list context, a
1537 list of values is returned, one per line of output. (You can set
1538 C<$/> to use a different line terminator.) The command is executed
1539 each time the pseudo-literal is evaluated. The status value of the
1540 command is returned in C<$?> (see L<perlvar> for the interpretation
1541 of C<$?>). Unlike in B<csh>, no translation is done on the return
1542 data--newlines remain newlines. Unlike in any of the shells, single
1543 quotes do not hide variable names in the command from interpretation.
1544 To pass a literal dollar-sign through to the shell you need to hide
1545 it with a backslash. The generalized form of backticks is C<qx//>.
1546 (Because backticks always undergo shell expansion as well, see
1547 L<perlsec> for security concerns.)
1549 In scalar context, evaluating a filehandle in angle brackets yields
1550 the next line from that file (the newline, if any, included), or
1551 C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
1552 (sometimes known as file-slurp mode) and the file is empty, it
1553 returns C<''> the first time, followed by C<undef> subsequently.
1555 Ordinarily you must assign the returned value to a variable, but
1556 there is one situation where an automatic assignment happens. If
1557 and only if the input symbol is the only thing inside the conditional
1558 of a C<while> statement (even if disguised as a C<for(;;)> loop),
1559 the value is automatically assigned to the global variable $_,
1560 destroying whatever was there previously. (This may seem like an
1561 odd thing to you, but you'll use the construct in almost every Perl
1562 script you write.) The $_ variables is not implicitly localized.
1563 You'll have to put a C<local $_;> before the loop if you want that
1566 The following lines are equivalent:
1568 while (defined($_ = <STDIN>)) { print; }
1569 while ($_ = <STDIN>) { print; }
1570 while (<STDIN>) { print; }
1571 for (;<STDIN>;) { print; }
1572 print while defined($_ = <STDIN>);
1573 print while ($_ = <STDIN>);
1574 print while <STDIN>;
1576 This also behaves similarly, but avoids $_ :
1578 while (my $line = <STDIN>) { print $line }
1580 In these loop constructs, the assigned value (whether assignment
1581 is automatic or explicit) is then tested to see whether it is
1582 defined. The defined test avoids problems where line has a string
1583 value that would be treated as false by Perl, for example a "" or
1584 a "0" with no trailing newline. If you really mean for such values
1585 to terminate the loop, they should be tested for explicitly:
1587 while (($_ = <STDIN>) ne '0') { ... }
1588 while (<STDIN>) { last unless $_; ... }
1590 In other boolean contexts, C<< <I<filehandle>> >> without an
1591 explicit C<defined> test or comparison elicit a warning if the B<-w>
1592 command-line switch (the C<$^W> variable) is in effect.
1594 The filehandles STDIN, STDOUT, and STDERR are predefined. (The
1595 filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
1596 in packages, where they would be interpreted as local identifiers
1597 rather than global.) Additional filehandles may be created with
1598 the open() function, amongst others. See L<perlopentut> and
1599 L<perlfunc/open> for details on this.
1601 If a <FILEHANDLE> is used in a context that is looking for
1602 a list, a list comprising all input lines is returned, one line per
1603 list element. It's easy to grow to a rather large data space this
1604 way, so use with care.
1606 <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
1607 See L<perlfunc/readline>.
1609 The null filehandle <> is special: it can be used to emulate the
1610 behavior of B<sed> and B<awk>. Input from <> comes either from
1611 standard input, or from each file listed on the command line. Here's
1612 how it works: the first time <> is evaluated, the @ARGV array is
1613 checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
1614 gives you standard input. The @ARGV array is then processed as a list
1615 of filenames. The loop
1618 ... # code for each line
1621 is equivalent to the following Perl-like pseudo code:
1623 unshift(@ARGV, '-') unless @ARGV;
1624 while ($ARGV = shift) {
1627 ... # code for each line
1631 except that it isn't so cumbersome to say, and will actually work.
1632 It really does shift the @ARGV array and put the current filename
1633 into the $ARGV variable. It also uses filehandle I<ARGV>
1634 internally--<> is just a synonym for <ARGV>, which
1635 is magical. (The pseudo code above doesn't work because it treats
1636 <ARGV> as non-magical.)
1638 You can modify @ARGV before the first <> as long as the array ends up
1639 containing the list of filenames you really want. Line numbers (C<$.>)
1640 continue as though the input were one big happy file. See the example
1641 in L<perlfunc/eof> for how to reset line numbers on each file.
1643 If you want to set @ARGV to your own list of files, go right ahead.
1644 This sets @ARGV to all plain text files if no @ARGV was given:
1646 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
1648 You can even set them to pipe commands. For example, this automatically
1649 filters compressed arguments through B<gzip>:
1651 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
1653 If you want to pass switches into your script, you can use one of the
1654 Getopts modules or put a loop on the front like this:
1656 while ($_ = $ARGV[0], /^-/) {
1659 if (/^-D(.*)/) { $debug = $1 }
1660 if (/^-v/) { $verbose++ }
1661 # ... # other switches
1665 # ... # code for each line
1668 The <> symbol will return C<undef> for end-of-file only once.
1669 If you call it again after this, it will assume you are processing another
1670 @ARGV list, and if you haven't set @ARGV, will read input from STDIN.
1672 If angle brackets contain is a simple scalar variable (e.g.,
1673 <$foo>), then that variable contains the name of the
1674 filehandle to input from, or its typeglob, or a reference to the
1680 If what's within the angle brackets is neither a filehandle nor a simple
1681 scalar variable containing a filehandle name, typeglob, or typeglob
1682 reference, it is interpreted as a filename pattern to be globbed, and
1683 either a list of filenames or the next filename in the list is returned,
1684 depending on context. This distinction is determined on syntactic
1685 grounds alone. That means C<< <$x> >> is always a readline() from
1686 an indirect handle, but C<< <$hash{key}> >> is always a glob().
1687 That's because $x is a simple scalar variable, but C<$hash{key}> is
1688 not--it's a hash element.
1690 One level of double-quote interpretation is done first, but you can't
1691 say C<< <$foo> >> because that's an indirect filehandle as explained
1692 in the previous paragraph. (In older versions of Perl, programmers
1693 would insert curly brackets to force interpretation as a filename glob:
1694 C<< <${foo}> >>. These days, it's considered cleaner to call the
1695 internal function directly as C<glob($foo)>, which is probably the right
1696 way to have done it in the first place.) For example:
1702 is roughly equivalent to:
1704 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
1710 except that the globbing is actually done internally using the standard
1711 C<File::Glob> extension. Of course, the shortest way to do the above is:
1715 A (file)glob evaluates its (embedded) argument only when it is
1716 starting a new list. All values must be read before it will start
1717 over. In list context, this isn't important because you automatically
1718 get them all anyway. However, in scalar context the operator returns
1719 the next value each time it's called, or C
1720 run out. As with filehandle reads, an automatic C<defined> is
1721 generated when the glob occurs in the test part of a C<while>,
1722 because legal glob returns (e.g. a file called F<0>) would otherwise
1723 terminate the loop. Again, C<undef> is returned only once. So if
1724 you're expecting a single value from a glob, it is much better to
1727 ($file) = <blurch*>;
1733 because the latter will alternate between returning a filename and
1736 It you're trying to do variable interpolation, it's definitely better
1737 to use the glob() function, because the older notation can cause people
1738 to become confused with the indirect filehandle notation.
1740 @files = glob("$dir/*.[ch]");
1741 @files = glob($files[$i]);
1743 =head2 Constant Folding
1745 Like C, Perl does a certain amount of expression evaluation at
1746 compile time whenever it determines that all arguments to an
1747 operator are static and have no side effects. In particular, string
1748 concatenation happens at compile time between literals that don't do
1749 variable substitution. Backslash interpolation also happens at
1750 compile time. You can say
1752 'Now is the time for all' . "\n" .
1753 'good men to come to.'
1755 and this all reduces to one string internally. Likewise, if
1758 foreach $file (@filenames) {
1759 if (-s $file > 5 + 100 * 2**16) { }
1762 the compiler will precompute the number which that expression
1763 represents so that the interpreter won't have to.
1765 =head2 Bitwise String Operators
1767 Bitstrings of any size may be manipulated by the bitwise operators
1770 If the operands to a binary bitwise op are strings of different
1771 sizes, B<|> and B<^> ops act as though the shorter operand had
1772 additional zero bits on the right, while the B<&> op acts as though
1773 the longer operand were truncated to the length of the shorter.
1774 The granularity for such extension or truncation is one or more
1777 # ASCII-based examples
1778 print "j p \n" ^ " a h"; # prints "JAPH\n"
1779 print "JA" | " ph\n"; # prints "japh\n"
1780 print "japh\nJunk" & '_____'; # prints "JAPH\n";
1781 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
1783 If you are intending to manipulate bitstrings, be certain that
1784 you're supplying bitstrings: If an operand is a number, that will imply
1785 a B<numeric> bitwise operation. You may explicitly show which type of
1786 operation you intend by using C<""> or C<0+>, as in the examples below.
1788 $foo = 150 | 105 ; # yields 255 (0x96 | 0x69 is 0xFF)
1789 $foo = '150' | 105 ; # yields 255
1790 $foo = 150 | '105'; # yields 255
1791 $foo = '150' | '105'; # yields string '155' (under ASCII)
1793 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
1794 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
1796 See L<perlfunc/vec> for information on how to manipulate individual bits
1799 =head2 Integer Arithmetic
1801 By default, Perl assumes that it must do most of its arithmetic in
1802 floating point. But by saying
1806 you may tell the compiler that it's okay to use integer operations
1807 (if it feels like it) from here to the end of the enclosing BLOCK.
1808 An inner BLOCK may countermand this by saying
1812 which lasts until the end of that BLOCK. Note that this doesn't
1813 mean everything is only an integer, merely that Perl may use integer
1814 operations if it is so inclined. For example, even under C<use
1815 integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731>
1818 Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
1819 and ">>") always produce integral results. (But see also L<Bitwise
1820 String Operators>.) However, C<use integer> still has meaning for
1821 them. By default, their results are interpreted as unsigned integers, but
1822 if C<use integer> is in effect, their results are interpreted
1823 as signed integers. For example, C<~0> usually evaluates to a large
1824 integral value. However, C<use integer; ~0> is C<-1> on twos-complement
1827 =head2 Floating-point Arithmetic
1829 While C<use integer> provides integer-only arithmetic, there is no
1830 analogous mechanism to provide automatic rounding or truncation to a
1831 certain number of decimal places. For rounding to a certain number
1832 of digits, sprintf() or printf() is usually the easiest route.
1835 Floating-point numbers are only approximations to what a mathematician
1836 would call real numbers. There are infinitely more reals than floats,
1837 so some corners must be cut. For example:
1839 printf "%.20g\n", 123456789123456789;
1840 # produces 123456789123456784
1842 Testing for exact equality of floating-point equality or inequality is
1843 not a good idea. Here's a (relatively expensive) work-around to compare
1844 whether two floating-point numbers are equal to a particular number of
1845 decimal places. See Knuth, volume II, for a more robust treatment of
1849 my ($X, $Y, $POINTS) = @_;
1851 $tX = sprintf("%.${POINTS}g", $X);
1852 $tY = sprintf("%.${POINTS}g", $Y);
1856 The POSIX module (part of the standard perl distribution) implements
1857 ceil(), floor(), and other mathematical and trigonometric functions.
1858 The Math::Complex module (part of the standard perl distribution)
1859 defines mathematical functions that work on both the reals and the
1860 imaginary numbers. Math::Complex not as efficient as POSIX, but
1861 POSIX can't work with complex numbers.
1863 Rounding in financial applications can have serious implications, and
1864 the rounding method used should be specified precisely. In these
1865 cases, it probably pays not to trust whichever system rounding is
1866 being used by Perl, but to instead implement the rounding function you
1869 =head2 Bigger Numbers
1871 The standard Math::BigInt and Math::BigFloat modules provide
1872 variable-precision arithmetic and overloaded operators, although
1873 they're currently pretty slow. At the cost of some space and
1874 considerable speed, they avoid the normal pitfalls associated with
1875 limited-precision representations.
1878 $x = Math::BigInt->new('123456789123456789');
1881 # prints +15241578780673678515622620750190521
1883 The non-standard modules SSLeay::BN and Math::Pari provide
1884 equivalent functionality (and much more) with a substantial
1885 performance savings.