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 Note that both "<<" and ">>" in Perl are implemented directly using
237 "<<" and ">>" in C. If C<use integer> (see L<Integer Arithmetic>) is
238 in force then signed C integers are used, else unsigned C integers are
239 used. Either way, the implementation isn't going to generate results
240 larger than the size of the integer type Perl was built with (32 bits
243 The result of overflowing the range of the integers is undefined
244 because it is undefined also in C. In other words, using 32-bit
245 integers, C<< 1 << 32 >> is undefined. Shifting by a negative number
246 of bits is also undefined.
248 =head2 Named Unary Operators
250 The various named unary operators are treated as functions with one
251 argument, with optional parentheses. These include the filetest
252 operators, like C<-f>, C<-M>, etc. See L<perlfunc>.
254 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
255 is followed by a left parenthesis as the next token, the operator and
256 arguments within parentheses are taken to be of highest precedence,
257 just like a normal function call. For example,
258 because named unary operators are higher precedence than ||:
260 chdir $foo || die; # (chdir $foo) || die
261 chdir($foo) || die; # (chdir $foo) || die
262 chdir ($foo) || die; # (chdir $foo) || die
263 chdir +($foo) || die; # (chdir $foo) || die
265 but, because * is higher precedence than named operators:
267 chdir $foo * 20; # chdir ($foo * 20)
268 chdir($foo) * 20; # (chdir $foo) * 20
269 chdir ($foo) * 20; # (chdir $foo) * 20
270 chdir +($foo) * 20; # chdir ($foo * 20)
272 rand 10 * 20; # rand (10 * 20)
273 rand(10) * 20; # (rand 10) * 20
274 rand (10) * 20; # (rand 10) * 20
275 rand +(10) * 20; # rand (10 * 20)
277 See also L<"Terms and List Operators (Leftward)">.
279 =head2 Relational Operators
281 Binary "<" returns true if the left argument is numerically less than
284 Binary ">" returns true if the left argument is numerically greater
285 than the right argument.
287 Binary "<=" returns true if the left argument is numerically less than
288 or equal to the right argument.
290 Binary ">=" returns true if the left argument is numerically greater
291 than or equal to the right argument.
293 Binary "lt" returns true if the left argument is stringwise less than
296 Binary "gt" returns true if the left argument is stringwise greater
297 than the right argument.
299 Binary "le" returns true if the left argument is stringwise less than
300 or equal to the right argument.
302 Binary "ge" returns true if the left argument is stringwise greater
303 than or equal to the right argument.
305 =head2 Equality Operators
307 Binary "==" returns true if the left argument is numerically equal to
310 Binary "!=" returns true if the left argument is numerically not equal
311 to the right argument.
313 Binary "<=>" returns -1, 0, or 1 depending on whether the left
314 argument is numerically less than, equal to, or greater than the right
315 argument. If your platform supports NaNs (not-a-numbers) as numeric
316 values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
317 "<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
318 returns true, as does NaN != anything else. If your platform doesn't
319 support NaNs then NaN is just a string with numeric value 0.
321 perl -le '$a = NaN; print "No NaN support here" if $a == $a'
322 perl -le '$a = NaN; print "NaN support here" if $a != $a'
324 Binary "eq" returns true if the left argument is stringwise equal to
327 Binary "ne" returns true if the left argument is stringwise not equal
328 to the right argument.
330 Binary "cmp" returns -1, 0, or 1 depending on whether the left
331 argument is stringwise less than, equal to, or greater than the right
334 "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
335 by the current locale if C<use locale> is in effect. See L<perllocale>.
339 Binary "&" returns its operators ANDed together bit by bit.
340 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
342 =head2 Bitwise Or and Exclusive Or
344 Binary "|" returns its operators ORed together bit by bit.
345 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
347 Binary "^" returns its operators XORed together bit by bit.
348 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
350 =head2 C-style Logical And
352 Binary "&&" performs a short-circuit logical AND operation. That is,
353 if the left operand is false, the right operand is not even evaluated.
354 Scalar or list context propagates down to the right operand if it
357 =head2 C-style Logical Or
359 Binary "||" performs a short-circuit logical OR operation. That is,
360 if the left operand is true, the right operand is not even evaluated.
361 Scalar or list context propagates down to the right operand if it
364 The C<||> and C<&&> operators differ from C's in that, rather than returning
365 0 or 1, they return the last value evaluated. Thus, a reasonably portable
366 way to find out the home directory (assuming it's not "0") might be:
368 $home = $ENV{'HOME'} || $ENV{'LOGDIR'} ||
369 (getpwuid($<))[7] || die "You're homeless!\n";
371 In particular, this means that you shouldn't use this
372 for selecting between two aggregates for assignment:
374 @a = @b || @c; # this is wrong
375 @a = scalar(@b) || @c; # really meant this
376 @a = @b ? @b : @c; # this works fine, though
378 As more readable alternatives to C<&&> and C<||> when used for
379 control flow, Perl provides C<and> and C<or> operators (see below).
380 The short-circuit behavior is identical. The precedence of "and" and
381 "or" is much lower, however, so that you can safely use them after a
382 list operator without the need for parentheses:
384 unlink "alpha", "beta", "gamma"
385 or gripe(), next LINE;
387 With the C-style operators that would have been written like this:
389 unlink("alpha", "beta", "gamma")
390 || (gripe(), next LINE);
392 Using "or" for assignment is unlikely to do what you want; see below.
394 =head2 Range Operators
396 Binary ".." is the range operator, which is really two different
397 operators depending on the context. In list context, it returns an
398 array of values counting (up by ones) from the left value to the right
399 value. If the left value is greater than the right value then it
400 returns the empty array. The range operator is useful for writing
401 C<foreach (1..10)> loops and for doing slice operations on arrays. In
402 the current implementation, no temporary array is created when the
403 range operator is used as the expression in C<foreach> loops, but older
404 versions of Perl might burn a lot of memory when you write something
407 for (1 .. 1_000_000) {
411 In scalar context, ".." returns a boolean value. The operator is
412 bistable, like a flip-flop, and emulates the line-range (comma) operator
413 of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
414 own boolean state. It is false as long as its left operand is false.
415 Once the left operand is true, the range operator stays true until the
416 right operand is true, I<AFTER> which the range operator becomes false
417 again. It doesn't become false till the next time the range operator is
418 evaluated. It can test the right operand and become false on the same
419 evaluation it became true (as in B<awk>), but it still returns true once.
420 If you don't want it to test the right operand till the next
421 evaluation, as in B<sed>, just use three dots ("...") instead of
422 two. In all other regards, "..." behaves just like ".." does.
424 The right operand is not evaluated while the operator is in the
425 "false" state, and the left operand is not evaluated while the
426 operator is in the "true" state. The precedence is a little lower
427 than || and &&. The value returned is either the empty string for
428 false, or a sequence number (beginning with 1) for true. The
429 sequence number is reset for each range encountered. The final
430 sequence number in a range has the string "E0" appended to it, which
431 doesn't affect its numeric value, but gives you something to search
432 for if you want to exclude the endpoint. You can exclude the
433 beginning point by waiting for the sequence number to be greater
434 than 1. If either operand of scalar ".." is a constant expression,
435 that operand is implicitly compared to the C<$.> variable, the
436 current line number. Examples:
438 As a scalar operator:
440 if (101 .. 200) { print; } # print 2nd hundred lines
441 next line if (1 .. /^$/); # skip header lines
442 s/^/> / if (/^$/ .. eof()); # quote body
444 # parse mail messages
446 $in_header = 1 .. /^$/;
447 $in_body = /^$/ .. eof();
448 # do something based on those
450 close ARGV if eof; # reset $. each file
455 for (101 .. 200) { print; } # print $_ 100 times
456 @foo = @foo[0 .. $#foo]; # an expensive no-op
457 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
459 The range operator (in list context) makes use of the magical
460 auto-increment algorithm if the operands are strings. You
463 @alphabet = ('A' .. 'Z');
465 to get all normal letters of the alphabet, or
467 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
469 to get a hexadecimal digit, or
471 @z2 = ('01' .. '31'); print $z2[$mday];
473 to get dates with leading zeros. If the final value specified is not
474 in the sequence that the magical increment would produce, the sequence
475 goes until the next value would be longer than the final value
478 =head2 Conditional Operator
480 Ternary "?:" is the conditional operator, just as in C. It works much
481 like an if-then-else. If the argument before the ? is true, the
482 argument before the : is returned, otherwise the argument after the :
483 is returned. For example:
485 printf "I have %d dog%s.\n", $n,
486 ($n == 1) ? '' : "s";
488 Scalar or list context propagates downward into the 2nd
489 or 3rd argument, whichever is selected.
491 $a = $ok ? $b : $c; # get a scalar
492 @a = $ok ? @b : @c; # get an array
493 $a = $ok ? @b : @c; # oops, that's just a count!
495 The operator may be assigned to if both the 2nd and 3rd arguments are
496 legal lvalues (meaning that you can assign to them):
498 ($a_or_b ? $a : $b) = $c;
500 Because this operator produces an assignable result, using assignments
501 without parentheses will get you in trouble. For example, this:
503 $a % 2 ? $a += 10 : $a += 2
507 (($a % 2) ? ($a += 10) : $a) += 2
511 ($a % 2) ? ($a += 10) : ($a += 2)
513 That should probably be written more simply as:
515 $a += ($a % 2) ? 10 : 2;
517 =head2 Assignment Operators
519 "=" is the ordinary assignment operator.
521 Assignment operators work as in C. That is,
529 although without duplicating any side effects that dereferencing the lvalue
530 might trigger, such as from tie(). Other assignment operators work similarly.
531 The following are recognized:
538 Although these are grouped by family, they all have the precedence
541 Unlike in C, the scalar assignment operator produces a valid lvalue.
542 Modifying an assignment is equivalent to doing the assignment and
543 then modifying the variable that was assigned to. This is useful
544 for modifying a copy of something, like this:
546 ($tmp = $global) =~ tr [A-Z] [a-z];
557 Similarly, a list assignment in list context produces the list of
558 lvalues assigned to, and a list assignment in scalar context returns
559 the number of elements produced by the expression on the right hand
560 side of the assignment.
562 =head2 Comma Operator
564 Binary "," is the comma operator. In scalar context it evaluates
565 its left argument, throws that value away, then evaluates its right
566 argument and returns that value. This is just like C's comma operator.
568 In list context, it's just the list argument separator, and inserts
569 both its arguments into the list.
571 The => digraph is mostly just a synonym for the comma operator. It's useful for
572 documenting arguments that come in pairs. As of release 5.001, it also forces
573 any word to the left of it to be interpreted as a string.
575 =head2 List Operators (Rightward)
577 On the right side of a list operator, it has very low precedence,
578 such that it controls all comma-separated expressions found there.
579 The only operators with lower precedence are the logical operators
580 "and", "or", and "not", which may be used to evaluate calls to list
581 operators without the need for extra parentheses:
583 open HANDLE, "filename"
584 or die "Can't open: $!\n";
586 See also discussion of list operators in L<Terms and List Operators (Leftward)>.
590 Unary "not" returns the logical negation of the expression to its right.
591 It's the equivalent of "!" except for the very low precedence.
595 Binary "and" returns the logical conjunction of the two surrounding
596 expressions. It's equivalent to && except for the very low
597 precedence. This means that it short-circuits: i.e., the right
598 expression is evaluated only if the left expression is true.
600 =head2 Logical or and Exclusive Or
602 Binary "or" returns the logical disjunction of the two surrounding
603 expressions. It's equivalent to || except for the very low precedence.
604 This makes it useful for control flow
606 print FH $data or die "Can't write to FH: $!";
608 This means that it short-circuits: i.e., the right expression is evaluated
609 only if the left expression is false. Due to its precedence, you should
610 probably avoid using this for assignment, only for control flow.
612 $a = $b or $c; # bug: this is wrong
613 ($a = $b) or $c; # really means this
614 $a = $b || $c; # better written this way
616 However, when it's a list-context assignment and you're trying to use
617 "||" for control flow, you probably need "or" so that the assignment
618 takes higher precedence.
620 @info = stat($file) || die; # oops, scalar sense of stat!
621 @info = stat($file) or die; # better, now @info gets its due
623 Then again, you could always use parentheses.
625 Binary "xor" returns the exclusive-OR of the two surrounding expressions.
626 It cannot short circuit, of course.
628 =head2 C Operators Missing From Perl
630 Here is what C has that Perl doesn't:
636 Address-of operator. (But see the "\" operator for taking a reference.)
640 Dereference-address operator. (Perl's prefix dereferencing
641 operators are typed: $, @, %, and &.)
645 Type-casting operator.
649 =head2 Quote and Quote-like Operators
651 While we usually think of quotes as literal values, in Perl they
652 function as operators, providing various kinds of interpolating and
653 pattern matching capabilities. Perl provides customary quote characters
654 for these behaviors, but also provides a way for you to choose your
655 quote character for any of them. In the following table, a C<{}> represents
656 any pair of delimiters you choose.
658 Customary Generic Meaning Interpolates
663 // m{} Pattern match yes*
665 s{}{} Substitution yes*
666 tr{}{} Transliteration no (but see below)
668 * unless the delimiter is ''.
670 Non-bracketing delimiters use the same character fore and aft, but the four
671 sorts of brackets (round, angle, square, curly) will all nest, which means
680 Note, however, that this does not always work for quoting Perl code:
682 $s = q{ if($a eq "}") ... }; # WRONG
684 is a syntax error. The C<Text::Balanced> module (from CPAN, and
685 starting from Perl 5.8 part of the standard distribution) is able
688 There can be whitespace between the operator and the quoting
689 characters, except when C<#> is being used as the quoting character.
690 C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
691 operator C<q> followed by a comment. Its argument will be taken
692 from the next line. This allows you to write:
694 s {foo} # Replace foo
697 The following escape sequences are available in constructs that interpolate
698 and in transliterations.
705 \a alarm (bell) (BEL)
707 \033 octal char (ESC)
709 \x{263a} wide hex char (SMILEY)
710 \c[ control char (ESC)
713 The following escape sequences are available in constructs that interpolate
714 but not in transliterations.
716 \l lowercase next char
717 \u uppercase next char
720 \E end case modification
721 \Q quote non-word characters till \E
723 If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u>
724 and C<\U> is taken from the current locale. See L<perllocale>. For
725 documentation of C<\N{name}>, see L<charnames>.
727 All systems use the virtual C<"\n"> to represent a line terminator,
728 called a "newline". There is no such thing as an unvarying, physical
729 newline character. It is only an illusion that the operating system,
730 device drivers, C libraries, and Perl all conspire to preserve. Not all
731 systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
732 on a Mac, these are reversed, and on systems without line terminator,
733 printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
734 you mean a "newline" for your system, but use the literal ASCII when you
735 need an exact character. For example, most networking protocols expect
736 and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
737 and although they often accept just C<"\012">, they seldom tolerate just
738 C<"\015">. If you get in the habit of using C<"\n"> for networking,
739 you may be burned some day.
741 For constructs that do interpolate, variables beginning with "C<$>"
742 or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
743 C<$href->{key}[0]> are also interpolated, as are array and hash slices.
744 But method calls such as C<$obj->meth> are not.
746 Interpolating an array or slice interpolates the elements in order,
747 separated by the value of C<$">, so is equivalent to interpolating
748 C<join $", @array>. "Punctuation" arrays such as C<@+> are only
749 interpolated if the name is enclosed in braces C<@{+}>.
751 You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
752 An unescaped C<$> or C<@> interpolates the corresponding variable,
753 while escaping will cause the literal string C<\$> to be inserted.
754 You'll need to write something like C<m/\Quser\E\@\Qhost/>.
756 Patterns are subject to an additional level of interpretation as a
757 regular expression. This is done as a second pass, after variables are
758 interpolated, so that regular expressions may be incorporated into the
759 pattern from the variables. If this is not what you want, use C<\Q> to
760 interpolate a variable literally.
762 Apart from the behavior described above, Perl does not expand
763 multiple levels of interpolation. In particular, contrary to the
764 expectations of shell programmers, back-quotes do I<NOT> interpolate
765 within double quotes, nor do single quotes impede evaluation of
766 variables when used within double quotes.
768 =head2 Regexp Quote-Like Operators
770 Here are the quote-like operators that apply to pattern
771 matching and related activities.
777 This is just like the C</pattern/> search, except that it matches only
778 once between calls to the reset() operator. This is a useful
779 optimization when you want to see only the first occurrence of
780 something in each file of a set of files, for instance. Only C<??>
781 patterns local to the current package are reset.
785 # blank line between header and body
788 reset if eof; # clear ?? status for next file
791 This usage is vaguely deprecated, which means it just might possibly
792 be removed in some distant future version of Perl, perhaps somewhere
793 around the year 2168.
795 =item m/PATTERN/cgimosx
797 =item /PATTERN/cgimosx
799 Searches a string for a pattern match, and in scalar context returns
800 true if it succeeds, false if it fails. If no string is specified
801 via the C<=~> or C<!~> operator, the $_ string is searched. (The
802 string specified with C<=~> need not be an lvalue--it may be the
803 result of an expression evaluation, but remember the C<=~> binds
804 rather tightly.) See also L<perlre>. See L<perllocale> for
805 discussion of additional considerations that apply when C<use locale>
810 c Do not reset search position on a failed match when /g is in effect.
811 g Match globally, i.e., find all occurrences.
812 i Do case-insensitive pattern matching.
813 m Treat string as multiple lines.
814 o Compile pattern only once.
815 s Treat string as single line.
816 x Use extended regular expressions.
818 If "/" is the delimiter then the initial C<m> is optional. With the C<m>
819 you can use any pair of non-alphanumeric, non-whitespace characters
820 as delimiters. This is particularly useful for matching path names
821 that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
822 the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
823 If "'" is the delimiter, no interpolation is performed on the PATTERN.
825 PATTERN may contain variables, which will be interpolated (and the
826 pattern recompiled) every time the pattern search is evaluated, except
827 for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
828 C<$|> are not interpolated because they look like end-of-string tests.)
829 If you want such a pattern to be compiled only once, add a C</o> after
830 the trailing delimiter. This avoids expensive run-time recompilations,
831 and is useful when the value you are interpolating won't change over
832 the life of the script. However, mentioning C</o> constitutes a promise
833 that you won't change the variables in the pattern. If you change them,
834 Perl won't even notice. See also L<"qr/STRING/imosx">.
836 If the PATTERN evaluates to the empty string, the last
837 I<successfully> matched regular expression is used instead.
839 If the C</g> option is not used, C<m//> in list context returns a
840 list consisting of the subexpressions matched by the parentheses in the
841 pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are
842 also set, and that this differs from Perl 4's behavior.) When there are
843 no parentheses in the pattern, the return value is the list C<(1)> for
844 success. With or without parentheses, an empty list is returned upon
849 open(TTY, '/dev/tty');
850 <TTY> =~ /^y/i && foo(); # do foo if desired
852 if (/Version: *([0-9.]*)/) { $version = $1; }
854 next if m#^/usr/spool/uucp#;
859 print if /$arg/o; # compile only once
862 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
864 This last example splits $foo into the first two words and the
865 remainder of the line, and assigns those three fields to $F1, $F2, and
866 $Etc. The conditional is true if any variables were assigned, i.e., if
869 The C</g> modifier specifies global pattern matching--that is,
870 matching as many times as possible within the string. How it behaves
871 depends on the context. In list context, it returns a list of the
872 substrings matched by any capturing parentheses in the regular
873 expression. If there are no parentheses, it returns a list of all
874 the matched strings, as if there were parentheses around the whole
877 In scalar context, each execution of C<m//g> finds the next match,
878 returning true if it matches, and false if there is no further match.
879 The position after the last match can be read or set using the pos()
880 function; see L<perlfunc/pos>. A failed match normally resets the
881 search position to the beginning of the string, but you can avoid that
882 by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
883 string also resets the search position.
885 You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
886 zero-width assertion that matches the exact position where the previous
887 C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion
888 still anchors at pos(), but the match is of course only attempted once.
889 Using C<\G> without C</g> on a target string that has not previously had a
890 C</g> match applied to it is the same as using the C<\A> assertion to match
891 the beginning of the string.
896 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
900 while (defined($paragraph = <>)) {
901 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
905 print "$sentences\n";
907 # using m//gc with \G
911 print $1 while /(o)/gc; print "', pos=", pos, "\n";
913 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
915 print $1 while /(p)/gc; print "', pos=", pos, "\n";
917 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
919 The last example should print:
929 Notice that the final match matched C<q> instead of C<p>, which a match
930 without the C<\G> anchor would have done. Also note that the final match
931 did not update C<pos> -- C<pos> is only updated on a C</g> match. If the
932 final match did indeed match C<p>, it's a good bet that you're running an
933 older (pre-5.6.0) Perl.
935 A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
936 combine several regexps like this to process a string part-by-part,
937 doing different actions depending on which regexp matched. Each
938 regexp tries to match where the previous one leaves off.
941 $url = new URI::URL "http://www/"; die if $url eq "xXx";
945 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
946 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
947 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
948 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
949 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
950 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
951 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
952 print ". That's all!\n";
955 Here is the output (split into several lines):
957 line-noise lowercase line-noise lowercase UPPERCASE line-noise
958 UPPERCASE line-noise lowercase line-noise lowercase line-noise
959 lowercase lowercase line-noise lowercase lowercase line-noise
960 MiXeD line-noise. That's all!
966 A single-quoted, literal string. A backslash represents a backslash
967 unless followed by the delimiter or another backslash, in which case
968 the delimiter or backslash is interpolated.
970 $foo = q!I said, "You said, 'She said it.'"!;
971 $bar = q('This is it.');
972 $baz = '\n'; # a two-character string
978 A double-quoted, interpolated string.
981 (*** The previous line contains the naughty word "$1".\n)
982 if /\b(tcl|java|python)\b/i; # :-)
983 $baz = "\n"; # a one-character string
985 =item qr/STRING/imosx
987 This operator quotes (and possibly compiles) its I<STRING> as a regular
988 expression. I<STRING> is interpolated the same way as I<PATTERN>
989 in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
990 is done. Returns a Perl value which may be used instead of the
991 corresponding C</STRING/imosx> expression.
995 $rex = qr/my.STRING/is;
1002 The result may be used as a subpattern in a match:
1005 $string =~ /foo${re}bar/; # can be interpolated in other patterns
1006 $string =~ $re; # or used standalone
1007 $string =~ /$re/; # or this way
1009 Since Perl may compile the pattern at the moment of execution of qr()
1010 operator, using qr() may have speed advantages in some situations,
1011 notably if the result of qr() is used standalone:
1014 my $patterns = shift;
1015 my @compiled = map qr/$_/i, @$patterns;
1018 foreach my $pat (@compiled) {
1019 $success = 1, last if /$pat/;
1025 Precompilation of the pattern into an internal representation at
1026 the moment of qr() avoids a need to recompile the pattern every
1027 time a match C</$pat/> is attempted. (Perl has many other internal
1028 optimizations, but none would be triggered in the above example if
1029 we did not use qr() operator.)
1033 i Do case-insensitive pattern matching.
1034 m Treat string as multiple lines.
1035 o Compile pattern only once.
1036 s Treat string as single line.
1037 x Use extended regular expressions.
1039 See L<perlre> for additional information on valid syntax for STRING, and
1040 for a detailed look at the semantics of regular expressions.
1046 A string which is (possibly) interpolated and then executed as a
1047 system command with C</bin/sh> or its equivalent. Shell wildcards,
1048 pipes, and redirections will be honored. The collected standard
1049 output of the command is returned; standard error is unaffected. In
1050 scalar context, it comes back as a single (potentially multi-line)
1051 string, or undef if the command failed. In list context, returns a
1052 list of lines (however you've defined lines with $/ or
1053 $INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
1055 Because backticks do not affect standard error, use shell file descriptor
1056 syntax (assuming the shell supports this) if you care to address this.
1057 To capture a command's STDERR and STDOUT together:
1059 $output = `cmd 2>&1`;
1061 To capture a command's STDOUT but discard its STDERR:
1063 $output = `cmd 2>/dev/null`;
1065 To capture a command's STDERR but discard its STDOUT (ordering is
1068 $output = `cmd 2>&1 1>/dev/null`;
1070 To exchange a command's STDOUT and STDERR in order to capture the STDERR
1071 but leave its STDOUT to come out the old STDERR:
1073 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
1075 To read both a command's STDOUT and its STDERR separately, it's easiest
1076 and safest to redirect them separately to files, and then read from those
1077 files when the program is done:
1079 system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr");
1081 Using single-quote as a delimiter protects the command from Perl's
1082 double-quote interpolation, passing it on to the shell instead:
1084 $perl_info = qx(ps $$); # that's Perl's $$
1085 $shell_info = qx'ps $$'; # that's the new shell's $$
1087 How that string gets evaluated is entirely subject to the command
1088 interpreter on your system. On most platforms, you will have to protect
1089 shell metacharacters if you want them treated literally. This is in
1090 practice difficult to do, as it's unclear how to escape which characters.
1091 See L<perlsec> for a clean and safe example of a manual fork() and exec()
1092 to emulate backticks safely.
1094 On some platforms (notably DOS-like ones), the shell may not be
1095 capable of dealing with multiline commands, so putting newlines in
1096 the string may not get you what you want. You may be able to evaluate
1097 multiple commands in a single line by separating them with the command
1098 separator character, if your shell supports that (e.g. C<;> on many Unix
1099 shells; C<&> on the Windows NT C<cmd> shell).
1101 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1102 output before starting the child process, but this may not be supported
1103 on some platforms (see L<perlport>). To be safe, you may need to set
1104 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1105 C<IO::Handle> on any open handles.
1107 Beware that some command shells may place restrictions on the length
1108 of the command line. You must ensure your strings don't exceed this
1109 limit after any necessary interpolations. See the platform-specific
1110 release notes for more details about your particular environment.
1112 Using this operator can lead to programs that are difficult to port,
1113 because the shell commands called vary between systems, and may in
1114 fact not be present at all. As one example, the C<type> command under
1115 the POSIX shell is very different from the C<type> command under DOS.
1116 That doesn't mean you should go out of your way to avoid backticks
1117 when they're the right way to get something done. Perl was made to be
1118 a glue language, and one of the things it glues together is commands.
1119 Just understand what you're getting yourself into.
1121 See L<"I/O Operators"> for more discussion.
1125 Evaluates to a list of the words extracted out of STRING, using embedded
1126 whitespace as the word delimiters. It can be understood as being roughly
1129 split(' ', q/STRING/);
1131 the difference being that it generates a real list at compile time. So
1136 is semantically equivalent to the list:
1140 Some frequently seen examples:
1142 use POSIX qw( setlocale localeconv )
1143 @EXPORT = qw( foo bar baz );
1145 A common mistake is to try to separate the words with comma or to
1146 put comments into a multi-line C<qw>-string. For this reason, the
1147 C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
1148 produces warnings if the STRING contains the "," or the "#" character.
1150 =item s/PATTERN/REPLACEMENT/egimosx
1152 Searches a string for a pattern, and if found, replaces that pattern
1153 with the replacement text and returns the number of substitutions
1154 made. Otherwise it returns false (specifically, the empty string).
1156 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
1157 variable is searched and modified. (The string specified with C<=~> must
1158 be scalar variable, an array element, a hash element, or an assignment
1159 to one of those, i.e., an lvalue.)
1161 If the delimiter chosen is a single quote, no interpolation is
1162 done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1163 PATTERN contains a $ that looks like a variable rather than an
1164 end-of-string test, the variable will be interpolated into the pattern
1165 at run-time. If you want the pattern compiled only once the first time
1166 the variable is interpolated, use the C</o> option. If the pattern
1167 evaluates to the empty string, the last successfully executed regular
1168 expression is used instead. See L<perlre> for further explanation on these.
1169 See L<perllocale> for discussion of additional considerations that apply
1170 when C<use locale> is in effect.
1174 e Evaluate the right side as an expression.
1175 g Replace globally, i.e., all occurrences.
1176 i Do case-insensitive pattern matching.
1177 m Treat string as multiple lines.
1178 o Compile pattern only once.
1179 s Treat string as single line.
1180 x Use extended regular expressions.
1182 Any non-alphanumeric, non-whitespace delimiter may replace the
1183 slashes. If single quotes are used, no interpretation is done on the
1184 replacement string (the C</e> modifier overrides this, however). Unlike
1185 Perl 4, Perl 5 treats backticks as normal delimiters; the replacement
1186 text is not evaluated as a command. If the
1187 PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
1188 pair of quotes, which may or may not be bracketing quotes, e.g.,
1189 C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
1190 replacement portion to be treated as a full-fledged Perl expression
1191 and evaluated right then and there. It is, however, syntax checked at
1192 compile-time. A second C<e> modifier will cause the replacement portion
1193 to be C<eval>ed before being run as a Perl expression.
1197 s/\bgreen\b/mauve/g; # don't change wintergreen
1199 $path =~ s|/usr/bin|/usr/local/bin|;
1201 s/Login: $foo/Login: $bar/; # run-time pattern
1203 ($foo = $bar) =~ s/this/that/; # copy first, then change
1205 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
1208 s/\d+/$&*2/e; # yields 'abc246xyz'
1209 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
1210 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1212 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
1213 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
1214 s/^=(\w+)/&pod($1)/ge; # use function call
1216 # expand variables in $_, but dynamics only, using
1217 # symbolic dereferencing
1220 # Add one to the value of any numbers in the string
1223 # This will expand any embedded scalar variable
1224 # (including lexicals) in $_ : First $1 is interpolated
1225 # to the variable name, and then evaluated
1228 # Delete (most) C comments.
1230 /\* # Match the opening delimiter.
1231 .*? # Match a minimal number of characters.
1232 \*/ # Match the closing delimiter.
1235 s/^\s*(.*?)\s*$/$1/; # trim white space in $_, expensively
1237 for ($variable) { # trim white space in $variable, cheap
1242 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
1244 Note the use of $ instead of \ in the last example. Unlike
1245 B<sed>, we use the \<I<digit>> form in only the left hand side.
1246 Anywhere else it's $<I<digit>>.
1248 Occasionally, you can't use just a C</g> to get all the changes
1249 to occur that you might want. Here are two common cases:
1251 # put commas in the right places in an integer
1252 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
1254 # expand tabs to 8-column spacing
1255 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
1257 =item tr/SEARCHLIST/REPLACEMENTLIST/cds
1259 =item y/SEARCHLIST/REPLACEMENTLIST/cds
1261 Transliterates all occurrences of the characters found in the search list
1262 with the corresponding character in the replacement list. It returns
1263 the number of characters replaced or deleted. If no string is
1264 specified via the =~ or !~ operator, the $_ string is transliterated. (The
1265 string specified with =~ must be a scalar variable, an array element, a
1266 hash element, or an assignment to one of those, i.e., an lvalue.)
1268 A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
1269 does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
1270 For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1271 SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1272 its own pair of quotes, which may or may not be bracketing quotes,
1273 e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
1275 Note that C<tr> does B<not> do regular expression character classes
1276 such as C<\d> or C<[:lower:]>. The <tr> operator is not equivalent to
1277 the tr(1) utility. If you want to map strings between lower/upper
1278 cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider
1279 using the C<s> operator if you need regular expressions.
1281 Note also that the whole range idea is rather unportable between
1282 character sets--and even within character sets they may cause results
1283 you probably didn't expect. A sound principle is to use only ranges
1284 that begin from and end at either alphabets of equal case (a-e, A-E),
1285 or digits (0-4). Anything else is unsafe. If in doubt, spell out the
1286 character sets in full.
1290 c Complement the SEARCHLIST.
1291 d Delete found but unreplaced characters.
1292 s Squash duplicate replaced characters.
1294 If the C</c> modifier is specified, the SEARCHLIST character set
1295 is complemented. If the C</d> modifier is specified, any characters
1296 specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
1297 (Note that this is slightly more flexible than the behavior of some
1298 B<tr> programs, which delete anything they find in the SEARCHLIST,
1299 period.) If the C</s> modifier is specified, sequences of characters
1300 that were transliterated to the same character are squashed down
1301 to a single instance of the character.
1303 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1304 exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1305 than the SEARCHLIST, the final character is replicated till it is long
1306 enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
1307 This latter is useful for counting characters in a class or for
1308 squashing character sequences in a class.
1312 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1314 $cnt = tr/*/*/; # count the stars in $_
1316 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1318 $cnt = tr/0-9//; # count the digits in $_
1320 tr/a-zA-Z//s; # bookkeeper -> bokeper
1322 ($HOST = $host) =~ tr/a-z/A-Z/;
1324 tr/a-zA-Z/ /cs; # change non-alphas to single space
1327 [\000-\177]; # delete 8th bit
1329 If multiple transliterations are given for a character, only the
1334 will transliterate any A to X.
1336 Because the transliteration table is built at compile time, neither
1337 the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
1338 interpolation. That means that if you want to use variables, you
1341 eval "tr/$oldlist/$newlist/";
1344 eval "tr/$oldlist/$newlist/, 1" or die $@;
1348 =head2 Gory details of parsing quoted constructs
1350 When presented with something that might have several different
1351 interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
1352 principle to pick the most probable interpretation. This strategy
1353 is so successful that Perl programmers often do not suspect the
1354 ambivalence of what they write. But from time to time, Perl's
1355 notions differ substantially from what the author honestly meant.
1357 This section hopes to clarify how Perl handles quoted constructs.
1358 Although the most common reason to learn this is to unravel labyrinthine
1359 regular expressions, because the initial steps of parsing are the
1360 same for all quoting operators, they are all discussed together.
1362 The most important Perl parsing rule is the first one discussed
1363 below: when processing a quoted construct, Perl first finds the end
1364 of that construct, then interprets its contents. If you understand
1365 this rule, you may skip the rest of this section on the first
1366 reading. The other rules are likely to contradict the user's
1367 expectations much less frequently than this first one.
1369 Some passes discussed below are performed concurrently, but because
1370 their results are the same, we consider them individually. For different
1371 quoting constructs, Perl performs different numbers of passes, from
1372 one to five, but these passes are always performed in the same order.
1376 =item Finding the end
1378 The first pass is finding the end of the quoted construct, whether
1379 it be a multicharacter delimiter C<"\nEOF\n"> in the C<<<EOF>
1380 construct, a C</> that terminates a C<qq//> construct, a C<]> which
1381 terminates C<qq[]> construct, or a C<< > >> which terminates a
1382 fileglob started with C<< < >>.
1384 When searching for single-character non-pairing delimiters, such
1385 as C</>, combinations of C<\\> and C<\/> are skipped. However,
1386 when searching for single-character pairing delimiter like C<[>,
1387 combinations of C<\\>, C<\]>, and C<\[> are all skipped, and nested
1388 C<[>, C<]> are skipped as well. When searching for multicharacter
1389 delimiters, nothing is skipped.
1391 For constructs with three-part delimiters (C<s///>, C<y///>, and
1392 C<tr///>), the search is repeated once more.
1394 During this search no attention is paid to the semantics of the construct.
1397 "$hash{"$foo/$bar"}"
1402 bar # NOT a comment, this slash / terminated m//!
1405 do not form legal quoted expressions. The quoted part ends on the
1406 first C<"> and C</>, and the rest happens to be a syntax error.
1407 Because the slash that terminated C<m//> was followed by a C<SPACE>,
1408 the example above is not C<m//x>, but rather C<m//> with no C</x>
1409 modifier. So the embedded C<#> is interpreted as a literal C<#>.
1411 =item Removal of backslashes before delimiters
1413 During the second pass, text between the starting and ending
1414 delimiters is copied to a safe location, and the C<\> is removed
1415 from combinations consisting of C<\> and delimiter--or delimiters,
1416 meaning both starting and ending delimiters will should these differ.
1417 This removal does not happen for multi-character delimiters.
1418 Note that the combination C<\\> is left intact, just as it was.
1420 Starting from this step no information about the delimiters is
1425 The next step is interpolation in the text obtained, which is now
1426 delimiter-independent. There are four different cases.
1430 =item C<<<'EOF'>, C<m''>, C<s'''>, C<tr///>, C<y///>
1432 No interpolation is performed.
1436 The only interpolation is removal of C<\> from pairs C<\\>.
1438 =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>
1440 C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
1441 converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
1442 is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
1443 The other combinations are replaced with appropriate expansions.
1445 Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
1446 is interpolated in the usual way. Something like C<"\Q\\E"> has
1447 no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
1448 result is the same as for C<"\\\\E">. As a general rule, backslashes
1449 between C<\Q> and C<\E> may lead to counterintuitive results. So,
1450 C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
1451 as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
1456 may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
1458 Interpolated scalars and arrays are converted internally to the C<join> and
1459 C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
1461 $foo . " XXX '" . (join $", @arr) . "'";
1463 All operations above are performed simultaneously, left to right.
1465 Because the result of C<"\Q STRING \E"> has all metacharacters
1466 quoted, there is no way to insert a literal C<$> or C<@> inside a
1467 C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
1468 C<"\\\$">; if not, it is interpreted as the start of an interpolated
1471 Note also that the interpolation code needs to make a decision on
1472 where the interpolated scalar ends. For instance, whether
1473 C<< "a $b -> {c}" >> really means:
1475 "a " . $b . " -> {c}";
1481 Most of the time, the longest possible text that does not include
1482 spaces between components and which contains matching braces or
1483 brackets. because the outcome may be determined by voting based
1484 on heuristic estimators, the result is not strictly predictable.
1485 Fortunately, it's usually correct for ambiguous cases.
1487 =item C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
1489 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
1490 happens (almost) as with C<qq//> constructs, but the substitution
1491 of C<\> followed by RE-special chars (including C<\>) is not
1492 performed. Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
1493 a C<#>-comment in a C<//x>-regular expression, no processing is
1494 performed whatsoever. This is the first step at which the presence
1495 of the C<//x> modifier is relevant.
1497 Interpolation has several quirks: C<$|>, C<$(>, and C<$)> are not
1498 interpolated, and constructs C<$var[SOMETHING]> are voted (by several
1499 different estimators) to be either an array element or C<$var>
1500 followed by an RE alternative. This is where the notation
1501 C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
1502 array element C<-9>, not as a regular expression from the variable
1503 C<$arr> followed by a digit, which would be the interpretation of
1504 C</$arr[0-9]/>. Since voting among different estimators may occur,
1505 the result is not predictable.
1507 It is at this step that C<\1> is begrudgingly converted to C<$1> in
1508 the replacement text of C<s///> to correct the incorrigible
1509 I<sed> hackers who haven't picked up the saner idiom yet. A warning
1510 is emitted if the C<use warnings> pragma or the B<-w> command-line flag
1511 (that is, the C<$^W> variable) was set.
1513 The lack of processing of C<\\> creates specific restrictions on
1514 the post-processed text. If the delimiter is C</>, one cannot get
1515 the combination C<\/> into the result of this step. C</> will
1516 finish the regular expression, C<\/> will be stripped to C</> on
1517 the previous step, and C<\\/> will be left as is. Because C</> is
1518 equivalent to C<\/> inside a regular expression, this does not
1519 matter unless the delimiter happens to be character special to the
1520 RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
1521 alphanumeric char, as in:
1525 In the RE above, which is intentionally obfuscated for illustration, the
1526 delimiter is C<m>, the modifier is C<mx>, and after backslash-removal the
1527 RE is the same as for C<m/ ^ a s* b /mx>). There's more than one
1528 reason you're encouraged to restrict your delimiters to non-alphanumeric,
1529 non-whitespace choices.
1533 This step is the last one for all constructs except regular expressions,
1534 which are processed further.
1536 =item Interpolation of regular expressions
1538 Previous steps were performed during the compilation of Perl code,
1539 but this one happens at run time--although it may be optimized to
1540 be calculated at compile time if appropriate. After preprocessing
1541 described above, and possibly after evaluation if catenation,
1542 joining, casing translation, or metaquoting are involved, the
1543 resulting I<string> is passed to the RE engine for compilation.
1545 Whatever happens in the RE engine might be better discussed in L<perlre>,
1546 but for the sake of continuity, we shall do so here.
1548 This is another step where the presence of the C<//x> modifier is
1549 relevant. The RE engine scans the string from left to right and
1550 converts it to a finite automaton.
1552 Backslashed characters are either replaced with corresponding
1553 literal strings (as with C<\{>), or else they generate special nodes
1554 in the finite automaton (as with C<\b>). Characters special to the
1555 RE engine (such as C<|>) generate corresponding nodes or groups of
1556 nodes. C<(?#...)> comments are ignored. All the rest is either
1557 converted to literal strings to match, or else is ignored (as is
1558 whitespace and C<#>-style comments if C<//x> is present).
1560 Parsing of the bracketed character class construct, C<[...]>, is
1561 rather different than the rule used for the rest of the pattern.
1562 The terminator of this construct is found using the same rules as
1563 for finding the terminator of a C<{}>-delimited construct, the only
1564 exception being that C<]> immediately following C<[> is treated as
1565 though preceded by a backslash. Similarly, the terminator of
1566 C<(?{...})> is found using the same rules as for finding the
1567 terminator of a C<{}>-delimited construct.
1569 It is possible to inspect both the string given to RE engine and the
1570 resulting finite automaton. See the arguments C<debug>/C<debugcolor>
1571 in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
1572 switch documented in L<perlrun/"Command Switches">.
1574 =item Optimization of regular expressions
1576 This step is listed for completeness only. Since it does not change
1577 semantics, details of this step are not documented and are subject
1578 to change without notice. This step is performed over the finite
1579 automaton that was generated during the previous pass.
1581 It is at this stage that C<split()> silently optimizes C</^/> to
1586 =head2 I/O Operators
1588 There are several I/O operators you should know about.
1590 A string enclosed by backticks (grave accents) first undergoes
1591 double-quote interpolation. It is then interpreted as an external
1592 command, and the output of that command is the value of the
1593 backtick string, like in a shell. In scalar context, a single string
1594 consisting of all output is returned. In list context, a list of
1595 values is returned, one per line of output. (You can set C<$/> to use
1596 a different line terminator.) The command is executed each time the
1597 pseudo-literal is evaluated. The status value of the command is
1598 returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
1599 Unlike in B<csh>, no translation is done on the return data--newlines
1600 remain newlines. Unlike in any of the shells, single quotes do not
1601 hide variable names in the command from interpretation. To pass a
1602 literal dollar-sign through to the shell you need to hide it with a
1603 backslash. The generalized form of backticks is C<qx//>. (Because
1604 backticks always undergo shell expansion as well, see L<perlsec> for
1607 In scalar context, evaluating a filehandle in angle brackets yields
1608 the next line from that file (the newline, if any, included), or
1609 C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
1610 (sometimes known as file-slurp mode) and the file is empty, it
1611 returns C<''> the first time, followed by C<undef> subsequently.
1613 Ordinarily you must assign the returned value to a variable, but
1614 there is one situation where an automatic assignment happens. If
1615 and only if the input symbol is the only thing inside the conditional
1616 of a C<while> statement (even if disguised as a C<for(;;)> loop),
1617 the value is automatically assigned to the global variable $_,
1618 destroying whatever was there previously. (This may seem like an
1619 odd thing to you, but you'll use the construct in almost every Perl
1620 script you write.) The $_ variable is not implicitly localized.
1621 You'll have to put a C<local $_;> before the loop if you want that
1624 The following lines are equivalent:
1626 while (defined($_ = <STDIN>)) { print; }
1627 while ($_ = <STDIN>) { print; }
1628 while (<STDIN>) { print; }
1629 for (;<STDIN>;) { print; }
1630 print while defined($_ = <STDIN>);
1631 print while ($_ = <STDIN>);
1632 print while <STDIN>;
1634 This also behaves similarly, but avoids $_ :
1636 while (my $line = <STDIN>) { print $line }
1638 In these loop constructs, the assigned value (whether assignment
1639 is automatic or explicit) is then tested to see whether it is
1640 defined. The defined test avoids problems where line has a string
1641 value that would be treated as false by Perl, for example a "" or
1642 a "0" with no trailing newline. If you really mean for such values
1643 to terminate the loop, they should be tested for explicitly:
1645 while (($_ = <STDIN>) ne '0') { ... }
1646 while (<STDIN>) { last unless $_; ... }
1648 In other boolean contexts, C<< <I<filehandle>> >> without an
1649 explicit C<defined> test or comparison elicit a warning if the
1650 C<use warnings> pragma or the B<-w>
1651 command-line switch (the C<$^W> variable) is in effect.
1653 The filehandles STDIN, STDOUT, and STDERR are predefined. (The
1654 filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
1655 in packages, where they would be interpreted as local identifiers
1656 rather than global.) Additional filehandles may be created with
1657 the open() function, amongst others. See L<perlopentut> and
1658 L<perlfunc/open> for details on this.
1660 If a <FILEHANDLE> is used in a context that is looking for
1661 a list, a list comprising all input lines is returned, one line per
1662 list element. It's easy to grow to a rather large data space this
1663 way, so use with care.
1665 <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
1666 See L<perlfunc/readline>.
1668 The null filehandle <> is special: it can be used to emulate the
1669 behavior of B<sed> and B<awk>. Input from <> comes either from
1670 standard input, or from each file listed on the command line. Here's
1671 how it works: the first time <> is evaluated, the @ARGV array is
1672 checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
1673 gives you standard input. The @ARGV array is then processed as a list
1674 of filenames. The loop
1677 ... # code for each line
1680 is equivalent to the following Perl-like pseudo code:
1682 unshift(@ARGV, '-') unless @ARGV;
1683 while ($ARGV = shift) {
1686 ... # code for each line
1690 except that it isn't so cumbersome to say, and will actually work.
1691 It really does shift the @ARGV array and put the current filename
1692 into the $ARGV variable. It also uses filehandle I<ARGV>
1693 internally--<> is just a synonym for <ARGV>, which
1694 is magical. (The pseudo code above doesn't work because it treats
1695 <ARGV> as non-magical.)
1697 You can modify @ARGV before the first <> as long as the array ends up
1698 containing the list of filenames you really want. Line numbers (C<$.>)
1699 continue as though the input were one big happy file. See the example
1700 in L<perlfunc/eof> for how to reset line numbers on each file.
1702 If you want to set @ARGV to your own list of files, go right ahead.
1703 This sets @ARGV to all plain text files if no @ARGV was given:
1705 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
1707 You can even set them to pipe commands. For example, this automatically
1708 filters compressed arguments through B<gzip>:
1710 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
1712 If you want to pass switches into your script, you can use one of the
1713 Getopts modules or put a loop on the front like this:
1715 while ($_ = $ARGV[0], /^-/) {
1718 if (/^-D(.*)/) { $debug = $1 }
1719 if (/^-v/) { $verbose++ }
1720 # ... # other switches
1724 # ... # code for each line
1727 The <> symbol will return C<undef> for end-of-file only once.
1728 If you call it again after this, it will assume you are processing another
1729 @ARGV list, and if you haven't set @ARGV, will read input from STDIN.
1731 If what the angle brackets contain is a simple scalar variable (e.g.,
1732 <$foo>), then that variable contains the name of the
1733 filehandle to input from, or its typeglob, or a reference to the
1739 If what's within the angle brackets is neither a filehandle nor a simple
1740 scalar variable containing a filehandle name, typeglob, or typeglob
1741 reference, it is interpreted as a filename pattern to be globbed, and
1742 either a list of filenames or the next filename in the list is returned,
1743 depending on context. This distinction is determined on syntactic
1744 grounds alone. That means C<< <$x> >> is always a readline() from
1745 an indirect handle, but C<< <$hash{key}> >> is always a glob().
1746 That's because $x is a simple scalar variable, but C<$hash{key}> is
1747 not--it's a hash element.
1749 One level of double-quote interpretation is done first, but you can't
1750 say C<< <$foo> >> because that's an indirect filehandle as explained
1751 in the previous paragraph. (In older versions of Perl, programmers
1752 would insert curly brackets to force interpretation as a filename glob:
1753 C<< <${foo}> >>. These days, it's considered cleaner to call the
1754 internal function directly as C<glob($foo)>, which is probably the right
1755 way to have done it in the first place.) For example:
1761 is roughly equivalent to:
1763 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
1769 except that the globbing is actually done internally using the standard
1770 C<File::Glob> extension. Of course, the shortest way to do the above is:
1774 A (file)glob evaluates its (embedded) argument only when it is
1775 starting a new list. All values must be read before it will start
1776 over. In list context, this isn't important because you automatically
1777 get them all anyway. However, in scalar context the operator returns
1778 the next value each time it's called, or C<undef> when the list has
1779 run out. As with filehandle reads, an automatic C<defined> is
1780 generated when the glob occurs in the test part of a C<while>,
1781 because legal glob returns (e.g. a file called F<0>) would otherwise
1782 terminate the loop. Again, C<undef> is returned only once. So if
1783 you're expecting a single value from a glob, it is much better to
1786 ($file) = <blurch*>;
1792 because the latter will alternate between returning a filename and
1795 If you're trying to do variable interpolation, it's definitely better
1796 to use the glob() function, because the older notation can cause people
1797 to become confused with the indirect filehandle notation.
1799 @files = glob("$dir/*.[ch]");
1800 @files = glob($files[$i]);
1802 =head2 Constant Folding
1804 Like C, Perl does a certain amount of expression evaluation at
1805 compile time whenever it determines that all arguments to an
1806 operator are static and have no side effects. In particular, string
1807 concatenation happens at compile time between literals that don't do
1808 variable substitution. Backslash interpolation also happens at
1809 compile time. You can say
1811 'Now is the time for all' . "\n" .
1812 'good men to come to.'
1814 and this all reduces to one string internally. Likewise, if
1817 foreach $file (@filenames) {
1818 if (-s $file > 5 + 100 * 2**16) { }
1821 the compiler will precompute the number which that expression
1822 represents so that the interpreter won't have to.
1824 =head2 Bitwise String Operators
1826 Bitstrings of any size may be manipulated by the bitwise operators
1829 If the operands to a binary bitwise op are strings of different
1830 sizes, B<|> and B<^> ops act as though the shorter operand had
1831 additional zero bits on the right, while the B<&> op acts as though
1832 the longer operand were truncated to the length of the shorter.
1833 The granularity for such extension or truncation is one or more
1836 # ASCII-based examples
1837 print "j p \n" ^ " a h"; # prints "JAPH\n"
1838 print "JA" | " ph\n"; # prints "japh\n"
1839 print "japh\nJunk" & '_____'; # prints "JAPH\n";
1840 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
1842 If you are intending to manipulate bitstrings, be certain that
1843 you're supplying bitstrings: If an operand is a number, that will imply
1844 a B<numeric> bitwise operation. You may explicitly show which type of
1845 operation you intend by using C<""> or C<0+>, as in the examples below.
1847 $foo = 150 | 105 ; # yields 255 (0x96 | 0x69 is 0xFF)
1848 $foo = '150' | 105 ; # yields 255
1849 $foo = 150 | '105'; # yields 255
1850 $foo = '150' | '105'; # yields string '155' (under ASCII)
1852 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
1853 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
1855 See L<perlfunc/vec> for information on how to manipulate individual bits
1858 =head2 Integer Arithmetic
1860 By default, Perl assumes that it must do most of its arithmetic in
1861 floating point. But by saying
1865 you may tell the compiler that it's okay to use integer operations
1866 (if it feels like it) from here to the end of the enclosing BLOCK.
1867 An inner BLOCK may countermand this by saying
1871 which lasts until the end of that BLOCK. Note that this doesn't
1872 mean everything is only an integer, merely that Perl may use integer
1873 operations if it is so inclined. For example, even under C<use
1874 integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731>
1877 Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
1878 and ">>") always produce integral results. (But see also
1879 L<Bitwise String Operators>.) However, C<use integer> still has meaning for
1880 them. By default, their results are interpreted as unsigned integers, but
1881 if C<use integer> is in effect, their results are interpreted
1882 as signed integers. For example, C<~0> usually evaluates to a large
1883 integral value. However, C<use integer; ~0> is C<-1> on twos-complement
1886 =head2 Floating-point Arithmetic
1888 While C<use integer> provides integer-only arithmetic, there is no
1889 analogous mechanism to provide automatic rounding or truncation to a
1890 certain number of decimal places. For rounding to a certain number
1891 of digits, sprintf() or printf() is usually the easiest route.
1894 Floating-point numbers are only approximations to what a mathematician
1895 would call real numbers. There are infinitely more reals than floats,
1896 so some corners must be cut. For example:
1898 printf "%.20g\n", 123456789123456789;
1899 # produces 123456789123456784
1901 Testing for exact equality of floating-point equality or inequality is
1902 not a good idea. Here's a (relatively expensive) work-around to compare
1903 whether two floating-point numbers are equal to a particular number of
1904 decimal places. See Knuth, volume II, for a more robust treatment of
1908 my ($X, $Y, $POINTS) = @_;
1910 $tX = sprintf("%.${POINTS}g", $X);
1911 $tY = sprintf("%.${POINTS}g", $Y);
1915 The POSIX module (part of the standard perl distribution) implements
1916 ceil(), floor(), and other mathematical and trigonometric functions.
1917 The Math::Complex module (part of the standard perl distribution)
1918 defines mathematical functions that work on both the reals and the
1919 imaginary numbers. Math::Complex not as efficient as POSIX, but
1920 POSIX can't work with complex numbers.
1922 Rounding in financial applications can have serious implications, and
1923 the rounding method used should be specified precisely. In these
1924 cases, it probably pays not to trust whichever system rounding is
1925 being used by Perl, but to instead implement the rounding function you
1928 =head2 Bigger Numbers
1930 The standard Math::BigInt and Math::BigFloat modules provide
1931 variable-precision arithmetic and overloaded operators, although
1932 they're currently pretty slow. At the cost of some space and
1933 considerable speed, they avoid the normal pitfalls associated with
1934 limited-precision representations.
1937 $x = Math::BigInt->new('123456789123456789');
1940 # prints +15241578780673678515622620750190521
1942 There are several modules that let you calculate with (bound only by
1943 memory and cpu-time) unlimited or fixed precision. There are also
1944 some non-standard modules that provide faster implementations via
1945 external C libraries.
1947 Here is a short, but incomplete summary:
1949 Math::Fraction big, unlimited fractions like 9973 / 12967
1950 Math::String treat string sequences like numbers
1951 Math::FixedPrecision calculate with a fixed precision
1952 Math::Currency for currency calculations
1953 Bit::Vector manipulate bit vectors fast (uses C)
1954 Math::BigIntFast Bit::Vector wrapper for big numbers
1955 Math::Pari provides access to the Pari C library
1956 Math::BigInteger uses an external C library
1957 Math::Cephes uses external Cephes C library (no big numbers)
1958 Math::Cephes::Fraction fractions via the Cephes library
1959 Math::GMP another one using an external C library