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perlport.pod nit for doc bug #8253.
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1=head1 NAME
2
3perlop - Perl operators and precedence
4
5=head1 SYNOPSIS
6
7Perl operators have the following associativity and precedence,
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8listed from highest precedence to lowest. Operators borrowed from
9C keep the same precedence relationship with each other, even where
10C's precedence is slightly screwy. (This makes learning Perl easier
11for C folks.) With very few exceptions, these all operate on scalar
12values only, not array values.
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13
14 left terms and list operators (leftward)
15 left ->
16 nonassoc ++ --
17 right **
18 right ! ~ \ and unary + and -
54310121 19 left =~ !~
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20 left * / % x
21 left + - .
22 left << >>
23 nonassoc named unary operators
24 nonassoc < > <= >= lt gt le ge
25 nonassoc == != <=> eq ne cmp
26 left &
27 left | ^
28 left &&
c963b151 29 left || //
137443ea 30 nonassoc .. ...
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31 right ?:
32 right = += -= *= etc.
33 left , =>
34 nonassoc list operators (rightward)
a5f75d66 35 right not
a0d0e21e 36 left and
c963b151 37 left or xor err
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38
39In the following sections, these operators are covered in precedence order.
40
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41Many operators can be overloaded for objects. See L<overload>.
42
cb1a09d0 43=head1 DESCRIPTION
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44
45=head2 Terms and List Operators (Leftward)
46
62c18ce2 47A TERM has the highest precedence in Perl. They include variables,
5f05dabc 48quote and quote-like operators, any expression in parentheses,
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49and any function whose arguments are parenthesized. Actually, there
50aren't really functions in this sense, just list operators and unary
51operators behaving as functions because you put parentheses around
52the arguments. These are all documented in L<perlfunc>.
53
54If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
55is followed by a left parenthesis as the next token, the operator and
56arguments within parentheses are taken to be of highest precedence,
57just like a normal function call.
58
59In the absence of parentheses, the precedence of list operators such as
60C<print>, C<sort>, or C<chmod> is either very high or very low depending on
54310121 61whether you are looking at the left side or the right side of the operator.
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62For example, in
63
64 @ary = (1, 3, sort 4, 2);
65 print @ary; # prints 1324
66
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67the commas on the right of the sort are evaluated before the sort,
68but the commas on the left are evaluated after. In other words,
69list operators tend to gobble up all arguments that follow, and
a0d0e21e 70then act like a simple TERM with regard to the preceding expression.
19799a22 71Be careful with parentheses:
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72
73 # These evaluate exit before doing the print:
74 print($foo, exit); # Obviously not what you want.
75 print $foo, exit; # Nor is this.
76
77 # These do the print before evaluating exit:
78 (print $foo), exit; # This is what you want.
79 print($foo), exit; # Or this.
80 print ($foo), exit; # Or even this.
81
82Also note that
83
84 print ($foo & 255) + 1, "\n";
85
54310121 86probably doesn't do what you expect at first glance. See
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87L<Named Unary Operators> for more discussion of this.
88
89Also parsed as terms are the C<do {}> and C<eval {}> constructs, as
54310121 90well as subroutine and method calls, and the anonymous
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91constructors C<[]> and C<{}>.
92
2ae324a7 93See also L<Quote and Quote-like Operators> toward the end of this section,
c07a80fd 94as well as L<"I/O Operators">.
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95
96=head2 The Arrow Operator
97
35f2feb0 98"C<< -> >>" is an infix dereference operator, just as it is in C
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99and C++. If the right side is either a C<[...]>, C<{...}>, or a
100C<(...)> subscript, then the left side must be either a hard or
101symbolic reference to an array, a hash, or a subroutine respectively.
102(Or technically speaking, a location capable of holding a hard
103reference, if it's an array or hash reference being used for
104assignment.) See L<perlreftut> and L<perlref>.
a0d0e21e 105
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106Otherwise, the right side is a method name or a simple scalar
107variable containing either the method name or a subroutine reference,
108and the left side must be either an object (a blessed reference)
109or a class name (that is, a package name). See L<perlobj>.
a0d0e21e 110
5f05dabc 111=head2 Auto-increment and Auto-decrement
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112
113"++" and "--" work as in C. That is, if placed before a variable, they
114increment or decrement the variable before returning the value, and if
115placed after, increment or decrement the variable after returning the value.
116
54310121 117The auto-increment operator has a little extra builtin magic to it. If
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118you increment a variable that is numeric, or that has ever been used in
119a numeric context, you get a normal increment. If, however, the
5f05dabc 120variable has been used in only string contexts since it was set, and
5a964f20 121has a value that is not the empty string and matches the pattern
9c0670e1 122C</^[a-zA-Z]*[0-9]*\z/>, the increment is done as a string, preserving each
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123character within its range, with carry:
124
125 print ++($foo = '99'); # prints '100'
126 print ++($foo = 'a0'); # prints 'a1'
127 print ++($foo = 'Az'); # prints 'Ba'
128 print ++($foo = 'zz'); # prints 'aaa'
129
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130C<undef> is always treated as numeric, and in particular is changed
131to C<0> before incrementing (so that a post-increment of an undef value
132will return C<0> rather than C<undef>).
133
5f05dabc 134The auto-decrement operator is not magical.
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135
136=head2 Exponentiation
137
19799a22 138Binary "**" is the exponentiation operator. It binds even more
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139tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is
140implemented using C's pow(3) function, which actually works on doubles
141internally.)
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142
143=head2 Symbolic Unary Operators
144
5f05dabc 145Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower
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146precedence version of this.
147
148Unary "-" performs arithmetic negation if the operand is numeric. If
149the operand is an identifier, a string consisting of a minus sign
150concatenated with the identifier is returned. Otherwise, if the string
151starts with a plus or minus, a string starting with the opposite sign
152is returned. One effect of these rules is that C<-bareword> is equivalent
153to C<"-bareword">.
154
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155Unary "~" performs bitwise negation, i.e., 1's complement. For
156example, C<0666 & ~027> is 0640. (See also L<Integer Arithmetic> and
157L<Bitwise String Operators>.) Note that the width of the result is
158platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64
159bits wide on a 64-bit platform, so if you are expecting a certain bit
160width, remember use the & operator to mask off the excess bits.
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161
162Unary "+" has no effect whatsoever, even on strings. It is useful
163syntactically for separating a function name from a parenthesized expression
164that would otherwise be interpreted as the complete list of function
5ba421f6 165arguments. (See examples above under L<Terms and List Operators (Leftward)>.)
a0d0e21e 166
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167Unary "\" creates a reference to whatever follows it. See L<perlreftut>
168and L<perlref>. Do not confuse this behavior with the behavior of
169backslash within a string, although both forms do convey the notion
170of protecting the next thing from interpolation.
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171
172=head2 Binding Operators
173
c07a80fd 174Binary "=~" binds a scalar expression to a pattern match. Certain operations
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175search or modify the string $_ by default. This operator makes that kind
176of operation work on some other string. The right argument is a search
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177pattern, substitution, or transliteration. The left argument is what is
178supposed to be searched, substituted, or transliterated instead of the default
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179$_. When used in scalar context, the return value generally indicates the
180success of the operation. Behavior in list context depends on the particular
181operator. See L</"Regexp Quote-Like Operators"> for details.
182
183If the right argument is an expression rather than a search pattern,
2c268ad5 184substitution, or transliteration, it is interpreted as a search pattern at run
573e01ca 185time.
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186
187Binary "!~" is just like "=~" except the return value is negated in
188the logical sense.
189
190=head2 Multiplicative Operators
191
192Binary "*" multiplies two numbers.
193
194Binary "/" divides two numbers.
195
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196Binary "%" computes the modulus of two numbers. Given integer
197operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
198C<$a> minus the largest multiple of C<$b> that is not greater than
199C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
200smallest multiple of C<$b> that is not less than C<$a> (i.e. the
6bb4e6d4 201result will be less than or equal to zero).
0412d526 202Note that when C<use integer> is in scope, "%" gives you direct access
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203to the modulus operator as implemented by your C compiler. This
204operator is not as well defined for negative operands, but it will
205execute faster.
206
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207Binary "x" is the repetition operator. In scalar context or if the left
208operand is not enclosed in parentheses, it returns a string consisting
209of the left operand repeated the number of times specified by the right
210operand. In list context, if the left operand is enclosed in
211parentheses, it repeats the list.
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212
213 print '-' x 80; # print row of dashes
214
215 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
216
217 @ones = (1) x 80; # a list of 80 1's
218 @ones = (5) x @ones; # set all elements to 5
219
220
221=head2 Additive Operators
222
223Binary "+" returns the sum of two numbers.
224
225Binary "-" returns the difference of two numbers.
226
227Binary "." concatenates two strings.
228
229=head2 Shift Operators
230
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231Binary "<<" returns the value of its left argument shifted left by the
232number of bits specified by the right argument. Arguments should be
982ce180 233integers. (See also L<Integer Arithmetic>.)
a0d0e21e 234
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235Binary ">>" returns the value of its left argument shifted right by
236the number of bits specified by the right argument. Arguments should
982ce180 237be integers. (See also L<Integer Arithmetic>.)
a0d0e21e 238
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239Note that both "<<" and ">>" in Perl are implemented directly using
240"<<" and ">>" in C. If C<use integer> (see L<Integer Arithmetic>) is
241in force then signed C integers are used, else unsigned C integers are
242used. Either way, the implementation isn't going to generate results
243larger than the size of the integer type Perl was built with (32 bits
244or 64 bits).
245
246The result of overflowing the range of the integers is undefined
247because it is undefined also in C. In other words, using 32-bit
248integers, C<< 1 << 32 >> is undefined. Shifting by a negative number
249of bits is also undefined.
250
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251=head2 Named Unary Operators
252
253The various named unary operators are treated as functions with one
254argument, with optional parentheses. These include the filetest
255operators, like C<-f>, C<-M>, etc. See L<perlfunc>.
256
257If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
258is followed by a left parenthesis as the next token, the operator and
259arguments within parentheses are taken to be of highest precedence,
3981b0eb 260just like a normal function call. For example,
261because named unary operators are higher precedence than ||:
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262
263 chdir $foo || die; # (chdir $foo) || die
264 chdir($foo) || die; # (chdir $foo) || die
265 chdir ($foo) || die; # (chdir $foo) || die
266 chdir +($foo) || die; # (chdir $foo) || die
267
3981b0eb 268but, because * is higher precedence than named operators:
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269
270 chdir $foo * 20; # chdir ($foo * 20)
271 chdir($foo) * 20; # (chdir $foo) * 20
272 chdir ($foo) * 20; # (chdir $foo) * 20
273 chdir +($foo) * 20; # chdir ($foo * 20)
274
275 rand 10 * 20; # rand (10 * 20)
276 rand(10) * 20; # (rand 10) * 20
277 rand (10) * 20; # (rand 10) * 20
278 rand +(10) * 20; # rand (10 * 20)
279
5ba421f6 280See also L<"Terms and List Operators (Leftward)">.
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281
282=head2 Relational Operators
283
35f2feb0 284Binary "<" returns true if the left argument is numerically less than
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285the right argument.
286
35f2feb0 287Binary ">" returns true if the left argument is numerically greater
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288than the right argument.
289
35f2feb0 290Binary "<=" returns true if the left argument is numerically less than
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291or equal to the right argument.
292
35f2feb0 293Binary ">=" returns true if the left argument is numerically greater
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294than or equal to the right argument.
295
296Binary "lt" returns true if the left argument is stringwise less than
297the right argument.
298
299Binary "gt" returns true if the left argument is stringwise greater
300than the right argument.
301
302Binary "le" returns true if the left argument is stringwise less than
303or equal to the right argument.
304
305Binary "ge" returns true if the left argument is stringwise greater
306than or equal to the right argument.
307
308=head2 Equality Operators
309
310Binary "==" returns true if the left argument is numerically equal to
311the right argument.
312
313Binary "!=" returns true if the left argument is numerically not equal
314to the right argument.
315
35f2feb0 316Binary "<=>" returns -1, 0, or 1 depending on whether the left
6ee5d4e7 317argument is numerically less than, equal to, or greater than the right
d4ad863d 318argument. If your platform supports NaNs (not-a-numbers) as numeric
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319values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
320"<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
321returns true, as does NaN != anything else. If your platform doesn't
322support NaNs then NaN is just a string with numeric value 0.
323
324 perl -le '$a = NaN; print "No NaN support here" if $a == $a'
325 perl -le '$a = NaN; print "NaN support here" if $a != $a'
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326
327Binary "eq" returns true if the left argument is stringwise equal to
328the right argument.
329
330Binary "ne" returns true if the left argument is stringwise not equal
331to the right argument.
332
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333Binary "cmp" returns -1, 0, or 1 depending on whether the left
334argument is stringwise less than, equal to, or greater than the right
335argument.
a0d0e21e 336
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337"lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
338by the current locale if C<use locale> is in effect. See L<perllocale>.
339
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340=head2 Bitwise And
341
2cdc098b 342Binary "&" returns its operands ANDed together bit by bit.
2c268ad5 343(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
a0d0e21e 344
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345Note that "&" has lower priority than relational operators, so for example
346the brackets are essential in a test like
347
348 print "Even\n" if ($x & 1) == 0;
349
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350=head2 Bitwise Or and Exclusive Or
351
2cdc098b 352Binary "|" returns its operands ORed together bit by bit.
2c268ad5 353(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
a0d0e21e 354
2cdc098b 355Binary "^" returns its operands XORed together bit by bit.
2c268ad5 356(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
a0d0e21e 357
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358Note that "|" and "^" have lower priority than relational operators, so
359for example the brackets are essential in a test like
360
361 print "false\n" if (8 | 2) != 10;
362
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363=head2 C-style Logical And
364
365Binary "&&" performs a short-circuit logical AND operation. That is,
366if the left operand is false, the right operand is not even evaluated.
367Scalar or list context propagates down to the right operand if it
368is evaluated.
369
370=head2 C-style Logical Or
371
372Binary "||" performs a short-circuit logical OR operation. That is,
373if the left operand is true, the right operand is not even evaluated.
374Scalar or list context propagates down to the right operand if it
375is evaluated.
376
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377=head2 C-style Logical Defined-Or
378
379Although it has no direct equivalent in C, Perl's C<//> operator is related
380to its C-style or. In fact, it's exactly the same as C<||>, except that it
381tests the left hand side's definedness instead of its truth. Thus, C<$a // $b>
382is similar to C<defined($a) || $b> (except that it returns the value of C<$a>
383rather than the value of C<defined($a)>) and is exactly equivalent to
384C<defined($a) ? $a : $b>. This is very useful for providing default values
385for variables. If you actually want to test if at least one of C<$a> and C<$b> is
386defined, use C<defined($a // $b)>.
387
388The C<||>, C<//> and C<&&> operators differ from C's in that, rather than returning
a0d0e21e 3890 or 1, they return the last value evaluated. Thus, a reasonably portable
c963b151 390way to find out the home directory might be:
a0d0e21e 391
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392 $home = $ENV{'HOME'} // $ENV{'LOGDIR'} //
393 (getpwuid($<))[7] // die "You're homeless!\n";
a0d0e21e 394
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395In particular, this means that you shouldn't use this
396for selecting between two aggregates for assignment:
397
398 @a = @b || @c; # this is wrong
399 @a = scalar(@b) || @c; # really meant this
400 @a = @b ? @b : @c; # this works fine, though
401
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402As more readable alternatives to C<&&>, C<//> and C<||> when used for
403control flow, Perl provides C<and>, C<err> and C<or> operators (see below).
404The short-circuit behavior is identical. The precedence of "and", "err"
405and "or" is much lower, however, so that you can safely use them after a
5a964f20 406list operator without the need for parentheses:
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407
408 unlink "alpha", "beta", "gamma"
409 or gripe(), next LINE;
410
411With the C-style operators that would have been written like this:
412
413 unlink("alpha", "beta", "gamma")
414 || (gripe(), next LINE);
415
eeb6a2c9 416Using "or" for assignment is unlikely to do what you want; see below.
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417
418=head2 Range Operators
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419
420Binary ".." is the range operator, which is really two different
5a964f20 421operators depending on the context. In list context, it returns an
54ae734e 422list of values counting (up by ones) from the left value to the right
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423value. If the left value is greater than the right value then it
424returns the empty array. The range operator is useful for writing
54ae734e 425C<foreach (1..10)> loops and for doing slice operations on arrays. In
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426the current implementation, no temporary array is created when the
427range operator is used as the expression in C<foreach> loops, but older
428versions of Perl might burn a lot of memory when you write something
429like this:
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430
431 for (1 .. 1_000_000) {
432 # code
54310121 433 }
a0d0e21e 434
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435The range operator also works on strings, using the magical auto-increment,
436see below.
437
5a964f20 438In scalar context, ".." returns a boolean value. The operator is
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439bistable, like a flip-flop, and emulates the line-range (comma) operator
440of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
441own boolean state. It is false as long as its left operand is false.
442Once the left operand is true, the range operator stays true until the
443right operand is true, I<AFTER> which the range operator becomes false
19799a22 444again. It doesn't become false till the next time the range operator is
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445evaluated. It can test the right operand and become false on the same
446evaluation it became true (as in B<awk>), but it still returns true once.
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447If you don't want it to test the right operand till the next
448evaluation, as in B<sed>, just use three dots ("...") instead of
449two. In all other regards, "..." behaves just like ".." does.
450
451The right operand is not evaluated while the operator is in the
452"false" state, and the left operand is not evaluated while the
453operator is in the "true" state. The precedence is a little lower
454than || and &&. The value returned is either the empty string for
455false, or a sequence number (beginning with 1) for true. The
456sequence number is reset for each range encountered. The final
457sequence number in a range has the string "E0" appended to it, which
458doesn't affect its numeric value, but gives you something to search
459for if you want to exclude the endpoint. You can exclude the
460beginning point by waiting for the sequence number to be greater
461than 1. If either operand of scalar ".." is a constant expression,
462that operand is implicitly compared to the C<$.> variable, the
463current line number. Examples:
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464
465As a scalar operator:
466
467 if (101 .. 200) { print; } # print 2nd hundred lines
468 next line if (1 .. /^$/); # skip header lines
469 s/^/> / if (/^$/ .. eof()); # quote body
470
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471 # parse mail messages
472 while (<>) {
473 $in_header = 1 .. /^$/;
474 $in_body = /^$/ .. eof();
475 # do something based on those
476 } continue {
477 close ARGV if eof; # reset $. each file
478 }
479
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480As a list operator:
481
482 for (101 .. 200) { print; } # print $_ 100 times
3e3baf6d 483 @foo = @foo[0 .. $#foo]; # an expensive no-op
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484 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
485
5a964f20 486The range operator (in list context) makes use of the magical
5f05dabc 487auto-increment algorithm if the operands are strings. You
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488can say
489
490 @alphabet = ('A' .. 'Z');
491
54ae734e 492to get all normal letters of the English alphabet, or
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493
494 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
495
496to get a hexadecimal digit, or
497
498 @z2 = ('01' .. '31'); print $z2[$mday];
499
500to get dates with leading zeros. If the final value specified is not
501in the sequence that the magical increment would produce, the sequence
502goes until the next value would be longer than the final value
503specified.
504
505=head2 Conditional Operator
506
507Ternary "?:" is the conditional operator, just as in C. It works much
508like an if-then-else. If the argument before the ? is true, the
509argument before the : is returned, otherwise the argument after the :
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510is returned. For example:
511
54310121 512 printf "I have %d dog%s.\n", $n,
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513 ($n == 1) ? '' : "s";
514
515Scalar or list context propagates downward into the 2nd
54310121 516or 3rd argument, whichever is selected.
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517
518 $a = $ok ? $b : $c; # get a scalar
519 @a = $ok ? @b : @c; # get an array
520 $a = $ok ? @b : @c; # oops, that's just a count!
521
522The operator may be assigned to if both the 2nd and 3rd arguments are
523legal lvalues (meaning that you can assign to them):
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524
525 ($a_or_b ? $a : $b) = $c;
526
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527Because this operator produces an assignable result, using assignments
528without parentheses will get you in trouble. For example, this:
529
530 $a % 2 ? $a += 10 : $a += 2
531
532Really means this:
533
534 (($a % 2) ? ($a += 10) : $a) += 2
535
536Rather than this:
537
538 ($a % 2) ? ($a += 10) : ($a += 2)
539
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540That should probably be written more simply as:
541
542 $a += ($a % 2) ? 10 : 2;
543
4633a7c4 544=head2 Assignment Operators
a0d0e21e
LW
545
546"=" is the ordinary assignment operator.
547
548Assignment operators work as in C. That is,
549
550 $a += 2;
551
552is equivalent to
553
554 $a = $a + 2;
555
556although without duplicating any side effects that dereferencing the lvalue
54310121
PP
557might trigger, such as from tie(). Other assignment operators work similarly.
558The following are recognized:
a0d0e21e
LW
559
560 **= += *= &= <<= &&=
561 -= /= |= >>= ||=
562 .= %= ^=
563 x=
564
19799a22 565Although these are grouped by family, they all have the precedence
a0d0e21e
LW
566of assignment.
567
b350dd2f
GS
568Unlike in C, the scalar assignment operator produces a valid lvalue.
569Modifying an assignment is equivalent to doing the assignment and
570then modifying the variable that was assigned to. This is useful
571for modifying a copy of something, like this:
a0d0e21e
LW
572
573 ($tmp = $global) =~ tr [A-Z] [a-z];
574
575Likewise,
576
577 ($a += 2) *= 3;
578
579is equivalent to
580
581 $a += 2;
582 $a *= 3;
583
b350dd2f
GS
584Similarly, a list assignment in list context produces the list of
585lvalues assigned to, and a list assignment in scalar context returns
586the number of elements produced by the expression on the right hand
587side of the assignment.
588
748a9306 589=head2 Comma Operator
a0d0e21e 590
5a964f20 591Binary "," is the comma operator. In scalar context it evaluates
a0d0e21e
LW
592its left argument, throws that value away, then evaluates its right
593argument and returns that value. This is just like C's comma operator.
594
5a964f20 595In list context, it's just the list argument separator, and inserts
a0d0e21e
LW
596both its arguments into the list.
597
35f2feb0 598The => digraph is mostly just a synonym for the comma operator. It's useful for
cb1a09d0 599documenting arguments that come in pairs. As of release 5.001, it also forces
4633a7c4 600any word to the left of it to be interpreted as a string.
748a9306 601
a0d0e21e
LW
602=head2 List Operators (Rightward)
603
604On the right side of a list operator, it has very low precedence,
605such that it controls all comma-separated expressions found there.
606The only operators with lower precedence are the logical operators
607"and", "or", and "not", which may be used to evaluate calls to list
608operators without the need for extra parentheses:
609
610 open HANDLE, "filename"
611 or die "Can't open: $!\n";
612
5ba421f6 613See also discussion of list operators in L<Terms and List Operators (Leftward)>.
a0d0e21e
LW
614
615=head2 Logical Not
616
617Unary "not" returns the logical negation of the expression to its right.
618It's the equivalent of "!" except for the very low precedence.
619
620=head2 Logical And
621
622Binary "and" returns the logical conjunction of the two surrounding
623expressions. It's equivalent to && except for the very low
5f05dabc 624precedence. This means that it short-circuits: i.e., the right
a0d0e21e
LW
625expression is evaluated only if the left expression is true.
626
c963b151 627=head2 Logical or, Defined or, and Exclusive Or
a0d0e21e
LW
628
629Binary "or" returns the logical disjunction of the two surrounding
5a964f20
TC
630expressions. It's equivalent to || except for the very low precedence.
631This makes it useful for control flow
632
633 print FH $data or die "Can't write to FH: $!";
634
635This means that it short-circuits: i.e., the right expression is evaluated
636only if the left expression is false. Due to its precedence, you should
637probably avoid using this for assignment, only for control flow.
638
639 $a = $b or $c; # bug: this is wrong
640 ($a = $b) or $c; # really means this
641 $a = $b || $c; # better written this way
642
19799a22 643However, when it's a list-context assignment and you're trying to use
5a964f20
TC
644"||" for control flow, you probably need "or" so that the assignment
645takes higher precedence.
646
647 @info = stat($file) || die; # oops, scalar sense of stat!
648 @info = stat($file) or die; # better, now @info gets its due
649
c963b151
BD
650Then again, you could always use parentheses.
651
652Binary "err" is equivalent to C<//>--it's just like binary "or", except it tests
653its left argument's definedness instead of its truth. There are two ways to
654remember "err": either because many functions return C<undef> on an B<err>or,
655or as a sort of correction: C<$a=($b err 'default')>
a0d0e21e
LW
656
657Binary "xor" returns the exclusive-OR of the two surrounding expressions.
658It cannot short circuit, of course.
659
660=head2 C Operators Missing From Perl
661
662Here is what C has that Perl doesn't:
663
664=over 8
665
666=item unary &
667
668Address-of operator. (But see the "\" operator for taking a reference.)
669
670=item unary *
671
54310121 672Dereference-address operator. (Perl's prefix dereferencing
a0d0e21e
LW
673operators are typed: $, @, %, and &.)
674
675=item (TYPE)
676
19799a22 677Type-casting operator.
a0d0e21e
LW
678
679=back
680
5f05dabc 681=head2 Quote and Quote-like Operators
a0d0e21e
LW
682
683While we usually think of quotes as literal values, in Perl they
684function as operators, providing various kinds of interpolating and
685pattern matching capabilities. Perl provides customary quote characters
686for these behaviors, but also provides a way for you to choose your
687quote character for any of them. In the following table, a C<{}> represents
87275199 688any pair of delimiters you choose.
a0d0e21e 689
2c268ad5
TP
690 Customary Generic Meaning Interpolates
691 '' q{} Literal no
692 "" qq{} Literal yes
af9219ee 693 `` qx{} Command yes*
2c268ad5 694 qw{} Word list no
af9219ee
MG
695 // m{} Pattern match yes*
696 qr{} Pattern yes*
697 s{}{} Substitution yes*
2c268ad5 698 tr{}{} Transliteration no (but see below)
7e3b091d 699 <<EOF here-doc yes*
a0d0e21e 700
af9219ee
MG
701 * unless the delimiter is ''.
702
87275199
GS
703Non-bracketing delimiters use the same character fore and aft, but the four
704sorts of brackets (round, angle, square, curly) will all nest, which means
705that
706
707 q{foo{bar}baz}
35f2feb0 708
87275199
GS
709is the same as
710
711 'foo{bar}baz'
712
713Note, however, that this does not always work for quoting Perl code:
714
715 $s = q{ if($a eq "}") ... }; # WRONG
716
83df6a1d
JH
717is a syntax error. The C<Text::Balanced> module (from CPAN, and
718starting from Perl 5.8 part of the standard distribution) is able
719to do this properly.
87275199 720
19799a22 721There can be whitespace between the operator and the quoting
fb73857a 722characters, except when C<#> is being used as the quoting character.
19799a22
GS
723C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
724operator C<q> followed by a comment. Its argument will be taken
725from the next line. This allows you to write:
fb73857a
PP
726
727 s {foo} # Replace foo
728 {bar} # with bar.
729
904501ec
MG
730The following escape sequences are available in constructs that interpolate
731and in transliterations.
a0d0e21e 732
6ee5d4e7 733 \t tab (HT, TAB)
5a964f20 734 \n newline (NL)
6ee5d4e7
PP
735 \r return (CR)
736 \f form feed (FF)
737 \b backspace (BS)
738 \a alarm (bell) (BEL)
739 \e escape (ESC)
a0ed51b3
LW
740 \033 octal char (ESC)
741 \x1b hex char (ESC)
742 \x{263a} wide hex char (SMILEY)
19799a22 743 \c[ control char (ESC)
95cc3e0c 744 \N{name} named Unicode character
2c268ad5 745
904501ec
MG
746The following escape sequences are available in constructs that interpolate
747but not in transliterations.
748
a0d0e21e
LW
749 \l lowercase next char
750 \u uppercase next char
751 \L lowercase till \E
752 \U uppercase till \E
753 \E end case modification
1d2dff63 754 \Q quote non-word characters till \E
a0d0e21e 755
95cc3e0c
JH
756If C<use locale> is in effect, the case map used by C<\l>, C<\L>,
757C<\u> and C<\U> is taken from the current locale. See L<perllocale>.
758If Unicode (for example, C<\N{}> or wide hex characters of 0x100 or
759beyond) is being used, the case map used by C<\l>, C<\L>, C<\u> and
760C<\U> is as defined by Unicode. For documentation of C<\N{name}>,
761see L<charnames>.
a034a98d 762
5a964f20
TC
763All systems use the virtual C<"\n"> to represent a line terminator,
764called a "newline". There is no such thing as an unvarying, physical
19799a22 765newline character. It is only an illusion that the operating system,
5a964f20
TC
766device drivers, C libraries, and Perl all conspire to preserve. Not all
767systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
768on a Mac, these are reversed, and on systems without line terminator,
769printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
770you mean a "newline" for your system, but use the literal ASCII when you
771need an exact character. For example, most networking protocols expect
2a380090 772and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
5a964f20
TC
773and although they often accept just C<"\012">, they seldom tolerate just
774C<"\015">. If you get in the habit of using C<"\n"> for networking,
775you may be burned some day.
776
904501ec
MG
777For constructs that do interpolate, variables beginning with "C<$>"
778or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
ad0f383a
A
779C<< $href->{key}[0] >> are also interpolated, as are array and hash slices.
780But method calls such as C<< $obj->meth >> are not.
af9219ee
MG
781
782Interpolating an array or slice interpolates the elements in order,
783separated by the value of C<$">, so is equivalent to interpolating
904501ec
MG
784C<join $", @array>. "Punctuation" arrays such as C<@+> are only
785interpolated if the name is enclosed in braces C<@{+}>.
af9219ee 786
1d2dff63
GS
787You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
788An unescaped C<$> or C<@> interpolates the corresponding variable,
789while escaping will cause the literal string C<\$> to be inserted.
790You'll need to write something like C<m/\Quser\E\@\Qhost/>.
791
a0d0e21e
LW
792Patterns are subject to an additional level of interpretation as a
793regular expression. This is done as a second pass, after variables are
794interpolated, so that regular expressions may be incorporated into the
795pattern from the variables. If this is not what you want, use C<\Q> to
796interpolate a variable literally.
797
19799a22
GS
798Apart from the behavior described above, Perl does not expand
799multiple levels of interpolation. In particular, contrary to the
800expectations of shell programmers, back-quotes do I<NOT> interpolate
801within double quotes, nor do single quotes impede evaluation of
802variables when used within double quotes.
a0d0e21e 803
5f05dabc 804=head2 Regexp Quote-Like Operators
cb1a09d0 805
5f05dabc 806Here are the quote-like operators that apply to pattern
cb1a09d0
AD
807matching and related activities.
808
a0d0e21e
LW
809=over 8
810
811=item ?PATTERN?
812
813This is just like the C</pattern/> search, except that it matches only
814once between calls to the reset() operator. This is a useful
5f05dabc 815optimization when you want to see only the first occurrence of
a0d0e21e
LW
816something in each file of a set of files, for instance. Only C<??>
817patterns local to the current package are reset.
818
5a964f20
TC
819 while (<>) {
820 if (?^$?) {
821 # blank line between header and body
822 }
823 } continue {
824 reset if eof; # clear ?? status for next file
825 }
826
483b4840 827This usage is vaguely deprecated, which means it just might possibly
19799a22
GS
828be removed in some distant future version of Perl, perhaps somewhere
829around the year 2168.
a0d0e21e 830
fb73857a 831=item m/PATTERN/cgimosx
a0d0e21e 832
fb73857a 833=item /PATTERN/cgimosx
a0d0e21e 834
5a964f20 835Searches a string for a pattern match, and in scalar context returns
19799a22
GS
836true if it succeeds, false if it fails. If no string is specified
837via the C<=~> or C<!~> operator, the $_ string is searched. (The
838string specified with C<=~> need not be an lvalue--it may be the
839result of an expression evaluation, but remember the C<=~> binds
840rather tightly.) See also L<perlre>. See L<perllocale> for
841discussion of additional considerations that apply when C<use locale>
842is in effect.
a0d0e21e
LW
843
844Options are:
845
fb73857a 846 c Do not reset search position on a failed match when /g is in effect.
5f05dabc 847 g Match globally, i.e., find all occurrences.
a0d0e21e
LW
848 i Do case-insensitive pattern matching.
849 m Treat string as multiple lines.
5f05dabc 850 o Compile pattern only once.
a0d0e21e
LW
851 s Treat string as single line.
852 x Use extended regular expressions.
853
854If "/" is the delimiter then the initial C<m> is optional. With the C<m>
01ae956f 855you can use any pair of non-alphanumeric, non-whitespace characters
19799a22
GS
856as delimiters. This is particularly useful for matching path names
857that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
7bac28a0 858the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
19799a22 859If "'" is the delimiter, no interpolation is performed on the PATTERN.
a0d0e21e
LW
860
861PATTERN may contain variables, which will be interpolated (and the
f70b4f9c 862pattern recompiled) every time the pattern search is evaluated, except
1f247705
GS
863for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
864C<$|> are not interpolated because they look like end-of-string tests.)
f70b4f9c
AB
865If you want such a pattern to be compiled only once, add a C</o> after
866the trailing delimiter. This avoids expensive run-time recompilations,
867and is useful when the value you are interpolating won't change over
868the life of the script. However, mentioning C</o> constitutes a promise
869that you won't change the variables in the pattern. If you change them,
13a2d996 870Perl won't even notice. See also L<"qr/STRING/imosx">.
a0d0e21e 871
5a964f20 872If the PATTERN evaluates to the empty string, the last
d65afb4b
HS
873I<successfully> matched regular expression is used instead. In this
874case, only the C<g> and C<c> flags on the empty pattern is honoured -
875the other flags are taken from the original pattern. If no match has
876previously succeeded, this will (silently) act instead as a genuine
877empty pattern (which will always match).
a0d0e21e 878
c963b151
BD
879Note that it's possible to confuse Perl into thinking C<//> (the empty
880regex) is really C<//> (the defined-or operator). Perl is usually pretty
881good about this, but some pathological cases might trigger this, such as
882C<$a///> (is that C<($a) / (//)> or C<$a // />?) and C<print $fh //>
883(C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl
884will assume you meant defined-or. If you meant the empty regex, just
885use parentheses or spaces to disambiguate, or even prefix the empty
886regex with an C<m> (so C<//> becomes C<m//>).
887
19799a22 888If the C</g> option is not used, C<m//> in list context returns a
a0d0e21e 889list consisting of the subexpressions matched by the parentheses in the
f7e33566
GS
890pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are
891also set, and that this differs from Perl 4's behavior.) When there are
892no parentheses in the pattern, the return value is the list C<(1)> for
893success. With or without parentheses, an empty list is returned upon
894failure.
a0d0e21e
LW
895
896Examples:
897
898 open(TTY, '/dev/tty');
899 <TTY> =~ /^y/i && foo(); # do foo if desired
900
901 if (/Version: *([0-9.]*)/) { $version = $1; }
902
903 next if m#^/usr/spool/uucp#;
904
905 # poor man's grep
906 $arg = shift;
907 while (<>) {
908 print if /$arg/o; # compile only once
909 }
910
911 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
912
913This last example splits $foo into the first two words and the
5f05dabc
PP
914remainder of the line, and assigns those three fields to $F1, $F2, and
915$Etc. The conditional is true if any variables were assigned, i.e., if
a0d0e21e
LW
916the pattern matched.
917
19799a22
GS
918The C</g> modifier specifies global pattern matching--that is,
919matching as many times as possible within the string. How it behaves
920depends on the context. In list context, it returns a list of the
921substrings matched by any capturing parentheses in the regular
922expression. If there are no parentheses, it returns a list of all
923the matched strings, as if there were parentheses around the whole
924pattern.
a0d0e21e 925
7e86de3e 926In scalar context, each execution of C<m//g> finds the next match,
19799a22 927returning true if it matches, and false if there is no further match.
7e86de3e
G
928The position after the last match can be read or set using the pos()
929function; see L<perlfunc/pos>. A failed match normally resets the
930search position to the beginning of the string, but you can avoid that
931by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
932string also resets the search position.
c90c0ff4
PP
933
934You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
935zero-width assertion that matches the exact position where the previous
5d43e42d
DC
936C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion
937still anchors at pos(), but the match is of course only attempted once.
938Using C<\G> without C</g> on a target string that has not previously had a
939C</g> match applied to it is the same as using the C<\A> assertion to match
fe4b3f22
RGS
940the beginning of the string. Note also that, currently, C<\G> is only
941properly supported when anchored at the very beginning of the pattern.
c90c0ff4
PP
942
943Examples:
a0d0e21e
LW
944
945 # list context
946 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
947
948 # scalar context
5d43e42d 949 $/ = "";
19799a22
GS
950 while (defined($paragraph = <>)) {
951 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
952 $sentences++;
a0d0e21e
LW
953 }
954 }
955 print "$sentences\n";
956
c90c0ff4 957 # using m//gc with \G
137443ea 958 $_ = "ppooqppqq";
44a8e56a
PP
959 while ($i++ < 2) {
960 print "1: '";
c90c0ff4 961 print $1 while /(o)/gc; print "', pos=", pos, "\n";
44a8e56a 962 print "2: '";
c90c0ff4 963 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
44a8e56a 964 print "3: '";
c90c0ff4 965 print $1 while /(p)/gc; print "', pos=", pos, "\n";
44a8e56a 966 }
5d43e42d 967 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
44a8e56a
PP
968
969The last example should print:
970
971 1: 'oo', pos=4
137443ea 972 2: 'q', pos=5
44a8e56a
PP
973 3: 'pp', pos=7
974 1: '', pos=7
137443ea
PP
975 2: 'q', pos=8
976 3: '', pos=8
5d43e42d
DC
977 Final: 'q', pos=8
978
979Notice that the final match matched C<q> instead of C<p>, which a match
980without the C<\G> anchor would have done. Also note that the final match
981did not update C<pos> -- C<pos> is only updated on a C</g> match. If the
982final match did indeed match C<p>, it's a good bet that you're running an
983older (pre-5.6.0) Perl.
44a8e56a 984
c90c0ff4 985A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
e7ea3e70 986combine several regexps like this to process a string part-by-part,
c90c0ff4
PP
987doing different actions depending on which regexp matched. Each
988regexp tries to match where the previous one leaves off.
e7ea3e70 989
3fe9a6f1 990 $_ = <<'EOL';
e7ea3e70 991 $url = new URI::URL "http://www/"; die if $url eq "xXx";
3fe9a6f1
PP
992 EOL
993 LOOP:
e7ea3e70 994 {
c90c0ff4
PP
995 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
996 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
997 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
998 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
999 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
1000 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
1001 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
e7ea3e70
IZ
1002 print ". That's all!\n";
1003 }
1004
1005Here is the output (split into several lines):
1006
1007 line-noise lowercase line-noise lowercase UPPERCASE line-noise
1008 UPPERCASE line-noise lowercase line-noise lowercase line-noise
1009 lowercase lowercase line-noise lowercase lowercase line-noise
1010 MiXeD line-noise. That's all!
44a8e56a 1011
a0d0e21e
LW
1012=item q/STRING/
1013
1014=item C<'STRING'>
1015
19799a22 1016A single-quoted, literal string. A backslash represents a backslash
68dc0745
PP
1017unless followed by the delimiter or another backslash, in which case
1018the delimiter or backslash is interpolated.
a0d0e21e
LW
1019
1020 $foo = q!I said, "You said, 'She said it.'"!;
1021 $bar = q('This is it.');
68dc0745 1022 $baz = '\n'; # a two-character string
a0d0e21e
LW
1023
1024=item qq/STRING/
1025
1026=item "STRING"
1027
1028A double-quoted, interpolated string.
1029
1030 $_ .= qq
1031 (*** The previous line contains the naughty word "$1".\n)
19799a22 1032 if /\b(tcl|java|python)\b/i; # :-)
68dc0745 1033 $baz = "\n"; # a one-character string
a0d0e21e 1034
eec2d3df
GS
1035=item qr/STRING/imosx
1036
322edccd 1037This operator quotes (and possibly compiles) its I<STRING> as a regular
19799a22
GS
1038expression. I<STRING> is interpolated the same way as I<PATTERN>
1039in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
1040is done. Returns a Perl value which may be used instead of the
1041corresponding C</STRING/imosx> expression.
4b6a7270
IZ
1042
1043For example,
1044
1045 $rex = qr/my.STRING/is;
1046 s/$rex/foo/;
1047
1048is equivalent to
1049
1050 s/my.STRING/foo/is;
1051
1052The result may be used as a subpattern in a match:
eec2d3df
GS
1053
1054 $re = qr/$pattern/;
0a92e3a8
GS
1055 $string =~ /foo${re}bar/; # can be interpolated in other patterns
1056 $string =~ $re; # or used standalone
4b6a7270
IZ
1057 $string =~ /$re/; # or this way
1058
1059Since Perl may compile the pattern at the moment of execution of qr()
19799a22 1060operator, using qr() may have speed advantages in some situations,
4b6a7270
IZ
1061notably if the result of qr() is used standalone:
1062
1063 sub match {
1064 my $patterns = shift;
1065 my @compiled = map qr/$_/i, @$patterns;
1066 grep {
1067 my $success = 0;
a7665c5e 1068 foreach my $pat (@compiled) {
4b6a7270
IZ
1069 $success = 1, last if /$pat/;
1070 }
1071 $success;
1072 } @_;
1073 }
1074
19799a22
GS
1075Precompilation of the pattern into an internal representation at
1076the moment of qr() avoids a need to recompile the pattern every
1077time a match C</$pat/> is attempted. (Perl has many other internal
1078optimizations, but none would be triggered in the above example if
1079we did not use qr() operator.)
eec2d3df
GS
1080
1081Options are:
1082
1083 i Do case-insensitive pattern matching.
1084 m Treat string as multiple lines.
1085 o Compile pattern only once.
1086 s Treat string as single line.
1087 x Use extended regular expressions.
1088
0a92e3a8
GS
1089See L<perlre> for additional information on valid syntax for STRING, and
1090for a detailed look at the semantics of regular expressions.
1091
a0d0e21e
LW
1092=item qx/STRING/
1093
1094=item `STRING`
1095
43dd4d21
JH
1096A string which is (possibly) interpolated and then executed as a
1097system command with C</bin/sh> or its equivalent. Shell wildcards,
1098pipes, and redirections will be honored. The collected standard
1099output of the command is returned; standard error is unaffected. In
1100scalar context, it comes back as a single (potentially multi-line)
1101string, or undef if the command failed. In list context, returns a
1102list of lines (however you've defined lines with $/ or
1103$INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
5a964f20
TC
1104
1105Because backticks do not affect standard error, use shell file descriptor
1106syntax (assuming the shell supports this) if you care to address this.
1107To capture a command's STDERR and STDOUT together:
a0d0e21e 1108
5a964f20
TC
1109 $output = `cmd 2>&1`;
1110
1111To capture a command's STDOUT but discard its STDERR:
1112
1113 $output = `cmd 2>/dev/null`;
1114
1115To capture a command's STDERR but discard its STDOUT (ordering is
1116important here):
1117
1118 $output = `cmd 2>&1 1>/dev/null`;
1119
1120To exchange a command's STDOUT and STDERR in order to capture the STDERR
1121but leave its STDOUT to come out the old STDERR:
1122
1123 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
1124
1125To read both a command's STDOUT and its STDERR separately, it's easiest
1126and safest to redirect them separately to files, and then read from those
1127files when the program is done:
1128
1129 system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr");
1130
1131Using single-quote as a delimiter protects the command from Perl's
1132double-quote interpolation, passing it on to the shell instead:
1133
1134 $perl_info = qx(ps $$); # that's Perl's $$
1135 $shell_info = qx'ps $$'; # that's the new shell's $$
1136
19799a22 1137How that string gets evaluated is entirely subject to the command
5a964f20
TC
1138interpreter on your system. On most platforms, you will have to protect
1139shell metacharacters if you want them treated literally. This is in
1140practice difficult to do, as it's unclear how to escape which characters.
1141See L<perlsec> for a clean and safe example of a manual fork() and exec()
1142to emulate backticks safely.
a0d0e21e 1143
bb32b41a
GS
1144On some platforms (notably DOS-like ones), the shell may not be
1145capable of dealing with multiline commands, so putting newlines in
1146the string may not get you what you want. You may be able to evaluate
1147multiple commands in a single line by separating them with the command
1148separator character, if your shell supports that (e.g. C<;> on many Unix
1149shells; C<&> on the Windows NT C<cmd> shell).
1150
0f897271
GS
1151Beginning with v5.6.0, Perl will attempt to flush all files opened for
1152output before starting the child process, but this may not be supported
1153on some platforms (see L<perlport>). To be safe, you may need to set
1154C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1155C<IO::Handle> on any open handles.
1156
bb32b41a
GS
1157Beware that some command shells may place restrictions on the length
1158of the command line. You must ensure your strings don't exceed this
1159limit after any necessary interpolations. See the platform-specific
1160release notes for more details about your particular environment.
1161
5a964f20
TC
1162Using this operator can lead to programs that are difficult to port,
1163because the shell commands called vary between systems, and may in
1164fact not be present at all. As one example, the C<type> command under
1165the POSIX shell is very different from the C<type> command under DOS.
1166That doesn't mean you should go out of your way to avoid backticks
1167when they're the right way to get something done. Perl was made to be
1168a glue language, and one of the things it glues together is commands.
1169Just understand what you're getting yourself into.
bb32b41a 1170
dc848c6f 1171See L<"I/O Operators"> for more discussion.
a0d0e21e 1172
945c54fd
JH
1173=item qw/STRING/
1174
1175Evaluates to a list of the words extracted out of STRING, using embedded
1176whitespace as the word delimiters. It can be understood as being roughly
1177equivalent to:
1178
1179 split(' ', q/STRING/);
1180
efb1e162
CW
1181the differences being that it generates a real list at compile time, and
1182in scalar context it returns the last element in the list. So
945c54fd
JH
1183this expression:
1184
1185 qw(foo bar baz)
1186
1187is semantically equivalent to the list:
1188
1189 'foo', 'bar', 'baz'
1190
1191Some frequently seen examples:
1192
1193 use POSIX qw( setlocale localeconv )
1194 @EXPORT = qw( foo bar baz );
1195
1196A common mistake is to try to separate the words with comma or to
1197put comments into a multi-line C<qw>-string. For this reason, the
1198C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
1199produces warnings if the STRING contains the "," or the "#" character.
1200
a0d0e21e
LW
1201=item s/PATTERN/REPLACEMENT/egimosx
1202
1203Searches a string for a pattern, and if found, replaces that pattern
1204with the replacement text and returns the number of substitutions
e37d713d 1205made. Otherwise it returns false (specifically, the empty string).
a0d0e21e
LW
1206
1207If no string is specified via the C<=~> or C<!~> operator, the C<$_>
1208variable is searched and modified. (The string specified with C<=~> must
5a964f20 1209be scalar variable, an array element, a hash element, or an assignment
5f05dabc 1210to one of those, i.e., an lvalue.)
a0d0e21e 1211
19799a22 1212If the delimiter chosen is a single quote, no interpolation is
a0d0e21e
LW
1213done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1214PATTERN contains a $ that looks like a variable rather than an
1215end-of-string test, the variable will be interpolated into the pattern
5f05dabc 1216at run-time. If you want the pattern compiled only once the first time
a0d0e21e 1217the variable is interpolated, use the C</o> option. If the pattern
5a964f20 1218evaluates to the empty string, the last successfully executed regular
a0d0e21e 1219expression is used instead. See L<perlre> for further explanation on these.
5a964f20 1220See L<perllocale> for discussion of additional considerations that apply
a034a98d 1221when C<use locale> is in effect.
a0d0e21e
LW
1222
1223Options are:
1224
1225 e Evaluate the right side as an expression.
5f05dabc 1226 g Replace globally, i.e., all occurrences.
a0d0e21e
LW
1227 i Do case-insensitive pattern matching.
1228 m Treat string as multiple lines.
5f05dabc 1229 o Compile pattern only once.
a0d0e21e
LW
1230 s Treat string as single line.
1231 x Use extended regular expressions.
1232
1233Any non-alphanumeric, non-whitespace delimiter may replace the
1234slashes. If single quotes are used, no interpretation is done on the
e37d713d 1235replacement string (the C</e> modifier overrides this, however). Unlike
54310121 1236Perl 4, Perl 5 treats backticks as normal delimiters; the replacement
e37d713d 1237text is not evaluated as a command. If the
a0d0e21e 1238PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
5f05dabc 1239pair of quotes, which may or may not be bracketing quotes, e.g.,
35f2feb0 1240C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
cec88af6
GS
1241replacement portion to be treated as a full-fledged Perl expression
1242and evaluated right then and there. It is, however, syntax checked at
1243compile-time. A second C<e> modifier will cause the replacement portion
1244to be C<eval>ed before being run as a Perl expression.
a0d0e21e
LW
1245
1246Examples:
1247
1248 s/\bgreen\b/mauve/g; # don't change wintergreen
1249
1250 $path =~ s|/usr/bin|/usr/local/bin|;
1251
1252 s/Login: $foo/Login: $bar/; # run-time pattern
1253
5a964f20 1254 ($foo = $bar) =~ s/this/that/; # copy first, then change
a0d0e21e 1255
5a964f20 1256 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
a0d0e21e
LW
1257
1258 $_ = 'abc123xyz';
1259 s/\d+/$&*2/e; # yields 'abc246xyz'
1260 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
1261 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1262
1263 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
1264 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
1265 s/^=(\w+)/&pod($1)/ge; # use function call
1266
5a964f20
TC
1267 # expand variables in $_, but dynamics only, using
1268 # symbolic dereferencing
1269 s/\$(\w+)/${$1}/g;
1270
cec88af6
GS
1271 # Add one to the value of any numbers in the string
1272 s/(\d+)/1 + $1/eg;
1273
1274 # This will expand any embedded scalar variable
1275 # (including lexicals) in $_ : First $1 is interpolated
1276 # to the variable name, and then evaluated
a0d0e21e
LW
1277 s/(\$\w+)/$1/eeg;
1278
5a964f20 1279 # Delete (most) C comments.
a0d0e21e 1280 $program =~ s {
4633a7c4
LW
1281 /\* # Match the opening delimiter.
1282 .*? # Match a minimal number of characters.
1283 \*/ # Match the closing delimiter.
a0d0e21e
LW
1284 } []gsx;
1285
5a964f20
TC
1286 s/^\s*(.*?)\s*$/$1/; # trim white space in $_, expensively
1287
1288 for ($variable) { # trim white space in $variable, cheap
1289 s/^\s+//;
1290 s/\s+$//;
1291 }
a0d0e21e
LW
1292
1293 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
1294
54310121 1295Note the use of $ instead of \ in the last example. Unlike
35f2feb0
GS
1296B<sed>, we use the \<I<digit>> form in only the left hand side.
1297Anywhere else it's $<I<digit>>.
a0d0e21e 1298
5f05dabc 1299Occasionally, you can't use just a C</g> to get all the changes
19799a22 1300to occur that you might want. Here are two common cases:
a0d0e21e
LW
1301
1302 # put commas in the right places in an integer
19799a22 1303 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
a0d0e21e
LW
1304
1305 # expand tabs to 8-column spacing
1306 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
1307
6940069f 1308=item tr/SEARCHLIST/REPLACEMENTLIST/cds
a0d0e21e 1309
6940069f 1310=item y/SEARCHLIST/REPLACEMENTLIST/cds
a0d0e21e 1311
2c268ad5 1312Transliterates all occurrences of the characters found in the search list
a0d0e21e
LW
1313with the corresponding character in the replacement list. It returns
1314the number of characters replaced or deleted. If no string is
2c268ad5 1315specified via the =~ or !~ operator, the $_ string is transliterated. (The
54310121
PP
1316string specified with =~ must be a scalar variable, an array element, a
1317hash element, or an assignment to one of those, i.e., an lvalue.)
8ada0baa 1318
2c268ad5
TP
1319A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
1320does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
54310121
PP
1321For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1322SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1323its own pair of quotes, which may or may not be bracketing quotes,
2c268ad5 1324e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
a0d0e21e 1325
cc255d5f
JH
1326Note that C<tr> does B<not> do regular expression character classes
1327such as C<\d> or C<[:lower:]>. The <tr> operator is not equivalent to
1328the tr(1) utility. If you want to map strings between lower/upper
1329cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider
1330using the C<s> operator if you need regular expressions.
1331
8ada0baa
JH
1332Note also that the whole range idea is rather unportable between
1333character sets--and even within character sets they may cause results
1334you probably didn't expect. A sound principle is to use only ranges
1335that begin from and end at either alphabets of equal case (a-e, A-E),
1336or digits (0-4). Anything else is unsafe. If in doubt, spell out the
1337character sets in full.
1338
a0d0e21e
LW
1339Options:
1340
1341 c Complement the SEARCHLIST.
1342 d Delete found but unreplaced characters.
1343 s Squash duplicate replaced characters.
1344
19799a22
GS
1345If the C</c> modifier is specified, the SEARCHLIST character set
1346is complemented. If the C</d> modifier is specified, any characters
1347specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
1348(Note that this is slightly more flexible than the behavior of some
1349B<tr> programs, which delete anything they find in the SEARCHLIST,
1350period.) If the C</s> modifier is specified, sequences of characters
1351that were transliterated to the same character are squashed down
1352to a single instance of the character.
a0d0e21e
LW
1353
1354If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1355exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1356than the SEARCHLIST, the final character is replicated till it is long
5a964f20 1357enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
a0d0e21e
LW
1358This latter is useful for counting characters in a class or for
1359squashing character sequences in a class.
1360
1361Examples:
1362
1363 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1364
1365 $cnt = tr/*/*/; # count the stars in $_
1366
1367 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1368
1369 $cnt = tr/0-9//; # count the digits in $_
1370
1371 tr/a-zA-Z//s; # bookkeeper -> bokeper
1372
1373 ($HOST = $host) =~ tr/a-z/A-Z/;
1374
1375 tr/a-zA-Z/ /cs; # change non-alphas to single space
1376
1377 tr [\200-\377]
1378 [\000-\177]; # delete 8th bit
1379
19799a22
GS
1380If multiple transliterations are given for a character, only the
1381first one is used:
748a9306
LW
1382
1383 tr/AAA/XYZ/
1384
2c268ad5 1385will transliterate any A to X.
748a9306 1386
19799a22 1387Because the transliteration table is built at compile time, neither
a0d0e21e 1388the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
19799a22
GS
1389interpolation. That means that if you want to use variables, you
1390must use an eval():
a0d0e21e
LW
1391
1392 eval "tr/$oldlist/$newlist/";
1393 die $@ if $@;
1394
1395 eval "tr/$oldlist/$newlist/, 1" or die $@;
1396
7e3b091d
DA
1397=item <<EOF
1398
1399A line-oriented form of quoting is based on the shell "here-document"
1400syntax. Following a C<< << >> you specify a string to terminate
1401the quoted material, and all lines following the current line down to
1402the terminating string are the value of the item. The terminating
1403string may be either an identifier (a word), or some quoted text. If
1404quoted, the type of quotes you use determines the treatment of the
1405text, just as in regular quoting. An unquoted identifier works like
1406double quotes. There must be no space between the C<< << >> and
1407the identifier, unless the identifier is quoted. (If you put a space it
1408will be treated as a null identifier, which is valid, and matches the first
1409empty line.) The terminating string must appear by itself (unquoted and
1410with no surrounding whitespace) on the terminating line.
1411
1412 print <<EOF;
1413 The price is $Price.
1414 EOF
1415
1416 print << "EOF"; # same as above
1417 The price is $Price.
1418 EOF
1419
1420 print << `EOC`; # execute commands
1421 echo hi there
1422 echo lo there
1423 EOC
1424
1425 print <<"foo", <<"bar"; # you can stack them
1426 I said foo.
1427 foo
1428 I said bar.
1429 bar
1430
1431 myfunc(<< "THIS", 23, <<'THAT');
1432 Here's a line
1433 or two.
1434 THIS
1435 and here's another.
1436 THAT
1437
1438Just don't forget that you have to put a semicolon on the end
1439to finish the statement, as Perl doesn't know you're not going to
1440try to do this:
1441
1442 print <<ABC
1443 179231
1444 ABC
1445 + 20;
1446
1447If you want your here-docs to be indented with the
1448rest of the code, you'll need to remove leading whitespace
1449from each line manually:
1450
1451 ($quote = <<'FINIS') =~ s/^\s+//gm;
1452 The Road goes ever on and on,
1453 down from the door where it began.
1454 FINIS
1455
1456If you use a here-doc within a delimited construct, such as in C<s///eg>,
1457the quoted material must come on the lines following the final delimiter.
1458So instead of
1459
1460 s/this/<<E . 'that'
1461 the other
1462 E
1463 . 'more '/eg;
1464
1465you have to write
1466
1467 s/this/<<E . 'that'
1468 . 'more '/eg;
1469 the other
1470 E
1471
1472If the terminating identifier is on the last line of the program, you
1473must be sure there is a newline after it; otherwise, Perl will give the
1474warning B<Can't find string terminator "END" anywhere before EOF...>.
1475
1476Additionally, the quoting rules for the identifier are not related to
1477Perl's quoting rules -- C<q()>, C<qq()>, and the like are not supported
1478in place of C<''> and C<"">, and the only interpolation is for backslashing
1479the quoting character:
1480
1481 print << "abc\"def";
1482 testing...
1483 abc"def
1484
1485Finally, quoted strings cannot span multiple lines. The general rule is
1486that the identifier must be a string literal. Stick with that, and you
1487should be safe.
1488
a0d0e21e
LW
1489=back
1490
75e14d17
IZ
1491=head2 Gory details of parsing quoted constructs
1492
19799a22
GS
1493When presented with something that might have several different
1494interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
1495principle to pick the most probable interpretation. This strategy
1496is so successful that Perl programmers often do not suspect the
1497ambivalence of what they write. But from time to time, Perl's
1498notions differ substantially from what the author honestly meant.
1499
1500This section hopes to clarify how Perl handles quoted constructs.
1501Although the most common reason to learn this is to unravel labyrinthine
1502regular expressions, because the initial steps of parsing are the
1503same for all quoting operators, they are all discussed together.
1504
1505The most important Perl parsing rule is the first one discussed
1506below: when processing a quoted construct, Perl first finds the end
1507of that construct, then interprets its contents. If you understand
1508this rule, you may skip the rest of this section on the first
1509reading. The other rules are likely to contradict the user's
1510expectations much less frequently than this first one.
1511
1512Some passes discussed below are performed concurrently, but because
1513their results are the same, we consider them individually. For different
1514quoting constructs, Perl performs different numbers of passes, from
1515one to five, but these passes are always performed in the same order.
75e14d17 1516
13a2d996 1517=over 4
75e14d17
IZ
1518
1519=item Finding the end
1520
19799a22
GS
1521The first pass is finding the end of the quoted construct, whether
1522it be a multicharacter delimiter C<"\nEOF\n"> in the C<<<EOF>
1523construct, a C</> that terminates a C<qq//> construct, a C<]> which
35f2feb0
GS
1524terminates C<qq[]> construct, or a C<< > >> which terminates a
1525fileglob started with C<< < >>.
75e14d17 1526
19799a22
GS
1527When searching for single-character non-pairing delimiters, such
1528as C</>, combinations of C<\\> and C<\/> are skipped. However,
1529when searching for single-character pairing delimiter like C<[>,
1530combinations of C<\\>, C<\]>, and C<\[> are all skipped, and nested
1531C<[>, C<]> are skipped as well. When searching for multicharacter
1532delimiters, nothing is skipped.
75e14d17 1533
19799a22
GS
1534For constructs with three-part delimiters (C<s///>, C<y///>, and
1535C<tr///>), the search is repeated once more.
75e14d17 1536
19799a22
GS
1537During this search no attention is paid to the semantics of the construct.
1538Thus:
75e14d17
IZ
1539
1540 "$hash{"$foo/$bar"}"
1541
2a94b7ce 1542or:
75e14d17
IZ
1543
1544 m/
2a94b7ce 1545 bar # NOT a comment, this slash / terminated m//!
75e14d17
IZ
1546 /x
1547
19799a22
GS
1548do not form legal quoted expressions. The quoted part ends on the
1549first C<"> and C</>, and the rest happens to be a syntax error.
1550Because the slash that terminated C<m//> was followed by a C<SPACE>,
1551the example above is not C<m//x>, but rather C<m//> with no C</x>
1552modifier. So the embedded C<#> is interpreted as a literal C<#>.
75e14d17
IZ
1553
1554=item Removal of backslashes before delimiters
1555
19799a22
GS
1556During the second pass, text between the starting and ending
1557delimiters is copied to a safe location, and the C<\> is removed
1558from combinations consisting of C<\> and delimiter--or delimiters,
1559meaning both starting and ending delimiters will should these differ.
1560This removal does not happen for multi-character delimiters.
1561Note that the combination C<\\> is left intact, just as it was.
75e14d17 1562
19799a22
GS
1563Starting from this step no information about the delimiters is
1564used in parsing.
75e14d17
IZ
1565
1566=item Interpolation
1567
19799a22
GS
1568The next step is interpolation in the text obtained, which is now
1569delimiter-independent. There are four different cases.
75e14d17 1570
13a2d996 1571=over 4
75e14d17
IZ
1572
1573=item C<<<'EOF'>, C<m''>, C<s'''>, C<tr///>, C<y///>
1574
1575No interpolation is performed.
1576
1577=item C<''>, C<q//>
1578
1579The only interpolation is removal of C<\> from pairs C<\\>.
1580
35f2feb0 1581=item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>
75e14d17 1582
19799a22
GS
1583C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
1584converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
1585is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
1586The other combinations are replaced with appropriate expansions.
2a94b7ce 1587
19799a22
GS
1588Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
1589is interpolated in the usual way. Something like C<"\Q\\E"> has
1590no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
1591result is the same as for C<"\\\\E">. As a general rule, backslashes
1592between C<\Q> and C<\E> may lead to counterintuitive results. So,
1593C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
1594as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
2a94b7ce
IZ
1595
1596 $str = '\t';
1597 return "\Q$str";
1598
1599may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
1600
19799a22 1601Interpolated scalars and arrays are converted internally to the C<join> and
92d29cee 1602C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
75e14d17 1603
19799a22 1604 $foo . " XXX '" . (join $", @arr) . "'";
75e14d17 1605
19799a22 1606All operations above are performed simultaneously, left to right.
75e14d17 1607
19799a22
GS
1608Because the result of C<"\Q STRING \E"> has all metacharacters
1609quoted, there is no way to insert a literal C<$> or C<@> inside a
1610C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
1611C<"\\\$">; if not, it is interpreted as the start of an interpolated
1612scalar.
75e14d17 1613
19799a22
GS
1614Note also that the interpolation code needs to make a decision on
1615where the interpolated scalar ends. For instance, whether
35f2feb0 1616C<< "a $b -> {c}" >> really means:
75e14d17
IZ
1617
1618 "a " . $b . " -> {c}";
1619
2a94b7ce 1620or:
75e14d17
IZ
1621
1622 "a " . $b -> {c};
1623
19799a22
GS
1624Most of the time, the longest possible text that does not include
1625spaces between components and which contains matching braces or
1626brackets. because the outcome may be determined by voting based
1627on heuristic estimators, the result is not strictly predictable.
1628Fortunately, it's usually correct for ambiguous cases.
75e14d17
IZ
1629
1630=item C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
1631
19799a22
GS
1632Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
1633happens (almost) as with C<qq//> constructs, but the substitution
1634of C<\> followed by RE-special chars (including C<\>) is not
1635performed. Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
1636a C<#>-comment in a C<//x>-regular expression, no processing is
1637performed whatsoever. This is the first step at which the presence
1638of the C<//x> modifier is relevant.
1639
1640Interpolation has several quirks: C<$|>, C<$(>, and C<$)> are not
1641interpolated, and constructs C<$var[SOMETHING]> are voted (by several
1642different estimators) to be either an array element or C<$var>
1643followed by an RE alternative. This is where the notation
1644C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
1645array element C<-9>, not as a regular expression from the variable
1646C<$arr> followed by a digit, which would be the interpretation of
1647C</$arr[0-9]/>. Since voting among different estimators may occur,
1648the result is not predictable.
1649
1650It is at this step that C<\1> is begrudgingly converted to C<$1> in
1651the replacement text of C<s///> to correct the incorrigible
1652I<sed> hackers who haven't picked up the saner idiom yet. A warning
9f1b1f2d
GS
1653is emitted if the C<use warnings> pragma or the B<-w> command-line flag
1654(that is, the C<$^W> variable) was set.
19799a22
GS
1655
1656The lack of processing of C<\\> creates specific restrictions on
1657the post-processed text. If the delimiter is C</>, one cannot get
1658the combination C<\/> into the result of this step. C</> will
1659finish the regular expression, C<\/> will be stripped to C</> on
1660the previous step, and C<\\/> will be left as is. Because C</> is
1661equivalent to C<\/> inside a regular expression, this does not
1662matter unless the delimiter happens to be character special to the
1663RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
1664alphanumeric char, as in:
2a94b7ce
IZ
1665
1666 m m ^ a \s* b mmx;
1667
19799a22 1668In the RE above, which is intentionally obfuscated for illustration, the
2a94b7ce 1669delimiter is C<m>, the modifier is C<mx>, and after backslash-removal the
aa863641 1670RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
19799a22
GS
1671reason you're encouraged to restrict your delimiters to non-alphanumeric,
1672non-whitespace choices.
75e14d17
IZ
1673
1674=back
1675
19799a22 1676This step is the last one for all constructs except regular expressions,
75e14d17
IZ
1677which are processed further.
1678
1679=item Interpolation of regular expressions
1680
19799a22
GS
1681Previous steps were performed during the compilation of Perl code,
1682but this one happens at run time--although it may be optimized to
1683be calculated at compile time if appropriate. After preprocessing
1684described above, and possibly after evaluation if catenation,
1685joining, casing translation, or metaquoting are involved, the
1686resulting I<string> is passed to the RE engine for compilation.
1687
1688Whatever happens in the RE engine might be better discussed in L<perlre>,
1689but for the sake of continuity, we shall do so here.
1690
1691This is another step where the presence of the C<//x> modifier is
1692relevant. The RE engine scans the string from left to right and
1693converts it to a finite automaton.
1694
1695Backslashed characters are either replaced with corresponding
1696literal strings (as with C<\{>), or else they generate special nodes
1697in the finite automaton (as with C<\b>). Characters special to the
1698RE engine (such as C<|>) generate corresponding nodes or groups of
1699nodes. C<(?#...)> comments are ignored. All the rest is either
1700converted to literal strings to match, or else is ignored (as is
1701whitespace and C<#>-style comments if C<//x> is present).
1702
1703Parsing of the bracketed character class construct, C<[...]>, is
1704rather different than the rule used for the rest of the pattern.
1705The terminator of this construct is found using the same rules as
1706for finding the terminator of a C<{}>-delimited construct, the only
1707exception being that C<]> immediately following C<[> is treated as
1708though preceded by a backslash. Similarly, the terminator of
1709C<(?{...})> is found using the same rules as for finding the
1710terminator of a C<{}>-delimited construct.
1711
1712It is possible to inspect both the string given to RE engine and the
1713resulting finite automaton. See the arguments C<debug>/C<debugcolor>
1714in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
4a4eefd0 1715switch documented in L<perlrun/"Command Switches">.
75e14d17
IZ
1716
1717=item Optimization of regular expressions
1718
7522fed5 1719This step is listed for completeness only. Since it does not change
75e14d17 1720semantics, details of this step are not documented and are subject
19799a22
GS
1721to change without notice. This step is performed over the finite
1722automaton that was generated during the previous pass.
2a94b7ce 1723
19799a22
GS
1724It is at this stage that C<split()> silently optimizes C</^/> to
1725mean C</^/m>.
75e14d17
IZ
1726
1727=back
1728
a0d0e21e
LW
1729=head2 I/O Operators
1730
54310121 1731There are several I/O operators you should know about.
fbad3eb5 1732
7b8d334a 1733A string enclosed by backticks (grave accents) first undergoes
19799a22
GS
1734double-quote interpolation. It is then interpreted as an external
1735command, and the output of that command is the value of the
e9c56f9b
JH
1736backtick string, like in a shell. In scalar context, a single string
1737consisting of all output is returned. In list context, a list of
1738values is returned, one per line of output. (You can set C<$/> to use
1739a different line terminator.) The command is executed each time the
1740pseudo-literal is evaluated. The status value of the command is
1741returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
1742Unlike in B<csh>, no translation is done on the return data--newlines
1743remain newlines. Unlike in any of the shells, single quotes do not
1744hide variable names in the command from interpretation. To pass a
1745literal dollar-sign through to the shell you need to hide it with a
1746backslash. The generalized form of backticks is C<qx//>. (Because
1747backticks always undergo shell expansion as well, see L<perlsec> for
1748security concerns.)
19799a22
GS
1749
1750In scalar context, evaluating a filehandle in angle brackets yields
1751the next line from that file (the newline, if any, included), or
1752C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
1753(sometimes known as file-slurp mode) and the file is empty, it
1754returns C<''> the first time, followed by C<undef> subsequently.
1755
1756Ordinarily you must assign the returned value to a variable, but
1757there is one situation where an automatic assignment happens. If
1758and only if the input symbol is the only thing inside the conditional
1759of a C<while> statement (even if disguised as a C<for(;;)> loop),
1760the value is automatically assigned to the global variable $_,
1761destroying whatever was there previously. (This may seem like an
1762odd thing to you, but you'll use the construct in almost every Perl
17b829fa 1763script you write.) The $_ variable is not implicitly localized.
19799a22
GS
1764You'll have to put a C<local $_;> before the loop if you want that
1765to happen.
1766
1767The following lines are equivalent:
a0d0e21e 1768
748a9306 1769 while (defined($_ = <STDIN>)) { print; }
7b8d334a 1770 while ($_ = <STDIN>) { print; }
a0d0e21e
LW
1771 while (<STDIN>) { print; }
1772 for (;<STDIN>;) { print; }
748a9306 1773 print while defined($_ = <STDIN>);
7b8d334a 1774 print while ($_ = <STDIN>);
a0d0e21e
LW
1775 print while <STDIN>;
1776
19799a22 1777This also behaves similarly, but avoids $_ :
7b8d334a
GS
1778
1779 while (my $line = <STDIN>) { print $line }
1780
19799a22
GS
1781In these loop constructs, the assigned value (whether assignment
1782is automatic or explicit) is then tested to see whether it is
1783defined. The defined test avoids problems where line has a string
1784value that would be treated as false by Perl, for example a "" or
1785a "0" with no trailing newline. If you really mean for such values
1786to terminate the loop, they should be tested for explicitly:
7b8d334a
GS
1787
1788 while (($_ = <STDIN>) ne '0') { ... }
1789 while (<STDIN>) { last unless $_; ... }
1790
35f2feb0 1791In other boolean contexts, C<< <I<filehandle>> >> without an
9f1b1f2d
GS
1792explicit C<defined> test or comparison elicit a warning if the
1793C<use warnings> pragma or the B<-w>
19799a22 1794command-line switch (the C<$^W> variable) is in effect.
7b8d334a 1795
5f05dabc 1796The filehandles STDIN, STDOUT, and STDERR are predefined. (The
19799a22
GS
1797filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
1798in packages, where they would be interpreted as local identifiers
1799rather than global.) Additional filehandles may be created with
1800the open() function, amongst others. See L<perlopentut> and
1801L<perlfunc/open> for details on this.
a0d0e21e 1802
35f2feb0 1803If a <FILEHANDLE> is used in a context that is looking for
19799a22
GS
1804a list, a list comprising all input lines is returned, one line per
1805list element. It's easy to grow to a rather large data space this
1806way, so use with care.
a0d0e21e 1807
35f2feb0 1808<FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
19799a22 1809See L<perlfunc/readline>.
fbad3eb5 1810
35f2feb0
GS
1811The null filehandle <> is special: it can be used to emulate the
1812behavior of B<sed> and B<awk>. Input from <> comes either from
a0d0e21e 1813standard input, or from each file listed on the command line. Here's
35f2feb0 1814how it works: the first time <> is evaluated, the @ARGV array is
5a964f20 1815checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
a0d0e21e
LW
1816gives you standard input. The @ARGV array is then processed as a list
1817of filenames. The loop
1818
1819 while (<>) {
1820 ... # code for each line
1821 }
1822
1823is equivalent to the following Perl-like pseudo code:
1824
3e3baf6d 1825 unshift(@ARGV, '-') unless @ARGV;
a0d0e21e
LW
1826 while ($ARGV = shift) {
1827 open(ARGV, $ARGV);
1828 while (<ARGV>) {
1829 ... # code for each line
1830 }
1831 }
1832
19799a22
GS
1833except that it isn't so cumbersome to say, and will actually work.
1834It really does shift the @ARGV array and put the current filename
1835into the $ARGV variable. It also uses filehandle I<ARGV>
35f2feb0 1836internally--<> is just a synonym for <ARGV>, which
19799a22 1837is magical. (The pseudo code above doesn't work because it treats
35f2feb0 1838<ARGV> as non-magical.)
a0d0e21e 1839
35f2feb0 1840You can modify @ARGV before the first <> as long as the array ends up
a0d0e21e 1841containing the list of filenames you really want. Line numbers (C<$.>)
19799a22
GS
1842continue as though the input were one big happy file. See the example
1843in L<perlfunc/eof> for how to reset line numbers on each file.
5a964f20
TC
1844
1845If you want to set @ARGV to your own list of files, go right ahead.
1846This sets @ARGV to all plain text files if no @ARGV was given:
1847
1848 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
a0d0e21e 1849
5a964f20
TC
1850You can even set them to pipe commands. For example, this automatically
1851filters compressed arguments through B<gzip>:
1852
1853 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
1854
1855If you want to pass switches into your script, you can use one of the
a0d0e21e
LW
1856Getopts modules or put a loop on the front like this:
1857
1858 while ($_ = $ARGV[0], /^-/) {
1859 shift;
1860 last if /^--$/;
1861 if (/^-D(.*)/) { $debug = $1 }
1862 if (/^-v/) { $verbose++ }
5a964f20 1863 # ... # other switches
a0d0e21e 1864 }
5a964f20 1865
a0d0e21e 1866 while (<>) {
5a964f20 1867 # ... # code for each line
a0d0e21e
LW
1868 }
1869
35f2feb0 1870The <> symbol will return C<undef> for end-of-file only once.
19799a22
GS
1871If you call it again after this, it will assume you are processing another
1872@ARGV list, and if you haven't set @ARGV, will read input from STDIN.
a0d0e21e 1873
b159ebd3 1874If what the angle brackets contain is a simple scalar variable (e.g.,
35f2feb0 1875<$foo>), then that variable contains the name of the
19799a22
GS
1876filehandle to input from, or its typeglob, or a reference to the
1877same. For example:
cb1a09d0
AD
1878
1879 $fh = \*STDIN;
1880 $line = <$fh>;
a0d0e21e 1881
5a964f20
TC
1882If what's within the angle brackets is neither a filehandle nor a simple
1883scalar variable containing a filehandle name, typeglob, or typeglob
1884reference, it is interpreted as a filename pattern to be globbed, and
1885either a list of filenames or the next filename in the list is returned,
19799a22 1886depending on context. This distinction is determined on syntactic
35f2feb0
GS
1887grounds alone. That means C<< <$x> >> is always a readline() from
1888an indirect handle, but C<< <$hash{key}> >> is always a glob().
5a964f20
TC
1889That's because $x is a simple scalar variable, but C<$hash{key}> is
1890not--it's a hash element.
1891
1892One level of double-quote interpretation is done first, but you can't
35f2feb0 1893say C<< <$foo> >> because that's an indirect filehandle as explained
5a964f20
TC
1894in the previous paragraph. (In older versions of Perl, programmers
1895would insert curly brackets to force interpretation as a filename glob:
35f2feb0 1896C<< <${foo}> >>. These days, it's considered cleaner to call the
5a964f20 1897internal function directly as C<glob($foo)>, which is probably the right
19799a22 1898way to have done it in the first place.) For example:
a0d0e21e
LW
1899
1900 while (<*.c>) {
1901 chmod 0644, $_;
1902 }
1903
3a4b19e4 1904is roughly equivalent to:
a0d0e21e
LW
1905
1906 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
1907 while (<FOO>) {
5b3eff12 1908 chomp;
a0d0e21e
LW
1909 chmod 0644, $_;
1910 }
1911
3a4b19e4
GS
1912except that the globbing is actually done internally using the standard
1913C<File::Glob> extension. Of course, the shortest way to do the above is:
a0d0e21e
LW
1914
1915 chmod 0644, <*.c>;
1916
19799a22
GS
1917A (file)glob evaluates its (embedded) argument only when it is
1918starting a new list. All values must be read before it will start
1919over. In list context, this isn't important because you automatically
1920get them all anyway. However, in scalar context the operator returns
069e01df 1921the next value each time it's called, or C<undef> when the list has
19799a22
GS
1922run out. As with filehandle reads, an automatic C<defined> is
1923generated when the glob occurs in the test part of a C<while>,
1924because legal glob returns (e.g. a file called F<0>) would otherwise
1925terminate the loop. Again, C<undef> is returned only once. So if
1926you're expecting a single value from a glob, it is much better to
1927say
4633a7c4
LW
1928
1929 ($file) = <blurch*>;
1930
1931than
1932
1933 $file = <blurch*>;
1934
1935because the latter will alternate between returning a filename and
19799a22 1936returning false.
4633a7c4 1937
b159ebd3 1938If you're trying to do variable interpolation, it's definitely better
4633a7c4 1939to use the glob() function, because the older notation can cause people
e37d713d 1940to become confused with the indirect filehandle notation.
4633a7c4
LW
1941
1942 @files = glob("$dir/*.[ch]");
1943 @files = glob($files[$i]);
1944
a0d0e21e
LW
1945=head2 Constant Folding
1946
1947Like C, Perl does a certain amount of expression evaluation at
19799a22 1948compile time whenever it determines that all arguments to an
a0d0e21e
LW
1949operator are static and have no side effects. In particular, string
1950concatenation happens at compile time between literals that don't do
19799a22 1951variable substitution. Backslash interpolation also happens at
a0d0e21e
LW
1952compile time. You can say
1953
1954 'Now is the time for all' . "\n" .
1955 'good men to come to.'
1956
54310121 1957and this all reduces to one string internally. Likewise, if
a0d0e21e
LW
1958you say
1959
1960 foreach $file (@filenames) {
5a964f20 1961 if (-s $file > 5 + 100 * 2**16) { }
54310121 1962 }
a0d0e21e 1963
19799a22
GS
1964the compiler will precompute the number which that expression
1965represents so that the interpreter won't have to.
a0d0e21e 1966
2c268ad5
TP
1967=head2 Bitwise String Operators
1968
1969Bitstrings of any size may be manipulated by the bitwise operators
1970(C<~ | & ^>).
1971
19799a22
GS
1972If the operands to a binary bitwise op are strings of different
1973sizes, B<|> and B<^> ops act as though the shorter operand had
1974additional zero bits on the right, while the B<&> op acts as though
1975the longer operand were truncated to the length of the shorter.
1976The granularity for such extension or truncation is one or more
1977bytes.
2c268ad5
TP
1978
1979 # ASCII-based examples
1980 print "j p \n" ^ " a h"; # prints "JAPH\n"
1981 print "JA" | " ph\n"; # prints "japh\n"
1982 print "japh\nJunk" & '_____'; # prints "JAPH\n";
1983 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
1984
19799a22 1985If you are intending to manipulate bitstrings, be certain that
2c268ad5 1986you're supplying bitstrings: If an operand is a number, that will imply
19799a22 1987a B<numeric> bitwise operation. You may explicitly show which type of
2c268ad5
TP
1988operation you intend by using C<""> or C<0+>, as in the examples below.
1989
1990 $foo = 150 | 105 ; # yields 255 (0x96 | 0x69 is 0xFF)
1991 $foo = '150' | 105 ; # yields 255
1992 $foo = 150 | '105'; # yields 255
1993 $foo = '150' | '105'; # yields string '155' (under ASCII)
1994
1995 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
1996 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
a0d0e21e 1997
1ae175c8
GS
1998See L<perlfunc/vec> for information on how to manipulate individual bits
1999in a bit vector.
2000
55497cff 2001=head2 Integer Arithmetic
a0d0e21e 2002
19799a22 2003By default, Perl assumes that it must do most of its arithmetic in
a0d0e21e
LW
2004floating point. But by saying
2005
2006 use integer;
2007
2008you may tell the compiler that it's okay to use integer operations
19799a22
GS
2009(if it feels like it) from here to the end of the enclosing BLOCK.
2010An inner BLOCK may countermand this by saying
a0d0e21e
LW
2011
2012 no integer;
2013
19799a22
GS
2014which lasts until the end of that BLOCK. Note that this doesn't
2015mean everything is only an integer, merely that Perl may use integer
2016operations if it is so inclined. For example, even under C<use
2017integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731>
2018or so.
2019
2020Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
13a2d996
SP
2021and ">>") always produce integral results. (But see also
2022L<Bitwise String Operators>.) However, C<use integer> still has meaning for
19799a22
GS
2023them. By default, their results are interpreted as unsigned integers, but
2024if C<use integer> is in effect, their results are interpreted
2025as signed integers. For example, C<~0> usually evaluates to a large
2026integral value. However, C<use integer; ~0> is C<-1> on twos-complement
2027machines.
68dc0745
PP
2028
2029=head2 Floating-point Arithmetic
2030
2031While C<use integer> provides integer-only arithmetic, there is no
19799a22
GS
2032analogous mechanism to provide automatic rounding or truncation to a
2033certain number of decimal places. For rounding to a certain number
2034of digits, sprintf() or printf() is usually the easiest route.
2035See L<perlfaq4>.
68dc0745 2036
5a964f20
TC
2037Floating-point numbers are only approximations to what a mathematician
2038would call real numbers. There are infinitely more reals than floats,
2039so some corners must be cut. For example:
2040
2041 printf "%.20g\n", 123456789123456789;
2042 # produces 123456789123456784
2043
2044Testing for exact equality of floating-point equality or inequality is
2045not a good idea. Here's a (relatively expensive) work-around to compare
2046whether two floating-point numbers are equal to a particular number of
2047decimal places. See Knuth, volume II, for a more robust treatment of
2048this topic.
2049
2050 sub fp_equal {
2051 my ($X, $Y, $POINTS) = @_;
2052 my ($tX, $tY);
2053 $tX = sprintf("%.${POINTS}g", $X);
2054 $tY = sprintf("%.${POINTS}g", $Y);
2055 return $tX eq $tY;
2056 }
2057
68dc0745 2058The POSIX module (part of the standard perl distribution) implements
19799a22
GS
2059ceil(), floor(), and other mathematical and trigonometric functions.
2060The Math::Complex module (part of the standard perl distribution)
2061defines mathematical functions that work on both the reals and the
2062imaginary numbers. Math::Complex not as efficient as POSIX, but
68dc0745
PP
2063POSIX can't work with complex numbers.
2064
2065Rounding in financial applications can have serious implications, and
2066the rounding method used should be specified precisely. In these
2067cases, it probably pays not to trust whichever system rounding is
2068being used by Perl, but to instead implement the rounding function you
2069need yourself.
5a964f20
TC
2070
2071=head2 Bigger Numbers
2072
2073The standard Math::BigInt and Math::BigFloat modules provide
19799a22 2074variable-precision arithmetic and overloaded operators, although
cd5c4fce 2075they're currently pretty slow. At the cost of some space and
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2076considerable speed, they avoid the normal pitfalls associated with
2077limited-precision representations.
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2078
2079 use Math::BigInt;
2080 $x = Math::BigInt->new('123456789123456789');
2081 print $x * $x;
2082
2083 # prints +15241578780673678515622620750190521
19799a22 2084
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2085There are several modules that let you calculate with (bound only by
2086memory and cpu-time) unlimited or fixed precision. There are also
2087some non-standard modules that provide faster implementations via
2088external C libraries.
2089
2090Here is a short, but incomplete summary:
2091
2092 Math::Fraction big, unlimited fractions like 9973 / 12967
2093 Math::String treat string sequences like numbers
2094 Math::FixedPrecision calculate with a fixed precision
2095 Math::Currency for currency calculations
2096 Bit::Vector manipulate bit vectors fast (uses C)
2097 Math::BigIntFast Bit::Vector wrapper for big numbers
2098 Math::Pari provides access to the Pari C library
2099 Math::BigInteger uses an external C library
2100 Math::Cephes uses external Cephes C library (no big numbers)
2101 Math::Cephes::Fraction fractions via the Cephes library
2102 Math::GMP another one using an external C library
2103
2104Choose wisely.
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2105
2106=cut