4 perlop - Perl operators and precedence
8 =head2 Operator Precedence and Associativity
9 X<operator, precedence> X<precedence> X<associativity>
11 Operator precedence and associativity work in Perl more or less like
12 they do in mathematics.
14 I<Operator precedence> means some operators are evaluated before
15 others. For example, in C<2 + 4 * 5>, the multiplication has higher
16 precedence so C<4 * 5> is evaluated first yielding C<2 + 20 ==
17 22> and not C<6 * 5 == 30>.
19 I<Operator associativity> defines what happens if a sequence of the
20 same operators is used one after another: whether the evaluator will
21 evaluate the left operations first or the right. For example, in C<8
22 - 4 - 2>, subtraction is left associative so Perl evaluates the
23 expression left to right. C<8 - 4> is evaluated first making the
24 expression C<4 - 2 == 2> and not C<8 - 2 == 6>.
26 Perl operators have the following associativity and precedence,
27 listed from highest precedence to lowest. Operators borrowed from
28 C keep the same precedence relationship with each other, even where
29 C's precedence is slightly screwy. (This makes learning Perl easier
30 for C folks.) With very few exceptions, these all operate on scalar
31 values only, not array values.
33 left terms and list operators (leftward)
37 right ! ~ \ and unary + and -
42 nonassoc named unary operators
43 nonassoc < > <= >= lt gt le ge
44 nonassoc == != <=> eq ne cmp ~~
53 nonassoc list operators (rightward)
58 In the following sections, these operators are covered in precedence order.
60 Many operators can be overloaded for objects. See L<overload>.
62 =head2 Terms and List Operators (Leftward)
63 X<list operator> X<operator, list> X<term>
65 A TERM has the highest precedence in Perl. They include variables,
66 quote and quote-like operators, any expression in parentheses,
67 and any function whose arguments are parenthesized. Actually, there
68 aren't really functions in this sense, just list operators and unary
69 operators behaving as functions because you put parentheses around
70 the arguments. These are all documented in L<perlfunc>.
72 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
73 is followed by a left parenthesis as the next token, the operator and
74 arguments within parentheses are taken to be of highest precedence,
75 just like a normal function call.
77 In the absence of parentheses, the precedence of list operators such as
78 C<print>, C<sort>, or C<chmod> is either very high or very low depending on
79 whether you are looking at the left side or the right side of the operator.
82 @ary = (1, 3, sort 4, 2);
83 print @ary; # prints 1324
85 the commas on the right of the sort are evaluated before the sort,
86 but the commas on the left are evaluated after. In other words,
87 list operators tend to gobble up all arguments that follow, and
88 then act like a simple TERM with regard to the preceding expression.
89 Be careful with parentheses:
91 # These evaluate exit before doing the print:
92 print($foo, exit); # Obviously not what you want.
93 print $foo, exit; # Nor is this.
95 # These do the print before evaluating exit:
96 (print $foo), exit; # This is what you want.
97 print($foo), exit; # Or this.
98 print ($foo), exit; # Or even this.
102 print ($foo & 255) + 1, "\n";
104 probably doesn't do what you expect at first glance. The parentheses
105 enclose the argument list for C<print> which is evaluated (printing
106 the result of C<$foo & 255>). Then one is added to the return value
107 of C<print> (usually 1). The result is something like this:
109 1 + 1, "\n"; # Obviously not what you meant.
111 To do what you meant properly, you must write:
113 print(($foo & 255) + 1, "\n");
115 See L<Named Unary Operators> for more discussion of this.
117 Also parsed as terms are the C<do {}> and C<eval {}> constructs, as
118 well as subroutine and method calls, and the anonymous
119 constructors C<[]> and C<{}>.
121 See also L<Quote and Quote-like Operators> toward the end of this section,
122 as well as L</"I/O Operators">.
124 =head2 The Arrow Operator
125 X<arrow> X<dereference> X<< -> >>
127 "C<< -> >>" is an infix dereference operator, just as it is in C
128 and C++. If the right side is either a C<[...]>, C<{...}>, or a
129 C<(...)> subscript, then the left side must be either a hard or
130 symbolic reference to an array, a hash, or a subroutine respectively.
131 (Or technically speaking, a location capable of holding a hard
132 reference, if it's an array or hash reference being used for
133 assignment.) See L<perlreftut> and L<perlref>.
135 Otherwise, the right side is a method name or a simple scalar
136 variable containing either the method name or a subroutine reference,
137 and the left side must be either an object (a blessed reference)
138 or a class name (that is, a package name). See L<perlobj>.
140 =head2 Auto-increment and Auto-decrement
141 X<increment> X<auto-increment> X<++> X<decrement> X<auto-decrement> X<-->
143 "++" and "--" work as in C. That is, if placed before a variable,
144 they increment or decrement the variable by one before returning the
145 value, and if placed after, increment or decrement after returning the
149 print $i++; # prints 0
150 print ++$j; # prints 1
152 Note that just as in C, Perl doesn't define B<when> the variable is
153 incremented or decremented. You just know it will be done sometime
154 before or after the value is returned. This also means that modifying
155 a variable twice in the same statement will lead to undefined behaviour.
156 Avoid statements like:
161 Perl will not guarantee what the result of the above statements is.
163 The auto-increment operator has a little extra builtin magic to it. If
164 you increment a variable that is numeric, or that has ever been used in
165 a numeric context, you get a normal increment. If, however, the
166 variable has been used in only string contexts since it was set, and
167 has a value that is not the empty string and matches the pattern
168 C</^[a-zA-Z]*[0-9]*\z/>, the increment is done as a string, preserving each
169 character within its range, with carry:
171 print ++($foo = '99'); # prints '100'
172 print ++($foo = 'a0'); # prints 'a1'
173 print ++($foo = 'Az'); # prints 'Ba'
174 print ++($foo = 'zz'); # prints 'aaa'
176 C<undef> is always treated as numeric, and in particular is changed
177 to C<0> before incrementing (so that a post-increment of an undef value
178 will return C<0> rather than C<undef>).
180 The auto-decrement operator is not magical.
182 =head2 Exponentiation
183 X<**> X<exponentiation> X<power>
185 Binary "**" is the exponentiation operator. It binds even more
186 tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is
187 implemented using C's pow(3) function, which actually works on doubles
190 =head2 Symbolic Unary Operators
191 X<unary operator> X<operator, unary>
193 Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower
194 precedence version of this.
197 Unary "-" performs arithmetic negation if the operand is numeric. If
198 the operand is an identifier, a string consisting of a minus sign
199 concatenated with the identifier is returned. Otherwise, if the string
200 starts with a plus or minus, a string starting with the opposite sign
201 is returned. One effect of these rules is that -bareword is equivalent
202 to the string "-bareword". If, however, the string begins with a
203 non-alphabetic character (excluding "+" or "-"), Perl will attempt to convert
204 the string to a numeric and the arithmetic negation is performed. If the
205 string cannot be cleanly converted to a numeric, Perl will give the warning
206 B<Argument "the string" isn't numeric in negation (-) at ...>.
207 X<-> X<negation, arithmetic>
209 Unary "~" performs bitwise negation, i.e., 1's complement. For
210 example, C<0666 & ~027> is 0640. (See also L<Integer Arithmetic> and
211 L<Bitwise String Operators>.) Note that the width of the result is
212 platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64
213 bits wide on a 64-bit platform, so if you are expecting a certain bit
214 width, remember to use the & operator to mask off the excess bits.
215 X<~> X<negation, binary>
217 Unary "+" has no effect whatsoever, even on strings. It is useful
218 syntactically for separating a function name from a parenthesized expression
219 that would otherwise be interpreted as the complete list of function
220 arguments. (See examples above under L<Terms and List Operators (Leftward)>.)
223 Unary "\" creates a reference to whatever follows it. See L<perlreftut>
224 and L<perlref>. Do not confuse this behavior with the behavior of
225 backslash within a string, although both forms do convey the notion
226 of protecting the next thing from interpolation.
227 X<\> X<reference> X<backslash>
229 =head2 Binding Operators
230 X<binding> X<operator, binding> X<=~> X<!~>
232 Binary "=~" binds a scalar expression to a pattern match. Certain operations
233 search or modify the string $_ by default. This operator makes that kind
234 of operation work on some other string. The right argument is a search
235 pattern, substitution, or transliteration. The left argument is what is
236 supposed to be searched, substituted, or transliterated instead of the default
237 $_. When used in scalar context, the return value generally indicates the
238 success of the operation. Behavior in list context depends on the particular
239 operator. See L</"Regexp Quote-Like Operators"> for details and
240 L<perlretut> for examples using these operators.
242 If the right argument is an expression rather than a search pattern,
243 substitution, or transliteration, it is interpreted as a search pattern at run
244 time. Note that this means that its contents will be interpolated twice, so
248 is not ok, as the regex engine will end up trying to compile the
249 pattern C<\>, which it will consider a syntax error.
251 Binary "!~" is just like "=~" except the return value is negated in
254 =head2 Multiplicative Operators
255 X<operator, multiplicative>
257 Binary "*" multiplies two numbers.
260 Binary "/" divides two numbers.
263 Binary "%" is the modulo operator, which computes the division
264 remainder of its first argument with respect to its second argument.
266 operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
267 C<$a> minus the largest multiple of C<$b> less than or equal to
268 C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
269 smallest multiple of C<$b> that is not less than C<$a> (i.e. the
270 result will be less than or equal to zero). If the operands
271 C<$a> and C<$b> are floating point values and the absolute value of
272 C<$b> (that is C<abs($b)>) is less than C<(UV_MAX + 1)>, only
273 the integer portion of C<$a> and C<$b> will be used in the operation
274 (Note: here C<UV_MAX> means the maximum of the unsigned integer type).
275 If the absolute value of the right operand (C<abs($b)>) is greater than
276 or equal to C<(UV_MAX + 1)>, "%" computes the floating-point remainder
277 C<$r> in the equation C<($r = $a - $i*$b)> where C<$i> is a certain
278 integer that makes C<$r> have the same sign as the right operand
279 C<$b> (B<not> as the left operand C<$a> like C function C<fmod()>)
280 and the absolute value less than that of C<$b>.
281 Note that when C<use integer> is in scope, "%" gives you direct access
282 to the modulo operator as implemented by your C compiler. This
283 operator is not as well defined for negative operands, but it will
285 X<%> X<remainder> X<modulo> X<mod>
287 Binary "x" is the repetition operator. In scalar context or if the left
288 operand is not enclosed in parentheses, it returns a string consisting
289 of the left operand repeated the number of times specified by the right
290 operand. In list context, if the left operand is enclosed in
291 parentheses or is a list formed by C<qw/STRING/>, it repeats the list.
292 If the right operand is zero or negative, it returns an empty string
293 or an empty list, depending on the context.
296 print '-' x 80; # print row of dashes
298 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
300 @ones = (1) x 80; # a list of 80 1's
301 @ones = (5) x @ones; # set all elements to 5
304 =head2 Additive Operators
305 X<operator, additive>
307 Binary "+" returns the sum of two numbers.
310 Binary "-" returns the difference of two numbers.
313 Binary "." concatenates two strings.
314 X<string, concatenation> X<concatenation>
315 X<cat> X<concat> X<concatenate> X<.>
317 =head2 Shift Operators
318 X<shift operator> X<operator, shift> X<<< << >>>
319 X<<< >> >>> X<right shift> X<left shift> X<bitwise shift>
320 X<shl> X<shr> X<shift, right> X<shift, left>
322 Binary "<<" returns the value of its left argument shifted left by the
323 number of bits specified by the right argument. Arguments should be
324 integers. (See also L<Integer Arithmetic>.)
326 Binary ">>" returns the value of its left argument shifted right by
327 the number of bits specified by the right argument. Arguments should
328 be integers. (See also L<Integer Arithmetic>.)
330 Note that both "<<" and ">>" in Perl are implemented directly using
331 "<<" and ">>" in C. If C<use integer> (see L<Integer Arithmetic>) is
332 in force then signed C integers are used, else unsigned C integers are
333 used. Either way, the implementation isn't going to generate results
334 larger than the size of the integer type Perl was built with (32 bits
337 The result of overflowing the range of the integers is undefined
338 because it is undefined also in C. In other words, using 32-bit
339 integers, C<< 1 << 32 >> is undefined. Shifting by a negative number
340 of bits is also undefined.
342 =head2 Named Unary Operators
343 X<operator, named unary>
345 The various named unary operators are treated as functions with one
346 argument, with optional parentheses.
348 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
349 is followed by a left parenthesis as the next token, the operator and
350 arguments within parentheses are taken to be of highest precedence,
351 just like a normal function call. For example,
352 because named unary operators are higher precedence than ||:
354 chdir $foo || die; # (chdir $foo) || die
355 chdir($foo) || die; # (chdir $foo) || die
356 chdir ($foo) || die; # (chdir $foo) || die
357 chdir +($foo) || die; # (chdir $foo) || die
359 but, because * is higher precedence than named operators:
361 chdir $foo * 20; # chdir ($foo * 20)
362 chdir($foo) * 20; # (chdir $foo) * 20
363 chdir ($foo) * 20; # (chdir $foo) * 20
364 chdir +($foo) * 20; # chdir ($foo * 20)
366 rand 10 * 20; # rand (10 * 20)
367 rand(10) * 20; # (rand 10) * 20
368 rand (10) * 20; # (rand 10) * 20
369 rand +(10) * 20; # rand (10 * 20)
371 Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are
372 treated like named unary operators, but they don't follow this functional
373 parenthesis rule. That means, for example, that C<-f($file).".bak"> is
374 equivalent to C<-f "$file.bak">.
375 X<-X> X<filetest> X<operator, filetest>
377 See also L<"Terms and List Operators (Leftward)">.
379 =head2 Relational Operators
380 X<relational operator> X<operator, relational>
382 Binary "<" returns true if the left argument is numerically less than
386 Binary ">" returns true if the left argument is numerically greater
387 than the right argument.
390 Binary "<=" returns true if the left argument is numerically less than
391 or equal to the right argument.
394 Binary ">=" returns true if the left argument is numerically greater
395 than or equal to the right argument.
398 Binary "lt" returns true if the left argument is stringwise less than
402 Binary "gt" returns true if the left argument is stringwise greater
403 than the right argument.
406 Binary "le" returns true if the left argument is stringwise less than
407 or equal to the right argument.
410 Binary "ge" returns true if the left argument is stringwise greater
411 than or equal to the right argument.
414 =head2 Equality Operators
415 X<equality> X<equal> X<equals> X<operator, equality>
417 Binary "==" returns true if the left argument is numerically equal to
421 Binary "!=" returns true if the left argument is numerically not equal
422 to the right argument.
425 Binary "<=>" returns -1, 0, or 1 depending on whether the left
426 argument is numerically less than, equal to, or greater than the right
427 argument. If your platform supports NaNs (not-a-numbers) as numeric
428 values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
429 "<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
430 returns true, as does NaN != anything else. If your platform doesn't
431 support NaNs then NaN is just a string with numeric value 0.
432 X<< <=> >> X<spaceship>
434 perl -le '$a = "NaN"; print "No NaN support here" if $a == $a'
435 perl -le '$a = "NaN"; print "NaN support here" if $a != $a'
437 Binary "eq" returns true if the left argument is stringwise equal to
441 Binary "ne" returns true if the left argument is stringwise not equal
442 to the right argument.
445 Binary "cmp" returns -1, 0, or 1 depending on whether the left
446 argument is stringwise less than, equal to, or greater than the right
450 Binary "~~" does a smart match between its arguments. Smart matching
451 is described in L<perlsyn/"Smart matching in detail">.
454 "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
455 by the current locale if C<use locale> is in effect. See L<perllocale>.
458 X<operator, bitwise, and> X<bitwise and> X<&>
460 Binary "&" returns its operands ANDed together bit by bit.
461 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
463 Note that "&" has lower priority than relational operators, so for example
464 the brackets are essential in a test like
466 print "Even\n" if ($x & 1) == 0;
468 =head2 Bitwise Or and Exclusive Or
469 X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor>
472 Binary "|" returns its operands ORed together bit by bit.
473 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
475 Binary "^" returns its operands XORed together bit by bit.
476 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
478 Note that "|" and "^" have lower priority than relational operators, so
479 for example the brackets are essential in a test like
481 print "false\n" if (8 | 2) != 10;
483 =head2 C-style Logical And
484 X<&&> X<logical and> X<operator, logical, and>
486 Binary "&&" performs a short-circuit logical AND operation. That is,
487 if the left operand is false, the right operand is not even evaluated.
488 Scalar or list context propagates down to the right operand if it
491 =head2 C-style Logical Or
492 X<||> X<operator, logical, or>
494 Binary "||" performs a short-circuit logical OR operation. That is,
495 if the left operand is true, the right operand is not even evaluated.
496 Scalar or list context propagates down to the right operand if it
499 =head2 C-style Logical Defined-Or
500 X<//> X<operator, logical, defined-or>
502 Although it has no direct equivalent in C, Perl's C<//> operator is related
503 to its C-style or. In fact, it's exactly the same as C<||>, except that it
504 tests the left hand side's definedness instead of its truth. Thus, C<$a // $b>
505 is similar to C<defined($a) || $b> (except that it returns the value of C<$a>
506 rather than the value of C<defined($a)>) and is exactly equivalent to
507 C<defined($a) ? $a : $b>. This is very useful for providing default values
508 for variables. If you actually want to test if at least one of C<$a> and
509 C<$b> is defined, use C<defined($a // $b)>.
511 The C<||>, C<//> and C<&&> operators return the last value evaluated
512 (unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably
513 portable way to find out the home directory might be:
515 $home = $ENV{'HOME'} // $ENV{'LOGDIR'} //
516 (getpwuid($<))[7] // die "You're homeless!\n";
518 In particular, this means that you shouldn't use this
519 for selecting between two aggregates for assignment:
521 @a = @b || @c; # this is wrong
522 @a = scalar(@b) || @c; # really meant this
523 @a = @b ? @b : @c; # this works fine, though
525 As more readable alternatives to C<&&> and C<||> when used for
526 control flow, Perl provides the C<and> and C<or> operators (see below).
527 The short-circuit behavior is identical. The precedence of "and"
528 and "or" is much lower, however, so that you can safely use them after a
529 list operator without the need for parentheses:
531 unlink "alpha", "beta", "gamma"
532 or gripe(), next LINE;
534 With the C-style operators that would have been written like this:
536 unlink("alpha", "beta", "gamma")
537 || (gripe(), next LINE);
539 Using "or" for assignment is unlikely to do what you want; see below.
541 =head2 Range Operators
542 X<operator, range> X<range> X<..> X<...>
544 Binary ".." is the range operator, which is really two different
545 operators depending on the context. In list context, it returns a
546 list of values counting (up by ones) from the left value to the right
547 value. If the left value is greater than the right value then it
548 returns the empty list. The range operator is useful for writing
549 C<foreach (1..10)> loops and for doing slice operations on arrays. In
550 the current implementation, no temporary array is created when the
551 range operator is used as the expression in C<foreach> loops, but older
552 versions of Perl might burn a lot of memory when you write something
555 for (1 .. 1_000_000) {
559 The range operator also works on strings, using the magical auto-increment,
562 In scalar context, ".." returns a boolean value. The operator is
563 bistable, like a flip-flop, and emulates the line-range (comma) operator
564 of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
565 own boolean state. It is false as long as its left operand is false.
566 Once the left operand is true, the range operator stays true until the
567 right operand is true, I<AFTER> which the range operator becomes false
568 again. It doesn't become false till the next time the range operator is
569 evaluated. It can test the right operand and become false on the same
570 evaluation it became true (as in B<awk>), but it still returns true once.
571 If you don't want it to test the right operand until the next
572 evaluation, as in B<sed>, just use three dots ("...") instead of
573 two. In all other regards, "..." behaves just like ".." does.
575 The right operand is not evaluated while the operator is in the
576 "false" state, and the left operand is not evaluated while the
577 operator is in the "true" state. The precedence is a little lower
578 than || and &&. The value returned is either the empty string for
579 false, or a sequence number (beginning with 1) for true. The
580 sequence number is reset for each range encountered. The final
581 sequence number in a range has the string "E0" appended to it, which
582 doesn't affect its numeric value, but gives you something to search
583 for if you want to exclude the endpoint. You can exclude the
584 beginning point by waiting for the sequence number to be greater
587 If either operand of scalar ".." is a constant expression,
588 that operand is considered true if it is equal (C<==>) to the current
589 input line number (the C<$.> variable).
591 To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>,
592 but that is only an issue if you use a floating point expression; when
593 implicitly using C<$.> as described in the previous paragraph, the
594 comparison is C<int(EXPR) == int($.)> which is only an issue when C<$.>
595 is set to a floating point value and you are not reading from a file.
596 Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what
597 you want in scalar context because each of the operands are evaluated
598 using their integer representation.
602 As a scalar operator:
604 if (101 .. 200) { print; } # print 2nd hundred lines, short for
605 # if ($. == 101 .. $. == 200) { print; }
607 next LINE if (1 .. /^$/); # skip header lines, short for
608 # next LINE if ($. == 1 .. /^$/);
609 # (typically in a loop labeled LINE)
611 s/^/> / if (/^$/ .. eof()); # quote body
613 # parse mail messages
615 $in_header = 1 .. /^$/;
616 $in_body = /^$/ .. eof;
623 close ARGV if eof; # reset $. each file
626 Here's a simple example to illustrate the difference between
627 the two range operators:
640 This program will print only the line containing "Bar". If
641 the range operator is changed to C<...>, it will also print the
644 And now some examples as a list operator:
646 for (101 .. 200) { print; } # print $_ 100 times
647 @foo = @foo[0 .. $#foo]; # an expensive no-op
648 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
650 The range operator (in list context) makes use of the magical
651 auto-increment algorithm if the operands are strings. You
654 @alphabet = ('A' .. 'Z');
656 to get all normal letters of the English alphabet, or
658 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
660 to get a hexadecimal digit, or
662 @z2 = ('01' .. '31'); print $z2[$mday];
664 to get dates with leading zeros.
666 If the final value specified is not in the sequence that the magical
667 increment would produce, the sequence goes until the next value would
668 be longer than the final value specified.
670 If the initial value specified isn't part of a magical increment
671 sequence (that is, a non-empty string matching "/^[a-zA-Z]*[0-9]*\z/"),
672 only the initial value will be returned. So the following will only
675 use charnames 'greek';
676 my @greek_small = ("\N{alpha}" .. "\N{omega}");
678 To get lower-case greek letters, use this instead:
680 my @greek_small = map { chr } ( ord("\N{alpha}") .. ord("\N{omega}") );
682 Because each operand is evaluated in integer form, C<2.18 .. 3.14> will
683 return two elements in list context.
685 @list = (2.18 .. 3.14); # same as @list = (2 .. 3);
687 =head2 Conditional Operator
688 X<operator, conditional> X<operator, ternary> X<ternary> X<?:>
690 Ternary "?:" is the conditional operator, just as in C. It works much
691 like an if-then-else. If the argument before the ? is true, the
692 argument before the : is returned, otherwise the argument after the :
693 is returned. For example:
695 printf "I have %d dog%s.\n", $n,
696 ($n == 1) ? '' : "s";
698 Scalar or list context propagates downward into the 2nd
699 or 3rd argument, whichever is selected.
701 $a = $ok ? $b : $c; # get a scalar
702 @a = $ok ? @b : @c; # get an array
703 $a = $ok ? @b : @c; # oops, that's just a count!
705 The operator may be assigned to if both the 2nd and 3rd arguments are
706 legal lvalues (meaning that you can assign to them):
708 ($a_or_b ? $a : $b) = $c;
710 Because this operator produces an assignable result, using assignments
711 without parentheses will get you in trouble. For example, this:
713 $a % 2 ? $a += 10 : $a += 2
717 (($a % 2) ? ($a += 10) : $a) += 2
721 ($a % 2) ? ($a += 10) : ($a += 2)
723 That should probably be written more simply as:
725 $a += ($a % 2) ? 10 : 2;
727 =head2 Assignment Operators
728 X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=>
729 X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=>
732 "=" is the ordinary assignment operator.
734 Assignment operators work as in C. That is,
742 although without duplicating any side effects that dereferencing the lvalue
743 might trigger, such as from tie(). Other assignment operators work similarly.
744 The following are recognized:
751 Although these are grouped by family, they all have the precedence
754 Unlike in C, the scalar assignment operator produces a valid lvalue.
755 Modifying an assignment is equivalent to doing the assignment and
756 then modifying the variable that was assigned to. This is useful
757 for modifying a copy of something, like this:
759 ($tmp = $global) =~ tr [A-Z] [a-z];
770 Similarly, a list assignment in list context produces the list of
771 lvalues assigned to, and a list assignment in scalar context returns
772 the number of elements produced by the expression on the right hand
773 side of the assignment.
775 =head2 Comma Operator
776 X<comma> X<operator, comma> X<,>
778 Binary "," is the comma operator. In scalar context it evaluates
779 its left argument, throws that value away, then evaluates its right
780 argument and returns that value. This is just like C's comma operator.
782 In list context, it's just the list argument separator, and inserts
783 both its arguments into the list. These arguments are also evaluated
786 The C<< => >> operator is a synonym for the comma except that it causes
787 its left operand to be interpreted as a string if it begins with a letter
788 or underscore and is composed only of letters, digits and underscores.
789 This includes operands that might otherwise be interpreted as operators,
790 constants, single number v-strings or function calls. If in doubt about
791 this behaviour, the left operand can be quoted explicitly.
793 Otherwise, the C<< => >> operator behaves exactly as the comma operator
794 or list argument separator, according to context.
798 use constant FOO => "something";
800 my %h = ( FOO => 23 );
808 my %h = ("something", 23);
810 The C<< => >> operator is helpful in documenting the correspondence
811 between keys and values in hashes, and other paired elements in lists.
813 %hash = ( $key => $value );
814 login( $username => $password );
816 =head2 Yada Yada Operator
817 X<...> X<... operator> X<yada yada operator>
819 The yada yada operator (noted C<...>) is a placeholder for code.
820 It parses without error, but when executed it throws an exception
821 with the text C<Unimplemented>:
826 Unimplemented at <file> line <line number>.
828 It takes no argument.
830 =head2 List Operators (Rightward)
831 X<operator, list, rightward> X<list operator>
833 On the right side of a list operator, it has very low precedence,
834 such that it controls all comma-separated expressions found there.
835 The only operators with lower precedence are the logical operators
836 "and", "or", and "not", which may be used to evaluate calls to list
837 operators without the need for extra parentheses:
839 open HANDLE, "filename"
840 or die "Can't open: $!\n";
842 See also discussion of list operators in L<Terms and List Operators (Leftward)>.
845 X<operator, logical, not> X<not>
847 Unary "not" returns the logical negation of the expression to its right.
848 It's the equivalent of "!" except for the very low precedence.
851 X<operator, logical, and> X<and>
853 Binary "and" returns the logical conjunction of the two surrounding
854 expressions. It's equivalent to && except for the very low
855 precedence. This means that it short-circuits: i.e., the right
856 expression is evaluated only if the left expression is true.
858 =head2 Logical or, Defined or, and Exclusive Or
859 X<operator, logical, or> X<operator, logical, xor>
860 X<operator, logical, defined or> X<operator, logical, exclusive or>
863 Binary "or" returns the logical disjunction of the two surrounding
864 expressions. It's equivalent to || except for the very low precedence.
865 This makes it useful for control flow
867 print FH $data or die "Can't write to FH: $!";
869 This means that it short-circuits: i.e., the right expression is evaluated
870 only if the left expression is false. Due to its precedence, you should
871 probably avoid using this for assignment, only for control flow.
873 $a = $b or $c; # bug: this is wrong
874 ($a = $b) or $c; # really means this
875 $a = $b || $c; # better written this way
877 However, when it's a list-context assignment and you're trying to use
878 "||" for control flow, you probably need "or" so that the assignment
879 takes higher precedence.
881 @info = stat($file) || die; # oops, scalar sense of stat!
882 @info = stat($file) or die; # better, now @info gets its due
884 Then again, you could always use parentheses.
886 Binary "xor" returns the exclusive-OR of the two surrounding expressions.
887 It cannot short circuit, of course.
889 =head2 C Operators Missing From Perl
890 X<operator, missing from perl> X<&> X<*>
891 X<typecasting> X<(TYPE)>
893 Here is what C has that Perl doesn't:
899 Address-of operator. (But see the "\" operator for taking a reference.)
903 Dereference-address operator. (Perl's prefix dereferencing
904 operators are typed: $, @, %, and &.)
908 Type-casting operator.
912 =head2 Quote and Quote-like Operators
913 X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m>
914 X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>>
915 X<escape sequence> X<escape>
918 While we usually think of quotes as literal values, in Perl they
919 function as operators, providing various kinds of interpolating and
920 pattern matching capabilities. Perl provides customary quote characters
921 for these behaviors, but also provides a way for you to choose your
922 quote character for any of them. In the following table, a C<{}> represents
923 any pair of delimiters you choose.
925 Customary Generic Meaning Interpolates
930 // m{} Pattern match yes*
932 s{}{} Substitution yes*
933 tr{}{} Transliteration no (but see below)
936 * unless the delimiter is ''.
938 Non-bracketing delimiters use the same character fore and aft, but the four
939 sorts of brackets (round, angle, square, curly) will all nest, which means
948 Note, however, that this does not always work for quoting Perl code:
950 $s = q{ if($a eq "}") ... }; # WRONG
952 is a syntax error. The C<Text::Balanced> module (from CPAN, and
953 starting from Perl 5.8 part of the standard distribution) is able
956 There can be whitespace between the operator and the quoting
957 characters, except when C<#> is being used as the quoting character.
958 C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
959 operator C<q> followed by a comment. Its argument will be taken
960 from the next line. This allows you to write:
962 s {foo} # Replace foo
965 The following escape sequences are available in constructs that interpolate
966 and in transliterations.
967 X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N>
974 \a alarm (bell) (BEL)
976 \033 octal char (example: ESC)
977 \x1b hex char (example: ESC)
978 \x{263a} wide hex char (example: SMILEY)
979 \c[ control char (example: ESC)
980 \N{name} named Unicode character
982 The character following C<\c> is mapped to some other character by
983 converting letters to upper case and then (on ASCII systems) by inverting
984 the 7th bit (0x40). The most interesting range is from '@' to '_'
985 (0x40 through 0x5F), resulting in a control character from 0x00
986 through 0x1F. A '?' maps to the DEL character. On EBCDIC systems only
987 '@', the letters, '[', '\', ']', '^', '_' and '?' will work, resulting
988 in 0x00 through 0x1F and 0x7F.
990 B<NOTE>: Unlike C and other languages, Perl has no \v escape sequence for
991 the vertical tab (VT - ASCII 11), but you may use C<\ck> or C<\x0b>.
993 The following escape sequences are available in constructs that interpolate
994 but not in transliterations.
995 X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q>
997 \l lowercase next char
998 \u uppercase next char
1000 \U uppercase till \E
1001 \E end case modification
1002 \Q quote non-word characters till \E
1004 If C<use locale> is in effect, the case map used by C<\l>, C<\L>,
1005 C<\u> and C<\U> is taken from the current locale. See L<perllocale>.
1006 If Unicode (for example, C<\N{}> or wide hex characters of 0x100 or
1007 beyond) is being used, the case map used by C<\l>, C<\L>, C<\u> and
1008 C<\U> is as defined by Unicode. For documentation of C<\N{name}>,
1011 All systems use the virtual C<"\n"> to represent a line terminator,
1012 called a "newline". There is no such thing as an unvarying, physical
1013 newline character. It is only an illusion that the operating system,
1014 device drivers, C libraries, and Perl all conspire to preserve. Not all
1015 systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
1016 on a Mac, these are reversed, and on systems without line terminator,
1017 printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
1018 you mean a "newline" for your system, but use the literal ASCII when you
1019 need an exact character. For example, most networking protocols expect
1020 and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
1021 and although they often accept just C<"\012">, they seldom tolerate just
1022 C<"\015">. If you get in the habit of using C<"\n"> for networking,
1023 you may be burned some day.
1024 X<newline> X<line terminator> X<eol> X<end of line>
1027 For constructs that do interpolate, variables beginning with "C<$>"
1028 or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
1029 C<< $href->{key}[0] >> are also interpolated, as are array and hash slices.
1030 But method calls such as C<< $obj->meth >> are not.
1032 Interpolating an array or slice interpolates the elements in order,
1033 separated by the value of C<$">, so is equivalent to interpolating
1034 C<join $", @array>. "Punctuation" arrays such as C<@*> are only
1035 interpolated if the name is enclosed in braces C<@{*}>, but special
1036 arrays C<@_>, C<@+>, and C<@-> are interpolated, even without braces.
1038 You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
1039 An unescaped C<$> or C<@> interpolates the corresponding variable,
1040 while escaping will cause the literal string C<\$> to be inserted.
1041 You'll need to write something like C<m/\Quser\E\@\Qhost/>.
1043 Patterns are subject to an additional level of interpretation as a
1044 regular expression. This is done as a second pass, after variables are
1045 interpolated, so that regular expressions may be incorporated into the
1046 pattern from the variables. If this is not what you want, use C<\Q> to
1047 interpolate a variable literally.
1049 Apart from the behavior described above, Perl does not expand
1050 multiple levels of interpolation. In particular, contrary to the
1051 expectations of shell programmers, back-quotes do I<NOT> interpolate
1052 within double quotes, nor do single quotes impede evaluation of
1053 variables when used within double quotes.
1055 =head2 Regexp Quote-Like Operators
1058 Here are the quote-like operators that apply to pattern
1059 matching and related activities.
1063 =item qr/STRING/msixpo
1064 X<qr> X</i> X</m> X</o> X</s> X</x> X</p>
1066 This operator quotes (and possibly compiles) its I<STRING> as a regular
1067 expression. I<STRING> is interpolated the same way as I<PATTERN>
1068 in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
1069 is done. Returns a Perl value which may be used instead of the
1070 corresponding C</STRING/msixpo> expression. The returned value is a
1071 normalized version of the original pattern. It magically differs from
1072 a string containing the same characters: C<ref(qr/x/)> returns "Regexp",
1073 even though dereferencing the result returns undef.
1077 $rex = qr/my.STRING/is;
1078 print $rex; # prints (?si-xm:my.STRING)
1085 The result may be used as a subpattern in a match:
1088 $string =~ /foo${re}bar/; # can be interpolated in other patterns
1089 $string =~ $re; # or used standalone
1090 $string =~ /$re/; # or this way
1092 Since Perl may compile the pattern at the moment of execution of qr()
1093 operator, using qr() may have speed advantages in some situations,
1094 notably if the result of qr() is used standalone:
1097 my $patterns = shift;
1098 my @compiled = map qr/$_/i, @$patterns;
1101 foreach my $pat (@compiled) {
1102 $success = 1, last if /$pat/;
1108 Precompilation of the pattern into an internal representation at
1109 the moment of qr() avoids a need to recompile the pattern every
1110 time a match C</$pat/> is attempted. (Perl has many other internal
1111 optimizations, but none would be triggered in the above example if
1112 we did not use qr() operator.)
1116 m Treat string as multiple lines.
1117 s Treat string as single line. (Make . match a newline)
1118 i Do case-insensitive pattern matching.
1119 x Use extended regular expressions.
1120 p When matching preserve a copy of the matched string so
1121 that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be defined.
1122 o Compile pattern only once.
1124 If a precompiled pattern is embedded in a larger pattern then the effect
1125 of 'msixp' will be propagated appropriately. The effect of the 'o'
1126 modifier has is not propagated, being restricted to those patterns
1127 explicitly using it.
1129 See L<perlre> for additional information on valid syntax for STRING, and
1130 for a detailed look at the semantics of regular expressions.
1132 =item m/PATTERN/msixpogc
1133 X<m> X<operator, match>
1134 X<regexp, options> X<regexp> X<regex, options> X<regex>
1135 X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c>
1137 =item /PATTERN/msixpogc
1139 Searches a string for a pattern match, and in scalar context returns
1140 true if it succeeds, false if it fails. If no string is specified
1141 via the C<=~> or C<!~> operator, the $_ string is searched. (The
1142 string specified with C<=~> need not be an lvalue--it may be the
1143 result of an expression evaluation, but remember the C<=~> binds
1144 rather tightly.) See also L<perlre>. See L<perllocale> for
1145 discussion of additional considerations that apply when C<use locale>
1148 Options are as described in C<qr//>; in addition, the following match
1149 process modifiers are available:
1151 g Match globally, i.e., find all occurrences.
1152 c Do not reset search position on a failed match when /g is in effect.
1154 If "/" is the delimiter then the initial C<m> is optional. With the C<m>
1155 you can use any pair of non-whitespace characters
1156 as delimiters. This is particularly useful for matching path names
1157 that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
1158 the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
1159 If "'" is the delimiter, no interpolation is performed on the PATTERN.
1160 When using a character valid in an identifier, whitespace is required
1163 PATTERN may contain variables, which will be interpolated (and the
1164 pattern recompiled) every time the pattern search is evaluated, except
1165 for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
1166 C<$|> are not interpolated because they look like end-of-string tests.)
1167 If you want such a pattern to be compiled only once, add a C</o> after
1168 the trailing delimiter. This avoids expensive run-time recompilations,
1169 and is useful when the value you are interpolating won't change over
1170 the life of the script. However, mentioning C</o> constitutes a promise
1171 that you won't change the variables in the pattern. If you change them,
1172 Perl won't even notice. See also L<"qr/STRING/msixpo">.
1174 =item The empty pattern //
1176 If the PATTERN evaluates to the empty string, the last
1177 I<successfully> matched regular expression is used instead. In this
1178 case, only the C<g> and C<c> flags on the empty pattern is honoured -
1179 the other flags are taken from the original pattern. If no match has
1180 previously succeeded, this will (silently) act instead as a genuine
1181 empty pattern (which will always match).
1183 Note that it's possible to confuse Perl into thinking C<//> (the empty
1184 regex) is really C<//> (the defined-or operator). Perl is usually pretty
1185 good about this, but some pathological cases might trigger this, such as
1186 C<$a///> (is that C<($a) / (//)> or C<$a // />?) and C<print $fh //>
1187 (C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl
1188 will assume you meant defined-or. If you meant the empty regex, just
1189 use parentheses or spaces to disambiguate, or even prefix the empty
1190 regex with an C<m> (so C<//> becomes C<m//>).
1192 =item Matching in list context
1194 If the C</g> option is not used, C<m//> in list context returns a
1195 list consisting of the subexpressions matched by the parentheses in the
1196 pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are
1197 also set, and that this differs from Perl 4's behavior.) When there are
1198 no parentheses in the pattern, the return value is the list C<(1)> for
1199 success. With or without parentheses, an empty list is returned upon
1204 open(TTY, '/dev/tty');
1205 <TTY> =~ /^y/i && foo(); # do foo if desired
1207 if (/Version: *([0-9.]*)/) { $version = $1; }
1209 next if m#^/usr/spool/uucp#;
1214 print if /$arg/o; # compile only once
1217 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
1219 This last example splits $foo into the first two words and the
1220 remainder of the line, and assigns those three fields to $F1, $F2, and
1221 $Etc. The conditional is true if any variables were assigned, i.e., if
1222 the pattern matched.
1224 The C</g> modifier specifies global pattern matching--that is,
1225 matching as many times as possible within the string. How it behaves
1226 depends on the context. In list context, it returns a list of the
1227 substrings matched by any capturing parentheses in the regular
1228 expression. If there are no parentheses, it returns a list of all
1229 the matched strings, as if there were parentheses around the whole
1232 In scalar context, each execution of C<m//g> finds the next match,
1233 returning true if it matches, and false if there is no further match.
1234 The position after the last match can be read or set using the pos()
1235 function; see L<perlfunc/pos>. A failed match normally resets the
1236 search position to the beginning of the string, but you can avoid that
1237 by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
1238 string also resets the search position.
1242 You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
1243 zero-width assertion that matches the exact position where the previous
1244 C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion
1245 still anchors at pos(), but the match is of course only attempted once.
1246 Using C<\G> without C</g> on a target string that has not previously had a
1247 C</g> match applied to it is the same as using the C<\A> assertion to match
1248 the beginning of the string. Note also that, currently, C<\G> is only
1249 properly supported when anchored at the very beginning of the pattern.
1254 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
1258 while (defined($paragraph = <>)) {
1259 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
1263 print "$sentences\n";
1265 # using m//gc with \G
1269 print $1 while /(o)/gc; print "', pos=", pos, "\n";
1271 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
1273 print $1 while /(p)/gc; print "', pos=", pos, "\n";
1275 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
1277 The last example should print:
1287 Notice that the final match matched C<q> instead of C<p>, which a match
1288 without the C<\G> anchor would have done. Also note that the final match
1289 did not update C<pos> -- C<pos> is only updated on a C</g> match. If the
1290 final match did indeed match C<p>, it's a good bet that you're running an
1291 older (pre-5.6.0) Perl.
1293 A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
1294 combine several regexps like this to process a string part-by-part,
1295 doing different actions depending on which regexp matched. Each
1296 regexp tries to match where the previous one leaves off.
1299 $url = URI::URL->new( "http://example.com/" ); die if $url eq "xXx";
1303 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
1304 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
1305 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
1306 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
1307 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
1308 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
1309 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
1310 print ". That's all!\n";
1313 Here is the output (split into several lines):
1315 line-noise lowercase line-noise lowercase UPPERCASE line-noise
1316 UPPERCASE line-noise lowercase line-noise lowercase line-noise
1317 lowercase lowercase line-noise lowercase lowercase line-noise
1318 MiXeD line-noise. That's all!
1323 This is just like the C</pattern/> search, except that it matches only
1324 once between calls to the reset() operator. This is a useful
1325 optimization when you want to see only the first occurrence of
1326 something in each file of a set of files, for instance. Only C<??>
1327 patterns local to the current package are reset.
1331 # blank line between header and body
1334 reset if eof; # clear ?? status for next file
1337 This usage is vaguely deprecated, which means it just might possibly
1338 be removed in some distant future version of Perl, perhaps somewhere
1339 around the year 2168.
1341 =item s/PATTERN/REPLACEMENT/msixpogce
1342 X<substitute> X<substitution> X<replace> X<regexp, replace>
1343 X<regexp, substitute> X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> X</e>
1345 Searches a string for a pattern, and if found, replaces that pattern
1346 with the replacement text and returns the number of substitutions
1347 made. Otherwise it returns false (specifically, the empty string).
1349 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
1350 variable is searched and modified. (The string specified with C<=~> must
1351 be scalar variable, an array element, a hash element, or an assignment
1352 to one of those, i.e., an lvalue.)
1354 If the delimiter chosen is a single quote, no interpolation is
1355 done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1356 PATTERN contains a $ that looks like a variable rather than an
1357 end-of-string test, the variable will be interpolated into the pattern
1358 at run-time. If you want the pattern compiled only once the first time
1359 the variable is interpolated, use the C</o> option. If the pattern
1360 evaluates to the empty string, the last successfully executed regular
1361 expression is used instead. See L<perlre> for further explanation on these.
1362 See L<perllocale> for discussion of additional considerations that apply
1363 when C<use locale> is in effect.
1365 Options are as with m// with the addition of the following replacement
1368 e Evaluate the right side as an expression.
1369 ee Evaluate the right side as a string then eval the result
1371 Any non-whitespace delimiter may replace the slashes. Add space after
1372 the C<s> when using a character allowed in identifiers. If single quotes
1373 are used, no interpretation is done on the replacement string (the C</e>
1374 modifier overrides this, however). Unlike Perl 4, Perl 5 treats backticks
1375 as normal delimiters; the replacement text is not evaluated as a command.
1376 If the PATTERN is delimited by bracketing quotes, the REPLACEMENT has
1377 its own pair of quotes, which may or may not be bracketing quotes, e.g.,
1378 C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
1379 replacement portion to be treated as a full-fledged Perl expression
1380 and evaluated right then and there. It is, however, syntax checked at
1381 compile-time. A second C<e> modifier will cause the replacement portion
1382 to be C<eval>ed before being run as a Perl expression.
1386 s/\bgreen\b/mauve/g; # don't change wintergreen
1388 $path =~ s|/usr/bin|/usr/local/bin|;
1390 s/Login: $foo/Login: $bar/; # run-time pattern
1392 ($foo = $bar) =~ s/this/that/; # copy first, then change
1394 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
1397 s/\d+/$&*2/e; # yields 'abc246xyz'
1398 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
1399 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1401 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
1402 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
1403 s/^=(\w+)/pod($1)/ge; # use function call
1405 # expand variables in $_, but dynamics only, using
1406 # symbolic dereferencing
1409 # Add one to the value of any numbers in the string
1412 # This will expand any embedded scalar variable
1413 # (including lexicals) in $_ : First $1 is interpolated
1414 # to the variable name, and then evaluated
1417 # Delete (most) C comments.
1419 /\* # Match the opening delimiter.
1420 .*? # Match a minimal number of characters.
1421 \*/ # Match the closing delimiter.
1424 s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_, expensively
1426 for ($variable) { # trim whitespace in $variable, cheap
1431 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
1433 Note the use of $ instead of \ in the last example. Unlike
1434 B<sed>, we use the \<I<digit>> form in only the left hand side.
1435 Anywhere else it's $<I<digit>>.
1437 Occasionally, you can't use just a C</g> to get all the changes
1438 to occur that you might want. Here are two common cases:
1440 # put commas in the right places in an integer
1441 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
1443 # expand tabs to 8-column spacing
1444 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
1448 =head2 Quote-Like Operators
1449 X<operator, quote-like>
1454 X<q> X<quote, single> X<'> X<''>
1458 A single-quoted, literal string. A backslash represents a backslash
1459 unless followed by the delimiter or another backslash, in which case
1460 the delimiter or backslash is interpolated.
1462 $foo = q!I said, "You said, 'She said it.'"!;
1463 $bar = q('This is it.');
1464 $baz = '\n'; # a two-character string
1467 X<qq> X<quote, double> X<"> X<"">
1471 A double-quoted, interpolated string.
1474 (*** The previous line contains the naughty word "$1".\n)
1475 if /\b(tcl|java|python)\b/i; # :-)
1476 $baz = "\n"; # a one-character string
1479 X<qx> X<`> X<``> X<backtick>
1483 A string which is (possibly) interpolated and then executed as a
1484 system command with C</bin/sh> or its equivalent. Shell wildcards,
1485 pipes, and redirections will be honored. The collected standard
1486 output of the command is returned; standard error is unaffected. In
1487 scalar context, it comes back as a single (potentially multi-line)
1488 string, or undef if the command failed. In list context, returns a
1489 list of lines (however you've defined lines with $/ or
1490 $INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
1492 Because backticks do not affect standard error, use shell file descriptor
1493 syntax (assuming the shell supports this) if you care to address this.
1494 To capture a command's STDERR and STDOUT together:
1496 $output = `cmd 2>&1`;
1498 To capture a command's STDOUT but discard its STDERR:
1500 $output = `cmd 2>/dev/null`;
1502 To capture a command's STDERR but discard its STDOUT (ordering is
1505 $output = `cmd 2>&1 1>/dev/null`;
1507 To exchange a command's STDOUT and STDERR in order to capture the STDERR
1508 but leave its STDOUT to come out the old STDERR:
1510 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
1512 To read both a command's STDOUT and its STDERR separately, it's easiest
1513 to redirect them separately to files, and then read from those files
1514 when the program is done:
1516 system("program args 1>program.stdout 2>program.stderr");
1518 The STDIN filehandle used by the command is inherited from Perl's STDIN.
1521 open BLAM, "blam" || die "Can't open: $!";
1522 open STDIN, "<&BLAM";
1525 will print the sorted contents of the file "blam".
1527 Using single-quote as a delimiter protects the command from Perl's
1528 double-quote interpolation, passing it on to the shell instead:
1530 $perl_info = qx(ps $$); # that's Perl's $$
1531 $shell_info = qx'ps $$'; # that's the new shell's $$
1533 How that string gets evaluated is entirely subject to the command
1534 interpreter on your system. On most platforms, you will have to protect
1535 shell metacharacters if you want them treated literally. This is in
1536 practice difficult to do, as it's unclear how to escape which characters.
1537 See L<perlsec> for a clean and safe example of a manual fork() and exec()
1538 to emulate backticks safely.
1540 On some platforms (notably DOS-like ones), the shell may not be
1541 capable of dealing with multiline commands, so putting newlines in
1542 the string may not get you what you want. You may be able to evaluate
1543 multiple commands in a single line by separating them with the command
1544 separator character, if your shell supports that (e.g. C<;> on many Unix
1545 shells; C<&> on the Windows NT C<cmd> shell).
1547 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1548 output before starting the child process, but this may not be supported
1549 on some platforms (see L<perlport>). To be safe, you may need to set
1550 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1551 C<IO::Handle> on any open handles.
1553 Beware that some command shells may place restrictions on the length
1554 of the command line. You must ensure your strings don't exceed this
1555 limit after any necessary interpolations. See the platform-specific
1556 release notes for more details about your particular environment.
1558 Using this operator can lead to programs that are difficult to port,
1559 because the shell commands called vary between systems, and may in
1560 fact not be present at all. As one example, the C<type> command under
1561 the POSIX shell is very different from the C<type> command under DOS.
1562 That doesn't mean you should go out of your way to avoid backticks
1563 when they're the right way to get something done. Perl was made to be
1564 a glue language, and one of the things it glues together is commands.
1565 Just understand what you're getting yourself into.
1567 See L</"I/O Operators"> for more discussion.
1570 X<qw> X<quote, list> X<quote, words>
1572 Evaluates to a list of the words extracted out of STRING, using embedded
1573 whitespace as the word delimiters. It can be understood as being roughly
1576 split(' ', q/STRING/);
1578 the differences being that it generates a real list at compile time, and
1579 in scalar context it returns the last element in the list. So
1584 is semantically equivalent to the list:
1588 Some frequently seen examples:
1590 use POSIX qw( setlocale localeconv )
1591 @EXPORT = qw( foo bar baz );
1593 A common mistake is to try to separate the words with comma or to
1594 put comments into a multi-line C<qw>-string. For this reason, the
1595 C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
1596 produces warnings if the STRING contains the "," or the "#" character.
1599 =item tr/SEARCHLIST/REPLACEMENTLIST/cds
1600 X<tr> X<y> X<transliterate> X</c> X</d> X</s>
1602 =item y/SEARCHLIST/REPLACEMENTLIST/cds
1604 Transliterates all occurrences of the characters found in the search list
1605 with the corresponding character in the replacement list. It returns
1606 the number of characters replaced or deleted. If no string is
1607 specified via the =~ or !~ operator, the $_ string is transliterated. (The
1608 string specified with =~ must be a scalar variable, an array element, a
1609 hash element, or an assignment to one of those, i.e., an lvalue.)
1611 A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
1612 does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
1613 For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1614 SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1615 its own pair of quotes, which may or may not be bracketing quotes,
1616 e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
1618 Note that C<tr> does B<not> do regular expression character classes
1619 such as C<\d> or C<[:lower:]>. The C<tr> operator is not equivalent to
1620 the tr(1) utility. If you want to map strings between lower/upper
1621 cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider
1622 using the C<s> operator if you need regular expressions.
1624 Note also that the whole range idea is rather unportable between
1625 character sets--and even within character sets they may cause results
1626 you probably didn't expect. A sound principle is to use only ranges
1627 that begin from and end at either alphabets of equal case (a-e, A-E),
1628 or digits (0-4). Anything else is unsafe. If in doubt, spell out the
1629 character sets in full.
1633 c Complement the SEARCHLIST.
1634 d Delete found but unreplaced characters.
1635 s Squash duplicate replaced characters.
1637 If the C</c> modifier is specified, the SEARCHLIST character set
1638 is complemented. If the C</d> modifier is specified, any characters
1639 specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
1640 (Note that this is slightly more flexible than the behavior of some
1641 B<tr> programs, which delete anything they find in the SEARCHLIST,
1642 period.) If the C</s> modifier is specified, sequences of characters
1643 that were transliterated to the same character are squashed down
1644 to a single instance of the character.
1646 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1647 exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1648 than the SEARCHLIST, the final character is replicated till it is long
1649 enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
1650 This latter is useful for counting characters in a class or for
1651 squashing character sequences in a class.
1655 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1657 $cnt = tr/*/*/; # count the stars in $_
1659 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1661 $cnt = tr/0-9//; # count the digits in $_
1663 tr/a-zA-Z//s; # bookkeeper -> bokeper
1665 ($HOST = $host) =~ tr/a-z/A-Z/;
1667 tr/a-zA-Z/ /cs; # change non-alphas to single space
1670 [\000-\177]; # delete 8th bit
1672 If multiple transliterations are given for a character, only the
1677 will transliterate any A to X.
1679 Because the transliteration table is built at compile time, neither
1680 the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
1681 interpolation. That means that if you want to use variables, you
1684 eval "tr/$oldlist/$newlist/";
1687 eval "tr/$oldlist/$newlist/, 1" or die $@;
1690 X<here-doc> X<heredoc> X<here-document> X<<< << >>>
1692 A line-oriented form of quoting is based on the shell "here-document"
1693 syntax. Following a C<< << >> you specify a string to terminate
1694 the quoted material, and all lines following the current line down to
1695 the terminating string are the value of the item.
1697 The terminating string may be either an identifier (a word), or some
1698 quoted text. An unquoted identifier works like double quotes.
1699 There may not be a space between the C<< << >> and the identifier,
1700 unless the identifier is explicitly quoted. (If you put a space it
1701 will be treated as a null identifier, which is valid, and matches the
1702 first empty line.) The terminating string must appear by itself
1703 (unquoted and with no surrounding whitespace) on the terminating line.
1705 If the terminating string is quoted, the type of quotes used determine
1706 the treatment of the text.
1712 Double quotes indicate that the text will be interpolated using exactly
1713 the same rules as normal double quoted strings.
1716 The price is $Price.
1719 print << "EOF"; # same as above
1720 The price is $Price.
1726 Single quotes indicate the text is to be treated literally with no
1727 interpolation of its content. This is similar to single quoted
1728 strings except that backslashes have no special meaning, with C<\\>
1729 being treated as two backslashes and not one as they would in every
1730 other quoting construct.
1732 This is the only form of quoting in perl where there is no need
1733 to worry about escaping content, something that code generators
1734 can and do make good use of.
1738 The content of the here doc is treated just as it would be if the
1739 string were embedded in backticks. Thus the content is interpolated
1740 as though it were double quoted and then executed via the shell, with
1741 the results of the execution returned.
1743 print << `EOC`; # execute command and get results
1749 It is possible to stack multiple here-docs in a row:
1751 print <<"foo", <<"bar"; # you can stack them
1757 myfunc(<< "THIS", 23, <<'THAT');
1764 Just don't forget that you have to put a semicolon on the end
1765 to finish the statement, as Perl doesn't know you're not going to
1773 If you want to remove the line terminator from your here-docs,
1776 chomp($string = <<'END');
1780 If you want your here-docs to be indented with the rest of the code,
1781 you'll need to remove leading whitespace from each line manually:
1783 ($quote = <<'FINIS') =~ s/^\s+//gm;
1784 The Road goes ever on and on,
1785 down from the door where it began.
1788 If you use a here-doc within a delimited construct, such as in C<s///eg>,
1789 the quoted material must come on the lines following the final delimiter.
1804 If the terminating identifier is on the last line of the program, you
1805 must be sure there is a newline after it; otherwise, Perl will give the
1806 warning B<Can't find string terminator "END" anywhere before EOF...>.
1808 Additionally, the quoting rules for the end of string identifier are not
1809 related to Perl's quoting rules -- C<q()>, C<qq()>, and the like are not
1810 supported in place of C<''> and C<"">, and the only interpolation is for
1811 backslashing the quoting character:
1813 print << "abc\"def";
1817 Finally, quoted strings cannot span multiple lines. The general rule is
1818 that the identifier must be a string literal. Stick with that, and you
1823 =head2 Gory details of parsing quoted constructs
1824 X<quote, gory details>
1826 When presented with something that might have several different
1827 interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
1828 principle to pick the most probable interpretation. This strategy
1829 is so successful that Perl programmers often do not suspect the
1830 ambivalence of what they write. But from time to time, Perl's
1831 notions differ substantially from what the author honestly meant.
1833 This section hopes to clarify how Perl handles quoted constructs.
1834 Although the most common reason to learn this is to unravel labyrinthine
1835 regular expressions, because the initial steps of parsing are the
1836 same for all quoting operators, they are all discussed together.
1838 The most important Perl parsing rule is the first one discussed
1839 below: when processing a quoted construct, Perl first finds the end
1840 of that construct, then interprets its contents. If you understand
1841 this rule, you may skip the rest of this section on the first
1842 reading. The other rules are likely to contradict the user's
1843 expectations much less frequently than this first one.
1845 Some passes discussed below are performed concurrently, but because
1846 their results are the same, we consider them individually. For different
1847 quoting constructs, Perl performs different numbers of passes, from
1848 one to four, but these passes are always performed in the same order.
1852 =item Finding the end
1854 The first pass is finding the end of the quoted construct, where
1855 the information about the delimiters is used in parsing.
1856 During this search, text between the starting and ending delimiters
1857 is copied to a safe location. The text copied gets delimiter-independent.
1859 If the construct is a here-doc, the ending delimiter is a line
1860 that has a terminating string as the content. Therefore C<<<EOF> is
1861 terminated by C<EOF> immediately followed by C<"\n"> and starting
1862 from the first column of the terminating line.
1863 When searching for the terminating line of a here-doc, nothing
1864 is skipped. In other words, lines after the here-doc syntax
1865 are compared with the terminating string line by line.
1867 For the constructs except here-docs, single characters are used as starting
1868 and ending delimiters. If the starting delimiter is an opening punctuation
1869 (that is C<(>, C<[>, C<{>, or C<< < >>), the ending delimiter is the
1870 corresponding closing punctuation (that is C<)>, C<]>, C<}>, or C<< > >>).
1871 If the starting delimiter is an unpaired character like C</> or a closing
1872 punctuation, the ending delimiter is same as the starting delimiter.
1873 Therefore a C</> terminates a C<qq//> construct, while a C<]> terminates
1874 C<qq[]> and C<qq]]> constructs.
1876 When searching for single-character delimiters, escaped delimiters
1877 and C<\\> are skipped. For example, while searching for terminating C</>,
1878 combinations of C<\\> and C<\/> are skipped. If the delimiters are
1879 bracketing, nested pairs are also skipped. For example, while searching
1880 for closing C<]> paired with the opening C<[>, combinations of C<\\>, C<\]>,
1881 and C<\[> are all skipped, and nested C<[> and C<]> are skipped as well.
1882 However, when backslashes are used as the delimiters (like C<qq\\> and
1883 C<tr\\\>), nothing is skipped.
1884 During the search for the end, backslashes that escape delimiters
1885 are removed (exactly speaking, they are not copied to the safe location).
1887 For constructs with three-part delimiters (C<s///>, C<y///>, and
1888 C<tr///>), the search is repeated once more.
1889 If the first delimiter is not an opening punctuation, three delimiters must
1890 be same such as C<s!!!> and C<tr)))>, in which case the second delimiter
1891 terminates the left part and starts the right part at once.
1892 If the left part is delimited by bracketing punctuations (that is C<()>,
1893 C<[]>, C<{}>, or C<< <> >>), the right part needs another pair of
1894 delimiters such as C<s(){}> and C<tr[]//>. In these cases, whitespaces
1895 and comments are allowed between both parts, though the comment must follow
1896 at least one whitespace; otherwise a character expected as the start of
1897 the comment may be regarded as the starting delimiter of the right part.
1899 During this search no attention is paid to the semantics of the construct.
1902 "$hash{"$foo/$bar"}"
1907 bar # NOT a comment, this slash / terminated m//!
1910 do not form legal quoted expressions. The quoted part ends on the
1911 first C<"> and C</>, and the rest happens to be a syntax error.
1912 Because the slash that terminated C<m//> was followed by a C<SPACE>,
1913 the example above is not C<m//x>, but rather C<m//> with no C</x>
1914 modifier. So the embedded C<#> is interpreted as a literal C<#>.
1916 Also no attention is paid to C<\c\> (multichar control char syntax) during
1917 this search. Thus the second C<\> in C<qq/\c\/> is interpreted as a part
1918 of C<\/>, and the following C</> is not recognized as a delimiter.
1919 Instead, use C<\034> or C<\x1c> at the end of quoted constructs.
1924 The next step is interpolation in the text obtained, which is now
1925 delimiter-independent. There are multiple cases.
1931 No interpolation is performed.
1932 Note that the combination C<\\> is left intact, since escaped delimiters
1933 are not available for here-docs.
1935 =item C<m''>, the pattern of C<s'''>
1937 No interpolation is performed at this stage.
1938 Any backslashed sequences including C<\\> are treated at the stage
1939 to L</"parsing regular expressions">.
1941 =item C<''>, C<q//>, C<tr'''>, C<y'''>, the replacement of C<s'''>
1943 The only interpolation is removal of C<\> from pairs of C<\\>.
1944 Therefore C<-> in C<tr'''> and C<y'''> is treated literally
1945 as a hyphen and no character range is available.
1946 C<\1> in the replacement of C<s'''> does not work as C<$1>.
1948 =item C<tr///>, C<y///>
1950 No variable interpolation occurs. String modifying combinations for
1951 case and quoting such as C<\Q>, C<\U>, and C<\E> are not recognized.
1952 The other escape sequences such as C<\200> and C<\t> and backslashed
1953 characters such as C<\\> and C<\-> are converted to appropriate literals.
1954 The character C<-> is treated specially and therefore C<\-> is treated
1957 =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF">
1959 C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
1960 converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
1961 is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
1962 The other escape sequences such as C<\200> and C<\t> and backslashed
1963 characters such as C<\\> and C<\-> are replaced with appropriate
1966 Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
1967 is interpolated in the usual way. Something like C<"\Q\\E"> has
1968 no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
1969 result is the same as for C<"\\\\E">. As a general rule, backslashes
1970 between C<\Q> and C<\E> may lead to counterintuitive results. So,
1971 C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
1972 as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
1977 may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
1979 Interpolated scalars and arrays are converted internally to the C<join> and
1980 C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
1982 $foo . " XXX '" . (join $", @arr) . "'";
1984 All operations above are performed simultaneously, left to right.
1986 Because the result of C<"\Q STRING \E"> has all metacharacters
1987 quoted, there is no way to insert a literal C<$> or C<@> inside a
1988 C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
1989 C<"\\\$">; if not, it is interpreted as the start of an interpolated
1992 Note also that the interpolation code needs to make a decision on
1993 where the interpolated scalar ends. For instance, whether
1994 C<< "a $b -> {c}" >> really means:
1996 "a " . $b . " -> {c}";
2002 Most of the time, the longest possible text that does not include
2003 spaces between components and which contains matching braces or
2004 brackets. because the outcome may be determined by voting based
2005 on heuristic estimators, the result is not strictly predictable.
2006 Fortunately, it's usually correct for ambiguous cases.
2008 =item the replacement of C<s///>
2010 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
2011 happens as with C<qq//> constructs.
2013 It is at this step that C<\1> is begrudgingly converted to C<$1> in
2014 the replacement text of C<s///>, in order to correct the incorrigible
2015 I<sed> hackers who haven't picked up the saner idiom yet. A warning
2016 is emitted if the C<use warnings> pragma or the B<-w> command-line flag
2017 (that is, the C<$^W> variable) was set.
2019 =item C<RE> in C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
2021 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\E>,
2022 and interpolation happens (almost) as with C<qq//> constructs.
2024 However any other combinations of C<\> followed by a character
2025 are not substituted but only skipped, in order to parse them
2026 as regular expressions at the following step.
2027 As C<\c> is skipped at this step, C<@> of C<\c@> in RE is possibly
2028 treated as an array symbol (for example C<@foo>),
2029 even though the same text in C<qq//> gives interpolation of C<\c@>.
2031 Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
2032 a C<#>-comment in a C<//x>-regular expression, no processing is
2033 performed whatsoever. This is the first step at which the presence
2034 of the C<//x> modifier is relevant.
2036 Interpolation in patterns has several quirks: C<$|>, C<$(>, C<$)>, C<@+>
2037 and C<@-> are not interpolated, and constructs C<$var[SOMETHING]> are
2038 voted (by several different estimators) to be either an array element
2039 or C<$var> followed by an RE alternative. This is where the notation
2040 C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
2041 array element C<-9>, not as a regular expression from the variable
2042 C<$arr> followed by a digit, which would be the interpretation of
2043 C</$arr[0-9]/>. Since voting among different estimators may occur,
2044 the result is not predictable.
2046 The lack of processing of C<\\> creates specific restrictions on
2047 the post-processed text. If the delimiter is C</>, one cannot get
2048 the combination C<\/> into the result of this step. C</> will
2049 finish the regular expression, C<\/> will be stripped to C</> on
2050 the previous step, and C<\\/> will be left as is. Because C</> is
2051 equivalent to C<\/> inside a regular expression, this does not
2052 matter unless the delimiter happens to be character special to the
2053 RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
2054 alphanumeric char, as in:
2058 In the RE above, which is intentionally obfuscated for illustration, the
2059 delimiter is C<m>, the modifier is C<mx>, and after delimiter-removal the
2060 RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
2061 reason you're encouraged to restrict your delimiters to non-alphanumeric,
2062 non-whitespace choices.
2066 This step is the last one for all constructs except regular expressions,
2067 which are processed further.
2069 =item parsing regular expressions
2072 Previous steps were performed during the compilation of Perl code,
2073 but this one happens at run time--although it may be optimized to
2074 be calculated at compile time if appropriate. After preprocessing
2075 described above, and possibly after evaluation if concatenation,
2076 joining, casing translation, or metaquoting are involved, the
2077 resulting I<string> is passed to the RE engine for compilation.
2079 Whatever happens in the RE engine might be better discussed in L<perlre>,
2080 but for the sake of continuity, we shall do so here.
2082 This is another step where the presence of the C<//x> modifier is
2083 relevant. The RE engine scans the string from left to right and
2084 converts it to a finite automaton.
2086 Backslashed characters are either replaced with corresponding
2087 literal strings (as with C<\{>), or else they generate special nodes
2088 in the finite automaton (as with C<\b>). Characters special to the
2089 RE engine (such as C<|>) generate corresponding nodes or groups of
2090 nodes. C<(?#...)> comments are ignored. All the rest is either
2091 converted to literal strings to match, or else is ignored (as is
2092 whitespace and C<#>-style comments if C<//x> is present).
2094 Parsing of the bracketed character class construct, C<[...]>, is
2095 rather different than the rule used for the rest of the pattern.
2096 The terminator of this construct is found using the same rules as
2097 for finding the terminator of a C<{}>-delimited construct, the only
2098 exception being that C<]> immediately following C<[> is treated as
2099 though preceded by a backslash. Similarly, the terminator of
2100 C<(?{...})> is found using the same rules as for finding the
2101 terminator of a C<{}>-delimited construct.
2103 It is possible to inspect both the string given to RE engine and the
2104 resulting finite automaton. See the arguments C<debug>/C<debugcolor>
2105 in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
2106 switch documented in L<perlrun/"Command Switches">.
2108 =item Optimization of regular expressions
2109 X<regexp, optimization>
2111 This step is listed for completeness only. Since it does not change
2112 semantics, details of this step are not documented and are subject
2113 to change without notice. This step is performed over the finite
2114 automaton that was generated during the previous pass.
2116 It is at this stage that C<split()> silently optimizes C</^/> to
2121 =head2 I/O Operators
2122 X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle>
2125 There are several I/O operators you should know about.
2127 A string enclosed by backticks (grave accents) first undergoes
2128 double-quote interpolation. It is then interpreted as an external
2129 command, and the output of that command is the value of the
2130 backtick string, like in a shell. In scalar context, a single string
2131 consisting of all output is returned. In list context, a list of
2132 values is returned, one per line of output. (You can set C<$/> to use
2133 a different line terminator.) The command is executed each time the
2134 pseudo-literal is evaluated. The status value of the command is
2135 returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
2136 Unlike in B<csh>, no translation is done on the return data--newlines
2137 remain newlines. Unlike in any of the shells, single quotes do not
2138 hide variable names in the command from interpretation. To pass a
2139 literal dollar-sign through to the shell you need to hide it with a
2140 backslash. The generalized form of backticks is C<qx//>. (Because
2141 backticks always undergo shell expansion as well, see L<perlsec> for
2143 X<qx> X<`> X<``> X<backtick> X<glob>
2145 In scalar context, evaluating a filehandle in angle brackets yields
2146 the next line from that file (the newline, if any, included), or
2147 C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
2148 (sometimes known as file-slurp mode) and the file is empty, it
2149 returns C<''> the first time, followed by C<undef> subsequently.
2151 Ordinarily you must assign the returned value to a variable, but
2152 there is one situation where an automatic assignment happens. If
2153 and only if the input symbol is the only thing inside the conditional
2154 of a C<while> statement (even if disguised as a C<for(;;)> loop),
2155 the value is automatically assigned to the global variable $_,
2156 destroying whatever was there previously. (This may seem like an
2157 odd thing to you, but you'll use the construct in almost every Perl
2158 script you write.) The $_ variable is not implicitly localized.
2159 You'll have to put a C<local $_;> before the loop if you want that
2162 The following lines are equivalent:
2164 while (defined($_ = <STDIN>)) { print; }
2165 while ($_ = <STDIN>) { print; }
2166 while (<STDIN>) { print; }
2167 for (;<STDIN>;) { print; }
2168 print while defined($_ = <STDIN>);
2169 print while ($_ = <STDIN>);
2170 print while <STDIN>;
2172 This also behaves similarly, but avoids $_ :
2174 while (my $line = <STDIN>) { print $line }
2176 In these loop constructs, the assigned value (whether assignment
2177 is automatic or explicit) is then tested to see whether it is
2178 defined. The defined test avoids problems where line has a string
2179 value that would be treated as false by Perl, for example a "" or
2180 a "0" with no trailing newline. If you really mean for such values
2181 to terminate the loop, they should be tested for explicitly:
2183 while (($_ = <STDIN>) ne '0') { ... }
2184 while (<STDIN>) { last unless $_; ... }
2186 In other boolean contexts, I<C<E<lt>filehandleE<gt>>> without an
2187 explicit C<defined> test or comparison elicit a warning if the
2188 C<use warnings> pragma or the B<-w>
2189 command-line switch (the C<$^W> variable) is in effect.
2191 The filehandles STDIN, STDOUT, and STDERR are predefined. (The
2192 filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
2193 in packages, where they would be interpreted as local identifiers
2194 rather than global.) Additional filehandles may be created with
2195 the open() function, amongst others. See L<perlopentut> and
2196 L<perlfunc/open> for details on this.
2197 X<stdin> X<stdout> X<sterr>
2199 If a <FILEHANDLE> is used in a context that is looking for
2200 a list, a list comprising all input lines is returned, one line per
2201 list element. It's easy to grow to a rather large data space this
2202 way, so use with care.
2204 <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
2205 See L<perlfunc/readline>.
2207 The null filehandle <> is special: it can be used to emulate the
2208 behavior of B<sed> and B<awk>. Input from <> comes either from
2209 standard input, or from each file listed on the command line. Here's
2210 how it works: the first time <> is evaluated, the @ARGV array is
2211 checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
2212 gives you standard input. The @ARGV array is then processed as a list
2213 of filenames. The loop
2216 ... # code for each line
2219 is equivalent to the following Perl-like pseudo code:
2221 unshift(@ARGV, '-') unless @ARGV;
2222 while ($ARGV = shift) {
2225 ... # code for each line
2229 except that it isn't so cumbersome to say, and will actually work.
2230 It really does shift the @ARGV array and put the current filename
2231 into the $ARGV variable. It also uses filehandle I<ARGV>
2232 internally--<> is just a synonym for <ARGV>, which
2233 is magical. (The pseudo code above doesn't work because it treats
2234 <ARGV> as non-magical.)
2236 Since the null filehandle uses the two argument form of L<perlfunc/open>
2237 it interprets special characters, so if you have a script like this:
2243 and call it with C<perl dangerous.pl 'rm -rfv *|'>, it actually opens a
2244 pipe, executes the C<rm> command and reads C<rm>'s output from that pipe.
2245 If you want all items in C<@ARGV> to be interpreted as file names, you
2246 can use the module C<ARGV::readonly> from CPAN.
2248 You can modify @ARGV before the first <> as long as the array ends up
2249 containing the list of filenames you really want. Line numbers (C<$.>)
2250 continue as though the input were one big happy file. See the example
2251 in L<perlfunc/eof> for how to reset line numbers on each file.
2253 If you want to set @ARGV to your own list of files, go right ahead.
2254 This sets @ARGV to all plain text files if no @ARGV was given:
2256 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
2258 You can even set them to pipe commands. For example, this automatically
2259 filters compressed arguments through B<gzip>:
2261 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
2263 If you want to pass switches into your script, you can use one of the
2264 Getopts modules or put a loop on the front like this:
2266 while ($_ = $ARGV[0], /^-/) {
2269 if (/^-D(.*)/) { $debug = $1 }
2270 if (/^-v/) { $verbose++ }
2271 # ... # other switches
2275 # ... # code for each line
2278 The <> symbol will return C<undef> for end-of-file only once.
2279 If you call it again after this, it will assume you are processing another
2280 @ARGV list, and if you haven't set @ARGV, will read input from STDIN.
2282 If what the angle brackets contain is a simple scalar variable (e.g.,
2283 <$foo>), then that variable contains the name of the
2284 filehandle to input from, or its typeglob, or a reference to the
2290 If what's within the angle brackets is neither a filehandle nor a simple
2291 scalar variable containing a filehandle name, typeglob, or typeglob
2292 reference, it is interpreted as a filename pattern to be globbed, and
2293 either a list of filenames or the next filename in the list is returned,
2294 depending on context. This distinction is determined on syntactic
2295 grounds alone. That means C<< <$x> >> is always a readline() from
2296 an indirect handle, but C<< <$hash{key}> >> is always a glob().
2297 That's because $x is a simple scalar variable, but C<$hash{key}> is
2298 not--it's a hash element. Even C<< <$x > >> (note the extra space)
2299 is treated as C<glob("$x ")>, not C<readline($x)>.
2301 One level of double-quote interpretation is done first, but you can't
2302 say C<< <$foo> >> because that's an indirect filehandle as explained
2303 in the previous paragraph. (In older versions of Perl, programmers
2304 would insert curly brackets to force interpretation as a filename glob:
2305 C<< <${foo}> >>. These days, it's considered cleaner to call the
2306 internal function directly as C<glob($foo)>, which is probably the right
2307 way to have done it in the first place.) For example:
2313 is roughly equivalent to:
2315 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
2321 except that the globbing is actually done internally using the standard
2322 C<File::Glob> extension. Of course, the shortest way to do the above is:
2326 A (file)glob evaluates its (embedded) argument only when it is
2327 starting a new list. All values must be read before it will start
2328 over. In list context, this isn't important because you automatically
2329 get them all anyway. However, in scalar context the operator returns
2330 the next value each time it's called, or C<undef> when the list has
2331 run out. As with filehandle reads, an automatic C<defined> is
2332 generated when the glob occurs in the test part of a C<while>,
2333 because legal glob returns (e.g. a file called F<0>) would otherwise
2334 terminate the loop. Again, C<undef> is returned only once. So if
2335 you're expecting a single value from a glob, it is much better to
2338 ($file) = <blurch*>;
2344 because the latter will alternate between returning a filename and
2347 If you're trying to do variable interpolation, it's definitely better
2348 to use the glob() function, because the older notation can cause people
2349 to become confused with the indirect filehandle notation.
2351 @files = glob("$dir/*.[ch]");
2352 @files = glob($files[$i]);
2354 =head2 Constant Folding
2355 X<constant folding> X<folding>
2357 Like C, Perl does a certain amount of expression evaluation at
2358 compile time whenever it determines that all arguments to an
2359 operator are static and have no side effects. In particular, string
2360 concatenation happens at compile time between literals that don't do
2361 variable substitution. Backslash interpolation also happens at
2362 compile time. You can say
2364 'Now is the time for all' . "\n" .
2365 'good men to come to.'
2367 and this all reduces to one string internally. Likewise, if
2370 foreach $file (@filenames) {
2371 if (-s $file > 5 + 100 * 2**16) { }
2374 the compiler will precompute the number which that expression
2375 represents so that the interpreter won't have to.
2380 Perl doesn't officially have a no-op operator, but the bare constants
2381 C<0> and C<1> are special-cased to not produce a warning in a void
2382 context, so you can for example safely do
2386 =head2 Bitwise String Operators
2387 X<operator, bitwise, string>
2389 Bitstrings of any size may be manipulated by the bitwise operators
2392 If the operands to a binary bitwise op are strings of different
2393 sizes, B<|> and B<^> ops act as though the shorter operand had
2394 additional zero bits on the right, while the B<&> op acts as though
2395 the longer operand were truncated to the length of the shorter.
2396 The granularity for such extension or truncation is one or more
2399 # ASCII-based examples
2400 print "j p \n" ^ " a h"; # prints "JAPH\n"
2401 print "JA" | " ph\n"; # prints "japh\n"
2402 print "japh\nJunk" & '_____'; # prints "JAPH\n";
2403 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
2405 If you are intending to manipulate bitstrings, be certain that
2406 you're supplying bitstrings: If an operand is a number, that will imply
2407 a B<numeric> bitwise operation. You may explicitly show which type of
2408 operation you intend by using C<""> or C<0+>, as in the examples below.
2410 $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
2411 $foo = '150' | 105; # yields 255
2412 $foo = 150 | '105'; # yields 255
2413 $foo = '150' | '105'; # yields string '155' (under ASCII)
2415 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
2416 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
2418 See L<perlfunc/vec> for information on how to manipulate individual bits
2421 =head2 Integer Arithmetic
2424 By default, Perl assumes that it must do most of its arithmetic in
2425 floating point. But by saying
2429 you may tell the compiler that it's okay to use integer operations
2430 (if it feels like it) from here to the end of the enclosing BLOCK.
2431 An inner BLOCK may countermand this by saying
2435 which lasts until the end of that BLOCK. Note that this doesn't
2436 mean everything is only an integer, merely that Perl may use integer
2437 operations if it is so inclined. For example, even under C<use
2438 integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731>
2441 Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
2442 and ">>") always produce integral results. (But see also
2443 L<Bitwise String Operators>.) However, C<use integer> still has meaning for
2444 them. By default, their results are interpreted as unsigned integers, but
2445 if C<use integer> is in effect, their results are interpreted
2446 as signed integers. For example, C<~0> usually evaluates to a large
2447 integral value. However, C<use integer; ~0> is C<-1> on two's-complement
2450 =head2 Floating-point Arithmetic
2451 X<floating-point> X<floating point> X<float> X<real>
2453 While C<use integer> provides integer-only arithmetic, there is no
2454 analogous mechanism to provide automatic rounding or truncation to a
2455 certain number of decimal places. For rounding to a certain number
2456 of digits, sprintf() or printf() is usually the easiest route.
2459 Floating-point numbers are only approximations to what a mathematician
2460 would call real numbers. There are infinitely more reals than floats,
2461 so some corners must be cut. For example:
2463 printf "%.20g\n", 123456789123456789;
2464 # produces 123456789123456784
2466 Testing for exact equality of floating-point equality or inequality is
2467 not a good idea. Here's a (relatively expensive) work-around to compare
2468 whether two floating-point numbers are equal to a particular number of
2469 decimal places. See Knuth, volume II, for a more robust treatment of
2473 my ($X, $Y, $POINTS) = @_;
2475 $tX = sprintf("%.${POINTS}g", $X);
2476 $tY = sprintf("%.${POINTS}g", $Y);
2480 The POSIX module (part of the standard perl distribution) implements
2481 ceil(), floor(), and other mathematical and trigonometric functions.
2482 The Math::Complex module (part of the standard perl distribution)
2483 defines mathematical functions that work on both the reals and the
2484 imaginary numbers. Math::Complex not as efficient as POSIX, but
2485 POSIX can't work with complex numbers.
2487 Rounding in financial applications can have serious implications, and
2488 the rounding method used should be specified precisely. In these
2489 cases, it probably pays not to trust whichever system rounding is
2490 being used by Perl, but to instead implement the rounding function you
2493 =head2 Bigger Numbers
2494 X<number, arbitrary precision>
2496 The standard Math::BigInt and Math::BigFloat modules provide
2497 variable-precision arithmetic and overloaded operators, although
2498 they're currently pretty slow. At the cost of some space and
2499 considerable speed, they avoid the normal pitfalls associated with
2500 limited-precision representations.
2503 $x = Math::BigInt->new('123456789123456789');
2506 # prints +15241578780673678515622620750190521
2508 There are several modules that let you calculate with (bound only by
2509 memory and cpu-time) unlimited or fixed precision. There are also
2510 some non-standard modules that provide faster implementations via
2511 external C libraries.
2513 Here is a short, but incomplete summary:
2515 Math::Fraction big, unlimited fractions like 9973 / 12967
2516 Math::String treat string sequences like numbers
2517 Math::FixedPrecision calculate with a fixed precision
2518 Math::Currency for currency calculations
2519 Bit::Vector manipulate bit vectors fast (uses C)
2520 Math::BigIntFast Bit::Vector wrapper for big numbers
2521 Math::Pari provides access to the Pari C library
2522 Math::BigInteger uses an external C library
2523 Math::Cephes uses external Cephes C library (no big numbers)
2524 Math::Cephes::Fraction fractions via the Cephes library
2525 Math::GMP another one using an external C library