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 $_. When used in scalar context, the return value generally indicates the
238 success of the operation. Behavior in list context depends on the particular
239 operator. See L</"Regexp Quote-Like Operators"> for details and
240 L<perlretut> for examples using these operators.
242 If the right argument is an expression rather than a search pattern,
243 substitution, or transliteration, it is interpreted as a search pattern at run
244 time. Note that this means that its contents will be interpolated twice, so
248 is not ok, as the regex engine will end up trying to compile the
249 pattern C<\>, which it will consider a syntax error.
251 Binary "!~" is just like "=~" except the return value is negated in
254 =head2 Multiplicative Operators
255 X<operator, multiplicative>
257 Binary "*" multiplies two numbers.
260 Binary "/" divides two numbers.
263 Binary "%" computes the modulus of two numbers. Given integer
264 operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
265 C<$a> minus the largest multiple of C<$b> that is not greater than
266 C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
267 smallest multiple of C<$b> that is not less than C<$a> (i.e. the
268 result will be less than or equal to zero). If the operands
269 C<$a> and C<$b> are floating point values and the absolute value of
270 C<$b> (that is C<abs($b)>) is less than C<(UV_MAX + 1)>, only
271 the integer portion of C<$a> and C<$b> will be used in the operation
272 (Note: here C<UV_MAX> means the maximum of the unsigned integer type).
273 If the absolute value of the right operand (C<abs($b)>) is greater than
274 or equal to C<(UV_MAX + 1)>, "%" computes the floating-point remainder
275 C<$r> in the equation C<($r = $a - $i*$b)> where C<$i> is a certain
276 integer that makes C<$r> should have the same sign as the right operand
277 C<$b> (B<not> as the left operand C<$a> like C function C<fmod()>)
278 and the absolute value less than that of C<$b>.
279 Note that when C<use integer> is in scope, "%" gives you direct access
280 to the modulus operator as implemented by your C compiler. This
281 operator is not as well defined for negative operands, but it will
283 X<%> X<remainder> X<modulus> X<mod>
285 Binary "x" is the repetition operator. In scalar context or if the left
286 operand is not enclosed in parentheses, it returns a string consisting
287 of the left operand repeated the number of times specified by the right
288 operand. In list context, if the left operand is enclosed in
289 parentheses or is a list formed by C<qw/STRING/>, it repeats the list.
290 If the right operand is zero or negative, it returns an empty string
291 or an empty list, depending on the context.
294 print '-' x 80; # print row of dashes
296 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
298 @ones = (1) x 80; # a list of 80 1's
299 @ones = (5) x @ones; # set all elements to 5
302 =head2 Additive Operators
303 X<operator, additive>
305 Binary "+" returns the sum of two numbers.
308 Binary "-" returns the difference of two numbers.
311 Binary "." concatenates two strings.
312 X<string, concatenation> X<concatenation>
313 X<cat> X<concat> X<concatenate> X<.>
315 =head2 Shift Operators
316 X<shift operator> X<operator, shift> X<<< << >>>
317 X<<< >> >>> X<right shift> X<left shift> X<bitwise shift>
318 X<shl> X<shr> X<shift, right> X<shift, left>
320 Binary "<<" returns the value of its left argument shifted left by the
321 number of bits specified by the right argument. Arguments should be
322 integers. (See also L<Integer Arithmetic>.)
324 Binary ">>" returns the value of its left argument shifted right by
325 the number of bits specified by the right argument. Arguments should
326 be integers. (See also L<Integer Arithmetic>.)
328 Note that both "<<" and ">>" in Perl are implemented directly using
329 "<<" and ">>" in C. If C<use integer> (see L<Integer Arithmetic>) is
330 in force then signed C integers are used, else unsigned C integers are
331 used. Either way, the implementation isn't going to generate results
332 larger than the size of the integer type Perl was built with (32 bits
335 The result of overflowing the range of the integers is undefined
336 because it is undefined also in C. In other words, using 32-bit
337 integers, C<< 1 << 32 >> is undefined. Shifting by a negative number
338 of bits is also undefined.
340 =head2 Named Unary Operators
341 X<operator, named unary>
343 The various named unary operators are treated as functions with one
344 argument, with optional parentheses.
346 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
347 is followed by a left parenthesis as the next token, the operator and
348 arguments within parentheses are taken to be of highest precedence,
349 just like a normal function call. For example,
350 because named unary operators are higher precedence than ||:
352 chdir $foo || die; # (chdir $foo) || die
353 chdir($foo) || die; # (chdir $foo) || die
354 chdir ($foo) || die; # (chdir $foo) || die
355 chdir +($foo) || die; # (chdir $foo) || die
357 but, because * is higher precedence than named operators:
359 chdir $foo * 20; # chdir ($foo * 20)
360 chdir($foo) * 20; # (chdir $foo) * 20
361 chdir ($foo) * 20; # (chdir $foo) * 20
362 chdir +($foo) * 20; # chdir ($foo * 20)
364 rand 10 * 20; # rand (10 * 20)
365 rand(10) * 20; # (rand 10) * 20
366 rand (10) * 20; # (rand 10) * 20
367 rand +(10) * 20; # rand (10 * 20)
369 Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are
370 treated like named unary operators, but they don't follow this functional
371 parenthesis rule. That means, for example, that C<-f($file).".bak"> is
372 equivalent to C<-f "$file.bak">.
373 X<-X> X<filetest> X<operator, filetest>
375 See also L<"Terms and List Operators (Leftward)">.
377 =head2 Relational Operators
378 X<relational operator> X<operator, relational>
380 Binary "<" returns true if the left argument is numerically less than
384 Binary ">" returns true if the left argument is numerically greater
385 than the right argument.
388 Binary "<=" returns true if the left argument is numerically less than
389 or equal to the right argument.
392 Binary ">=" returns true if the left argument is numerically greater
393 than or equal to the right argument.
396 Binary "lt" returns true if the left argument is stringwise less than
400 Binary "gt" returns true if the left argument is stringwise greater
401 than the right argument.
404 Binary "le" returns true if the left argument is stringwise less than
405 or equal to the right argument.
408 Binary "ge" returns true if the left argument is stringwise greater
409 than or equal to the right argument.
412 =head2 Equality Operators
413 X<equality> X<equal> X<equals> X<operator, equality>
415 Binary "==" returns true if the left argument is numerically equal to
419 Binary "!=" returns true if the left argument is numerically not equal
420 to the right argument.
423 Binary "<=>" returns -1, 0, or 1 depending on whether the left
424 argument is numerically less than, equal to, or greater than the right
425 argument. If your platform supports NaNs (not-a-numbers) as numeric
426 values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
427 "<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
428 returns true, as does NaN != anything else. If your platform doesn't
429 support NaNs then NaN is just a string with numeric value 0.
430 X<< <=> >> X<spaceship>
432 perl -le '$a = "NaN"; print "No NaN support here" if $a == $a'
433 perl -le '$a = "NaN"; print "NaN support here" if $a != $a'
435 Binary "eq" returns true if the left argument is stringwise equal to
439 Binary "ne" returns true if the left argument is stringwise not equal
440 to the right argument.
443 Binary "cmp" returns -1, 0, or 1 depending on whether the left
444 argument is stringwise less than, equal to, or greater than the right
448 Binary "~~" does a smart match between its arguments. Smart matching
449 is described in L<perlsyn/"Smart matching in detail">.
452 "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
453 by the current locale if C<use locale> is in effect. See L<perllocale>.
456 X<operator, bitwise, and> X<bitwise and> X<&>
458 Binary "&" returns its operands ANDed together bit by bit.
459 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
461 Note that "&" has lower priority than relational operators, so for example
462 the brackets are essential in a test like
464 print "Even\n" if ($x & 1) == 0;
466 =head2 Bitwise Or and Exclusive Or
467 X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor>
470 Binary "|" returns its operands ORed together bit by bit.
471 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
473 Binary "^" returns its operands XORed together bit by bit.
474 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
476 Note that "|" and "^" have lower priority than relational operators, so
477 for example the brackets are essential in a test like
479 print "false\n" if (8 | 2) != 10;
481 =head2 C-style Logical And
482 X<&&> X<logical and> X<operator, logical, and>
484 Binary "&&" performs a short-circuit logical AND operation. That is,
485 if the left operand is false, the right operand is not even evaluated.
486 Scalar or list context propagates down to the right operand if it
489 =head2 C-style Logical Or
490 X<||> X<operator, logical, or>
492 Binary "||" performs a short-circuit logical OR operation. That is,
493 if the left operand is true, the right operand is not even evaluated.
494 Scalar or list context propagates down to the right operand if it
497 =head2 C-style Logical Defined-Or
498 X<//> X<operator, logical, defined-or>
500 Although it has no direct equivalent in C, Perl's C<//> operator is related
501 to its C-style or. In fact, it's exactly the same as C<||>, except that it
502 tests the left hand side's definedness instead of its truth. Thus, C<$a // $b>
503 is similar to C<defined($a) || $b> (except that it returns the value of C<$a>
504 rather than the value of C<defined($a)>) and is exactly equivalent to
505 C<defined($a) ? $a : $b>. This is very useful for providing default values
506 for variables. If you actually want to test if at least one of C<$a> and
507 C<$b> is defined, use C<defined($a // $b)>.
509 The C<||>, C<//> and C<&&> operators return the last value evaluated
510 (unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably
511 portable way to find out the home directory might be:
513 $home = $ENV{'HOME'} // $ENV{'LOGDIR'} //
514 (getpwuid($<))[7] // die "You're homeless!\n";
516 In particular, this means that you shouldn't use this
517 for selecting between two aggregates for assignment:
519 @a = @b || @c; # this is wrong
520 @a = scalar(@b) || @c; # really meant this
521 @a = @b ? @b : @c; # this works fine, though
523 As more readable alternatives to C<&&> and C<||> when used for
524 control flow, Perl provides the C<and> and C<or> operators (see below).
525 The short-circuit behavior is identical. The precedence of "and"
526 and "or" is much lower, however, so that you can safely use them after a
527 list operator without the need for parentheses:
529 unlink "alpha", "beta", "gamma"
530 or gripe(), next LINE;
532 With the C-style operators that would have been written like this:
534 unlink("alpha", "beta", "gamma")
535 || (gripe(), next LINE);
537 Using "or" for assignment is unlikely to do what you want; see below.
539 =head2 Range Operators
540 X<operator, range> X<range> X<..> X<...>
542 Binary ".." is the range operator, which is really two different
543 operators depending on the context. In list context, it returns a
544 list of values counting (up by ones) from the left value to the right
545 value. If the left value is greater than the right value then it
546 returns the empty list. The range operator is useful for writing
547 C<foreach (1..10)> loops and for doing slice operations on arrays. In
548 the current implementation, no temporary array is created when the
549 range operator is used as the expression in C<foreach> loops, but older
550 versions of Perl might burn a lot of memory when you write something
553 for (1 .. 1_000_000) {
557 The range operator also works on strings, using the magical auto-increment,
560 In scalar context, ".." returns a boolean value. The operator is
561 bistable, like a flip-flop, and emulates the line-range (comma) operator
562 of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
563 own boolean state. It is false as long as its left operand is false.
564 Once the left operand is true, the range operator stays true until the
565 right operand is true, I<AFTER> which the range operator becomes false
566 again. It doesn't become false till the next time the range operator is
567 evaluated. It can test the right operand and become false on the same
568 evaluation it became true (as in B<awk>), but it still returns true once.
569 If you don't want it to test the right operand till the next
570 evaluation, as in B<sed>, just use three dots ("...") instead of
571 two. In all other regards, "..." behaves just like ".." does.
573 The right operand is not evaluated while the operator is in the
574 "false" state, and the left operand is not evaluated while the
575 operator is in the "true" state. The precedence is a little lower
576 than || and &&. The value returned is either the empty string for
577 false, or a sequence number (beginning with 1) for true. The
578 sequence number is reset for each range encountered. The final
579 sequence number in a range has the string "E0" appended to it, which
580 doesn't affect its numeric value, but gives you something to search
581 for if you want to exclude the endpoint. You can exclude the
582 beginning point by waiting for the sequence number to be greater
585 If either operand of scalar ".." is a constant expression,
586 that operand is considered true if it is equal (C<==>) to the current
587 input line number (the C<$.> variable).
589 To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>,
590 but that is only an issue if you use a floating point expression; when
591 implicitly using C<$.> as described in the previous paragraph, the
592 comparison is C<int(EXPR) == int($.)> which is only an issue when C<$.>
593 is set to a floating point value and you are not reading from a file.
594 Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what
595 you want in scalar context because each of the operands are evaluated
596 using their integer representation.
600 As a scalar operator:
602 if (101 .. 200) { print; } # print 2nd hundred lines, short for
603 # if ($. == 101 .. $. == 200) ...
605 next LINE if (1 .. /^$/); # skip header lines, short for
606 # ... if ($. == 1 .. /^$/);
607 # (typically in a loop labeled LINE)
609 s/^/> / if (/^$/ .. eof()); # quote body
611 # parse mail messages
613 $in_header = 1 .. /^$/;
614 $in_body = /^$/ .. eof;
621 close ARGV if eof; # reset $. each file
624 Here's a simple example to illustrate the difference between
625 the two range operators:
638 This program will print only the line containing "Bar". If
639 the range operator is changed to C<...>, it will also print the
642 And now some examples as a list operator:
644 for (101 .. 200) { print; } # print $_ 100 times
645 @foo = @foo[0 .. $#foo]; # an expensive no-op
646 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
648 The range operator (in list context) makes use of the magical
649 auto-increment algorithm if the operands are strings. You
652 @alphabet = ('A' .. 'Z');
654 to get all normal letters of the English alphabet, or
656 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
658 to get a hexadecimal digit, or
660 @z2 = ('01' .. '31'); print $z2[$mday];
662 to get dates with leading zeros.
664 If the final value specified is not in the sequence that the magical
665 increment would produce, the sequence goes until the next value would
666 be longer than the final value specified.
668 If the initial value specified isn't part of a magical increment
669 sequence (that is, a non-empty string matching "/^[a-zA-Z]*[0-9]*\z/"),
670 only the initial value will be returned. So the following will only
673 use charnames 'greek';
674 my @greek_small = ("\N{alpha}" .. "\N{omega}");
676 To get lower-case greek letters, use this instead:
678 my @greek_small = map { chr } ( ord("\N{alpha}") .. ord("\N{omega}") );
680 Because each operand is evaluated in integer form, C<2.18 .. 3.14> will
681 return two elements in list context.
683 @list = (2.18 .. 3.14); # same as @list = (2 .. 3);
685 =head2 Conditional Operator
686 X<operator, conditional> X<operator, ternary> X<ternary> X<?:>
688 Ternary "?:" is the conditional operator, just as in C. It works much
689 like an if-then-else. If the argument before the ? is true, the
690 argument before the : is returned, otherwise the argument after the :
691 is returned. For example:
693 printf "I have %d dog%s.\n", $n,
694 ($n == 1) ? '' : "s";
696 Scalar or list context propagates downward into the 2nd
697 or 3rd argument, whichever is selected.
699 $a = $ok ? $b : $c; # get a scalar
700 @a = $ok ? @b : @c; # get an array
701 $a = $ok ? @b : @c; # oops, that's just a count!
703 The operator may be assigned to if both the 2nd and 3rd arguments are
704 legal lvalues (meaning that you can assign to them):
706 ($a_or_b ? $a : $b) = $c;
708 Because this operator produces an assignable result, using assignments
709 without parentheses will get you in trouble. For example, this:
711 $a % 2 ? $a += 10 : $a += 2
715 (($a % 2) ? ($a += 10) : $a) += 2
719 ($a % 2) ? ($a += 10) : ($a += 2)
721 That should probably be written more simply as:
723 $a += ($a % 2) ? 10 : 2;
725 =head2 Assignment Operators
726 X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=>
727 X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=>
730 "=" is the ordinary assignment operator.
732 Assignment operators work as in C. That is,
740 although without duplicating any side effects that dereferencing the lvalue
741 might trigger, such as from tie(). Other assignment operators work similarly.
742 The following are recognized:
749 Although these are grouped by family, they all have the precedence
752 Unlike in C, the scalar assignment operator produces a valid lvalue.
753 Modifying an assignment is equivalent to doing the assignment and
754 then modifying the variable that was assigned to. This is useful
755 for modifying a copy of something, like this:
757 ($tmp = $global) =~ tr [A-Z] [a-z];
768 Similarly, a list assignment in list context produces the list of
769 lvalues assigned to, and a list assignment in scalar context returns
770 the number of elements produced by the expression on the right hand
771 side of the assignment.
773 =head2 Comma Operator
774 X<comma> X<operator, comma> X<,>
776 Binary "," is the comma operator. In scalar context it evaluates
777 its left argument, throws that value away, then evaluates its right
778 argument and returns that value. This is just like C's comma operator.
780 In list context, it's just the list argument separator, and inserts
781 both its arguments into the list. These arguments are also evaluated
784 The C<< => >> operator is a synonym for the comma, but forces any word
785 (consisting entirely of word characters) to its left to be interpreted
786 as a string (as of 5.001). This includes words that might otherwise be
787 considered a constant or function call.
789 use constant FOO => "something";
791 my %h = ( FOO => 23 );
799 my %h = ("something", 23);
801 If the argument on the left is not a word, it is first interpreted as
802 an expression, and then the string value of that is used.
804 The C<< => >> operator is helpful in documenting the correspondence
805 between keys and values in hashes, and other paired elements in lists.
807 %hash = ( $key => $value );
808 login( $username => $password );
810 =head2 List Operators (Rightward)
811 X<operator, list, rightward> X<list operator>
813 On the right side of a list operator, it has very low precedence,
814 such that it controls all comma-separated expressions found there.
815 The only operators with lower precedence are the logical operators
816 "and", "or", and "not", which may be used to evaluate calls to list
817 operators without the need for extra parentheses:
819 open HANDLE, "filename"
820 or die "Can't open: $!\n";
822 See also discussion of list operators in L<Terms and List Operators (Leftward)>.
825 X<operator, logical, not> X<not>
827 Unary "not" returns the logical negation of the expression to its right.
828 It's the equivalent of "!" except for the very low precedence.
831 X<operator, logical, and> X<and>
833 Binary "and" returns the logical conjunction of the two surrounding
834 expressions. It's equivalent to && except for the very low
835 precedence. This means that it short-circuits: i.e., the right
836 expression is evaluated only if the left expression is true.
838 =head2 Logical or, Defined or, and Exclusive Or
839 X<operator, logical, or> X<operator, logical, xor>
840 X<operator, logical, defined or> X<operator, logical, exclusive or>
843 Binary "or" returns the logical disjunction of the two surrounding
844 expressions. It's equivalent to || except for the very low precedence.
845 This makes it useful for control flow
847 print FH $data or die "Can't write to FH: $!";
849 This means that it short-circuits: i.e., the right expression is evaluated
850 only if the left expression is false. Due to its precedence, you should
851 probably avoid using this for assignment, only for control flow.
853 $a = $b or $c; # bug: this is wrong
854 ($a = $b) or $c; # really means this
855 $a = $b || $c; # better written this way
857 However, when it's a list-context assignment and you're trying to use
858 "||" for control flow, you probably need "or" so that the assignment
859 takes higher precedence.
861 @info = stat($file) || die; # oops, scalar sense of stat!
862 @info = stat($file) or die; # better, now @info gets its due
864 Then again, you could always use parentheses.
866 Binary "xor" returns the exclusive-OR of the two surrounding expressions.
867 It cannot short circuit, of course.
869 =head2 C Operators Missing From Perl
870 X<operator, missing from perl> X<&> X<*>
871 X<typecasting> X<(TYPE)>
873 Here is what C has that Perl doesn't:
879 Address-of operator. (But see the "\" operator for taking a reference.)
883 Dereference-address operator. (Perl's prefix dereferencing
884 operators are typed: $, @, %, and &.)
888 Type-casting operator.
892 =head2 Quote and Quote-like Operators
893 X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m>
894 X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>>
895 X<escape sequence> X<escape>
898 While we usually think of quotes as literal values, in Perl they
899 function as operators, providing various kinds of interpolating and
900 pattern matching capabilities. Perl provides customary quote characters
901 for these behaviors, but also provides a way for you to choose your
902 quote character for any of them. In the following table, a C<{}> represents
903 any pair of delimiters you choose.
905 Customary Generic Meaning Interpolates
910 // m{} Pattern match yes*
912 s{}{} Substitution yes*
913 tr{}{} Transliteration no (but see below)
916 * unless the delimiter is ''.
918 Non-bracketing delimiters use the same character fore and aft, but the four
919 sorts of brackets (round, angle, square, curly) will all nest, which means
928 Note, however, that this does not always work for quoting Perl code:
930 $s = q{ if($a eq "}") ... }; # WRONG
932 is a syntax error. The C<Text::Balanced> module (from CPAN, and
933 starting from Perl 5.8 part of the standard distribution) is able
936 There can be whitespace between the operator and the quoting
937 characters, except when C<#> is being used as the quoting character.
938 C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
939 operator C<q> followed by a comment. Its argument will be taken
940 from the next line. This allows you to write:
942 s {foo} # Replace foo
945 The following escape sequences are available in constructs that interpolate
946 and in transliterations.
947 X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N>
954 \a alarm (bell) (BEL)
956 \033 octal char (example: ESC)
957 \x1b hex char (example: ESC)
958 \x{263a} wide hex char (example: SMILEY)
959 \c[ control char (example: ESC)
960 \N{name} named Unicode character
962 The character following C<\c> is mapped to some other character by
963 converting letters to upper case and then (on ASCII systems) by inverting
964 the 7th bit (0x40). The most interesting range is from '@' to '_'
965 (0x40 through 0x5F), resulting in a control character from 0x00
966 through 0x1F. A '?' maps to the DEL character. On EBCDIC systems only
967 '@', the letters, '[', '\', ']', '^', '_' and '?' will work, resulting
968 in 0x00 through 0x1F and 0x7F.
970 B<NOTE>: Unlike C and other languages, Perl has no \v escape sequence for
971 the vertical tab (VT - ASCII 11), but you may use C<\ck> or C<\x0b>.
973 The following escape sequences are available in constructs that interpolate
974 but not in transliterations.
975 X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q>
977 \l lowercase next char
978 \u uppercase next char
981 \E end case modification
982 \Q quote non-word characters till \E
984 If C<use locale> is in effect, the case map used by C<\l>, C<\L>,
985 C<\u> and C<\U> is taken from the current locale. See L<perllocale>.
986 If Unicode (for example, C<\N{}> or wide hex characters of 0x100 or
987 beyond) is being used, the case map used by C<\l>, C<\L>, C<\u> and
988 C<\U> is as defined by Unicode. For documentation of C<\N{name}>,
991 All systems use the virtual C<"\n"> to represent a line terminator,
992 called a "newline". There is no such thing as an unvarying, physical
993 newline character. It is only an illusion that the operating system,
994 device drivers, C libraries, and Perl all conspire to preserve. Not all
995 systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
996 on a Mac, these are reversed, and on systems without line terminator,
997 printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
998 you mean a "newline" for your system, but use the literal ASCII when you
999 need an exact character. For example, most networking protocols expect
1000 and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
1001 and although they often accept just C<"\012">, they seldom tolerate just
1002 C<"\015">. If you get in the habit of using C<"\n"> for networking,
1003 you may be burned some day.
1004 X<newline> X<line terminator> X<eol> X<end of line>
1007 For constructs that do interpolate, variables beginning with "C<$>"
1008 or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
1009 C<< $href->{key}[0] >> are also interpolated, as are array and hash slices.
1010 But method calls such as C<< $obj->meth >> are not.
1012 Interpolating an array or slice interpolates the elements in order,
1013 separated by the value of C<$">, so is equivalent to interpolating
1014 C<join $", @array>. "Punctuation" arrays such as C<@*> are only
1015 interpolated if the name is enclosed in braces C<@{*}>, but special
1016 arrays C<@_>, C<@+>, and C<@-> are interpolated, even without braces.
1018 You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
1019 An unescaped C<$> or C<@> interpolates the corresponding variable,
1020 while escaping will cause the literal string C<\$> to be inserted.
1021 You'll need to write something like C<m/\Quser\E\@\Qhost/>.
1023 Patterns are subject to an additional level of interpretation as a
1024 regular expression. This is done as a second pass, after variables are
1025 interpolated, so that regular expressions may be incorporated into the
1026 pattern from the variables. If this is not what you want, use C<\Q> to
1027 interpolate a variable literally.
1029 Apart from the behavior described above, Perl does not expand
1030 multiple levels of interpolation. In particular, contrary to the
1031 expectations of shell programmers, back-quotes do I<NOT> interpolate
1032 within double quotes, nor do single quotes impede evaluation of
1033 variables when used within double quotes.
1035 =head2 Regexp Quote-Like Operators
1038 Here are the quote-like operators that apply to pattern
1039 matching and related activities.
1043 =item qr/STRING/msixpo
1044 X<qr> X</i> X</m> X</o> X</s> X</x> X</p>
1046 This operator quotes (and possibly compiles) its I<STRING> as a regular
1047 expression. I<STRING> is interpolated the same way as I<PATTERN>
1048 in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
1049 is done. Returns a Perl value which may be used instead of the
1050 corresponding C</STRING/msixpo> expression. The returned value is a
1051 normalized version of the original pattern. It magically differs from
1052 a string containing the same characters: C<ref(qr/x/)> returns "Regexp",
1053 even though dereferencing the result returns undef.
1057 $rex = qr/my.STRING/is;
1058 print $rex; # prints (?si-xm:my.STRING)
1065 The result may be used as a subpattern in a match:
1068 $string =~ /foo${re}bar/; # can be interpolated in other patterns
1069 $string =~ $re; # or used standalone
1070 $string =~ /$re/; # or this way
1072 Since Perl may compile the pattern at the moment of execution of qr()
1073 operator, using qr() may have speed advantages in some situations,
1074 notably if the result of qr() is used standalone:
1077 my $patterns = shift;
1078 my @compiled = map qr/$_/i, @$patterns;
1081 foreach my $pat (@compiled) {
1082 $success = 1, last if /$pat/;
1088 Precompilation of the pattern into an internal representation at
1089 the moment of qr() avoids a need to recompile the pattern every
1090 time a match C</$pat/> is attempted. (Perl has many other internal
1091 optimizations, but none would be triggered in the above example if
1092 we did not use qr() operator.)
1096 m Treat string as multiple lines.
1097 s Treat string as single line. (Make . match a newline)
1098 i Do case-insensitive pattern matching.
1099 x Use extended regular expressions.
1100 p When matching preserve a copy of the matched string so
1101 that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be defined.
1102 o Compile pattern only once.
1104 If a precompiled pattern is embedded in a larger pattern then the effect
1105 of 'msixp' will be propagated appropriately. The effect of the 'o'
1106 modifier has is not propagated, being restricted to those patterns
1107 explicitly using it.
1109 See L<perlre> for additional information on valid syntax for STRING, and
1110 for a detailed look at the semantics of regular expressions.
1112 =item m/PATTERN/msixpogc
1113 X<m> X<operator, match>
1114 X<regexp, options> X<regexp> X<regex, options> X<regex>
1115 X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c>
1117 =item /PATTERN/msixpogc
1119 Searches a string for a pattern match, and in scalar context returns
1120 true if it succeeds, false if it fails. If no string is specified
1121 via the C<=~> or C<!~> operator, the $_ string is searched. (The
1122 string specified with C<=~> need not be an lvalue--it may be the
1123 result of an expression evaluation, but remember the C<=~> binds
1124 rather tightly.) See also L<perlre>. See L<perllocale> for
1125 discussion of additional considerations that apply when C<use locale>
1128 Options are as described in C<qr//>; in addition, the following match
1129 process modifiers are available:
1131 g Match globally, i.e., find all occurrences.
1132 c Do not reset search position on a failed match when /g is in effect.
1134 If "/" is the delimiter then the initial C<m> is optional. With the C<m>
1135 you can use any pair of non-alphanumeric, non-whitespace characters
1136 as delimiters. This is particularly useful for matching path names
1137 that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
1138 the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
1139 If "'" is the delimiter, no interpolation is performed on the PATTERN.
1141 PATTERN may contain variables, which will be interpolated (and the
1142 pattern recompiled) every time the pattern search is evaluated, except
1143 for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
1144 C<$|> are not interpolated because they look like end-of-string tests.)
1145 If you want such a pattern to be compiled only once, add a C</o> after
1146 the trailing delimiter. This avoids expensive run-time recompilations,
1147 and is useful when the value you are interpolating won't change over
1148 the life of the script. However, mentioning C</o> constitutes a promise
1149 that you won't change the variables in the pattern. If you change them,
1150 Perl won't even notice. See also L<"qr/STRING/msixpo">.
1152 If the PATTERN evaluates to the empty string, the last
1153 I<successfully> matched regular expression is used instead. In this
1154 case, only the C<g> and C<c> flags on the empty pattern is honoured -
1155 the other flags are taken from the original pattern. If no match has
1156 previously succeeded, this will (silently) act instead as a genuine
1157 empty pattern (which will always match).
1159 Note that it's possible to confuse Perl into thinking C<//> (the empty
1160 regex) is really C<//> (the defined-or operator). Perl is usually pretty
1161 good about this, but some pathological cases might trigger this, such as
1162 C<$a///> (is that C<($a) / (//)> or C<$a // />?) and C<print $fh //>
1163 (C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl
1164 will assume you meant defined-or. If you meant the empty regex, just
1165 use parentheses or spaces to disambiguate, or even prefix the empty
1166 regex with an C<m> (so C<//> becomes C<m//>).
1168 If the C</g> option is not used, C<m//> in list context returns a
1169 list consisting of the subexpressions matched by the parentheses in the
1170 pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are
1171 also set, and that this differs from Perl 4's behavior.) When there are
1172 no parentheses in the pattern, the return value is the list C<(1)> for
1173 success. With or without parentheses, an empty list is returned upon
1178 open(TTY, '/dev/tty');
1179 <TTY> =~ /^y/i && foo(); # do foo if desired
1181 if (/Version: *([0-9.]*)/) { $version = $1; }
1183 next if m#^/usr/spool/uucp#;
1188 print if /$arg/o; # compile only once
1191 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
1193 This last example splits $foo into the first two words and the
1194 remainder of the line, and assigns those three fields to $F1, $F2, and
1195 $Etc. The conditional is true if any variables were assigned, i.e., if
1196 the pattern matched.
1198 The C</g> modifier specifies global pattern matching--that is,
1199 matching as many times as possible within the string. How it behaves
1200 depends on the context. In list context, it returns a list of the
1201 substrings matched by any capturing parentheses in the regular
1202 expression. If there are no parentheses, it returns a list of all
1203 the matched strings, as if there were parentheses around the whole
1206 In scalar context, each execution of C<m//g> finds the next match,
1207 returning true if it matches, and false if there is no further match.
1208 The position after the last match can be read or set using the pos()
1209 function; see L<perlfunc/pos>. A failed match normally resets the
1210 search position to the beginning of the string, but you can avoid that
1211 by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
1212 string also resets the search position.
1214 You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
1215 zero-width assertion that matches the exact position where the previous
1216 C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion
1217 still anchors at pos(), but the match is of course only attempted once.
1218 Using C<\G> without C</g> on a target string that has not previously had a
1219 C</g> match applied to it is the same as using the C<\A> assertion to match
1220 the beginning of the string. Note also that, currently, C<\G> is only
1221 properly supported when anchored at the very beginning of the pattern.
1226 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
1230 while (defined($paragraph = <>)) {
1231 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
1235 print "$sentences\n";
1237 # using m//gc with \G
1241 print $1 while /(o)/gc; print "', pos=", pos, "\n";
1243 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
1245 print $1 while /(p)/gc; print "', pos=", pos, "\n";
1247 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
1249 The last example should print:
1259 Notice that the final match matched C<q> instead of C<p>, which a match
1260 without the C<\G> anchor would have done. Also note that the final match
1261 did not update C<pos> -- C<pos> is only updated on a C</g> match. If the
1262 final match did indeed match C<p>, it's a good bet that you're running an
1263 older (pre-5.6.0) Perl.
1265 A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
1266 combine several regexps like this to process a string part-by-part,
1267 doing different actions depending on which regexp matched. Each
1268 regexp tries to match where the previous one leaves off.
1271 $url = URI::URL->new( "http://www/" ); die if $url eq "xXx";
1275 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
1276 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
1277 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
1278 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
1279 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
1280 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
1281 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
1282 print ". That's all!\n";
1285 Here is the output (split into several lines):
1287 line-noise lowercase line-noise lowercase UPPERCASE line-noise
1288 UPPERCASE line-noise lowercase line-noise lowercase line-noise
1289 lowercase lowercase line-noise lowercase lowercase line-noise
1290 MiXeD line-noise. That's all!
1295 This is just like the C</pattern/> search, except that it matches only
1296 once between calls to the reset() operator. This is a useful
1297 optimization when you want to see only the first occurrence of
1298 something in each file of a set of files, for instance. Only C<??>
1299 patterns local to the current package are reset.
1303 # blank line between header and body
1306 reset if eof; # clear ?? status for next file
1309 This usage is vaguely deprecated, which means it just might possibly
1310 be removed in some distant future version of Perl, perhaps somewhere
1311 around the year 2168.
1313 =item s/PATTERN/REPLACEMENT/msixpogce
1314 X<substitute> X<substitution> X<replace> X<regexp, replace>
1315 X<regexp, substitute> X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> X</e>
1317 Searches a string for a pattern, and if found, replaces that pattern
1318 with the replacement text and returns the number of substitutions
1319 made. Otherwise it returns false (specifically, the empty string).
1321 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
1322 variable is searched and modified. (The string specified with C<=~> must
1323 be scalar variable, an array element, a hash element, or an assignment
1324 to one of those, i.e., an lvalue.)
1326 If the delimiter chosen is a single quote, no interpolation is
1327 done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1328 PATTERN contains a $ that looks like a variable rather than an
1329 end-of-string test, the variable will be interpolated into the pattern
1330 at run-time. If you want the pattern compiled only once the first time
1331 the variable is interpolated, use the C</o> option. If the pattern
1332 evaluates to the empty string, the last successfully executed regular
1333 expression is used instead. See L<perlre> for further explanation on these.
1334 See L<perllocale> for discussion of additional considerations that apply
1335 when C<use locale> is in effect.
1337 Options are as with m// with the addition of the following replacement
1340 e Evaluate the right side as an expression.
1341 ee Evaluate the right side as a string then eval the result
1343 Any non-alphanumeric, non-whitespace delimiter may replace the
1344 slashes. If single quotes are used, no interpretation is done on the
1345 replacement string (the C</e> modifier overrides this, however). Unlike
1346 Perl 4, Perl 5 treats backticks as normal delimiters; the replacement
1347 text is not evaluated as a command. If the
1348 PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
1349 pair of quotes, which may or may not be bracketing quotes, e.g.,
1350 C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
1351 replacement portion to be treated as a full-fledged Perl expression
1352 and evaluated right then and there. It is, however, syntax checked at
1353 compile-time. A second C<e> modifier will cause the replacement portion
1354 to be C<eval>ed before being run as a Perl expression.
1358 s/\bgreen\b/mauve/g; # don't change wintergreen
1360 $path =~ s|/usr/bin|/usr/local/bin|;
1362 s/Login: $foo/Login: $bar/; # run-time pattern
1364 ($foo = $bar) =~ s/this/that/; # copy first, then change
1366 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
1369 s/\d+/$&*2/e; # yields 'abc246xyz'
1370 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
1371 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1373 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
1374 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
1375 s/^=(\w+)/pod($1)/ge; # use function call
1377 # expand variables in $_, but dynamics only, using
1378 # symbolic dereferencing
1381 # Add one to the value of any numbers in the string
1384 # This will expand any embedded scalar variable
1385 # (including lexicals) in $_ : First $1 is interpolated
1386 # to the variable name, and then evaluated
1389 # Delete (most) C comments.
1391 /\* # Match the opening delimiter.
1392 .*? # Match a minimal number of characters.
1393 \*/ # Match the closing delimiter.
1396 s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_, expensively
1398 for ($variable) { # trim whitespace in $variable, cheap
1403 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
1405 Note the use of $ instead of \ in the last example. Unlike
1406 B<sed>, we use the \<I<digit>> form in only the left hand side.
1407 Anywhere else it's $<I<digit>>.
1409 Occasionally, you can't use just a C</g> to get all the changes
1410 to occur that you might want. Here are two common cases:
1412 # put commas in the right places in an integer
1413 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
1415 # expand tabs to 8-column spacing
1416 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
1420 =head2 Quote-Like Operators
1421 X<operator, quote-like>
1426 X<q> X<quote, single> X<'> X<''>
1430 A single-quoted, literal string. A backslash represents a backslash
1431 unless followed by the delimiter or another backslash, in which case
1432 the delimiter or backslash is interpolated.
1434 $foo = q!I said, "You said, 'She said it.'"!;
1435 $bar = q('This is it.');
1436 $baz = '\n'; # a two-character string
1439 X<qq> X<quote, double> X<"> X<"">
1443 A double-quoted, interpolated string.
1446 (*** The previous line contains the naughty word "$1".\n)
1447 if /\b(tcl|java|python)\b/i; # :-)
1448 $baz = "\n"; # a one-character string
1451 X<qx> X<`> X<``> X<backtick>
1455 A string which is (possibly) interpolated and then executed as a
1456 system command with C</bin/sh> or its equivalent. Shell wildcards,
1457 pipes, and redirections will be honored. The collected standard
1458 output of the command is returned; standard error is unaffected. In
1459 scalar context, it comes back as a single (potentially multi-line)
1460 string, or undef if the command failed. In list context, returns a
1461 list of lines (however you've defined lines with $/ or
1462 $INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
1464 Because backticks do not affect standard error, use shell file descriptor
1465 syntax (assuming the shell supports this) if you care to address this.
1466 To capture a command's STDERR and STDOUT together:
1468 $output = `cmd 2>&1`;
1470 To capture a command's STDOUT but discard its STDERR:
1472 $output = `cmd 2>/dev/null`;
1474 To capture a command's STDERR but discard its STDOUT (ordering is
1477 $output = `cmd 2>&1 1>/dev/null`;
1479 To exchange a command's STDOUT and STDERR in order to capture the STDERR
1480 but leave its STDOUT to come out the old STDERR:
1482 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
1484 To read both a command's STDOUT and its STDERR separately, it's easiest
1485 to redirect them separately to files, and then read from those files
1486 when the program is done:
1488 system("program args 1>program.stdout 2>program.stderr");
1490 The STDIN filehandle used by the command is inherited from Perl's STDIN.
1493 open BLAM, "blam" || die "Can't open: $!";
1494 open STDIN, "<&BLAM";
1497 will print the sorted contents of the file "blam".
1499 Using single-quote as a delimiter protects the command from Perl's
1500 double-quote interpolation, passing it on to the shell instead:
1502 $perl_info = qx(ps $$); # that's Perl's $$
1503 $shell_info = qx'ps $$'; # that's the new shell's $$
1505 How that string gets evaluated is entirely subject to the command
1506 interpreter on your system. On most platforms, you will have to protect
1507 shell metacharacters if you want them treated literally. This is in
1508 practice difficult to do, as it's unclear how to escape which characters.
1509 See L<perlsec> for a clean and safe example of a manual fork() and exec()
1510 to emulate backticks safely.
1512 On some platforms (notably DOS-like ones), the shell may not be
1513 capable of dealing with multiline commands, so putting newlines in
1514 the string may not get you what you want. You may be able to evaluate
1515 multiple commands in a single line by separating them with the command
1516 separator character, if your shell supports that (e.g. C<;> on many Unix
1517 shells; C<&> on the Windows NT C<cmd> shell).
1519 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1520 output before starting the child process, but this may not be supported
1521 on some platforms (see L<perlport>). To be safe, you may need to set
1522 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1523 C<IO::Handle> on any open handles.
1525 Beware that some command shells may place restrictions on the length
1526 of the command line. You must ensure your strings don't exceed this
1527 limit after any necessary interpolations. See the platform-specific
1528 release notes for more details about your particular environment.
1530 Using this operator can lead to programs that are difficult to port,
1531 because the shell commands called vary between systems, and may in
1532 fact not be present at all. As one example, the C<type> command under
1533 the POSIX shell is very different from the C<type> command under DOS.
1534 That doesn't mean you should go out of your way to avoid backticks
1535 when they're the right way to get something done. Perl was made to be
1536 a glue language, and one of the things it glues together is commands.
1537 Just understand what you're getting yourself into.
1539 See L</"I/O Operators"> for more discussion.
1542 X<qw> X<quote, list> X<quote, words>
1544 Evaluates to a list of the words extracted out of STRING, using embedded
1545 whitespace as the word delimiters. It can be understood as being roughly
1548 split(' ', q/STRING/);
1550 the differences being that it generates a real list at compile time, and
1551 in scalar context it returns the last element in the list. So
1556 is semantically equivalent to the list:
1560 Some frequently seen examples:
1562 use POSIX qw( setlocale localeconv )
1563 @EXPORT = qw( foo bar baz );
1565 A common mistake is to try to separate the words with comma or to
1566 put comments into a multi-line C<qw>-string. For this reason, the
1567 C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
1568 produces warnings if the STRING contains the "," or the "#" character.
1571 =item tr/SEARCHLIST/REPLACEMENTLIST/cds
1572 X<tr> X<y> X<transliterate> X</c> X</d> X</s>
1574 =item y/SEARCHLIST/REPLACEMENTLIST/cds
1576 Transliterates all occurrences of the characters found in the search list
1577 with the corresponding character in the replacement list. It returns
1578 the number of characters replaced or deleted. If no string is
1579 specified via the =~ or !~ operator, the $_ string is transliterated. (The
1580 string specified with =~ must be a scalar variable, an array element, a
1581 hash element, or an assignment to one of those, i.e., an lvalue.)
1583 A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
1584 does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
1585 For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1586 SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1587 its own pair of quotes, which may or may not be bracketing quotes,
1588 e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
1590 Note that C<tr> does B<not> do regular expression character classes
1591 such as C<\d> or C<[:lower:]>. The C<tr> operator is not equivalent to
1592 the tr(1) utility. If you want to map strings between lower/upper
1593 cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider
1594 using the C<s> operator if you need regular expressions.
1596 Note also that the whole range idea is rather unportable between
1597 character sets--and even within character sets they may cause results
1598 you probably didn't expect. A sound principle is to use only ranges
1599 that begin from and end at either alphabets of equal case (a-e, A-E),
1600 or digits (0-4). Anything else is unsafe. If in doubt, spell out the
1601 character sets in full.
1605 c Complement the SEARCHLIST.
1606 d Delete found but unreplaced characters.
1607 s Squash duplicate replaced characters.
1609 If the C</c> modifier is specified, the SEARCHLIST character set
1610 is complemented. If the C</d> modifier is specified, any characters
1611 specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
1612 (Note that this is slightly more flexible than the behavior of some
1613 B<tr> programs, which delete anything they find in the SEARCHLIST,
1614 period.) If the C</s> modifier is specified, sequences of characters
1615 that were transliterated to the same character are squashed down
1616 to a single instance of the character.
1618 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1619 exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1620 than the SEARCHLIST, the final character is replicated till it is long
1621 enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
1622 This latter is useful for counting characters in a class or for
1623 squashing character sequences in a class.
1627 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1629 $cnt = tr/*/*/; # count the stars in $_
1631 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1633 $cnt = tr/0-9//; # count the digits in $_
1635 tr/a-zA-Z//s; # bookkeeper -> bokeper
1637 ($HOST = $host) =~ tr/a-z/A-Z/;
1639 tr/a-zA-Z/ /cs; # change non-alphas to single space
1642 [\000-\177]; # delete 8th bit
1644 If multiple transliterations are given for a character, only the
1649 will transliterate any A to X.
1651 Because the transliteration table is built at compile time, neither
1652 the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
1653 interpolation. That means that if you want to use variables, you
1656 eval "tr/$oldlist/$newlist/";
1659 eval "tr/$oldlist/$newlist/, 1" or die $@;
1662 X<here-doc> X<heredoc> X<here-document> X<<< << >>>
1664 A line-oriented form of quoting is based on the shell "here-document"
1665 syntax. Following a C<< << >> you specify a string to terminate
1666 the quoted material, and all lines following the current line down to
1667 the terminating string are the value of the item.
1669 The terminating string may be either an identifier (a word), or some
1670 quoted text. An unquoted identifier works like double quotes.
1671 There may not be a space between the C<< << >> and the identifier,
1672 unless the identifier is explicitly quoted. (If you put a space it
1673 will be treated as a null identifier, which is valid, and matches the
1674 first empty line.) The terminating string must appear by itself
1675 (unquoted and with no surrounding whitespace) on the terminating line.
1677 If the terminating string is quoted, the type of quotes used determine
1678 the treatment of the text.
1684 Double quotes indicate that the text will be interpolated using exactly
1685 the same rules as normal double quoted strings.
1688 The price is $Price.
1691 print << "EOF"; # same as above
1692 The price is $Price.
1698 Single quotes indicate the text is to be treated literally with no
1699 interpolation of its content. This is similar to single quoted
1700 strings except that backslashes have no special meaning, with C<\\>
1701 being treated as two backslashes and not one as they would in every
1702 other quoting construct.
1704 This is the only form of quoting in perl where there is no need
1705 to worry about escaping content, something that code generators
1706 can and do make good use of.
1710 The content of the here doc is treated just as it would be if the
1711 string were embedded in backticks. Thus the content is interpolated
1712 as though it were double quoted and then executed via the shell, with
1713 the results of the execution returned.
1715 print << `EOC`; # execute command and get results
1721 It is possible to stack multiple here-docs in a row:
1723 print <<"foo", <<"bar"; # you can stack them
1729 myfunc(<< "THIS", 23, <<'THAT');
1736 Just don't forget that you have to put a semicolon on the end
1737 to finish the statement, as Perl doesn't know you're not going to
1745 If you want to remove the line terminator from your here-docs,
1748 chomp($string = <<'END');
1752 If you want your here-docs to be indented with the rest of the code,
1753 you'll need to remove leading whitespace from each line manually:
1755 ($quote = <<'FINIS') =~ s/^\s+//gm;
1756 The Road goes ever on and on,
1757 down from the door where it began.
1760 If you use a here-doc within a delimited construct, such as in C<s///eg>,
1761 the quoted material must come on the lines following the final delimiter.
1776 If the terminating identifier is on the last line of the program, you
1777 must be sure there is a newline after it; otherwise, Perl will give the
1778 warning B<Can't find string terminator "END" anywhere before EOF...>.
1780 Additionally, the quoting rules for the end of string identifier are not
1781 related to Perl's quoting rules -- C<q()>, C<qq()>, and the like are not
1782 supported in place of C<''> and C<"">, and the only interpolation is for
1783 backslashing the quoting character:
1785 print << "abc\"def";
1789 Finally, quoted strings cannot span multiple lines. The general rule is
1790 that the identifier must be a string literal. Stick with that, and you
1795 =head2 Gory details of parsing quoted constructs
1796 X<quote, gory details>
1798 When presented with something that might have several different
1799 interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
1800 principle to pick the most probable interpretation. This strategy
1801 is so successful that Perl programmers often do not suspect the
1802 ambivalence of what they write. But from time to time, Perl's
1803 notions differ substantially from what the author honestly meant.
1805 This section hopes to clarify how Perl handles quoted constructs.
1806 Although the most common reason to learn this is to unravel labyrinthine
1807 regular expressions, because the initial steps of parsing are the
1808 same for all quoting operators, they are all discussed together.
1810 The most important Perl parsing rule is the first one discussed
1811 below: when processing a quoted construct, Perl first finds the end
1812 of that construct, then interprets its contents. If you understand
1813 this rule, you may skip the rest of this section on the first
1814 reading. The other rules are likely to contradict the user's
1815 expectations much less frequently than this first one.
1817 Some passes discussed below are performed concurrently, but because
1818 their results are the same, we consider them individually. For different
1819 quoting constructs, Perl performs different numbers of passes, from
1820 one to four, but these passes are always performed in the same order.
1824 =item Finding the end
1826 The first pass is finding the end of the quoted construct, where
1827 the information about the delimiters is used in parsing.
1828 During this search, text between the starting and ending delimiters
1829 is copied to a safe location. The text copied gets delimiter-independent.
1831 If the construct is a here-doc, the ending delimiter is a line
1832 that has a terminating string as the content. Therefore C<<<EOF> is
1833 terminated by C<EOF> immediately followed by C<"\n"> and starting
1834 from the first column of the terminating line.
1835 When searching for the terminating line of a here-doc, nothing
1836 is skipped. In other words, lines after the here-doc syntax
1837 are compared with the terminating string line by line.
1839 For the constructs except here-docs, single characters are used as starting
1840 and ending delimiters. If the starting delimiter is an opening punctuation
1841 (that is C<(>, C<[>, C<{>, or C<< < >>), the ending delimiter is the
1842 corresponding closing punctuation (that is C<)>, C<]>, C<}>, or C<< > >>).
1843 If the starting delimiter is an unpaired character like C</> or a closing
1844 punctuation, the ending delimiter is same as the starting delimiter.
1845 Therefore a C</> terminates a C<qq//> construct, while a C<]> terminates
1846 C<qq[]> and C<qq]]> constructs.
1848 When searching for single-character delimiters, escaped delimiters
1849 and C<\\> are skipped. For example, while searching for terminating C</>,
1850 combinations of C<\\> and C<\/> are skipped. If the delimiters are
1851 bracketing, nested pairs are also skipped. For example, while searching
1852 for closing C<]> paired with the opening C<[>, combinations of C<\\>, C<\]>,
1853 and C<\[> are all skipped, and nested C<[> and C<]> are skipped as well.
1854 However, when backslashes are used as the delimiters (like C<qq\\> and
1855 C<tr\\\>), nothing is skipped.
1856 During the search for the end, backslashes that escape delimiters
1857 are removed (exactly speaking, they are not copied to the safe location).
1859 For constructs with three-part delimiters (C<s///>, C<y///>, and
1860 C<tr///>), the search is repeated once more.
1861 If the first delimiter is not an opening punctuation, three delimiters must
1862 be same such as C<s!!!> and C<tr)))>, in which case the second delimiter
1863 terminates the left part and starts the right part at once.
1864 If the left part is delimited by bracketing punctuations (that is C<()>,
1865 C<[]>, C<{}>, or C<< <> >>), the right part needs another pair of
1866 delimiters such as C<s(){}> and C<tr[]//>. In these cases, whitespaces
1867 and comments are allowed between both parts, though the comment must follow
1868 at least one whitespace; otherwise a character expected as the start of
1869 the comment may be regarded as the starting delimiter of the right part.
1871 During this search no attention is paid to the semantics of the construct.
1874 "$hash{"$foo/$bar"}"
1879 bar # NOT a comment, this slash / terminated m//!
1882 do not form legal quoted expressions. The quoted part ends on the
1883 first C<"> and C</>, and the rest happens to be a syntax error.
1884 Because the slash that terminated C<m//> was followed by a C<SPACE>,
1885 the example above is not C<m//x>, but rather C<m//> with no C</x>
1886 modifier. So the embedded C<#> is interpreted as a literal C<#>.
1888 Also no attention is paid to C<\c\> (multichar control char syntax) during
1889 this search. Thus the second C<\> in C<qq/\c\/> is interpreted as a part
1890 of C<\/>, and the following C</> is not recognized as a delimiter.
1891 Instead, use C<\034> or C<\x1c> at the end of quoted constructs.
1896 The next step is interpolation in the text obtained, which is now
1897 delimiter-independent. There are multiple cases.
1903 No interpolation is performed.
1904 Note that the combination C<\\> is left intact, since escaped delimiters
1905 are not available for here-docs.
1907 =item C<m''>, the pattern of C<s'''>
1909 No interpolation is performed at this stage.
1910 Any backslashed sequences including C<\\> are treated at the stage
1911 to L</"parsing regular expressions">.
1913 =item C<''>, C<q//>, C<tr'''>, C<y'''>, the replacement of C<s'''>
1915 The only interpolation is removal of C<\> from pairs of C<\\>.
1916 Therefore C<-> in C<tr'''> and C<y'''> is treated literally
1917 as a hyphen and no character range is available.
1918 C<\1> in the replacement of C<s'''> does not work as C<$1>.
1920 =item C<tr///>, C<y///>
1922 No variable interpolation occurs. String modifying combinations for
1923 case and quoting such as C<\Q>, C<\U>, and C<\E> are not recognized.
1924 The other escape sequences such as C<\200> and C<\t> and backslashed
1925 characters such as C<\\> and C<\-> are converted to appropriate literals.
1926 The character C<-> is treated specially and therefore C<\-> is treated
1929 =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF">
1931 C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
1932 converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
1933 is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
1934 The other escape sequences such as C<\200> and C<\t> and backslashed
1935 characters such as C<\\> and C<\-> are replaced with appropriate
1938 Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
1939 is interpolated in the usual way. Something like C<"\Q\\E"> has
1940 no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
1941 result is the same as for C<"\\\\E">. As a general rule, backslashes
1942 between C<\Q> and C<\E> may lead to counterintuitive results. So,
1943 C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
1944 as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
1949 may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
1951 Interpolated scalars and arrays are converted internally to the C<join> and
1952 C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
1954 $foo . " XXX '" . (join $", @arr) . "'";
1956 All operations above are performed simultaneously, left to right.
1958 Because the result of C<"\Q STRING \E"> has all metacharacters
1959 quoted, there is no way to insert a literal C<$> or C<@> inside a
1960 C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
1961 C<"\\\$">; if not, it is interpreted as the start of an interpolated
1964 Note also that the interpolation code needs to make a decision on
1965 where the interpolated scalar ends. For instance, whether
1966 C<< "a $b -> {c}" >> really means:
1968 "a " . $b . " -> {c}";
1974 Most of the time, the longest possible text that does not include
1975 spaces between components and which contains matching braces or
1976 brackets. because the outcome may be determined by voting based
1977 on heuristic estimators, the result is not strictly predictable.
1978 Fortunately, it's usually correct for ambiguous cases.
1980 =item the replacement of C<s///>
1982 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
1983 happens as with C<qq//> constructs.
1985 It is at this step that C<\1> is begrudgingly converted to C<$1> in
1986 the replacement text of C<s///>, in order to correct the incorrigible
1987 I<sed> hackers who haven't picked up the saner idiom yet. A warning
1988 is emitted if the C<use warnings> pragma or the B<-w> command-line flag
1989 (that is, the C<$^W> variable) was set.
1991 =item C<RE> in C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
1993 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\E>,
1994 and interpolation happens (almost) as with C<qq//> constructs.
1996 However any other combinations of C<\> followed by a character
1997 are not substituted but only skipped, in order to parse them
1998 as regular expressions at the following step.
1999 As C<\c> is skipped at this step, C<@> of C<\c@> in RE is possibly
2000 treated as an array symbol (for example C<@foo>),
2001 even though the same text in C<qq//> gives interpolation of C<\c@>.
2003 Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
2004 a C<#>-comment in a C<//x>-regular expression, no processing is
2005 performed whatsoever. This is the first step at which the presence
2006 of the C<//x> modifier is relevant.
2008 Interpolation in patterns has several quirks: C<$|>, C<$(>, C<$)>, C<@+>
2009 and C<@-> are not interpolated, and constructs C<$var[SOMETHING]> are
2010 voted (by several different estimators) to be either an array element
2011 or C<$var> followed by an RE alternative. This is where the notation
2012 C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
2013 array element C<-9>, not as a regular expression from the variable
2014 C<$arr> followed by a digit, which would be the interpretation of
2015 C</$arr[0-9]/>. Since voting among different estimators may occur,
2016 the result is not predictable.
2018 The lack of processing of C<\\> creates specific restrictions on
2019 the post-processed text. If the delimiter is C</>, one cannot get
2020 the combination C<\/> into the result of this step. C</> will
2021 finish the regular expression, C<\/> will be stripped to C</> on
2022 the previous step, and C<\\/> will be left as is. Because C</> is
2023 equivalent to C<\/> inside a regular expression, this does not
2024 matter unless the delimiter happens to be character special to the
2025 RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
2026 alphanumeric char, as in:
2030 In the RE above, which is intentionally obfuscated for illustration, the
2031 delimiter is C<m>, the modifier is C<mx>, and after delimiter-removal the
2032 RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
2033 reason you're encouraged to restrict your delimiters to non-alphanumeric,
2034 non-whitespace choices.
2038 This step is the last one for all constructs except regular expressions,
2039 which are processed further.
2041 =item parsing regular expressions
2044 Previous steps were performed during the compilation of Perl code,
2045 but this one happens at run time--although it may be optimized to
2046 be calculated at compile time if appropriate. After preprocessing
2047 described above, and possibly after evaluation if concatenation,
2048 joining, casing translation, or metaquoting are involved, the
2049 resulting I<string> is passed to the RE engine for compilation.
2051 Whatever happens in the RE engine might be better discussed in L<perlre>,
2052 but for the sake of continuity, we shall do so here.
2054 This is another step where the presence of the C<//x> modifier is
2055 relevant. The RE engine scans the string from left to right and
2056 converts it to a finite automaton.
2058 Backslashed characters are either replaced with corresponding
2059 literal strings (as with C<\{>), or else they generate special nodes
2060 in the finite automaton (as with C<\b>). Characters special to the
2061 RE engine (such as C<|>) generate corresponding nodes or groups of
2062 nodes. C<(?#...)> comments are ignored. All the rest is either
2063 converted to literal strings to match, or else is ignored (as is
2064 whitespace and C<#>-style comments if C<//x> is present).
2066 Parsing of the bracketed character class construct, C<[...]>, is
2067 rather different than the rule used for the rest of the pattern.
2068 The terminator of this construct is found using the same rules as
2069 for finding the terminator of a C<{}>-delimited construct, the only
2070 exception being that C<]> immediately following C<[> is treated as
2071 though preceded by a backslash. Similarly, the terminator of
2072 C<(?{...})> is found using the same rules as for finding the
2073 terminator of a C<{}>-delimited construct.
2075 It is possible to inspect both the string given to RE engine and the
2076 resulting finite automaton. See the arguments C<debug>/C<debugcolor>
2077 in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
2078 switch documented in L<perlrun/"Command Switches">.
2080 =item Optimization of regular expressions
2081 X<regexp, optimization>
2083 This step is listed for completeness only. Since it does not change
2084 semantics, details of this step are not documented and are subject
2085 to change without notice. This step is performed over the finite
2086 automaton that was generated during the previous pass.
2088 It is at this stage that C<split()> silently optimizes C</^/> to
2093 =head2 I/O Operators
2094 X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle>
2097 There are several I/O operators you should know about.
2099 A string enclosed by backticks (grave accents) first undergoes
2100 double-quote interpolation. It is then interpreted as an external
2101 command, and the output of that command is the value of the
2102 backtick string, like in a shell. In scalar context, a single string
2103 consisting of all output is returned. In list context, a list of
2104 values is returned, one per line of output. (You can set C<$/> to use
2105 a different line terminator.) The command is executed each time the
2106 pseudo-literal is evaluated. The status value of the command is
2107 returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
2108 Unlike in B<csh>, no translation is done on the return data--newlines
2109 remain newlines. Unlike in any of the shells, single quotes do not
2110 hide variable names in the command from interpretation. To pass a
2111 literal dollar-sign through to the shell you need to hide it with a
2112 backslash. The generalized form of backticks is C<qx//>. (Because
2113 backticks always undergo shell expansion as well, see L<perlsec> for
2115 X<qx> X<`> X<``> X<backtick> X<glob>
2117 In scalar context, evaluating a filehandle in angle brackets yields
2118 the next line from that file (the newline, if any, included), or
2119 C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
2120 (sometimes known as file-slurp mode) and the file is empty, it
2121 returns C<''> the first time, followed by C<undef> subsequently.
2123 Ordinarily you must assign the returned value to a variable, but
2124 there is one situation where an automatic assignment happens. If
2125 and only if the input symbol is the only thing inside the conditional
2126 of a C<while> statement (even if disguised as a C<for(;;)> loop),
2127 the value is automatically assigned to the global variable $_,
2128 destroying whatever was there previously. (This may seem like an
2129 odd thing to you, but you'll use the construct in almost every Perl
2130 script you write.) The $_ variable is not implicitly localized.
2131 You'll have to put a C<local $_;> before the loop if you want that
2134 The following lines are equivalent:
2136 while (defined($_ = <STDIN>)) { print; }
2137 while ($_ = <STDIN>) { print; }
2138 while (<STDIN>) { print; }
2139 for (;<STDIN>;) { print; }
2140 print while defined($_ = <STDIN>);
2141 print while ($_ = <STDIN>);
2142 print while <STDIN>;
2144 This also behaves similarly, but avoids $_ :
2146 while (my $line = <STDIN>) { print $line }
2148 In these loop constructs, the assigned value (whether assignment
2149 is automatic or explicit) is then tested to see whether it is
2150 defined. The defined test avoids problems where line has a string
2151 value that would be treated as false by Perl, for example a "" or
2152 a "0" with no trailing newline. If you really mean for such values
2153 to terminate the loop, they should be tested for explicitly:
2155 while (($_ = <STDIN>) ne '0') { ... }
2156 while (<STDIN>) { last unless $_; ... }
2158 In other boolean contexts, C<< <I<filehandle>> >> without an
2159 explicit C<defined> test or comparison elicit a warning if the
2160 C<use warnings> pragma or the B<-w>
2161 command-line switch (the C<$^W> variable) is in effect.
2163 The filehandles STDIN, STDOUT, and STDERR are predefined. (The
2164 filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
2165 in packages, where they would be interpreted as local identifiers
2166 rather than global.) Additional filehandles may be created with
2167 the open() function, amongst others. See L<perlopentut> and
2168 L<perlfunc/open> for details on this.
2169 X<stdin> X<stdout> X<sterr>
2171 If a <FILEHANDLE> is used in a context that is looking for
2172 a list, a list comprising all input lines is returned, one line per
2173 list element. It's easy to grow to a rather large data space this
2174 way, so use with care.
2176 <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
2177 See L<perlfunc/readline>.
2179 The null filehandle <> is special: it can be used to emulate the
2180 behavior of B<sed> and B<awk>. Input from <> comes either from
2181 standard input, or from each file listed on the command line. Here's
2182 how it works: the first time <> is evaluated, the @ARGV array is
2183 checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
2184 gives you standard input. The @ARGV array is then processed as a list
2185 of filenames. The loop
2188 ... # code for each line
2191 is equivalent to the following Perl-like pseudo code:
2193 unshift(@ARGV, '-') unless @ARGV;
2194 while ($ARGV = shift) {
2197 ... # code for each line
2201 except that it isn't so cumbersome to say, and will actually work.
2202 It really does shift the @ARGV array and put the current filename
2203 into the $ARGV variable. It also uses filehandle I<ARGV>
2204 internally--<> is just a synonym for <ARGV>, which
2205 is magical. (The pseudo code above doesn't work because it treats
2206 <ARGV> as non-magical.)
2208 You can modify @ARGV before the first <> as long as the array ends up
2209 containing the list of filenames you really want. Line numbers (C<$.>)
2210 continue as though the input were one big happy file. See the example
2211 in L<perlfunc/eof> for how to reset line numbers on each file.
2213 If you want to set @ARGV to your own list of files, go right ahead.
2214 This sets @ARGV to all plain text files if no @ARGV was given:
2216 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
2218 You can even set them to pipe commands. For example, this automatically
2219 filters compressed arguments through B<gzip>:
2221 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
2223 If you want to pass switches into your script, you can use one of the
2224 Getopts modules or put a loop on the front like this:
2226 while ($_ = $ARGV[0], /^-/) {
2229 if (/^-D(.*)/) { $debug = $1 }
2230 if (/^-v/) { $verbose++ }
2231 # ... # other switches
2235 # ... # code for each line
2238 The <> symbol will return C<undef> for end-of-file only once.
2239 If you call it again after this, it will assume you are processing another
2240 @ARGV list, and if you haven't set @ARGV, will read input from STDIN.
2242 If what the angle brackets contain is a simple scalar variable (e.g.,
2243 <$foo>), then that variable contains the name of the
2244 filehandle to input from, or its typeglob, or a reference to the
2250 If what's within the angle brackets is neither a filehandle nor a simple
2251 scalar variable containing a filehandle name, typeglob, or typeglob
2252 reference, it is interpreted as a filename pattern to be globbed, and
2253 either a list of filenames or the next filename in the list is returned,
2254 depending on context. This distinction is determined on syntactic
2255 grounds alone. That means C<< <$x> >> is always a readline() from
2256 an indirect handle, but C<< <$hash{key}> >> is always a glob().
2257 That's because $x is a simple scalar variable, but C<$hash{key}> is
2258 not--it's a hash element. Even C<< <$x > >> (note the extra space)
2259 is treated as C<glob("$x ")>, not C<readline($x)>.
2261 One level of double-quote interpretation is done first, but you can't
2262 say C<< <$foo> >> because that's an indirect filehandle as explained
2263 in the previous paragraph. (In older versions of Perl, programmers
2264 would insert curly brackets to force interpretation as a filename glob:
2265 C<< <${foo}> >>. These days, it's considered cleaner to call the
2266 internal function directly as C<glob($foo)>, which is probably the right
2267 way to have done it in the first place.) For example:
2273 is roughly equivalent to:
2275 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
2281 except that the globbing is actually done internally using the standard
2282 C<File::Glob> extension. Of course, the shortest way to do the above is:
2286 A (file)glob evaluates its (embedded) argument only when it is
2287 starting a new list. All values must be read before it will start
2288 over. In list context, this isn't important because you automatically
2289 get them all anyway. However, in scalar context the operator returns
2290 the next value each time it's called, or C<undef> when the list has
2291 run out. As with filehandle reads, an automatic C<defined> is
2292 generated when the glob occurs in the test part of a C<while>,
2293 because legal glob returns (e.g. a file called F<0>) would otherwise
2294 terminate the loop. Again, C<undef> is returned only once. So if
2295 you're expecting a single value from a glob, it is much better to
2298 ($file) = <blurch*>;
2304 because the latter will alternate between returning a filename and
2307 If you're trying to do variable interpolation, it's definitely better
2308 to use the glob() function, because the older notation can cause people
2309 to become confused with the indirect filehandle notation.
2311 @files = glob("$dir/*.[ch]");
2312 @files = glob($files[$i]);
2314 =head2 Constant Folding
2315 X<constant folding> X<folding>
2317 Like C, Perl does a certain amount of expression evaluation at
2318 compile time whenever it determines that all arguments to an
2319 operator are static and have no side effects. In particular, string
2320 concatenation happens at compile time between literals that don't do
2321 variable substitution. Backslash interpolation also happens at
2322 compile time. You can say
2324 'Now is the time for all' . "\n" .
2325 'good men to come to.'
2327 and this all reduces to one string internally. Likewise, if
2330 foreach $file (@filenames) {
2331 if (-s $file > 5 + 100 * 2**16) { }
2334 the compiler will precompute the number which that expression
2335 represents so that the interpreter won't have to.
2340 Perl doesn't officially have a no-op operator, but the bare constants
2341 C<0> and C<1> are special-cased to not produce a warning in a void
2342 context, so you can for example safely do
2346 =head2 Bitwise String Operators
2347 X<operator, bitwise, string>
2349 Bitstrings of any size may be manipulated by the bitwise operators
2352 If the operands to a binary bitwise op are strings of different
2353 sizes, B<|> and B<^> ops act as though the shorter operand had
2354 additional zero bits on the right, while the B<&> op acts as though
2355 the longer operand were truncated to the length of the shorter.
2356 The granularity for such extension or truncation is one or more
2359 # ASCII-based examples
2360 print "j p \n" ^ " a h"; # prints "JAPH\n"
2361 print "JA" | " ph\n"; # prints "japh\n"
2362 print "japh\nJunk" & '_____'; # prints "JAPH\n";
2363 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
2365 If you are intending to manipulate bitstrings, be certain that
2366 you're supplying bitstrings: If an operand is a number, that will imply
2367 a B<numeric> bitwise operation. You may explicitly show which type of
2368 operation you intend by using C<""> or C<0+>, as in the examples below.
2370 $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
2371 $foo = '150' | 105; # yields 255
2372 $foo = 150 | '105'; # yields 255
2373 $foo = '150' | '105'; # yields string '155' (under ASCII)
2375 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
2376 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
2378 See L<perlfunc/vec> for information on how to manipulate individual bits
2381 =head2 Integer Arithmetic
2384 By default, Perl assumes that it must do most of its arithmetic in
2385 floating point. But by saying
2389 you may tell the compiler that it's okay to use integer operations
2390 (if it feels like it) from here to the end of the enclosing BLOCK.
2391 An inner BLOCK may countermand this by saying
2395 which lasts until the end of that BLOCK. Note that this doesn't
2396 mean everything is only an integer, merely that Perl may use integer
2397 operations if it is so inclined. For example, even under C<use
2398 integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731>
2401 Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
2402 and ">>") always produce integral results. (But see also
2403 L<Bitwise String Operators>.) However, C<use integer> still has meaning for
2404 them. By default, their results are interpreted as unsigned integers, but
2405 if C<use integer> is in effect, their results are interpreted
2406 as signed integers. For example, C<~0> usually evaluates to a large
2407 integral value. However, C<use integer; ~0> is C<-1> on two's-complement
2410 =head2 Floating-point Arithmetic
2411 X<floating-point> X<floating point> X<float> X<real>
2413 While C<use integer> provides integer-only arithmetic, there is no
2414 analogous mechanism to provide automatic rounding or truncation to a
2415 certain number of decimal places. For rounding to a certain number
2416 of digits, sprintf() or printf() is usually the easiest route.
2419 Floating-point numbers are only approximations to what a mathematician
2420 would call real numbers. There are infinitely more reals than floats,
2421 so some corners must be cut. For example:
2423 printf "%.20g\n", 123456789123456789;
2424 # produces 123456789123456784
2426 Testing for exact equality of floating-point equality or inequality is
2427 not a good idea. Here's a (relatively expensive) work-around to compare
2428 whether two floating-point numbers are equal to a particular number of
2429 decimal places. See Knuth, volume II, for a more robust treatment of
2433 my ($X, $Y, $POINTS) = @_;
2435 $tX = sprintf("%.${POINTS}g", $X);
2436 $tY = sprintf("%.${POINTS}g", $Y);
2440 The POSIX module (part of the standard perl distribution) implements
2441 ceil(), floor(), and other mathematical and trigonometric functions.
2442 The Math::Complex module (part of the standard perl distribution)
2443 defines mathematical functions that work on both the reals and the
2444 imaginary numbers. Math::Complex not as efficient as POSIX, but
2445 POSIX can't work with complex numbers.
2447 Rounding in financial applications can have serious implications, and
2448 the rounding method used should be specified precisely. In these
2449 cases, it probably pays not to trust whichever system rounding is
2450 being used by Perl, but to instead implement the rounding function you
2453 =head2 Bigger Numbers
2454 X<number, arbitrary precision>
2456 The standard Math::BigInt and Math::BigFloat modules provide
2457 variable-precision arithmetic and overloaded operators, although
2458 they're currently pretty slow. At the cost of some space and
2459 considerable speed, they avoid the normal pitfalls associated with
2460 limited-precision representations.
2463 $x = Math::BigInt->new('123456789123456789');
2466 # prints +15241578780673678515622620750190521
2468 There are several modules that let you calculate with (bound only by
2469 memory and cpu-time) unlimited or fixed precision. There are also
2470 some non-standard modules that provide faster implementations via
2471 external C libraries.
2473 Here is a short, but incomplete summary:
2475 Math::Fraction big, unlimited fractions like 9973 / 12967
2476 Math::String treat string sequences like numbers
2477 Math::FixedPrecision calculate with a fixed precision
2478 Math::Currency for currency calculations
2479 Bit::Vector manipulate bit vectors fast (uses C)
2480 Math::BigIntFast Bit::Vector wrapper for big numbers
2481 Math::Pari provides access to the Pari C library
2482 Math::BigInteger uses an external C library
2483 Math::Cephes uses external Cephes C library (no big numbers)
2484 Math::Cephes::Fraction fractions via the Cephes library
2485 Math::GMP another one using an external C library