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 (exluding "+" 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 $_. When used in scalar context, the return value generally indicates the
238 success of the operation. Behavior in list context depends on the particular
239 operator. See L</"Regexp Quote-Like Operators"> for details and
240 L<perlretut> for examples using these operators.
242 If the right argument is an expression rather than a search pattern,
243 substitution, or transliteration, it is interpreted as a search pattern at run
246 Binary "!~" is just like "=~" except the return value is negated in
249 =head2 Multiplicative Operators
250 X<operator, multiplicative>
252 Binary "*" multiplies two numbers.
255 Binary "/" divides two numbers.
258 Binary "%" computes the modulus of two numbers. Given integer
259 operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
260 C<$a> minus the largest multiple of C<$b> that is not greater than
261 C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
262 smallest multiple of C<$b> that is not less than C<$a> (i.e. the
263 result will be less than or equal to zero).
264 Note that when C<use integer> is in scope, "%" gives you direct access
265 to the modulus operator as implemented by your C compiler. This
266 operator is not as well defined for negative operands, but it will
268 X<%> X<remainder> X<modulus> X<mod>
270 Binary "x" is the repetition operator. In scalar context or if the left
271 operand is not enclosed in parentheses, it returns a string consisting
272 of the left operand repeated the number of times specified by the right
273 operand. In list context, if the left operand is enclosed in
274 parentheses or is a list formed by C<qw/STRING/>, it repeats the list.
275 If the right operand is zero or negative, it returns an empty string
276 or an empty list, depending on the context.
279 print '-' x 80; # print row of dashes
281 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
283 @ones = (1) x 80; # a list of 80 1's
284 @ones = (5) x @ones; # set all elements to 5
287 =head2 Additive Operators
288 X<operator, additive>
290 Binary "+" returns the sum of two numbers.
293 Binary "-" returns the difference of two numbers.
296 Binary "." concatenates two strings.
297 X<string, concatenation> X<concatenation>
298 X<cat> X<concat> X<concatenate> X<.>
300 =head2 Shift Operators
301 X<shift operator> X<operator, shift> X<<< << >>>
302 X<<< >> >>> X<right shift> X<left shift> X<bitwise shift>
303 X<shl> X<shr> X<shift, right> X<shift, left>
305 Binary "<<" returns the value of its left argument shifted left by the
306 number of bits specified by the right argument. Arguments should be
307 integers. (See also L<Integer Arithmetic>.)
309 Binary ">>" returns the value of its left argument shifted right by
310 the number of bits specified by the right argument. Arguments should
311 be integers. (See also L<Integer Arithmetic>.)
313 Note that both "<<" and ">>" in Perl are implemented directly using
314 "<<" and ">>" in C. If C<use integer> (see L<Integer Arithmetic>) is
315 in force then signed C integers are used, else unsigned C integers are
316 used. Either way, the implementation isn't going to generate results
317 larger than the size of the integer type Perl was built with (32 bits
320 The result of overflowing the range of the integers is undefined
321 because it is undefined also in C. In other words, using 32-bit
322 integers, C<< 1 << 32 >> is undefined. Shifting by a negative number
323 of bits is also undefined.
325 =head2 Named Unary Operators
326 X<operator, named unary>
328 The various named unary operators are treated as functions with one
329 argument, with optional parentheses.
331 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
332 is followed by a left parenthesis as the next token, the operator and
333 arguments within parentheses are taken to be of highest precedence,
334 just like a normal function call. For example,
335 because named unary operators are higher precedence than ||:
337 chdir $foo || die; # (chdir $foo) || die
338 chdir($foo) || die; # (chdir $foo) || die
339 chdir ($foo) || die; # (chdir $foo) || die
340 chdir +($foo) || die; # (chdir $foo) || die
342 but, because * is higher precedence than named operators:
344 chdir $foo * 20; # chdir ($foo * 20)
345 chdir($foo) * 20; # (chdir $foo) * 20
346 chdir ($foo) * 20; # (chdir $foo) * 20
347 chdir +($foo) * 20; # chdir ($foo * 20)
349 rand 10 * 20; # rand (10 * 20)
350 rand(10) * 20; # (rand 10) * 20
351 rand (10) * 20; # (rand 10) * 20
352 rand +(10) * 20; # rand (10 * 20)
354 Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are
355 treated like named unary operators, but they don't follow this functional
356 parenthesis rule. That means, for example, that C<-f($file).".bak"> is
357 equivalent to C<-f "$file.bak">.
358 X<-X> X<filetest> X<operator, filetest>
360 See also L<"Terms and List Operators (Leftward)">.
362 =head2 Relational Operators
363 X<relational operator> X<operator, relational>
365 Binary "<" returns true if the left argument is numerically less than
369 Binary ">" returns true if the left argument is numerically greater
370 than the right argument.
373 Binary "<=" returns true if the left argument is numerically less than
374 or equal to the right argument.
377 Binary ">=" returns true if the left argument is numerically greater
378 than or equal to the right argument.
381 Binary "lt" returns true if the left argument is stringwise less than
385 Binary "gt" returns true if the left argument is stringwise greater
386 than the right argument.
389 Binary "le" returns true if the left argument is stringwise less than
390 or equal to the right argument.
393 Binary "ge" returns true if the left argument is stringwise greater
394 than or equal to the right argument.
397 =head2 Equality Operators
398 X<equality> X<equal> X<equals> X<operator, equality>
400 Binary "==" returns true if the left argument is numerically equal to
404 Binary "!=" returns true if the left argument is numerically not equal
405 to the right argument.
408 Binary "<=>" returns -1, 0, or 1 depending on whether the left
409 argument is numerically less than, equal to, or greater than the right
410 argument. If your platform supports NaNs (not-a-numbers) as numeric
411 values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
412 "<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
413 returns true, as does NaN != anything else. If your platform doesn't
414 support NaNs then NaN is just a string with numeric value 0.
415 X<< <=> >> X<spaceship>
417 perl -le '$a = "NaN"; print "No NaN support here" if $a == $a'
418 perl -le '$a = "NaN"; print "NaN support here" if $a != $a'
420 Binary "eq" returns true if the left argument is stringwise equal to
424 Binary "ne" returns true if the left argument is stringwise not equal
425 to the right argument.
428 Binary "cmp" returns -1, 0, or 1 depending on whether the left
429 argument is stringwise less than, equal to, or greater than the right
433 Binary "~~" does a smart match between its arguments. Smart matching
434 is described in L<perlsyn/"Smart Matching in Detail">.
435 This operator is only available if you enable the "~~" feature:
436 see L<feature> for more information.
439 "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
440 by the current locale if C<use locale> is in effect. See L<perllocale>.
443 X<operator, bitwise, and> X<bitwise and> X<&>
445 Binary "&" returns its operands ANDed together bit by bit.
446 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
448 Note that "&" has lower priority than relational operators, so for example
449 the brackets are essential in a test like
451 print "Even\n" if ($x & 1) == 0;
453 =head2 Bitwise Or and Exclusive Or
454 X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor>
457 Binary "|" returns its operands ORed together bit by bit.
458 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
460 Binary "^" returns its operands XORed together bit by bit.
461 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
463 Note that "|" and "^" have lower priority than relational operators, so
464 for example the brackets are essential in a test like
466 print "false\n" if (8 | 2) != 10;
468 =head2 C-style Logical And
469 X<&&> X<logical and> X<operator, logical, and>
471 Binary "&&" performs a short-circuit logical AND operation. That is,
472 if the left operand is false, the right operand is not even evaluated.
473 Scalar or list context propagates down to the right operand if it
476 =head2 C-style Logical Or
477 X<||> X<operator, logical, or>
479 Binary "||" performs a short-circuit logical OR operation. That is,
480 if the left operand is true, the right operand is not even evaluated.
481 Scalar or list context propagates down to the right operand if it
484 =head2 C-style Logical Defined-Or
485 X<//> X<operator, logical, defined-or>
487 Although it has no direct equivalent in C, Perl's C<//> operator is related
488 to its C-style or. In fact, it's exactly the same as C<||>, except that it
489 tests the left hand side's definedness instead of its truth. Thus, C<$a // $b>
490 is similar to C<defined($a) || $b> (except that it returns the value of C<$a>
491 rather than the value of C<defined($a)>) and is exactly equivalent to
492 C<defined($a) ? $a : $b>. This is very useful for providing default values
493 for variables. If you actually want to test if at least one of C<$a> and
494 C<$b> is defined, use C<defined($a // $b)>.
496 The C<||>, C<//> and C<&&> operators return the last value evaluated
497 (unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably
498 portable way to find out the home directory might be:
500 $home = $ENV{'HOME'} // $ENV{'LOGDIR'} //
501 (getpwuid($<))[7] // die "You're homeless!\n";
503 In particular, this means that you shouldn't use this
504 for selecting between two aggregates for assignment:
506 @a = @b || @c; # this is wrong
507 @a = scalar(@b) || @c; # really meant this
508 @a = @b ? @b : @c; # this works fine, though
510 As more readable alternatives to C<&&>, C<//> and C<||> when used for
511 control flow, Perl provides C<and>, C<err> and C<or> operators (see below).
512 The short-circuit behavior is identical. The precedence of "and", "err"
513 and "or" is much lower, however, so that you can safely use them after a
514 list operator without the need for parentheses:
516 unlink "alpha", "beta", "gamma"
517 or gripe(), next LINE;
519 With the C-style operators that would have been written like this:
521 unlink("alpha", "beta", "gamma")
522 || (gripe(), next LINE);
524 Using "or" for assignment is unlikely to do what you want; see below.
526 =head2 Range Operators
527 X<operator, range> X<range> X<..> X<...>
529 Binary ".." is the range operator, which is really two different
530 operators depending on the context. In list context, it returns a
531 list of values counting (up by ones) from the left value to the right
532 value. If the left value is greater than the right value then it
533 returns the empty list. The range operator is useful for writing
534 C<foreach (1..10)> loops and for doing slice operations on arrays. In
535 the current implementation, no temporary array is created when the
536 range operator is used as the expression in C<foreach> loops, but older
537 versions of Perl might burn a lot of memory when you write something
540 for (1 .. 1_000_000) {
544 The range operator also works on strings, using the magical auto-increment,
547 In scalar context, ".." returns a boolean value. The operator is
548 bistable, like a flip-flop, and emulates the line-range (comma) operator
549 of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
550 own boolean state. It is false as long as its left operand is false.
551 Once the left operand is true, the range operator stays true until the
552 right operand is true, I<AFTER> which the range operator becomes false
553 again. It doesn't become false till the next time the range operator is
554 evaluated. It can test the right operand and become false on the same
555 evaluation it became true (as in B<awk>), but it still returns true once.
556 If you don't want it to test the right operand till the next
557 evaluation, as in B<sed>, just use three dots ("...") instead of
558 two. In all other regards, "..." behaves just like ".." does.
560 The right operand is not evaluated while the operator is in the
561 "false" state, and the left operand is not evaluated while the
562 operator is in the "true" state. The precedence is a little lower
563 than || and &&. The value returned is either the empty string for
564 false, or a sequence number (beginning with 1) for true. The
565 sequence number is reset for each range encountered. The final
566 sequence number in a range has the string "E0" appended to it, which
567 doesn't affect its numeric value, but gives you something to search
568 for if you want to exclude the endpoint. You can exclude the
569 beginning point by waiting for the sequence number to be greater
572 If either operand of scalar ".." is a constant expression,
573 that operand is considered true if it is equal (C<==>) to the current
574 input line number (the C<$.> variable).
576 To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>,
577 but that is only an issue if you use a floating point expression; when
578 implicitly using C<$.> as described in the previous paragraph, the
579 comparison is C<int(EXPR) == int($.)> which is only an issue when C<$.>
580 is set to a floating point value and you are not reading from a file.
581 Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what
582 you want in scalar context because each of the operands are evaluated
583 using their integer representation.
587 As a scalar operator:
589 if (101 .. 200) { print; } # print 2nd hundred lines, short for
590 # if ($. == 101 .. $. == 200) ...
592 next LINE if (1 .. /^$/); # skip header lines, short for
593 # ... if ($. == 1 .. /^$/);
594 # (typically in a loop labeled LINE)
596 s/^/> / if (/^$/ .. eof()); # quote body
598 # parse mail messages
600 $in_header = 1 .. /^$/;
601 $in_body = /^$/ .. eof;
608 close ARGV if eof; # reset $. each file
611 Here's a simple example to illustrate the difference between
612 the two range operators:
625 This program will print only the line containing "Bar". If
626 the range operator is changed to C<...>, it will also print the
629 And now some examples as a list operator:
631 for (101 .. 200) { print; } # print $_ 100 times
632 @foo = @foo[0 .. $#foo]; # an expensive no-op
633 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
635 The range operator (in list context) makes use of the magical
636 auto-increment algorithm if the operands are strings. You
639 @alphabet = ('A' .. 'Z');
641 to get all normal letters of the English alphabet, or
643 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
645 to get a hexadecimal digit, or
647 @z2 = ('01' .. '31'); print $z2[$mday];
649 to get dates with leading zeros. If the final value specified is not
650 in the sequence that the magical increment would produce, the sequence
651 goes until the next value would be longer than the final value
654 Because each operand is evaluated in integer form, C<2.18 .. 3.14> will
655 return two elements in list context.
657 @list = (2.18 .. 3.14); # same as @list = (2 .. 3);
659 =head2 Conditional Operator
660 X<operator, conditional> X<operator, ternary> X<ternary> X<?:>
662 Ternary "?:" is the conditional operator, just as in C. It works much
663 like an if-then-else. If the argument before the ? is true, the
664 argument before the : is returned, otherwise the argument after the :
665 is returned. For example:
667 printf "I have %d dog%s.\n", $n,
668 ($n == 1) ? '' : "s";
670 Scalar or list context propagates downward into the 2nd
671 or 3rd argument, whichever is selected.
673 $a = $ok ? $b : $c; # get a scalar
674 @a = $ok ? @b : @c; # get an array
675 $a = $ok ? @b : @c; # oops, that's just a count!
677 The operator may be assigned to if both the 2nd and 3rd arguments are
678 legal lvalues (meaning that you can assign to them):
680 ($a_or_b ? $a : $b) = $c;
682 Because this operator produces an assignable result, using assignments
683 without parentheses will get you in trouble. For example, this:
685 $a % 2 ? $a += 10 : $a += 2
689 (($a % 2) ? ($a += 10) : $a) += 2
693 ($a % 2) ? ($a += 10) : ($a += 2)
695 That should probably be written more simply as:
697 $a += ($a % 2) ? 10 : 2;
699 =head2 Assignment Operators
700 X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=>
701 X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=>
704 "=" is the ordinary assignment operator.
706 Assignment operators work as in C. That is,
714 although without duplicating any side effects that dereferencing the lvalue
715 might trigger, such as from tie(). Other assignment operators work similarly.
716 The following are recognized:
723 Although these are grouped by family, they all have the precedence
726 Unlike in C, the scalar assignment operator produces a valid lvalue.
727 Modifying an assignment is equivalent to doing the assignment and
728 then modifying the variable that was assigned to. This is useful
729 for modifying a copy of something, like this:
731 ($tmp = $global) =~ tr [A-Z] [a-z];
742 Similarly, a list assignment in list context produces the list of
743 lvalues assigned to, and a list assignment in scalar context returns
744 the number of elements produced by the expression on the right hand
745 side of the assignment.
747 =head2 Comma Operator
748 X<comma> X<operator, comma> X<,>
750 Binary "," is the comma operator. In scalar context it evaluates
751 its left argument, throws that value away, then evaluates its right
752 argument and returns that value. This is just like C's comma operator.
754 In list context, it's just the list argument separator, and inserts
755 both its arguments into the list. These arguments are also evaluated
758 The C<< => >> operator is a synonym for the comma, but forces any word
759 (consisting entirely of word characters) to its left to be interpreted
760 as a string (as of 5.001). This includes words that might otherwise be
761 considered a constant or function call.
763 use constant FOO => "something";
765 my %h = ( FOO => 23 );
773 my %h = ("something", 23);
775 If the argument on the left is not a word, it is first interpreted as
776 an expression, and then the string value of that is used.
778 The C<< => >> operator is helpful in documenting the correspondence
779 between keys and values in hashes, and other paired elements in lists.
781 %hash = ( $key => $value );
782 login( $username => $password );
784 =head2 List Operators (Rightward)
785 X<operator, list, rightward> X<list operator>
787 On the right side of a list operator, it has very low precedence,
788 such that it controls all comma-separated expressions found there.
789 The only operators with lower precedence are the logical operators
790 "and", "or", and "not", which may be used to evaluate calls to list
791 operators without the need for extra parentheses:
793 open HANDLE, "filename"
794 or die "Can't open: $!\n";
796 See also discussion of list operators in L<Terms and List Operators (Leftward)>.
799 X<operator, logical, not> X<not>
801 Unary "not" returns the logical negation of the expression to its right.
802 It's the equivalent of "!" except for the very low precedence.
805 X<operator, logical, and> X<and>
807 Binary "and" returns the logical conjunction of the two surrounding
808 expressions. It's equivalent to && except for the very low
809 precedence. This means that it short-circuits: i.e., the right
810 expression is evaluated only if the left expression is true.
812 =head2 Logical or, Defined or, and Exclusive Or
813 X<operator, logical, or> X<operator, logical, xor> X<operator, logical, err>
814 X<operator, logical, defined or> X<operator, logical, exclusive or>
817 Binary "or" returns the logical disjunction of the two surrounding
818 expressions. It's equivalent to || except for the very low precedence.
819 This makes it useful for control flow
821 print FH $data or die "Can't write to FH: $!";
823 This means that it short-circuits: i.e., the right expression is evaluated
824 only if the left expression is false. Due to its precedence, you should
825 probably avoid using this for assignment, only for control flow.
827 $a = $b or $c; # bug: this is wrong
828 ($a = $b) or $c; # really means this
829 $a = $b || $c; # better written this way
831 However, when it's a list-context assignment and you're trying to use
832 "||" for control flow, you probably need "or" so that the assignment
833 takes higher precedence.
835 @info = stat($file) || die; # oops, scalar sense of stat!
836 @info = stat($file) or die; # better, now @info gets its due
838 Then again, you could always use parentheses.
840 Binary "err" is equivalent to C<//>--it's just like binary "or", except it
841 tests its left argument's definedness instead of its truth. There are two
842 ways to remember "err": either because many functions return C<undef> on
843 an B<err>or, or as a sort of correction: C<$a = ($b err 'default')>. This
844 keyword is only available when the 'err' feature is enabled: see
845 L<feature> for more information.
847 Binary "xor" returns the exclusive-OR of the two surrounding expressions.
848 It cannot short circuit, of course.
850 =head2 C Operators Missing From Perl
851 X<operator, missing from perl> X<&> X<*>
852 X<typecasting> X<(TYPE)>
854 Here is what C has that Perl doesn't:
860 Address-of operator. (But see the "\" operator for taking a reference.)
864 Dereference-address operator. (Perl's prefix dereferencing
865 operators are typed: $, @, %, and &.)
869 Type-casting operator.
873 =head2 Quote and Quote-like Operators
874 X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m>
875 X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>>
876 X<escape sequence> X<escape>
879 While we usually think of quotes as literal values, in Perl they
880 function as operators, providing various kinds of interpolating and
881 pattern matching capabilities. Perl provides customary quote characters
882 for these behaviors, but also provides a way for you to choose your
883 quote character for any of them. In the following table, a C<{}> represents
884 any pair of delimiters you choose.
886 Customary Generic Meaning Interpolates
891 // m{} Pattern match yes*
893 s{}{} Substitution yes*
894 tr{}{} Transliteration no (but see below)
897 * unless the delimiter is ''.
899 Non-bracketing delimiters use the same character fore and aft, but the four
900 sorts of brackets (round, angle, square, curly) will all nest, which means
909 Note, however, that this does not always work for quoting Perl code:
911 $s = q{ if($a eq "}") ... }; # WRONG
913 is a syntax error. The C<Text::Balanced> module (from CPAN, and
914 starting from Perl 5.8 part of the standard distribution) is able
917 There can be whitespace between the operator and the quoting
918 characters, except when C<#> is being used as the quoting character.
919 C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
920 operator C<q> followed by a comment. Its argument will be taken
921 from the next line. This allows you to write:
923 s {foo} # Replace foo
926 The following escape sequences are available in constructs that interpolate
927 and in transliterations.
928 X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N>
935 \a alarm (bell) (BEL)
937 \033 octal char (ESC)
939 \x{263a} wide hex char (SMILEY)
940 \c[ control char (ESC)
941 \N{name} named Unicode character
943 B<NOTE>: Unlike C and other languages, Perl has no \v escape sequence for
944 the vertical tab (VT - ASCII 11).
946 The following escape sequences are available in constructs that interpolate
947 but not in transliterations.
948 X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q>
950 \l lowercase next char
951 \u uppercase next char
954 \E end case modification
955 \Q quote non-word characters till \E
957 If C<use locale> is in effect, the case map used by C<\l>, C<\L>,
958 C<\u> and C<\U> is taken from the current locale. See L<perllocale>.
959 If Unicode (for example, C<\N{}> or wide hex characters of 0x100 or
960 beyond) is being used, the case map used by C<\l>, C<\L>, C<\u> and
961 C<\U> is as defined by Unicode. For documentation of C<\N{name}>,
964 All systems use the virtual C<"\n"> to represent a line terminator,
965 called a "newline". There is no such thing as an unvarying, physical
966 newline character. It is only an illusion that the operating system,
967 device drivers, C libraries, and Perl all conspire to preserve. Not all
968 systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
969 on a Mac, these are reversed, and on systems without line terminator,
970 printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
971 you mean a "newline" for your system, but use the literal ASCII when you
972 need an exact character. For example, most networking protocols expect
973 and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
974 and although they often accept just C<"\012">, they seldom tolerate just
975 C<"\015">. If you get in the habit of using C<"\n"> for networking,
976 you may be burned some day.
977 X<newline> X<line terminator> X<eol> X<end of line>
980 For constructs that do interpolate, variables beginning with "C<$>"
981 or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
982 C<< $href->{key}[0] >> are also interpolated, as are array and hash slices.
983 But method calls such as C<< $obj->meth >> are not.
985 Interpolating an array or slice interpolates the elements in order,
986 separated by the value of C<$">, so is equivalent to interpolating
987 C<join $", @array>. "Punctuation" arrays such as C<@+> are only
988 interpolated if the name is enclosed in braces C<@{+}>.
990 You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
991 An unescaped C<$> or C<@> interpolates the corresponding variable,
992 while escaping will cause the literal string C<\$> to be inserted.
993 You'll need to write something like C<m/\Quser\E\@\Qhost/>.
995 Patterns are subject to an additional level of interpretation as a
996 regular expression. This is done as a second pass, after variables are
997 interpolated, so that regular expressions may be incorporated into the
998 pattern from the variables. If this is not what you want, use C<\Q> to
999 interpolate a variable literally.
1001 Apart from the behavior described above, Perl does not expand
1002 multiple levels of interpolation. In particular, contrary to the
1003 expectations of shell programmers, back-quotes do I<NOT> interpolate
1004 within double quotes, nor do single quotes impede evaluation of
1005 variables when used within double quotes.
1007 =head2 Regexp Quote-Like Operators
1010 Here are the quote-like operators that apply to pattern
1011 matching and related activities.
1018 This is just like the C</pattern/> search, except that it matches only
1019 once between calls to the reset() operator. This is a useful
1020 optimization when you want to see only the first occurrence of
1021 something in each file of a set of files, for instance. Only C<??>
1022 patterns local to the current package are reset.
1026 # blank line between header and body
1029 reset if eof; # clear ?? status for next file
1032 This usage is vaguely deprecated, which means it just might possibly
1033 be removed in some distant future version of Perl, perhaps somewhere
1034 around the year 2168.
1036 =item m/PATTERN/cgimosx
1037 X<m> X<operator, match>
1038 X<regexp, options> X<regexp> X<regex, options> X<regex>
1039 X</c> X</i> X</m> X</o> X</s> X</x>
1041 =item /PATTERN/cgimosx
1043 Searches a string for a pattern match, and in scalar context returns
1044 true if it succeeds, false if it fails. If no string is specified
1045 via the C<=~> or C<!~> operator, the $_ string is searched. (The
1046 string specified with C<=~> need not be an lvalue--it may be the
1047 result of an expression evaluation, but remember the C<=~> binds
1048 rather tightly.) See also L<perlre>. See L<perllocale> for
1049 discussion of additional considerations that apply when C<use locale>
1054 c Do not reset search position on a failed match when /g is in effect.
1055 g Match globally, i.e., find all occurrences.
1056 i Do case-insensitive pattern matching.
1057 m Treat string as multiple lines.
1058 o Compile pattern only once.
1059 s Treat string as single line.
1060 x Use extended regular expressions.
1062 If "/" is the delimiter then the initial C<m> is optional. With the C<m>
1063 you can use any pair of non-alphanumeric, non-whitespace characters
1064 as delimiters. This is particularly useful for matching path names
1065 that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
1066 the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
1067 If "'" is the delimiter, no interpolation is performed on the PATTERN.
1069 PATTERN may contain variables, which will be interpolated (and the
1070 pattern recompiled) every time the pattern search is evaluated, except
1071 for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
1072 C<$|> are not interpolated because they look like end-of-string tests.)
1073 If you want such a pattern to be compiled only once, add a C</o> after
1074 the trailing delimiter. This avoids expensive run-time recompilations,
1075 and is useful when the value you are interpolating won't change over
1076 the life of the script. However, mentioning C</o> constitutes a promise
1077 that you won't change the variables in the pattern. If you change them,
1078 Perl won't even notice. See also L<"qr/STRING/imosx">.
1080 If the PATTERN evaluates to the empty string, the last
1081 I<successfully> matched regular expression is used instead. In this
1082 case, only the C<g> and C<c> flags on the empty pattern is honoured -
1083 the other flags are taken from the original pattern. If no match has
1084 previously succeeded, this will (silently) act instead as a genuine
1085 empty pattern (which will always match).
1087 Note that it's possible to confuse Perl into thinking C<//> (the empty
1088 regex) is really C<//> (the defined-or operator). Perl is usually pretty
1089 good about this, but some pathological cases might trigger this, such as
1090 C<$a///> (is that C<($a) / (//)> or C<$a // />?) and C<print $fh //>
1091 (C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl
1092 will assume you meant defined-or. If you meant the empty regex, just
1093 use parentheses or spaces to disambiguate, or even prefix the empty
1094 regex with an C<m> (so C<//> becomes C<m//>).
1096 If the C</g> option is not used, C<m//> in list context returns a
1097 list consisting of the subexpressions matched by the parentheses in the
1098 pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are
1099 also set, and that this differs from Perl 4's behavior.) When there are
1100 no parentheses in the pattern, the return value is the list C<(1)> for
1101 success. With or without parentheses, an empty list is returned upon
1106 open(TTY, '/dev/tty');
1107 <TTY> =~ /^y/i && foo(); # do foo if desired
1109 if (/Version: *([0-9.]*)/) { $version = $1; }
1111 next if m#^/usr/spool/uucp#;
1116 print if /$arg/o; # compile only once
1119 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
1121 This last example splits $foo into the first two words and the
1122 remainder of the line, and assigns those three fields to $F1, $F2, and
1123 $Etc. The conditional is true if any variables were assigned, i.e., if
1124 the pattern matched.
1126 The C</g> modifier specifies global pattern matching--that is,
1127 matching as many times as possible within the string. How it behaves
1128 depends on the context. In list context, it returns a list of the
1129 substrings matched by any capturing parentheses in the regular
1130 expression. If there are no parentheses, it returns a list of all
1131 the matched strings, as if there were parentheses around the whole
1134 In scalar context, each execution of C<m//g> finds the next match,
1135 returning true if it matches, and false if there is no further match.
1136 The position after the last match can be read or set using the pos()
1137 function; see L<perlfunc/pos>. A failed match normally resets the
1138 search position to the beginning of the string, but you can avoid that
1139 by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
1140 string also resets the search position.
1142 You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
1143 zero-width assertion that matches the exact position where the previous
1144 C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion
1145 still anchors at pos(), but the match is of course only attempted once.
1146 Using C<\G> without C</g> on a target string that has not previously had a
1147 C</g> match applied to it is the same as using the C<\A> assertion to match
1148 the beginning of the string. Note also that, currently, C<\G> is only
1149 properly supported when anchored at the very beginning of the pattern.
1154 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
1158 while (defined($paragraph = <>)) {
1159 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
1163 print "$sentences\n";
1165 # using m//gc with \G
1169 print $1 while /(o)/gc; print "', pos=", pos, "\n";
1171 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
1173 print $1 while /(p)/gc; print "', pos=", pos, "\n";
1175 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
1177 The last example should print:
1187 Notice that the final match matched C<q> instead of C<p>, which a match
1188 without the C<\G> anchor would have done. Also note that the final match
1189 did not update C<pos> -- C<pos> is only updated on a C</g> match. If the
1190 final match did indeed match C<p>, it's a good bet that you're running an
1191 older (pre-5.6.0) Perl.
1193 A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
1194 combine several regexps like this to process a string part-by-part,
1195 doing different actions depending on which regexp matched. Each
1196 regexp tries to match where the previous one leaves off.
1199 $url = new URI::URL "http://www/"; die if $url eq "xXx";
1203 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
1204 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
1205 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
1206 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
1207 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
1208 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
1209 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
1210 print ". That's all!\n";
1213 Here is the output (split into several lines):
1215 line-noise lowercase line-noise lowercase UPPERCASE line-noise
1216 UPPERCASE line-noise lowercase line-noise lowercase line-noise
1217 lowercase lowercase line-noise lowercase lowercase line-noise
1218 MiXeD line-noise. That's all!
1221 X<q> X<quote, double> X<'> X<''>
1225 A single-quoted, literal string. A backslash represents a backslash
1226 unless followed by the delimiter or another backslash, in which case
1227 the delimiter or backslash is interpolated.
1229 $foo = q!I said, "You said, 'She said it.'"!;
1230 $bar = q('This is it.');
1231 $baz = '\n'; # a two-character string
1234 X<qq> X<quote, double> X<"> X<"">
1238 A double-quoted, interpolated string.
1241 (*** The previous line contains the naughty word "$1".\n)
1242 if /\b(tcl|java|python)\b/i; # :-)
1243 $baz = "\n"; # a one-character string
1245 =item qr/STRING/imosx
1246 X<qr> X</i> X</m> X</o> X</s> X</x>
1248 This operator quotes (and possibly compiles) its I<STRING> as a regular
1249 expression. I<STRING> is interpolated the same way as I<PATTERN>
1250 in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
1251 is done. Returns a Perl value which may be used instead of the
1252 corresponding C</STRING/imosx> expression.
1256 $rex = qr/my.STRING/is;
1263 The result may be used as a subpattern in a match:
1266 $string =~ /foo${re}bar/; # can be interpolated in other patterns
1267 $string =~ $re; # or used standalone
1268 $string =~ /$re/; # or this way
1270 Since Perl may compile the pattern at the moment of execution of qr()
1271 operator, using qr() may have speed advantages in some situations,
1272 notably if the result of qr() is used standalone:
1275 my $patterns = shift;
1276 my @compiled = map qr/$_/i, @$patterns;
1279 foreach my $pat (@compiled) {
1280 $success = 1, last if /$pat/;
1286 Precompilation of the pattern into an internal representation at
1287 the moment of qr() avoids a need to recompile the pattern every
1288 time a match C</$pat/> is attempted. (Perl has many other internal
1289 optimizations, but none would be triggered in the above example if
1290 we did not use qr() operator.)
1294 i Do case-insensitive pattern matching.
1295 m Treat string as multiple lines.
1296 o Compile pattern only once.
1297 s Treat string as single line.
1298 x Use extended regular expressions.
1300 See L<perlre> for additional information on valid syntax for STRING, and
1301 for a detailed look at the semantics of regular expressions.
1304 X<qx> X<`> X<``> X<backtick>
1308 A string which is (possibly) interpolated and then executed as a
1309 system command with C</bin/sh> or its equivalent. Shell wildcards,
1310 pipes, and redirections will be honored. The collected standard
1311 output of the command is returned; standard error is unaffected. In
1312 scalar context, it comes back as a single (potentially multi-line)
1313 string, or undef if the command failed. In list context, returns a
1314 list of lines (however you've defined lines with $/ or
1315 $INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
1317 Because backticks do not affect standard error, use shell file descriptor
1318 syntax (assuming the shell supports this) if you care to address this.
1319 To capture a command's STDERR and STDOUT together:
1321 $output = `cmd 2>&1`;
1323 To capture a command's STDOUT but discard its STDERR:
1325 $output = `cmd 2>/dev/null`;
1327 To capture a command's STDERR but discard its STDOUT (ordering is
1330 $output = `cmd 2>&1 1>/dev/null`;
1332 To exchange a command's STDOUT and STDERR in order to capture the STDERR
1333 but leave its STDOUT to come out the old STDERR:
1335 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
1337 To read both a command's STDOUT and its STDERR separately, it's easiest
1338 to redirect them separately to files, and then read from those files
1339 when the program is done:
1341 system("program args 1>program.stdout 2>program.stderr");
1343 Using single-quote as a delimiter protects the command from Perl's
1344 double-quote interpolation, passing it on to the shell instead:
1346 $perl_info = qx(ps $$); # that's Perl's $$
1347 $shell_info = qx'ps $$'; # that's the new shell's $$
1349 How that string gets evaluated is entirely subject to the command
1350 interpreter on your system. On most platforms, you will have to protect
1351 shell metacharacters if you want them treated literally. This is in
1352 practice difficult to do, as it's unclear how to escape which characters.
1353 See L<perlsec> for a clean and safe example of a manual fork() and exec()
1354 to emulate backticks safely.
1356 On some platforms (notably DOS-like ones), the shell may not be
1357 capable of dealing with multiline commands, so putting newlines in
1358 the string may not get you what you want. You may be able to evaluate
1359 multiple commands in a single line by separating them with the command
1360 separator character, if your shell supports that (e.g. C<;> on many Unix
1361 shells; C<&> on the Windows NT C<cmd> shell).
1363 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1364 output before starting the child process, but this may not be supported
1365 on some platforms (see L<perlport>). To be safe, you may need to set
1366 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1367 C<IO::Handle> on any open handles.
1369 Beware that some command shells may place restrictions on the length
1370 of the command line. You must ensure your strings don't exceed this
1371 limit after any necessary interpolations. See the platform-specific
1372 release notes for more details about your particular environment.
1374 Using this operator can lead to programs that are difficult to port,
1375 because the shell commands called vary between systems, and may in
1376 fact not be present at all. As one example, the C<type> command under
1377 the POSIX shell is very different from the C<type> command under DOS.
1378 That doesn't mean you should go out of your way to avoid backticks
1379 when they're the right way to get something done. Perl was made to be
1380 a glue language, and one of the things it glues together is commands.
1381 Just understand what you're getting yourself into.
1383 See L<"I/O Operators"> for more discussion.
1386 X<qw> X<quote, list> X<quote, words>
1388 Evaluates to a list of the words extracted out of STRING, using embedded
1389 whitespace as the word delimiters. It can be understood as being roughly
1392 split(' ', q/STRING/);
1394 the differences being that it generates a real list at compile time, and
1395 in scalar context it returns the last element in the list. So
1400 is semantically equivalent to the list:
1404 Some frequently seen examples:
1406 use POSIX qw( setlocale localeconv )
1407 @EXPORT = qw( foo bar baz );
1409 A common mistake is to try to separate the words with comma or to
1410 put comments into a multi-line C<qw>-string. For this reason, the
1411 C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
1412 produces warnings if the STRING contains the "," or the "#" character.
1414 =item s/PATTERN/REPLACEMENT/egimosx
1415 X<substitute> X<substitution> X<replace> X<regexp, replace>
1416 X<regexp, substitute> X</e> X</g> X</i> X</m> X</o> X</s> X</x>
1418 Searches a string for a pattern, and if found, replaces that pattern
1419 with the replacement text and returns the number of substitutions
1420 made. Otherwise it returns false (specifically, the empty string).
1422 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
1423 variable is searched and modified. (The string specified with C<=~> must
1424 be scalar variable, an array element, a hash element, or an assignment
1425 to one of those, i.e., an lvalue.)
1427 If the delimiter chosen is a single quote, no interpolation is
1428 done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1429 PATTERN contains a $ that looks like a variable rather than an
1430 end-of-string test, the variable will be interpolated into the pattern
1431 at run-time. If you want the pattern compiled only once the first time
1432 the variable is interpolated, use the C</o> option. If the pattern
1433 evaluates to the empty string, the last successfully executed regular
1434 expression is used instead. See L<perlre> for further explanation on these.
1435 See L<perllocale> for discussion of additional considerations that apply
1436 when C<use locale> is in effect.
1440 e Evaluate the right side as an expression.
1441 g Replace globally, i.e., all occurrences.
1442 i Do case-insensitive pattern matching.
1443 m Treat string as multiple lines.
1444 o Compile pattern only once.
1445 s Treat string as single line.
1446 x Use extended regular expressions.
1448 Any non-alphanumeric, non-whitespace delimiter may replace the
1449 slashes. If single quotes are used, no interpretation is done on the
1450 replacement string (the C</e> modifier overrides this, however). Unlike
1451 Perl 4, Perl 5 treats backticks as normal delimiters; the replacement
1452 text is not evaluated as a command. If the
1453 PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
1454 pair of quotes, which may or may not be bracketing quotes, e.g.,
1455 C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
1456 replacement portion to be treated as a full-fledged Perl expression
1457 and evaluated right then and there. It is, however, syntax checked at
1458 compile-time. A second C<e> modifier will cause the replacement portion
1459 to be C<eval>ed before being run as a Perl expression.
1463 s/\bgreen\b/mauve/g; # don't change wintergreen
1465 $path =~ s|/usr/bin|/usr/local/bin|;
1467 s/Login: $foo/Login: $bar/; # run-time pattern
1469 ($foo = $bar) =~ s/this/that/; # copy first, then change
1471 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
1474 s/\d+/$&*2/e; # yields 'abc246xyz'
1475 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
1476 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1478 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
1479 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
1480 s/^=(\w+)/&pod($1)/ge; # use function call
1482 # expand variables in $_, but dynamics only, using
1483 # symbolic dereferencing
1486 # Add one to the value of any numbers in the string
1489 # This will expand any embedded scalar variable
1490 # (including lexicals) in $_ : First $1 is interpolated
1491 # to the variable name, and then evaluated
1494 # Delete (most) C comments.
1496 /\* # Match the opening delimiter.
1497 .*? # Match a minimal number of characters.
1498 \*/ # Match the closing delimiter.
1501 s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_, expensively
1503 for ($variable) { # trim whitespace in $variable, cheap
1508 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
1510 Note the use of $ instead of \ in the last example. Unlike
1511 B<sed>, we use the \<I<digit>> form in only the left hand side.
1512 Anywhere else it's $<I<digit>>.
1514 Occasionally, you can't use just a C</g> to get all the changes
1515 to occur that you might want. Here are two common cases:
1517 # put commas in the right places in an integer
1518 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
1520 # expand tabs to 8-column spacing
1521 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
1523 =item tr/SEARCHLIST/REPLACEMENTLIST/cds
1524 X<tr> X<y> X<transliterate> X</c> X</d> X</s>
1526 =item y/SEARCHLIST/REPLACEMENTLIST/cds
1528 Transliterates all occurrences of the characters found in the search list
1529 with the corresponding character in the replacement list. It returns
1530 the number of characters replaced or deleted. If no string is
1531 specified via the =~ or !~ operator, the $_ string is transliterated. (The
1532 string specified with =~ must be a scalar variable, an array element, a
1533 hash element, or an assignment to one of those, i.e., an lvalue.)
1535 A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
1536 does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
1537 For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1538 SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1539 its own pair of quotes, which may or may not be bracketing quotes,
1540 e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
1542 Note that C<tr> does B<not> do regular expression character classes
1543 such as C<\d> or C<[:lower:]>. The <tr> operator is not equivalent to
1544 the tr(1) utility. If you want to map strings between lower/upper
1545 cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider
1546 using the C<s> operator if you need regular expressions.
1548 Note also that the whole range idea is rather unportable between
1549 character sets--and even within character sets they may cause results
1550 you probably didn't expect. A sound principle is to use only ranges
1551 that begin from and end at either alphabets of equal case (a-e, A-E),
1552 or digits (0-4). Anything else is unsafe. If in doubt, spell out the
1553 character sets in full.
1557 c Complement the SEARCHLIST.
1558 d Delete found but unreplaced characters.
1559 s Squash duplicate replaced characters.
1561 If the C</c> modifier is specified, the SEARCHLIST character set
1562 is complemented. If the C</d> modifier is specified, any characters
1563 specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
1564 (Note that this is slightly more flexible than the behavior of some
1565 B<tr> programs, which delete anything they find in the SEARCHLIST,
1566 period.) If the C</s> modifier is specified, sequences of characters
1567 that were transliterated to the same character are squashed down
1568 to a single instance of the character.
1570 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1571 exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1572 than the SEARCHLIST, the final character is replicated till it is long
1573 enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
1574 This latter is useful for counting characters in a class or for
1575 squashing character sequences in a class.
1579 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1581 $cnt = tr/*/*/; # count the stars in $_
1583 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1585 $cnt = tr/0-9//; # count the digits in $_
1587 tr/a-zA-Z//s; # bookkeeper -> bokeper
1589 ($HOST = $host) =~ tr/a-z/A-Z/;
1591 tr/a-zA-Z/ /cs; # change non-alphas to single space
1594 [\000-\177]; # delete 8th bit
1596 If multiple transliterations are given for a character, only the
1601 will transliterate any A to X.
1603 Because the transliteration table is built at compile time, neither
1604 the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
1605 interpolation. That means that if you want to use variables, you
1608 eval "tr/$oldlist/$newlist/";
1611 eval "tr/$oldlist/$newlist/, 1" or die $@;
1614 X<here-doc> X<heredoc> X<here-document> X<<< << >>>
1616 A line-oriented form of quoting is based on the shell "here-document"
1617 syntax. Following a C<< << >> you specify a string to terminate
1618 the quoted material, and all lines following the current line down to
1619 the terminating string are the value of the item. The terminating
1620 string may be either an identifier (a word), or some quoted text. If
1621 quoted, the type of quotes you use determines the treatment of the
1622 text, just as in regular quoting. An unquoted identifier works like
1623 double quotes. There must be no space between the C<< << >> and
1624 the identifier, unless the identifier is quoted. (If you put a space it
1625 will be treated as a null identifier, which is valid, and matches the first
1626 empty line.) The terminating string must appear by itself (unquoted and
1627 with no surrounding whitespace) on the terminating line.
1630 The price is $Price.
1633 print << "EOF"; # same as above
1634 The price is $Price.
1637 print << `EOC`; # execute commands
1642 print <<"foo", <<"bar"; # you can stack them
1648 myfunc(<< "THIS", 23, <<'THAT');
1655 Just don't forget that you have to put a semicolon on the end
1656 to finish the statement, as Perl doesn't know you're not going to
1664 If you want your here-docs to be indented with the
1665 rest of the code, you'll need to remove leading whitespace
1666 from each line manually:
1668 ($quote = <<'FINIS') =~ s/^\s+//gm;
1669 The Road goes ever on and on,
1670 down from the door where it began.
1673 If you use a here-doc within a delimited construct, such as in C<s///eg>,
1674 the quoted material must come on the lines following the final delimiter.
1689 If the terminating identifier is on the last line of the program, you
1690 must be sure there is a newline after it; otherwise, Perl will give the
1691 warning B<Can't find string terminator "END" anywhere before EOF...>.
1693 Additionally, the quoting rules for the identifier are not related to
1694 Perl's quoting rules -- C<q()>, C<qq()>, and the like are not supported
1695 in place of C<''> and C<"">, and the only interpolation is for backslashing
1696 the quoting character:
1698 print << "abc\"def";
1702 Finally, quoted strings cannot span multiple lines. The general rule is
1703 that the identifier must be a string literal. Stick with that, and you
1708 =head2 Gory details of parsing quoted constructs
1709 X<quote, gory details>
1711 When presented with something that might have several different
1712 interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
1713 principle to pick the most probable interpretation. This strategy
1714 is so successful that Perl programmers often do not suspect the
1715 ambivalence of what they write. But from time to time, Perl's
1716 notions differ substantially from what the author honestly meant.
1718 This section hopes to clarify how Perl handles quoted constructs.
1719 Although the most common reason to learn this is to unravel labyrinthine
1720 regular expressions, because the initial steps of parsing are the
1721 same for all quoting operators, they are all discussed together.
1723 The most important Perl parsing rule is the first one discussed
1724 below: when processing a quoted construct, Perl first finds the end
1725 of that construct, then interprets its contents. If you understand
1726 this rule, you may skip the rest of this section on the first
1727 reading. The other rules are likely to contradict the user's
1728 expectations much less frequently than this first one.
1730 Some passes discussed below are performed concurrently, but because
1731 their results are the same, we consider them individually. For different
1732 quoting constructs, Perl performs different numbers of passes, from
1733 one to five, but these passes are always performed in the same order.
1737 =item Finding the end
1739 The first pass is finding the end of the quoted construct, whether
1740 it be a multicharacter delimiter C<"\nEOF\n"> in the C<<<EOF>
1741 construct, a C</> that terminates a C<qq//> construct, a C<]> which
1742 terminates C<qq[]> construct, or a C<< > >> which terminates a
1743 fileglob started with C<< < >>.
1745 When searching for single-character non-pairing delimiters, such
1746 as C</>, combinations of C<\\> and C<\/> are skipped. However,
1747 when searching for single-character pairing delimiter like C<[>,
1748 combinations of C<\\>, C<\]>, and C<\[> are all skipped, and nested
1749 C<[>, C<]> are skipped as well. When searching for multicharacter
1750 delimiters, nothing is skipped.
1752 For constructs with three-part delimiters (C<s///>, C<y///>, and
1753 C<tr///>), the search is repeated once more.
1755 During this search no attention is paid to the semantics of the construct.
1758 "$hash{"$foo/$bar"}"
1763 bar # NOT a comment, this slash / terminated m//!
1766 do not form legal quoted expressions. The quoted part ends on the
1767 first C<"> and C</>, and the rest happens to be a syntax error.
1768 Because the slash that terminated C<m//> was followed by a C<SPACE>,
1769 the example above is not C<m//x>, but rather C<m//> with no C</x>
1770 modifier. So the embedded C<#> is interpreted as a literal C<#>.
1772 Also no attention is paid to C<\c\> during this search.
1773 Thus the second C<\> in C<qq/\c\/> is interpreted as a part of C<\/>,
1774 and the following C</> is not recognized as a delimiter.
1775 Instead, use C<\034> or C<\x1c> at the end of quoted constructs.
1777 =item Removal of backslashes before delimiters
1779 During the second pass, text between the starting and ending
1780 delimiters is copied to a safe location, and the C<\> is removed
1781 from combinations consisting of C<\> and delimiter--or delimiters,
1782 meaning both starting and ending delimiters will should these differ.
1783 This removal does not happen for multi-character delimiters.
1784 Note that the combination C<\\> is left intact, just as it was.
1786 Starting from this step no information about the delimiters is
1792 The next step is interpolation in the text obtained, which is now
1793 delimiter-independent. There are four different cases.
1797 =item C<<<'EOF'>, C<m''>, C<s'''>, C<tr///>, C<y///>
1799 No interpolation is performed.
1803 The only interpolation is removal of C<\> from pairs C<\\>.
1805 =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>
1807 C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
1808 converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
1809 is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
1810 The other combinations are replaced with appropriate expansions.
1812 Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
1813 is interpolated in the usual way. Something like C<"\Q\\E"> has
1814 no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
1815 result is the same as for C<"\\\\E">. As a general rule, backslashes
1816 between C<\Q> and C<\E> may lead to counterintuitive results. So,
1817 C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
1818 as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
1823 may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
1825 Interpolated scalars and arrays are converted internally to the C<join> and
1826 C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
1828 $foo . " XXX '" . (join $", @arr) . "'";
1830 All operations above are performed simultaneously, left to right.
1832 Because the result of C<"\Q STRING \E"> has all metacharacters
1833 quoted, there is no way to insert a literal C<$> or C<@> inside a
1834 C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
1835 C<"\\\$">; if not, it is interpreted as the start of an interpolated
1838 Note also that the interpolation code needs to make a decision on
1839 where the interpolated scalar ends. For instance, whether
1840 C<< "a $b -> {c}" >> really means:
1842 "a " . $b . " -> {c}";
1848 Most of the time, the longest possible text that does not include
1849 spaces between components and which contains matching braces or
1850 brackets. because the outcome may be determined by voting based
1851 on heuristic estimators, the result is not strictly predictable.
1852 Fortunately, it's usually correct for ambiguous cases.
1854 =item C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
1856 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
1857 happens (almost) as with C<qq//> constructs, but the substitution
1858 of C<\> followed by RE-special chars (including C<\>) is not
1859 performed. Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
1860 a C<#>-comment in a C<//x>-regular expression, no processing is
1861 performed whatsoever. This is the first step at which the presence
1862 of the C<//x> modifier is relevant.
1864 Interpolation has several quirks: C<$|>, C<$(>, and C<$)> are not
1865 interpolated, and constructs C<$var[SOMETHING]> are voted (by several
1866 different estimators) to be either an array element or C<$var>
1867 followed by an RE alternative. This is where the notation
1868 C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
1869 array element C<-9>, not as a regular expression from the variable
1870 C<$arr> followed by a digit, which would be the interpretation of
1871 C</$arr[0-9]/>. Since voting among different estimators may occur,
1872 the result is not predictable.
1874 It is at this step that C<\1> is begrudgingly converted to C<$1> in
1875 the replacement text of C<s///> to correct the incorrigible
1876 I<sed> hackers who haven't picked up the saner idiom yet. A warning
1877 is emitted if the C<use warnings> pragma or the B<-w> command-line flag
1878 (that is, the C<$^W> variable) was set.
1880 The lack of processing of C<\\> creates specific restrictions on
1881 the post-processed text. If the delimiter is C</>, one cannot get
1882 the combination C<\/> into the result of this step. C</> will
1883 finish the regular expression, C<\/> will be stripped to C</> on
1884 the previous step, and C<\\/> will be left as is. Because C</> is
1885 equivalent to C<\/> inside a regular expression, this does not
1886 matter unless the delimiter happens to be character special to the
1887 RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
1888 alphanumeric char, as in:
1892 In the RE above, which is intentionally obfuscated for illustration, the
1893 delimiter is C<m>, the modifier is C<mx>, and after backslash-removal the
1894 RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
1895 reason you're encouraged to restrict your delimiters to non-alphanumeric,
1896 non-whitespace choices.
1900 This step is the last one for all constructs except regular expressions,
1901 which are processed further.
1903 =item Interpolation of regular expressions
1904 X<regexp, interpolation>
1906 Previous steps were performed during the compilation of Perl code,
1907 but this one happens at run time--although it may be optimized to
1908 be calculated at compile time if appropriate. After preprocessing
1909 described above, and possibly after evaluation if catenation,
1910 joining, casing translation, or metaquoting are involved, the
1911 resulting I<string> is passed to the RE engine for compilation.
1913 Whatever happens in the RE engine might be better discussed in L<perlre>,
1914 but for the sake of continuity, we shall do so here.
1916 This is another step where the presence of the C<//x> modifier is
1917 relevant. The RE engine scans the string from left to right and
1918 converts it to a finite automaton.
1920 Backslashed characters are either replaced with corresponding
1921 literal strings (as with C<\{>), or else they generate special nodes
1922 in the finite automaton (as with C<\b>). Characters special to the
1923 RE engine (such as C<|>) generate corresponding nodes or groups of
1924 nodes. C<(?#...)> comments are ignored. All the rest is either
1925 converted to literal strings to match, or else is ignored (as is
1926 whitespace and C<#>-style comments if C<//x> is present).
1928 Parsing of the bracketed character class construct, C<[...]>, is
1929 rather different than the rule used for the rest of the pattern.
1930 The terminator of this construct is found using the same rules as
1931 for finding the terminator of a C<{}>-delimited construct, the only
1932 exception being that C<]> immediately following C<[> is treated as
1933 though preceded by a backslash. Similarly, the terminator of
1934 C<(?{...})> is found using the same rules as for finding the
1935 terminator of a C<{}>-delimited construct.
1937 It is possible to inspect both the string given to RE engine and the
1938 resulting finite automaton. See the arguments C<debug>/C<debugcolor>
1939 in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
1940 switch documented in L<perlrun/"Command Switches">.
1942 =item Optimization of regular expressions
1943 X<regexp, optimization>
1945 This step is listed for completeness only. Since it does not change
1946 semantics, details of this step are not documented and are subject
1947 to change without notice. This step is performed over the finite
1948 automaton that was generated during the previous pass.
1950 It is at this stage that C<split()> silently optimizes C</^/> to
1955 =head2 I/O Operators
1956 X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle>
1959 There are several I/O operators you should know about.
1961 A string enclosed by backticks (grave accents) first undergoes
1962 double-quote interpolation. It is then interpreted as an external
1963 command, and the output of that command is the value of the
1964 backtick string, like in a shell. In scalar context, a single string
1965 consisting of all output is returned. In list context, a list of
1966 values is returned, one per line of output. (You can set C<$/> to use
1967 a different line terminator.) The command is executed each time the
1968 pseudo-literal is evaluated. The status value of the command is
1969 returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
1970 Unlike in B<csh>, no translation is done on the return data--newlines
1971 remain newlines. Unlike in any of the shells, single quotes do not
1972 hide variable names in the command from interpretation. To pass a
1973 literal dollar-sign through to the shell you need to hide it with a
1974 backslash. The generalized form of backticks is C<qx//>. (Because
1975 backticks always undergo shell expansion as well, see L<perlsec> for
1977 X<qx> X<`> X<``> X<backtick> X<glob>
1979 In scalar context, evaluating a filehandle in angle brackets yields
1980 the next line from that file (the newline, if any, included), or
1981 C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
1982 (sometimes known as file-slurp mode) and the file is empty, it
1983 returns C<''> the first time, followed by C<undef> subsequently.
1985 Ordinarily you must assign the returned value to a variable, but
1986 there is one situation where an automatic assignment happens. If
1987 and only if the input symbol is the only thing inside the conditional
1988 of a C<while> statement (even if disguised as a C<for(;;)> loop),
1989 the value is automatically assigned to the global variable $_,
1990 destroying whatever was there previously. (This may seem like an
1991 odd thing to you, but you'll use the construct in almost every Perl
1992 script you write.) The $_ variable is not implicitly localized.
1993 You'll have to put a C<local $_;> before the loop if you want that
1996 The following lines are equivalent:
1998 while (defined($_ = <STDIN>)) { print; }
1999 while ($_ = <STDIN>) { print; }
2000 while (<STDIN>) { print; }
2001 for (;<STDIN>;) { print; }
2002 print while defined($_ = <STDIN>);
2003 print while ($_ = <STDIN>);
2004 print while <STDIN>;
2006 This also behaves similarly, but avoids $_ :
2008 while (my $line = <STDIN>) { print $line }
2010 In these loop constructs, the assigned value (whether assignment
2011 is automatic or explicit) is then tested to see whether it is
2012 defined. The defined test avoids problems where line has a string
2013 value that would be treated as false by Perl, for example a "" or
2014 a "0" with no trailing newline. If you really mean for such values
2015 to terminate the loop, they should be tested for explicitly:
2017 while (($_ = <STDIN>) ne '0') { ... }
2018 while (<STDIN>) { last unless $_; ... }
2020 In other boolean contexts, C<< <I<filehandle>> >> without an
2021 explicit C<defined> test or comparison elicit a warning if the
2022 C<use warnings> pragma or the B<-w>
2023 command-line switch (the C<$^W> variable) is in effect.
2025 The filehandles STDIN, STDOUT, and STDERR are predefined. (The
2026 filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
2027 in packages, where they would be interpreted as local identifiers
2028 rather than global.) Additional filehandles may be created with
2029 the open() function, amongst others. See L<perlopentut> and
2030 L<perlfunc/open> for details on this.
2031 X<stdin> X<stdout> X<sterr>
2033 If a <FILEHANDLE> is used in a context that is looking for
2034 a list, a list comprising all input lines is returned, one line per
2035 list element. It's easy to grow to a rather large data space this
2036 way, so use with care.
2038 <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
2039 See L<perlfunc/readline>.
2041 The null filehandle <> is special: it can be used to emulate the
2042 behavior of B<sed> and B<awk>. Input from <> comes either from
2043 standard input, or from each file listed on the command line. Here's
2044 how it works: the first time <> is evaluated, the @ARGV array is
2045 checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
2046 gives you standard input. The @ARGV array is then processed as a list
2047 of filenames. The loop
2050 ... # code for each line
2053 is equivalent to the following Perl-like pseudo code:
2055 unshift(@ARGV, '-') unless @ARGV;
2056 while ($ARGV = shift) {
2059 ... # code for each line
2063 except that it isn't so cumbersome to say, and will actually work.
2064 It really does shift the @ARGV array and put the current filename
2065 into the $ARGV variable. It also uses filehandle I<ARGV>
2066 internally--<> is just a synonym for <ARGV>, which
2067 is magical. (The pseudo code above doesn't work because it treats
2068 <ARGV> as non-magical.)
2070 You can modify @ARGV before the first <> as long as the array ends up
2071 containing the list of filenames you really want. Line numbers (C<$.>)
2072 continue as though the input were one big happy file. See the example
2073 in L<perlfunc/eof> for how to reset line numbers on each file.
2075 If you want to set @ARGV to your own list of files, go right ahead.
2076 This sets @ARGV to all plain text files if no @ARGV was given:
2078 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
2080 You can even set them to pipe commands. For example, this automatically
2081 filters compressed arguments through B<gzip>:
2083 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
2085 If you want to pass switches into your script, you can use one of the
2086 Getopts modules or put a loop on the front like this:
2088 while ($_ = $ARGV[0], /^-/) {
2091 if (/^-D(.*)/) { $debug = $1 }
2092 if (/^-v/) { $verbose++ }
2093 # ... # other switches
2097 # ... # code for each line
2100 The <> symbol will return C<undef> for end-of-file only once.
2101 If you call it again after this, it will assume you are processing another
2102 @ARGV list, and if you haven't set @ARGV, will read input from STDIN.
2104 If what the angle brackets contain is a simple scalar variable (e.g.,
2105 <$foo>), then that variable contains the name of the
2106 filehandle to input from, or its typeglob, or a reference to the
2112 If what's within the angle brackets is neither a filehandle nor a simple
2113 scalar variable containing a filehandle name, typeglob, or typeglob
2114 reference, it is interpreted as a filename pattern to be globbed, and
2115 either a list of filenames or the next filename in the list is returned,
2116 depending on context. This distinction is determined on syntactic
2117 grounds alone. That means C<< <$x> >> is always a readline() from
2118 an indirect handle, but C<< <$hash{key}> >> is always a glob().
2119 That's because $x is a simple scalar variable, but C<$hash{key}> is
2120 not--it's a hash element. Even C<< <$x > >> (note the extra space)
2121 is treated as C<glob("$x ")>, not C<readline($x)>.
2123 One level of double-quote interpretation is done first, but you can't
2124 say C<< <$foo> >> because that's an indirect filehandle as explained
2125 in the previous paragraph. (In older versions of Perl, programmers
2126 would insert curly brackets to force interpretation as a filename glob:
2127 C<< <${foo}> >>. These days, it's considered cleaner to call the
2128 internal function directly as C<glob($foo)>, which is probably the right
2129 way to have done it in the first place.) For example:
2135 is roughly equivalent to:
2137 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
2143 except that the globbing is actually done internally using the standard
2144 C<File::Glob> extension. Of course, the shortest way to do the above is:
2148 A (file)glob evaluates its (embedded) argument only when it is
2149 starting a new list. All values must be read before it will start
2150 over. In list context, this isn't important because you automatically
2151 get them all anyway. However, in scalar context the operator returns
2152 the next value each time it's called, or C<undef> when the list has
2153 run out. As with filehandle reads, an automatic C<defined> is
2154 generated when the glob occurs in the test part of a C<while>,
2155 because legal glob returns (e.g. a file called F<0>) would otherwise
2156 terminate the loop. Again, C<undef> is returned only once. So if
2157 you're expecting a single value from a glob, it is much better to
2160 ($file) = <blurch*>;
2166 because the latter will alternate between returning a filename and
2169 If you're trying to do variable interpolation, it's definitely better
2170 to use the glob() function, because the older notation can cause people
2171 to become confused with the indirect filehandle notation.
2173 @files = glob("$dir/*.[ch]");
2174 @files = glob($files[$i]);
2176 =head2 Constant Folding
2177 X<constant folding> X<folding>
2179 Like C, Perl does a certain amount of expression evaluation at
2180 compile time whenever it determines that all arguments to an
2181 operator are static and have no side effects. In particular, string
2182 concatenation happens at compile time between literals that don't do
2183 variable substitution. Backslash interpolation also happens at
2184 compile time. You can say
2186 'Now is the time for all' . "\n" .
2187 'good men to come to.'
2189 and this all reduces to one string internally. Likewise, if
2192 foreach $file (@filenames) {
2193 if (-s $file > 5 + 100 * 2**16) { }
2196 the compiler will precompute the number which that expression
2197 represents so that the interpreter won't have to.
2202 Perl doesn't officially have a no-op operator, but the bare constants
2203 C<0> and C<1> are special-cased to not produce a warning in a void
2204 context, so you can for example safely do
2208 =head2 Bitwise String Operators
2209 X<operator, bitwise, string>
2211 Bitstrings of any size may be manipulated by the bitwise operators
2214 If the operands to a binary bitwise op are strings of different
2215 sizes, B<|> and B<^> ops act as though the shorter operand had
2216 additional zero bits on the right, while the B<&> op acts as though
2217 the longer operand were truncated to the length of the shorter.
2218 The granularity for such extension or truncation is one or more
2221 # ASCII-based examples
2222 print "j p \n" ^ " a h"; # prints "JAPH\n"
2223 print "JA" | " ph\n"; # prints "japh\n"
2224 print "japh\nJunk" & '_____'; # prints "JAPH\n";
2225 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
2227 If you are intending to manipulate bitstrings, be certain that
2228 you're supplying bitstrings: If an operand is a number, that will imply
2229 a B<numeric> bitwise operation. You may explicitly show which type of
2230 operation you intend by using C<""> or C<0+>, as in the examples below.
2232 $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
2233 $foo = '150' | 105; # yields 255
2234 $foo = 150 | '105'; # yields 255
2235 $foo = '150' | '105'; # yields string '155' (under ASCII)
2237 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
2238 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
2240 See L<perlfunc/vec> for information on how to manipulate individual bits
2243 =head2 Integer Arithmetic
2246 By default, Perl assumes that it must do most of its arithmetic in
2247 floating point. But by saying
2251 you may tell the compiler that it's okay to use integer operations
2252 (if it feels like it) from here to the end of the enclosing BLOCK.
2253 An inner BLOCK may countermand this by saying
2257 which lasts until the end of that BLOCK. Note that this doesn't
2258 mean everything is only an integer, merely that Perl may use integer
2259 operations if it is so inclined. For example, even under C<use
2260 integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731>
2263 Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
2264 and ">>") always produce integral results. (But see also
2265 L<Bitwise String Operators>.) However, C<use integer> still has meaning for
2266 them. By default, their results are interpreted as unsigned integers, but
2267 if C<use integer> is in effect, their results are interpreted
2268 as signed integers. For example, C<~0> usually evaluates to a large
2269 integral value. However, C<use integer; ~0> is C<-1> on two's-complement
2272 =head2 Floating-point Arithmetic
2273 X<floating-point> X<floating point> X<float> X<real>
2275 While C<use integer> provides integer-only arithmetic, there is no
2276 analogous mechanism to provide automatic rounding or truncation to a
2277 certain number of decimal places. For rounding to a certain number
2278 of digits, sprintf() or printf() is usually the easiest route.
2281 Floating-point numbers are only approximations to what a mathematician
2282 would call real numbers. There are infinitely more reals than floats,
2283 so some corners must be cut. For example:
2285 printf "%.20g\n", 123456789123456789;
2286 # produces 123456789123456784
2288 Testing for exact equality of floating-point equality or inequality is
2289 not a good idea. Here's a (relatively expensive) work-around to compare
2290 whether two floating-point numbers are equal to a particular number of
2291 decimal places. See Knuth, volume II, for a more robust treatment of
2295 my ($X, $Y, $POINTS) = @_;
2297 $tX = sprintf("%.${POINTS}g", $X);
2298 $tY = sprintf("%.${POINTS}g", $Y);
2302 The POSIX module (part of the standard perl distribution) implements
2303 ceil(), floor(), and other mathematical and trigonometric functions.
2304 The Math::Complex module (part of the standard perl distribution)
2305 defines mathematical functions that work on both the reals and the
2306 imaginary numbers. Math::Complex not as efficient as POSIX, but
2307 POSIX can't work with complex numbers.
2309 Rounding in financial applications can have serious implications, and
2310 the rounding method used should be specified precisely. In these
2311 cases, it probably pays not to trust whichever system rounding is
2312 being used by Perl, but to instead implement the rounding function you
2315 =head2 Bigger Numbers
2316 X<number, arbitrary precision>
2318 The standard Math::BigInt and Math::BigFloat modules provide
2319 variable-precision arithmetic and overloaded operators, although
2320 they're currently pretty slow. At the cost of some space and
2321 considerable speed, they avoid the normal pitfalls associated with
2322 limited-precision representations.
2325 $x = Math::BigInt->new('123456789123456789');
2328 # prints +15241578780673678515622620750190521
2330 There are several modules that let you calculate with (bound only by
2331 memory and cpu-time) unlimited or fixed precision. There are also
2332 some non-standard modules that provide faster implementations via
2333 external C libraries.
2335 Here is a short, but incomplete summary:
2337 Math::Fraction big, unlimited fractions like 9973 / 12967
2338 Math::String treat string sequences like numbers
2339 Math::FixedPrecision calculate with a fixed precision
2340 Math::Currency for currency calculations
2341 Bit::Vector manipulate bit vectors fast (uses C)
2342 Math::BigIntFast Bit::Vector wrapper for big numbers
2343 Math::Pari provides access to the Pari C library
2344 Math::BigInteger uses an external C library
2345 Math::Cephes uses external Cephes C library (no big numbers)
2346 Math::Cephes::Fraction fractions via the Cephes library
2347 Math::GMP another one using an external C library
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