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 .. $. == 200) ...
591 next line if (1 .. /^$/); # skip header lines, short for
592 # ... if ($. == 1 .. /^$/);
593 s/^/> / if (/^$/ .. eof()); # quote body
595 # parse mail messages
597 $in_header = 1 .. /^$/;
598 $in_body = /^$/ .. eof;
605 close ARGV if eof; # reset $. each file
608 Here's a simple example to illustrate the difference between
609 the two range operators:
624 This program will print only the line containing "Bar". If
625 the range operator is changed to C<...>, it will also print the
628 And now some examples as a list operator:
630 for (101 .. 200) { print; } # print $_ 100 times
631 @foo = @foo[0 .. $#foo]; # an expensive no-op
632 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
634 The range operator (in list context) makes use of the magical
635 auto-increment algorithm if the operands are strings. You
638 @alphabet = ('A' .. 'Z');
640 to get all normal letters of the English alphabet, or
642 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
644 to get a hexadecimal digit, or
646 @z2 = ('01' .. '31'); print $z2[$mday];
648 to get dates with leading zeros. If the final value specified is not
649 in the sequence that the magical increment would produce, the sequence
650 goes until the next value would be longer than the final value
653 Because each operand is evaluated in integer form, C<2.18 .. 3.14> will
654 return two elements in list context.
656 @list = (2.18 .. 3.14); # same as @list = (2 .. 3);
658 =head2 Conditional Operator
659 X<operator, conditional> X<operator, ternary> X<ternary> X<?:>
661 Ternary "?:" is the conditional operator, just as in C. It works much
662 like an if-then-else. If the argument before the ? is true, the
663 argument before the : is returned, otherwise the argument after the :
664 is returned. For example:
666 printf "I have %d dog%s.\n", $n,
667 ($n == 1) ? '' : "s";
669 Scalar or list context propagates downward into the 2nd
670 or 3rd argument, whichever is selected.
672 $a = $ok ? $b : $c; # get a scalar
673 @a = $ok ? @b : @c; # get an array
674 $a = $ok ? @b : @c; # oops, that's just a count!
676 The operator may be assigned to if both the 2nd and 3rd arguments are
677 legal lvalues (meaning that you can assign to them):
679 ($a_or_b ? $a : $b) = $c;
681 Because this operator produces an assignable result, using assignments
682 without parentheses will get you in trouble. For example, this:
684 $a % 2 ? $a += 10 : $a += 2
688 (($a % 2) ? ($a += 10) : $a) += 2
692 ($a % 2) ? ($a += 10) : ($a += 2)
694 That should probably be written more simply as:
696 $a += ($a % 2) ? 10 : 2;
698 =head2 Assignment Operators
699 X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=>
700 X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=>
703 "=" is the ordinary assignment operator.
705 Assignment operators work as in C. That is,
713 although without duplicating any side effects that dereferencing the lvalue
714 might trigger, such as from tie(). Other assignment operators work similarly.
715 The following are recognized:
722 Although these are grouped by family, they all have the precedence
725 Unlike in C, the scalar assignment operator produces a valid lvalue.
726 Modifying an assignment is equivalent to doing the assignment and
727 then modifying the variable that was assigned to. This is useful
728 for modifying a copy of something, like this:
730 ($tmp = $global) =~ tr [A-Z] [a-z];
741 Similarly, a list assignment in list context produces the list of
742 lvalues assigned to, and a list assignment in scalar context returns
743 the number of elements produced by the expression on the right hand
744 side of the assignment.
746 =head2 Comma Operator
747 X<comma> X<operator, comma> X<,>
749 Binary "," is the comma operator. In scalar context it evaluates
750 its left argument, throws that value away, then evaluates its right
751 argument and returns that value. This is just like C's comma operator.
753 In list context, it's just the list argument separator, and inserts
754 both its arguments into the list.
756 The C<< => >> operator is a synonym for the comma, but forces any word
757 (consisting entirely of word characters) to its left to be interpreted
758 as a string (as of 5.001). This includes words that might otherwise be
759 considered a constant or function call.
761 use constant FOO => "something";
763 my %h = ( FOO => 23 );
771 my %h = ("something", 23);
773 If the argument on the left is not a word, it is first interpreted as
774 an expression, and then the string value of that is used.
776 The C<< => >> operator is helpful in documenting the correspondence
777 between keys and values in hashes, and other paired elements in lists.
779 %hash = ( $key => $value );
780 login( $username => $password );
782 =head2 List Operators (Rightward)
783 X<operator, list, rightward> X<list operator>
785 On the right side of a list operator, it has very low precedence,
786 such that it controls all comma-separated expressions found there.
787 The only operators with lower precedence are the logical operators
788 "and", "or", and "not", which may be used to evaluate calls to list
789 operators without the need for extra parentheses:
791 open HANDLE, "filename"
792 or die "Can't open: $!\n";
794 See also discussion of list operators in L<Terms and List Operators (Leftward)>.
797 X<operator, logical, not> X<not>
799 Unary "not" returns the logical negation of the expression to its right.
800 It's the equivalent of "!" except for the very low precedence.
803 X<operator, logical, and> X<and>
805 Binary "and" returns the logical conjunction of the two surrounding
806 expressions. It's equivalent to && except for the very low
807 precedence. This means that it short-circuits: i.e., the right
808 expression is evaluated only if the left expression is true.
810 =head2 Logical or, Defined or, and Exclusive Or
811 X<operator, logical, or> X<operator, logical, xor> X<operator, logical, err>
812 X<operator, logical, defined or> X<operator, logical, exclusive or>
815 Binary "or" returns the logical disjunction of the two surrounding
816 expressions. It's equivalent to || except for the very low precedence.
817 This makes it useful for control flow
819 print FH $data or die "Can't write to FH: $!";
821 This means that it short-circuits: i.e., the right expression is evaluated
822 only if the left expression is false. Due to its precedence, you should
823 probably avoid using this for assignment, only for control flow.
825 $a = $b or $c; # bug: this is wrong
826 ($a = $b) or $c; # really means this
827 $a = $b || $c; # better written this way
829 However, when it's a list-context assignment and you're trying to use
830 "||" for control flow, you probably need "or" so that the assignment
831 takes higher precedence.
833 @info = stat($file) || die; # oops, scalar sense of stat!
834 @info = stat($file) or die; # better, now @info gets its due
836 Then again, you could always use parentheses.
838 Binary "err" is equivalent to C<//>--it's just like binary "or", except it tests
839 its left argument's definedness instead of its truth. There are two ways to
840 remember "err": either because many functions return C<undef> on an B<err>or,
841 or as a sort of correction: C<$a=($b err 'default')>
843 Binary "xor" returns the exclusive-OR of the two surrounding expressions.
844 It cannot short circuit, of course.
846 =head2 C Operators Missing From Perl
847 X<operator, missing from perl> X<&> X<*>
848 X<typecasting> X<(TYPE)>
850 Here is what C has that Perl doesn't:
856 Address-of operator. (But see the "\" operator for taking a reference.)
860 Dereference-address operator. (Perl's prefix dereferencing
861 operators are typed: $, @, %, and &.)
865 Type-casting operator.
869 =head2 Quote and Quote-like Operators
870 X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m>
871 X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>>
872 X<escape sequence> X<escape>
875 While we usually think of quotes as literal values, in Perl they
876 function as operators, providing various kinds of interpolating and
877 pattern matching capabilities. Perl provides customary quote characters
878 for these behaviors, but also provides a way for you to choose your
879 quote character for any of them. In the following table, a C<{}> represents
880 any pair of delimiters you choose.
882 Customary Generic Meaning Interpolates
887 // m{} Pattern match yes*
889 s{}{} Substitution yes*
890 tr{}{} Transliteration no (but see below)
893 * unless the delimiter is ''.
895 Non-bracketing delimiters use the same character fore and aft, but the four
896 sorts of brackets (round, angle, square, curly) will all nest, which means
905 Note, however, that this does not always work for quoting Perl code:
907 $s = q{ if($a eq "}") ... }; # WRONG
909 is a syntax error. The C<Text::Balanced> module (from CPAN, and
910 starting from Perl 5.8 part of the standard distribution) is able
913 There can be whitespace between the operator and the quoting
914 characters, except when C<#> is being used as the quoting character.
915 C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
916 operator C<q> followed by a comment. Its argument will be taken
917 from the next line. This allows you to write:
919 s {foo} # Replace foo
922 The following escape sequences are available in constructs that interpolate
923 and in transliterations.
924 X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N>
931 \a alarm (bell) (BEL)
933 \033 octal char (ESC)
935 \x{263a} wide hex char (SMILEY)
936 \c[ control char (ESC)
937 \N{name} named Unicode character
939 B<NOTE>: Unlike C and other languages, Perl has no \v escape sequence for
940 the vertical tab (VT - ASCII 11).
942 The following escape sequences are available in constructs that interpolate
943 but not in transliterations.
944 X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q>
946 \l lowercase next char
947 \u uppercase next char
950 \E end case modification
951 \Q quote non-word characters till \E
953 If C<use locale> is in effect, the case map used by C<\l>, C<\L>,
954 C<\u> and C<\U> is taken from the current locale. See L<perllocale>.
955 If Unicode (for example, C<\N{}> or wide hex characters of 0x100 or
956 beyond) is being used, the case map used by C<\l>, C<\L>, C<\u> and
957 C<\U> is as defined by Unicode. For documentation of C<\N{name}>,
960 All systems use the virtual C<"\n"> to represent a line terminator,
961 called a "newline". There is no such thing as an unvarying, physical
962 newline character. It is only an illusion that the operating system,
963 device drivers, C libraries, and Perl all conspire to preserve. Not all
964 systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
965 on a Mac, these are reversed, and on systems without line terminator,
966 printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
967 you mean a "newline" for your system, but use the literal ASCII when you
968 need an exact character. For example, most networking protocols expect
969 and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
970 and although they often accept just C<"\012">, they seldom tolerate just
971 C<"\015">. If you get in the habit of using C<"\n"> for networking,
972 you may be burned some day.
973 X<newline> X<line terminator> X<eol> X<end of line>
976 For constructs that do interpolate, variables beginning with "C<$>"
977 or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
978 C<< $href->{key}[0] >> are also interpolated, as are array and hash slices.
979 But method calls such as C<< $obj->meth >> are not.
981 Interpolating an array or slice interpolates the elements in order,
982 separated by the value of C<$">, so is equivalent to interpolating
983 C<join $", @array>. "Punctuation" arrays such as C<@+> are only
984 interpolated if the name is enclosed in braces C<@{+}>.
986 You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
987 An unescaped C<$> or C<@> interpolates the corresponding variable,
988 while escaping will cause the literal string C<\$> to be inserted.
989 You'll need to write something like C<m/\Quser\E\@\Qhost/>.
991 Patterns are subject to an additional level of interpretation as a
992 regular expression. This is done as a second pass, after variables are
993 interpolated, so that regular expressions may be incorporated into the
994 pattern from the variables. If this is not what you want, use C<\Q> to
995 interpolate a variable literally.
997 Apart from the behavior described above, Perl does not expand
998 multiple levels of interpolation. In particular, contrary to the
999 expectations of shell programmers, back-quotes do I<NOT> interpolate
1000 within double quotes, nor do single quotes impede evaluation of
1001 variables when used within double quotes.
1003 =head2 Regexp Quote-Like Operators
1006 Here are the quote-like operators that apply to pattern
1007 matching and related activities.
1014 This is just like the C</pattern/> search, except that it matches only
1015 once between calls to the reset() operator. This is a useful
1016 optimization when you want to see only the first occurrence of
1017 something in each file of a set of files, for instance. Only C<??>
1018 patterns local to the current package are reset.
1022 # blank line between header and body
1025 reset if eof; # clear ?? status for next file
1028 This usage is vaguely deprecated, which means it just might possibly
1029 be removed in some distant future version of Perl, perhaps somewhere
1030 around the year 2168.
1032 =item m/PATTERN/cgimosx
1033 X<m> X<operator, match>
1034 X<regexp, options> X<regexp> X<regex, options> X<regex>
1035 X</c> X</i> X</m> X</o> X</s> X</x>
1037 =item /PATTERN/cgimosx
1039 Searches a string for a pattern match, and in scalar context returns
1040 true if it succeeds, false if it fails. If no string is specified
1041 via the C<=~> or C<!~> operator, the $_ string is searched. (The
1042 string specified with C<=~> need not be an lvalue--it may be the
1043 result of an expression evaluation, but remember the C<=~> binds
1044 rather tightly.) See also L<perlre>. See L<perllocale> for
1045 discussion of additional considerations that apply when C<use locale>
1050 c Do not reset search position on a failed match when /g is in effect.
1051 g Match globally, i.e., find all occurrences.
1052 i Do case-insensitive pattern matching.
1053 m Treat string as multiple lines.
1054 o Compile pattern only once.
1055 s Treat string as single line.
1056 x Use extended regular expressions.
1058 If "/" is the delimiter then the initial C<m> is optional. With the C<m>
1059 you can use any pair of non-alphanumeric, non-whitespace characters
1060 as delimiters. This is particularly useful for matching path names
1061 that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
1062 the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
1063 If "'" is the delimiter, no interpolation is performed on the PATTERN.
1065 PATTERN may contain variables, which will be interpolated (and the
1066 pattern recompiled) every time the pattern search is evaluated, except
1067 for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
1068 C<$|> are not interpolated because they look like end-of-string tests.)
1069 If you want such a pattern to be compiled only once, add a C</o> after
1070 the trailing delimiter. This avoids expensive run-time recompilations,
1071 and is useful when the value you are interpolating won't change over
1072 the life of the script. However, mentioning C</o> constitutes a promise
1073 that you won't change the variables in the pattern. If you change them,
1074 Perl won't even notice. See also L<"qr/STRING/imosx">.
1076 If the PATTERN evaluates to the empty string, the last
1077 I<successfully> matched regular expression is used instead. In this
1078 case, only the C<g> and C<c> flags on the empty pattern is honoured -
1079 the other flags are taken from the original pattern. If no match has
1080 previously succeeded, this will (silently) act instead as a genuine
1081 empty pattern (which will always match).
1083 Note that it's possible to confuse Perl into thinking C<//> (the empty
1084 regex) is really C<//> (the defined-or operator). Perl is usually pretty
1085 good about this, but some pathological cases might trigger this, such as
1086 C<$a///> (is that C<($a) / (//)> or C<$a // />?) and C<print $fh //>
1087 (C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl
1088 will assume you meant defined-or. If you meant the empty regex, just
1089 use parentheses or spaces to disambiguate, or even prefix the empty
1090 regex with an C<m> (so C<//> becomes C<m//>).
1092 If the C</g> option is not used, C<m//> in list context returns a
1093 list consisting of the subexpressions matched by the parentheses in the
1094 pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are
1095 also set, and that this differs from Perl 4's behavior.) When there are
1096 no parentheses in the pattern, the return value is the list C<(1)> for
1097 success. With or without parentheses, an empty list is returned upon
1102 open(TTY, '/dev/tty');
1103 <TTY> =~ /^y/i && foo(); # do foo if desired
1105 if (/Version: *([0-9.]*)/) { $version = $1; }
1107 next if m#^/usr/spool/uucp#;
1112 print if /$arg/o; # compile only once
1115 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
1117 This last example splits $foo into the first two words and the
1118 remainder of the line, and assigns those three fields to $F1, $F2, and
1119 $Etc. The conditional is true if any variables were assigned, i.e., if
1120 the pattern matched.
1122 The C</g> modifier specifies global pattern matching--that is,
1123 matching as many times as possible within the string. How it behaves
1124 depends on the context. In list context, it returns a list of the
1125 substrings matched by any capturing parentheses in the regular
1126 expression. If there are no parentheses, it returns a list of all
1127 the matched strings, as if there were parentheses around the whole
1130 In scalar context, each execution of C<m//g> finds the next match,
1131 returning true if it matches, and false if there is no further match.
1132 The position after the last match can be read or set using the pos()
1133 function; see L<perlfunc/pos>. A failed match normally resets the
1134 search position to the beginning of the string, but you can avoid that
1135 by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
1136 string also resets the search position.
1138 You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
1139 zero-width assertion that matches the exact position where the previous
1140 C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion
1141 still anchors at pos(), but the match is of course only attempted once.
1142 Using C<\G> without C</g> on a target string that has not previously had a
1143 C</g> match applied to it is the same as using the C<\A> assertion to match
1144 the beginning of the string. Note also that, currently, C<\G> is only
1145 properly supported when anchored at the very beginning of the pattern.
1150 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
1154 while (defined($paragraph = <>)) {
1155 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
1159 print "$sentences\n";
1161 # using m//gc with \G
1165 print $1 while /(o)/gc; print "', pos=", pos, "\n";
1167 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
1169 print $1 while /(p)/gc; print "', pos=", pos, "\n";
1171 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
1173 The last example should print:
1183 Notice that the final match matched C<q> instead of C<p>, which a match
1184 without the C<\G> anchor would have done. Also note that the final match
1185 did not update C<pos> -- C<pos> is only updated on a C</g> match. If the
1186 final match did indeed match C<p>, it's a good bet that you're running an
1187 older (pre-5.6.0) Perl.
1189 A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
1190 combine several regexps like this to process a string part-by-part,
1191 doing different actions depending on which regexp matched. Each
1192 regexp tries to match where the previous one leaves off.
1195 $url = new URI::URL "http://www/"; die if $url eq "xXx";
1199 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
1200 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
1201 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
1202 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
1203 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
1204 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
1205 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
1206 print ". That's all!\n";
1209 Here is the output (split into several lines):
1211 line-noise lowercase line-noise lowercase UPPERCASE line-noise
1212 UPPERCASE line-noise lowercase line-noise lowercase line-noise
1213 lowercase lowercase line-noise lowercase lowercase line-noise
1214 MiXeD line-noise. That's all!
1217 X<q> X<quote, double> X<'> X<''>
1221 A single-quoted, literal string. A backslash represents a backslash
1222 unless followed by the delimiter or another backslash, in which case
1223 the delimiter or backslash is interpolated.
1225 $foo = q!I said, "You said, 'She said it.'"!;
1226 $bar = q('This is it.');
1227 $baz = '\n'; # a two-character string
1230 X<qq> X<quote, double> X<"> X<"">
1234 A double-quoted, interpolated string.
1237 (*** The previous line contains the naughty word "$1".\n)
1238 if /\b(tcl|java|python)\b/i; # :-)
1239 $baz = "\n"; # a one-character string
1241 =item qr/STRING/imosx
1242 X<qr> X</i> X</m> X</o> X</s> X</x>
1244 This operator quotes (and possibly compiles) its I<STRING> as a regular
1245 expression. I<STRING> is interpolated the same way as I<PATTERN>
1246 in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
1247 is done. Returns a Perl value which may be used instead of the
1248 corresponding C</STRING/imosx> expression.
1252 $rex = qr/my.STRING/is;
1259 The result may be used as a subpattern in a match:
1262 $string =~ /foo${re}bar/; # can be interpolated in other patterns
1263 $string =~ $re; # or used standalone
1264 $string =~ /$re/; # or this way
1266 Since Perl may compile the pattern at the moment of execution of qr()
1267 operator, using qr() may have speed advantages in some situations,
1268 notably if the result of qr() is used standalone:
1271 my $patterns = shift;
1272 my @compiled = map qr/$_/i, @$patterns;
1275 foreach my $pat (@compiled) {
1276 $success = 1, last if /$pat/;
1282 Precompilation of the pattern into an internal representation at
1283 the moment of qr() avoids a need to recompile the pattern every
1284 time a match C</$pat/> is attempted. (Perl has many other internal
1285 optimizations, but none would be triggered in the above example if
1286 we did not use qr() operator.)
1290 i Do case-insensitive pattern matching.
1291 m Treat string as multiple lines.
1292 o Compile pattern only once.
1293 s Treat string as single line.
1294 x Use extended regular expressions.
1296 See L<perlre> for additional information on valid syntax for STRING, and
1297 for a detailed look at the semantics of regular expressions.
1300 X<qx> X<`> X<``> X<backtick>
1304 A string which is (possibly) interpolated and then executed as a
1305 system command with C</bin/sh> or its equivalent. Shell wildcards,
1306 pipes, and redirections will be honored. The collected standard
1307 output of the command is returned; standard error is unaffected. In
1308 scalar context, it comes back as a single (potentially multi-line)
1309 string, or undef if the command failed. In list context, returns a
1310 list of lines (however you've defined lines with $/ or
1311 $INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
1313 Because backticks do not affect standard error, use shell file descriptor
1314 syntax (assuming the shell supports this) if you care to address this.
1315 To capture a command's STDERR and STDOUT together:
1317 $output = `cmd 2>&1`;
1319 To capture a command's STDOUT but discard its STDERR:
1321 $output = `cmd 2>/dev/null`;
1323 To capture a command's STDERR but discard its STDOUT (ordering is
1326 $output = `cmd 2>&1 1>/dev/null`;
1328 To exchange a command's STDOUT and STDERR in order to capture the STDERR
1329 but leave its STDOUT to come out the old STDERR:
1331 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
1333 To read both a command's STDOUT and its STDERR separately, it's easiest
1334 to redirect them separately to files, and then read from those files
1335 when the program is done:
1337 system("program args 1>program.stdout 2>program.stderr");
1339 Using single-quote as a delimiter protects the command from Perl's
1340 double-quote interpolation, passing it on to the shell instead:
1342 $perl_info = qx(ps $$); # that's Perl's $$
1343 $shell_info = qx'ps $$'; # that's the new shell's $$
1345 How that string gets evaluated is entirely subject to the command
1346 interpreter on your system. On most platforms, you will have to protect
1347 shell metacharacters if you want them treated literally. This is in
1348 practice difficult to do, as it's unclear how to escape which characters.
1349 See L<perlsec> for a clean and safe example of a manual fork() and exec()
1350 to emulate backticks safely.
1352 On some platforms (notably DOS-like ones), the shell may not be
1353 capable of dealing with multiline commands, so putting newlines in
1354 the string may not get you what you want. You may be able to evaluate
1355 multiple commands in a single line by separating them with the command
1356 separator character, if your shell supports that (e.g. C<;> on many Unix
1357 shells; C<&> on the Windows NT C<cmd> shell).
1359 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1360 output before starting the child process, but this may not be supported
1361 on some platforms (see L<perlport>). To be safe, you may need to set
1362 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1363 C<IO::Handle> on any open handles.
1365 Beware that some command shells may place restrictions on the length
1366 of the command line. You must ensure your strings don't exceed this
1367 limit after any necessary interpolations. See the platform-specific
1368 release notes for more details about your particular environment.
1370 Using this operator can lead to programs that are difficult to port,
1371 because the shell commands called vary between systems, and may in
1372 fact not be present at all. As one example, the C<type> command under
1373 the POSIX shell is very different from the C<type> command under DOS.
1374 That doesn't mean you should go out of your way to avoid backticks
1375 when they're the right way to get something done. Perl was made to be
1376 a glue language, and one of the things it glues together is commands.
1377 Just understand what you're getting yourself into.
1379 See L<"I/O Operators"> for more discussion.
1382 X<qw> X<quote, list> X<quote, words>
1384 Evaluates to a list of the words extracted out of STRING, using embedded
1385 whitespace as the word delimiters. It can be understood as being roughly
1388 split(' ', q/STRING/);
1390 the differences being that it generates a real list at compile time, and
1391 in scalar context it returns the last element in the list. So
1396 is semantically equivalent to the list:
1400 Some frequently seen examples:
1402 use POSIX qw( setlocale localeconv )
1403 @EXPORT = qw( foo bar baz );
1405 A common mistake is to try to separate the words with comma or to
1406 put comments into a multi-line C<qw>-string. For this reason, the
1407 C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
1408 produces warnings if the STRING contains the "," or the "#" character.
1410 =item s/PATTERN/REPLACEMENT/egimosx
1411 X<substitute> X<substitution> X<replace> X<regexp, replace>
1412 X<regexp, substitute> X</e> X</g> X</i> X</m> X</o> X</s> X</x>
1414 Searches a string for a pattern, and if found, replaces that pattern
1415 with the replacement text and returns the number of substitutions
1416 made. Otherwise it returns false (specifically, the empty string).
1418 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
1419 variable is searched and modified. (The string specified with C<=~> must
1420 be scalar variable, an array element, a hash element, or an assignment
1421 to one of those, i.e., an lvalue.)
1423 If the delimiter chosen is a single quote, no interpolation is
1424 done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1425 PATTERN contains a $ that looks like a variable rather than an
1426 end-of-string test, the variable will be interpolated into the pattern
1427 at run-time. If you want the pattern compiled only once the first time
1428 the variable is interpolated, use the C</o> option. If the pattern
1429 evaluates to the empty string, the last successfully executed regular
1430 expression is used instead. See L<perlre> for further explanation on these.
1431 See L<perllocale> for discussion of additional considerations that apply
1432 when C<use locale> is in effect.
1436 e Evaluate the right side as an expression.
1437 g Replace globally, i.e., all occurrences.
1438 i Do case-insensitive pattern matching.
1439 m Treat string as multiple lines.
1440 o Compile pattern only once.
1441 s Treat string as single line.
1442 x Use extended regular expressions.
1444 Any non-alphanumeric, non-whitespace delimiter may replace the
1445 slashes. If single quotes are used, no interpretation is done on the
1446 replacement string (the C</e> modifier overrides this, however). Unlike
1447 Perl 4, Perl 5 treats backticks as normal delimiters; the replacement
1448 text is not evaluated as a command. If the
1449 PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
1450 pair of quotes, which may or may not be bracketing quotes, e.g.,
1451 C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
1452 replacement portion to be treated as a full-fledged Perl expression
1453 and evaluated right then and there. It is, however, syntax checked at
1454 compile-time. A second C<e> modifier will cause the replacement portion
1455 to be C<eval>ed before being run as a Perl expression.
1459 s/\bgreen\b/mauve/g; # don't change wintergreen
1461 $path =~ s|/usr/bin|/usr/local/bin|;
1463 s/Login: $foo/Login: $bar/; # run-time pattern
1465 ($foo = $bar) =~ s/this/that/; # copy first, then change
1467 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
1470 s/\d+/$&*2/e; # yields 'abc246xyz'
1471 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
1472 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1474 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
1475 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
1476 s/^=(\w+)/&pod($1)/ge; # use function call
1478 # expand variables in $_, but dynamics only, using
1479 # symbolic dereferencing
1482 # Add one to the value of any numbers in the string
1485 # This will expand any embedded scalar variable
1486 # (including lexicals) in $_ : First $1 is interpolated
1487 # to the variable name, and then evaluated
1490 # Delete (most) C comments.
1492 /\* # Match the opening delimiter.
1493 .*? # Match a minimal number of characters.
1494 \*/ # Match the closing delimiter.
1497 s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_, expensively
1499 for ($variable) { # trim whitespace in $variable, cheap
1504 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
1506 Note the use of $ instead of \ in the last example. Unlike
1507 B<sed>, we use the \<I<digit>> form in only the left hand side.
1508 Anywhere else it's $<I<digit>>.
1510 Occasionally, you can't use just a C</g> to get all the changes
1511 to occur that you might want. Here are two common cases:
1513 # put commas in the right places in an integer
1514 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
1516 # expand tabs to 8-column spacing
1517 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
1519 =item tr/SEARCHLIST/REPLACEMENTLIST/cds
1520 X<tr> X<y> X<transliterate> X</c> X</d> X</s>
1522 =item y/SEARCHLIST/REPLACEMENTLIST/cds
1524 Transliterates all occurrences of the characters found in the search list
1525 with the corresponding character in the replacement list. It returns
1526 the number of characters replaced or deleted. If no string is
1527 specified via the =~ or !~ operator, the $_ string is transliterated. (The
1528 string specified with =~ must be a scalar variable, an array element, a
1529 hash element, or an assignment to one of those, i.e., an lvalue.)
1531 A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
1532 does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
1533 For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1534 SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1535 its own pair of quotes, which may or may not be bracketing quotes,
1536 e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
1538 Note that C<tr> does B<not> do regular expression character classes
1539 such as C<\d> or C<[:lower:]>. The <tr> operator is not equivalent to
1540 the tr(1) utility. If you want to map strings between lower/upper
1541 cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider
1542 using the C<s> operator if you need regular expressions.
1544 Note also that the whole range idea is rather unportable between
1545 character sets--and even within character sets they may cause results
1546 you probably didn't expect. A sound principle is to use only ranges
1547 that begin from and end at either alphabets of equal case (a-e, A-E),
1548 or digits (0-4). Anything else is unsafe. If in doubt, spell out the
1549 character sets in full.
1553 c Complement the SEARCHLIST.
1554 d Delete found but unreplaced characters.
1555 s Squash duplicate replaced characters.
1557 If the C</c> modifier is specified, the SEARCHLIST character set
1558 is complemented. If the C</d> modifier is specified, any characters
1559 specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
1560 (Note that this is slightly more flexible than the behavior of some
1561 B<tr> programs, which delete anything they find in the SEARCHLIST,
1562 period.) If the C</s> modifier is specified, sequences of characters
1563 that were transliterated to the same character are squashed down
1564 to a single instance of the character.
1566 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1567 exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1568 than the SEARCHLIST, the final character is replicated till it is long
1569 enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
1570 This latter is useful for counting characters in a class or for
1571 squashing character sequences in a class.
1575 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1577 $cnt = tr/*/*/; # count the stars in $_
1579 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1581 $cnt = tr/0-9//; # count the digits in $_
1583 tr/a-zA-Z//s; # bookkeeper -> bokeper
1585 ($HOST = $host) =~ tr/a-z/A-Z/;
1587 tr/a-zA-Z/ /cs; # change non-alphas to single space
1590 [\000-\177]; # delete 8th bit
1592 If multiple transliterations are given for a character, only the
1597 will transliterate any A to X.
1599 Because the transliteration table is built at compile time, neither
1600 the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
1601 interpolation. That means that if you want to use variables, you
1604 eval "tr/$oldlist/$newlist/";
1607 eval "tr/$oldlist/$newlist/, 1" or die $@;
1610 X<here-doc> X<heredoc> X<here-document> X<<< << >>>
1612 A line-oriented form of quoting is based on the shell "here-document"
1613 syntax. Following a C<< << >> you specify a string to terminate
1614 the quoted material, and all lines following the current line down to
1615 the terminating string are the value of the item. The terminating
1616 string may be either an identifier (a word), or some quoted text. If
1617 quoted, the type of quotes you use determines the treatment of the
1618 text, just as in regular quoting. An unquoted identifier works like
1619 double quotes. There must be no space between the C<< << >> and
1620 the identifier, unless the identifier is quoted. (If you put a space it
1621 will be treated as a null identifier, which is valid, and matches the first
1622 empty line.) The terminating string must appear by itself (unquoted and
1623 with no surrounding whitespace) on the terminating line.
1626 The price is $Price.
1629 print << "EOF"; # same as above
1630 The price is $Price.
1633 print << `EOC`; # execute commands
1638 print <<"foo", <<"bar"; # you can stack them
1644 myfunc(<< "THIS", 23, <<'THAT');
1651 Just don't forget that you have to put a semicolon on the end
1652 to finish the statement, as Perl doesn't know you're not going to
1660 If you want your here-docs to be indented with the
1661 rest of the code, you'll need to remove leading whitespace
1662 from each line manually:
1664 ($quote = <<'FINIS') =~ s/^\s+//gm;
1665 The Road goes ever on and on,
1666 down from the door where it began.
1669 If you use a here-doc within a delimited construct, such as in C<s///eg>,
1670 the quoted material must come on the lines following the final delimiter.
1685 If the terminating identifier is on the last line of the program, you
1686 must be sure there is a newline after it; otherwise, Perl will give the
1687 warning B<Can't find string terminator "END" anywhere before EOF...>.
1689 Additionally, the quoting rules for the identifier are not related to
1690 Perl's quoting rules -- C<q()>, C<qq()>, and the like are not supported
1691 in place of C<''> and C<"">, and the only interpolation is for backslashing
1692 the quoting character:
1694 print << "abc\"def";
1698 Finally, quoted strings cannot span multiple lines. The general rule is
1699 that the identifier must be a string literal. Stick with that, and you
1704 =head2 Gory details of parsing quoted constructs
1705 X<quote, gory details>
1707 When presented with something that might have several different
1708 interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
1709 principle to pick the most probable interpretation. This strategy
1710 is so successful that Perl programmers often do not suspect the
1711 ambivalence of what they write. But from time to time, Perl's
1712 notions differ substantially from what the author honestly meant.
1714 This section hopes to clarify how Perl handles quoted constructs.
1715 Although the most common reason to learn this is to unravel labyrinthine
1716 regular expressions, because the initial steps of parsing are the
1717 same for all quoting operators, they are all discussed together.
1719 The most important Perl parsing rule is the first one discussed
1720 below: when processing a quoted construct, Perl first finds the end
1721 of that construct, then interprets its contents. If you understand
1722 this rule, you may skip the rest of this section on the first
1723 reading. The other rules are likely to contradict the user's
1724 expectations much less frequently than this first one.
1726 Some passes discussed below are performed concurrently, but because
1727 their results are the same, we consider them individually. For different
1728 quoting constructs, Perl performs different numbers of passes, from
1729 one to five, but these passes are always performed in the same order.
1733 =item Finding the end
1735 The first pass is finding the end of the quoted construct, whether
1736 it be a multicharacter delimiter C<"\nEOF\n"> in the C<<<EOF>
1737 construct, a C</> that terminates a C<qq//> construct, a C<]> which
1738 terminates C<qq[]> construct, or a C<< > >> which terminates a
1739 fileglob started with C<< < >>.
1741 When searching for single-character non-pairing delimiters, such
1742 as C</>, combinations of C<\\> and C<\/> are skipped. However,
1743 when searching for single-character pairing delimiter like C<[>,
1744 combinations of C<\\>, C<\]>, and C<\[> are all skipped, and nested
1745 C<[>, C<]> are skipped as well. When searching for multicharacter
1746 delimiters, nothing is skipped.
1748 For constructs with three-part delimiters (C<s///>, C<y///>, and
1749 C<tr///>), the search is repeated once more.
1751 During this search no attention is paid to the semantics of the construct.
1754 "$hash{"$foo/$bar"}"
1759 bar # NOT a comment, this slash / terminated m//!
1762 do not form legal quoted expressions. The quoted part ends on the
1763 first C<"> and C</>, and the rest happens to be a syntax error.
1764 Because the slash that terminated C<m//> was followed by a C<SPACE>,
1765 the example above is not C<m//x>, but rather C<m//> with no C</x>
1766 modifier. So the embedded C<#> is interpreted as a literal C<#>.
1768 Also no attention is paid to C<\c\> during this search.
1769 Thus the second C<\> in C<qq/\c\/> is interpreted as a part of C<\/>,
1770 and the following C</> is not recognized as a delimiter.
1771 Instead, use C<\034> or C<\x1c> at the end of quoted constructs.
1773 =item Removal of backslashes before delimiters
1775 During the second pass, text between the starting and ending
1776 delimiters is copied to a safe location, and the C<\> is removed
1777 from combinations consisting of C<\> and delimiter--or delimiters,
1778 meaning both starting and ending delimiters will should these differ.
1779 This removal does not happen for multi-character delimiters.
1780 Note that the combination C<\\> is left intact, just as it was.
1782 Starting from this step no information about the delimiters is
1788 The next step is interpolation in the text obtained, which is now
1789 delimiter-independent. There are four different cases.
1793 =item C<<<'EOF'>, C<m''>, C<s'''>, C<tr///>, C<y///>
1795 No interpolation is performed.
1799 The only interpolation is removal of C<\> from pairs C<\\>.
1801 =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>
1803 C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
1804 converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
1805 is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
1806 The other combinations are replaced with appropriate expansions.
1808 Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
1809 is interpolated in the usual way. Something like C<"\Q\\E"> has
1810 no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
1811 result is the same as for C<"\\\\E">. As a general rule, backslashes
1812 between C<\Q> and C<\E> may lead to counterintuitive results. So,
1813 C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
1814 as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
1819 may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
1821 Interpolated scalars and arrays are converted internally to the C<join> and
1822 C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
1824 $foo . " XXX '" . (join $", @arr) . "'";
1826 All operations above are performed simultaneously, left to right.
1828 Because the result of C<"\Q STRING \E"> has all metacharacters
1829 quoted, there is no way to insert a literal C<$> or C<@> inside a
1830 C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
1831 C<"\\\$">; if not, it is interpreted as the start of an interpolated
1834 Note also that the interpolation code needs to make a decision on
1835 where the interpolated scalar ends. For instance, whether
1836 C<< "a $b -> {c}" >> really means:
1838 "a " . $b . " -> {c}";
1844 Most of the time, the longest possible text that does not include
1845 spaces between components and which contains matching braces or
1846 brackets. because the outcome may be determined by voting based
1847 on heuristic estimators, the result is not strictly predictable.
1848 Fortunately, it's usually correct for ambiguous cases.
1850 =item C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
1852 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
1853 happens (almost) as with C<qq//> constructs, but the substitution
1854 of C<\> followed by RE-special chars (including C<\>) is not
1855 performed. Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
1856 a C<#>-comment in a C<//x>-regular expression, no processing is
1857 performed whatsoever. This is the first step at which the presence
1858 of the C<//x> modifier is relevant.
1860 Interpolation has several quirks: C<$|>, C<$(>, and C<$)> are not
1861 interpolated, and constructs C<$var[SOMETHING]> are voted (by several
1862 different estimators) to be either an array element or C<$var>
1863 followed by an RE alternative. This is where the notation
1864 C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
1865 array element C<-9>, not as a regular expression from the variable
1866 C<$arr> followed by a digit, which would be the interpretation of
1867 C</$arr[0-9]/>. Since voting among different estimators may occur,
1868 the result is not predictable.
1870 It is at this step that C<\1> is begrudgingly converted to C<$1> in
1871 the replacement text of C<s///> to correct the incorrigible
1872 I<sed> hackers who haven't picked up the saner idiom yet. A warning
1873 is emitted if the C<use warnings> pragma or the B<-w> command-line flag
1874 (that is, the C<$^W> variable) was set.
1876 The lack of processing of C<\\> creates specific restrictions on
1877 the post-processed text. If the delimiter is C</>, one cannot get
1878 the combination C<\/> into the result of this step. C</> will
1879 finish the regular expression, C<\/> will be stripped to C</> on
1880 the previous step, and C<\\/> will be left as is. Because C</> is
1881 equivalent to C<\/> inside a regular expression, this does not
1882 matter unless the delimiter happens to be character special to the
1883 RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
1884 alphanumeric char, as in:
1888 In the RE above, which is intentionally obfuscated for illustration, the
1889 delimiter is C<m>, the modifier is C<mx>, and after backslash-removal the
1890 RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
1891 reason you're encouraged to restrict your delimiters to non-alphanumeric,
1892 non-whitespace choices.
1896 This step is the last one for all constructs except regular expressions,
1897 which are processed further.
1899 =item Interpolation of regular expressions
1900 X<regexp, interpolation>
1902 Previous steps were performed during the compilation of Perl code,
1903 but this one happens at run time--although it may be optimized to
1904 be calculated at compile time if appropriate. After preprocessing
1905 described above, and possibly after evaluation if catenation,
1906 joining, casing translation, or metaquoting are involved, the
1907 resulting I<string> is passed to the RE engine for compilation.
1909 Whatever happens in the RE engine might be better discussed in L<perlre>,
1910 but for the sake of continuity, we shall do so here.
1912 This is another step where the presence of the C<//x> modifier is
1913 relevant. The RE engine scans the string from left to right and
1914 converts it to a finite automaton.
1916 Backslashed characters are either replaced with corresponding
1917 literal strings (as with C<\{>), or else they generate special nodes
1918 in the finite automaton (as with C<\b>). Characters special to the
1919 RE engine (such as C<|>) generate corresponding nodes or groups of
1920 nodes. C<(?#...)> comments are ignored. All the rest is either
1921 converted to literal strings to match, or else is ignored (as is
1922 whitespace and C<#>-style comments if C<//x> is present).
1924 Parsing of the bracketed character class construct, C<[...]>, is
1925 rather different than the rule used for the rest of the pattern.
1926 The terminator of this construct is found using the same rules as
1927 for finding the terminator of a C<{}>-delimited construct, the only
1928 exception being that C<]> immediately following C<[> is treated as
1929 though preceded by a backslash. Similarly, the terminator of
1930 C<(?{...})> is found using the same rules as for finding the
1931 terminator of a C<{}>-delimited construct.
1933 It is possible to inspect both the string given to RE engine and the
1934 resulting finite automaton. See the arguments C<debug>/C<debugcolor>
1935 in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
1936 switch documented in L<perlrun/"Command Switches">.
1938 =item Optimization of regular expressions
1939 X<regexp, optimization>
1941 This step is listed for completeness only. Since it does not change
1942 semantics, details of this step are not documented and are subject
1943 to change without notice. This step is performed over the finite
1944 automaton that was generated during the previous pass.
1946 It is at this stage that C<split()> silently optimizes C</^/> to
1951 =head2 I/O Operators
1952 X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle>
1955 There are several I/O operators you should know about.
1957 A string enclosed by backticks (grave accents) first undergoes
1958 double-quote interpolation. It is then interpreted as an external
1959 command, and the output of that command is the value of the
1960 backtick string, like in a shell. In scalar context, a single string
1961 consisting of all output is returned. In list context, a list of
1962 values is returned, one per line of output. (You can set C<$/> to use
1963 a different line terminator.) The command is executed each time the
1964 pseudo-literal is evaluated. The status value of the command is
1965 returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
1966 Unlike in B<csh>, no translation is done on the return data--newlines
1967 remain newlines. Unlike in any of the shells, single quotes do not
1968 hide variable names in the command from interpretation. To pass a
1969 literal dollar-sign through to the shell you need to hide it with a
1970 backslash. The generalized form of backticks is C<qx//>. (Because
1971 backticks always undergo shell expansion as well, see L<perlsec> for
1973 X<qx> X<`> X<``> X<backtick> X<glob>
1975 In scalar context, evaluating a filehandle in angle brackets yields
1976 the next line from that file (the newline, if any, included), or
1977 C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
1978 (sometimes known as file-slurp mode) and the file is empty, it
1979 returns C<''> the first time, followed by C<undef> subsequently.
1981 Ordinarily you must assign the returned value to a variable, but
1982 there is one situation where an automatic assignment happens. If
1983 and only if the input symbol is the only thing inside the conditional
1984 of a C<while> statement (even if disguised as a C<for(;;)> loop),
1985 the value is automatically assigned to the global variable $_,
1986 destroying whatever was there previously. (This may seem like an
1987 odd thing to you, but you'll use the construct in almost every Perl
1988 script you write.) The $_ variable is not implicitly localized.
1989 You'll have to put a C<local $_;> before the loop if you want that
1992 The following lines are equivalent:
1994 while (defined($_ = <STDIN>)) { print; }
1995 while ($_ = <STDIN>) { print; }
1996 while (<STDIN>) { print; }
1997 for (;<STDIN>;) { print; }
1998 print while defined($_ = <STDIN>);
1999 print while ($_ = <STDIN>);
2000 print while <STDIN>;
2002 This also behaves similarly, but avoids $_ :
2004 while (my $line = <STDIN>) { print $line }
2006 In these loop constructs, the assigned value (whether assignment
2007 is automatic or explicit) is then tested to see whether it is
2008 defined. The defined test avoids problems where line has a string
2009 value that would be treated as false by Perl, for example a "" or
2010 a "0" with no trailing newline. If you really mean for such values
2011 to terminate the loop, they should be tested for explicitly:
2013 while (($_ = <STDIN>) ne '0') { ... }
2014 while (<STDIN>) { last unless $_; ... }
2016 In other boolean contexts, C<< <I<filehandle>> >> without an
2017 explicit C<defined> test or comparison elicit a warning if the
2018 C<use warnings> pragma or the B<-w>
2019 command-line switch (the C<$^W> variable) is in effect.
2021 The filehandles STDIN, STDOUT, and STDERR are predefined. (The
2022 filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
2023 in packages, where they would be interpreted as local identifiers
2024 rather than global.) Additional filehandles may be created with
2025 the open() function, amongst others. See L<perlopentut> and
2026 L<perlfunc/open> for details on this.
2027 X<stdin> X<stdout> X<sterr>
2029 If a <FILEHANDLE> is used in a context that is looking for
2030 a list, a list comprising all input lines is returned, one line per
2031 list element. It's easy to grow to a rather large data space this
2032 way, so use with care.
2034 <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
2035 See L<perlfunc/readline>.
2037 The null filehandle <> is special: it can be used to emulate the
2038 behavior of B<sed> and B<awk>. Input from <> comes either from
2039 standard input, or from each file listed on the command line. Here's
2040 how it works: the first time <> is evaluated, the @ARGV array is
2041 checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
2042 gives you standard input. The @ARGV array is then processed as a list
2043 of filenames. The loop
2046 ... # code for each line
2049 is equivalent to the following Perl-like pseudo code:
2051 unshift(@ARGV, '-') unless @ARGV;
2052 while ($ARGV = shift) {
2055 ... # code for each line
2059 except that it isn't so cumbersome to say, and will actually work.
2060 It really does shift the @ARGV array and put the current filename
2061 into the $ARGV variable. It also uses filehandle I<ARGV>
2062 internally--<> is just a synonym for <ARGV>, which
2063 is magical. (The pseudo code above doesn't work because it treats
2064 <ARGV> as non-magical.)
2066 You can modify @ARGV before the first <> as long as the array ends up
2067 containing the list of filenames you really want. Line numbers (C<$.>)
2068 continue as though the input were one big happy file. See the example
2069 in L<perlfunc/eof> for how to reset line numbers on each file.
2071 If you want to set @ARGV to your own list of files, go right ahead.
2072 This sets @ARGV to all plain text files if no @ARGV was given:
2074 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
2076 You can even set them to pipe commands. For example, this automatically
2077 filters compressed arguments through B<gzip>:
2079 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
2081 If you want to pass switches into your script, you can use one of the
2082 Getopts modules or put a loop on the front like this:
2084 while ($_ = $ARGV[0], /^-/) {
2087 if (/^-D(.*)/) { $debug = $1 }
2088 if (/^-v/) { $verbose++ }
2089 # ... # other switches
2093 # ... # code for each line
2096 The <> symbol will return C<undef> for end-of-file only once.
2097 If you call it again after this, it will assume you are processing another
2098 @ARGV list, and if you haven't set @ARGV, will read input from STDIN.
2100 If what the angle brackets contain is a simple scalar variable (e.g.,
2101 <$foo>), then that variable contains the name of the
2102 filehandle to input from, or its typeglob, or a reference to the
2108 If what's within the angle brackets is neither a filehandle nor a simple
2109 scalar variable containing a filehandle name, typeglob, or typeglob
2110 reference, it is interpreted as a filename pattern to be globbed, and
2111 either a list of filenames or the next filename in the list is returned,
2112 depending on context. This distinction is determined on syntactic
2113 grounds alone. That means C<< <$x> >> is always a readline() from
2114 an indirect handle, but C<< <$hash{key}> >> is always a glob().
2115 That's because $x is a simple scalar variable, but C<$hash{key}> is
2116 not--it's a hash element. Even C<< <$x > >> (note the extra space)
2117 is treated as C<glob("$x ")>, not C<readline($x)>.
2119 One level of double-quote interpretation is done first, but you can't
2120 say C<< <$foo> >> because that's an indirect filehandle as explained
2121 in the previous paragraph. (In older versions of Perl, programmers
2122 would insert curly brackets to force interpretation as a filename glob:
2123 C<< <${foo}> >>. These days, it's considered cleaner to call the
2124 internal function directly as C<glob($foo)>, which is probably the right
2125 way to have done it in the first place.) For example:
2131 is roughly equivalent to:
2133 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
2139 except that the globbing is actually done internally using the standard
2140 C<File::Glob> extension. Of course, the shortest way to do the above is:
2144 A (file)glob evaluates its (embedded) argument only when it is
2145 starting a new list. All values must be read before it will start
2146 over. In list context, this isn't important because you automatically
2147 get them all anyway. However, in scalar context the operator returns
2148 the next value each time it's called, or C<undef> when the list has
2149 run out. As with filehandle reads, an automatic C<defined> is
2150 generated when the glob occurs in the test part of a C<while>,
2151 because legal glob returns (e.g. a file called F<0>) would otherwise
2152 terminate the loop. Again, C<undef> is returned only once. So if
2153 you're expecting a single value from a glob, it is much better to
2156 ($file) = <blurch*>;
2162 because the latter will alternate between returning a filename and
2165 If you're trying to do variable interpolation, it's definitely better
2166 to use the glob() function, because the older notation can cause people
2167 to become confused with the indirect filehandle notation.
2169 @files = glob("$dir/*.[ch]");
2170 @files = glob($files[$i]);
2172 =head2 Constant Folding
2173 X<constant folding> X<folding>
2175 Like C, Perl does a certain amount of expression evaluation at
2176 compile time whenever it determines that all arguments to an
2177 operator are static and have no side effects. In particular, string
2178 concatenation happens at compile time between literals that don't do
2179 variable substitution. Backslash interpolation also happens at
2180 compile time. You can say
2182 'Now is the time for all' . "\n" .
2183 'good men to come to.'
2185 and this all reduces to one string internally. Likewise, if
2188 foreach $file (@filenames) {
2189 if (-s $file > 5 + 100 * 2**16) { }
2192 the compiler will precompute the number which that expression
2193 represents so that the interpreter won't have to.
2198 Perl doesn't officially have a no-op operator, but the bare constants
2199 C<0> and C<1> are special-cased to not produce a warning in a void
2200 context, so you can for example safely do
2204 =head2 Bitwise String Operators
2205 X<operator, bitwise, string>
2207 Bitstrings of any size may be manipulated by the bitwise operators
2210 If the operands to a binary bitwise op are strings of different
2211 sizes, B<|> and B<^> ops act as though the shorter operand had
2212 additional zero bits on the right, while the B<&> op acts as though
2213 the longer operand were truncated to the length of the shorter.
2214 The granularity for such extension or truncation is one or more
2217 # ASCII-based examples
2218 print "j p \n" ^ " a h"; # prints "JAPH\n"
2219 print "JA" | " ph\n"; # prints "japh\n"
2220 print "japh\nJunk" & '_____'; # prints "JAPH\n";
2221 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
2223 If you are intending to manipulate bitstrings, be certain that
2224 you're supplying bitstrings: If an operand is a number, that will imply
2225 a B<numeric> bitwise operation. You may explicitly show which type of
2226 operation you intend by using C<""> or C<0+>, as in the examples below.
2228 $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
2229 $foo = '150' | 105; # yields 255
2230 $foo = 150 | '105'; # yields 255
2231 $foo = '150' | '105'; # yields string '155' (under ASCII)
2233 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
2234 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
2236 See L<perlfunc/vec> for information on how to manipulate individual bits
2239 =head2 Integer Arithmetic
2242 By default, Perl assumes that it must do most of its arithmetic in
2243 floating point. But by saying
2247 you may tell the compiler that it's okay to use integer operations
2248 (if it feels like it) from here to the end of the enclosing BLOCK.
2249 An inner BLOCK may countermand this by saying
2253 which lasts until the end of that BLOCK. Note that this doesn't
2254 mean everything is only an integer, merely that Perl may use integer
2255 operations if it is so inclined. For example, even under C<use
2256 integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731>
2259 Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
2260 and ">>") always produce integral results. (But see also
2261 L<Bitwise String Operators>.) However, C<use integer> still has meaning for
2262 them. By default, their results are interpreted as unsigned integers, but
2263 if C<use integer> is in effect, their results are interpreted
2264 as signed integers. For example, C<~0> usually evaluates to a large
2265 integral value. However, C<use integer; ~0> is C<-1> on twos-complement
2268 =head2 Floating-point Arithmetic
2269 X<floating-point> X<floating point> X<float> X<real>
2271 While C<use integer> provides integer-only arithmetic, there is no
2272 analogous mechanism to provide automatic rounding or truncation to a
2273 certain number of decimal places. For rounding to a certain number
2274 of digits, sprintf() or printf() is usually the easiest route.
2277 Floating-point numbers are only approximations to what a mathematician
2278 would call real numbers. There are infinitely more reals than floats,
2279 so some corners must be cut. For example:
2281 printf "%.20g\n", 123456789123456789;
2282 # produces 123456789123456784
2284 Testing for exact equality of floating-point equality or inequality is
2285 not a good idea. Here's a (relatively expensive) work-around to compare
2286 whether two floating-point numbers are equal to a particular number of
2287 decimal places. See Knuth, volume II, for a more robust treatment of
2291 my ($X, $Y, $POINTS) = @_;
2293 $tX = sprintf("%.${POINTS}g", $X);
2294 $tY = sprintf("%.${POINTS}g", $Y);
2298 The POSIX module (part of the standard perl distribution) implements
2299 ceil(), floor(), and other mathematical and trigonometric functions.
2300 The Math::Complex module (part of the standard perl distribution)
2301 defines mathematical functions that work on both the reals and the
2302 imaginary numbers. Math::Complex not as efficient as POSIX, but
2303 POSIX can't work with complex numbers.
2305 Rounding in financial applications can have serious implications, and
2306 the rounding method used should be specified precisely. In these
2307 cases, it probably pays not to trust whichever system rounding is
2308 being used by Perl, but to instead implement the rounding function you
2311 =head2 Bigger Numbers
2312 X<number, arbitrary precision>
2314 The standard Math::BigInt and Math::BigFloat modules provide
2315 variable-precision arithmetic and overloaded operators, although
2316 they're currently pretty slow. At the cost of some space and
2317 considerable speed, they avoid the normal pitfalls associated with
2318 limited-precision representations.
2321 $x = Math::BigInt->new('123456789123456789');
2324 # prints +15241578780673678515622620750190521
2326 There are several modules that let you calculate with (bound only by
2327 memory and cpu-time) unlimited or fixed precision. There are also
2328 some non-standard modules that provide faster implementations via
2329 external C libraries.
2331 Here is a short, but incomplete summary:
2333 Math::Fraction big, unlimited fractions like 9973 / 12967
2334 Math::String treat string sequences like numbers
2335 Math::FixedPrecision calculate with a fixed precision
2336 Math::Currency for currency calculations
2337 Bit::Vector manipulate bit vectors fast (uses C)
2338 Math::BigIntFast Bit::Vector wrapper for big numbers
2339 Math::Pari provides access to the Pari C library
2340 Math::BigInteger uses an external C library
2341 Math::Cephes uses external Cephes C library (no big numbers)
2342 Math::Cephes::Fraction fractions via the Cephes library
2343 Math::GMP another one using an external C library
projects / p5sagit/p5-mst-13.2.git / blob