3 perlop - Perl operators and precedence
7 Perl operators have the following associativity and precedence,
8 listed from highest precedence to lowest. Operators borrowed from
9 C keep the same precedence relationship with each other, even where
10 C's precedence is slightly screwy. (This makes learning Perl easier
11 for C folks.) With very few exceptions, these all operate on scalar
12 values only, not array values.
14 left terms and list operators (leftward)
18 right ! ~ \ and unary + and -
23 nonassoc named unary operators
24 nonassoc < > <= >= lt gt le ge
25 nonassoc == != <=> eq ne cmp
34 nonassoc list operators (rightward)
39 In the following sections, these operators are covered in precedence order.
41 Many operators can be overloaded for objects. See L<overload>.
45 =head2 Terms and List Operators (Leftward)
47 A TERM has the highest precedence in Perl. They include variables,
48 quote and quote-like operators, any expression in parentheses,
49 and any function whose arguments are parenthesized. Actually, there
50 aren't really functions in this sense, just list operators and unary
51 operators behaving as functions because you put parentheses around
52 the arguments. These are all documented in L<perlfunc>.
54 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
55 is followed by a left parenthesis as the next token, the operator and
56 arguments within parentheses are taken to be of highest precedence,
57 just like a normal function call.
59 In the absence of parentheses, the precedence of list operators such as
60 C<print>, C<sort>, or C<chmod> is either very high or very low depending on
61 whether you are looking at the left side or the right side of the operator.
64 @ary = (1, 3, sort 4, 2);
65 print @ary; # prints 1324
67 the commas on the right of the sort are evaluated before the sort,
68 but the commas on the left are evaluated after. In other words,
69 list operators tend to gobble up all arguments that follow, and
70 then act like a simple TERM with regard to the preceding expression.
71 Be careful with parentheses:
73 # These evaluate exit before doing the print:
74 print($foo, exit); # Obviously not what you want.
75 print $foo, exit; # Nor is this.
77 # These do the print before evaluating exit:
78 (print $foo), exit; # This is what you want.
79 print($foo), exit; # Or this.
80 print ($foo), exit; # Or even this.
84 print ($foo & 255) + 1, "\n";
86 probably doesn't do what you expect at first glance. See
87 L<Named Unary Operators> for more discussion of this.
89 Also parsed as terms are the C<do {}> and C<eval {}> constructs, as
90 well as subroutine and method calls, and the anonymous
91 constructors C<[]> and C<{}>.
93 See also L<Quote and Quote-like Operators> toward the end of this section,
94 as well as L<"I/O Operators">.
96 =head2 The Arrow Operator
98 "C<< -> >>" is an infix dereference operator, just as it is in C
99 and C++. If the right side is either a C<[...]>, C<{...}>, or a
100 C<(...)> subscript, then the left side must be either a hard or
101 symbolic reference to an array, a hash, or a subroutine respectively.
102 (Or technically speaking, a location capable of holding a hard
103 reference, if it's an array or hash reference being used for
104 assignment.) See L<perlreftut> and L<perlref>.
106 Otherwise, the right side is a method name or a simple scalar
107 variable containing either the method name or a subroutine reference,
108 and the left side must be either an object (a blessed reference)
109 or a class name (that is, a package name). See L<perlobj>.
111 =head2 Auto-increment and Auto-decrement
113 "++" and "--" work as in C. That is, if placed before a variable, they
114 increment or decrement the variable before returning the value, and if
115 placed after, increment or decrement the variable after returning the value.
117 The auto-increment operator has a little extra builtin magic to it. If
118 you increment a variable that is numeric, or that has ever been used in
119 a numeric context, you get a normal increment. If, however, the
120 variable has been used in only string contexts since it was set, and
121 has a value that is not the empty string and matches the pattern
122 C</^[a-zA-Z]*[0-9]*\z/>, the increment is done as a string, preserving each
123 character within its range, with carry:
125 print ++($foo = '99'); # prints '100'
126 print ++($foo = 'a0'); # prints 'a1'
127 print ++($foo = 'Az'); # prints 'Ba'
128 print ++($foo = 'zz'); # prints 'aaa'
130 The auto-decrement operator is not magical.
132 =head2 Exponentiation
134 Binary "**" is the exponentiation operator. It binds even more
135 tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is
136 implemented using C's pow(3) function, which actually works on doubles
139 =head2 Symbolic Unary Operators
141 Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower
142 precedence version of this.
144 Unary "-" performs arithmetic negation if the operand is numeric. If
145 the operand is an identifier, a string consisting of a minus sign
146 concatenated with the identifier is returned. Otherwise, if the string
147 starts with a plus or minus, a string starting with the opposite sign
148 is returned. One effect of these rules is that C<-bareword> is equivalent
151 Unary "~" performs bitwise negation, i.e., 1's complement. For
152 example, C<0666 & ~027> is 0640. (See also L<Integer Arithmetic> and
153 L<Bitwise String Operators>.) Note that the width of the result is
154 platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64
155 bits wide on a 64-bit platform, so if you are expecting a certain bit
156 width, remember use the & operator to mask off the excess bits.
158 Unary "+" has no effect whatsoever, even on strings. It is useful
159 syntactically for separating a function name from a parenthesized expression
160 that would otherwise be interpreted as the complete list of function
161 arguments. (See examples above under L<Terms and List Operators (Leftward)>.)
163 Unary "\" creates a reference to whatever follows it. See L<perlreftut>
164 and L<perlref>. Do not confuse this behavior with the behavior of
165 backslash within a string, although both forms do convey the notion
166 of protecting the next thing from interpolation.
168 =head2 Binding Operators
170 Binary "=~" binds a scalar expression to a pattern match. Certain operations
171 search or modify the string $_ by default. This operator makes that kind
172 of operation work on some other string. The right argument is a search
173 pattern, substitution, or transliteration. The left argument is what is
174 supposed to be searched, substituted, or transliterated instead of the default
175 $_. When used in scalar context, the return value generally indicates the
176 success of the operation. Behavior in list context depends on the particular
177 operator. See L</"Regexp Quote-Like Operators"> for details.
179 If the right argument is an expression rather than a search pattern,
180 substitution, or transliteration, it is interpreted as a search pattern at run
181 time. This can be less efficient than an explicit search, because the
182 pattern must be compiled every time the expression is evaluated.
184 Binary "!~" is just like "=~" except the return value is negated in
187 =head2 Multiplicative Operators
189 Binary "*" multiplies two numbers.
191 Binary "/" divides two numbers.
193 Binary "%" computes the modulus of two numbers. Given integer
194 operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
195 C<$a> minus the largest multiple of C<$b> that is not greater than
196 C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
197 smallest multiple of C<$b> that is not less than C<$a> (i.e. the
198 result will be less than or equal to zero).
199 Note than when C<use integer> is in scope, "%" gives you direct access
200 to the modulus operator as implemented by your C compiler. This
201 operator is not as well defined for negative operands, but it will
204 Binary "x" is the repetition operator. In scalar context or if the left
205 operand is not enclosed in parentheses, it returns a string consisting
206 of the left operand repeated the number of times specified by the right
207 operand. In list context, if the left operand is enclosed in
208 parentheses, it repeats the list.
210 print '-' x 80; # print row of dashes
212 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
214 @ones = (1) x 80; # a list of 80 1's
215 @ones = (5) x @ones; # set all elements to 5
218 =head2 Additive Operators
220 Binary "+" returns the sum of two numbers.
222 Binary "-" returns the difference of two numbers.
224 Binary "." concatenates two strings.
226 =head2 Shift Operators
228 Binary "<<" returns the value of its left argument shifted left by the
229 number of bits specified by the right argument. Arguments should be
230 integers. (See also L<Integer Arithmetic>.)
232 Binary ">>" returns the value of its left argument shifted right by
233 the number of bits specified by the right argument. Arguments should
234 be integers. (See also L<Integer Arithmetic>.)
236 =head2 Named Unary Operators
238 The various named unary operators are treated as functions with one
239 argument, with optional parentheses. These include the filetest
240 operators, like C<-f>, C<-M>, etc. See L<perlfunc>.
242 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
243 is followed by a left parenthesis as the next token, the operator and
244 arguments within parentheses are taken to be of highest precedence,
245 just like a normal function call. For example,
246 because named unary operators are higher precedence than ||:
248 chdir $foo || die; # (chdir $foo) || die
249 chdir($foo) || die; # (chdir $foo) || die
250 chdir ($foo) || die; # (chdir $foo) || die
251 chdir +($foo) || die; # (chdir $foo) || die
253 but, because * is higher precedence than named operators:
255 chdir $foo * 20; # chdir ($foo * 20)
256 chdir($foo) * 20; # (chdir $foo) * 20
257 chdir ($foo) * 20; # (chdir $foo) * 20
258 chdir +($foo) * 20; # chdir ($foo * 20)
260 rand 10 * 20; # rand (10 * 20)
261 rand(10) * 20; # (rand 10) * 20
262 rand (10) * 20; # (rand 10) * 20
263 rand +(10) * 20; # rand (10 * 20)
265 See also L<"Terms and List Operators (Leftward)">.
267 =head2 Relational Operators
269 Binary "<" returns true if the left argument is numerically less than
272 Binary ">" returns true if the left argument is numerically greater
273 than the right argument.
275 Binary "<=" returns true if the left argument is numerically less than
276 or equal to the right argument.
278 Binary ">=" returns true if the left argument is numerically greater
279 than or equal to the right argument.
281 Binary "lt" returns true if the left argument is stringwise less than
284 Binary "gt" returns true if the left argument is stringwise greater
285 than the right argument.
287 Binary "le" returns true if the left argument is stringwise less than
288 or equal to the right argument.
290 Binary "ge" returns true if the left argument is stringwise greater
291 than or equal to the right argument.
293 =head2 Equality Operators
295 Binary "==" returns true if the left argument is numerically equal to
298 Binary "!=" returns true if the left argument is numerically not equal
299 to the right argument.
301 Binary "<=>" returns -1, 0, or 1 depending on whether the left
302 argument is numerically less than, equal to, or greater than the right
303 argument. If your platform supports NaNs (not-a-numbers) as numeric
304 values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
305 "<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
306 returns true, as does NaN != anything else. If your platform doesn't
307 support NaNs then NaN is just a string with numeric value 0.
309 perl -le '$a = NaN; print "No NaN support here" if $a == $a'
310 perl -le '$a = NaN; print "NaN support here" if $a != $a'
312 Binary "eq" returns true if the left argument is stringwise equal to
315 Binary "ne" returns true if the left argument is stringwise not equal
316 to the right argument.
318 Binary "cmp" returns -1, 0, or 1 depending on whether the left
319 argument is stringwise less than, equal to, or greater than the right
322 "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
323 by the current locale if C<use locale> is in effect. See L<perllocale>.
327 Binary "&" returns its operators ANDed together bit by bit.
328 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
330 =head2 Bitwise Or and Exclusive Or
332 Binary "|" returns its operators ORed together bit by bit.
333 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
335 Binary "^" returns its operators XORed together bit by bit.
336 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
338 =head2 C-style Logical And
340 Binary "&&" performs a short-circuit logical AND operation. That is,
341 if the left operand is false, the right operand is not even evaluated.
342 Scalar or list context propagates down to the right operand if it
345 =head2 C-style Logical Or
347 Binary "||" performs a short-circuit logical OR operation. That is,
348 if the left operand is true, the right operand is not even evaluated.
349 Scalar or list context propagates down to the right operand if it
352 The C<||> and C<&&> operators differ from C's in that, rather than returning
353 0 or 1, they return the last value evaluated. Thus, a reasonably portable
354 way to find out the home directory (assuming it's not "0") might be:
356 $home = $ENV{'HOME'} || $ENV{'LOGDIR'} ||
357 (getpwuid($<))[7] || die "You're homeless!\n";
359 In particular, this means that you shouldn't use this
360 for selecting between two aggregates for assignment:
362 @a = @b || @c; # this is wrong
363 @a = scalar(@b) || @c; # really meant this
364 @a = @b ? @b : @c; # this works fine, though
366 As more readable alternatives to C<&&> and C<||> when used for
367 control flow, Perl provides C<and> and C<or> operators (see below).
368 The short-circuit behavior is identical. The precedence of "and" and
369 "or" is much lower, however, so that you can safely use them after a
370 list operator without the need for parentheses:
372 unlink "alpha", "beta", "gamma"
373 or gripe(), next LINE;
375 With the C-style operators that would have been written like this:
377 unlink("alpha", "beta", "gamma")
378 || (gripe(), next LINE);
380 Using "or" for assignment is unlikely to do what you want; see below.
382 =head2 Range Operators
384 Binary ".." is the range operator, which is really two different
385 operators depending on the context. In list context, it returns an
386 array of values counting (up by ones) from the left value to the right
387 value. If the left value is greater than the right value then it
388 returns the empty array. The range operator is useful for writing
389 C<foreach (1..10)> loops and for doing slice operations on arrays. In
390 the current implementation, no temporary array is created when the
391 range operator is used as the expression in C<foreach> loops, but older
392 versions of Perl might burn a lot of memory when you write something
395 for (1 .. 1_000_000) {
399 In scalar context, ".." returns a boolean value. The operator is
400 bistable, like a flip-flop, and emulates the line-range (comma) operator
401 of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
402 own boolean state. It is false as long as its left operand is false.
403 Once the left operand is true, the range operator stays true until the
404 right operand is true, I<AFTER> which the range operator becomes false
405 again. It doesn't become false till the next time the range operator is
406 evaluated. It can test the right operand and become false on the same
407 evaluation it became true (as in B<awk>), but it still returns true once.
408 If you don't want it to test the right operand till the next
409 evaluation, as in B<sed>, just use three dots ("...") instead of
410 two. In all other regards, "..." behaves just like ".." does.
412 The right operand is not evaluated while the operator is in the
413 "false" state, and the left operand is not evaluated while the
414 operator is in the "true" state. The precedence is a little lower
415 than || and &&. The value returned is either the empty string for
416 false, or a sequence number (beginning with 1) for true. The
417 sequence number is reset for each range encountered. The final
418 sequence number in a range has the string "E0" appended to it, which
419 doesn't affect its numeric value, but gives you something to search
420 for if you want to exclude the endpoint. You can exclude the
421 beginning point by waiting for the sequence number to be greater
422 than 1. If either operand of scalar ".." is a constant expression,
423 that operand is implicitly compared to the C<$.> variable, the
424 current line number. Examples:
426 As a scalar operator:
428 if (101 .. 200) { print; } # print 2nd hundred lines
429 next line if (1 .. /^$/); # skip header lines
430 s/^/> / if (/^$/ .. eof()); # quote body
432 # parse mail messages
434 $in_header = 1 .. /^$/;
435 $in_body = /^$/ .. eof();
436 # do something based on those
438 close ARGV if eof; # reset $. each file
443 for (101 .. 200) { print; } # print $_ 100 times
444 @foo = @foo[0 .. $#foo]; # an expensive no-op
445 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
447 The range operator (in list context) makes use of the magical
448 auto-increment algorithm if the operands are strings. You
451 @alphabet = ('A' .. 'Z');
453 to get all normal letters of the alphabet, or
455 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
457 to get a hexadecimal digit, or
459 @z2 = ('01' .. '31'); print $z2[$mday];
461 to get dates with leading zeros. If the final value specified is not
462 in the sequence that the magical increment would produce, the sequence
463 goes until the next value would be longer than the final value
466 =head2 Conditional Operator
468 Ternary "?:" is the conditional operator, just as in C. It works much
469 like an if-then-else. If the argument before the ? is true, the
470 argument before the : is returned, otherwise the argument after the :
471 is returned. For example:
473 printf "I have %d dog%s.\n", $n,
474 ($n == 1) ? '' : "s";
476 Scalar or list context propagates downward into the 2nd
477 or 3rd argument, whichever is selected.
479 $a = $ok ? $b : $c; # get a scalar
480 @a = $ok ? @b : @c; # get an array
481 $a = $ok ? @b : @c; # oops, that's just a count!
483 The operator may be assigned to if both the 2nd and 3rd arguments are
484 legal lvalues (meaning that you can assign to them):
486 ($a_or_b ? $a : $b) = $c;
488 Because this operator produces an assignable result, using assignments
489 without parentheses will get you in trouble. For example, this:
491 $a % 2 ? $a += 10 : $a += 2
495 (($a % 2) ? ($a += 10) : $a) += 2
499 ($a % 2) ? ($a += 10) : ($a += 2)
501 That should probably be written more simply as:
503 $a += ($a % 2) ? 10 : 2;
505 =head2 Assignment Operators
507 "=" is the ordinary assignment operator.
509 Assignment operators work as in C. That is,
517 although without duplicating any side effects that dereferencing the lvalue
518 might trigger, such as from tie(). Other assignment operators work similarly.
519 The following are recognized:
526 Although these are grouped by family, they all have the precedence
529 Unlike in C, the scalar assignment operator produces a valid lvalue.
530 Modifying an assignment is equivalent to doing the assignment and
531 then modifying the variable that was assigned to. This is useful
532 for modifying a copy of something, like this:
534 ($tmp = $global) =~ tr [A-Z] [a-z];
545 Similarly, a list assignment in list context produces the list of
546 lvalues assigned to, and a list assignment in scalar context returns
547 the number of elements produced by the expression on the right hand
548 side of the assignment.
550 =head2 Comma Operator
552 Binary "," is the comma operator. In scalar context it evaluates
553 its left argument, throws that value away, then evaluates its right
554 argument and returns that value. This is just like C's comma operator.
556 In list context, it's just the list argument separator, and inserts
557 both its arguments into the list.
559 The => digraph is mostly just a synonym for the comma operator. It's useful for
560 documenting arguments that come in pairs. As of release 5.001, it also forces
561 any word to the left of it to be interpreted as a string.
563 =head2 List Operators (Rightward)
565 On the right side of a list operator, it has very low precedence,
566 such that it controls all comma-separated expressions found there.
567 The only operators with lower precedence are the logical operators
568 "and", "or", and "not", which may be used to evaluate calls to list
569 operators without the need for extra parentheses:
571 open HANDLE, "filename"
572 or die "Can't open: $!\n";
574 See also discussion of list operators in L<Terms and List Operators (Leftward)>.
578 Unary "not" returns the logical negation of the expression to its right.
579 It's the equivalent of "!" except for the very low precedence.
583 Binary "and" returns the logical conjunction of the two surrounding
584 expressions. It's equivalent to && except for the very low
585 precedence. This means that it short-circuits: i.e., the right
586 expression is evaluated only if the left expression is true.
588 =head2 Logical or and Exclusive Or
590 Binary "or" returns the logical disjunction of the two surrounding
591 expressions. It's equivalent to || except for the very low precedence.
592 This makes it useful for control flow
594 print FH $data or die "Can't write to FH: $!";
596 This means that it short-circuits: i.e., the right expression is evaluated
597 only if the left expression is false. Due to its precedence, you should
598 probably avoid using this for assignment, only for control flow.
600 $a = $b or $c; # bug: this is wrong
601 ($a = $b) or $c; # really means this
602 $a = $b || $c; # better written this way
604 However, when it's a list-context assignment and you're trying to use
605 "||" for control flow, you probably need "or" so that the assignment
606 takes higher precedence.
608 @info = stat($file) || die; # oops, scalar sense of stat!
609 @info = stat($file) or die; # better, now @info gets its due
611 Then again, you could always use parentheses.
613 Binary "xor" returns the exclusive-OR of the two surrounding expressions.
614 It cannot short circuit, of course.
616 =head2 C Operators Missing From Perl
618 Here is what C has that Perl doesn't:
624 Address-of operator. (But see the "\" operator for taking a reference.)
628 Dereference-address operator. (Perl's prefix dereferencing
629 operators are typed: $, @, %, and &.)
633 Type-casting operator.
637 =head2 Quote and Quote-like Operators
639 While we usually think of quotes as literal values, in Perl they
640 function as operators, providing various kinds of interpolating and
641 pattern matching capabilities. Perl provides customary quote characters
642 for these behaviors, but also provides a way for you to choose your
643 quote character for any of them. In the following table, a C<{}> represents
644 any pair of delimiters you choose.
646 Customary Generic Meaning Interpolates
649 `` qx{} Command yes (unless '' is delimiter)
651 // m{} Pattern match yes (unless '' is delimiter)
652 qr{} Pattern yes (unless '' is delimiter)
653 s{}{} Substitution yes (unless '' is delimiter)
654 tr{}{} Transliteration no (but see below)
656 Non-bracketing delimiters use the same character fore and aft, but the four
657 sorts of brackets (round, angle, square, curly) will all nest, which means
666 Note, however, that this does not always work for quoting Perl code:
668 $s = q{ if($a eq "}") ... }; # WRONG
670 is a syntax error. The C<Text::Balanced> module (from CPAN, and
671 starting from Perl 5.8 part of the standard distribution) is able
674 There can be whitespace between the operator and the quoting
675 characters, except when C<#> is being used as the quoting character.
676 C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
677 operator C<q> followed by a comment. Its argument will be taken
678 from the next line. This allows you to write:
680 s {foo} # Replace foo
683 For constructs that do interpolate, variables beginning with "C<$>"
684 or "C<@>" are interpolated, as are the following escape sequences. Within
685 a transliteration, the first eleven of these sequences may be used.
692 \a alarm (bell) (BEL)
694 \033 octal char (ESC)
696 \x{263a} wide hex char (SMILEY)
697 \c[ control char (ESC)
700 \l lowercase next char
701 \u uppercase next char
704 \E end case modification
705 \Q quote non-word characters till \E
707 If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u>
708 and C<\U> is taken from the current locale. See L<perllocale>. For
709 documentation of C<\N{name}>, see L<charnames>.
711 All systems use the virtual C<"\n"> to represent a line terminator,
712 called a "newline". There is no such thing as an unvarying, physical
713 newline character. It is only an illusion that the operating system,
714 device drivers, C libraries, and Perl all conspire to preserve. Not all
715 systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
716 on a Mac, these are reversed, and on systems without line terminator,
717 printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
718 you mean a "newline" for your system, but use the literal ASCII when you
719 need an exact character. For example, most networking protocols expect
720 and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
721 and although they often accept just C<"\012">, they seldom tolerate just
722 C<"\015">. If you get in the habit of using C<"\n"> for networking,
723 you may be burned some day.
725 You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
726 An unescaped C<$> or C<@> interpolates the corresponding variable,
727 while escaping will cause the literal string C<\$> to be inserted.
728 You'll need to write something like C<m/\Quser\E\@\Qhost/>.
730 Patterns are subject to an additional level of interpretation as a
731 regular expression. This is done as a second pass, after variables are
732 interpolated, so that regular expressions may be incorporated into the
733 pattern from the variables. If this is not what you want, use C<\Q> to
734 interpolate a variable literally.
736 Apart from the behavior described above, Perl does not expand
737 multiple levels of interpolation. In particular, contrary to the
738 expectations of shell programmers, back-quotes do I<NOT> interpolate
739 within double quotes, nor do single quotes impede evaluation of
740 variables when used within double quotes.
742 =head2 Regexp Quote-Like Operators
744 Here are the quote-like operators that apply to pattern
745 matching and related activities.
751 This is just like the C</pattern/> search, except that it matches only
752 once between calls to the reset() operator. This is a useful
753 optimization when you want to see only the first occurrence of
754 something in each file of a set of files, for instance. Only C<??>
755 patterns local to the current package are reset.
759 # blank line between header and body
762 reset if eof; # clear ?? status for next file
765 This usage is vaguely deprecated, which means it just might possibly
766 be removed in some distant future version of Perl, perhaps somewhere
767 around the year 2168.
769 =item m/PATTERN/cgimosx
771 =item /PATTERN/cgimosx
773 Searches a string for a pattern match, and in scalar context returns
774 true if it succeeds, false if it fails. If no string is specified
775 via the C<=~> or C<!~> operator, the $_ string is searched. (The
776 string specified with C<=~> need not be an lvalue--it may be the
777 result of an expression evaluation, but remember the C<=~> binds
778 rather tightly.) See also L<perlre>. See L<perllocale> for
779 discussion of additional considerations that apply when C<use locale>
784 c Do not reset search position on a failed match when /g is in effect.
785 g Match globally, i.e., find all occurrences.
786 i Do case-insensitive pattern matching.
787 m Treat string as multiple lines.
788 o Compile pattern only once.
789 s Treat string as single line.
790 x Use extended regular expressions.
792 If "/" is the delimiter then the initial C<m> is optional. With the C<m>
793 you can use any pair of non-alphanumeric, non-whitespace characters
794 as delimiters. This is particularly useful for matching path names
795 that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
796 the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
797 If "'" is the delimiter, no interpolation is performed on the PATTERN.
799 PATTERN may contain variables, which will be interpolated (and the
800 pattern recompiled) every time the pattern search is evaluated, except
801 for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
802 C<$|> are not interpolated because they look like end-of-string tests.)
803 If you want such a pattern to be compiled only once, add a C</o> after
804 the trailing delimiter. This avoids expensive run-time recompilations,
805 and is useful when the value you are interpolating won't change over
806 the life of the script. However, mentioning C</o> constitutes a promise
807 that you won't change the variables in the pattern. If you change them,
808 Perl won't even notice. See also L<"qr/STRING/imosx">.
810 If the PATTERN evaluates to the empty string, the last
811 I<successfully> matched regular expression is used instead.
813 If the C</g> option is not used, C<m//> in list context returns a
814 list consisting of the subexpressions matched by the parentheses in the
815 pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are
816 also set, and that this differs from Perl 4's behavior.) When there are
817 no parentheses in the pattern, the return value is the list C<(1)> for
818 success. With or without parentheses, an empty list is returned upon
823 open(TTY, '/dev/tty');
824 <TTY> =~ /^y/i && foo(); # do foo if desired
826 if (/Version: *([0-9.]*)/) { $version = $1; }
828 next if m#^/usr/spool/uucp#;
833 print if /$arg/o; # compile only once
836 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
838 This last example splits $foo into the first two words and the
839 remainder of the line, and assigns those three fields to $F1, $F2, and
840 $Etc. The conditional is true if any variables were assigned, i.e., if
843 The C</g> modifier specifies global pattern matching--that is,
844 matching as many times as possible within the string. How it behaves
845 depends on the context. In list context, it returns a list of the
846 substrings matched by any capturing parentheses in the regular
847 expression. If there are no parentheses, it returns a list of all
848 the matched strings, as if there were parentheses around the whole
851 In scalar context, each execution of C<m//g> finds the next match,
852 returning true if it matches, and false if there is no further match.
853 The position after the last match can be read or set using the pos()
854 function; see L<perlfunc/pos>. A failed match normally resets the
855 search position to the beginning of the string, but you can avoid that
856 by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
857 string also resets the search position.
859 You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
860 zero-width assertion that matches the exact position where the previous
861 C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion
862 still anchors at pos(), but the match is of course only attempted once.
863 Using C<\G> without C</g> on a target string that has not previously had a
864 C</g> match applied to it is the same as using the C<\A> assertion to match
865 the beginning of the string.
870 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
874 while (defined($paragraph = <>)) {
875 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
879 print "$sentences\n";
881 # using m//gc with \G
885 print $1 while /(o)/gc; print "', pos=", pos, "\n";
887 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
889 print $1 while /(p)/gc; print "', pos=", pos, "\n";
891 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
893 The last example should print:
903 Notice that the final match matched C<q> instead of C<p>, which a match
904 without the C<\G> anchor would have done. Also note that the final match
905 did not update C<pos> -- C<pos> is only updated on a C</g> match. If the
906 final match did indeed match C<p>, it's a good bet that you're running an
907 older (pre-5.6.0) Perl.
909 A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
910 combine several regexps like this to process a string part-by-part,
911 doing different actions depending on which regexp matched. Each
912 regexp tries to match where the previous one leaves off.
915 $url = new URI::URL "http://www/"; die if $url eq "xXx";
919 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
920 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
921 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
922 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
923 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
924 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
925 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
926 print ". That's all!\n";
929 Here is the output (split into several lines):
931 line-noise lowercase line-noise lowercase UPPERCASE line-noise
932 UPPERCASE line-noise lowercase line-noise lowercase line-noise
933 lowercase lowercase line-noise lowercase lowercase line-noise
934 MiXeD line-noise. That's all!
940 A single-quoted, literal string. A backslash represents a backslash
941 unless followed by the delimiter or another backslash, in which case
942 the delimiter or backslash is interpolated.
944 $foo = q!I said, "You said, 'She said it.'"!;
945 $bar = q('This is it.');
946 $baz = '\n'; # a two-character string
952 A double-quoted, interpolated string.
955 (*** The previous line contains the naughty word "$1".\n)
956 if /\b(tcl|java|python)\b/i; # :-)
957 $baz = "\n"; # a one-character string
959 =item qr/STRING/imosx
961 This operator quotes (and possibly compiles) its I<STRING> as a regular
962 expression. I<STRING> is interpolated the same way as I<PATTERN>
963 in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
964 is done. Returns a Perl value which may be used instead of the
965 corresponding C</STRING/imosx> expression.
969 $rex = qr/my.STRING/is;
976 The result may be used as a subpattern in a match:
979 $string =~ /foo${re}bar/; # can be interpolated in other patterns
980 $string =~ $re; # or used standalone
981 $string =~ /$re/; # or this way
983 Since Perl may compile the pattern at the moment of execution of qr()
984 operator, using qr() may have speed advantages in some situations,
985 notably if the result of qr() is used standalone:
988 my $patterns = shift;
989 my @compiled = map qr/$_/i, @$patterns;
992 foreach my $pat (@compiled) {
993 $success = 1, last if /$pat/;
999 Precompilation of the pattern into an internal representation at
1000 the moment of qr() avoids a need to recompile the pattern every
1001 time a match C</$pat/> is attempted. (Perl has many other internal
1002 optimizations, but none would be triggered in the above example if
1003 we did not use qr() operator.)
1007 i Do case-insensitive pattern matching.
1008 m Treat string as multiple lines.
1009 o Compile pattern only once.
1010 s Treat string as single line.
1011 x Use extended regular expressions.
1013 See L<perlre> for additional information on valid syntax for STRING, and
1014 for a detailed look at the semantics of regular expressions.
1020 A string which is (possibly) interpolated and then executed as a
1021 system command with C</bin/sh> or its equivalent. Shell wildcards,
1022 pipes, and redirections will be honored. The collected standard
1023 output of the command is returned; standard error is unaffected. In
1024 scalar context, it comes back as a single (potentially multi-line)
1025 string, or undef if the command failed. In list context, returns a
1026 list of lines (however you've defined lines with $/ or
1027 $INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
1029 Because backticks do not affect standard error, use shell file descriptor
1030 syntax (assuming the shell supports this) if you care to address this.
1031 To capture a command's STDERR and STDOUT together:
1033 $output = `cmd 2>&1`;
1035 To capture a command's STDOUT but discard its STDERR:
1037 $output = `cmd 2>/dev/null`;
1039 To capture a command's STDERR but discard its STDOUT (ordering is
1042 $output = `cmd 2>&1 1>/dev/null`;
1044 To exchange a command's STDOUT and STDERR in order to capture the STDERR
1045 but leave its STDOUT to come out the old STDERR:
1047 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
1049 To read both a command's STDOUT and its STDERR separately, it's easiest
1050 and safest to redirect them separately to files, and then read from those
1051 files when the program is done:
1053 system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr");
1055 Using single-quote as a delimiter protects the command from Perl's
1056 double-quote interpolation, passing it on to the shell instead:
1058 $perl_info = qx(ps $$); # that's Perl's $$
1059 $shell_info = qx'ps $$'; # that's the new shell's $$
1061 How that string gets evaluated is entirely subject to the command
1062 interpreter on your system. On most platforms, you will have to protect
1063 shell metacharacters if you want them treated literally. This is in
1064 practice difficult to do, as it's unclear how to escape which characters.
1065 See L<perlsec> for a clean and safe example of a manual fork() and exec()
1066 to emulate backticks safely.
1068 On some platforms (notably DOS-like ones), the shell may not be
1069 capable of dealing with multiline commands, so putting newlines in
1070 the string may not get you what you want. You may be able to evaluate
1071 multiple commands in a single line by separating them with the command
1072 separator character, if your shell supports that (e.g. C<;> on many Unix
1073 shells; C<&> on the Windows NT C<cmd> shell).
1075 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1076 output before starting the child process, but this may not be supported
1077 on some platforms (see L<perlport>). To be safe, you may need to set
1078 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1079 C<IO::Handle> on any open handles.
1081 Beware that some command shells may place restrictions on the length
1082 of the command line. You must ensure your strings don't exceed this
1083 limit after any necessary interpolations. See the platform-specific
1084 release notes for more details about your particular environment.
1086 Using this operator can lead to programs that are difficult to port,
1087 because the shell commands called vary between systems, and may in
1088 fact not be present at all. As one example, the C<type> command under
1089 the POSIX shell is very different from the C<type> command under DOS.
1090 That doesn't mean you should go out of your way to avoid backticks
1091 when they're the right way to get something done. Perl was made to be
1092 a glue language, and one of the things it glues together is commands.
1093 Just understand what you're getting yourself into.
1095 See L<"I/O Operators"> for more discussion.
1099 Evaluates to a list of the words extracted out of STRING, using embedded
1100 whitespace as the word delimiters. It can be understood as being roughly
1103 split(' ', q/STRING/);
1105 the difference being that it generates a real list at compile time. So
1110 is semantically equivalent to the list:
1114 Some frequently seen examples:
1116 use POSIX qw( setlocale localeconv )
1117 @EXPORT = qw( foo bar baz );
1119 A common mistake is to try to separate the words with comma or to
1120 put comments into a multi-line C<qw>-string. For this reason, the
1121 C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
1122 produces warnings if the STRING contains the "," or the "#" character.
1124 =item s/PATTERN/REPLACEMENT/egimosx
1126 Searches a string for a pattern, and if found, replaces that pattern
1127 with the replacement text and returns the number of substitutions
1128 made. Otherwise it returns false (specifically, the empty string).
1130 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
1131 variable is searched and modified. (The string specified with C<=~> must
1132 be scalar variable, an array element, a hash element, or an assignment
1133 to one of those, i.e., an lvalue.)
1135 If the delimiter chosen is a single quote, no interpolation is
1136 done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1137 PATTERN contains a $ that looks like a variable rather than an
1138 end-of-string test, the variable will be interpolated into the pattern
1139 at run-time. If you want the pattern compiled only once the first time
1140 the variable is interpolated, use the C</o> option. If the pattern
1141 evaluates to the empty string, the last successfully executed regular
1142 expression is used instead. See L<perlre> for further explanation on these.
1143 See L<perllocale> for discussion of additional considerations that apply
1144 when C<use locale> is in effect.
1148 e Evaluate the right side as an expression.
1149 g Replace globally, i.e., all occurrences.
1150 i Do case-insensitive pattern matching.
1151 m Treat string as multiple lines.
1152 o Compile pattern only once.
1153 s Treat string as single line.
1154 x Use extended regular expressions.
1156 Any non-alphanumeric, non-whitespace delimiter may replace the
1157 slashes. If single quotes are used, no interpretation is done on the
1158 replacement string (the C</e> modifier overrides this, however). Unlike
1159 Perl 4, Perl 5 treats backticks as normal delimiters; the replacement
1160 text is not evaluated as a command. If the
1161 PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
1162 pair of quotes, which may or may not be bracketing quotes, e.g.,
1163 C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
1164 replacement portion to be treated as a full-fledged Perl expression
1165 and evaluated right then and there. It is, however, syntax checked at
1166 compile-time. A second C<e> modifier will cause the replacement portion
1167 to be C<eval>ed before being run as a Perl expression.
1171 s/\bgreen\b/mauve/g; # don't change wintergreen
1173 $path =~ s|/usr/bin|/usr/local/bin|;
1175 s/Login: $foo/Login: $bar/; # run-time pattern
1177 ($foo = $bar) =~ s/this/that/; # copy first, then change
1179 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
1182 s/\d+/$&*2/e; # yields 'abc246xyz'
1183 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
1184 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1186 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
1187 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
1188 s/^=(\w+)/&pod($1)/ge; # use function call
1190 # expand variables in $_, but dynamics only, using
1191 # symbolic dereferencing
1194 # Add one to the value of any numbers in the string
1197 # This will expand any embedded scalar variable
1198 # (including lexicals) in $_ : First $1 is interpolated
1199 # to the variable name, and then evaluated
1202 # Delete (most) C comments.
1204 /\* # Match the opening delimiter.
1205 .*? # Match a minimal number of characters.
1206 \*/ # Match the closing delimiter.
1209 s/^\s*(.*?)\s*$/$1/; # trim white space in $_, expensively
1211 for ($variable) { # trim white space in $variable, cheap
1216 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
1218 Note the use of $ instead of \ in the last example. Unlike
1219 B<sed>, we use the \<I<digit>> form in only the left hand side.
1220 Anywhere else it's $<I<digit>>.
1222 Occasionally, you can't use just a C</g> to get all the changes
1223 to occur that you might want. Here are two common cases:
1225 # put commas in the right places in an integer
1226 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
1228 # expand tabs to 8-column spacing
1229 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
1231 =item tr/SEARCHLIST/REPLACEMENTLIST/cds
1233 =item y/SEARCHLIST/REPLACEMENTLIST/cds
1235 Transliterates all occurrences of the characters found in the search list
1236 with the corresponding character in the replacement list. It returns
1237 the number of characters replaced or deleted. If no string is
1238 specified via the =~ or !~ operator, the $_ string is transliterated. (The
1239 string specified with =~ must be a scalar variable, an array element, a
1240 hash element, or an assignment to one of those, i.e., an lvalue.)
1242 A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
1243 does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
1244 For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1245 SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1246 its own pair of quotes, which may or may not be bracketing quotes,
1247 e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
1249 Note that C<tr> does B<not> do regular expression character classes
1250 such as C<\d> or C<[:lower:]>. The <tr> operator is not equivalent to
1251 the tr(1) utility. If you want to map strings between lower/upper
1252 cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider
1253 using the C<s> operator if you need regular expressions.
1255 Note also that the whole range idea is rather unportable between
1256 character sets--and even within character sets they may cause results
1257 you probably didn't expect. A sound principle is to use only ranges
1258 that begin from and end at either alphabets of equal case (a-e, A-E),
1259 or digits (0-4). Anything else is unsafe. If in doubt, spell out the
1260 character sets in full.
1264 c Complement the SEARCHLIST.
1265 d Delete found but unreplaced characters.
1266 s Squash duplicate replaced characters.
1268 If the C</c> modifier is specified, the SEARCHLIST character set
1269 is complemented. If the C</d> modifier is specified, any characters
1270 specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
1271 (Note that this is slightly more flexible than the behavior of some
1272 B<tr> programs, which delete anything they find in the SEARCHLIST,
1273 period.) If the C</s> modifier is specified, sequences of characters
1274 that were transliterated to the same character are squashed down
1275 to a single instance of the character.
1277 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1278 exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1279 than the SEARCHLIST, the final character is replicated till it is long
1280 enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
1281 This latter is useful for counting characters in a class or for
1282 squashing character sequences in a class.
1286 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1288 $cnt = tr/*/*/; # count the stars in $_
1290 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1292 $cnt = tr/0-9//; # count the digits in $_
1294 tr/a-zA-Z//s; # bookkeeper -> bokeper
1296 ($HOST = $host) =~ tr/a-z/A-Z/;
1298 tr/a-zA-Z/ /cs; # change non-alphas to single space
1301 [\000-\177]; # delete 8th bit
1303 If multiple transliterations are given for a character, only the
1308 will transliterate any A to X.
1310 Because the transliteration table is built at compile time, neither
1311 the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
1312 interpolation. That means that if you want to use variables, you
1315 eval "tr/$oldlist/$newlist/";
1318 eval "tr/$oldlist/$newlist/, 1" or die $@;
1322 =head2 Gory details of parsing quoted constructs
1324 When presented with something that might have several different
1325 interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
1326 principle to pick the most probable interpretation. This strategy
1327 is so successful that Perl programmers often do not suspect the
1328 ambivalence of what they write. But from time to time, Perl's
1329 notions differ substantially from what the author honestly meant.
1331 This section hopes to clarify how Perl handles quoted constructs.
1332 Although the most common reason to learn this is to unravel labyrinthine
1333 regular expressions, because the initial steps of parsing are the
1334 same for all quoting operators, they are all discussed together.
1336 The most important Perl parsing rule is the first one discussed
1337 below: when processing a quoted construct, Perl first finds the end
1338 of that construct, then interprets its contents. If you understand
1339 this rule, you may skip the rest of this section on the first
1340 reading. The other rules are likely to contradict the user's
1341 expectations much less frequently than this first one.
1343 Some passes discussed below are performed concurrently, but because
1344 their results are the same, we consider them individually. For different
1345 quoting constructs, Perl performs different numbers of passes, from
1346 one to five, but these passes are always performed in the same order.
1350 =item Finding the end
1352 The first pass is finding the end of the quoted construct, whether
1353 it be a multicharacter delimiter C<"\nEOF\n"> in the C<<<EOF>
1354 construct, a C</> that terminates a C<qq//> construct, a C<]> which
1355 terminates C<qq[]> construct, or a C<< > >> which terminates a
1356 fileglob started with C<< < >>.
1358 When searching for single-character non-pairing delimiters, such
1359 as C</>, combinations of C<\\> and C<\/> are skipped. However,
1360 when searching for single-character pairing delimiter like C<[>,
1361 combinations of C<\\>, C<\]>, and C<\[> are all skipped, and nested
1362 C<[>, C<]> are skipped as well. When searching for multicharacter
1363 delimiters, nothing is skipped.
1365 For constructs with three-part delimiters (C<s///>, C<y///>, and
1366 C<tr///>), the search is repeated once more.
1368 During this search no attention is paid to the semantics of the construct.
1371 "$hash{"$foo/$bar"}"
1376 bar # NOT a comment, this slash / terminated m//!
1379 do not form legal quoted expressions. The quoted part ends on the
1380 first C<"> and C</>, and the rest happens to be a syntax error.
1381 Because the slash that terminated C<m//> was followed by a C<SPACE>,
1382 the example above is not C<m//x>, but rather C<m//> with no C</x>
1383 modifier. So the embedded C<#> is interpreted as a literal C<#>.
1385 =item Removal of backslashes before delimiters
1387 During the second pass, text between the starting and ending
1388 delimiters is copied to a safe location, and the C<\> is removed
1389 from combinations consisting of C<\> and delimiter--or delimiters,
1390 meaning both starting and ending delimiters will should these differ.
1391 This removal does not happen for multi-character delimiters.
1392 Note that the combination C<\\> is left intact, just as it was.
1394 Starting from this step no information about the delimiters is
1399 The next step is interpolation in the text obtained, which is now
1400 delimiter-independent. There are four different cases.
1404 =item C<<<'EOF'>, C<m''>, C<s'''>, C<tr///>, C<y///>
1406 No interpolation is performed.
1410 The only interpolation is removal of C<\> from pairs C<\\>.
1412 =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>
1414 C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
1415 converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
1416 is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
1417 The other combinations are replaced with appropriate expansions.
1419 Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
1420 is interpolated in the usual way. Something like C<"\Q\\E"> has
1421 no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
1422 result is the same as for C<"\\\\E">. As a general rule, backslashes
1423 between C<\Q> and C<\E> may lead to counterintuitive results. So,
1424 C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
1425 as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
1430 may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
1432 Interpolated scalars and arrays are converted internally to the C<join> and
1433 C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
1435 $foo . " XXX '" . (join $", @arr) . "'";
1437 All operations above are performed simultaneously, left to right.
1439 Because the result of C<"\Q STRING \E"> has all metacharacters
1440 quoted, there is no way to insert a literal C<$> or C<@> inside a
1441 C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
1442 C<"\\\$">; if not, it is interpreted as the start of an interpolated
1445 Note also that the interpolation code needs to make a decision on
1446 where the interpolated scalar ends. For instance, whether
1447 C<< "a $b -> {c}" >> really means:
1449 "a " . $b . " -> {c}";
1455 Most of the time, the longest possible text that does not include
1456 spaces between components and which contains matching braces or
1457 brackets. because the outcome may be determined by voting based
1458 on heuristic estimators, the result is not strictly predictable.
1459 Fortunately, it's usually correct for ambiguous cases.
1461 =item C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
1463 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
1464 happens (almost) as with C<qq//> constructs, but the substitution
1465 of C<\> followed by RE-special chars (including C<\>) is not
1466 performed. Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
1467 a C<#>-comment in a C<//x>-regular expression, no processing is
1468 performed whatsoever. This is the first step at which the presence
1469 of the C<//x> modifier is relevant.
1471 Interpolation has several quirks: C<$|>, C<$(>, and C<$)> are not
1472 interpolated, and constructs C<$var[SOMETHING]> are voted (by several
1473 different estimators) to be either an array element or C<$var>
1474 followed by an RE alternative. This is where the notation
1475 C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
1476 array element C<-9>, not as a regular expression from the variable
1477 C<$arr> followed by a digit, which would be the interpretation of
1478 C</$arr[0-9]/>. Since voting among different estimators may occur,
1479 the result is not predictable.
1481 It is at this step that C<\1> is begrudgingly converted to C<$1> in
1482 the replacement text of C<s///> to correct the incorrigible
1483 I<sed> hackers who haven't picked up the saner idiom yet. A warning
1484 is emitted if the C<use warnings> pragma or the B<-w> command-line flag
1485 (that is, the C<$^W> variable) was set.
1487 The lack of processing of C<\\> creates specific restrictions on
1488 the post-processed text. If the delimiter is C</>, one cannot get
1489 the combination C<\/> into the result of this step. C</> will
1490 finish the regular expression, C<\/> will be stripped to C</> on
1491 the previous step, and C<\\/> will be left as is. Because C</> is
1492 equivalent to C<\/> inside a regular expression, this does not
1493 matter unless the delimiter happens to be character special to the
1494 RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
1495 alphanumeric char, as in:
1499 In the RE above, which is intentionally obfuscated for illustration, the
1500 delimiter is C<m>, the modifier is C<mx>, and after backslash-removal the
1501 RE is the same as for C<m/ ^ a s* b /mx>). There's more than one
1502 reason you're encouraged to restrict your delimiters to non-alphanumeric,
1503 non-whitespace choices.
1507 This step is the last one for all constructs except regular expressions,
1508 which are processed further.
1510 =item Interpolation of regular expressions
1512 Previous steps were performed during the compilation of Perl code,
1513 but this one happens at run time--although it may be optimized to
1514 be calculated at compile time if appropriate. After preprocessing
1515 described above, and possibly after evaluation if catenation,
1516 joining, casing translation, or metaquoting are involved, the
1517 resulting I<string> is passed to the RE engine for compilation.
1519 Whatever happens in the RE engine might be better discussed in L<perlre>,
1520 but for the sake of continuity, we shall do so here.
1522 This is another step where the presence of the C<//x> modifier is
1523 relevant. The RE engine scans the string from left to right and
1524 converts it to a finite automaton.
1526 Backslashed characters are either replaced with corresponding
1527 literal strings (as with C<\{>), or else they generate special nodes
1528 in the finite automaton (as with C<\b>). Characters special to the
1529 RE engine (such as C<|>) generate corresponding nodes or groups of
1530 nodes. C<(?#...)> comments are ignored. All the rest is either
1531 converted to literal strings to match, or else is ignored (as is
1532 whitespace and C<#>-style comments if C<//x> is present).
1534 Parsing of the bracketed character class construct, C<[...]>, is
1535 rather different than the rule used for the rest of the pattern.
1536 The terminator of this construct is found using the same rules as
1537 for finding the terminator of a C<{}>-delimited construct, the only
1538 exception being that C<]> immediately following C<[> is treated as
1539 though preceded by a backslash. Similarly, the terminator of
1540 C<(?{...})> is found using the same rules as for finding the
1541 terminator of a C<{}>-delimited construct.
1543 It is possible to inspect both the string given to RE engine and the
1544 resulting finite automaton. See the arguments C<debug>/C<debugcolor>
1545 in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
1546 switch documented in L<perlrun/"Command Switches">.
1548 =item Optimization of regular expressions
1550 This step is listed for completeness only. Since it does not change
1551 semantics, details of this step are not documented and are subject
1552 to change without notice. This step is performed over the finite
1553 automaton that was generated during the previous pass.
1555 It is at this stage that C<split()> silently optimizes C</^/> to
1560 =head2 I/O Operators
1562 There are several I/O operators you should know about.
1564 A string enclosed by backticks (grave accents) first undergoes
1565 double-quote interpolation. It is then interpreted as an external
1566 command, and the output of that command is the value of the
1567 backtick string, like in a shell. In scalar context, a single string
1568 consisting of all output is returned. In list context, a list of
1569 values is returned, one per line of output. (You can set C<$/> to use
1570 a different line terminator.) The command is executed each time the
1571 pseudo-literal is evaluated. The status value of the command is
1572 returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
1573 Unlike in B<csh>, no translation is done on the return data--newlines
1574 remain newlines. Unlike in any of the shells, single quotes do not
1575 hide variable names in the command from interpretation. To pass a
1576 literal dollar-sign through to the shell you need to hide it with a
1577 backslash. The generalized form of backticks is C<qx//>. (Because
1578 backticks always undergo shell expansion as well, see L<perlsec> for
1581 In scalar context, evaluating a filehandle in angle brackets yields
1582 the next line from that file (the newline, if any, included), or
1583 C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
1584 (sometimes known as file-slurp mode) and the file is empty, it
1585 returns C<''> the first time, followed by C<undef> subsequently.
1587 Ordinarily you must assign the returned value to a variable, but
1588 there is one situation where an automatic assignment happens. If
1589 and only if the input symbol is the only thing inside the conditional
1590 of a C<while> statement (even if disguised as a C<for(;;)> loop),
1591 the value is automatically assigned to the global variable $_,
1592 destroying whatever was there previously. (This may seem like an
1593 odd thing to you, but you'll use the construct in almost every Perl
1594 script you write.) The $_ variable is not implicitly localized.
1595 You'll have to put a C<local $_;> before the loop if you want that
1598 The following lines are equivalent:
1600 while (defined($_ = <STDIN>)) { print; }
1601 while ($_ = <STDIN>) { print; }
1602 while (<STDIN>) { print; }
1603 for (;<STDIN>;) { print; }
1604 print while defined($_ = <STDIN>);
1605 print while ($_ = <STDIN>);
1606 print while <STDIN>;
1608 This also behaves similarly, but avoids $_ :
1610 while (my $line = <STDIN>) { print $line }
1612 In these loop constructs, the assigned value (whether assignment
1613 is automatic or explicit) is then tested to see whether it is
1614 defined. The defined test avoids problems where line has a string
1615 value that would be treated as false by Perl, for example a "" or
1616 a "0" with no trailing newline. If you really mean for such values
1617 to terminate the loop, they should be tested for explicitly:
1619 while (($_ = <STDIN>) ne '0') { ... }
1620 while (<STDIN>) { last unless $_; ... }
1622 In other boolean contexts, C<< <I<filehandle>> >> without an
1623 explicit C<defined> test or comparison elicit a warning if the
1624 C<use warnings> pragma or the B<-w>
1625 command-line switch (the C<$^W> variable) is in effect.
1627 The filehandles STDIN, STDOUT, and STDERR are predefined. (The
1628 filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
1629 in packages, where they would be interpreted as local identifiers
1630 rather than global.) Additional filehandles may be created with
1631 the open() function, amongst others. See L<perlopentut> and
1632 L<perlfunc/open> for details on this.
1634 If a <FILEHANDLE> is used in a context that is looking for
1635 a list, a list comprising all input lines is returned, one line per
1636 list element. It's easy to grow to a rather large data space this
1637 way, so use with care.
1639 <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
1640 See L<perlfunc/readline>.
1642 The null filehandle <> is special: it can be used to emulate the
1643 behavior of B<sed> and B<awk>. Input from <> comes either from
1644 standard input, or from each file listed on the command line. Here's
1645 how it works: the first time <> is evaluated, the @ARGV array is
1646 checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
1647 gives you standard input. The @ARGV array is then processed as a list
1648 of filenames. The loop
1651 ... # code for each line
1654 is equivalent to the following Perl-like pseudo code:
1656 unshift(@ARGV, '-') unless @ARGV;
1657 while ($ARGV = shift) {
1660 ... # code for each line
1664 except that it isn't so cumbersome to say, and will actually work.
1665 It really does shift the @ARGV array and put the current filename
1666 into the $ARGV variable. It also uses filehandle I<ARGV>
1667 internally--<> is just a synonym for <ARGV>, which
1668 is magical. (The pseudo code above doesn't work because it treats
1669 <ARGV> as non-magical.)
1671 You can modify @ARGV before the first <> as long as the array ends up
1672 containing the list of filenames you really want. Line numbers (C<$.>)
1673 continue as though the input were one big happy file. See the example
1674 in L<perlfunc/eof> for how to reset line numbers on each file.
1676 If you want to set @ARGV to your own list of files, go right ahead.
1677 This sets @ARGV to all plain text files if no @ARGV was given:
1679 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
1681 You can even set them to pipe commands. For example, this automatically
1682 filters compressed arguments through B<gzip>:
1684 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
1686 If you want to pass switches into your script, you can use one of the
1687 Getopts modules or put a loop on the front like this:
1689 while ($_ = $ARGV[0], /^-/) {
1692 if (/^-D(.*)/) { $debug = $1 }
1693 if (/^-v/) { $verbose++ }
1694 # ... # other switches
1698 # ... # code for each line
1701 The <> symbol will return C<undef> for end-of-file only once.
1702 If you call it again after this, it will assume you are processing another
1703 @ARGV list, and if you haven't set @ARGV, will read input from STDIN.
1705 If what the angle brackets contain is a simple scalar variable (e.g.,
1706 <$foo>), then that variable contains the name of the
1707 filehandle to input from, or its typeglob, or a reference to the
1713 If what's within the angle brackets is neither a filehandle nor a simple
1714 scalar variable containing a filehandle name, typeglob, or typeglob
1715 reference, it is interpreted as a filename pattern to be globbed, and
1716 either a list of filenames or the next filename in the list is returned,
1717 depending on context. This distinction is determined on syntactic
1718 grounds alone. That means C<< <$x> >> is always a readline() from
1719 an indirect handle, but C<< <$hash{key}> >> is always a glob().
1720 That's because $x is a simple scalar variable, but C<$hash{key}> is
1721 not--it's a hash element.
1723 One level of double-quote interpretation is done first, but you can't
1724 say C<< <$foo> >> because that's an indirect filehandle as explained
1725 in the previous paragraph. (In older versions of Perl, programmers
1726 would insert curly brackets to force interpretation as a filename glob:
1727 C<< <${foo}> >>. These days, it's considered cleaner to call the
1728 internal function directly as C<glob($foo)>, which is probably the right
1729 way to have done it in the first place.) For example:
1735 is roughly equivalent to:
1737 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
1743 except that the globbing is actually done internally using the standard
1744 C<File::Glob> extension. Of course, the shortest way to do the above is:
1748 A (file)glob evaluates its (embedded) argument only when it is
1749 starting a new list. All values must be read before it will start
1750 over. In list context, this isn't important because you automatically
1751 get them all anyway. However, in scalar context the operator returns
1752 the next value each time it's called, or C<undef> when the list has
1753 run out. As with filehandle reads, an automatic C<defined> is
1754 generated when the glob occurs in the test part of a C<while>,
1755 because legal glob returns (e.g. a file called F<0>) would otherwise
1756 terminate the loop. Again, C<undef> is returned only once. So if
1757 you're expecting a single value from a glob, it is much better to
1760 ($file) = <blurch*>;
1766 because the latter will alternate between returning a filename and
1769 If you're trying to do variable interpolation, it's definitely better
1770 to use the glob() function, because the older notation can cause people
1771 to become confused with the indirect filehandle notation.
1773 @files = glob("$dir/*.[ch]");
1774 @files = glob($files[$i]);
1776 =head2 Constant Folding
1778 Like C, Perl does a certain amount of expression evaluation at
1779 compile time whenever it determines that all arguments to an
1780 operator are static and have no side effects. In particular, string
1781 concatenation happens at compile time between literals that don't do
1782 variable substitution. Backslash interpolation also happens at
1783 compile time. You can say
1785 'Now is the time for all' . "\n" .
1786 'good men to come to.'
1788 and this all reduces to one string internally. Likewise, if
1791 foreach $file (@filenames) {
1792 if (-s $file > 5 + 100 * 2**16) { }
1795 the compiler will precompute the number which that expression
1796 represents so that the interpreter won't have to.
1798 =head2 Bitwise String Operators
1800 Bitstrings of any size may be manipulated by the bitwise operators
1803 If the operands to a binary bitwise op are strings of different
1804 sizes, B<|> and B<^> ops act as though the shorter operand had
1805 additional zero bits on the right, while the B<&> op acts as though
1806 the longer operand were truncated to the length of the shorter.
1807 The granularity for such extension or truncation is one or more
1810 # ASCII-based examples
1811 print "j p \n" ^ " a h"; # prints "JAPH\n"
1812 print "JA" | " ph\n"; # prints "japh\n"
1813 print "japh\nJunk" & '_____'; # prints "JAPH\n";
1814 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
1816 If you are intending to manipulate bitstrings, be certain that
1817 you're supplying bitstrings: If an operand is a number, that will imply
1818 a B<numeric> bitwise operation. You may explicitly show which type of
1819 operation you intend by using C<""> or C<0+>, as in the examples below.
1821 $foo = 150 | 105 ; # yields 255 (0x96 | 0x69 is 0xFF)
1822 $foo = '150' | 105 ; # yields 255
1823 $foo = 150 | '105'; # yields 255
1824 $foo = '150' | '105'; # yields string '155' (under ASCII)
1826 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
1827 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
1829 See L<perlfunc/vec> for information on how to manipulate individual bits
1832 =head2 Integer Arithmetic
1834 By default, Perl assumes that it must do most of its arithmetic in
1835 floating point. But by saying
1839 you may tell the compiler that it's okay to use integer operations
1840 (if it feels like it) from here to the end of the enclosing BLOCK.
1841 An inner BLOCK may countermand this by saying
1845 which lasts until the end of that BLOCK. Note that this doesn't
1846 mean everything is only an integer, merely that Perl may use integer
1847 operations if it is so inclined. For example, even under C<use
1848 integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731>
1851 Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
1852 and ">>") always produce integral results. (But see also
1853 L<Bitwise String Operators>.) However, C<use integer> still has meaning for
1854 them. By default, their results are interpreted as unsigned integers, but
1855 if C<use integer> is in effect, their results are interpreted
1856 as signed integers. For example, C<~0> usually evaluates to a large
1857 integral value. However, C<use integer; ~0> is C<-1> on twos-complement
1860 =head2 Floating-point Arithmetic
1862 While C<use integer> provides integer-only arithmetic, there is no
1863 analogous mechanism to provide automatic rounding or truncation to a
1864 certain number of decimal places. For rounding to a certain number
1865 of digits, sprintf() or printf() is usually the easiest route.
1868 Floating-point numbers are only approximations to what a mathematician
1869 would call real numbers. There are infinitely more reals than floats,
1870 so some corners must be cut. For example:
1872 printf "%.20g\n", 123456789123456789;
1873 # produces 123456789123456784
1875 Testing for exact equality of floating-point equality or inequality is
1876 not a good idea. Here's a (relatively expensive) work-around to compare
1877 whether two floating-point numbers are equal to a particular number of
1878 decimal places. See Knuth, volume II, for a more robust treatment of
1882 my ($X, $Y, $POINTS) = @_;
1884 $tX = sprintf("%.${POINTS}g", $X);
1885 $tY = sprintf("%.${POINTS}g", $Y);
1889 The POSIX module (part of the standard perl distribution) implements
1890 ceil(), floor(), and other mathematical and trigonometric functions.
1891 The Math::Complex module (part of the standard perl distribution)
1892 defines mathematical functions that work on both the reals and the
1893 imaginary numbers. Math::Complex not as efficient as POSIX, but
1894 POSIX can't work with complex numbers.
1896 Rounding in financial applications can have serious implications, and
1897 the rounding method used should be specified precisely. In these
1898 cases, it probably pays not to trust whichever system rounding is
1899 being used by Perl, but to instead implement the rounding function you
1902 =head2 Bigger Numbers
1904 The standard Math::BigInt and Math::BigFloat modules provide
1905 variable-precision arithmetic and overloaded operators, although
1906 they're currently pretty slow. At the cost of some space and
1907 considerable speed, they avoid the normal pitfalls associated with
1908 limited-precision representations.
1911 $x = Math::BigInt->new('123456789123456789');
1914 # prints +15241578780673678515622620750190521
1916 There are several modules that let you calculate with (bound only by
1917 memory and cpu-time) unlimited or fixed precision. There are also
1918 some non-standard modules that provide faster implementations via
1919 external C libraries.
1921 Here is a short, but incomplete summary:
1923 Math::Fraction big, unlimited fractions like 9973 / 12967
1924 Math::String treat string sequences like numbers
1925 Math::FixedPrecision calculate with a fixed precision
1926 Math::Currency for currency calculations
1927 Bit::Vector manipulate bit vectors fast (uses C)
1928 Math::BigIntFast Bit::Vector wrapper for big numbers
1929 Math::Pari provides access to the Pari C library
1930 Math::BigInteger uses an external C library
1931 Math::Cephes uses external Cephes C library (no big numbers)
1932 Math::Cephes::Fraction fractions via the Cephes library
1933 Math::GMP another one using an external C library