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 .. $. == 200) ...
607 next LINE if (1 .. /^$/); # skip header lines, short for
608 # ... 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 Operators
817 X<...> X<... operator> X<!!!> X<!!! operator> X<???> X<??? operator>
818 X<yada yada operator>
820 The yada yada operators are placeholders for code. They parse without error,
821 but when executed either throw an exception or a warning.
823 The C<...> operator takes no arguments. When executed, it throws an exception
824 with the text C<Unimplemented>:
829 Unimplemented at <file> line <line number>.
831 The C<!!!> operator is similar, but it takes one argument, a string to use as
832 the text of the exception:
834 sub bar { !!! "Don't call me, Ishmael!" }
837 Don't call me, Ishmael! at <file> line <line number>.
839 The C<???> operator also takes one argument, but it emits a warning instead of
840 throwing an exception:
842 sub baz { ??? "Who are you? What do you want?" }
844 say "Why are you here?";
846 Who are you? What do you want? at <file> line <line number>.
849 =head2 List Operators (Rightward)
850 X<operator, list, rightward> X<list operator>
852 On the right side of a list operator, it has very low precedence,
853 such that it controls all comma-separated expressions found there.
854 The only operators with lower precedence are the logical operators
855 "and", "or", and "not", which may be used to evaluate calls to list
856 operators without the need for extra parentheses:
858 open HANDLE, "filename"
859 or die "Can't open: $!\n";
861 See also discussion of list operators in L<Terms and List Operators (Leftward)>.
864 X<operator, logical, not> X<not>
866 Unary "not" returns the logical negation of the expression to its right.
867 It's the equivalent of "!" except for the very low precedence.
870 X<operator, logical, and> X<and>
872 Binary "and" returns the logical conjunction of the two surrounding
873 expressions. It's equivalent to && except for the very low
874 precedence. This means that it short-circuits: i.e., the right
875 expression is evaluated only if the left expression is true.
877 =head2 Logical or, Defined or, and Exclusive Or
878 X<operator, logical, or> X<operator, logical, xor>
879 X<operator, logical, defined or> X<operator, logical, exclusive or>
882 Binary "or" returns the logical disjunction of the two surrounding
883 expressions. It's equivalent to || except for the very low precedence.
884 This makes it useful for control flow
886 print FH $data or die "Can't write to FH: $!";
888 This means that it short-circuits: i.e., the right expression is evaluated
889 only if the left expression is false. Due to its precedence, you should
890 probably avoid using this for assignment, only for control flow.
892 $a = $b or $c; # bug: this is wrong
893 ($a = $b) or $c; # really means this
894 $a = $b || $c; # better written this way
896 However, when it's a list-context assignment and you're trying to use
897 "||" for control flow, you probably need "or" so that the assignment
898 takes higher precedence.
900 @info = stat($file) || die; # oops, scalar sense of stat!
901 @info = stat($file) or die; # better, now @info gets its due
903 Then again, you could always use parentheses.
905 Binary "xor" returns the exclusive-OR of the two surrounding expressions.
906 It cannot short circuit, of course.
908 =head2 C Operators Missing From Perl
909 X<operator, missing from perl> X<&> X<*>
910 X<typecasting> X<(TYPE)>
912 Here is what C has that Perl doesn't:
918 Address-of operator. (But see the "\" operator for taking a reference.)
922 Dereference-address operator. (Perl's prefix dereferencing
923 operators are typed: $, @, %, and &.)
927 Type-casting operator.
931 =head2 Quote and Quote-like Operators
932 X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m>
933 X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>>
934 X<escape sequence> X<escape>
937 While we usually think of quotes as literal values, in Perl they
938 function as operators, providing various kinds of interpolating and
939 pattern matching capabilities. Perl provides customary quote characters
940 for these behaviors, but also provides a way for you to choose your
941 quote character for any of them. In the following table, a C<{}> represents
942 any pair of delimiters you choose.
944 Customary Generic Meaning Interpolates
949 // m{} Pattern match yes*
951 s{}{} Substitution yes*
952 tr{}{} Transliteration no (but see below)
955 * unless the delimiter is ''.
957 Non-bracketing delimiters use the same character fore and aft, but the four
958 sorts of brackets (round, angle, square, curly) will all nest, which means
967 Note, however, that this does not always work for quoting Perl code:
969 $s = q{ if($a eq "}") ... }; # WRONG
971 is a syntax error. The C<Text::Balanced> module (from CPAN, and
972 starting from Perl 5.8 part of the standard distribution) is able
975 There can be whitespace between the operator and the quoting
976 characters, except when C<#> is being used as the quoting character.
977 C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
978 operator C<q> followed by a comment. Its argument will be taken
979 from the next line. This allows you to write:
981 s {foo} # Replace foo
984 The following escape sequences are available in constructs that interpolate
985 and in transliterations.
986 X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N>
993 \a alarm (bell) (BEL)
995 \033 octal char (example: ESC)
996 \x1b hex char (example: ESC)
997 \x{263a} wide hex char (example: SMILEY)
998 \c[ control char (example: ESC)
999 \N{name} named Unicode character
1001 The character following C<\c> is mapped to some other character by
1002 converting letters to upper case and then (on ASCII systems) by inverting
1003 the 7th bit (0x40). The most interesting range is from '@' to '_'
1004 (0x40 through 0x5F), resulting in a control character from 0x00
1005 through 0x1F. A '?' maps to the DEL character. On EBCDIC systems only
1006 '@', the letters, '[', '\', ']', '^', '_' and '?' will work, resulting
1007 in 0x00 through 0x1F and 0x7F.
1009 B<NOTE>: Unlike C and other languages, Perl has no \v escape sequence for
1010 the vertical tab (VT - ASCII 11), but you may use C<\ck> or C<\x0b>.
1012 The following escape sequences are available in constructs that interpolate
1013 but not in transliterations.
1014 X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q>
1016 \l lowercase next char
1017 \u uppercase next char
1018 \L lowercase till \E
1019 \U uppercase till \E
1020 \E end case modification
1021 \Q quote non-word characters till \E
1023 If C<use locale> is in effect, the case map used by C<\l>, C<\L>,
1024 C<\u> and C<\U> is taken from the current locale. See L<perllocale>.
1025 If Unicode (for example, C<\N{}> or wide hex characters of 0x100 or
1026 beyond) is being used, the case map used by C<\l>, C<\L>, C<\u> and
1027 C<\U> is as defined by Unicode. For documentation of C<\N{name}>,
1030 All systems use the virtual C<"\n"> to represent a line terminator,
1031 called a "newline". There is no such thing as an unvarying, physical
1032 newline character. It is only an illusion that the operating system,
1033 device drivers, C libraries, and Perl all conspire to preserve. Not all
1034 systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
1035 on a Mac, these are reversed, and on systems without line terminator,
1036 printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
1037 you mean a "newline" for your system, but use the literal ASCII when you
1038 need an exact character. For example, most networking protocols expect
1039 and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
1040 and although they often accept just C<"\012">, they seldom tolerate just
1041 C<"\015">. If you get in the habit of using C<"\n"> for networking,
1042 you may be burned some day.
1043 X<newline> X<line terminator> X<eol> X<end of line>
1046 For constructs that do interpolate, variables beginning with "C<$>"
1047 or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
1048 C<< $href->{key}[0] >> are also interpolated, as are array and hash slices.
1049 But method calls such as C<< $obj->meth >> are not.
1051 Interpolating an array or slice interpolates the elements in order,
1052 separated by the value of C<$">, so is equivalent to interpolating
1053 C<join $", @array>. "Punctuation" arrays such as C<@*> are only
1054 interpolated if the name is enclosed in braces C<@{*}>, but special
1055 arrays C<@_>, C<@+>, and C<@-> are interpolated, even without braces.
1057 You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
1058 An unescaped C<$> or C<@> interpolates the corresponding variable,
1059 while escaping will cause the literal string C<\$> to be inserted.
1060 You'll need to write something like C<m/\Quser\E\@\Qhost/>.
1062 Patterns are subject to an additional level of interpretation as a
1063 regular expression. This is done as a second pass, after variables are
1064 interpolated, so that regular expressions may be incorporated into the
1065 pattern from the variables. If this is not what you want, use C<\Q> to
1066 interpolate a variable literally.
1068 Apart from the behavior described above, Perl does not expand
1069 multiple levels of interpolation. In particular, contrary to the
1070 expectations of shell programmers, back-quotes do I<NOT> interpolate
1071 within double quotes, nor do single quotes impede evaluation of
1072 variables when used within double quotes.
1074 =head2 Regexp Quote-Like Operators
1077 Here are the quote-like operators that apply to pattern
1078 matching and related activities.
1082 =item qr/STRING/msixpo
1083 X<qr> X</i> X</m> X</o> X</s> X</x> X</p>
1085 This operator quotes (and possibly compiles) its I<STRING> as a regular
1086 expression. I<STRING> is interpolated the same way as I<PATTERN>
1087 in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
1088 is done. Returns a Perl value which may be used instead of the
1089 corresponding C</STRING/msixpo> expression. The returned value is a
1090 normalized version of the original pattern. It magically differs from
1091 a string containing the same characters: C<ref(qr/x/)> returns "Regexp",
1092 even though dereferencing the result returns undef.
1096 $rex = qr/my.STRING/is;
1097 print $rex; # prints (?si-xm:my.STRING)
1104 The result may be used as a subpattern in a match:
1107 $string =~ /foo${re}bar/; # can be interpolated in other patterns
1108 $string =~ $re; # or used standalone
1109 $string =~ /$re/; # or this way
1111 Since Perl may compile the pattern at the moment of execution of qr()
1112 operator, using qr() may have speed advantages in some situations,
1113 notably if the result of qr() is used standalone:
1116 my $patterns = shift;
1117 my @compiled = map qr/$_/i, @$patterns;
1120 foreach my $pat (@compiled) {
1121 $success = 1, last if /$pat/;
1127 Precompilation of the pattern into an internal representation at
1128 the moment of qr() avoids a need to recompile the pattern every
1129 time a match C</$pat/> is attempted. (Perl has many other internal
1130 optimizations, but none would be triggered in the above example if
1131 we did not use qr() operator.)
1135 m Treat string as multiple lines.
1136 s Treat string as single line. (Make . match a newline)
1137 i Do case-insensitive pattern matching.
1138 x Use extended regular expressions.
1139 p When matching preserve a copy of the matched string so
1140 that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be defined.
1141 o Compile pattern only once.
1143 If a precompiled pattern is embedded in a larger pattern then the effect
1144 of 'msixp' will be propagated appropriately. The effect of the 'o'
1145 modifier has is not propagated, being restricted to those patterns
1146 explicitly using it.
1148 See L<perlre> for additional information on valid syntax for STRING, and
1149 for a detailed look at the semantics of regular expressions.
1151 =item m/PATTERN/msixpogc
1152 X<m> X<operator, match>
1153 X<regexp, options> X<regexp> X<regex, options> X<regex>
1154 X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c>
1156 =item /PATTERN/msixpogc
1158 Searches a string for a pattern match, and in scalar context returns
1159 true if it succeeds, false if it fails. If no string is specified
1160 via the C<=~> or C<!~> operator, the $_ string is searched. (The
1161 string specified with C<=~> need not be an lvalue--it may be the
1162 result of an expression evaluation, but remember the C<=~> binds
1163 rather tightly.) See also L<perlre>. See L<perllocale> for
1164 discussion of additional considerations that apply when C<use locale>
1167 Options are as described in C<qr//>; in addition, the following match
1168 process modifiers are available:
1170 g Match globally, i.e., find all occurrences.
1171 c Do not reset search position on a failed match when /g is in effect.
1173 If "/" is the delimiter then the initial C<m> is optional. With the C<m>
1174 you can use any pair of non-alphanumeric, non-whitespace characters
1175 as delimiters. This is particularly useful for matching path names
1176 that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
1177 the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
1178 If "'" is the delimiter, no interpolation is performed on the PATTERN.
1180 PATTERN may contain variables, which will be interpolated (and the
1181 pattern recompiled) every time the pattern search is evaluated, except
1182 for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
1183 C<$|> are not interpolated because they look like end-of-string tests.)
1184 If you want such a pattern to be compiled only once, add a C</o> after
1185 the trailing delimiter. This avoids expensive run-time recompilations,
1186 and is useful when the value you are interpolating won't change over
1187 the life of the script. However, mentioning C</o> constitutes a promise
1188 that you won't change the variables in the pattern. If you change them,
1189 Perl won't even notice. See also L<"qr/STRING/msixpo">.
1191 =item The empty pattern //
1193 If the PATTERN evaluates to the empty string, the last
1194 I<successfully> matched regular expression is used instead. In this
1195 case, only the C<g> and C<c> flags on the empty pattern is honoured -
1196 the other flags are taken from the original pattern. If no match has
1197 previously succeeded, this will (silently) act instead as a genuine
1198 empty pattern (which will always match).
1200 Note that it's possible to confuse Perl into thinking C<//> (the empty
1201 regex) is really C<//> (the defined-or operator). Perl is usually pretty
1202 good about this, but some pathological cases might trigger this, such as
1203 C<$a///> (is that C<($a) / (//)> or C<$a // />?) and C<print $fh //>
1204 (C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl
1205 will assume you meant defined-or. If you meant the empty regex, just
1206 use parentheses or spaces to disambiguate, or even prefix the empty
1207 regex with an C<m> (so C<//> becomes C<m//>).
1209 =item Matching in list context
1211 If the C</g> option is not used, C<m//> in list context returns a
1212 list consisting of the subexpressions matched by the parentheses in the
1213 pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are
1214 also set, and that this differs from Perl 4's behavior.) When there are
1215 no parentheses in the pattern, the return value is the list C<(1)> for
1216 success. With or without parentheses, an empty list is returned upon
1221 open(TTY, '/dev/tty');
1222 <TTY> =~ /^y/i && foo(); # do foo if desired
1224 if (/Version: *([0-9.]*)/) { $version = $1; }
1226 next if m#^/usr/spool/uucp#;
1231 print if /$arg/o; # compile only once
1234 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
1236 This last example splits $foo into the first two words and the
1237 remainder of the line, and assigns those three fields to $F1, $F2, and
1238 $Etc. The conditional is true if any variables were assigned, i.e., if
1239 the pattern matched.
1241 The C</g> modifier specifies global pattern matching--that is,
1242 matching as many times as possible within the string. How it behaves
1243 depends on the context. In list context, it returns a list of the
1244 substrings matched by any capturing parentheses in the regular
1245 expression. If there are no parentheses, it returns a list of all
1246 the matched strings, as if there were parentheses around the whole
1249 In scalar context, each execution of C<m//g> finds the next match,
1250 returning true if it matches, and false if there is no further match.
1251 The position after the last match can be read or set using the pos()
1252 function; see L<perlfunc/pos>. A failed match normally resets the
1253 search position to the beginning of the string, but you can avoid that
1254 by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
1255 string also resets the search position.
1259 You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
1260 zero-width assertion that matches the exact position where the previous
1261 C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion
1262 still anchors at pos(), but the match is of course only attempted once.
1263 Using C<\G> without C</g> on a target string that has not previously had a
1264 C</g> match applied to it is the same as using the C<\A> assertion to match
1265 the beginning of the string. Note also that, currently, C<\G> is only
1266 properly supported when anchored at the very beginning of the pattern.
1271 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
1275 while (defined($paragraph = <>)) {
1276 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
1280 print "$sentences\n";
1282 # using m//gc with \G
1286 print $1 while /(o)/gc; print "', pos=", pos, "\n";
1288 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
1290 print $1 while /(p)/gc; print "', pos=", pos, "\n";
1292 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
1294 The last example should print:
1304 Notice that the final match matched C<q> instead of C<p>, which a match
1305 without the C<\G> anchor would have done. Also note that the final match
1306 did not update C<pos> -- C<pos> is only updated on a C</g> match. If the
1307 final match did indeed match C<p>, it's a good bet that you're running an
1308 older (pre-5.6.0) Perl.
1310 A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
1311 combine several regexps like this to process a string part-by-part,
1312 doing different actions depending on which regexp matched. Each
1313 regexp tries to match where the previous one leaves off.
1316 $url = URI::URL->new( "http://www/" ); die if $url eq "xXx";
1320 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
1321 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
1322 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
1323 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
1324 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
1325 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
1326 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
1327 print ". That's all!\n";
1330 Here is the output (split into several lines):
1332 line-noise lowercase line-noise lowercase UPPERCASE line-noise
1333 UPPERCASE line-noise lowercase line-noise lowercase line-noise
1334 lowercase lowercase line-noise lowercase lowercase line-noise
1335 MiXeD line-noise. That's all!
1340 This is just like the C</pattern/> search, except that it matches only
1341 once between calls to the reset() operator. This is a useful
1342 optimization when you want to see only the first occurrence of
1343 something in each file of a set of files, for instance. Only C<??>
1344 patterns local to the current package are reset.
1348 # blank line between header and body
1351 reset if eof; # clear ?? status for next file
1354 This usage is vaguely deprecated, which means it just might possibly
1355 be removed in some distant future version of Perl, perhaps somewhere
1356 around the year 2168.
1358 =item s/PATTERN/REPLACEMENT/msixpogce
1359 X<substitute> X<substitution> X<replace> X<regexp, replace>
1360 X<regexp, substitute> X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> X</e>
1362 Searches a string for a pattern, and if found, replaces that pattern
1363 with the replacement text and returns the number of substitutions
1364 made. Otherwise it returns false (specifically, the empty string).
1366 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
1367 variable is searched and modified. (The string specified with C<=~> must
1368 be scalar variable, an array element, a hash element, or an assignment
1369 to one of those, i.e., an lvalue.)
1371 If the delimiter chosen is a single quote, no interpolation is
1372 done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1373 PATTERN contains a $ that looks like a variable rather than an
1374 end-of-string test, the variable will be interpolated into the pattern
1375 at run-time. If you want the pattern compiled only once the first time
1376 the variable is interpolated, use the C</o> option. If the pattern
1377 evaluates to the empty string, the last successfully executed regular
1378 expression is used instead. See L<perlre> for further explanation on these.
1379 See L<perllocale> for discussion of additional considerations that apply
1380 when C<use locale> is in effect.
1382 Options are as with m// with the addition of the following replacement
1385 e Evaluate the right side as an expression.
1386 ee Evaluate the right side as a string then eval the result
1388 Any non-alphanumeric, non-whitespace delimiter may replace the
1389 slashes. If single quotes are used, no interpretation is done on the
1390 replacement string (the C</e> modifier overrides this, however). Unlike
1391 Perl 4, Perl 5 treats backticks as normal delimiters; the replacement
1392 text is not evaluated as a command. If the
1393 PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
1394 pair of quotes, which may or may not be bracketing quotes, e.g.,
1395 C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
1396 replacement portion to be treated as a full-fledged Perl expression
1397 and evaluated right then and there. It is, however, syntax checked at
1398 compile-time. A second C<e> modifier will cause the replacement portion
1399 to be C<eval>ed before being run as a Perl expression.
1403 s/\bgreen\b/mauve/g; # don't change wintergreen
1405 $path =~ s|/usr/bin|/usr/local/bin|;
1407 s/Login: $foo/Login: $bar/; # run-time pattern
1409 ($foo = $bar) =~ s/this/that/; # copy first, then change
1411 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
1414 s/\d+/$&*2/e; # yields 'abc246xyz'
1415 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
1416 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1418 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
1419 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
1420 s/^=(\w+)/pod($1)/ge; # use function call
1422 # expand variables in $_, but dynamics only, using
1423 # symbolic dereferencing
1426 # Add one to the value of any numbers in the string
1429 # This will expand any embedded scalar variable
1430 # (including lexicals) in $_ : First $1 is interpolated
1431 # to the variable name, and then evaluated
1434 # Delete (most) C comments.
1436 /\* # Match the opening delimiter.
1437 .*? # Match a minimal number of characters.
1438 \*/ # Match the closing delimiter.
1441 s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_, expensively
1443 for ($variable) { # trim whitespace in $variable, cheap
1448 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
1450 Note the use of $ instead of \ in the last example. Unlike
1451 B<sed>, we use the \<I<digit>> form in only the left hand side.
1452 Anywhere else it's $<I<digit>>.
1454 Occasionally, you can't use just a C</g> to get all the changes
1455 to occur that you might want. Here are two common cases:
1457 # put commas in the right places in an integer
1458 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
1460 # expand tabs to 8-column spacing
1461 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
1465 =head2 Quote-Like Operators
1466 X<operator, quote-like>
1471 X<q> X<quote, single> X<'> X<''>
1475 A single-quoted, literal string. A backslash represents a backslash
1476 unless followed by the delimiter or another backslash, in which case
1477 the delimiter or backslash is interpolated.
1479 $foo = q!I said, "You said, 'She said it.'"!;
1480 $bar = q('This is it.');
1481 $baz = '\n'; # a two-character string
1484 X<qq> X<quote, double> X<"> X<"">
1488 A double-quoted, interpolated string.
1491 (*** The previous line contains the naughty word "$1".\n)
1492 if /\b(tcl|java|python)\b/i; # :-)
1493 $baz = "\n"; # a one-character string
1496 X<qx> X<`> X<``> X<backtick>
1500 A string which is (possibly) interpolated and then executed as a
1501 system command with C</bin/sh> or its equivalent. Shell wildcards,
1502 pipes, and redirections will be honored. The collected standard
1503 output of the command is returned; standard error is unaffected. In
1504 scalar context, it comes back as a single (potentially multi-line)
1505 string, or undef if the command failed. In list context, returns a
1506 list of lines (however you've defined lines with $/ or
1507 $INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
1509 Because backticks do not affect standard error, use shell file descriptor
1510 syntax (assuming the shell supports this) if you care to address this.
1511 To capture a command's STDERR and STDOUT together:
1513 $output = `cmd 2>&1`;
1515 To capture a command's STDOUT but discard its STDERR:
1517 $output = `cmd 2>/dev/null`;
1519 To capture a command's STDERR but discard its STDOUT (ordering is
1522 $output = `cmd 2>&1 1>/dev/null`;
1524 To exchange a command's STDOUT and STDERR in order to capture the STDERR
1525 but leave its STDOUT to come out the old STDERR:
1527 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
1529 To read both a command's STDOUT and its STDERR separately, it's easiest
1530 to redirect them separately to files, and then read from those files
1531 when the program is done:
1533 system("program args 1>program.stdout 2>program.stderr");
1535 The STDIN filehandle used by the command is inherited from Perl's STDIN.
1538 open BLAM, "blam" || die "Can't open: $!";
1539 open STDIN, "<&BLAM";
1542 will print the sorted contents of the file "blam".
1544 Using single-quote as a delimiter protects the command from Perl's
1545 double-quote interpolation, passing it on to the shell instead:
1547 $perl_info = qx(ps $$); # that's Perl's $$
1548 $shell_info = qx'ps $$'; # that's the new shell's $$
1550 How that string gets evaluated is entirely subject to the command
1551 interpreter on your system. On most platforms, you will have to protect
1552 shell metacharacters if you want them treated literally. This is in
1553 practice difficult to do, as it's unclear how to escape which characters.
1554 See L<perlsec> for a clean and safe example of a manual fork() and exec()
1555 to emulate backticks safely.
1557 On some platforms (notably DOS-like ones), the shell may not be
1558 capable of dealing with multiline commands, so putting newlines in
1559 the string may not get you what you want. You may be able to evaluate
1560 multiple commands in a single line by separating them with the command
1561 separator character, if your shell supports that (e.g. C<;> on many Unix
1562 shells; C<&> on the Windows NT C<cmd> shell).
1564 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1565 output before starting the child process, but this may not be supported
1566 on some platforms (see L<perlport>). To be safe, you may need to set
1567 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1568 C<IO::Handle> on any open handles.
1570 Beware that some command shells may place restrictions on the length
1571 of the command line. You must ensure your strings don't exceed this
1572 limit after any necessary interpolations. See the platform-specific
1573 release notes for more details about your particular environment.
1575 Using this operator can lead to programs that are difficult to port,
1576 because the shell commands called vary between systems, and may in
1577 fact not be present at all. As one example, the C<type> command under
1578 the POSIX shell is very different from the C<type> command under DOS.
1579 That doesn't mean you should go out of your way to avoid backticks
1580 when they're the right way to get something done. Perl was made to be
1581 a glue language, and one of the things it glues together is commands.
1582 Just understand what you're getting yourself into.
1584 See L</"I/O Operators"> for more discussion.
1587 X<qw> X<quote, list> X<quote, words>
1589 Evaluates to a list of the words extracted out of STRING, using embedded
1590 whitespace as the word delimiters. It can be understood as being roughly
1593 split(' ', q/STRING/);
1595 the differences being that it generates a real list at compile time, and
1596 in scalar context it returns the last element in the list. So
1601 is semantically equivalent to the list:
1605 Some frequently seen examples:
1607 use POSIX qw( setlocale localeconv )
1608 @EXPORT = qw( foo bar baz );
1610 A common mistake is to try to separate the words with comma or to
1611 put comments into a multi-line C<qw>-string. For this reason, the
1612 C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
1613 produces warnings if the STRING contains the "," or the "#" character.
1616 =item tr/SEARCHLIST/REPLACEMENTLIST/cds
1617 X<tr> X<y> X<transliterate> X</c> X</d> X</s>
1619 =item y/SEARCHLIST/REPLACEMENTLIST/cds
1621 Transliterates all occurrences of the characters found in the search list
1622 with the corresponding character in the replacement list. It returns
1623 the number of characters replaced or deleted. If no string is
1624 specified via the =~ or !~ operator, the $_ string is transliterated. (The
1625 string specified with =~ must be a scalar variable, an array element, a
1626 hash element, or an assignment to one of those, i.e., an lvalue.)
1628 A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
1629 does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
1630 For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1631 SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1632 its own pair of quotes, which may or may not be bracketing quotes,
1633 e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
1635 Note that C<tr> does B<not> do regular expression character classes
1636 such as C<\d> or C<[:lower:]>. The C<tr> operator is not equivalent to
1637 the tr(1) utility. If you want to map strings between lower/upper
1638 cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider
1639 using the C<s> operator if you need regular expressions.
1641 Note also that the whole range idea is rather unportable between
1642 character sets--and even within character sets they may cause results
1643 you probably didn't expect. A sound principle is to use only ranges
1644 that begin from and end at either alphabets of equal case (a-e, A-E),
1645 or digits (0-4). Anything else is unsafe. If in doubt, spell out the
1646 character sets in full.
1650 c Complement the SEARCHLIST.
1651 d Delete found but unreplaced characters.
1652 s Squash duplicate replaced characters.
1654 If the C</c> modifier is specified, the SEARCHLIST character set
1655 is complemented. If the C</d> modifier is specified, any characters
1656 specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
1657 (Note that this is slightly more flexible than the behavior of some
1658 B<tr> programs, which delete anything they find in the SEARCHLIST,
1659 period.) If the C</s> modifier is specified, sequences of characters
1660 that were transliterated to the same character are squashed down
1661 to a single instance of the character.
1663 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1664 exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1665 than the SEARCHLIST, the final character is replicated till it is long
1666 enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
1667 This latter is useful for counting characters in a class or for
1668 squashing character sequences in a class.
1672 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1674 $cnt = tr/*/*/; # count the stars in $_
1676 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1678 $cnt = tr/0-9//; # count the digits in $_
1680 tr/a-zA-Z//s; # bookkeeper -> bokeper
1682 ($HOST = $host) =~ tr/a-z/A-Z/;
1684 tr/a-zA-Z/ /cs; # change non-alphas to single space
1687 [\000-\177]; # delete 8th bit
1689 If multiple transliterations are given for a character, only the
1694 will transliterate any A to X.
1696 Because the transliteration table is built at compile time, neither
1697 the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
1698 interpolation. That means that if you want to use variables, you
1701 eval "tr/$oldlist/$newlist/";
1704 eval "tr/$oldlist/$newlist/, 1" or die $@;
1707 X<here-doc> X<heredoc> X<here-document> X<<< << >>>
1709 A line-oriented form of quoting is based on the shell "here-document"
1710 syntax. Following a C<< << >> you specify a string to terminate
1711 the quoted material, and all lines following the current line down to
1712 the terminating string are the value of the item.
1714 The terminating string may be either an identifier (a word), or some
1715 quoted text. An unquoted identifier works like double quotes.
1716 There may not be a space between the C<< << >> and the identifier,
1717 unless the identifier is explicitly quoted. (If you put a space it
1718 will be treated as a null identifier, which is valid, and matches the
1719 first empty line.) The terminating string must appear by itself
1720 (unquoted and with no surrounding whitespace) on the terminating line.
1722 If the terminating string is quoted, the type of quotes used determine
1723 the treatment of the text.
1729 Double quotes indicate that the text will be interpolated using exactly
1730 the same rules as normal double quoted strings.
1733 The price is $Price.
1736 print << "EOF"; # same as above
1737 The price is $Price.
1743 Single quotes indicate the text is to be treated literally with no
1744 interpolation of its content. This is similar to single quoted
1745 strings except that backslashes have no special meaning, with C<\\>
1746 being treated as two backslashes and not one as they would in every
1747 other quoting construct.
1749 This is the only form of quoting in perl where there is no need
1750 to worry about escaping content, something that code generators
1751 can and do make good use of.
1755 The content of the here doc is treated just as it would be if the
1756 string were embedded in backticks. Thus the content is interpolated
1757 as though it were double quoted and then executed via the shell, with
1758 the results of the execution returned.
1760 print << `EOC`; # execute command and get results
1766 It is possible to stack multiple here-docs in a row:
1768 print <<"foo", <<"bar"; # you can stack them
1774 myfunc(<< "THIS", 23, <<'THAT');
1781 Just don't forget that you have to put a semicolon on the end
1782 to finish the statement, as Perl doesn't know you're not going to
1790 If you want to remove the line terminator from your here-docs,
1793 chomp($string = <<'END');
1797 If you want your here-docs to be indented with the rest of the code,
1798 you'll need to remove leading whitespace from each line manually:
1800 ($quote = <<'FINIS') =~ s/^\s+//gm;
1801 The Road goes ever on and on,
1802 down from the door where it began.
1805 If you use a here-doc within a delimited construct, such as in C<s///eg>,
1806 the quoted material must come on the lines following the final delimiter.
1821 If the terminating identifier is on the last line of the program, you
1822 must be sure there is a newline after it; otherwise, Perl will give the
1823 warning B<Can't find string terminator "END" anywhere before EOF...>.
1825 Additionally, the quoting rules for the end of string identifier are not
1826 related to Perl's quoting rules -- C<q()>, C<qq()>, and the like are not
1827 supported in place of C<''> and C<"">, and the only interpolation is for
1828 backslashing the quoting character:
1830 print << "abc\"def";
1834 Finally, quoted strings cannot span multiple lines. The general rule is
1835 that the identifier must be a string literal. Stick with that, and you
1840 =head2 Gory details of parsing quoted constructs
1841 X<quote, gory details>
1843 When presented with something that might have several different
1844 interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
1845 principle to pick the most probable interpretation. This strategy
1846 is so successful that Perl programmers often do not suspect the
1847 ambivalence of what they write. But from time to time, Perl's
1848 notions differ substantially from what the author honestly meant.
1850 This section hopes to clarify how Perl handles quoted constructs.
1851 Although the most common reason to learn this is to unravel labyrinthine
1852 regular expressions, because the initial steps of parsing are the
1853 same for all quoting operators, they are all discussed together.
1855 The most important Perl parsing rule is the first one discussed
1856 below: when processing a quoted construct, Perl first finds the end
1857 of that construct, then interprets its contents. If you understand
1858 this rule, you may skip the rest of this section on the first
1859 reading. The other rules are likely to contradict the user's
1860 expectations much less frequently than this first one.
1862 Some passes discussed below are performed concurrently, but because
1863 their results are the same, we consider them individually. For different
1864 quoting constructs, Perl performs different numbers of passes, from
1865 one to four, but these passes are always performed in the same order.
1869 =item Finding the end
1871 The first pass is finding the end of the quoted construct, where
1872 the information about the delimiters is used in parsing.
1873 During this search, text between the starting and ending delimiters
1874 is copied to a safe location. The text copied gets delimiter-independent.
1876 If the construct is a here-doc, the ending delimiter is a line
1877 that has a terminating string as the content. Therefore C<<<EOF> is
1878 terminated by C<EOF> immediately followed by C<"\n"> and starting
1879 from the first column of the terminating line.
1880 When searching for the terminating line of a here-doc, nothing
1881 is skipped. In other words, lines after the here-doc syntax
1882 are compared with the terminating string line by line.
1884 For the constructs except here-docs, single characters are used as starting
1885 and ending delimiters. If the starting delimiter is an opening punctuation
1886 (that is C<(>, C<[>, C<{>, or C<< < >>), the ending delimiter is the
1887 corresponding closing punctuation (that is C<)>, C<]>, C<}>, or C<< > >>).
1888 If the starting delimiter is an unpaired character like C</> or a closing
1889 punctuation, the ending delimiter is same as the starting delimiter.
1890 Therefore a C</> terminates a C<qq//> construct, while a C<]> terminates
1891 C<qq[]> and C<qq]]> constructs.
1893 When searching for single-character delimiters, escaped delimiters
1894 and C<\\> are skipped. For example, while searching for terminating C</>,
1895 combinations of C<\\> and C<\/> are skipped. If the delimiters are
1896 bracketing, nested pairs are also skipped. For example, while searching
1897 for closing C<]> paired with the opening C<[>, combinations of C<\\>, C<\]>,
1898 and C<\[> are all skipped, and nested C<[> and C<]> are skipped as well.
1899 However, when backslashes are used as the delimiters (like C<qq\\> and
1900 C<tr\\\>), nothing is skipped.
1901 During the search for the end, backslashes that escape delimiters
1902 are removed (exactly speaking, they are not copied to the safe location).
1904 For constructs with three-part delimiters (C<s///>, C<y///>, and
1905 C<tr///>), the search is repeated once more.
1906 If the first delimiter is not an opening punctuation, three delimiters must
1907 be same such as C<s!!!> and C<tr)))>, in which case the second delimiter
1908 terminates the left part and starts the right part at once.
1909 If the left part is delimited by bracketing punctuations (that is C<()>,
1910 C<[]>, C<{}>, or C<< <> >>), the right part needs another pair of
1911 delimiters such as C<s(){}> and C<tr[]//>. In these cases, whitespaces
1912 and comments are allowed between both parts, though the comment must follow
1913 at least one whitespace; otherwise a character expected as the start of
1914 the comment may be regarded as the starting delimiter of the right part.
1916 During this search no attention is paid to the semantics of the construct.
1919 "$hash{"$foo/$bar"}"
1924 bar # NOT a comment, this slash / terminated m//!
1927 do not form legal quoted expressions. The quoted part ends on the
1928 first C<"> and C</>, and the rest happens to be a syntax error.
1929 Because the slash that terminated C<m//> was followed by a C<SPACE>,
1930 the example above is not C<m//x>, but rather C<m//> with no C</x>
1931 modifier. So the embedded C<#> is interpreted as a literal C<#>.
1933 Also no attention is paid to C<\c\> (multichar control char syntax) during
1934 this search. Thus the second C<\> in C<qq/\c\/> is interpreted as a part
1935 of C<\/>, and the following C</> is not recognized as a delimiter.
1936 Instead, use C<\034> or C<\x1c> at the end of quoted constructs.
1941 The next step is interpolation in the text obtained, which is now
1942 delimiter-independent. There are multiple cases.
1948 No interpolation is performed.
1949 Note that the combination C<\\> is left intact, since escaped delimiters
1950 are not available for here-docs.
1952 =item C<m''>, the pattern of C<s'''>
1954 No interpolation is performed at this stage.
1955 Any backslashed sequences including C<\\> are treated at the stage
1956 to L</"parsing regular expressions">.
1958 =item C<''>, C<q//>, C<tr'''>, C<y'''>, the replacement of C<s'''>
1960 The only interpolation is removal of C<\> from pairs of C<\\>.
1961 Therefore C<-> in C<tr'''> and C<y'''> is treated literally
1962 as a hyphen and no character range is available.
1963 C<\1> in the replacement of C<s'''> does not work as C<$1>.
1965 =item C<tr///>, C<y///>
1967 No variable interpolation occurs. String modifying combinations for
1968 case and quoting such as C<\Q>, C<\U>, and C<\E> are not recognized.
1969 The other escape sequences such as C<\200> and C<\t> and backslashed
1970 characters such as C<\\> and C<\-> are converted to appropriate literals.
1971 The character C<-> is treated specially and therefore C<\-> is treated
1974 =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF">
1976 C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
1977 converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
1978 is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
1979 The other escape sequences such as C<\200> and C<\t> and backslashed
1980 characters such as C<\\> and C<\-> are replaced with appropriate
1983 Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
1984 is interpolated in the usual way. Something like C<"\Q\\E"> has
1985 no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
1986 result is the same as for C<"\\\\E">. As a general rule, backslashes
1987 between C<\Q> and C<\E> may lead to counterintuitive results. So,
1988 C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
1989 as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
1994 may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
1996 Interpolated scalars and arrays are converted internally to the C<join> and
1997 C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
1999 $foo . " XXX '" . (join $", @arr) . "'";
2001 All operations above are performed simultaneously, left to right.
2003 Because the result of C<"\Q STRING \E"> has all metacharacters
2004 quoted, there is no way to insert a literal C<$> or C<@> inside a
2005 C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
2006 C<"\\\$">; if not, it is interpreted as the start of an interpolated
2009 Note also that the interpolation code needs to make a decision on
2010 where the interpolated scalar ends. For instance, whether
2011 C<< "a $b -> {c}" >> really means:
2013 "a " . $b . " -> {c}";
2019 Most of the time, the longest possible text that does not include
2020 spaces between components and which contains matching braces or
2021 brackets. because the outcome may be determined by voting based
2022 on heuristic estimators, the result is not strictly predictable.
2023 Fortunately, it's usually correct for ambiguous cases.
2025 =item the replacement of C<s///>
2027 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
2028 happens as with C<qq//> constructs.
2030 It is at this step that C<\1> is begrudgingly converted to C<$1> in
2031 the replacement text of C<s///>, in order to correct the incorrigible
2032 I<sed> hackers who haven't picked up the saner idiom yet. A warning
2033 is emitted if the C<use warnings> pragma or the B<-w> command-line flag
2034 (that is, the C<$^W> variable) was set.
2036 =item C<RE> in C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
2038 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\E>,
2039 and interpolation happens (almost) as with C<qq//> constructs.
2041 However any other combinations of C<\> followed by a character
2042 are not substituted but only skipped, in order to parse them
2043 as regular expressions at the following step.
2044 As C<\c> is skipped at this step, C<@> of C<\c@> in RE is possibly
2045 treated as an array symbol (for example C<@foo>),
2046 even though the same text in C<qq//> gives interpolation of C<\c@>.
2048 Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
2049 a C<#>-comment in a C<//x>-regular expression, no processing is
2050 performed whatsoever. This is the first step at which the presence
2051 of the C<//x> modifier is relevant.
2053 Interpolation in patterns has several quirks: C<$|>, C<$(>, C<$)>, C<@+>
2054 and C<@-> are not interpolated, and constructs C<$var[SOMETHING]> are
2055 voted (by several different estimators) to be either an array element
2056 or C<$var> followed by an RE alternative. This is where the notation
2057 C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
2058 array element C<-9>, not as a regular expression from the variable
2059 C<$arr> followed by a digit, which would be the interpretation of
2060 C</$arr[0-9]/>. Since voting among different estimators may occur,
2061 the result is not predictable.
2063 The lack of processing of C<\\> creates specific restrictions on
2064 the post-processed text. If the delimiter is C</>, one cannot get
2065 the combination C<\/> into the result of this step. C</> will
2066 finish the regular expression, C<\/> will be stripped to C</> on
2067 the previous step, and C<\\/> will be left as is. Because C</> is
2068 equivalent to C<\/> inside a regular expression, this does not
2069 matter unless the delimiter happens to be character special to the
2070 RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
2071 alphanumeric char, as in:
2075 In the RE above, which is intentionally obfuscated for illustration, the
2076 delimiter is C<m>, the modifier is C<mx>, and after delimiter-removal the
2077 RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
2078 reason you're encouraged to restrict your delimiters to non-alphanumeric,
2079 non-whitespace choices.
2083 This step is the last one for all constructs except regular expressions,
2084 which are processed further.
2086 =item parsing regular expressions
2089 Previous steps were performed during the compilation of Perl code,
2090 but this one happens at run time--although it may be optimized to
2091 be calculated at compile time if appropriate. After preprocessing
2092 described above, and possibly after evaluation if concatenation,
2093 joining, casing translation, or metaquoting are involved, the
2094 resulting I<string> is passed to the RE engine for compilation.
2096 Whatever happens in the RE engine might be better discussed in L<perlre>,
2097 but for the sake of continuity, we shall do so here.
2099 This is another step where the presence of the C<//x> modifier is
2100 relevant. The RE engine scans the string from left to right and
2101 converts it to a finite automaton.
2103 Backslashed characters are either replaced with corresponding
2104 literal strings (as with C<\{>), or else they generate special nodes
2105 in the finite automaton (as with C<\b>). Characters special to the
2106 RE engine (such as C<|>) generate corresponding nodes or groups of
2107 nodes. C<(?#...)> comments are ignored. All the rest is either
2108 converted to literal strings to match, or else is ignored (as is
2109 whitespace and C<#>-style comments if C<//x> is present).
2111 Parsing of the bracketed character class construct, C<[...]>, is
2112 rather different than the rule used for the rest of the pattern.
2113 The terminator of this construct is found using the same rules as
2114 for finding the terminator of a C<{}>-delimited construct, the only
2115 exception being that C<]> immediately following C<[> is treated as
2116 though preceded by a backslash. Similarly, the terminator of
2117 C<(?{...})> is found using the same rules as for finding the
2118 terminator of a C<{}>-delimited construct.
2120 It is possible to inspect both the string given to RE engine and the
2121 resulting finite automaton. See the arguments C<debug>/C<debugcolor>
2122 in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
2123 switch documented in L<perlrun/"Command Switches">.
2125 =item Optimization of regular expressions
2126 X<regexp, optimization>
2128 This step is listed for completeness only. Since it does not change
2129 semantics, details of this step are not documented and are subject
2130 to change without notice. This step is performed over the finite
2131 automaton that was generated during the previous pass.
2133 It is at this stage that C<split()> silently optimizes C</^/> to
2138 =head2 I/O Operators
2139 X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle>
2142 There are several I/O operators you should know about.
2144 A string enclosed by backticks (grave accents) first undergoes
2145 double-quote interpolation. It is then interpreted as an external
2146 command, and the output of that command is the value of the
2147 backtick string, like in a shell. In scalar context, a single string
2148 consisting of all output is returned. In list context, a list of
2149 values is returned, one per line of output. (You can set C<$/> to use
2150 a different line terminator.) The command is executed each time the
2151 pseudo-literal is evaluated. The status value of the command is
2152 returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
2153 Unlike in B<csh>, no translation is done on the return data--newlines
2154 remain newlines. Unlike in any of the shells, single quotes do not
2155 hide variable names in the command from interpretation. To pass a
2156 literal dollar-sign through to the shell you need to hide it with a
2157 backslash. The generalized form of backticks is C<qx//>. (Because
2158 backticks always undergo shell expansion as well, see L<perlsec> for
2160 X<qx> X<`> X<``> X<backtick> X<glob>
2162 In scalar context, evaluating a filehandle in angle brackets yields
2163 the next line from that file (the newline, if any, included), or
2164 C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
2165 (sometimes known as file-slurp mode) and the file is empty, it
2166 returns C<''> the first time, followed by C<undef> subsequently.
2168 Ordinarily you must assign the returned value to a variable, but
2169 there is one situation where an automatic assignment happens. If
2170 and only if the input symbol is the only thing inside the conditional
2171 of a C<while> statement (even if disguised as a C<for(;;)> loop),
2172 the value is automatically assigned to the global variable $_,
2173 destroying whatever was there previously. (This may seem like an
2174 odd thing to you, but you'll use the construct in almost every Perl
2175 script you write.) The $_ variable is not implicitly localized.
2176 You'll have to put a C<local $_;> before the loop if you want that
2179 The following lines are equivalent:
2181 while (defined($_ = <STDIN>)) { print; }
2182 while ($_ = <STDIN>) { print; }
2183 while (<STDIN>) { print; }
2184 for (;<STDIN>;) { print; }
2185 print while defined($_ = <STDIN>);
2186 print while ($_ = <STDIN>);
2187 print while <STDIN>;
2189 This also behaves similarly, but avoids $_ :
2191 while (my $line = <STDIN>) { print $line }
2193 In these loop constructs, the assigned value (whether assignment
2194 is automatic or explicit) is then tested to see whether it is
2195 defined. The defined test avoids problems where line has a string
2196 value that would be treated as false by Perl, for example a "" or
2197 a "0" with no trailing newline. If you really mean for such values
2198 to terminate the loop, they should be tested for explicitly:
2200 while (($_ = <STDIN>) ne '0') { ... }
2201 while (<STDIN>) { last unless $_; ... }
2203 In other boolean contexts, C<< <I<filehandle>> >> without an
2204 explicit C<defined> test or comparison elicit a warning if the
2205 C<use warnings> pragma or the B<-w>
2206 command-line switch (the C<$^W> variable) is in effect.
2208 The filehandles STDIN, STDOUT, and STDERR are predefined. (The
2209 filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
2210 in packages, where they would be interpreted as local identifiers
2211 rather than global.) Additional filehandles may be created with
2212 the open() function, amongst others. See L<perlopentut> and
2213 L<perlfunc/open> for details on this.
2214 X<stdin> X<stdout> X<sterr>
2216 If a <FILEHANDLE> is used in a context that is looking for
2217 a list, a list comprising all input lines is returned, one line per
2218 list element. It's easy to grow to a rather large data space this
2219 way, so use with care.
2221 <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
2222 See L<perlfunc/readline>.
2224 The null filehandle <> is special: it can be used to emulate the
2225 behavior of B<sed> and B<awk>. Input from <> comes either from
2226 standard input, or from each file listed on the command line. Here's
2227 how it works: the first time <> is evaluated, the @ARGV array is
2228 checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
2229 gives you standard input. The @ARGV array is then processed as a list
2230 of filenames. The loop
2233 ... # code for each line
2236 is equivalent to the following Perl-like pseudo code:
2238 unshift(@ARGV, '-') unless @ARGV;
2239 while ($ARGV = shift) {
2242 ... # code for each line
2246 except that it isn't so cumbersome to say, and will actually work.
2247 It really does shift the @ARGV array and put the current filename
2248 into the $ARGV variable. It also uses filehandle I<ARGV>
2249 internally--<> is just a synonym for <ARGV>, which
2250 is magical. (The pseudo code above doesn't work because it treats
2251 <ARGV> as non-magical.)
2253 Since the null filehandle uses the two argument form of L<perlfunc/open>
2254 it interprets special characters, so if you have a script like this:
2260 and call it with C<perl dangerous.pl 'rm -rfv *|'>, it actually opens a
2261 pipe, executes the C<rm> command and reads C<rm>'s output from that pipe.
2262 If you want all items in C<@ARGV> to be interpreted as file names, you
2263 can use the module C<ARGV::readonly> from CPAN.
2265 You can modify @ARGV before the first <> as long as the array ends up
2266 containing the list of filenames you really want. Line numbers (C<$.>)
2267 continue as though the input were one big happy file. See the example
2268 in L<perlfunc/eof> for how to reset line numbers on each file.
2270 If you want to set @ARGV to your own list of files, go right ahead.
2271 This sets @ARGV to all plain text files if no @ARGV was given:
2273 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
2275 You can even set them to pipe commands. For example, this automatically
2276 filters compressed arguments through B<gzip>:
2278 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
2280 If you want to pass switches into your script, you can use one of the
2281 Getopts modules or put a loop on the front like this:
2283 while ($_ = $ARGV[0], /^-/) {
2286 if (/^-D(.*)/) { $debug = $1 }
2287 if (/^-v/) { $verbose++ }
2288 # ... # other switches
2292 # ... # code for each line
2295 The <> symbol will return C<undef> for end-of-file only once.
2296 If you call it again after this, it will assume you are processing another
2297 @ARGV list, and if you haven't set @ARGV, will read input from STDIN.
2299 If what the angle brackets contain is a simple scalar variable (e.g.,
2300 <$foo>), then that variable contains the name of the
2301 filehandle to input from, or its typeglob, or a reference to the
2307 If what's within the angle brackets is neither a filehandle nor a simple
2308 scalar variable containing a filehandle name, typeglob, or typeglob
2309 reference, it is interpreted as a filename pattern to be globbed, and
2310 either a list of filenames or the next filename in the list is returned,
2311 depending on context. This distinction is determined on syntactic
2312 grounds alone. That means C<< <$x> >> is always a readline() from
2313 an indirect handle, but C<< <$hash{key}> >> is always a glob().
2314 That's because $x is a simple scalar variable, but C<$hash{key}> is
2315 not--it's a hash element. Even C<< <$x > >> (note the extra space)
2316 is treated as C<glob("$x ")>, not C<readline($x)>.
2318 One level of double-quote interpretation is done first, but you can't
2319 say C<< <$foo> >> because that's an indirect filehandle as explained
2320 in the previous paragraph. (In older versions of Perl, programmers
2321 would insert curly brackets to force interpretation as a filename glob:
2322 C<< <${foo}> >>. These days, it's considered cleaner to call the
2323 internal function directly as C<glob($foo)>, which is probably the right
2324 way to have done it in the first place.) For example:
2330 is roughly equivalent to:
2332 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
2338 except that the globbing is actually done internally using the standard
2339 C<File::Glob> extension. Of course, the shortest way to do the above is:
2343 A (file)glob evaluates its (embedded) argument only when it is
2344 starting a new list. All values must be read before it will start
2345 over. In list context, this isn't important because you automatically
2346 get them all anyway. However, in scalar context the operator returns
2347 the next value each time it's called, or C<undef> when the list has
2348 run out. As with filehandle reads, an automatic C<defined> is
2349 generated when the glob occurs in the test part of a C<while>,
2350 because legal glob returns (e.g. a file called F<0>) would otherwise
2351 terminate the loop. Again, C<undef> is returned only once. So if
2352 you're expecting a single value from a glob, it is much better to
2355 ($file) = <blurch*>;
2361 because the latter will alternate between returning a filename and
2364 If you're trying to do variable interpolation, it's definitely better
2365 to use the glob() function, because the older notation can cause people
2366 to become confused with the indirect filehandle notation.
2368 @files = glob("$dir/*.[ch]");
2369 @files = glob($files[$i]);
2371 =head2 Constant Folding
2372 X<constant folding> X<folding>
2374 Like C, Perl does a certain amount of expression evaluation at
2375 compile time whenever it determines that all arguments to an
2376 operator are static and have no side effects. In particular, string
2377 concatenation happens at compile time between literals that don't do
2378 variable substitution. Backslash interpolation also happens at
2379 compile time. You can say
2381 'Now is the time for all' . "\n" .
2382 'good men to come to.'
2384 and this all reduces to one string internally. Likewise, if
2387 foreach $file (@filenames) {
2388 if (-s $file > 5 + 100 * 2**16) { }
2391 the compiler will precompute the number which that expression
2392 represents so that the interpreter won't have to.
2397 Perl doesn't officially have a no-op operator, but the bare constants
2398 C<0> and C<1> are special-cased to not produce a warning in a void
2399 context, so you can for example safely do
2403 =head2 Bitwise String Operators
2404 X<operator, bitwise, string>
2406 Bitstrings of any size may be manipulated by the bitwise operators
2409 If the operands to a binary bitwise op are strings of different
2410 sizes, B<|> and B<^> ops act as though the shorter operand had
2411 additional zero bits on the right, while the B<&> op acts as though
2412 the longer operand were truncated to the length of the shorter.
2413 The granularity for such extension or truncation is one or more
2416 # ASCII-based examples
2417 print "j p \n" ^ " a h"; # prints "JAPH\n"
2418 print "JA" | " ph\n"; # prints "japh\n"
2419 print "japh\nJunk" & '_____'; # prints "JAPH\n";
2420 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
2422 If you are intending to manipulate bitstrings, be certain that
2423 you're supplying bitstrings: If an operand is a number, that will imply
2424 a B<numeric> bitwise operation. You may explicitly show which type of
2425 operation you intend by using C<""> or C<0+>, as in the examples below.
2427 $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
2428 $foo = '150' | 105; # yields 255
2429 $foo = 150 | '105'; # yields 255
2430 $foo = '150' | '105'; # yields string '155' (under ASCII)
2432 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
2433 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
2435 See L<perlfunc/vec> for information on how to manipulate individual bits
2438 =head2 Integer Arithmetic
2441 By default, Perl assumes that it must do most of its arithmetic in
2442 floating point. But by saying
2446 you may tell the compiler that it's okay to use integer operations
2447 (if it feels like it) from here to the end of the enclosing BLOCK.
2448 An inner BLOCK may countermand this by saying
2452 which lasts until the end of that BLOCK. Note that this doesn't
2453 mean everything is only an integer, merely that Perl may use integer
2454 operations if it is so inclined. For example, even under C<use
2455 integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731>
2458 Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
2459 and ">>") always produce integral results. (But see also
2460 L<Bitwise String Operators>.) However, C<use integer> still has meaning for
2461 them. By default, their results are interpreted as unsigned integers, but
2462 if C<use integer> is in effect, their results are interpreted
2463 as signed integers. For example, C<~0> usually evaluates to a large
2464 integral value. However, C<use integer; ~0> is C<-1> on two's-complement
2467 =head2 Floating-point Arithmetic
2468 X<floating-point> X<floating point> X<float> X<real>
2470 While C<use integer> provides integer-only arithmetic, there is no
2471 analogous mechanism to provide automatic rounding or truncation to a
2472 certain number of decimal places. For rounding to a certain number
2473 of digits, sprintf() or printf() is usually the easiest route.
2476 Floating-point numbers are only approximations to what a mathematician
2477 would call real numbers. There are infinitely more reals than floats,
2478 so some corners must be cut. For example:
2480 printf "%.20g\n", 123456789123456789;
2481 # produces 123456789123456784
2483 Testing for exact equality of floating-point equality or inequality is
2484 not a good idea. Here's a (relatively expensive) work-around to compare
2485 whether two floating-point numbers are equal to a particular number of
2486 decimal places. See Knuth, volume II, for a more robust treatment of
2490 my ($X, $Y, $POINTS) = @_;
2492 $tX = sprintf("%.${POINTS}g", $X);
2493 $tY = sprintf("%.${POINTS}g", $Y);
2497 The POSIX module (part of the standard perl distribution) implements
2498 ceil(), floor(), and other mathematical and trigonometric functions.
2499 The Math::Complex module (part of the standard perl distribution)
2500 defines mathematical functions that work on both the reals and the
2501 imaginary numbers. Math::Complex not as efficient as POSIX, but
2502 POSIX can't work with complex numbers.
2504 Rounding in financial applications can have serious implications, and
2505 the rounding method used should be specified precisely. In these
2506 cases, it probably pays not to trust whichever system rounding is
2507 being used by Perl, but to instead implement the rounding function you
2510 =head2 Bigger Numbers
2511 X<number, arbitrary precision>
2513 The standard Math::BigInt and Math::BigFloat modules provide
2514 variable-precision arithmetic and overloaded operators, although
2515 they're currently pretty slow. At the cost of some space and
2516 considerable speed, they avoid the normal pitfalls associated with
2517 limited-precision representations.
2520 $x = Math::BigInt->new('123456789123456789');
2523 # prints +15241578780673678515622620750190521
2525 There are several modules that let you calculate with (bound only by
2526 memory and cpu-time) unlimited or fixed precision. There are also
2527 some non-standard modules that provide faster implementations via
2528 external C libraries.
2530 Here is a short, but incomplete summary:
2532 Math::Fraction big, unlimited fractions like 9973 / 12967
2533 Math::String treat string sequences like numbers
2534 Math::FixedPrecision calculate with a fixed precision
2535 Math::Currency for currency calculations
2536 Bit::Vector manipulate bit vectors fast (uses C)
2537 Math::BigIntFast Bit::Vector wrapper for big numbers
2538 Math::Pari provides access to the Pari C library
2539 Math::BigInteger uses an external C library
2540 Math::Cephes uses external Cephes C library (no big numbers)
2541 Math::Cephes::Fraction fractions via the Cephes library
2542 Math::GMP another one using an external C library