3 perlop - Perl operators and precedence
7 =head2 Operator Precedence and Associativity
9 Operator precedence and associativity work in Perl more or less like
10 they do in mathematics.
12 I<Operator precedence> means some operators are evaluated before
13 others. For example, in C<2 + 4 * 5>, the multiplication has higher
14 precedence so C<4 * 5> is evaluated first yielding C<2 + 20 ==
15 22> and not C<6 * 5 == 30>.
17 I<Operator associativity> defines what happens if a sequence of the
18 same operators is used one after another: whether the evaluator will
19 evaluate the left operations first or the right. For example, in C<8
20 - 4 - 2>, subtraction is left associative so Perl evaluates the
21 expression left to right. C<8 - 4> is evaluated first making the
22 expression C<4 - 2 == 2> and not C<8 - 2 == 6>.
24 Perl operators have the following associativity and precedence,
25 listed from highest precedence to lowest. Operators borrowed from
26 C keep the same precedence relationship with each other, even where
27 C's precedence is slightly screwy. (This makes learning Perl easier
28 for C folks.) With very few exceptions, these all operate on scalar
29 values only, not array values.
31 left terms and list operators (leftward)
35 right ! ~ \ and unary + and -
40 nonassoc named unary operators
41 nonassoc < > <= >= lt gt le ge
42 nonassoc == != <=> eq ne cmp
51 nonassoc list operators (rightward)
56 In the following sections, these operators are covered in precedence order.
58 Many operators can be overloaded for objects. See L<overload>.
60 =head2 Terms and List Operators (Leftward)
62 A TERM has the highest precedence in Perl. They include variables,
63 quote and quote-like operators, any expression in parentheses,
64 and any function whose arguments are parenthesized. Actually, there
65 aren't really functions in this sense, just list operators and unary
66 operators behaving as functions because you put parentheses around
67 the arguments. These are all documented in L<perlfunc>.
69 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
70 is followed by a left parenthesis as the next token, the operator and
71 arguments within parentheses are taken to be of highest precedence,
72 just like a normal function call.
74 In the absence of parentheses, the precedence of list operators such as
75 C<print>, C<sort>, or C<chmod> is either very high or very low depending on
76 whether you are looking at the left side or the right side of the operator.
79 @ary = (1, 3, sort 4, 2);
80 print @ary; # prints 1324
82 the commas on the right of the sort are evaluated before the sort,
83 but the commas on the left are evaluated after. In other words,
84 list operators tend to gobble up all arguments that follow, and
85 then act like a simple TERM with regard to the preceding expression.
86 Be careful with parentheses:
88 # These evaluate exit before doing the print:
89 print($foo, exit); # Obviously not what you want.
90 print $foo, exit; # Nor is this.
92 # These do the print before evaluating exit:
93 (print $foo), exit; # This is what you want.
94 print($foo), exit; # Or this.
95 print ($foo), exit; # Or even this.
99 print ($foo & 255) + 1, "\n";
101 probably doesn't do what you expect at first glance. The parentheses
102 enclose the argument list for C<print> which is evaluated (printing
103 the result of C<$foo & 255>). Then one is added to the return value
104 of C<print> (usually 1). The result is something like this:
106 1 + 1, "\n"; # Obviously not what you meant.
108 To do what you meant properly, you must write:
110 print(($foo & 255) + 1, "\n");
112 See L<Named Unary Operators> for more discussion of this.
114 Also parsed as terms are the C<do {}> and C<eval {}> constructs, as
115 well as subroutine and method calls, and the anonymous
116 constructors C<[]> and C<{}>.
118 See also L<Quote and Quote-like Operators> toward the end of this section,
119 as well as L<"I/O Operators">.
121 =head2 The Arrow Operator
123 "C<< -> >>" is an infix dereference operator, just as it is in C
124 and C++. If the right side is either a C<[...]>, C<{...}>, or a
125 C<(...)> subscript, then the left side must be either a hard or
126 symbolic reference to an array, a hash, or a subroutine respectively.
127 (Or technically speaking, a location capable of holding a hard
128 reference, if it's an array or hash reference being used for
129 assignment.) See L<perlreftut> and L<perlref>.
131 Otherwise, the right side is a method name or a simple scalar
132 variable containing either the method name or a subroutine reference,
133 and the left side must be either an object (a blessed reference)
134 or a class name (that is, a package name). See L<perlobj>.
136 =head2 Auto-increment and Auto-decrement
138 "++" and "--" work as in C. That is, if placed before a variable,
139 they increment or decrement the variable by one before returning the
140 value, and if placed after, increment or decrement after returning the
144 print $i++; # prints 0
145 print ++$j; # prints 1
147 Note that just as in C, Perl doesn't define B<when> the variable is
148 incremented or decremented. You just know it will be done sometime
149 before or after the value is returned. This also means that modifying
150 a variable twice in the same statement will lead to undefined behaviour.
151 Avoid statements like:
156 Perl will not guarantee what the result of the above statements is.
158 The auto-increment operator has a little extra builtin magic to it. If
159 you increment a variable that is numeric, or that has ever been used in
160 a numeric context, you get a normal increment. If, however, the
161 variable has been used in only string contexts since it was set, and
162 has a value that is not the empty string and matches the pattern
163 C</^[a-zA-Z]*[0-9]*\z/>, the increment is done as a string, preserving each
164 character within its range, with carry:
166 print ++($foo = '99'); # prints '100'
167 print ++($foo = 'a0'); # prints 'a1'
168 print ++($foo = 'Az'); # prints 'Ba'
169 print ++($foo = 'zz'); # prints 'aaa'
171 C<undef> is always treated as numeric, and in particular is changed
172 to C<0> before incrementing (so that a post-increment of an undef value
173 will return C<0> rather than C<undef>).
175 The auto-decrement operator is not magical.
177 =head2 Exponentiation
179 Binary "**" is the exponentiation operator. It binds even more
180 tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is
181 implemented using C's pow(3) function, which actually works on doubles
184 =head2 Symbolic Unary Operators
186 Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower
187 precedence version of this.
189 Unary "-" performs arithmetic negation if the operand is numeric. If
190 the operand is an identifier, a string consisting of a minus sign
191 concatenated with the identifier is returned. Otherwise, if the string
192 starts with a plus or minus, a string starting with the opposite sign
193 is returned. One effect of these rules is that -bareword is equivalent
196 Unary "~" performs bitwise negation, i.e., 1's complement. For
197 example, C<0666 & ~027> is 0640. (See also L<Integer Arithmetic> and
198 L<Bitwise String Operators>.) Note that the width of the result is
199 platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64
200 bits wide on a 64-bit platform, so if you are expecting a certain bit
201 width, remember to use the & operator to mask off the excess bits.
203 Unary "+" has no effect whatsoever, even on strings. It is useful
204 syntactically for separating a function name from a parenthesized expression
205 that would otherwise be interpreted as the complete list of function
206 arguments. (See examples above under L<Terms and List Operators (Leftward)>.)
208 Unary "\" creates a reference to whatever follows it. See L<perlreftut>
209 and L<perlref>. Do not confuse this behavior with the behavior of
210 backslash within a string, although both forms do convey the notion
211 of protecting the next thing from interpolation.
213 =head2 Binding Operators
215 Binary "=~" binds a scalar expression to a pattern match. Certain operations
216 search or modify the string $_ by default. This operator makes that kind
217 of operation work on some other string. The right argument is a search
218 pattern, substitution, or transliteration. The left argument is what is
219 supposed to be searched, substituted, or transliterated instead of the default
220 $_. When used in scalar context, the return value generally indicates the
221 success of the operation. Behavior in list context depends on the particular
222 operator. See L</"Regexp Quote-Like Operators"> for details and
223 L<perlretut> for examples using these operators.
225 If the right argument is an expression rather than a search pattern,
226 substitution, or transliteration, it is interpreted as a search pattern at run
229 Binary "!~" is just like "=~" except the return value is negated in
232 =head2 Multiplicative Operators
234 Binary "*" multiplies two numbers.
236 Binary "/" divides two numbers.
238 Binary "%" computes the modulus of two numbers. Given integer
239 operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
240 C<$a> minus the largest multiple of C<$b> that is not greater than
241 C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
242 smallest multiple of C<$b> that is not less than C<$a> (i.e. the
243 result will be less than or equal to zero).
244 Note that when C<use integer> is in scope, "%" gives you direct access
245 to the modulus operator as implemented by your C compiler. This
246 operator is not as well defined for negative operands, but it will
249 Binary "x" is the repetition operator. In scalar context or if the left
250 operand is not enclosed in parentheses, it returns a string consisting
251 of the left operand repeated the number of times specified by the right
252 operand. In list context, if the left operand is enclosed in
253 parentheses, it repeats the list. If the right operand is zero or
254 negative, it returns an empty string or an empty list, depending on the
257 print '-' x 80; # print row of dashes
259 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
261 @ones = (1) x 80; # a list of 80 1's
262 @ones = (5) x @ones; # set all elements to 5
265 =head2 Additive Operators
267 Binary "+" returns the sum of two numbers.
269 Binary "-" returns the difference of two numbers.
271 Binary "." concatenates two strings.
273 =head2 Shift Operators
275 Binary "<<" returns the value of its left argument shifted left by the
276 number of bits specified by the right argument. Arguments should be
277 integers. (See also L<Integer Arithmetic>.)
279 Binary ">>" returns the value of its left argument shifted right by
280 the number of bits specified by the right argument. Arguments should
281 be integers. (See also L<Integer Arithmetic>.)
283 Note that both "<<" and ">>" in Perl are implemented directly using
284 "<<" and ">>" in C. If C<use integer> (see L<Integer Arithmetic>) is
285 in force then signed C integers are used, else unsigned C integers are
286 used. Either way, the implementation isn't going to generate results
287 larger than the size of the integer type Perl was built with (32 bits
290 The result of overflowing the range of the integers is undefined
291 because it is undefined also in C. In other words, using 32-bit
292 integers, C<< 1 << 32 >> is undefined. Shifting by a negative number
293 of bits is also undefined.
295 =head2 Named Unary Operators
297 The various named unary operators are treated as functions with one
298 argument, with optional parentheses.
300 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
301 is followed by a left parenthesis as the next token, the operator and
302 arguments within parentheses are taken to be of highest precedence,
303 just like a normal function call. For example,
304 because named unary operators are higher precedence than ||:
306 chdir $foo || die; # (chdir $foo) || die
307 chdir($foo) || die; # (chdir $foo) || die
308 chdir ($foo) || die; # (chdir $foo) || die
309 chdir +($foo) || die; # (chdir $foo) || die
311 but, because * is higher precedence than named operators:
313 chdir $foo * 20; # chdir ($foo * 20)
314 chdir($foo) * 20; # (chdir $foo) * 20
315 chdir ($foo) * 20; # (chdir $foo) * 20
316 chdir +($foo) * 20; # chdir ($foo * 20)
318 rand 10 * 20; # rand (10 * 20)
319 rand(10) * 20; # (rand 10) * 20
320 rand (10) * 20; # (rand 10) * 20
321 rand +(10) * 20; # rand (10 * 20)
323 Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are
324 treated like named unary operators, but they don't follow this functional
325 parenthesis rule. That means, for example, that C<-f($file).".bak"> is
326 equivalent to C<-f "$file.bak">.
328 See also L<"Terms and List Operators (Leftward)">.
330 =head2 Relational Operators
332 Binary "<" returns true if the left argument is numerically less than
335 Binary ">" returns true if the left argument is numerically greater
336 than the right argument.
338 Binary "<=" returns true if the left argument is numerically less than
339 or equal to the right argument.
341 Binary ">=" returns true if the left argument is numerically greater
342 than or equal to the right argument.
344 Binary "lt" returns true if the left argument is stringwise less than
347 Binary "gt" returns true if the left argument is stringwise greater
348 than the right argument.
350 Binary "le" returns true if the left argument is stringwise less than
351 or equal to the right argument.
353 Binary "ge" returns true if the left argument is stringwise greater
354 than or equal to the right argument.
356 =head2 Equality Operators
358 Binary "==" returns true if the left argument is numerically equal to
361 Binary "!=" returns true if the left argument is numerically not equal
362 to the right argument.
364 Binary "<=>" returns -1, 0, or 1 depending on whether the left
365 argument is numerically less than, equal to, or greater than the right
366 argument. If your platform supports NaNs (not-a-numbers) as numeric
367 values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
368 "<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
369 returns true, as does NaN != anything else. If your platform doesn't
370 support NaNs then NaN is just a string with numeric value 0.
372 perl -le '$a = NaN; print "No NaN support here" if $a == $a'
373 perl -le '$a = NaN; print "NaN support here" if $a != $a'
375 Binary "eq" returns true if the left argument is stringwise equal to
378 Binary "ne" returns true if the left argument is stringwise not equal
379 to the right argument.
381 Binary "cmp" returns -1, 0, or 1 depending on whether the left
382 argument is stringwise less than, equal to, or greater than the right
385 "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
386 by the current locale if C<use locale> is in effect. See L<perllocale>.
390 Binary "&" returns its operands ANDed together bit by bit.
391 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
393 Note that "&" has lower priority than relational operators, so for example
394 the brackets are essential in a test like
396 print "Even\n" if ($x & 1) == 0;
398 =head2 Bitwise Or and Exclusive Or
400 Binary "|" returns its operands ORed together bit by bit.
401 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
403 Binary "^" returns its operands XORed together bit by bit.
404 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
406 Note that "|" and "^" have lower priority than relational operators, so
407 for example the brackets are essential in a test like
409 print "false\n" if (8 | 2) != 10;
411 =head2 C-style Logical And
413 Binary "&&" performs a short-circuit logical AND operation. That is,
414 if the left operand is false, the right operand is not even evaluated.
415 Scalar or list context propagates down to the right operand if it
418 =head2 C-style Logical Or
420 Binary "||" performs a short-circuit logical OR operation. That is,
421 if the left operand is true, the right operand is not even evaluated.
422 Scalar or list context propagates down to the right operand if it
425 =head2 C-style Logical Defined-Or
427 Although it has no direct equivalent in C, Perl's C<//> operator is related
428 to its C-style or. In fact, it's exactly the same as C<||>, except that it
429 tests the left hand side's definedness instead of its truth. Thus, C<$a // $b>
430 is similar to C<defined($a) || $b> (except that it returns the value of C<$a>
431 rather than the value of C<defined($a)>) and is exactly equivalent to
432 C<defined($a) ? $a : $b>. This is very useful for providing default values
433 for variables. If you actually want to test if at least one of C<$a> and
434 C<$b> is defined, use C<defined($a // $b)>.
436 The C<||>, C<//> and C<&&> operators return the last value evaluated
437 (unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably
438 portable way to find out the home directory might be:
440 $home = $ENV{'HOME'} // $ENV{'LOGDIR'} //
441 (getpwuid($<))[7] // die "You're homeless!\n";
443 In particular, this means that you shouldn't use this
444 for selecting between two aggregates for assignment:
446 @a = @b || @c; # this is wrong
447 @a = scalar(@b) || @c; # really meant this
448 @a = @b ? @b : @c; # this works fine, though
450 As more readable alternatives to C<&&>, C<//> and C<||> when used for
451 control flow, Perl provides C<and>, C<err> and C<or> operators (see below).
452 The short-circuit behavior is identical. The precedence of "and", "err"
453 and "or" is much lower, however, so that you can safely use them after a
454 list operator without the need for parentheses:
456 unlink "alpha", "beta", "gamma"
457 or gripe(), next LINE;
459 With the C-style operators that would have been written like this:
461 unlink("alpha", "beta", "gamma")
462 || (gripe(), next LINE);
464 Using "or" for assignment is unlikely to do what you want; see below.
466 =head2 Range Operators
468 Binary ".." is the range operator, which is really two different
469 operators depending on the context. In list context, it returns a
470 list of values counting (up by ones) from the left value to the right
471 value. If the left value is greater than the right value then it
472 returns the empty list. The range operator is useful for writing
473 C<foreach (1..10)> loops and for doing slice operations on arrays. In
474 the current implementation, no temporary array is created when the
475 range operator is used as the expression in C<foreach> loops, but older
476 versions of Perl might burn a lot of memory when you write something
479 for (1 .. 1_000_000) {
483 The range operator also works on strings, using the magical auto-increment,
486 In scalar context, ".." returns a boolean value. The operator is
487 bistable, like a flip-flop, and emulates the line-range (comma) operator
488 of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
489 own boolean state. It is false as long as its left operand is false.
490 Once the left operand is true, the range operator stays true until the
491 right operand is true, I<AFTER> which the range operator becomes false
492 again. It doesn't become false till the next time the range operator is
493 evaluated. It can test the right operand and become false on the same
494 evaluation it became true (as in B<awk>), but it still returns true once.
495 If you don't want it to test the right operand till the next
496 evaluation, as in B<sed>, just use three dots ("...") instead of
497 two. In all other regards, "..." behaves just like ".." does.
499 The right operand is not evaluated while the operator is in the
500 "false" state, and the left operand is not evaluated while the
501 operator is in the "true" state. The precedence is a little lower
502 than || and &&. The value returned is either the empty string for
503 false, or a sequence number (beginning with 1) for true. The
504 sequence number is reset for each range encountered. The final
505 sequence number in a range has the string "E0" appended to it, which
506 doesn't affect its numeric value, but gives you something to search
507 for if you want to exclude the endpoint. You can exclude the
508 beginning point by waiting for the sequence number to be greater
511 If either operand of scalar ".." is a constant expression,
512 that operand is considered true if it is equal (C<==>) to the current
513 input line number (the C<$.> variable).
515 To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>,
516 but that is only an issue if you use a floating point expression; when
517 implicitly using C<$.> as described in the previous paragraph, the
518 comparison is C<int(EXPR) == int($.)> which is only an issue when C<$.>
519 is set to a floating point value and you are not reading from a file.
520 Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what
521 you want in scalar context because each of the operands are evaluated
522 using their integer representation.
526 As a scalar operator:
528 if (101 .. 200) { print; } # print 2nd hundred lines, short for
529 # if ($. == 101 .. $. == 200) ...
530 next line if (1 .. /^$/); # skip header lines, short for
531 # ... if ($. == 1 .. /^$/);
532 s/^/> / if (/^$/ .. eof()); # quote body
534 # parse mail messages
536 $in_header = 1 .. /^$/;
537 $in_body = /^$/ .. eof;
544 close ARGV if eof; # reset $. each file
547 Here's a simple example to illustrate the difference between
548 the two range operators:
563 This program will print only the line containing "Bar". If
564 the range operator is changed to C<...>, it will also print the
567 And now some examples as a list operator:
569 for (101 .. 200) { print; } # print $_ 100 times
570 @foo = @foo[0 .. $#foo]; # an expensive no-op
571 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
573 The range operator (in list context) makes use of the magical
574 auto-increment algorithm if the operands are strings. You
577 @alphabet = ('A' .. 'Z');
579 to get all normal letters of the English alphabet, or
581 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
583 to get a hexadecimal digit, or
585 @z2 = ('01' .. '31'); print $z2[$mday];
587 to get dates with leading zeros. If the final value specified is not
588 in the sequence that the magical increment would produce, the sequence
589 goes until the next value would be longer than the final value
592 Because each operand is evaluated in integer form, C<2.18 .. 3.14> will
593 return two elements in list context.
595 @list = (2.18 .. 3.14); # same as @list = (2 .. 3);
597 =head2 Conditional Operator
599 Ternary "?:" is the conditional operator, just as in C. It works much
600 like an if-then-else. If the argument before the ? is true, the
601 argument before the : is returned, otherwise the argument after the :
602 is returned. For example:
604 printf "I have %d dog%s.\n", $n,
605 ($n == 1) ? '' : "s";
607 Scalar or list context propagates downward into the 2nd
608 or 3rd argument, whichever is selected.
610 $a = $ok ? $b : $c; # get a scalar
611 @a = $ok ? @b : @c; # get an array
612 $a = $ok ? @b : @c; # oops, that's just a count!
614 The operator may be assigned to if both the 2nd and 3rd arguments are
615 legal lvalues (meaning that you can assign to them):
617 ($a_or_b ? $a : $b) = $c;
619 Because this operator produces an assignable result, using assignments
620 without parentheses will get you in trouble. For example, this:
622 $a % 2 ? $a += 10 : $a += 2
626 (($a % 2) ? ($a += 10) : $a) += 2
630 ($a % 2) ? ($a += 10) : ($a += 2)
632 That should probably be written more simply as:
634 $a += ($a % 2) ? 10 : 2;
636 =head2 Assignment Operators
638 "=" is the ordinary assignment operator.
640 Assignment operators work as in C. That is,
648 although without duplicating any side effects that dereferencing the lvalue
649 might trigger, such as from tie(). Other assignment operators work similarly.
650 The following are recognized:
657 Although these are grouped by family, they all have the precedence
660 Unlike in C, the scalar assignment operator produces a valid lvalue.
661 Modifying an assignment is equivalent to doing the assignment and
662 then modifying the variable that was assigned to. This is useful
663 for modifying a copy of something, like this:
665 ($tmp = $global) =~ tr [A-Z] [a-z];
676 Similarly, a list assignment in list context produces the list of
677 lvalues assigned to, and a list assignment in scalar context returns
678 the number of elements produced by the expression on the right hand
679 side of the assignment.
681 =head2 Comma Operator
683 Binary "," is the comma operator. In scalar context it evaluates
684 its left argument, throws that value away, then evaluates its right
685 argument and returns that value. This is just like C's comma operator.
687 In list context, it's just the list argument separator, and inserts
688 both its arguments into the list.
690 The C<< => >> operator is a synonym for the comma, but forces any word
691 to its left to be interpreted as a string (as of 5.001). It is helpful
692 in documenting the correspondence between keys and values in hashes,
693 and other paired elements in lists.
695 =head2 List Operators (Rightward)
697 On the right side of a list operator, it has very low precedence,
698 such that it controls all comma-separated expressions found there.
699 The only operators with lower precedence are the logical operators
700 "and", "or", and "not", which may be used to evaluate calls to list
701 operators without the need for extra parentheses:
703 open HANDLE, "filename"
704 or die "Can't open: $!\n";
706 See also discussion of list operators in L<Terms and List Operators (Leftward)>.
710 Unary "not" returns the logical negation of the expression to its right.
711 It's the equivalent of "!" except for the very low precedence.
715 Binary "and" returns the logical conjunction of the two surrounding
716 expressions. It's equivalent to && except for the very low
717 precedence. This means that it short-circuits: i.e., the right
718 expression is evaluated only if the left expression is true.
720 =head2 Logical or, Defined or, and Exclusive Or
722 Binary "or" returns the logical disjunction of the two surrounding
723 expressions. It's equivalent to || except for the very low precedence.
724 This makes it useful for control flow
726 print FH $data or die "Can't write to FH: $!";
728 This means that it short-circuits: i.e., the right expression is evaluated
729 only if the left expression is false. Due to its precedence, you should
730 probably avoid using this for assignment, only for control flow.
732 $a = $b or $c; # bug: this is wrong
733 ($a = $b) or $c; # really means this
734 $a = $b || $c; # better written this way
736 However, when it's a list-context assignment and you're trying to use
737 "||" for control flow, you probably need "or" so that the assignment
738 takes higher precedence.
740 @info = stat($file) || die; # oops, scalar sense of stat!
741 @info = stat($file) or die; # better, now @info gets its due
743 Then again, you could always use parentheses.
745 Binary "err" is equivalent to C<//>--it's just like binary "or", except it tests
746 its left argument's definedness instead of its truth. There are two ways to
747 remember "err": either because many functions return C<undef> on an B<err>or,
748 or as a sort of correction: C<$a=($b err 'default')>
750 Binary "xor" returns the exclusive-OR of the two surrounding expressions.
751 It cannot short circuit, of course.
753 =head2 C Operators Missing From Perl
755 Here is what C has that Perl doesn't:
761 Address-of operator. (But see the "\" operator for taking a reference.)
765 Dereference-address operator. (Perl's prefix dereferencing
766 operators are typed: $, @, %, and &.)
770 Type-casting operator.
774 =head2 Quote and Quote-like Operators
776 While we usually think of quotes as literal values, in Perl they
777 function as operators, providing various kinds of interpolating and
778 pattern matching capabilities. Perl provides customary quote characters
779 for these behaviors, but also provides a way for you to choose your
780 quote character for any of them. In the following table, a C<{}> represents
781 any pair of delimiters you choose.
783 Customary Generic Meaning Interpolates
788 // m{} Pattern match yes*
790 s{}{} Substitution yes*
791 tr{}{} Transliteration no (but see below)
794 * unless the delimiter is ''.
796 Non-bracketing delimiters use the same character fore and aft, but the four
797 sorts of brackets (round, angle, square, curly) will all nest, which means
806 Note, however, that this does not always work for quoting Perl code:
808 $s = q{ if($a eq "}") ... }; # WRONG
810 is a syntax error. The C<Text::Balanced> module (from CPAN, and
811 starting from Perl 5.8 part of the standard distribution) is able
814 There can be whitespace between the operator and the quoting
815 characters, except when C<#> is being used as the quoting character.
816 C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
817 operator C<q> followed by a comment. Its argument will be taken
818 from the next line. This allows you to write:
820 s {foo} # Replace foo
823 The following escape sequences are available in constructs that interpolate
824 and in transliterations.
831 \a alarm (bell) (BEL)
833 \033 octal char (ESC)
835 \x{263a} wide hex char (SMILEY)
836 \c[ control char (ESC)
837 \N{name} named Unicode character
839 B<NOTE>: Unlike C and other languages, Perl has no \v escape sequence for
840 the vertical tab (VT - ASCII 11).
842 The following escape sequences are available in constructs that interpolate
843 but not in transliterations.
845 \l lowercase next char
846 \u uppercase next char
849 \E end case modification
850 \Q quote non-word characters till \E
852 If C<use locale> is in effect, the case map used by C<\l>, C<\L>,
853 C<\u> and C<\U> is taken from the current locale. See L<perllocale>.
854 If Unicode (for example, C<\N{}> or wide hex characters of 0x100 or
855 beyond) is being used, the case map used by C<\l>, C<\L>, C<\u> and
856 C<\U> is as defined by Unicode. For documentation of C<\N{name}>,
859 All systems use the virtual C<"\n"> to represent a line terminator,
860 called a "newline". There is no such thing as an unvarying, physical
861 newline character. It is only an illusion that the operating system,
862 device drivers, C libraries, and Perl all conspire to preserve. Not all
863 systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
864 on a Mac, these are reversed, and on systems without line terminator,
865 printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
866 you mean a "newline" for your system, but use the literal ASCII when you
867 need an exact character. For example, most networking protocols expect
868 and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
869 and although they often accept just C<"\012">, they seldom tolerate just
870 C<"\015">. If you get in the habit of using C<"\n"> for networking,
871 you may be burned some day.
873 For constructs that do interpolate, variables beginning with "C<$>"
874 or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
875 C<< $href->{key}[0] >> are also interpolated, as are array and hash slices.
876 But method calls such as C<< $obj->meth >> are not.
878 Interpolating an array or slice interpolates the elements in order,
879 separated by the value of C<$">, so is equivalent to interpolating
880 C<join $", @array>. "Punctuation" arrays such as C<@+> are only
881 interpolated if the name is enclosed in braces C<@{+}>.
883 You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
884 An unescaped C<$> or C<@> interpolates the corresponding variable,
885 while escaping will cause the literal string C<\$> to be inserted.
886 You'll need to write something like C<m/\Quser\E\@\Qhost/>.
888 Patterns are subject to an additional level of interpretation as a
889 regular expression. This is done as a second pass, after variables are
890 interpolated, so that regular expressions may be incorporated into the
891 pattern from the variables. If this is not what you want, use C<\Q> to
892 interpolate a variable literally.
894 Apart from the behavior described above, Perl does not expand
895 multiple levels of interpolation. In particular, contrary to the
896 expectations of shell programmers, back-quotes do I<NOT> interpolate
897 within double quotes, nor do single quotes impede evaluation of
898 variables when used within double quotes.
900 =head2 Regexp Quote-Like Operators
902 Here are the quote-like operators that apply to pattern
903 matching and related activities.
909 This is just like the C</pattern/> search, except that it matches only
910 once between calls to the reset() operator. This is a useful
911 optimization when you want to see only the first occurrence of
912 something in each file of a set of files, for instance. Only C<??>
913 patterns local to the current package are reset.
917 # blank line between header and body
920 reset if eof; # clear ?? status for next file
923 This usage is vaguely deprecated, which means it just might possibly
924 be removed in some distant future version of Perl, perhaps somewhere
925 around the year 2168.
927 =item m/PATTERN/cgimosx
929 =item /PATTERN/cgimosx
931 Searches a string for a pattern match, and in scalar context returns
932 true if it succeeds, false if it fails. If no string is specified
933 via the C<=~> or C<!~> operator, the $_ string is searched. (The
934 string specified with C<=~> need not be an lvalue--it may be the
935 result of an expression evaluation, but remember the C<=~> binds
936 rather tightly.) See also L<perlre>. See L<perllocale> for
937 discussion of additional considerations that apply when C<use locale>
942 c Do not reset search position on a failed match when /g is in effect.
943 g Match globally, i.e., find all occurrences.
944 i Do case-insensitive pattern matching.
945 m Treat string as multiple lines.
946 o Compile pattern only once.
947 s Treat string as single line.
948 x Use extended regular expressions.
950 If "/" is the delimiter then the initial C<m> is optional. With the C<m>
951 you can use any pair of non-alphanumeric, non-whitespace characters
952 as delimiters. This is particularly useful for matching path names
953 that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
954 the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
955 If "'" is the delimiter, no interpolation is performed on the PATTERN.
957 PATTERN may contain variables, which will be interpolated (and the
958 pattern recompiled) every time the pattern search is evaluated, except
959 for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
960 C<$|> are not interpolated because they look like end-of-string tests.)
961 If you want such a pattern to be compiled only once, add a C</o> after
962 the trailing delimiter. This avoids expensive run-time recompilations,
963 and is useful when the value you are interpolating won't change over
964 the life of the script. However, mentioning C</o> constitutes a promise
965 that you won't change the variables in the pattern. If you change them,
966 Perl won't even notice. See also L<"qr/STRING/imosx">.
968 If the PATTERN evaluates to the empty string, the last
969 I<successfully> matched regular expression is used instead. In this
970 case, only the C<g> and C<c> flags on the empty pattern is honoured -
971 the other flags are taken from the original pattern. If no match has
972 previously succeeded, this will (silently) act instead as a genuine
973 empty pattern (which will always match).
975 Note that it's possible to confuse Perl into thinking C<//> (the empty
976 regex) is really C<//> (the defined-or operator). Perl is usually pretty
977 good about this, but some pathological cases might trigger this, such as
978 C<$a///> (is that C<($a) / (//)> or C<$a // />?) and C<print $fh //>
979 (C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl
980 will assume you meant defined-or. If you meant the empty regex, just
981 use parentheses or spaces to disambiguate, or even prefix the empty
982 regex with an C<m> (so C<//> becomes C<m//>).
984 If the C</g> option is not used, C<m//> in list context returns a
985 list consisting of the subexpressions matched by the parentheses in the
986 pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are
987 also set, and that this differs from Perl 4's behavior.) When there are
988 no parentheses in the pattern, the return value is the list C<(1)> for
989 success. With or without parentheses, an empty list is returned upon
994 open(TTY, '/dev/tty');
995 <TTY> =~ /^y/i && foo(); # do foo if desired
997 if (/Version: *([0-9.]*)/) { $version = $1; }
999 next if m#^/usr/spool/uucp#;
1004 print if /$arg/o; # compile only once
1007 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
1009 This last example splits $foo into the first two words and the
1010 remainder of the line, and assigns those three fields to $F1, $F2, and
1011 $Etc. The conditional is true if any variables were assigned, i.e., if
1012 the pattern matched.
1014 The C</g> modifier specifies global pattern matching--that is,
1015 matching as many times as possible within the string. How it behaves
1016 depends on the context. In list context, it returns a list of the
1017 substrings matched by any capturing parentheses in the regular
1018 expression. If there are no parentheses, it returns a list of all
1019 the matched strings, as if there were parentheses around the whole
1022 In scalar context, each execution of C<m//g> finds the next match,
1023 returning true if it matches, and false if there is no further match.
1024 The position after the last match can be read or set using the pos()
1025 function; see L<perlfunc/pos>. A failed match normally resets the
1026 search position to the beginning of the string, but you can avoid that
1027 by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
1028 string also resets the search position.
1030 You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
1031 zero-width assertion that matches the exact position where the previous
1032 C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion
1033 still anchors at pos(), but the match is of course only attempted once.
1034 Using C<\G> without C</g> on a target string that has not previously had a
1035 C</g> match applied to it is the same as using the C<\A> assertion to match
1036 the beginning of the string. Note also that, currently, C<\G> is only
1037 properly supported when anchored at the very beginning of the pattern.
1042 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
1046 while (defined($paragraph = <>)) {
1047 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
1051 print "$sentences\n";
1053 # using m//gc with \G
1057 print $1 while /(o)/gc; print "', pos=", pos, "\n";
1059 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
1061 print $1 while /(p)/gc; print "', pos=", pos, "\n";
1063 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
1065 The last example should print:
1075 Notice that the final match matched C<q> instead of C<p>, which a match
1076 without the C<\G> anchor would have done. Also note that the final match
1077 did not update C<pos> -- C<pos> is only updated on a C</g> match. If the
1078 final match did indeed match C<p>, it's a good bet that you're running an
1079 older (pre-5.6.0) Perl.
1081 A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
1082 combine several regexps like this to process a string part-by-part,
1083 doing different actions depending on which regexp matched. Each
1084 regexp tries to match where the previous one leaves off.
1087 $url = new URI::URL "http://www/"; die if $url eq "xXx";
1091 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
1092 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
1093 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
1094 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
1095 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
1096 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
1097 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
1098 print ". That's all!\n";
1101 Here is the output (split into several lines):
1103 line-noise lowercase line-noise lowercase UPPERCASE line-noise
1104 UPPERCASE line-noise lowercase line-noise lowercase line-noise
1105 lowercase lowercase line-noise lowercase lowercase line-noise
1106 MiXeD line-noise. That's all!
1112 A single-quoted, literal string. A backslash represents a backslash
1113 unless followed by the delimiter or another backslash, in which case
1114 the delimiter or backslash is interpolated.
1116 $foo = q!I said, "You said, 'She said it.'"!;
1117 $bar = q('This is it.');
1118 $baz = '\n'; # a two-character string
1124 A double-quoted, interpolated string.
1127 (*** The previous line contains the naughty word "$1".\n)
1128 if /\b(tcl|java|python)\b/i; # :-)
1129 $baz = "\n"; # a one-character string
1131 =item qr/STRING/imosx
1133 This operator quotes (and possibly compiles) its I<STRING> as a regular
1134 expression. I<STRING> is interpolated the same way as I<PATTERN>
1135 in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
1136 is done. Returns a Perl value which may be used instead of the
1137 corresponding C</STRING/imosx> expression.
1141 $rex = qr/my.STRING/is;
1148 The result may be used as a subpattern in a match:
1151 $string =~ /foo${re}bar/; # can be interpolated in other patterns
1152 $string =~ $re; # or used standalone
1153 $string =~ /$re/; # or this way
1155 Since Perl may compile the pattern at the moment of execution of qr()
1156 operator, using qr() may have speed advantages in some situations,
1157 notably if the result of qr() is used standalone:
1160 my $patterns = shift;
1161 my @compiled = map qr/$_/i, @$patterns;
1164 foreach my $pat (@compiled) {
1165 $success = 1, last if /$pat/;
1171 Precompilation of the pattern into an internal representation at
1172 the moment of qr() avoids a need to recompile the pattern every
1173 time a match C</$pat/> is attempted. (Perl has many other internal
1174 optimizations, but none would be triggered in the above example if
1175 we did not use qr() operator.)
1179 i Do case-insensitive pattern matching.
1180 m Treat string as multiple lines.
1181 o Compile pattern only once.
1182 s Treat string as single line.
1183 x Use extended regular expressions.
1185 See L<perlre> for additional information on valid syntax for STRING, and
1186 for a detailed look at the semantics of regular expressions.
1192 A string which is (possibly) interpolated and then executed as a
1193 system command with C</bin/sh> or its equivalent. Shell wildcards,
1194 pipes, and redirections will be honored. The collected standard
1195 output of the command is returned; standard error is unaffected. In
1196 scalar context, it comes back as a single (potentially multi-line)
1197 string, or undef if the command failed. In list context, returns a
1198 list of lines (however you've defined lines with $/ or
1199 $INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
1201 Because backticks do not affect standard error, use shell file descriptor
1202 syntax (assuming the shell supports this) if you care to address this.
1203 To capture a command's STDERR and STDOUT together:
1205 $output = `cmd 2>&1`;
1207 To capture a command's STDOUT but discard its STDERR:
1209 $output = `cmd 2>/dev/null`;
1211 To capture a command's STDERR but discard its STDOUT (ordering is
1214 $output = `cmd 2>&1 1>/dev/null`;
1216 To exchange a command's STDOUT and STDERR in order to capture the STDERR
1217 but leave its STDOUT to come out the old STDERR:
1219 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
1221 To read both a command's STDOUT and its STDERR separately, it's easiest
1222 to redirect them separately to files, and then read from those files
1223 when the program is done:
1225 system("program args 1>program.stdout 2>program.stderr");
1227 Using single-quote as a delimiter protects the command from Perl's
1228 double-quote interpolation, passing it on to the shell instead:
1230 $perl_info = qx(ps $$); # that's Perl's $$
1231 $shell_info = qx'ps $$'; # that's the new shell's $$
1233 How that string gets evaluated is entirely subject to the command
1234 interpreter on your system. On most platforms, you will have to protect
1235 shell metacharacters if you want them treated literally. This is in
1236 practice difficult to do, as it's unclear how to escape which characters.
1237 See L<perlsec> for a clean and safe example of a manual fork() and exec()
1238 to emulate backticks safely.
1240 On some platforms (notably DOS-like ones), the shell may not be
1241 capable of dealing with multiline commands, so putting newlines in
1242 the string may not get you what you want. You may be able to evaluate
1243 multiple commands in a single line by separating them with the command
1244 separator character, if your shell supports that (e.g. C<;> on many Unix
1245 shells; C<&> on the Windows NT C<cmd> shell).
1247 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1248 output before starting the child process, but this may not be supported
1249 on some platforms (see L<perlport>). To be safe, you may need to set
1250 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1251 C<IO::Handle> on any open handles.
1253 Beware that some command shells may place restrictions on the length
1254 of the command line. You must ensure your strings don't exceed this
1255 limit after any necessary interpolations. See the platform-specific
1256 release notes for more details about your particular environment.
1258 Using this operator can lead to programs that are difficult to port,
1259 because the shell commands called vary between systems, and may in
1260 fact not be present at all. As one example, the C<type> command under
1261 the POSIX shell is very different from the C<type> command under DOS.
1262 That doesn't mean you should go out of your way to avoid backticks
1263 when they're the right way to get something done. Perl was made to be
1264 a glue language, and one of the things it glues together is commands.
1265 Just understand what you're getting yourself into.
1267 See L<"I/O Operators"> for more discussion.
1271 Evaluates to a list of the words extracted out of STRING, using embedded
1272 whitespace as the word delimiters. It can be understood as being roughly
1275 split(' ', q/STRING/);
1277 the differences being that it generates a real list at compile time, and
1278 in scalar context it returns the last element in the list. So
1283 is semantically equivalent to the list:
1287 Some frequently seen examples:
1289 use POSIX qw( setlocale localeconv )
1290 @EXPORT = qw( foo bar baz );
1292 A common mistake is to try to separate the words with comma or to
1293 put comments into a multi-line C<qw>-string. For this reason, the
1294 C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
1295 produces warnings if the STRING contains the "," or the "#" character.
1297 =item s/PATTERN/REPLACEMENT/egimosx
1299 Searches a string for a pattern, and if found, replaces that pattern
1300 with the replacement text and returns the number of substitutions
1301 made. Otherwise it returns false (specifically, the empty string).
1303 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
1304 variable is searched and modified. (The string specified with C<=~> must
1305 be scalar variable, an array element, a hash element, or an assignment
1306 to one of those, i.e., an lvalue.)
1308 If the delimiter chosen is a single quote, no interpolation is
1309 done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1310 PATTERN contains a $ that looks like a variable rather than an
1311 end-of-string test, the variable will be interpolated into the pattern
1312 at run-time. If you want the pattern compiled only once the first time
1313 the variable is interpolated, use the C</o> option. If the pattern
1314 evaluates to the empty string, the last successfully executed regular
1315 expression is used instead. See L<perlre> for further explanation on these.
1316 See L<perllocale> for discussion of additional considerations that apply
1317 when C<use locale> is in effect.
1321 e Evaluate the right side as an expression.
1322 g Replace globally, i.e., all occurrences.
1323 i Do case-insensitive pattern matching.
1324 m Treat string as multiple lines.
1325 o Compile pattern only once.
1326 s Treat string as single line.
1327 x Use extended regular expressions.
1329 Any non-alphanumeric, non-whitespace delimiter may replace the
1330 slashes. If single quotes are used, no interpretation is done on the
1331 replacement string (the C</e> modifier overrides this, however). Unlike
1332 Perl 4, Perl 5 treats backticks as normal delimiters; the replacement
1333 text is not evaluated as a command. If the
1334 PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
1335 pair of quotes, which may or may not be bracketing quotes, e.g.,
1336 C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
1337 replacement portion to be treated as a full-fledged Perl expression
1338 and evaluated right then and there. It is, however, syntax checked at
1339 compile-time. A second C<e> modifier will cause the replacement portion
1340 to be C<eval>ed before being run as a Perl expression.
1344 s/\bgreen\b/mauve/g; # don't change wintergreen
1346 $path =~ s|/usr/bin|/usr/local/bin|;
1348 s/Login: $foo/Login: $bar/; # run-time pattern
1350 ($foo = $bar) =~ s/this/that/; # copy first, then change
1352 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
1355 s/\d+/$&*2/e; # yields 'abc246xyz'
1356 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
1357 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1359 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
1360 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
1361 s/^=(\w+)/&pod($1)/ge; # use function call
1363 # expand variables in $_, but dynamics only, using
1364 # symbolic dereferencing
1367 # Add one to the value of any numbers in the string
1370 # This will expand any embedded scalar variable
1371 # (including lexicals) in $_ : First $1 is interpolated
1372 # to the variable name, and then evaluated
1375 # Delete (most) C comments.
1377 /\* # Match the opening delimiter.
1378 .*? # Match a minimal number of characters.
1379 \*/ # Match the closing delimiter.
1382 s/^\s*(.*?)\s*$/$1/; # trim white space in $_, expensively
1384 for ($variable) { # trim white space in $variable, cheap
1389 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
1391 Note the use of $ instead of \ in the last example. Unlike
1392 B<sed>, we use the \<I<digit>> form in only the left hand side.
1393 Anywhere else it's $<I<digit>>.
1395 Occasionally, you can't use just a C</g> to get all the changes
1396 to occur that you might want. Here are two common cases:
1398 # put commas in the right places in an integer
1399 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
1401 # expand tabs to 8-column spacing
1402 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
1404 =item tr/SEARCHLIST/REPLACEMENTLIST/cds
1406 =item y/SEARCHLIST/REPLACEMENTLIST/cds
1408 Transliterates all occurrences of the characters found in the search list
1409 with the corresponding character in the replacement list. It returns
1410 the number of characters replaced or deleted. If no string is
1411 specified via the =~ or !~ operator, the $_ string is transliterated. (The
1412 string specified with =~ must be a scalar variable, an array element, a
1413 hash element, or an assignment to one of those, i.e., an lvalue.)
1415 A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
1416 does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
1417 For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1418 SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1419 its own pair of quotes, which may or may not be bracketing quotes,
1420 e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
1422 Note that C<tr> does B<not> do regular expression character classes
1423 such as C<\d> or C<[:lower:]>. The <tr> operator is not equivalent to
1424 the tr(1) utility. If you want to map strings between lower/upper
1425 cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider
1426 using the C<s> operator if you need regular expressions.
1428 Note also that the whole range idea is rather unportable between
1429 character sets--and even within character sets they may cause results
1430 you probably didn't expect. A sound principle is to use only ranges
1431 that begin from and end at either alphabets of equal case (a-e, A-E),
1432 or digits (0-4). Anything else is unsafe. If in doubt, spell out the
1433 character sets in full.
1437 c Complement the SEARCHLIST.
1438 d Delete found but unreplaced characters.
1439 s Squash duplicate replaced characters.
1441 If the C</c> modifier is specified, the SEARCHLIST character set
1442 is complemented. If the C</d> modifier is specified, any characters
1443 specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
1444 (Note that this is slightly more flexible than the behavior of some
1445 B<tr> programs, which delete anything they find in the SEARCHLIST,
1446 period.) If the C</s> modifier is specified, sequences of characters
1447 that were transliterated to the same character are squashed down
1448 to a single instance of the character.
1450 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1451 exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1452 than the SEARCHLIST, the final character is replicated till it is long
1453 enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
1454 This latter is useful for counting characters in a class or for
1455 squashing character sequences in a class.
1459 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1461 $cnt = tr/*/*/; # count the stars in $_
1463 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1465 $cnt = tr/0-9//; # count the digits in $_
1467 tr/a-zA-Z//s; # bookkeeper -> bokeper
1469 ($HOST = $host) =~ tr/a-z/A-Z/;
1471 tr/a-zA-Z/ /cs; # change non-alphas to single space
1474 [\000-\177]; # delete 8th bit
1476 If multiple transliterations are given for a character, only the
1481 will transliterate any A to X.
1483 Because the transliteration table is built at compile time, neither
1484 the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
1485 interpolation. That means that if you want to use variables, you
1488 eval "tr/$oldlist/$newlist/";
1491 eval "tr/$oldlist/$newlist/, 1" or die $@;
1495 A line-oriented form of quoting is based on the shell "here-document"
1496 syntax. Following a C<< << >> you specify a string to terminate
1497 the quoted material, and all lines following the current line down to
1498 the terminating string are the value of the item. The terminating
1499 string may be either an identifier (a word), or some quoted text. If
1500 quoted, the type of quotes you use determines the treatment of the
1501 text, just as in regular quoting. An unquoted identifier works like
1502 double quotes. There must be no space between the C<< << >> and
1503 the identifier, unless the identifier is quoted. (If you put a space it
1504 will be treated as a null identifier, which is valid, and matches the first
1505 empty line.) The terminating string must appear by itself (unquoted and
1506 with no surrounding whitespace) on the terminating line.
1509 The price is $Price.
1512 print << "EOF"; # same as above
1513 The price is $Price.
1516 print << `EOC`; # execute commands
1521 print <<"foo", <<"bar"; # you can stack them
1527 myfunc(<< "THIS", 23, <<'THAT');
1534 Just don't forget that you have to put a semicolon on the end
1535 to finish the statement, as Perl doesn't know you're not going to
1543 If you want your here-docs to be indented with the
1544 rest of the code, you'll need to remove leading whitespace
1545 from each line manually:
1547 ($quote = <<'FINIS') =~ s/^\s+//gm;
1548 The Road goes ever on and on,
1549 down from the door where it began.
1552 If you use a here-doc within a delimited construct, such as in C<s///eg>,
1553 the quoted material must come on the lines following the final delimiter.
1568 If the terminating identifier is on the last line of the program, you
1569 must be sure there is a newline after it; otherwise, Perl will give the
1570 warning B<Can't find string terminator "END" anywhere before EOF...>.
1572 Additionally, the quoting rules for the identifier are not related to
1573 Perl's quoting rules -- C<q()>, C<qq()>, and the like are not supported
1574 in place of C<''> and C<"">, and the only interpolation is for backslashing
1575 the quoting character:
1577 print << "abc\"def";
1581 Finally, quoted strings cannot span multiple lines. The general rule is
1582 that the identifier must be a string literal. Stick with that, and you
1587 =head2 Gory details of parsing quoted constructs
1589 When presented with something that might have several different
1590 interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
1591 principle to pick the most probable interpretation. This strategy
1592 is so successful that Perl programmers often do not suspect the
1593 ambivalence of what they write. But from time to time, Perl's
1594 notions differ substantially from what the author honestly meant.
1596 This section hopes to clarify how Perl handles quoted constructs.
1597 Although the most common reason to learn this is to unravel labyrinthine
1598 regular expressions, because the initial steps of parsing are the
1599 same for all quoting operators, they are all discussed together.
1601 The most important Perl parsing rule is the first one discussed
1602 below: when processing a quoted construct, Perl first finds the end
1603 of that construct, then interprets its contents. If you understand
1604 this rule, you may skip the rest of this section on the first
1605 reading. The other rules are likely to contradict the user's
1606 expectations much less frequently than this first one.
1608 Some passes discussed below are performed concurrently, but because
1609 their results are the same, we consider them individually. For different
1610 quoting constructs, Perl performs different numbers of passes, from
1611 one to five, but these passes are always performed in the same order.
1615 =item Finding the end
1617 The first pass is finding the end of the quoted construct, whether
1618 it be a multicharacter delimiter C<"\nEOF\n"> in the C<<<EOF>
1619 construct, a C</> that terminates a C<qq//> construct, a C<]> which
1620 terminates C<qq[]> construct, or a C<< > >> which terminates a
1621 fileglob started with C<< < >>.
1623 When searching for single-character non-pairing delimiters, such
1624 as C</>, combinations of C<\\> and C<\/> are skipped. However,
1625 when searching for single-character pairing delimiter like C<[>,
1626 combinations of C<\\>, C<\]>, and C<\[> are all skipped, and nested
1627 C<[>, C<]> are skipped as well. When searching for multicharacter
1628 delimiters, nothing is skipped.
1630 For constructs with three-part delimiters (C<s///>, C<y///>, and
1631 C<tr///>), the search is repeated once more.
1633 During this search no attention is paid to the semantics of the construct.
1636 "$hash{"$foo/$bar"}"
1641 bar # NOT a comment, this slash / terminated m//!
1644 do not form legal quoted expressions. The quoted part ends on the
1645 first C<"> and C</>, and the rest happens to be a syntax error.
1646 Because the slash that terminated C<m//> was followed by a C<SPACE>,
1647 the example above is not C<m//x>, but rather C<m//> with no C</x>
1648 modifier. So the embedded C<#> is interpreted as a literal C<#>.
1650 =item Removal of backslashes before delimiters
1652 During the second pass, text between the starting and ending
1653 delimiters is copied to a safe location, and the C<\> is removed
1654 from combinations consisting of C<\> and delimiter--or delimiters,
1655 meaning both starting and ending delimiters will should these differ.
1656 This removal does not happen for multi-character delimiters.
1657 Note that the combination C<\\> is left intact, just as it was.
1659 Starting from this step no information about the delimiters is
1664 The next step is interpolation in the text obtained, which is now
1665 delimiter-independent. There are four different cases.
1669 =item C<<<'EOF'>, C<m''>, C<s'''>, C<tr///>, C<y///>
1671 No interpolation is performed.
1675 The only interpolation is removal of C<\> from pairs C<\\>.
1677 =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>
1679 C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
1680 converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
1681 is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
1682 The other combinations are replaced with appropriate expansions.
1684 Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
1685 is interpolated in the usual way. Something like C<"\Q\\E"> has
1686 no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
1687 result is the same as for C<"\\\\E">. As a general rule, backslashes
1688 between C<\Q> and C<\E> may lead to counterintuitive results. So,
1689 C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
1690 as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
1695 may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
1697 Interpolated scalars and arrays are converted internally to the C<join> and
1698 C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
1700 $foo . " XXX '" . (join $", @arr) . "'";
1702 All operations above are performed simultaneously, left to right.
1704 Because the result of C<"\Q STRING \E"> has all metacharacters
1705 quoted, there is no way to insert a literal C<$> or C<@> inside a
1706 C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
1707 C<"\\\$">; if not, it is interpreted as the start of an interpolated
1710 Note also that the interpolation code needs to make a decision on
1711 where the interpolated scalar ends. For instance, whether
1712 C<< "a $b -> {c}" >> really means:
1714 "a " . $b . " -> {c}";
1720 Most of the time, the longest possible text that does not include
1721 spaces between components and which contains matching braces or
1722 brackets. because the outcome may be determined by voting based
1723 on heuristic estimators, the result is not strictly predictable.
1724 Fortunately, it's usually correct for ambiguous cases.
1726 =item C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
1728 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
1729 happens (almost) as with C<qq//> constructs, but the substitution
1730 of C<\> followed by RE-special chars (including C<\>) is not
1731 performed. Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
1732 a C<#>-comment in a C<//x>-regular expression, no processing is
1733 performed whatsoever. This is the first step at which the presence
1734 of the C<//x> modifier is relevant.
1736 Interpolation has several quirks: C<$|>, C<$(>, and C<$)> are not
1737 interpolated, and constructs C<$var[SOMETHING]> are voted (by several
1738 different estimators) to be either an array element or C<$var>
1739 followed by an RE alternative. This is where the notation
1740 C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
1741 array element C<-9>, not as a regular expression from the variable
1742 C<$arr> followed by a digit, which would be the interpretation of
1743 C</$arr[0-9]/>. Since voting among different estimators may occur,
1744 the result is not predictable.
1746 It is at this step that C<\1> is begrudgingly converted to C<$1> in
1747 the replacement text of C<s///> to correct the incorrigible
1748 I<sed> hackers who haven't picked up the saner idiom yet. A warning
1749 is emitted if the C<use warnings> pragma or the B<-w> command-line flag
1750 (that is, the C<$^W> variable) was set.
1752 The lack of processing of C<\\> creates specific restrictions on
1753 the post-processed text. If the delimiter is C</>, one cannot get
1754 the combination C<\/> into the result of this step. C</> will
1755 finish the regular expression, C<\/> will be stripped to C</> on
1756 the previous step, and C<\\/> will be left as is. Because C</> is
1757 equivalent to C<\/> inside a regular expression, this does not
1758 matter unless the delimiter happens to be character special to the
1759 RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
1760 alphanumeric char, as in:
1764 In the RE above, which is intentionally obfuscated for illustration, the
1765 delimiter is C<m>, the modifier is C<mx>, and after backslash-removal the
1766 RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
1767 reason you're encouraged to restrict your delimiters to non-alphanumeric,
1768 non-whitespace choices.
1772 This step is the last one for all constructs except regular expressions,
1773 which are processed further.
1775 =item Interpolation of regular expressions
1777 Previous steps were performed during the compilation of Perl code,
1778 but this one happens at run time--although it may be optimized to
1779 be calculated at compile time if appropriate. After preprocessing
1780 described above, and possibly after evaluation if catenation,
1781 joining, casing translation, or metaquoting are involved, the
1782 resulting I<string> is passed to the RE engine for compilation.
1784 Whatever happens in the RE engine might be better discussed in L<perlre>,
1785 but for the sake of continuity, we shall do so here.
1787 This is another step where the presence of the C<//x> modifier is
1788 relevant. The RE engine scans the string from left to right and
1789 converts it to a finite automaton.
1791 Backslashed characters are either replaced with corresponding
1792 literal strings (as with C<\{>), or else they generate special nodes
1793 in the finite automaton (as with C<\b>). Characters special to the
1794 RE engine (such as C<|>) generate corresponding nodes or groups of
1795 nodes. C<(?#...)> comments are ignored. All the rest is either
1796 converted to literal strings to match, or else is ignored (as is
1797 whitespace and C<#>-style comments if C<//x> is present).
1799 Parsing of the bracketed character class construct, C<[...]>, is
1800 rather different than the rule used for the rest of the pattern.
1801 The terminator of this construct is found using the same rules as
1802 for finding the terminator of a C<{}>-delimited construct, the only
1803 exception being that C<]> immediately following C<[> is treated as
1804 though preceded by a backslash. Similarly, the terminator of
1805 C<(?{...})> is found using the same rules as for finding the
1806 terminator of a C<{}>-delimited construct.
1808 It is possible to inspect both the string given to RE engine and the
1809 resulting finite automaton. See the arguments C<debug>/C<debugcolor>
1810 in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
1811 switch documented in L<perlrun/"Command Switches">.
1813 =item Optimization of regular expressions
1815 This step is listed for completeness only. Since it does not change
1816 semantics, details of this step are not documented and are subject
1817 to change without notice. This step is performed over the finite
1818 automaton that was generated during the previous pass.
1820 It is at this stage that C<split()> silently optimizes C</^/> to
1825 =head2 I/O Operators
1827 There are several I/O operators you should know about.
1829 A string enclosed by backticks (grave accents) first undergoes
1830 double-quote interpolation. It is then interpreted as an external
1831 command, and the output of that command is the value of the
1832 backtick string, like in a shell. In scalar context, a single string
1833 consisting of all output is returned. In list context, a list of
1834 values is returned, one per line of output. (You can set C<$/> to use
1835 a different line terminator.) The command is executed each time the
1836 pseudo-literal is evaluated. The status value of the command is
1837 returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
1838 Unlike in B<csh>, no translation is done on the return data--newlines
1839 remain newlines. Unlike in any of the shells, single quotes do not
1840 hide variable names in the command from interpretation. To pass a
1841 literal dollar-sign through to the shell you need to hide it with a
1842 backslash. The generalized form of backticks is C<qx//>. (Because
1843 backticks always undergo shell expansion as well, see L<perlsec> for
1846 In scalar context, evaluating a filehandle in angle brackets yields
1847 the next line from that file (the newline, if any, included), or
1848 C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
1849 (sometimes known as file-slurp mode) and the file is empty, it
1850 returns C<''> the first time, followed by C<undef> subsequently.
1852 Ordinarily you must assign the returned value to a variable, but
1853 there is one situation where an automatic assignment happens. If
1854 and only if the input symbol is the only thing inside the conditional
1855 of a C<while> statement (even if disguised as a C<for(;;)> loop),
1856 the value is automatically assigned to the global variable $_,
1857 destroying whatever was there previously. (This may seem like an
1858 odd thing to you, but you'll use the construct in almost every Perl
1859 script you write.) The $_ variable is not implicitly localized.
1860 You'll have to put a C<local $_;> before the loop if you want that
1863 The following lines are equivalent:
1865 while (defined($_ = <STDIN>)) { print; }
1866 while ($_ = <STDIN>) { print; }
1867 while (<STDIN>) { print; }
1868 for (;<STDIN>;) { print; }
1869 print while defined($_ = <STDIN>);
1870 print while ($_ = <STDIN>);
1871 print while <STDIN>;
1873 This also behaves similarly, but avoids $_ :
1875 while (my $line = <STDIN>) { print $line }
1877 In these loop constructs, the assigned value (whether assignment
1878 is automatic or explicit) is then tested to see whether it is
1879 defined. The defined test avoids problems where line has a string
1880 value that would be treated as false by Perl, for example a "" or
1881 a "0" with no trailing newline. If you really mean for such values
1882 to terminate the loop, they should be tested for explicitly:
1884 while (($_ = <STDIN>) ne '0') { ... }
1885 while (<STDIN>) { last unless $_; ... }
1887 In other boolean contexts, C<< <I<filehandle>> >> without an
1888 explicit C<defined> test or comparison elicit a warning if the
1889 C<use warnings> pragma or the B<-w>
1890 command-line switch (the C<$^W> variable) is in effect.
1892 The filehandles STDIN, STDOUT, and STDERR are predefined. (The
1893 filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
1894 in packages, where they would be interpreted as local identifiers
1895 rather than global.) Additional filehandles may be created with
1896 the open() function, amongst others. See L<perlopentut> and
1897 L<perlfunc/open> for details on this.
1899 If a <FILEHANDLE> is used in a context that is looking for
1900 a list, a list comprising all input lines is returned, one line per
1901 list element. It's easy to grow to a rather large data space this
1902 way, so use with care.
1904 <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
1905 See L<perlfunc/readline>.
1907 The null filehandle <> is special: it can be used to emulate the
1908 behavior of B<sed> and B<awk>. Input from <> comes either from
1909 standard input, or from each file listed on the command line. Here's
1910 how it works: the first time <> is evaluated, the @ARGV array is
1911 checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
1912 gives you standard input. The @ARGV array is then processed as a list
1913 of filenames. The loop
1916 ... # code for each line
1919 is equivalent to the following Perl-like pseudo code:
1921 unshift(@ARGV, '-') unless @ARGV;
1922 while ($ARGV = shift) {
1925 ... # code for each line
1929 except that it isn't so cumbersome to say, and will actually work.
1930 It really does shift the @ARGV array and put the current filename
1931 into the $ARGV variable. It also uses filehandle I<ARGV>
1932 internally--<> is just a synonym for <ARGV>, which
1933 is magical. (The pseudo code above doesn't work because it treats
1934 <ARGV> as non-magical.)
1936 You can modify @ARGV before the first <> as long as the array ends up
1937 containing the list of filenames you really want. Line numbers (C<$.>)
1938 continue as though the input were one big happy file. See the example
1939 in L<perlfunc/eof> for how to reset line numbers on each file.
1941 If you want to set @ARGV to your own list of files, go right ahead.
1942 This sets @ARGV to all plain text files if no @ARGV was given:
1944 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
1946 You can even set them to pipe commands. For example, this automatically
1947 filters compressed arguments through B<gzip>:
1949 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
1951 If you want to pass switches into your script, you can use one of the
1952 Getopts modules or put a loop on the front like this:
1954 while ($_ = $ARGV[0], /^-/) {
1957 if (/^-D(.*)/) { $debug = $1 }
1958 if (/^-v/) { $verbose++ }
1959 # ... # other switches
1963 # ... # code for each line
1966 The <> symbol will return C<undef> for end-of-file only once.
1967 If you call it again after this, it will assume you are processing another
1968 @ARGV list, and if you haven't set @ARGV, will read input from STDIN.
1970 If what the angle brackets contain is a simple scalar variable (e.g.,
1971 <$foo>), then that variable contains the name of the
1972 filehandle to input from, or its typeglob, or a reference to the
1978 If what's within the angle brackets is neither a filehandle nor a simple
1979 scalar variable containing a filehandle name, typeglob, or typeglob
1980 reference, it is interpreted as a filename pattern to be globbed, and
1981 either a list of filenames or the next filename in the list is returned,
1982 depending on context. This distinction is determined on syntactic
1983 grounds alone. That means C<< <$x> >> is always a readline() from
1984 an indirect handle, but C<< <$hash{key}> >> is always a glob().
1985 That's because $x is a simple scalar variable, but C<$hash{key}> is
1986 not--it's a hash element.
1988 One level of double-quote interpretation is done first, but you can't
1989 say C<< <$foo> >> because that's an indirect filehandle as explained
1990 in the previous paragraph. (In older versions of Perl, programmers
1991 would insert curly brackets to force interpretation as a filename glob:
1992 C<< <${foo}> >>. These days, it's considered cleaner to call the
1993 internal function directly as C<glob($foo)>, which is probably the right
1994 way to have done it in the first place.) For example:
2000 is roughly equivalent to:
2002 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
2008 except that the globbing is actually done internally using the standard
2009 C<File::Glob> extension. Of course, the shortest way to do the above is:
2013 A (file)glob evaluates its (embedded) argument only when it is
2014 starting a new list. All values must be read before it will start
2015 over. In list context, this isn't important because you automatically
2016 get them all anyway. However, in scalar context the operator returns
2017 the next value each time it's called, or C<undef> when the list has
2018 run out. As with filehandle reads, an automatic C<defined> is
2019 generated when the glob occurs in the test part of a C<while>,
2020 because legal glob returns (e.g. a file called F<0>) would otherwise
2021 terminate the loop. Again, C<undef> is returned only once. So if
2022 you're expecting a single value from a glob, it is much better to
2025 ($file) = <blurch*>;
2031 because the latter will alternate between returning a filename and
2034 If you're trying to do variable interpolation, it's definitely better
2035 to use the glob() function, because the older notation can cause people
2036 to become confused with the indirect filehandle notation.
2038 @files = glob("$dir/*.[ch]");
2039 @files = glob($files[$i]);
2041 =head2 Constant Folding
2043 Like C, Perl does a certain amount of expression evaluation at
2044 compile time whenever it determines that all arguments to an
2045 operator are static and have no side effects. In particular, string
2046 concatenation happens at compile time between literals that don't do
2047 variable substitution. Backslash interpolation also happens at
2048 compile time. You can say
2050 'Now is the time for all' . "\n" .
2051 'good men to come to.'
2053 and this all reduces to one string internally. Likewise, if
2056 foreach $file (@filenames) {
2057 if (-s $file > 5 + 100 * 2**16) { }
2060 the compiler will precompute the number which that expression
2061 represents so that the interpreter won't have to.
2065 Perl doesn't officially have a no-op operator, but the bare constants
2066 C<0> and C<1> are special-cased to not produce a warning in a void
2067 context, so you can for example safely do
2071 =head2 Bitwise String Operators
2073 Bitstrings of any size may be manipulated by the bitwise operators
2076 If the operands to a binary bitwise op are strings of different
2077 sizes, B<|> and B<^> ops act as though the shorter operand had
2078 additional zero bits on the right, while the B<&> op acts as though
2079 the longer operand were truncated to the length of the shorter.
2080 The granularity for such extension or truncation is one or more
2083 # ASCII-based examples
2084 print "j p \n" ^ " a h"; # prints "JAPH\n"
2085 print "JA" | " ph\n"; # prints "japh\n"
2086 print "japh\nJunk" & '_____'; # prints "JAPH\n";
2087 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
2089 If you are intending to manipulate bitstrings, be certain that
2090 you're supplying bitstrings: If an operand is a number, that will imply
2091 a B<numeric> bitwise operation. You may explicitly show which type of
2092 operation you intend by using C<""> or C<0+>, as in the examples below.
2094 $foo = 150 | 105 ; # yields 255 (0x96 | 0x69 is 0xFF)
2095 $foo = '150' | 105 ; # yields 255
2096 $foo = 150 | '105'; # yields 255
2097 $foo = '150' | '105'; # yields string '155' (under ASCII)
2099 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
2100 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
2102 See L<perlfunc/vec> for information on how to manipulate individual bits
2105 =head2 Integer Arithmetic
2107 By default, Perl assumes that it must do most of its arithmetic in
2108 floating point. But by saying
2112 you may tell the compiler that it's okay to use integer operations
2113 (if it feels like it) from here to the end of the enclosing BLOCK.
2114 An inner BLOCK may countermand this by saying
2118 which lasts until the end of that BLOCK. Note that this doesn't
2119 mean everything is only an integer, merely that Perl may use integer
2120 operations if it is so inclined. For example, even under C<use
2121 integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731>
2124 Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
2125 and ">>") always produce integral results. (But see also
2126 L<Bitwise String Operators>.) However, C<use integer> still has meaning for
2127 them. By default, their results are interpreted as unsigned integers, but
2128 if C<use integer> is in effect, their results are interpreted
2129 as signed integers. For example, C<~0> usually evaluates to a large
2130 integral value. However, C<use integer; ~0> is C<-1> on twos-complement
2133 =head2 Floating-point Arithmetic
2135 While C<use integer> provides integer-only arithmetic, there is no
2136 analogous mechanism to provide automatic rounding or truncation to a
2137 certain number of decimal places. For rounding to a certain number
2138 of digits, sprintf() or printf() is usually the easiest route.
2141 Floating-point numbers are only approximations to what a mathematician
2142 would call real numbers. There are infinitely more reals than floats,
2143 so some corners must be cut. For example:
2145 printf "%.20g\n", 123456789123456789;
2146 # produces 123456789123456784
2148 Testing for exact equality of floating-point equality or inequality is
2149 not a good idea. Here's a (relatively expensive) work-around to compare
2150 whether two floating-point numbers are equal to a particular number of
2151 decimal places. See Knuth, volume II, for a more robust treatment of
2155 my ($X, $Y, $POINTS) = @_;
2157 $tX = sprintf("%.${POINTS}g", $X);
2158 $tY = sprintf("%.${POINTS}g", $Y);
2162 The POSIX module (part of the standard perl distribution) implements
2163 ceil(), floor(), and other mathematical and trigonometric functions.
2164 The Math::Complex module (part of the standard perl distribution)
2165 defines mathematical functions that work on both the reals and the
2166 imaginary numbers. Math::Complex not as efficient as POSIX, but
2167 POSIX can't work with complex numbers.
2169 Rounding in financial applications can have serious implications, and
2170 the rounding method used should be specified precisely. In these
2171 cases, it probably pays not to trust whichever system rounding is
2172 being used by Perl, but to instead implement the rounding function you
2175 =head2 Bigger Numbers
2177 The standard Math::BigInt and Math::BigFloat modules provide
2178 variable-precision arithmetic and overloaded operators, although
2179 they're currently pretty slow. At the cost of some space and
2180 considerable speed, they avoid the normal pitfalls associated with
2181 limited-precision representations.
2184 $x = Math::BigInt->new('123456789123456789');
2187 # prints +15241578780673678515622620750190521
2189 There are several modules that let you calculate with (bound only by
2190 memory and cpu-time) unlimited or fixed precision. There are also
2191 some non-standard modules that provide faster implementations via
2192 external C libraries.
2194 Here is a short, but incomplete summary:
2196 Math::Fraction big, unlimited fractions like 9973 / 12967
2197 Math::String treat string sequences like numbers
2198 Math::FixedPrecision calculate with a fixed precision
2199 Math::Currency for currency calculations
2200 Bit::Vector manipulate bit vectors fast (uses C)
2201 Math::BigIntFast Bit::Vector wrapper for big numbers
2202 Math::Pari provides access to the Pari C library
2203 Math::BigInteger uses an external C library
2204 Math::Cephes uses external Cephes C library (no big numbers)
2205 Math::Cephes::Fraction fractions via the Cephes library
2206 Math::GMP another one using an external C library