extra code in pp_concat, Take 2
[p5sagit/p5-mst-13.2.git] / pod / perlop.pod
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a0d0e21e 1=head1 NAME
2
3perlop - Perl operators and precedence
4
d042e63d 5=head1 DESCRIPTION
6
7=head2 Operator Precedence and Associativity
8
9Operator precedence and associativity work in Perl more or less like
10they do in mathematics.
11
12I<Operator precedence> means some operators are evaluated before
13others. For example, in C<2 + 4 * 5>, the multiplication has higher
14precedence so C<4 * 5> is evaluated first yielding C<2 + 20 ==
1522> and not C<6 * 5 == 30>.
16
17I<Operator associativity> defines what happens if a sequence of the
18same operators is used one after another: whether the evaluator will
19evaluate the left operations first or the right. For example, in C<8
20- 4 - 2>, subtraction is left associative so Perl evaluates the
21expression left to right. C<8 - 4> is evaluated first making the
22expression C<4 - 2 == 2> and not C<8 - 2 == 6>.
a0d0e21e 23
24Perl operators have the following associativity and precedence,
19799a22 25listed from highest precedence to lowest. Operators borrowed from
26C keep the same precedence relationship with each other, even where
27C's precedence is slightly screwy. (This makes learning Perl easier
28for C folks.) With very few exceptions, these all operate on scalar
29values only, not array values.
a0d0e21e 30
31 left terms and list operators (leftward)
32 left ->
33 nonassoc ++ --
34 right **
35 right ! ~ \ and unary + and -
54310121 36 left =~ !~
a0d0e21e 37 left * / % x
38 left + - .
39 left << >>
40 nonassoc named unary operators
41 nonassoc < > <= >= lt gt le ge
42 nonassoc == != <=> eq ne cmp
43 left &
44 left | ^
45 left &&
c963b151 46 left || //
137443ea 47 nonassoc .. ...
a0d0e21e 48 right ?:
49 right = += -= *= etc.
50 left , =>
51 nonassoc list operators (rightward)
a5f75d66 52 right not
a0d0e21e 53 left and
c963b151 54 left or xor err
a0d0e21e 55
56In the following sections, these operators are covered in precedence order.
57
5a964f20 58Many operators can be overloaded for objects. See L<overload>.
59
a0d0e21e 60=head2 Terms and List Operators (Leftward)
61
62c18ce2 62A TERM has the highest precedence in Perl. They include variables,
5f05dabc 63quote and quote-like operators, any expression in parentheses,
a0d0e21e 64and any function whose arguments are parenthesized. Actually, there
65aren't really functions in this sense, just list operators and unary
66operators behaving as functions because you put parentheses around
67the arguments. These are all documented in L<perlfunc>.
68
69If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
70is followed by a left parenthesis as the next token, the operator and
71arguments within parentheses are taken to be of highest precedence,
72just like a normal function call.
73
74In the absence of parentheses, the precedence of list operators such as
75C<print>, C<sort>, or C<chmod> is either very high or very low depending on
54310121 76whether you are looking at the left side or the right side of the operator.
a0d0e21e 77For example, in
78
79 @ary = (1, 3, sort 4, 2);
80 print @ary; # prints 1324
81
19799a22 82the commas on the right of the sort are evaluated before the sort,
83but the commas on the left are evaluated after. In other words,
84list operators tend to gobble up all arguments that follow, and
a0d0e21e 85then act like a simple TERM with regard to the preceding expression.
19799a22 86Be careful with parentheses:
a0d0e21e 87
88 # These evaluate exit before doing the print:
89 print($foo, exit); # Obviously not what you want.
90 print $foo, exit; # Nor is this.
91
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 ($foo), exit; # Or even this.
96
97Also note that
98
99 print ($foo & 255) + 1, "\n";
100
d042e63d 101probably doesn't do what you expect at first glance. The parentheses
102enclose the argument list for C<print> which is evaluated (printing
103the result of C<$foo & 255>). Then one is added to the return value
104of C<print> (usually 1). The result is something like this:
105
106 1 + 1, "\n"; # Obviously not what you meant.
107
108To do what you meant properly, you must write:
109
110 print(($foo & 255) + 1, "\n");
111
112See L<Named Unary Operators> for more discussion of this.
a0d0e21e 113
114Also parsed as terms are the C<do {}> and C<eval {}> constructs, as
54310121 115well as subroutine and method calls, and the anonymous
a0d0e21e 116constructors C<[]> and C<{}>.
117
2ae324a7 118See also L<Quote and Quote-like Operators> toward the end of this section,
c07a80fd 119as well as L<"I/O Operators">.
a0d0e21e 120
121=head2 The Arrow Operator
122
35f2feb0 123"C<< -> >>" is an infix dereference operator, just as it is in C
19799a22 124and C++. If the right side is either a C<[...]>, C<{...}>, or a
125C<(...)> subscript, then the left side must be either a hard or
126symbolic reference to an array, a hash, or a subroutine respectively.
127(Or technically speaking, a location capable of holding a hard
128reference, if it's an array or hash reference being used for
129assignment.) See L<perlreftut> and L<perlref>.
a0d0e21e 130
19799a22 131Otherwise, the right side is a method name or a simple scalar
132variable containing either the method name or a subroutine reference,
133and the left side must be either an object (a blessed reference)
134or a class name (that is, a package name). See L<perlobj>.
a0d0e21e 135
5f05dabc 136=head2 Auto-increment and Auto-decrement
a0d0e21e 137
d042e63d 138"++" and "--" work as in C. That is, if placed before a variable,
139they increment or decrement the variable by one before returning the
140value, and if placed after, increment or decrement after returning the
141value.
142
143 $i = 0; $j = 0;
144 print $i++; # prints 0
145 print ++$j; # prints 1
a0d0e21e 146
b033823e 147Note that just as in C, Perl doesn't define B<when> the variable is
148incremented or decremented. You just know it will be done sometime
149before or after the value is returned. This also means that modifying
150a variable twice in the same statement will lead to undefined behaviour.
151Avoid statements like:
152
153 $i = $i ++;
154 print ++ $i + $i ++;
155
156Perl will not guarantee what the result of the above statements is.
157
54310121 158The auto-increment operator has a little extra builtin magic to it. If
a0d0e21e 159you increment a variable that is numeric, or that has ever been used in
160a numeric context, you get a normal increment. If, however, the
5f05dabc 161variable has been used in only string contexts since it was set, and
5a964f20 162has a value that is not the empty string and matches the pattern
9c0670e1 163C</^[a-zA-Z]*[0-9]*\z/>, the increment is done as a string, preserving each
a0d0e21e 164character within its range, with carry:
165
166 print ++($foo = '99'); # prints '100'
167 print ++($foo = 'a0'); # prints 'a1'
168 print ++($foo = 'Az'); # prints 'Ba'
169 print ++($foo = 'zz'); # prints 'aaa'
170
6a61d433 171C<undef> is always treated as numeric, and in particular is changed
172to C<0> before incrementing (so that a post-increment of an undef value
173will return C<0> rather than C<undef>).
174
5f05dabc 175The auto-decrement operator is not magical.
a0d0e21e 176
177=head2 Exponentiation
178
19799a22 179Binary "**" is the exponentiation operator. It binds even more
cb1a09d0 180tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is
181implemented using C's pow(3) function, which actually works on doubles
182internally.)
a0d0e21e 183
184=head2 Symbolic Unary Operators
185
5f05dabc 186Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower
a0d0e21e 187precedence version of this.
188
189Unary "-" performs arithmetic negation if the operand is numeric. If
190the operand is an identifier, a string consisting of a minus sign
191concatenated with the identifier is returned. Otherwise, if the string
192starts with a plus or minus, a string starting with the opposite sign
bff5667c 193is returned. One effect of these rules is that -bareword is equivalent
194to "-bareword".
a0d0e21e 195
972b05a9 196Unary "~" performs bitwise negation, i.e., 1's complement. For
197example, C<0666 & ~027> is 0640. (See also L<Integer Arithmetic> and
198L<Bitwise String Operators>.) Note that the width of the result is
199platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64
200bits wide on a 64-bit platform, so if you are expecting a certain bit
d042e63d 201width, remember to use the & operator to mask off the excess bits.
a0d0e21e 202
203Unary "+" has no effect whatsoever, even on strings. It is useful
204syntactically for separating a function name from a parenthesized expression
205that would otherwise be interpreted as the complete list of function
5ba421f6 206arguments. (See examples above under L<Terms and List Operators (Leftward)>.)
a0d0e21e 207
19799a22 208Unary "\" creates a reference to whatever follows it. See L<perlreftut>
209and L<perlref>. Do not confuse this behavior with the behavior of
210backslash within a string, although both forms do convey the notion
211of protecting the next thing from interpolation.
a0d0e21e 212
213=head2 Binding Operators
214
c07a80fd 215Binary "=~" binds a scalar expression to a pattern match. Certain operations
cb1a09d0 216search or modify the string $_ by default. This operator makes that kind
217of operation work on some other string. The right argument is a search
2c268ad5 218pattern, substitution, or transliteration. The left argument is what is
219supposed to be searched, substituted, or transliterated instead of the default
f8bab1e9 220$_. When used in scalar context, the return value generally indicates the
221success of the operation. Behavior in list context depends on the particular
d7782e69 222operator. See L</"Regexp Quote-Like Operators"> for details and
223L<perlretut> for examples using these operators.
f8bab1e9 224
225If the right argument is an expression rather than a search pattern,
2c268ad5 226substitution, or transliteration, it is interpreted as a search pattern at run
573e01ca 227time.
a0d0e21e 228
229Binary "!~" is just like "=~" except the return value is negated in
230the logical sense.
231
232=head2 Multiplicative Operators
233
234Binary "*" multiplies two numbers.
235
236Binary "/" divides two numbers.
237
54310121 238Binary "%" computes the modulus of two numbers. Given integer
239operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
240C<$a> minus the largest multiple of C<$b> that is not greater than
241C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
242smallest multiple of C<$b> that is not less than C<$a> (i.e. the
6bb4e6d4 243result will be less than or equal to zero).
0412d526 244Note that when C<use integer> is in scope, "%" gives you direct access
55d729e4 245to the modulus operator as implemented by your C compiler. This
246operator is not as well defined for negative operands, but it will
247execute faster.
248
62d10b70 249Binary "x" is the repetition operator. In scalar context or if the left
250operand is not enclosed in parentheses, it returns a string consisting
251of the left operand repeated the number of times specified by the right
252operand. In list context, if the left operand is enclosed in
58a9d1fc 253parentheses, it repeats the list. If the right operand is zero or
254negative, it returns an empty string or an empty list, depending on the
255context.
a0d0e21e 256
257 print '-' x 80; # print row of dashes
258
259 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
260
261 @ones = (1) x 80; # a list of 80 1's
262 @ones = (5) x @ones; # set all elements to 5
263
264
265=head2 Additive Operators
266
267Binary "+" returns the sum of two numbers.
268
269Binary "-" returns the difference of two numbers.
270
271Binary "." concatenates two strings.
272
273=head2 Shift Operators
274
55497cff 275Binary "<<" returns the value of its left argument shifted left by the
276number of bits specified by the right argument. Arguments should be
982ce180 277integers. (See also L<Integer Arithmetic>.)
a0d0e21e 278
55497cff 279Binary ">>" returns the value of its left argument shifted right by
280the number of bits specified by the right argument. Arguments should
982ce180 281be integers. (See also L<Integer Arithmetic>.)
a0d0e21e 282
b16cf6df 283Note that both "<<" and ">>" in Perl are implemented directly using
284"<<" and ">>" in C. If C<use integer> (see L<Integer Arithmetic>) is
285in force then signed C integers are used, else unsigned C integers are
286used. Either way, the implementation isn't going to generate results
287larger than the size of the integer type Perl was built with (32 bits
288or 64 bits).
289
290The result of overflowing the range of the integers is undefined
291because it is undefined also in C. In other words, using 32-bit
292integers, C<< 1 << 32 >> is undefined. Shifting by a negative number
293of bits is also undefined.
294
a0d0e21e 295=head2 Named Unary Operators
296
297The various named unary operators are treated as functions with one
568e6d8b 298argument, with optional parentheses.
a0d0e21e 299
300If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
301is followed by a left parenthesis as the next token, the operator and
302arguments within parentheses are taken to be of highest precedence,
3981b0eb 303just like a normal function call. For example,
304because named unary operators are higher precedence than ||:
a0d0e21e 305
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
310
3981b0eb 311but, because * is higher precedence than named operators:
a0d0e21e 312
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 ($foo * 20)
317
318 rand 10 * 20; # rand (10 * 20)
319 rand(10) * 20; # (rand 10) * 20
320 rand (10) * 20; # (rand 10) * 20
321 rand +(10) * 20; # rand (10 * 20)
322
568e6d8b 323Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are
324treated like named unary operators, but they don't follow this functional
325parenthesis rule. That means, for example, that C<-f($file).".bak"> is
326equivalent to C<-f "$file.bak">.
327
5ba421f6 328See also L<"Terms and List Operators (Leftward)">.
a0d0e21e 329
330=head2 Relational Operators
331
35f2feb0 332Binary "<" returns true if the left argument is numerically less than
a0d0e21e 333the right argument.
334
35f2feb0 335Binary ">" returns true if the left argument is numerically greater
a0d0e21e 336than the right argument.
337
35f2feb0 338Binary "<=" returns true if the left argument is numerically less than
a0d0e21e 339or equal to the right argument.
340
35f2feb0 341Binary ">=" returns true if the left argument is numerically greater
a0d0e21e 342than or equal to the right argument.
343
344Binary "lt" returns true if the left argument is stringwise less than
345the right argument.
346
347Binary "gt" returns true if the left argument is stringwise greater
348than the right argument.
349
350Binary "le" returns true if the left argument is stringwise less than
351or equal to the right argument.
352
353Binary "ge" returns true if the left argument is stringwise greater
354than or equal to the right argument.
355
356=head2 Equality Operators
357
358Binary "==" returns true if the left argument is numerically equal to
359the right argument.
360
361Binary "!=" returns true if the left argument is numerically not equal
362to the right argument.
363
35f2feb0 364Binary "<=>" returns -1, 0, or 1 depending on whether the left
6ee5d4e7 365argument is numerically less than, equal to, or greater than the right
d4ad863d 366argument. If your platform supports NaNs (not-a-numbers) as numeric
7d3a9d88 367values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
368"<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
369returns true, as does NaN != anything else. If your platform doesn't
370support NaNs then NaN is just a string with numeric value 0.
371
372 perl -le '$a = NaN; print "No NaN support here" if $a == $a'
373 perl -le '$a = NaN; print "NaN support here" if $a != $a'
a0d0e21e 374
375Binary "eq" returns true if the left argument is stringwise equal to
376the right argument.
377
378Binary "ne" returns true if the left argument is stringwise not equal
379to the right argument.
380
d4ad863d 381Binary "cmp" returns -1, 0, or 1 depending on whether the left
382argument is stringwise less than, equal to, or greater than the right
383argument.
a0d0e21e 384
a034a98d 385"lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
386by the current locale if C<use locale> is in effect. See L<perllocale>.
387
a0d0e21e 388=head2 Bitwise And
389
2cdc098b 390Binary "&" returns its operands ANDed together bit by bit.
2c268ad5 391(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
a0d0e21e 392
2cdc098b 393Note that "&" has lower priority than relational operators, so for example
394the brackets are essential in a test like
395
396 print "Even\n" if ($x & 1) == 0;
397
a0d0e21e 398=head2 Bitwise Or and Exclusive Or
399
2cdc098b 400Binary "|" returns its operands ORed together bit by bit.
2c268ad5 401(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
a0d0e21e 402
2cdc098b 403Binary "^" returns its operands XORed together bit by bit.
2c268ad5 404(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
a0d0e21e 405
2cdc098b 406Note that "|" and "^" have lower priority than relational operators, so
407for example the brackets are essential in a test like
408
409 print "false\n" if (8 | 2) != 10;
410
a0d0e21e 411=head2 C-style Logical And
412
413Binary "&&" performs a short-circuit logical AND operation. That is,
414if the left operand is false, the right operand is not even evaluated.
415Scalar or list context propagates down to the right operand if it
416is evaluated.
417
418=head2 C-style Logical Or
419
420Binary "||" performs a short-circuit logical OR operation. That is,
421if the left operand is true, the right operand is not even evaluated.
422Scalar or list context propagates down to the right operand if it
423is evaluated.
424
c963b151 425=head2 C-style Logical Defined-Or
426
427Although it has no direct equivalent in C, Perl's C<//> operator is related
428to its C-style or. In fact, it's exactly the same as C<||>, except that it
429tests the left hand side's definedness instead of its truth. Thus, C<$a // $b>
430is similar to C<defined($a) || $b> (except that it returns the value of C<$a>
431rather than the value of C<defined($a)>) and is exactly equivalent to
432C<defined($a) ? $a : $b>. This is very useful for providing default values
d042e63d 433for variables. If you actually want to test if at least one of C<$a> and
434C<$b> is defined, use C<defined($a // $b)>.
c963b151 435
d042e63d 436The 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
438portable way to find out the home directory might be:
a0d0e21e 439
c963b151 440 $home = $ENV{'HOME'} // $ENV{'LOGDIR'} //
441 (getpwuid($<))[7] // die "You're homeless!\n";
a0d0e21e 442
5a964f20 443In particular, this means that you shouldn't use this
444for selecting between two aggregates for assignment:
445
446 @a = @b || @c; # this is wrong
447 @a = scalar(@b) || @c; # really meant this
448 @a = @b ? @b : @c; # this works fine, though
449
c963b151 450As more readable alternatives to C<&&>, C<//> and C<||> when used for
451control flow, Perl provides C<and>, C<err> and C<or> operators (see below).
452The short-circuit behavior is identical. The precedence of "and", "err"
453and "or" is much lower, however, so that you can safely use them after a
5a964f20 454list operator without the need for parentheses:
a0d0e21e 455
456 unlink "alpha", "beta", "gamma"
457 or gripe(), next LINE;
458
459With the C-style operators that would have been written like this:
460
461 unlink("alpha", "beta", "gamma")
462 || (gripe(), next LINE);
463
eeb6a2c9 464Using "or" for assignment is unlikely to do what you want; see below.
5a964f20 465
466=head2 Range Operators
a0d0e21e 467
468Binary ".." is the range operator, which is really two different
fb53bbb2 469operators depending on the context. In list context, it returns a
54ae734e 470list of values counting (up by ones) from the left value to the right
2cdbc966 471value. If the left value is greater than the right value then it
fb53bbb2 472returns the empty list. The range operator is useful for writing
54ae734e 473C<foreach (1..10)> loops and for doing slice operations on arrays. In
2cdbc966 474the current implementation, no temporary array is created when the
475range operator is used as the expression in C<foreach> loops, but older
476versions of Perl might burn a lot of memory when you write something
477like this:
a0d0e21e 478
479 for (1 .. 1_000_000) {
480 # code
54310121 481 }
a0d0e21e 482
54ae734e 483The range operator also works on strings, using the magical auto-increment,
484see below.
485
5a964f20 486In scalar context, ".." returns a boolean value. The operator is
a0d0e21e 487bistable, like a flip-flop, and emulates the line-range (comma) operator
488of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
489own boolean state. It is false as long as its left operand is false.
490Once the left operand is true, the range operator stays true until the
491right operand is true, I<AFTER> which the range operator becomes false
19799a22 492again. It doesn't become false till the next time the range operator is
a0d0e21e 493evaluated. It can test the right operand and become false on the same
494evaluation it became true (as in B<awk>), but it still returns true once.
19799a22 495If you don't want it to test the right operand till the next
496evaluation, as in B<sed>, just use three dots ("...") instead of
497two. In all other regards, "..." behaves just like ".." does.
498
499The right operand is not evaluated while the operator is in the
500"false" state, and the left operand is not evaluated while the
501operator is in the "true" state. The precedence is a little lower
502than || and &&. The value returned is either the empty string for
503false, or a sequence number (beginning with 1) for true. The
504sequence number is reset for each range encountered. The final
505sequence number in a range has the string "E0" appended to it, which
506doesn't affect its numeric value, but gives you something to search
507for if you want to exclude the endpoint. You can exclude the
508beginning point by waiting for the sequence number to be greater
df5f8116 509than 1.
510
511If either operand of scalar ".." is a constant expression,
512that operand is considered true if it is equal (C<==>) to the current
513input line number (the C<$.> variable).
514
515To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>,
516but that is only an issue if you use a floating point expression; when
517implicitly using C<$.> as described in the previous paragraph, the
518comparison is C<int(EXPR) == int($.)> which is only an issue when C<$.>
519is set to a floating point value and you are not reading from a file.
520Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what
521you want in scalar context because each of the operands are evaluated
522using their integer representation.
523
524Examples:
a0d0e21e 525
526As a scalar operator:
527
df5f8116 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 ($. == 1 .. /^$/);
a0d0e21e 532 s/^/> / if (/^$/ .. eof()); # quote body
533
5a964f20 534 # parse mail messages
535 while (<>) {
536 $in_header = 1 .. /^$/;
df5f8116 537 $in_body = /^$/ .. eof;
538 if ($in_header) {
539 # ...
540 } else { # in body
541 # ...
542 }
5a964f20 543 } continue {
df5f8116 544 close ARGV if eof; # reset $. each file
5a964f20 545 }
546
acf31ca5 547Here's a simple example to illustrate the difference between
548the two range operators:
549
550 @lines = (" - Foo",
551 "01 - Bar",
552 "1 - Baz",
553 " - Quux");
554
555 foreach(@lines)
556 {
557 if (/0/ .. /1/)
558 {
559 print "$_\n";
560 }
561 }
562
563This program will print only the line containing "Bar". If
564the range operator is changed to C<...>, it will also print the
565"Baz" line.
566
567And now some examples as a list operator:
a0d0e21e 568
569 for (101 .. 200) { print; } # print $_ 100 times
3e3baf6d 570 @foo = @foo[0 .. $#foo]; # an expensive no-op
a0d0e21e 571 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
572
5a964f20 573The range operator (in list context) makes use of the magical
5f05dabc 574auto-increment algorithm if the operands are strings. You
a0d0e21e 575can say
576
577 @alphabet = ('A' .. 'Z');
578
54ae734e 579to get all normal letters of the English alphabet, or
a0d0e21e 580
581 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
582
583to get a hexadecimal digit, or
584
585 @z2 = ('01' .. '31'); print $z2[$mday];
586
587to get dates with leading zeros. If the final value specified is not
588in the sequence that the magical increment would produce, the sequence
589goes until the next value would be longer than the final value
590specified.
591
df5f8116 592Because each operand is evaluated in integer form, C<2.18 .. 3.14> will
593return two elements in list context.
594
595 @list = (2.18 .. 3.14); # same as @list = (2 .. 3);
596
a0d0e21e 597=head2 Conditional Operator
598
599Ternary "?:" is the conditional operator, just as in C. It works much
600like an if-then-else. If the argument before the ? is true, the
601argument before the : is returned, otherwise the argument after the :
cb1a09d0 602is returned. For example:
603
54310121 604 printf "I have %d dog%s.\n", $n,
cb1a09d0 605 ($n == 1) ? '' : "s";
606
607Scalar or list context propagates downward into the 2nd
54310121 608or 3rd argument, whichever is selected.
cb1a09d0 609
610 $a = $ok ? $b : $c; # get a scalar
611 @a = $ok ? @b : @c; # get an array
612 $a = $ok ? @b : @c; # oops, that's just a count!
613
614The operator may be assigned to if both the 2nd and 3rd arguments are
615legal lvalues (meaning that you can assign to them):
a0d0e21e 616
617 ($a_or_b ? $a : $b) = $c;
618
5a964f20 619Because this operator produces an assignable result, using assignments
620without parentheses will get you in trouble. For example, this:
621
622 $a % 2 ? $a += 10 : $a += 2
623
624Really means this:
625
626 (($a % 2) ? ($a += 10) : $a) += 2
627
628Rather than this:
629
630 ($a % 2) ? ($a += 10) : ($a += 2)
631
19799a22 632That should probably be written more simply as:
633
634 $a += ($a % 2) ? 10 : 2;
635
4633a7c4 636=head2 Assignment Operators
a0d0e21e 637
638"=" is the ordinary assignment operator.
639
640Assignment operators work as in C. That is,
641
642 $a += 2;
643
644is equivalent to
645
646 $a = $a + 2;
647
648although without duplicating any side effects that dereferencing the lvalue
54310121 649might trigger, such as from tie(). Other assignment operators work similarly.
650The following are recognized:
a0d0e21e 651
652 **= += *= &= <<= &&=
653 -= /= |= >>= ||=
654 .= %= ^=
655 x=
656
19799a22 657Although these are grouped by family, they all have the precedence
a0d0e21e 658of assignment.
659
b350dd2f 660Unlike in C, the scalar assignment operator produces a valid lvalue.
661Modifying an assignment is equivalent to doing the assignment and
662then modifying the variable that was assigned to. This is useful
663for modifying a copy of something, like this:
a0d0e21e 664
665 ($tmp = $global) =~ tr [A-Z] [a-z];
666
667Likewise,
668
669 ($a += 2) *= 3;
670
671is equivalent to
672
673 $a += 2;
674 $a *= 3;
675
b350dd2f 676Similarly, a list assignment in list context produces the list of
677lvalues assigned to, and a list assignment in scalar context returns
678the number of elements produced by the expression on the right hand
679side of the assignment.
680
748a9306 681=head2 Comma Operator
a0d0e21e 682
5a964f20 683Binary "," is the comma operator. In scalar context it evaluates
a0d0e21e 684its left argument, throws that value away, then evaluates its right
685argument and returns that value. This is just like C's comma operator.
686
5a964f20 687In list context, it's just the list argument separator, and inserts
a0d0e21e 688both its arguments into the list.
689
d042e63d 690The C<< => >> operator is a synonym for the comma, but forces any word
719b43e8 691(consisting entirely of word characters) to its left to be interpreted
692as a string (as of 5.001). If the argument on the left is not a word,
693it is first interpreted as an expression, and then the string value of
694that is used.
695
696The C<< => >> operator is helpful in documenting the correspondence
697between keys and values in hashes, and other paired elements in lists.
748a9306 698
a0d0e21e 699=head2 List Operators (Rightward)
700
701On the right side of a list operator, it has very low precedence,
702such that it controls all comma-separated expressions found there.
703The only operators with lower precedence are the logical operators
704"and", "or", and "not", which may be used to evaluate calls to list
705operators without the need for extra parentheses:
706
707 open HANDLE, "filename"
708 or die "Can't open: $!\n";
709
5ba421f6 710See also discussion of list operators in L<Terms and List Operators (Leftward)>.
a0d0e21e 711
712=head2 Logical Not
713
714Unary "not" returns the logical negation of the expression to its right.
715It's the equivalent of "!" except for the very low precedence.
716
717=head2 Logical And
718
719Binary "and" returns the logical conjunction of the two surrounding
720expressions. It's equivalent to && except for the very low
5f05dabc 721precedence. This means that it short-circuits: i.e., the right
a0d0e21e 722expression is evaluated only if the left expression is true.
723
c963b151 724=head2 Logical or, Defined or, and Exclusive Or
a0d0e21e 725
726Binary "or" returns the logical disjunction of the two surrounding
5a964f20 727expressions. It's equivalent to || except for the very low precedence.
728This makes it useful for control flow
729
730 print FH $data or die "Can't write to FH: $!";
731
732This means that it short-circuits: i.e., the right expression is evaluated
733only if the left expression is false. Due to its precedence, you should
734probably avoid using this for assignment, only for control flow.
735
736 $a = $b or $c; # bug: this is wrong
737 ($a = $b) or $c; # really means this
738 $a = $b || $c; # better written this way
739
19799a22 740However, when it's a list-context assignment and you're trying to use
5a964f20 741"||" for control flow, you probably need "or" so that the assignment
742takes higher precedence.
743
744 @info = stat($file) || die; # oops, scalar sense of stat!
745 @info = stat($file) or die; # better, now @info gets its due
746
c963b151 747Then again, you could always use parentheses.
748
749Binary "err" is equivalent to C<//>--it's just like binary "or", except it tests
750its left argument's definedness instead of its truth. There are two ways to
751remember "err": either because many functions return C<undef> on an B<err>or,
752or as a sort of correction: C<$a=($b err 'default')>
a0d0e21e 753
754Binary "xor" returns the exclusive-OR of the two surrounding expressions.
755It cannot short circuit, of course.
756
757=head2 C Operators Missing From Perl
758
759Here is what C has that Perl doesn't:
760
761=over 8
762
763=item unary &
764
765Address-of operator. (But see the "\" operator for taking a reference.)
766
767=item unary *
768
54310121 769Dereference-address operator. (Perl's prefix dereferencing
a0d0e21e 770operators are typed: $, @, %, and &.)
771
772=item (TYPE)
773
19799a22 774Type-casting operator.
a0d0e21e 775
776=back
777
5f05dabc 778=head2 Quote and Quote-like Operators
a0d0e21e 779
780While we usually think of quotes as literal values, in Perl they
781function as operators, providing various kinds of interpolating and
782pattern matching capabilities. Perl provides customary quote characters
783for these behaviors, but also provides a way for you to choose your
784quote character for any of them. In the following table, a C<{}> represents
87275199 785any pair of delimiters you choose.
a0d0e21e 786
2c268ad5 787 Customary Generic Meaning Interpolates
788 '' q{} Literal no
789 "" qq{} Literal yes
af9219ee 790 `` qx{} Command yes*
2c268ad5 791 qw{} Word list no
af9219ee 792 // m{} Pattern match yes*
793 qr{} Pattern yes*
794 s{}{} Substitution yes*
2c268ad5 795 tr{}{} Transliteration no (but see below)
7e3b091d 796 <<EOF here-doc yes*
a0d0e21e 797
af9219ee 798 * unless the delimiter is ''.
799
87275199 800Non-bracketing delimiters use the same character fore and aft, but the four
801sorts of brackets (round, angle, square, curly) will all nest, which means
802that
803
804 q{foo{bar}baz}
35f2feb0 805
87275199 806is the same as
807
808 'foo{bar}baz'
809
810Note, however, that this does not always work for quoting Perl code:
811
812 $s = q{ if($a eq "}") ... }; # WRONG
813
83df6a1d 814is a syntax error. The C<Text::Balanced> module (from CPAN, and
815starting from Perl 5.8 part of the standard distribution) is able
816to do this properly.
87275199 817
19799a22 818There can be whitespace between the operator and the quoting
fb73857a 819characters, except when C<#> is being used as the quoting character.
19799a22 820C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
821operator C<q> followed by a comment. Its argument will be taken
822from the next line. This allows you to write:
fb73857a 823
824 s {foo} # Replace foo
825 {bar} # with bar.
826
904501ec 827The following escape sequences are available in constructs that interpolate
828and in transliterations.
a0d0e21e 829
6ee5d4e7 830 \t tab (HT, TAB)
5a964f20 831 \n newline (NL)
6ee5d4e7 832 \r return (CR)
833 \f form feed (FF)
834 \b backspace (BS)
835 \a alarm (bell) (BEL)
836 \e escape (ESC)
a0ed51b3 837 \033 octal char (ESC)
838 \x1b hex char (ESC)
839 \x{263a} wide hex char (SMILEY)
19799a22 840 \c[ control char (ESC)
95cc3e0c 841 \N{name} named Unicode character
2c268ad5 842
4c77eaa2 843B<NOTE>: Unlike C and other languages, Perl has no \v escape sequence for
844the vertical tab (VT - ASCII 11).
845
904501ec 846The following escape sequences are available in constructs that interpolate
847but not in transliterations.
848
a0d0e21e 849 \l lowercase next char
850 \u uppercase next char
851 \L lowercase till \E
852 \U uppercase till \E
853 \E end case modification
1d2dff63 854 \Q quote non-word characters till \E
a0d0e21e 855
95cc3e0c 856If C<use locale> is in effect, the case map used by C<\l>, C<\L>,
857C<\u> and C<\U> is taken from the current locale. See L<perllocale>.
858If Unicode (for example, C<\N{}> or wide hex characters of 0x100 or
859beyond) is being used, the case map used by C<\l>, C<\L>, C<\u> and
860C<\U> is as defined by Unicode. For documentation of C<\N{name}>,
861see L<charnames>.
a034a98d 862
5a964f20 863All systems use the virtual C<"\n"> to represent a line terminator,
864called a "newline". There is no such thing as an unvarying, physical
19799a22 865newline character. It is only an illusion that the operating system,
5a964f20 866device drivers, C libraries, and Perl all conspire to preserve. Not all
867systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
868on a Mac, these are reversed, and on systems without line terminator,
869printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
870you mean a "newline" for your system, but use the literal ASCII when you
871need an exact character. For example, most networking protocols expect
2a380090 872and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
5a964f20 873and although they often accept just C<"\012">, they seldom tolerate just
874C<"\015">. If you get in the habit of using C<"\n"> for networking,
875you may be burned some day.
876
904501ec 877For constructs that do interpolate, variables beginning with "C<$>"
878or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
ad0f383a 879C<< $href->{key}[0] >> are also interpolated, as are array and hash slices.
880But method calls such as C<< $obj->meth >> are not.
af9219ee 881
882Interpolating an array or slice interpolates the elements in order,
883separated by the value of C<$">, so is equivalent to interpolating
904501ec 884C<join $", @array>. "Punctuation" arrays such as C<@+> are only
885interpolated if the name is enclosed in braces C<@{+}>.
af9219ee 886
1d2dff63 887You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
888An unescaped C<$> or C<@> interpolates the corresponding variable,
889while escaping will cause the literal string C<\$> to be inserted.
890You'll need to write something like C<m/\Quser\E\@\Qhost/>.
891
a0d0e21e 892Patterns are subject to an additional level of interpretation as a
893regular expression. This is done as a second pass, after variables are
894interpolated, so that regular expressions may be incorporated into the
895pattern from the variables. If this is not what you want, use C<\Q> to
896interpolate a variable literally.
897
19799a22 898Apart from the behavior described above, Perl does not expand
899multiple levels of interpolation. In particular, contrary to the
900expectations of shell programmers, back-quotes do I<NOT> interpolate
901within double quotes, nor do single quotes impede evaluation of
902variables when used within double quotes.
a0d0e21e 903
5f05dabc 904=head2 Regexp Quote-Like Operators
cb1a09d0 905
5f05dabc 906Here are the quote-like operators that apply to pattern
cb1a09d0 907matching and related activities.
908
a0d0e21e 909=over 8
910
911=item ?PATTERN?
912
913This is just like the C</pattern/> search, except that it matches only
914once between calls to the reset() operator. This is a useful
5f05dabc 915optimization when you want to see only the first occurrence of
a0d0e21e 916something in each file of a set of files, for instance. Only C<??>
917patterns local to the current package are reset.
918
5a964f20 919 while (<>) {
920 if (?^$?) {
921 # blank line between header and body
922 }
923 } continue {
924 reset if eof; # clear ?? status for next file
925 }
926
483b4840 927This usage is vaguely deprecated, which means it just might possibly
19799a22 928be removed in some distant future version of Perl, perhaps somewhere
929around the year 2168.
a0d0e21e 930
fb73857a 931=item m/PATTERN/cgimosx
a0d0e21e 932
fb73857a 933=item /PATTERN/cgimosx
a0d0e21e 934
5a964f20 935Searches a string for a pattern match, and in scalar context returns
19799a22 936true if it succeeds, false if it fails. If no string is specified
937via the C<=~> or C<!~> operator, the $_ string is searched. (The
938string specified with C<=~> need not be an lvalue--it may be the
939result of an expression evaluation, but remember the C<=~> binds
940rather tightly.) See also L<perlre>. See L<perllocale> for
941discussion of additional considerations that apply when C<use locale>
942is in effect.
a0d0e21e 943
944Options are:
945
fb73857a 946 c Do not reset search position on a failed match when /g is in effect.
5f05dabc 947 g Match globally, i.e., find all occurrences.
a0d0e21e 948 i Do case-insensitive pattern matching.
949 m Treat string as multiple lines.
5f05dabc 950 o Compile pattern only once.
a0d0e21e 951 s Treat string as single line.
952 x Use extended regular expressions.
953
954If "/" is the delimiter then the initial C<m> is optional. With the C<m>
01ae956f 955you can use any pair of non-alphanumeric, non-whitespace characters
19799a22 956as delimiters. This is particularly useful for matching path names
957that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
7bac28a0 958the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
19799a22 959If "'" is the delimiter, no interpolation is performed on the PATTERN.
a0d0e21e 960
961PATTERN may contain variables, which will be interpolated (and the
f70b4f9c 962pattern recompiled) every time the pattern search is evaluated, except
1f247705 963for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
964C<$|> are not interpolated because they look like end-of-string tests.)
f70b4f9c 965If you want such a pattern to be compiled only once, add a C</o> after
966the trailing delimiter. This avoids expensive run-time recompilations,
967and is useful when the value you are interpolating won't change over
968the life of the script. However, mentioning C</o> constitutes a promise
969that you won't change the variables in the pattern. If you change them,
13a2d996 970Perl won't even notice. See also L<"qr/STRING/imosx">.
a0d0e21e 971
5a964f20 972If the PATTERN evaluates to the empty string, the last
d65afb4b 973I<successfully> matched regular expression is used instead. In this
974case, only the C<g> and C<c> flags on the empty pattern is honoured -
975the other flags are taken from the original pattern. If no match has
976previously succeeded, this will (silently) act instead as a genuine
977empty pattern (which will always match).
a0d0e21e 978
c963b151 979Note that it's possible to confuse Perl into thinking C<//> (the empty
980regex) is really C<//> (the defined-or operator). Perl is usually pretty
981good about this, but some pathological cases might trigger this, such as
982C<$a///> (is that C<($a) / (//)> or C<$a // />?) and C<print $fh //>
983(C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl
984will assume you meant defined-or. If you meant the empty regex, just
985use parentheses or spaces to disambiguate, or even prefix the empty
986regex with an C<m> (so C<//> becomes C<m//>).
987
19799a22 988If the C</g> option is not used, C<m//> in list context returns a
a0d0e21e 989list consisting of the subexpressions matched by the parentheses in the
f7e33566 990pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are
991also set, and that this differs from Perl 4's behavior.) When there are
992no parentheses in the pattern, the return value is the list C<(1)> for
993success. With or without parentheses, an empty list is returned upon
994failure.
a0d0e21e 995
996Examples:
997
998 open(TTY, '/dev/tty');
999 <TTY> =~ /^y/i && foo(); # do foo if desired
1000
1001 if (/Version: *([0-9.]*)/) { $version = $1; }
1002
1003 next if m#^/usr/spool/uucp#;
1004
1005 # poor man's grep
1006 $arg = shift;
1007 while (<>) {
1008 print if /$arg/o; # compile only once
1009 }
1010
1011 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
1012
1013This last example splits $foo into the first two words and the
5f05dabc 1014remainder of the line, and assigns those three fields to $F1, $F2, and
1015$Etc. The conditional is true if any variables were assigned, i.e., if
a0d0e21e 1016the pattern matched.
1017
19799a22 1018The C</g> modifier specifies global pattern matching--that is,
1019matching as many times as possible within the string. How it behaves
1020depends on the context. In list context, it returns a list of the
1021substrings matched by any capturing parentheses in the regular
1022expression. If there are no parentheses, it returns a list of all
1023the matched strings, as if there were parentheses around the whole
1024pattern.
a0d0e21e 1025
7e86de3e 1026In scalar context, each execution of C<m//g> finds the next match,
19799a22 1027returning true if it matches, and false if there is no further match.
7e86de3e 1028The position after the last match can be read or set using the pos()
1029function; see L<perlfunc/pos>. A failed match normally resets the
1030search position to the beginning of the string, but you can avoid that
1031by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
1032string also resets the search position.
c90c0ff4 1033
1034You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
1035zero-width assertion that matches the exact position where the previous
5d43e42d 1036C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion
1037still anchors at pos(), but the match is of course only attempted once.
1038Using C<\G> without C</g> on a target string that has not previously had a
1039C</g> match applied to it is the same as using the C<\A> assertion to match
fe4b3f22 1040the beginning of the string. Note also that, currently, C<\G> is only
1041properly supported when anchored at the very beginning of the pattern.
c90c0ff4 1042
1043Examples:
a0d0e21e 1044
1045 # list context
1046 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
1047
1048 # scalar context
5d43e42d 1049 $/ = "";
19799a22 1050 while (defined($paragraph = <>)) {
1051 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
1052 $sentences++;
a0d0e21e 1053 }
1054 }
1055 print "$sentences\n";
1056
c90c0ff4 1057 # using m//gc with \G
137443ea 1058 $_ = "ppooqppqq";
44a8e56a 1059 while ($i++ < 2) {
1060 print "1: '";
c90c0ff4 1061 print $1 while /(o)/gc; print "', pos=", pos, "\n";
44a8e56a 1062 print "2: '";
c90c0ff4 1063 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
44a8e56a 1064 print "3: '";
c90c0ff4 1065 print $1 while /(p)/gc; print "', pos=", pos, "\n";
44a8e56a 1066 }
5d43e42d 1067 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
44a8e56a 1068
1069The last example should print:
1070
1071 1: 'oo', pos=4
137443ea 1072 2: 'q', pos=5
44a8e56a 1073 3: 'pp', pos=7
1074 1: '', pos=7
137443ea 1075 2: 'q', pos=8
1076 3: '', pos=8
5d43e42d 1077 Final: 'q', pos=8
1078
1079Notice that the final match matched C<q> instead of C<p>, which a match
1080without the C<\G> anchor would have done. Also note that the final match
1081did not update C<pos> -- C<pos> is only updated on a C</g> match. If the
1082final match did indeed match C<p>, it's a good bet that you're running an
1083older (pre-5.6.0) Perl.
44a8e56a 1084
c90c0ff4 1085A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
e7ea3e70 1086combine several regexps like this to process a string part-by-part,
c90c0ff4 1087doing different actions depending on which regexp matched. Each
1088regexp tries to match where the previous one leaves off.
e7ea3e70 1089
3fe9a6f1 1090 $_ = <<'EOL';
e7ea3e70 1091 $url = new URI::URL "http://www/"; die if $url eq "xXx";
3fe9a6f1 1092 EOL
1093 LOOP:
e7ea3e70 1094 {
c90c0ff4 1095 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
1096 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
1097 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
1098 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
1099 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
1100 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
1101 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
e7ea3e70 1102 print ". That's all!\n";
1103 }
1104
1105Here is the output (split into several lines):
1106
1107 line-noise lowercase line-noise lowercase UPPERCASE line-noise
1108 UPPERCASE line-noise lowercase line-noise lowercase line-noise
1109 lowercase lowercase line-noise lowercase lowercase line-noise
1110 MiXeD line-noise. That's all!
44a8e56a 1111
a0d0e21e 1112=item q/STRING/
1113
1114=item C<'STRING'>
1115
19799a22 1116A single-quoted, literal string. A backslash represents a backslash
68dc0745 1117unless followed by the delimiter or another backslash, in which case
1118the delimiter or backslash is interpolated.
a0d0e21e 1119
1120 $foo = q!I said, "You said, 'She said it.'"!;
1121 $bar = q('This is it.');
68dc0745 1122 $baz = '\n'; # a two-character string
a0d0e21e 1123
1124=item qq/STRING/
1125
1126=item "STRING"
1127
1128A double-quoted, interpolated string.
1129
1130 $_ .= qq
1131 (*** The previous line contains the naughty word "$1".\n)
19799a22 1132 if /\b(tcl|java|python)\b/i; # :-)
68dc0745 1133 $baz = "\n"; # a one-character string
a0d0e21e 1134
eec2d3df 1135=item qr/STRING/imosx
1136
322edccd 1137This operator quotes (and possibly compiles) its I<STRING> as a regular
19799a22 1138expression. I<STRING> is interpolated the same way as I<PATTERN>
1139in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
1140is done. Returns a Perl value which may be used instead of the
1141corresponding C</STRING/imosx> expression.
4b6a7270 1142
1143For example,
1144
1145 $rex = qr/my.STRING/is;
1146 s/$rex/foo/;
1147
1148is equivalent to
1149
1150 s/my.STRING/foo/is;
1151
1152The result may be used as a subpattern in a match:
eec2d3df 1153
1154 $re = qr/$pattern/;
0a92e3a8 1155 $string =~ /foo${re}bar/; # can be interpolated in other patterns
1156 $string =~ $re; # or used standalone
4b6a7270 1157 $string =~ /$re/; # or this way
1158
1159Since Perl may compile the pattern at the moment of execution of qr()
19799a22 1160operator, using qr() may have speed advantages in some situations,
4b6a7270 1161notably if the result of qr() is used standalone:
1162
1163 sub match {
1164 my $patterns = shift;
1165 my @compiled = map qr/$_/i, @$patterns;
1166 grep {
1167 my $success = 0;
a7665c5e 1168 foreach my $pat (@compiled) {
4b6a7270 1169 $success = 1, last if /$pat/;
1170 }
1171 $success;
1172 } @_;
1173 }
1174
19799a22 1175Precompilation of the pattern into an internal representation at
1176the moment of qr() avoids a need to recompile the pattern every
1177time a match C</$pat/> is attempted. (Perl has many other internal
1178optimizations, but none would be triggered in the above example if
1179we did not use qr() operator.)
eec2d3df 1180
1181Options are:
1182
1183 i Do case-insensitive pattern matching.
1184 m Treat string as multiple lines.
1185 o Compile pattern only once.
1186 s Treat string as single line.
1187 x Use extended regular expressions.
1188
0a92e3a8 1189See L<perlre> for additional information on valid syntax for STRING, and
1190for a detailed look at the semantics of regular expressions.
1191
a0d0e21e 1192=item qx/STRING/
1193
1194=item `STRING`
1195
43dd4d21 1196A string which is (possibly) interpolated and then executed as a
1197system command with C</bin/sh> or its equivalent. Shell wildcards,
1198pipes, and redirections will be honored. The collected standard
1199output of the command is returned; standard error is unaffected. In
1200scalar context, it comes back as a single (potentially multi-line)
1201string, or undef if the command failed. In list context, returns a
1202list of lines (however you've defined lines with $/ or
1203$INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
5a964f20 1204
1205Because backticks do not affect standard error, use shell file descriptor
1206syntax (assuming the shell supports this) if you care to address this.
1207To capture a command's STDERR and STDOUT together:
a0d0e21e 1208
5a964f20 1209 $output = `cmd 2>&1`;
1210
1211To capture a command's STDOUT but discard its STDERR:
1212
1213 $output = `cmd 2>/dev/null`;
1214
1215To capture a command's STDERR but discard its STDOUT (ordering is
1216important here):
1217
1218 $output = `cmd 2>&1 1>/dev/null`;
1219
1220To exchange a command's STDOUT and STDERR in order to capture the STDERR
1221but leave its STDOUT to come out the old STDERR:
1222
1223 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
1224
1225To read both a command's STDOUT and its STDERR separately, it's easiest
2359510d 1226to redirect them separately to files, and then read from those files
1227when the program is done:
5a964f20 1228
2359510d 1229 system("program args 1>program.stdout 2>program.stderr");
5a964f20 1230
1231Using single-quote as a delimiter protects the command from Perl's
1232double-quote interpolation, passing it on to the shell instead:
1233
1234 $perl_info = qx(ps $$); # that's Perl's $$
1235 $shell_info = qx'ps $$'; # that's the new shell's $$
1236
19799a22 1237How that string gets evaluated is entirely subject to the command
5a964f20 1238interpreter on your system. On most platforms, you will have to protect
1239shell metacharacters if you want them treated literally. This is in
1240practice difficult to do, as it's unclear how to escape which characters.
1241See L<perlsec> for a clean and safe example of a manual fork() and exec()
1242to emulate backticks safely.
a0d0e21e 1243
bb32b41a 1244On some platforms (notably DOS-like ones), the shell may not be
1245capable of dealing with multiline commands, so putting newlines in
1246the string may not get you what you want. You may be able to evaluate
1247multiple commands in a single line by separating them with the command
1248separator character, if your shell supports that (e.g. C<;> on many Unix
1249shells; C<&> on the Windows NT C<cmd> shell).
1250
0f897271 1251Beginning with v5.6.0, Perl will attempt to flush all files opened for
1252output before starting the child process, but this may not be supported
1253on some platforms (see L<perlport>). To be safe, you may need to set
1254C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1255C<IO::Handle> on any open handles.
1256
bb32b41a 1257Beware that some command shells may place restrictions on the length
1258of the command line. You must ensure your strings don't exceed this
1259limit after any necessary interpolations. See the platform-specific
1260release notes for more details about your particular environment.
1261
5a964f20 1262Using this operator can lead to programs that are difficult to port,
1263because the shell commands called vary between systems, and may in
1264fact not be present at all. As one example, the C<type> command under
1265the POSIX shell is very different from the C<type> command under DOS.
1266That doesn't mean you should go out of your way to avoid backticks
1267when they're the right way to get something done. Perl was made to be
1268a glue language, and one of the things it glues together is commands.
1269Just understand what you're getting yourself into.
bb32b41a 1270
dc848c6f 1271See L<"I/O Operators"> for more discussion.
a0d0e21e 1272
945c54fd 1273=item qw/STRING/
1274
1275Evaluates to a list of the words extracted out of STRING, using embedded
1276whitespace as the word delimiters. It can be understood as being roughly
1277equivalent to:
1278
1279 split(' ', q/STRING/);
1280
efb1e162 1281the differences being that it generates a real list at compile time, and
1282in scalar context it returns the last element in the list. So
945c54fd 1283this expression:
1284
1285 qw(foo bar baz)
1286
1287is semantically equivalent to the list:
1288
1289 'foo', 'bar', 'baz'
1290
1291Some frequently seen examples:
1292
1293 use POSIX qw( setlocale localeconv )
1294 @EXPORT = qw( foo bar baz );
1295
1296A common mistake is to try to separate the words with comma or to
1297put comments into a multi-line C<qw>-string. For this reason, the
1298C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
1299produces warnings if the STRING contains the "," or the "#" character.
1300
a0d0e21e 1301=item s/PATTERN/REPLACEMENT/egimosx
1302
1303Searches a string for a pattern, and if found, replaces that pattern
1304with the replacement text and returns the number of substitutions
e37d713d 1305made. Otherwise it returns false (specifically, the empty string).
a0d0e21e 1306
1307If no string is specified via the C<=~> or C<!~> operator, the C<$_>
1308variable is searched and modified. (The string specified with C<=~> must
5a964f20 1309be scalar variable, an array element, a hash element, or an assignment
5f05dabc 1310to one of those, i.e., an lvalue.)
a0d0e21e 1311
19799a22 1312If the delimiter chosen is a single quote, no interpolation is
a0d0e21e 1313done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1314PATTERN contains a $ that looks like a variable rather than an
1315end-of-string test, the variable will be interpolated into the pattern
5f05dabc 1316at run-time. If you want the pattern compiled only once the first time
a0d0e21e 1317the variable is interpolated, use the C</o> option. If the pattern
5a964f20 1318evaluates to the empty string, the last successfully executed regular
a0d0e21e 1319expression is used instead. See L<perlre> for further explanation on these.
5a964f20 1320See L<perllocale> for discussion of additional considerations that apply
a034a98d 1321when C<use locale> is in effect.
a0d0e21e 1322
1323Options are:
1324
1325 e Evaluate the right side as an expression.
5f05dabc 1326 g Replace globally, i.e., all occurrences.
a0d0e21e 1327 i Do case-insensitive pattern matching.
1328 m Treat string as multiple lines.
5f05dabc 1329 o Compile pattern only once.
a0d0e21e 1330 s Treat string as single line.
1331 x Use extended regular expressions.
1332
1333Any non-alphanumeric, non-whitespace delimiter may replace the
1334slashes. If single quotes are used, no interpretation is done on the
e37d713d 1335replacement string (the C</e> modifier overrides this, however). Unlike
54310121 1336Perl 4, Perl 5 treats backticks as normal delimiters; the replacement
e37d713d 1337text is not evaluated as a command. If the
a0d0e21e 1338PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
5f05dabc 1339pair of quotes, which may or may not be bracketing quotes, e.g.,
35f2feb0 1340C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
cec88af6 1341replacement portion to be treated as a full-fledged Perl expression
1342and evaluated right then and there. It is, however, syntax checked at
1343compile-time. A second C<e> modifier will cause the replacement portion
1344to be C<eval>ed before being run as a Perl expression.
a0d0e21e 1345
1346Examples:
1347
1348 s/\bgreen\b/mauve/g; # don't change wintergreen
1349
1350 $path =~ s|/usr/bin|/usr/local/bin|;
1351
1352 s/Login: $foo/Login: $bar/; # run-time pattern
1353
5a964f20 1354 ($foo = $bar) =~ s/this/that/; # copy first, then change
a0d0e21e 1355
5a964f20 1356 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
a0d0e21e 1357
1358 $_ = 'abc123xyz';
1359 s/\d+/$&*2/e; # yields 'abc246xyz'
1360 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
1361 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1362
1363 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
1364 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
1365 s/^=(\w+)/&pod($1)/ge; # use function call
1366
5a964f20 1367 # expand variables in $_, but dynamics only, using
1368 # symbolic dereferencing
1369 s/\$(\w+)/${$1}/g;
1370
cec88af6 1371 # Add one to the value of any numbers in the string
1372 s/(\d+)/1 + $1/eg;
1373
1374 # This will expand any embedded scalar variable
1375 # (including lexicals) in $_ : First $1 is interpolated
1376 # to the variable name, and then evaluated
a0d0e21e 1377 s/(\$\w+)/$1/eeg;
1378
5a964f20 1379 # Delete (most) C comments.
a0d0e21e 1380 $program =~ s {
4633a7c4 1381 /\* # Match the opening delimiter.
1382 .*? # Match a minimal number of characters.
1383 \*/ # Match the closing delimiter.
a0d0e21e 1384 } []gsx;
1385
6b0ac556 1386 s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_, expensively
5a964f20 1387
6b0ac556 1388 for ($variable) { # trim whitespace in $variable, cheap
5a964f20 1389 s/^\s+//;
1390 s/\s+$//;
1391 }
a0d0e21e 1392
1393 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
1394
54310121 1395Note the use of $ instead of \ in the last example. Unlike
35f2feb0 1396B<sed>, we use the \<I<digit>> form in only the left hand side.
1397Anywhere else it's $<I<digit>>.
a0d0e21e 1398
5f05dabc 1399Occasionally, you can't use just a C</g> to get all the changes
19799a22 1400to occur that you might want. Here are two common cases:
a0d0e21e 1401
1402 # put commas in the right places in an integer
19799a22 1403 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
a0d0e21e 1404
1405 # expand tabs to 8-column spacing
1406 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
1407
6940069f 1408=item tr/SEARCHLIST/REPLACEMENTLIST/cds
a0d0e21e 1409
6940069f 1410=item y/SEARCHLIST/REPLACEMENTLIST/cds
a0d0e21e 1411
2c268ad5 1412Transliterates all occurrences of the characters found in the search list
a0d0e21e 1413with the corresponding character in the replacement list. It returns
1414the number of characters replaced or deleted. If no string is
2c268ad5 1415specified via the =~ or !~ operator, the $_ string is transliterated. (The
54310121 1416string specified with =~ must be a scalar variable, an array element, a
1417hash element, or an assignment to one of those, i.e., an lvalue.)
8ada0baa 1418
2c268ad5 1419A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
1420does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
54310121 1421For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1422SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1423its own pair of quotes, which may or may not be bracketing quotes,
2c268ad5 1424e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
a0d0e21e 1425
cc255d5f 1426Note that C<tr> does B<not> do regular expression character classes
1427such as C<\d> or C<[:lower:]>. The <tr> operator is not equivalent to
1428the tr(1) utility. If you want to map strings between lower/upper
1429cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider
1430using the C<s> operator if you need regular expressions.
1431
8ada0baa 1432Note also that the whole range idea is rather unportable between
1433character sets--and even within character sets they may cause results
1434you probably didn't expect. A sound principle is to use only ranges
1435that begin from and end at either alphabets of equal case (a-e, A-E),
1436or digits (0-4). Anything else is unsafe. If in doubt, spell out the
1437character sets in full.
1438
a0d0e21e 1439Options:
1440
1441 c Complement the SEARCHLIST.
1442 d Delete found but unreplaced characters.
1443 s Squash duplicate replaced characters.
1444
19799a22 1445If the C</c> modifier is specified, the SEARCHLIST character set
1446is complemented. If the C</d> modifier is specified, any characters
1447specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
1448(Note that this is slightly more flexible than the behavior of some
1449B<tr> programs, which delete anything they find in the SEARCHLIST,
1450period.) If the C</s> modifier is specified, sequences of characters
1451that were transliterated to the same character are squashed down
1452to a single instance of the character.
a0d0e21e 1453
1454If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1455exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1456than the SEARCHLIST, the final character is replicated till it is long
5a964f20 1457enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
a0d0e21e 1458This latter is useful for counting characters in a class or for
1459squashing character sequences in a class.
1460
1461Examples:
1462
1463 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1464
1465 $cnt = tr/*/*/; # count the stars in $_
1466
1467 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1468
1469 $cnt = tr/0-9//; # count the digits in $_
1470
1471 tr/a-zA-Z//s; # bookkeeper -> bokeper
1472
1473 ($HOST = $host) =~ tr/a-z/A-Z/;
1474
1475 tr/a-zA-Z/ /cs; # change non-alphas to single space
1476
1477 tr [\200-\377]
1478 [\000-\177]; # delete 8th bit
1479
19799a22 1480If multiple transliterations are given for a character, only the
1481first one is used:
748a9306 1482
1483 tr/AAA/XYZ/
1484
2c268ad5 1485will transliterate any A to X.
748a9306 1486
19799a22 1487Because the transliteration table is built at compile time, neither
a0d0e21e 1488the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
19799a22 1489interpolation. That means that if you want to use variables, you
1490must use an eval():
a0d0e21e 1491
1492 eval "tr/$oldlist/$newlist/";
1493 die $@ if $@;
1494
1495 eval "tr/$oldlist/$newlist/, 1" or die $@;
1496
7e3b091d 1497=item <<EOF
1498
1499A line-oriented form of quoting is based on the shell "here-document"
1500syntax. Following a C<< << >> you specify a string to terminate
1501the quoted material, and all lines following the current line down to
1502the terminating string are the value of the item. The terminating
1503string may be either an identifier (a word), or some quoted text. If
1504quoted, the type of quotes you use determines the treatment of the
1505text, just as in regular quoting. An unquoted identifier works like
1506double quotes. There must be no space between the C<< << >> and
1507the identifier, unless the identifier is quoted. (If you put a space it
1508will be treated as a null identifier, which is valid, and matches the first
1509empty line.) The terminating string must appear by itself (unquoted and
1510with no surrounding whitespace) on the terminating line.
1511
1512 print <<EOF;
1513 The price is $Price.
1514 EOF
1515
1516 print << "EOF"; # same as above
1517 The price is $Price.
1518 EOF
1519
1520 print << `EOC`; # execute commands
1521 echo hi there
1522 echo lo there
1523 EOC
1524
1525 print <<"foo", <<"bar"; # you can stack them
1526 I said foo.
1527 foo
1528 I said bar.
1529 bar
1530
1531 myfunc(<< "THIS", 23, <<'THAT');
1532 Here's a line
1533 or two.
1534 THIS
1535 and here's another.
1536 THAT
1537
1538Just don't forget that you have to put a semicolon on the end
1539to finish the statement, as Perl doesn't know you're not going to
1540try to do this:
1541
1542 print <<ABC
1543 179231
1544 ABC
1545 + 20;
1546
1547If you want your here-docs to be indented with the
1548rest of the code, you'll need to remove leading whitespace
1549from each line manually:
1550
1551 ($quote = <<'FINIS') =~ s/^\s+//gm;
1552 The Road goes ever on and on,
1553 down from the door where it began.
1554 FINIS
1555
1556If you use a here-doc within a delimited construct, such as in C<s///eg>,
1557the quoted material must come on the lines following the final delimiter.
1558So instead of
1559
1560 s/this/<<E . 'that'
1561 the other
1562 E
1563 . 'more '/eg;
1564
1565you have to write
1566
1567 s/this/<<E . 'that'
1568 . 'more '/eg;
1569 the other
1570 E
1571
1572If the terminating identifier is on the last line of the program, you
1573must be sure there is a newline after it; otherwise, Perl will give the
1574warning B<Can't find string terminator "END" anywhere before EOF...>.
1575
1576Additionally, the quoting rules for the identifier are not related to
1577Perl's quoting rules -- C<q()>, C<qq()>, and the like are not supported
1578in place of C<''> and C<"">, and the only interpolation is for backslashing
1579the quoting character:
1580
1581 print << "abc\"def";
1582 testing...
1583 abc"def
1584
1585Finally, quoted strings cannot span multiple lines. The general rule is
1586that the identifier must be a string literal. Stick with that, and you
1587should be safe.
1588
a0d0e21e 1589=back
1590
75e14d17 1591=head2 Gory details of parsing quoted constructs
1592
19799a22 1593When presented with something that might have several different
1594interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
1595principle to pick the most probable interpretation. This strategy
1596is so successful that Perl programmers often do not suspect the
1597ambivalence of what they write. But from time to time, Perl's
1598notions differ substantially from what the author honestly meant.
1599
1600This section hopes to clarify how Perl handles quoted constructs.
1601Although the most common reason to learn this is to unravel labyrinthine
1602regular expressions, because the initial steps of parsing are the
1603same for all quoting operators, they are all discussed together.
1604
1605The most important Perl parsing rule is the first one discussed
1606below: when processing a quoted construct, Perl first finds the end
1607of that construct, then interprets its contents. If you understand
1608this rule, you may skip the rest of this section on the first
1609reading. The other rules are likely to contradict the user's
1610expectations much less frequently than this first one.
1611
1612Some passes discussed below are performed concurrently, but because
1613their results are the same, we consider them individually. For different
1614quoting constructs, Perl performs different numbers of passes, from
1615one to five, but these passes are always performed in the same order.
75e14d17 1616
13a2d996 1617=over 4
75e14d17 1618
1619=item Finding the end
1620
19799a22 1621The first pass is finding the end of the quoted construct, whether
1622it be a multicharacter delimiter C<"\nEOF\n"> in the C<<<EOF>
1623construct, a C</> that terminates a C<qq//> construct, a C<]> which
35f2feb0 1624terminates C<qq[]> construct, or a C<< > >> which terminates a
1625fileglob started with C<< < >>.
75e14d17 1626
19799a22 1627When searching for single-character non-pairing delimiters, such
1628as C</>, combinations of C<\\> and C<\/> are skipped. However,
1629when searching for single-character pairing delimiter like C<[>,
1630combinations of C<\\>, C<\]>, and C<\[> are all skipped, and nested
1631C<[>, C<]> are skipped as well. When searching for multicharacter
1632delimiters, nothing is skipped.
75e14d17 1633
19799a22 1634For constructs with three-part delimiters (C<s///>, C<y///>, and
1635C<tr///>), the search is repeated once more.
75e14d17 1636
19799a22 1637During this search no attention is paid to the semantics of the construct.
1638Thus:
75e14d17 1639
1640 "$hash{"$foo/$bar"}"
1641
2a94b7ce 1642or:
75e14d17 1643
1644 m/
2a94b7ce 1645 bar # NOT a comment, this slash / terminated m//!
75e14d17 1646 /x
1647
19799a22 1648do not form legal quoted expressions. The quoted part ends on the
1649first C<"> and C</>, and the rest happens to be a syntax error.
1650Because the slash that terminated C<m//> was followed by a C<SPACE>,
1651the example above is not C<m//x>, but rather C<m//> with no C</x>
1652modifier. So the embedded C<#> is interpreted as a literal C<#>.
75e14d17 1653
1654=item Removal of backslashes before delimiters
1655
19799a22 1656During the second pass, text between the starting and ending
1657delimiters is copied to a safe location, and the C<\> is removed
1658from combinations consisting of C<\> and delimiter--or delimiters,
1659meaning both starting and ending delimiters will should these differ.
1660This removal does not happen for multi-character delimiters.
1661Note that the combination C<\\> is left intact, just as it was.
75e14d17 1662
19799a22 1663Starting from this step no information about the delimiters is
1664used in parsing.
75e14d17 1665
1666=item Interpolation
1667
19799a22 1668The next step is interpolation in the text obtained, which is now
1669delimiter-independent. There are four different cases.
75e14d17 1670
13a2d996 1671=over 4
75e14d17 1672
1673=item C<<<'EOF'>, C<m''>, C<s'''>, C<tr///>, C<y///>
1674
1675No interpolation is performed.
1676
1677=item C<''>, C<q//>
1678
1679The only interpolation is removal of C<\> from pairs C<\\>.
1680
35f2feb0 1681=item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>
75e14d17 1682
19799a22 1683C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
1684converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
1685is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
1686The other combinations are replaced with appropriate expansions.
2a94b7ce 1687
19799a22 1688Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
1689is interpolated in the usual way. Something like C<"\Q\\E"> has
1690no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
1691result is the same as for C<"\\\\E">. As a general rule, backslashes
1692between C<\Q> and C<\E> may lead to counterintuitive results. So,
1693C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
1694as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
2a94b7ce 1695
1696 $str = '\t';
1697 return "\Q$str";
1698
1699may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
1700
19799a22 1701Interpolated scalars and arrays are converted internally to the C<join> and
92d29cee 1702C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
75e14d17 1703
19799a22 1704 $foo . " XXX '" . (join $", @arr) . "'";
75e14d17 1705
19799a22 1706All operations above are performed simultaneously, left to right.
75e14d17 1707
19799a22 1708Because the result of C<"\Q STRING \E"> has all metacharacters
1709quoted, there is no way to insert a literal C<$> or C<@> inside a
1710C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
1711C<"\\\$">; if not, it is interpreted as the start of an interpolated
1712scalar.
75e14d17 1713
19799a22 1714Note also that the interpolation code needs to make a decision on
1715where the interpolated scalar ends. For instance, whether
35f2feb0 1716C<< "a $b -> {c}" >> really means:
75e14d17 1717
1718 "a " . $b . " -> {c}";
1719
2a94b7ce 1720or:
75e14d17 1721
1722 "a " . $b -> {c};
1723
19799a22 1724Most of the time, the longest possible text that does not include
1725spaces between components and which contains matching braces or
1726brackets. because the outcome may be determined by voting based
1727on heuristic estimators, the result is not strictly predictable.
1728Fortunately, it's usually correct for ambiguous cases.
75e14d17 1729
1730=item C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
1731
19799a22 1732Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
1733happens (almost) as with C<qq//> constructs, but the substitution
1734of C<\> followed by RE-special chars (including C<\>) is not
1735performed. Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
1736a C<#>-comment in a C<//x>-regular expression, no processing is
1737performed whatsoever. This is the first step at which the presence
1738of the C<//x> modifier is relevant.
1739
1740Interpolation has several quirks: C<$|>, C<$(>, and C<$)> are not
1741interpolated, and constructs C<$var[SOMETHING]> are voted (by several
1742different estimators) to be either an array element or C<$var>
1743followed by an RE alternative. This is where the notation
1744C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
1745array element C<-9>, not as a regular expression from the variable
1746C<$arr> followed by a digit, which would be the interpretation of
1747C</$arr[0-9]/>. Since voting among different estimators may occur,
1748the result is not predictable.
1749
1750It is at this step that C<\1> is begrudgingly converted to C<$1> in
1751the replacement text of C<s///> to correct the incorrigible
1752I<sed> hackers who haven't picked up the saner idiom yet. A warning
9f1b1f2d 1753is emitted if the C<use warnings> pragma or the B<-w> command-line flag
1754(that is, the C<$^W> variable) was set.
19799a22 1755
1756The lack of processing of C<\\> creates specific restrictions on
1757the post-processed text. If the delimiter is C</>, one cannot get
1758the combination C<\/> into the result of this step. C</> will
1759finish the regular expression, C<\/> will be stripped to C</> on
1760the previous step, and C<\\/> will be left as is. Because C</> is
1761equivalent to C<\/> inside a regular expression, this does not
1762matter unless the delimiter happens to be character special to the
1763RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
1764alphanumeric char, as in:
2a94b7ce 1765
1766 m m ^ a \s* b mmx;
1767
19799a22 1768In the RE above, which is intentionally obfuscated for illustration, the
2a94b7ce 1769delimiter is C<m>, the modifier is C<mx>, and after backslash-removal the
aa863641 1770RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
19799a22 1771reason you're encouraged to restrict your delimiters to non-alphanumeric,
1772non-whitespace choices.
75e14d17 1773
1774=back
1775
19799a22 1776This step is the last one for all constructs except regular expressions,
75e14d17 1777which are processed further.
1778
1779=item Interpolation of regular expressions
1780
19799a22 1781Previous steps were performed during the compilation of Perl code,
1782but this one happens at run time--although it may be optimized to
1783be calculated at compile time if appropriate. After preprocessing
1784described above, and possibly after evaluation if catenation,
1785joining, casing translation, or metaquoting are involved, the
1786resulting I<string> is passed to the RE engine for compilation.
1787
1788Whatever happens in the RE engine might be better discussed in L<perlre>,
1789but for the sake of continuity, we shall do so here.
1790
1791This is another step where the presence of the C<//x> modifier is
1792relevant. The RE engine scans the string from left to right and
1793converts it to a finite automaton.
1794
1795Backslashed characters are either replaced with corresponding
1796literal strings (as with C<\{>), or else they generate special nodes
1797in the finite automaton (as with C<\b>). Characters special to the
1798RE engine (such as C<|>) generate corresponding nodes or groups of
1799nodes. C<(?#...)> comments are ignored. All the rest is either
1800converted to literal strings to match, or else is ignored (as is
1801whitespace and C<#>-style comments if C<//x> is present).
1802
1803Parsing of the bracketed character class construct, C<[...]>, is
1804rather different than the rule used for the rest of the pattern.
1805The terminator of this construct is found using the same rules as
1806for finding the terminator of a C<{}>-delimited construct, the only
1807exception being that C<]> immediately following C<[> is treated as
1808though preceded by a backslash. Similarly, the terminator of
1809C<(?{...})> is found using the same rules as for finding the
1810terminator of a C<{}>-delimited construct.
1811
1812It is possible to inspect both the string given to RE engine and the
1813resulting finite automaton. See the arguments C<debug>/C<debugcolor>
1814in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
4a4eefd0 1815switch documented in L<perlrun/"Command Switches">.
75e14d17 1816
1817=item Optimization of regular expressions
1818
7522fed5 1819This step is listed for completeness only. Since it does not change
75e14d17 1820semantics, details of this step are not documented and are subject
19799a22 1821to change without notice. This step is performed over the finite
1822automaton that was generated during the previous pass.
2a94b7ce 1823
19799a22 1824It is at this stage that C<split()> silently optimizes C</^/> to
1825mean C</^/m>.
75e14d17 1826
1827=back
1828
a0d0e21e 1829=head2 I/O Operators
1830
54310121 1831There are several I/O operators you should know about.
fbad3eb5 1832
7b8d334a 1833A string enclosed by backticks (grave accents) first undergoes
19799a22 1834double-quote interpolation. It is then interpreted as an external
1835command, and the output of that command is the value of the
e9c56f9b 1836backtick string, like in a shell. In scalar context, a single string
1837consisting of all output is returned. In list context, a list of
1838values is returned, one per line of output. (You can set C<$/> to use
1839a different line terminator.) The command is executed each time the
1840pseudo-literal is evaluated. The status value of the command is
1841returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
1842Unlike in B<csh>, no translation is done on the return data--newlines
1843remain newlines. Unlike in any of the shells, single quotes do not
1844hide variable names in the command from interpretation. To pass a
1845literal dollar-sign through to the shell you need to hide it with a
1846backslash. The generalized form of backticks is C<qx//>. (Because
1847backticks always undergo shell expansion as well, see L<perlsec> for
1848security concerns.)
19799a22 1849
1850In scalar context, evaluating a filehandle in angle brackets yields
1851the next line from that file (the newline, if any, included), or
1852C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
1853(sometimes known as file-slurp mode) and the file is empty, it
1854returns C<''> the first time, followed by C<undef> subsequently.
1855
1856Ordinarily you must assign the returned value to a variable, but
1857there is one situation where an automatic assignment happens. If
1858and only if the input symbol is the only thing inside the conditional
1859of a C<while> statement (even if disguised as a C<for(;;)> loop),
1860the value is automatically assigned to the global variable $_,
1861destroying whatever was there previously. (This may seem like an
1862odd thing to you, but you'll use the construct in almost every Perl
17b829fa 1863script you write.) The $_ variable is not implicitly localized.
19799a22 1864You'll have to put a C<local $_;> before the loop if you want that
1865to happen.
1866
1867The following lines are equivalent:
a0d0e21e 1868
748a9306 1869 while (defined($_ = <STDIN>)) { print; }
7b8d334a 1870 while ($_ = <STDIN>) { print; }
a0d0e21e 1871 while (<STDIN>) { print; }
1872 for (;<STDIN>;) { print; }
748a9306 1873 print while defined($_ = <STDIN>);
7b8d334a 1874 print while ($_ = <STDIN>);
a0d0e21e 1875 print while <STDIN>;
1876
19799a22 1877This also behaves similarly, but avoids $_ :
7b8d334a 1878
1879 while (my $line = <STDIN>) { print $line }
1880
19799a22 1881In these loop constructs, the assigned value (whether assignment
1882is automatic or explicit) is then tested to see whether it is
1883defined. The defined test avoids problems where line has a string
1884value that would be treated as false by Perl, for example a "" or
1885a "0" with no trailing newline. If you really mean for such values
1886to terminate the loop, they should be tested for explicitly:
7b8d334a 1887
1888 while (($_ = <STDIN>) ne '0') { ... }
1889 while (<STDIN>) { last unless $_; ... }
1890
35f2feb0 1891In other boolean contexts, C<< <I<filehandle>> >> without an
9f1b1f2d 1892explicit C<defined> test or comparison elicit a warning if the
1893C<use warnings> pragma or the B<-w>
19799a22 1894command-line switch (the C<$^W> variable) is in effect.
7b8d334a 1895
5f05dabc 1896The filehandles STDIN, STDOUT, and STDERR are predefined. (The
19799a22 1897filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
1898in packages, where they would be interpreted as local identifiers
1899rather than global.) Additional filehandles may be created with
1900the open() function, amongst others. See L<perlopentut> and
1901L<perlfunc/open> for details on this.
a0d0e21e 1902
35f2feb0 1903If a <FILEHANDLE> is used in a context that is looking for
19799a22 1904a list, a list comprising all input lines is returned, one line per
1905list element. It's easy to grow to a rather large data space this
1906way, so use with care.
a0d0e21e 1907
35f2feb0 1908<FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
19799a22 1909See L<perlfunc/readline>.
fbad3eb5 1910
35f2feb0 1911The null filehandle <> is special: it can be used to emulate the
1912behavior of B<sed> and B<awk>. Input from <> comes either from
a0d0e21e 1913standard input, or from each file listed on the command line. Here's
35f2feb0 1914how it works: the first time <> is evaluated, the @ARGV array is
5a964f20 1915checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
a0d0e21e 1916gives you standard input. The @ARGV array is then processed as a list
1917of filenames. The loop
1918
1919 while (<>) {
1920 ... # code for each line
1921 }
1922
1923is equivalent to the following Perl-like pseudo code:
1924
3e3baf6d 1925 unshift(@ARGV, '-') unless @ARGV;
a0d0e21e 1926 while ($ARGV = shift) {
1927 open(ARGV, $ARGV);
1928 while (<ARGV>) {
1929 ... # code for each line
1930 }
1931 }
1932
19799a22 1933except that it isn't so cumbersome to say, and will actually work.
1934It really does shift the @ARGV array and put the current filename
1935into the $ARGV variable. It also uses filehandle I<ARGV>
35f2feb0 1936internally--<> is just a synonym for <ARGV>, which
19799a22 1937is magical. (The pseudo code above doesn't work because it treats
35f2feb0 1938<ARGV> as non-magical.)
a0d0e21e 1939
35f2feb0 1940You can modify @ARGV before the first <> as long as the array ends up
a0d0e21e 1941containing the list of filenames you really want. Line numbers (C<$.>)
19799a22 1942continue as though the input were one big happy file. See the example
1943in L<perlfunc/eof> for how to reset line numbers on each file.
5a964f20 1944
1945If you want to set @ARGV to your own list of files, go right ahead.
1946This sets @ARGV to all plain text files if no @ARGV was given:
1947
1948 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
a0d0e21e 1949
5a964f20 1950You can even set them to pipe commands. For example, this automatically
1951filters compressed arguments through B<gzip>:
1952
1953 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
1954
1955If you want to pass switches into your script, you can use one of the
a0d0e21e 1956Getopts modules or put a loop on the front like this:
1957
1958 while ($_ = $ARGV[0], /^-/) {
1959 shift;
1960 last if /^--$/;
1961 if (/^-D(.*)/) { $debug = $1 }
1962 if (/^-v/) { $verbose++ }
5a964f20 1963 # ... # other switches
a0d0e21e 1964 }
5a964f20 1965
a0d0e21e 1966 while (<>) {
5a964f20 1967 # ... # code for each line
a0d0e21e 1968 }
1969
35f2feb0 1970The <> symbol will return C<undef> for end-of-file only once.
19799a22 1971If you call it again after this, it will assume you are processing another
1972@ARGV list, and if you haven't set @ARGV, will read input from STDIN.
a0d0e21e 1973
b159ebd3 1974If what the angle brackets contain is a simple scalar variable (e.g.,
35f2feb0 1975<$foo>), then that variable contains the name of the
19799a22 1976filehandle to input from, or its typeglob, or a reference to the
1977same. For example:
cb1a09d0 1978
1979 $fh = \*STDIN;
1980 $line = <$fh>;
a0d0e21e 1981
5a964f20 1982If what's within the angle brackets is neither a filehandle nor a simple
1983scalar variable containing a filehandle name, typeglob, or typeglob
1984reference, it is interpreted as a filename pattern to be globbed, and
1985either a list of filenames or the next filename in the list is returned,
19799a22 1986depending on context. This distinction is determined on syntactic
35f2feb0 1987grounds alone. That means C<< <$x> >> is always a readline() from
1988an indirect handle, but C<< <$hash{key}> >> is always a glob().
5a964f20 1989That's because $x is a simple scalar variable, but C<$hash{key}> is
1990not--it's a hash element.
1991
1992One level of double-quote interpretation is done first, but you can't
35f2feb0 1993say C<< <$foo> >> because that's an indirect filehandle as explained
5a964f20 1994in the previous paragraph. (In older versions of Perl, programmers
1995would insert curly brackets to force interpretation as a filename glob:
35f2feb0 1996C<< <${foo}> >>. These days, it's considered cleaner to call the
5a964f20 1997internal function directly as C<glob($foo)>, which is probably the right
19799a22 1998way to have done it in the first place.) For example:
a0d0e21e 1999
2000 while (<*.c>) {
2001 chmod 0644, $_;
2002 }
2003
3a4b19e4 2004is roughly equivalent to:
a0d0e21e 2005
2006 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
2007 while (<FOO>) {
5b3eff12 2008 chomp;
a0d0e21e 2009 chmod 0644, $_;
2010 }
2011
3a4b19e4 2012except that the globbing is actually done internally using the standard
2013C<File::Glob> extension. Of course, the shortest way to do the above is:
a0d0e21e 2014
2015 chmod 0644, <*.c>;
2016
19799a22 2017A (file)glob evaluates its (embedded) argument only when it is
2018starting a new list. All values must be read before it will start
2019over. In list context, this isn't important because you automatically
2020get them all anyway. However, in scalar context the operator returns
069e01df 2021the next value each time it's called, or C<undef> when the list has
19799a22 2022run out. As with filehandle reads, an automatic C<defined> is
2023generated when the glob occurs in the test part of a C<while>,
2024because legal glob returns (e.g. a file called F<0>) would otherwise
2025terminate the loop. Again, C<undef> is returned only once. So if
2026you're expecting a single value from a glob, it is much better to
2027say
4633a7c4 2028
2029 ($file) = <blurch*>;
2030
2031than
2032
2033 $file = <blurch*>;
2034
2035because the latter will alternate between returning a filename and
19799a22 2036returning false.
4633a7c4 2037
b159ebd3 2038If you're trying to do variable interpolation, it's definitely better
4633a7c4 2039to use the glob() function, because the older notation can cause people
e37d713d 2040to become confused with the indirect filehandle notation.
4633a7c4 2041
2042 @files = glob("$dir/*.[ch]");
2043 @files = glob($files[$i]);
2044
a0d0e21e 2045=head2 Constant Folding
2046
2047Like C, Perl does a certain amount of expression evaluation at
19799a22 2048compile time whenever it determines that all arguments to an
a0d0e21e 2049operator are static and have no side effects. In particular, string
2050concatenation happens at compile time between literals that don't do
19799a22 2051variable substitution. Backslash interpolation also happens at
a0d0e21e 2052compile time. You can say
2053
2054 'Now is the time for all' . "\n" .
2055 'good men to come to.'
2056
54310121 2057and this all reduces to one string internally. Likewise, if
a0d0e21e 2058you say
2059
2060 foreach $file (@filenames) {
5a964f20 2061 if (-s $file > 5 + 100 * 2**16) { }
54310121 2062 }
a0d0e21e 2063
19799a22 2064the compiler will precompute the number which that expression
2065represents so that the interpreter won't have to.
a0d0e21e 2066
fd1abbef 2067=head2 No-ops
2068
2069Perl doesn't officially have a no-op operator, but the bare constants
2070C<0> and C<1> are special-cased to not produce a warning in a void
2071context, so you can for example safely do
2072
2073 1 while foo();
2074
2c268ad5 2075=head2 Bitwise String Operators
2076
2077Bitstrings of any size may be manipulated by the bitwise operators
2078(C<~ | & ^>).
2079
19799a22 2080If the operands to a binary bitwise op are strings of different
2081sizes, B<|> and B<^> ops act as though the shorter operand had
2082additional zero bits on the right, while the B<&> op acts as though
2083the longer operand were truncated to the length of the shorter.
2084The granularity for such extension or truncation is one or more
2085bytes.
2c268ad5 2086
2087 # ASCII-based examples
2088 print "j p \n" ^ " a h"; # prints "JAPH\n"
2089 print "JA" | " ph\n"; # prints "japh\n"
2090 print "japh\nJunk" & '_____'; # prints "JAPH\n";
2091 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
2092
19799a22 2093If you are intending to manipulate bitstrings, be certain that
2c268ad5 2094you're supplying bitstrings: If an operand is a number, that will imply
19799a22 2095a B<numeric> bitwise operation. You may explicitly show which type of
2c268ad5 2096operation you intend by using C<""> or C<0+>, as in the examples below.
2097
2098 $foo = 150 | 105 ; # yields 255 (0x96 | 0x69 is 0xFF)
2099 $foo = '150' | 105 ; # yields 255
2100 $foo = 150 | '105'; # yields 255
2101 $foo = '150' | '105'; # yields string '155' (under ASCII)
2102
2103 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
2104 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
a0d0e21e 2105
1ae175c8 2106See L<perlfunc/vec> for information on how to manipulate individual bits
2107in a bit vector.
2108
55497cff 2109=head2 Integer Arithmetic
a0d0e21e 2110
19799a22 2111By default, Perl assumes that it must do most of its arithmetic in
a0d0e21e 2112floating point. But by saying
2113
2114 use integer;
2115
2116you may tell the compiler that it's okay to use integer operations
19799a22 2117(if it feels like it) from here to the end of the enclosing BLOCK.
2118An inner BLOCK may countermand this by saying
a0d0e21e 2119
2120 no integer;
2121
19799a22 2122which lasts until the end of that BLOCK. Note that this doesn't
2123mean everything is only an integer, merely that Perl may use integer
2124operations if it is so inclined. For example, even under C<use
2125integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731>
2126or so.
2127
2128Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
13a2d996 2129and ">>") always produce integral results. (But see also
2130L<Bitwise String Operators>.) However, C<use integer> still has meaning for
19799a22 2131them. By default, their results are interpreted as unsigned integers, but
2132if C<use integer> is in effect, their results are interpreted
2133as signed integers. For example, C<~0> usually evaluates to a large
2134integral value. However, C<use integer; ~0> is C<-1> on twos-complement
2135machines.
68dc0745 2136
2137=head2 Floating-point Arithmetic
2138
2139While C<use integer> provides integer-only arithmetic, there is no
19799a22 2140analogous mechanism to provide automatic rounding or truncation to a
2141certain number of decimal places. For rounding to a certain number
2142of digits, sprintf() or printf() is usually the easiest route.
2143See L<perlfaq4>.
68dc0745 2144
5a964f20 2145Floating-point numbers are only approximations to what a mathematician
2146would call real numbers. There are infinitely more reals than floats,
2147so some corners must be cut. For example:
2148
2149 printf "%.20g\n", 123456789123456789;
2150 # produces 123456789123456784
2151
2152Testing for exact equality of floating-point equality or inequality is
2153not a good idea. Here's a (relatively expensive) work-around to compare
2154whether two floating-point numbers are equal to a particular number of
2155decimal places. See Knuth, volume II, for a more robust treatment of
2156this topic.
2157
2158 sub fp_equal {
2159 my ($X, $Y, $POINTS) = @_;
2160 my ($tX, $tY);
2161 $tX = sprintf("%.${POINTS}g", $X);
2162 $tY = sprintf("%.${POINTS}g", $Y);
2163 return $tX eq $tY;
2164 }
2165
68dc0745 2166The POSIX module (part of the standard perl distribution) implements
19799a22 2167ceil(), floor(), and other mathematical and trigonometric functions.
2168The Math::Complex module (part of the standard perl distribution)
2169defines mathematical functions that work on both the reals and the
2170imaginary numbers. Math::Complex not as efficient as POSIX, but
68dc0745 2171POSIX can't work with complex numbers.
2172
2173Rounding in financial applications can have serious implications, and
2174the rounding method used should be specified precisely. In these
2175cases, it probably pays not to trust whichever system rounding is
2176being used by Perl, but to instead implement the rounding function you
2177need yourself.
5a964f20 2178
2179=head2 Bigger Numbers
2180
2181The standard Math::BigInt and Math::BigFloat modules provide
19799a22 2182variable-precision arithmetic and overloaded operators, although
cd5c4fce 2183they're currently pretty slow. At the cost of some space and
19799a22 2184considerable speed, they avoid the normal pitfalls associated with
2185limited-precision representations.
5a964f20 2186
2187 use Math::BigInt;
2188 $x = Math::BigInt->new('123456789123456789');
2189 print $x * $x;
2190
2191 # prints +15241578780673678515622620750190521
19799a22 2192
cd5c4fce 2193There are several modules that let you calculate with (bound only by
2194memory and cpu-time) unlimited or fixed precision. There are also
2195some non-standard modules that provide faster implementations via
2196external C libraries.
2197
2198Here is a short, but incomplete summary:
2199
2200 Math::Fraction big, unlimited fractions like 9973 / 12967
2201 Math::String treat string sequences like numbers
2202 Math::FixedPrecision calculate with a fixed precision
2203 Math::Currency for currency calculations
2204 Bit::Vector manipulate bit vectors fast (uses C)
2205 Math::BigIntFast Bit::Vector wrapper for big numbers
2206 Math::Pari provides access to the Pari C library
2207 Math::BigInteger uses an external C library
2208 Math::Cephes uses external Cephes C library (no big numbers)
2209 Math::Cephes::Fraction fractions via the Cephes library
2210 Math::GMP another one using an external C library
2211
2212Choose wisely.
16070b82 2213
2214=cut