3 perlop - Perl operators and precedence
7 Perl operators have the following associativity and precedence,
8 listed from highest precedence to lowest. Note that all operators
9 borrowed from C keep the same precedence relationship with each other,
10 even where C's precedence is slightly screwy. (This makes learning
11 Perl easier for C folks.) With very few exceptions, these all
12 operate on scalar values only, not array values.
14 left terms and list operators (leftward)
18 right ! ~ \ and unary + and -
23 nonassoc named unary operators
24 nonassoc < > <= >= lt gt le ge
25 nonassoc == != <=> eq ne cmp
34 nonassoc list operators (rightward)
39 In the following sections, these operators are covered in precedence order.
41 Many operators can be overloaded for objects. See L<overload>.
45 =head2 Terms and List Operators (Leftward)
47 A TERM has the highest precedence in Perl. They includes variables,
48 quote and quote-like operators, any expression in parentheses,
49 and any function whose arguments are parenthesized. Actually, there
50 aren't really functions in this sense, just list operators and unary
51 operators behaving as functions because you put parentheses around
52 the arguments. These are all documented in L<perlfunc>.
54 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
55 is followed by a left parenthesis as the next token, the operator and
56 arguments within parentheses are taken to be of highest precedence,
57 just like a normal function call.
59 In the absence of parentheses, the precedence of list operators such as
60 C<print>, C<sort>, or C<chmod> is either very high or very low depending on
61 whether you are looking at the left side or the right side of the operator.
64 @ary = (1, 3, sort 4, 2);
65 print @ary; # prints 1324
67 the commas on the right of the sort are evaluated before the sort, but
68 the commas on the left are evaluated after. In other words, list
69 operators tend to gobble up all the arguments that follow them, and
70 then act like a simple TERM with regard to the preceding expression.
71 Note that you have to be careful with parentheses:
73 # These evaluate exit before doing the print:
74 print($foo, exit); # Obviously not what you want.
75 print $foo, exit; # Nor is this.
77 # These do the print before evaluating exit:
78 (print $foo), exit; # This is what you want.
79 print($foo), exit; # Or this.
80 print ($foo), exit; # Or even this.
84 print ($foo & 255) + 1, "\n";
86 probably doesn't do what you expect at first glance. See
87 L<Named Unary Operators> for more discussion of this.
89 Also parsed as terms are the C<do {}> and C<eval {}> constructs, as
90 well as subroutine and method calls, and the anonymous
91 constructors C<[]> and C<{}>.
93 See also L<Quote and Quote-like Operators> toward the end of this section,
94 as well as L<"I/O Operators">.
96 =head2 The Arrow Operator
98 Just as in C and C++, "C<-E<gt>>" is an infix dereference operator. If the
99 right side is either a C<[...]> or C<{...}> subscript, then the left side
100 must be either a hard or symbolic reference to an array or hash (or
101 a location capable of holding a hard reference, if it's an lvalue (assignable)).
104 Otherwise, the right side is a method name or a simple scalar variable
105 containing the method name, and the left side must either be an object
106 (a blessed reference) or a class name (that is, a package name).
109 =head2 Auto-increment and Auto-decrement
111 "++" and "--" work as in C. That is, if placed before a variable, they
112 increment or decrement the variable before returning the value, and if
113 placed after, increment or decrement the variable after returning the value.
115 The auto-increment operator has a little extra builtin magic to it. If
116 you increment a variable that is numeric, or that has ever been used in
117 a numeric context, you get a normal increment. If, however, the
118 variable has been used in only string contexts since it was set, and
119 has a value that is not the empty string and matches the pattern
120 C</^[a-zA-Z]*[0-9]*$/>, the increment is done as a string, preserving each
121 character within its range, with carry:
123 print ++($foo = '99'); # prints '100'
124 print ++($foo = 'a0'); # prints 'a1'
125 print ++($foo = 'Az'); # prints 'Ba'
126 print ++($foo = 'zz'); # prints 'aaa'
128 The auto-decrement operator is not magical.
130 =head2 Exponentiation
132 Binary "**" is the exponentiation operator. Note that it binds even more
133 tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is
134 implemented using C's pow(3) function, which actually works on doubles
137 =head2 Symbolic Unary Operators
139 Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower
140 precedence version of this.
142 Unary "-" performs arithmetic negation if the operand is numeric. If
143 the operand is an identifier, a string consisting of a minus sign
144 concatenated with the identifier is returned. Otherwise, if the string
145 starts with a plus or minus, a string starting with the opposite sign
146 is returned. One effect of these rules is that C<-bareword> is equivalent
149 Unary "~" performs bitwise negation, i.e., 1's complement. For example,
150 C<0666 &~ 027> is 0640. (See also L<Integer Arithmetic> and L<Bitwise
153 Unary "+" has no effect whatsoever, even on strings. It is useful
154 syntactically for separating a function name from a parenthesized expression
155 that would otherwise be interpreted as the complete list of function
156 arguments. (See examples above under L<Terms and List Operators (Leftward)>.)
158 Unary "\" creates a reference to whatever follows it. See L<perlref>.
159 Do not confuse this behavior with the behavior of backslash within a
160 string, although both forms do convey the notion of protecting the next
161 thing from interpretation.
163 =head2 Binding Operators
165 Binary "=~" binds a scalar expression to a pattern match. Certain operations
166 search or modify the string $_ by default. This operator makes that kind
167 of operation work on some other string. The right argument is a search
168 pattern, substitution, or transliteration. The left argument is what is
169 supposed to be searched, substituted, or transliterated instead of the default
170 $_. The return value indicates the success of the operation. (If the
171 right argument is an expression rather than a search pattern,
172 substitution, or transliteration, it is interpreted as a search pattern at run
173 time. This can be is less efficient than an explicit search, because the
174 pattern must be compiled every time the expression is evaluated.
176 Binary "!~" is just like "=~" except the return value is negated in
179 =head2 Multiplicative Operators
181 Binary "*" multiplies two numbers.
183 Binary "/" divides two numbers.
185 Binary "%" computes the modulus of two numbers. Given integer
186 operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
187 C<$a> minus the largest multiple of C<$b> that is not greater than
188 C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
189 smallest multiple of C<$b> that is not less than C<$a> (i.e. the
190 result will be less than or equal to zero). If C<use integer> is
191 in effect, the native hardware will be used instead of this rule,
192 which may be construed a bug that will be fixed at some point.
194 Note than when C<use integer> is in scope, "%" give you direct access
195 to the modulus operator as implemented by your C compiler. This
196 operator is not as well defined for negative operands, but it will
199 Binary "x" is the repetition operator. In scalar context, it
200 returns a string consisting of the left operand repeated the number of
201 times specified by the right operand. In list context, if the left
202 operand is a list in parentheses, it repeats the list.
204 print '-' x 80; # print row of dashes
206 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
208 @ones = (1) x 80; # a list of 80 1's
209 @ones = (5) x @ones; # set all elements to 5
212 =head2 Additive Operators
214 Binary "+" returns the sum of two numbers.
216 Binary "-" returns the difference of two numbers.
218 Binary "." concatenates two strings.
220 =head2 Shift Operators
222 Binary "<<" returns the value of its left argument shifted left by the
223 number of bits specified by the right argument. Arguments should be
224 integers. (See also L<Integer Arithmetic>.)
226 Binary ">>" returns the value of its left argument shifted right by
227 the number of bits specified by the right argument. Arguments should
228 be integers. (See also L<Integer Arithmetic>.)
230 =head2 Named Unary Operators
232 The various named unary operators are treated as functions with one
233 argument, with optional parentheses. These include the filetest
234 operators, like C<-f>, C<-M>, etc. See L<perlfunc>.
236 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
237 is followed by a left parenthesis as the next token, the operator and
238 arguments within parentheses are taken to be of highest precedence,
239 just like a normal function call. Examples:
241 chdir $foo || die; # (chdir $foo) || die
242 chdir($foo) || die; # (chdir $foo) || die
243 chdir ($foo) || die; # (chdir $foo) || die
244 chdir +($foo) || die; # (chdir $foo) || die
246 but, because * is higher precedence than ||:
248 chdir $foo * 20; # chdir ($foo * 20)
249 chdir($foo) * 20; # (chdir $foo) * 20
250 chdir ($foo) * 20; # (chdir $foo) * 20
251 chdir +($foo) * 20; # chdir ($foo * 20)
253 rand 10 * 20; # rand (10 * 20)
254 rand(10) * 20; # (rand 10) * 20
255 rand (10) * 20; # (rand 10) * 20
256 rand +(10) * 20; # rand (10 * 20)
258 See also L<"Terms and List Operators (Leftward)">.
260 =head2 Relational Operators
262 Binary "E<lt>" returns true if the left argument is numerically less than
265 Binary "E<gt>" returns true if the left argument is numerically greater
266 than the right argument.
268 Binary "E<lt>=" returns true if the left argument is numerically less than
269 or equal to the right argument.
271 Binary "E<gt>=" returns true if the left argument is numerically greater
272 than or equal to the right argument.
274 Binary "lt" returns true if the left argument is stringwise less than
277 Binary "gt" returns true if the left argument is stringwise greater
278 than the right argument.
280 Binary "le" returns true if the left argument is stringwise less than
281 or equal to the right argument.
283 Binary "ge" returns true if the left argument is stringwise greater
284 than or equal to the right argument.
286 =head2 Equality Operators
288 Binary "==" returns true if the left argument is numerically equal to
291 Binary "!=" returns true if the left argument is numerically not equal
292 to the right argument.
294 Binary "E<lt>=E<gt>" returns -1, 0, or 1 depending on whether the left
295 argument is numerically less than, equal to, or greater than the right
298 Binary "eq" returns true if the left argument is stringwise equal to
301 Binary "ne" returns true if the left argument is stringwise not equal
302 to the right argument.
304 Binary "cmp" returns -1, 0, or 1 depending on whether the left argument is stringwise
305 less than, equal to, or greater than the right argument.
307 "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
308 by the current locale if C<use locale> is in effect. See L<perllocale>.
312 Binary "&" returns its operators ANDed together bit by bit.
313 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
315 =head2 Bitwise Or and Exclusive Or
317 Binary "|" returns its operators ORed together bit by bit.
318 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
320 Binary "^" returns its operators XORed together bit by bit.
321 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
323 =head2 C-style Logical And
325 Binary "&&" performs a short-circuit logical AND operation. That is,
326 if the left operand is false, the right operand is not even evaluated.
327 Scalar or list context propagates down to the right operand if it
330 =head2 C-style Logical Or
332 Binary "||" performs a short-circuit logical OR operation. That is,
333 if the left operand is true, the right operand is not even evaluated.
334 Scalar or list context propagates down to the right operand if it
337 The C<||> and C<&&> operators differ from C's in that, rather than returning
338 0 or 1, they return the last value evaluated. Thus, a reasonably portable
339 way to find out the home directory (assuming it's not "0") might be:
341 $home = $ENV{'HOME'} || $ENV{'LOGDIR'} ||
342 (getpwuid($<))[7] || die "You're homeless!\n";
344 In particular, this means that you shouldn't use this
345 for selecting between two aggregates for assignment:
347 @a = @b || @c; # this is wrong
348 @a = scalar(@b) || @c; # really meant this
349 @a = @b ? @b : @c; # this works fine, though
351 As more readable alternatives to C<&&> and C<||> when used for
352 control flow, Perl provides C<and> and C<or> operators (see below).
353 The short-circuit behavior is identical. The precedence of "and" and
354 "or" is much lower, however, so that you can safely use them after a
355 list operator without the need for parentheses:
357 unlink "alpha", "beta", "gamma"
358 or gripe(), next LINE;
360 With the C-style operators that would have been written like this:
362 unlink("alpha", "beta", "gamma")
363 || (gripe(), next LINE);
365 Use "or" for assignment is unlikely to do what you want; see below.
367 =head2 Range Operators
369 Binary ".." is the range operator, which is really two different
370 operators depending on the context. In list context, it returns an
371 array of values counting (by ones) from the left value to the right
372 value. This is useful for writing C<for (1..10)> loops and for doing
373 slice operations on arrays. Be aware that under the current implementation,
374 a temporary array is created, so you'll burn a lot of memory if you
375 write something like this:
377 for (1 .. 1_000_000) {
381 In scalar context, ".." returns a boolean value. The operator is
382 bistable, like a flip-flop, and emulates the line-range (comma) operator
383 of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
384 own boolean state. It is false as long as its left operand is false.
385 Once the left operand is true, the range operator stays true until the
386 right operand is true, I<AFTER> which the range operator becomes false
387 again. (It doesn't become false till the next time the range operator is
388 evaluated. It can test the right operand and become false on the same
389 evaluation it became true (as in B<awk>), but it still returns true once.
390 If you don't want it to test the right operand till the next evaluation
391 (as in B<sed>), use three dots ("...") instead of two.) The right
392 operand is not evaluated while the operator is in the "false" state, and
393 the left operand is not evaluated while the operator is in the "true"
394 state. The precedence is a little lower than || and &&. The value
395 returned is either the empty string for false, or a sequence number
396 (beginning with 1) for true. The sequence number is reset for each range
397 encountered. The final sequence number in a range has the string "E0"
398 appended to it, which doesn't affect its numeric value, but gives you
399 something to search for if you want to exclude the endpoint. You can
400 exclude the beginning point by waiting for the sequence number to be
401 greater than 1. If either operand of scalar ".." is a numeric literal,
402 that operand is implicitly compared to the C<$.> variable, the current
403 line number. Examples:
405 As a scalar operator:
407 if (101 .. 200) { print; } # print 2nd hundred lines
408 next line if (1 .. /^$/); # skip header lines
409 s/^/> / if (/^$/ .. eof()); # quote body
411 # parse mail messages
413 $in_header = 1 .. /^$/;
414 $in_body = /^$/ .. eof();
415 # do something based on those
417 close ARGV if eof; # reset $. each file
422 for (101 .. 200) { print; } # print $_ 100 times
423 @foo = @foo[0 .. $#foo]; # an expensive no-op
424 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
426 The range operator (in list context) makes use of the magical
427 auto-increment algorithm if the operands are strings. You
430 @alphabet = ('A' .. 'Z');
432 to get all the letters of the alphabet, or
434 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
436 to get a hexadecimal digit, or
438 @z2 = ('01' .. '31'); print $z2[$mday];
440 to get dates with leading zeros. If the final value specified is not
441 in the sequence that the magical increment would produce, the sequence
442 goes until the next value would be longer than the final value
445 =head2 Conditional Operator
447 Ternary "?:" is the conditional operator, just as in C. It works much
448 like an if-then-else. If the argument before the ? is true, the
449 argument before the : is returned, otherwise the argument after the :
450 is returned. For example:
452 printf "I have %d dog%s.\n", $n,
453 ($n == 1) ? '' : "s";
455 Scalar or list context propagates downward into the 2nd
456 or 3rd argument, whichever is selected.
458 $a = $ok ? $b : $c; # get a scalar
459 @a = $ok ? @b : @c; # get an array
460 $a = $ok ? @b : @c; # oops, that's just a count!
462 The operator may be assigned to if both the 2nd and 3rd arguments are
463 legal lvalues (meaning that you can assign to them):
465 ($a_or_b ? $a : $b) = $c;
467 This is not necessarily guaranteed to contribute to the readability of your program.
469 Because this operator produces an assignable result, using assignments
470 without parentheses will get you in trouble. For example, this:
472 $a % 2 ? $a += 10 : $a += 2
476 (($a % 2) ? ($a += 10) : $a) += 2
480 ($a % 2) ? ($a += 10) : ($a += 2)
482 =head2 Assignment Operators
484 "=" is the ordinary assignment operator.
486 Assignment operators work as in C. That is,
494 although without duplicating any side effects that dereferencing the lvalue
495 might trigger, such as from tie(). Other assignment operators work similarly.
496 The following are recognized:
503 Note that while these are grouped by family, they all have the precedence
506 Unlike in C, the assignment operator produces a valid lvalue. Modifying
507 an assignment is equivalent to doing the assignment and then modifying
508 the variable that was assigned to. This is useful for modifying
509 a copy of something, like this:
511 ($tmp = $global) =~ tr [A-Z] [a-z];
522 =head2 Comma Operator
524 Binary "," is the comma operator. In scalar context it evaluates
525 its left argument, throws that value away, then evaluates its right
526 argument and returns that value. This is just like C's comma operator.
528 In list context, it's just the list argument separator, and inserts
529 both its arguments into the list.
531 The =E<gt> digraph is mostly just a synonym for the comma operator. It's useful for
532 documenting arguments that come in pairs. As of release 5.001, it also forces
533 any word to the left of it to be interpreted as a string.
535 =head2 List Operators (Rightward)
537 On the right side of a list operator, it has very low precedence,
538 such that it controls all comma-separated expressions found there.
539 The only operators with lower precedence are the logical operators
540 "and", "or", and "not", which may be used to evaluate calls to list
541 operators without the need for extra parentheses:
543 open HANDLE, "filename"
544 or die "Can't open: $!\n";
546 See also discussion of list operators in L<Terms and List Operators (Leftward)>.
550 Unary "not" returns the logical negation of the expression to its right.
551 It's the equivalent of "!" except for the very low precedence.
555 Binary "and" returns the logical conjunction of the two surrounding
556 expressions. It's equivalent to && except for the very low
557 precedence. This means that it short-circuits: i.e., the right
558 expression is evaluated only if the left expression is true.
560 =head2 Logical or and Exclusive Or
562 Binary "or" returns the logical disjunction of the two surrounding
563 expressions. It's equivalent to || except for the very low precedence.
564 This makes it useful for control flow
566 print FH $data or die "Can't write to FH: $!";
568 This means that it short-circuits: i.e., the right expression is evaluated
569 only if the left expression is false. Due to its precedence, you should
570 probably avoid using this for assignment, only for control flow.
572 $a = $b or $c; # bug: this is wrong
573 ($a = $b) or $c; # really means this
574 $a = $b || $c; # better written this way
576 However, when it's a list context assignment and you're trying to use
577 "||" for control flow, you probably need "or" so that the assignment
578 takes higher precedence.
580 @info = stat($file) || die; # oops, scalar sense of stat!
581 @info = stat($file) or die; # better, now @info gets its due
583 Then again, you could always use parentheses.
585 Binary "xor" returns the exclusive-OR of the two surrounding expressions.
586 It cannot short circuit, of course.
588 =head2 C Operators Missing From Perl
590 Here is what C has that Perl doesn't:
596 Address-of operator. (But see the "\" operator for taking a reference.)
600 Dereference-address operator. (Perl's prefix dereferencing
601 operators are typed: $, @, %, and &.)
605 Type casting operator.
609 =head2 Quote and Quote-like Operators
611 While we usually think of quotes as literal values, in Perl they
612 function as operators, providing various kinds of interpolating and
613 pattern matching capabilities. Perl provides customary quote characters
614 for these behaviors, but also provides a way for you to choose your
615 quote character for any of them. In the following table, a C<{}> represents
616 any pair of delimiters you choose. Non-bracketing delimiters use
617 the same character fore and aft, but the 4 sorts of brackets
618 (round, angle, square, curly) will all nest.
620 Customary Generic Meaning Interpolates
625 // m{} Pattern match yes
626 s{}{} Substitution yes
627 tr{}{} Transliteration no (but see below)
629 Note that there can be whitespace between the operator and the quoting
630 characters, except when C<#> is being used as the quoting character.
631 C<q#foo#> is parsed as being the string C<foo>, while C<q #foo#> is the
632 operator C<q> followed by a comment. Its argument will be taken from the
633 next line. This allows you to write:
635 s {foo} # Replace foo
638 For constructs that do interpolation, variables beginning with "C<$>"
639 or "C<@>" are interpolated, as are the following sequences. Within
640 a transliteration, the first ten of these sequences may be used.
647 \a alarm (bell) (BEL)
653 \l lowercase next char
654 \u uppercase next char
657 \E end case modification
658 \Q quote non-word characters till \E
660 If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u>
661 and C<\U> is taken from the current locale. See L<perllocale>.
663 All systems use the virtual C<"\n"> to represent a line terminator,
664 called a "newline". There is no such thing as an unvarying, physical
665 newline character. It is an illusion that the operating system,
666 device drivers, C libraries, and Perl all conspire to preserve. Not all
667 systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
668 on a Mac, these are reversed, and on systems without line terminator,
669 printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
670 you mean a "newline" for your system, but use the literal ASCII when you
671 need an exact character. For example, most networking protocols expect
672 and prefer a CR+LF (C<"\012\015"> or C<"\cJ\cM">) for line terminators,
673 and although they often accept just C<"\012">, they seldom tolerate just
674 C<"\015">. If you get in the habit of using C<"\n"> for networking,
675 you may be burned some day.
677 You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
678 An unescaped C<$> or C<@> interpolates the corresponding variable,
679 while escaping will cause the literal string C<\$> to be inserted.
680 You'll need to write something like C<m/\Quser\E\@\Qhost/>.
682 Patterns are subject to an additional level of interpretation as a
683 regular expression. This is done as a second pass, after variables are
684 interpolated, so that regular expressions may be incorporated into the
685 pattern from the variables. If this is not what you want, use C<\Q> to
686 interpolate a variable literally.
688 Apart from the above, there are no multiple levels of interpolation. In
689 particular, contrary to the expectations of shell programmers, back-quotes
690 do I<NOT> interpolate within double quotes, nor do single quotes impede
691 evaluation of variables when used within double quotes.
693 =head2 Regexp Quote-Like Operators
695 Here are the quote-like operators that apply to pattern
696 matching and related activities.
702 This is just like the C</pattern/> search, except that it matches only
703 once between calls to the reset() operator. This is a useful
704 optimization when you want to see only the first occurrence of
705 something in each file of a set of files, for instance. Only C<??>
706 patterns local to the current package are reset.
710 # blank line between header and body
713 reset if eof; # clear ?? status for next file
716 This usage is vaguely deprecated, and may be removed in some future
719 =item m/PATTERN/cgimosx
721 =item /PATTERN/cgimosx
723 Searches a string for a pattern match, and in scalar context returns
724 true (1) or false (''). If no string is specified via the C<=~> or
725 C<!~> operator, the $_ string is searched. (The string specified with
726 C<=~> need not be an lvalue--it may be the result of an expression
727 evaluation, but remember the C<=~> binds rather tightly.) See also
729 See L<perllocale> for discussion of additional considerations that apply
730 when C<use locale> is in effect.
734 c Do not reset search position on a failed match when /g is in effect.
735 g Match globally, i.e., find all occurrences.
736 i Do case-insensitive pattern matching.
737 m Treat string as multiple lines.
738 o Compile pattern only once.
739 s Treat string as single line.
740 t Taint $1 etc. if target string is tainted.
741 x Use extended regular expressions.
743 If "/" is the delimiter then the initial C<m> is optional. With the C<m>
744 you can use any pair of non-alphanumeric, non-whitespace characters as
745 delimiters. This is particularly useful for matching Unix path names
746 that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
747 the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
749 PATTERN may contain variables, which will be interpolated (and the
750 pattern recompiled) every time the pattern search is evaluated. (Note
751 that C<$)> and C<$|> might not be interpolated because they look like
752 end-of-string tests.) If you want such a pattern to be compiled only
753 once, add a C</o> after the trailing delimiter. This avoids expensive
754 run-time recompilations, and is useful when the value you are
755 interpolating won't change over the life of the script. However, mentioning
756 C</o> constitutes a promise that you won't change the variables in the pattern.
757 If you change them, Perl won't even notice.
759 If the PATTERN evaluates to the empty string, the last
760 I<successfully> matched regular expression is used instead.
762 If used in a context that requires a list value, a pattern match returns a
763 list consisting of the subexpressions matched by the parentheses in the
764 pattern, i.e., (C<$1>, $2, $3...). (Note that here $1 etc. are also set, and
765 that this differs from Perl 4's behavior.) If there are no parentheses,
766 the return value is the list C<(1)> for success or C<('')> upon failure.
767 With parentheses, C<()> is returned upon failure.
771 open(TTY, '/dev/tty');
772 <TTY> =~ /^y/i && foo(); # do foo if desired
774 if (/Version: *([0-9.]*)/) { $version = $1; }
776 next if m#^/usr/spool/uucp#;
781 print if /$arg/o; # compile only once
784 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
786 This last example splits $foo into the first two words and the
787 remainder of the line, and assigns those three fields to $F1, $F2, and
788 $Etc. The conditional is true if any variables were assigned, i.e., if
791 The C</g> modifier specifies global pattern matching--that is, matching
792 as many times as possible within the string. How it behaves depends on
793 the context. In list context, it returns a list of all the
794 substrings matched by all the parentheses in the regular expression.
795 If there are no parentheses, it returns a list of all the matched
796 strings, as if there were parentheses around the whole pattern.
798 In scalar context, C<m//g> iterates through the string, returning TRUE
799 each time it matches, and FALSE when it eventually runs out of matches.
800 (In other words, it remembers where it left off last time and restarts
801 the search at that point. You can actually find the current match
802 position of a string or set it using the pos() function; see
803 L<perlfunc/pos>.) A failed match normally resets the search position to
804 the beginning of the string, but you can avoid that by adding the C</c>
805 modifier (e.g. C<m//gc>). Modifying the target string also resets the
808 You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
809 zero-width assertion that matches the exact position where the previous
810 C<m//g>, if any, left off. The C<\G> assertion is not supported without
811 the C</g> modifier; currently, without C</g>, C<\G> behaves just like
812 C<\A>, but that's accidental and may change in the future.
817 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
820 $/ = ""; $* = 1; # $* deprecated in modern perls
821 while (defined($paragraph = <>)) {
822 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
826 print "$sentences\n";
828 # using m//gc with \G
832 print $1 while /(o)/gc; print "', pos=", pos, "\n";
834 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
836 print $1 while /(p)/gc; print "', pos=", pos, "\n";
839 The last example should print:
848 A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
849 combine several regexps like this to process a string part-by-part,
850 doing different actions depending on which regexp matched. Each
851 regexp tries to match where the previous one leaves off.
854 $url = new URI::URL "http://www/"; die if $url eq "xXx";
858 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
859 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
860 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
861 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
862 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
863 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
864 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
865 print ". That's all!\n";
868 Here is the output (split into several lines):
870 line-noise lowercase line-noise lowercase UPPERCASE line-noise
871 UPPERCASE line-noise lowercase line-noise lowercase line-noise
872 lowercase lowercase line-noise lowercase lowercase line-noise
873 MiXeD line-noise. That's all!
879 A single-quoted, literal string. A backslash represents a backslash
880 unless followed by the delimiter or another backslash, in which case
881 the delimiter or backslash is interpolated.
883 $foo = q!I said, "You said, 'She said it.'"!;
884 $bar = q('This is it.');
885 $baz = '\n'; # a two-character string
891 A double-quoted, interpolated string.
894 (*** The previous line contains the naughty word "$1".\n)
895 if /(tcl|rexx|python)/; # :-)
896 $baz = "\n"; # a one-character string
902 A string which is (possibly) interpolated and then executed as a system
903 command with C</bin/sh> or its equivalent. Shell wildcards, pipes,
904 and redirections will be honored. The collected standard output of the
905 command is returned; standard error is unaffected. In scalar context,
906 it comes back as a single (potentially multi-line) string. In list
907 context, returns a list of lines (however you've defined lines with $/
908 or $INPUT_RECORD_SEPARATOR).
910 Because backticks do not affect standard error, use shell file descriptor
911 syntax (assuming the shell supports this) if you care to address this.
912 To capture a command's STDERR and STDOUT together:
914 $output = `cmd 2>&1`;
916 To capture a command's STDOUT but discard its STDERR:
918 $output = `cmd 2>/dev/null`;
920 To capture a command's STDERR but discard its STDOUT (ordering is
923 $output = `cmd 2>&1 1>/dev/null`;
925 To exchange a command's STDOUT and STDERR in order to capture the STDERR
926 but leave its STDOUT to come out the old STDERR:
928 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
930 To read both a command's STDOUT and its STDERR separately, it's easiest
931 and safest to redirect them separately to files, and then read from those
932 files when the program is done:
934 system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr");
936 Using single-quote as a delimiter protects the command from Perl's
937 double-quote interpolation, passing it on to the shell instead:
939 $perl_info = qx(ps $$); # that's Perl's $$
940 $shell_info = qx'ps $$'; # that's the new shell's $$
942 Note that how the string gets evaluated is entirely subject to the command
943 interpreter on your system. On most platforms, you will have to protect
944 shell metacharacters if you want them treated literally. This is in
945 practice difficult to do, as it's unclear how to escape which characters.
946 See L<perlsec> for a clean and safe example of a manual fork() and exec()
947 to emulate backticks safely.
949 On some platforms (notably DOS-like ones), the shell may not be
950 capable of dealing with multiline commands, so putting newlines in
951 the string may not get you what you want. You may be able to evaluate
952 multiple commands in a single line by separating them with the command
953 separator character, if your shell supports that (e.g. C<;> on many Unix
954 shells; C<&> on the Windows NT C<cmd> shell).
956 Beware that some command shells may place restrictions on the length
957 of the command line. You must ensure your strings don't exceed this
958 limit after any necessary interpolations. See the platform-specific
959 release notes for more details about your particular environment.
961 Using this operator can lead to programs that are difficult to port,
962 because the shell commands called vary between systems, and may in
963 fact not be present at all. As one example, the C<type> command under
964 the POSIX shell is very different from the C<type> command under DOS.
965 That doesn't mean you should go out of your way to avoid backticks
966 when they're the right way to get something done. Perl was made to be
967 a glue language, and one of the things it glues together is commands.
968 Just understand what you're getting yourself into.
970 See L<"I/O Operators"> for more discussion.
974 Returns a list of the words extracted out of STRING, using embedded
975 whitespace as the word delimiters. It is exactly equivalent to
977 split(' ', q/STRING/);
979 This equivalency means that if used in scalar context, you'll get split's
980 (unfortunate) scalar context behavior, complete with mysterious warnings.
982 Some frequently seen examples:
984 use POSIX qw( setlocale localeconv )
985 @EXPORT = qw( foo bar baz );
987 A common mistake is to try to separate the words with comma or to put
988 comments into a multi-line C<qw>-string. For this reason the C<-w>
989 switch produce warnings if the STRING contains the "," or the "#"
992 =item s/PATTERN/REPLACEMENT/egimosx
994 Searches a string for a pattern, and if found, replaces that pattern
995 with the replacement text and returns the number of substitutions
996 made. Otherwise it returns false (specifically, the empty string).
998 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
999 variable is searched and modified. (The string specified with C<=~> must
1000 be scalar variable, an array element, a hash element, or an assignment
1001 to one of those, i.e., an lvalue.)
1003 If the delimiter chosen is single quote, no variable interpolation is
1004 done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1005 PATTERN contains a $ that looks like a variable rather than an
1006 end-of-string test, the variable will be interpolated into the pattern
1007 at run-time. If you want the pattern compiled only once the first time
1008 the variable is interpolated, use the C</o> option. If the pattern
1009 evaluates to the empty string, the last successfully executed regular
1010 expression is used instead. See L<perlre> for further explanation on these.
1011 See L<perllocale> for discussion of additional considerations that apply
1012 when C<use locale> is in effect.
1016 e Evaluate the right side as an expression.
1017 g Replace globally, i.e., all occurrences.
1018 i Do case-insensitive pattern matching.
1019 m Treat string as multiple lines.
1020 o Compile pattern only once.
1021 s Treat string as single line.
1022 x Use extended regular expressions.
1024 Any non-alphanumeric, non-whitespace delimiter may replace the
1025 slashes. If single quotes are used, no interpretation is done on the
1026 replacement string (the C</e> modifier overrides this, however). Unlike
1027 Perl 4, Perl 5 treats backticks as normal delimiters; the replacement
1028 text is not evaluated as a command. If the
1029 PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
1030 pair of quotes, which may or may not be bracketing quotes, e.g.,
1031 C<s(foo)(bar)> or C<sE<lt>fooE<gt>/bar/>. A C</e> will cause the
1032 replacement portion to be interpreted as a full-fledged Perl expression
1033 and eval()ed right then and there. It is, however, syntax checked at
1038 s/\bgreen\b/mauve/g; # don't change wintergreen
1040 $path =~ s|/usr/bin|/usr/local/bin|;
1042 s/Login: $foo/Login: $bar/; # run-time pattern
1044 ($foo = $bar) =~ s/this/that/; # copy first, then change
1046 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
1049 s/\d+/$&*2/e; # yields 'abc246xyz'
1050 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
1051 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1053 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
1054 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
1055 s/^=(\w+)/&pod($1)/ge; # use function call
1057 # expand variables in $_, but dynamics only, using
1058 # symbolic dereferencing
1061 # /e's can even nest; this will expand
1062 # any embedded scalar variable (including lexicals) in $_
1065 # Delete (most) C comments.
1067 /\* # Match the opening delimiter.
1068 .*? # Match a minimal number of characters.
1069 \*/ # Match the closing delimiter.
1072 s/^\s*(.*?)\s*$/$1/; # trim white space in $_, expensively
1074 for ($variable) { # trim white space in $variable, cheap
1079 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
1081 Note the use of $ instead of \ in the last example. Unlike
1082 B<sed>, we use the \E<lt>I<digit>E<gt> form in only the left hand side.
1083 Anywhere else it's $E<lt>I<digit>E<gt>.
1085 Occasionally, you can't use just a C</g> to get all the changes
1086 to occur. Here are two common cases:
1088 # put commas in the right places in an integer
1089 1 while s/(.*\d)(\d\d\d)/$1,$2/g; # perl4
1090 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g; # perl5
1092 # expand tabs to 8-column spacing
1093 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
1096 =item tr/SEARCHLIST/REPLACEMENTLIST/cds
1098 =item y/SEARCHLIST/REPLACEMENTLIST/cds
1100 Transliterates all occurrences of the characters found in the search list
1101 with the corresponding character in the replacement list. It returns
1102 the number of characters replaced or deleted. If no string is
1103 specified via the =~ or !~ operator, the $_ string is transliterated. (The
1104 string specified with =~ must be a scalar variable, an array element, a
1105 hash element, or an assignment to one of those, i.e., an lvalue.)
1106 A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
1107 does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
1108 For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1109 SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1110 its own pair of quotes, which may or may not be bracketing quotes,
1111 e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
1115 c Complement the SEARCHLIST.
1116 d Delete found but unreplaced characters.
1117 s Squash duplicate replaced characters.
1119 If the C</c> modifier is specified, the SEARCHLIST character set is
1120 complemented. If the C</d> modifier is specified, any characters specified
1121 by SEARCHLIST not found in REPLACEMENTLIST are deleted. (Note
1122 that this is slightly more flexible than the behavior of some B<tr>
1123 programs, which delete anything they find in the SEARCHLIST, period.)
1124 If the C</s> modifier is specified, sequences of characters that were
1125 transliterated to the same character are squashed down to a single instance of the
1128 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1129 exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1130 than the SEARCHLIST, the final character is replicated till it is long
1131 enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
1132 This latter is useful for counting characters in a class or for
1133 squashing character sequences in a class.
1137 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1139 $cnt = tr/*/*/; # count the stars in $_
1141 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1143 $cnt = tr/0-9//; # count the digits in $_
1145 tr/a-zA-Z//s; # bookkeeper -> bokeper
1147 ($HOST = $host) =~ tr/a-z/A-Z/;
1149 tr/a-zA-Z/ /cs; # change non-alphas to single space
1152 [\000-\177]; # delete 8th bit
1154 If multiple transliterations are given for a character, only the first one is used:
1158 will transliterate any A to X.
1160 Note that because the transliteration table is built at compile time, neither
1161 the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
1162 interpolation. That means that if you want to use variables, you must use
1165 eval "tr/$oldlist/$newlist/";
1168 eval "tr/$oldlist/$newlist/, 1" or die $@;
1172 =head2 I/O Operators
1174 There are several I/O operators you should know about.
1175 A string enclosed by backticks (grave accents) first undergoes
1176 variable substitution just like a double quoted string. It is then
1177 interpreted as a command, and the output of that command is the value
1178 of the pseudo-literal, like in a shell. In scalar context, a single
1179 string consisting of all the output is returned. In list context,
1180 a list of values is returned, one for each line of output. (You can
1181 set C<$/> to use a different line terminator.) The command is executed
1182 each time the pseudo-literal is evaluated. The status value of the
1183 command is returned in C<$?> (see L<perlvar> for the interpretation
1184 of C<$?>). Unlike in B<csh>, no translation is done on the return
1185 data--newlines remain newlines. Unlike in any of the shells, single
1186 quotes do not hide variable names in the command from interpretation.
1187 To pass a $ through to the shell you need to hide it with a backslash.
1188 The generalized form of backticks is C<qx//>. (Because backticks
1189 always undergo shell expansion as well, see L<perlsec> for
1192 Evaluating a filehandle in angle brackets yields the next line from
1193 that file (newline, if any, included), or C<undef> at end of file.
1194 Ordinarily you must assign that value to a variable, but there is one
1195 situation where an automatic assignment happens. I<If and ONLY if> the
1196 input symbol is the only thing inside the conditional of a C<while> or
1197 C<for(;;)> loop, the value is automatically assigned to the variable
1198 C<$_>. In these loop constructs, the assigned value (whether assignment
1199 is automatic or explicit) is then tested to see if it is defined.
1200 The defined test avoids problems where line has a string value
1201 that would be treated as false by perl e.g. "" or "0" with no trailing
1202 newline. (This may seem like an odd thing to you, but you'll use the
1203 construct in almost every Perl script you write.) Anyway, the following
1204 lines are equivalent to each other:
1206 while (defined($_ = <STDIN>)) { print; }
1207 while ($_ = <STDIN>) { print; }
1208 while (<STDIN>) { print; }
1209 for (;<STDIN>;) { print; }
1210 print while defined($_ = <STDIN>);
1211 print while ($_ = <STDIN>);
1212 print while <STDIN>;
1214 and this also behaves similarly, but avoids the use of $_ :
1216 while (my $line = <STDIN>) { print $line }
1218 If you really mean such values to terminate the loop they should be
1219 tested for explicitly:
1221 while (($_ = <STDIN>) ne '0') { ... }
1222 while (<STDIN>) { last unless $_; ... }
1224 In other boolean contexts, C<E<lt>I<filehandle>E<gt>> without explicit C<defined>
1225 test or comparison will solicit a warning if C<-w> is in effect.
1227 The filehandles STDIN, STDOUT, and STDERR are predefined. (The
1228 filehandles C<stdin>, C<stdout>, and C<stderr> will also work except in
1229 packages, where they would be interpreted as local identifiers rather
1230 than global.) Additional filehandles may be created with the open()
1231 function. See L<perlfunc/open()> for details on this.
1233 If a E<lt>FILEHANDLEE<gt> is used in a context that is looking for a list, a
1234 list consisting of all the input lines is returned, one line per list
1235 element. It's easy to make a I<LARGE> data space this way, so use with
1238 The null filehandle E<lt>E<gt> is special and can be used to emulate the
1239 behavior of B<sed> and B<awk>. Input from E<lt>E<gt> comes either from
1240 standard input, or from each file listed on the command line. Here's
1241 how it works: the first time E<lt>E<gt> is evaluated, the @ARGV array is
1242 checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
1243 gives you standard input. The @ARGV array is then processed as a list
1244 of filenames. The loop
1247 ... # code for each line
1250 is equivalent to the following Perl-like pseudo code:
1252 unshift(@ARGV, '-') unless @ARGV;
1253 while ($ARGV = shift) {
1256 ... # code for each line
1260 except that it isn't so cumbersome to say, and will actually work. It
1261 really does shift array @ARGV and put the current filename into variable
1262 $ARGV. It also uses filehandle I<ARGV> internally--E<lt>E<gt> is just a
1263 synonym for E<lt>ARGVE<gt>, which is magical. (The pseudo code above
1264 doesn't work because it treats E<lt>ARGVE<gt> as non-magical.)
1266 You can modify @ARGV before the first E<lt>E<gt> as long as the array ends up
1267 containing the list of filenames you really want. Line numbers (C<$.>)
1268 continue as if the input were one big happy file. (But see example
1269 under C<eof> for how to reset line numbers on each file.)
1271 If you want to set @ARGV to your own list of files, go right ahead.
1272 This sets @ARGV to all plain text files if no @ARGV was given:
1274 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
1276 You can even set them to pipe commands. For example, this automatically
1277 filters compressed arguments through B<gzip>:
1279 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
1281 If you want to pass switches into your script, you can use one of the
1282 Getopts modules or put a loop on the front like this:
1284 while ($_ = $ARGV[0], /^-/) {
1287 if (/^-D(.*)/) { $debug = $1 }
1288 if (/^-v/) { $verbose++ }
1289 # ... # other switches
1293 # ... # code for each line
1296 The E<lt>E<gt> symbol will return C<undef> for end-of-file only once.
1297 If you call it again after this it will assume you are processing another
1298 @ARGV list, and if you haven't set @ARGV, will input from STDIN.
1300 If the string inside the angle brackets is a reference to a scalar
1301 variable (e.g., E<lt>$fooE<gt>), then that variable contains the name of the
1302 filehandle to input from, or its typeglob, or a reference to the same. For example:
1307 If what's within the angle brackets is neither a filehandle nor a simple
1308 scalar variable containing a filehandle name, typeglob, or typeglob
1309 reference, it is interpreted as a filename pattern to be globbed, and
1310 either a list of filenames or the next filename in the list is returned,
1311 depending on context. This distinction is determined on syntactic
1312 grounds alone. That means C<E<lt>$xE<gt>> is always a readline from
1313 an indirect handle, but C<E<lt>$hash{key}E<gt>> is always a glob.
1314 That's because $x is a simple scalar variable, but C<$hash{key}> is
1315 not--it's a hash element.
1317 One level of double-quote interpretation is done first, but you can't
1318 say C<E<lt>$fooE<gt>> because that's an indirect filehandle as explained
1319 in the previous paragraph. (In older versions of Perl, programmers
1320 would insert curly brackets to force interpretation as a filename glob:
1321 C<E<lt>${foo}E<gt>>. These days, it's considered cleaner to call the
1322 internal function directly as C<glob($foo)>, which is probably the right
1323 way to have done it in the first place.) Example:
1331 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
1337 In fact, it's currently implemented that way. (Which means it will not
1338 work on filenames with spaces in them unless you have csh(1) on your
1339 machine.) Of course, the shortest way to do the above is:
1343 Because globbing invokes a shell, it's often faster to call readdir() yourself
1344 and do your own grep() on the filenames. Furthermore, due to its current
1345 implementation of using a shell, the glob() routine may get "Arg list too
1346 long" errors (unless you've installed tcsh(1L) as F</bin/csh>).
1348 A glob evaluates its (embedded) argument only when it is starting a new
1349 list. All values must be read before it will start over. In a list
1350 context this isn't important, because you automatically get them all
1351 anyway. In scalar context, however, the operator returns the next value
1352 each time it is called, or a C<undef> value if you've just run out. As
1353 for filehandles an automatic C<defined> is generated when the glob
1354 occurs in the test part of a C<while> or C<for> - because legal glob returns
1355 (e.g. a file called F<0>) would otherwise terminate the loop.
1356 Again, C<undef> is returned only once. So if you're expecting a single value
1357 from a glob, it is much better to say
1359 ($file) = <blurch*>;
1365 because the latter will alternate between returning a filename and
1368 It you're trying to do variable interpolation, it's definitely better
1369 to use the glob() function, because the older notation can cause people
1370 to become confused with the indirect filehandle notation.
1372 @files = glob("$dir/*.[ch]");
1373 @files = glob($files[$i]);
1375 =head2 Constant Folding
1377 Like C, Perl does a certain amount of expression evaluation at
1378 compile time, whenever it determines that all arguments to an
1379 operator are static and have no side effects. In particular, string
1380 concatenation happens at compile time between literals that don't do
1381 variable substitution. Backslash interpretation also happens at
1382 compile time. You can say
1384 'Now is the time for all' . "\n" .
1385 'good men to come to.'
1387 and this all reduces to one string internally. Likewise, if
1390 foreach $file (@filenames) {
1391 if (-s $file > 5 + 100 * 2**16) { }
1394 the compiler will precompute the number that
1395 expression represents so that the interpreter
1398 =head2 Bitwise String Operators
1400 Bitstrings of any size may be manipulated by the bitwise operators
1403 If the operands to a binary bitwise op are strings of different sizes,
1404 B<or> and B<xor> ops will act as if the shorter operand had additional
1405 zero bits on the right, while the B<and> op will act as if the longer
1406 operand were truncated to the length of the shorter.
1408 # ASCII-based examples
1409 print "j p \n" ^ " a h"; # prints "JAPH\n"
1410 print "JA" | " ph\n"; # prints "japh\n"
1411 print "japh\nJunk" & '_____'; # prints "JAPH\n";
1412 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
1414 If you are intending to manipulate bitstrings, you should be certain that
1415 you're supplying bitstrings: If an operand is a number, that will imply
1416 a B<numeric> bitwise operation. You may explicitly show which type of
1417 operation you intend by using C<""> or C<0+>, as in the examples below.
1419 $foo = 150 | 105 ; # yields 255 (0x96 | 0x69 is 0xFF)
1420 $foo = '150' | 105 ; # yields 255
1421 $foo = 150 | '105'; # yields 255
1422 $foo = '150' | '105'; # yields string '155' (under ASCII)
1424 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
1425 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
1427 =head2 Integer Arithmetic
1429 By default Perl assumes that it must do most of its arithmetic in
1430 floating point. But by saying
1434 you may tell the compiler that it's okay to use integer operations
1435 from here to the end of the enclosing BLOCK. An inner BLOCK may
1436 countermand this by saying
1440 which lasts until the end of that BLOCK.
1442 The bitwise operators ("&", "|", "^", "~", "<<", and ">>") always
1443 produce integral results. (But see also L<Bitwise String Operators>.)
1444 However, C<use integer> still has meaning
1445 for them. By default, their results are interpreted as unsigned
1446 integers. However, if C<use integer> is in effect, their results are
1447 interpreted as signed integers. For example, C<~0> usually evaluates
1448 to a large integral value. However, C<use integer; ~0> is -1 on twos-complement machines.
1450 =head2 Floating-point Arithmetic
1452 While C<use integer> provides integer-only arithmetic, there is no
1453 similar ways to provide rounding or truncation at a certain number of
1454 decimal places. For rounding to a certain number of digits, sprintf()
1455 or printf() is usually the easiest route.
1457 Floating-point numbers are only approximations to what a mathematician
1458 would call real numbers. There are infinitely more reals than floats,
1459 so some corners must be cut. For example:
1461 printf "%.20g\n", 123456789123456789;
1462 # produces 123456789123456784
1464 Testing for exact equality of floating-point equality or inequality is
1465 not a good idea. Here's a (relatively expensive) work-around to compare
1466 whether two floating-point numbers are equal to a particular number of
1467 decimal places. See Knuth, volume II, for a more robust treatment of
1471 my ($X, $Y, $POINTS) = @_;
1473 $tX = sprintf("%.${POINTS}g", $X);
1474 $tY = sprintf("%.${POINTS}g", $Y);
1478 The POSIX module (part of the standard perl distribution) implements
1479 ceil(), floor(), and a number of other mathematical and trigonometric
1480 functions. The Math::Complex module (part of the standard perl
1481 distribution) defines a number of mathematical functions that can also
1482 work on real numbers. Math::Complex not as efficient as POSIX, but
1483 POSIX can't work with complex numbers.
1485 Rounding in financial applications can have serious implications, and
1486 the rounding method used should be specified precisely. In these
1487 cases, it probably pays not to trust whichever system rounding is
1488 being used by Perl, but to instead implement the rounding function you
1491 =head2 Bigger Numbers
1493 The standard Math::BigInt and Math::BigFloat modules provide
1494 variable precision arithmetic and overloaded operators.
1495 At the cost of some space and considerable speed, they
1496 avoid the normal pitfalls associated with limited-precision
1500 $x = Math::BigInt->new('123456789123456789');
1503 # prints +15241578780673678515622620750190521