Commit | Line | Data |
---|---|---|
a0d0e21e | 1 | =head1 NAME |
d74e8afc | 2 | X<operator> |
a0d0e21e LW |
3 | |
4 | perlop - Perl operators and precedence | |
5 | ||
d042e63d MS |
6 | =head1 DESCRIPTION |
7 | ||
89d205f2 | 8 | =head2 Operator Precedence and Associativity |
d74e8afc | 9 | X<operator, precedence> X<precedence> X<associativity> |
d042e63d MS |
10 | |
11 | Operator precedence and associativity work in Perl more or less like | |
12 | they do in mathematics. | |
13 | ||
14 | I<Operator precedence> means some operators are evaluated before | |
15 | others. For example, in C<2 + 4 * 5>, the multiplication has higher | |
16 | precedence so C<4 * 5> is evaluated first yielding C<2 + 20 == | |
17 | 22> and not C<6 * 5 == 30>. | |
18 | ||
19 | I<Operator associativity> defines what happens if a sequence of the | |
20 | same operators is used one after another: whether the evaluator will | |
21 | evaluate the left operations first or the right. For example, in C<8 | |
22 | - 4 - 2>, subtraction is left associative so Perl evaluates the | |
23 | expression left to right. C<8 - 4> is evaluated first making the | |
24 | expression C<4 - 2 == 2> and not C<8 - 2 == 6>. | |
a0d0e21e LW |
25 | |
26 | Perl operators have the following associativity and precedence, | |
19799a22 GS |
27 | listed from highest precedence to lowest. Operators borrowed from |
28 | C keep the same precedence relationship with each other, even where | |
29 | C's precedence is slightly screwy. (This makes learning Perl easier | |
30 | for C folks.) With very few exceptions, these all operate on scalar | |
31 | values only, not array values. | |
a0d0e21e LW |
32 | |
33 | left terms and list operators (leftward) | |
34 | left -> | |
35 | nonassoc ++ -- | |
36 | right ** | |
37 | right ! ~ \ and unary + and - | |
54310121 | 38 | left =~ !~ |
a0d0e21e LW |
39 | left * / % x |
40 | left + - . | |
41 | left << >> | |
42 | nonassoc named unary operators | |
43 | nonassoc < > <= >= lt gt le ge | |
0d863452 | 44 | nonassoc == != <=> eq ne cmp ~~ |
a0d0e21e LW |
45 | left & |
46 | left | ^ | |
47 | left && | |
c963b151 | 48 | left || // |
137443ea | 49 | nonassoc .. ... |
a0d0e21e LW |
50 | right ?: |
51 | right = += -= *= etc. | |
52 | left , => | |
53 | nonassoc list operators (rightward) | |
a5f75d66 | 54 | right not |
a0d0e21e | 55 | left and |
f23102e2 | 56 | left or xor |
a0d0e21e LW |
57 | |
58 | In the following sections, these operators are covered in precedence order. | |
59 | ||
5a964f20 TC |
60 | Many operators can be overloaded for objects. See L<overload>. |
61 | ||
a0d0e21e | 62 | =head2 Terms and List Operators (Leftward) |
d74e8afc | 63 | X<list operator> X<operator, list> X<term> |
a0d0e21e | 64 | |
62c18ce2 | 65 | A TERM has the highest precedence in Perl. They include variables, |
5f05dabc | 66 | quote and quote-like operators, any expression in parentheses, |
a0d0e21e LW |
67 | and any function whose arguments are parenthesized. Actually, there |
68 | aren't really functions in this sense, just list operators and unary | |
69 | operators behaving as functions because you put parentheses around | |
70 | the arguments. These are all documented in L<perlfunc>. | |
71 | ||
72 | If any list operator (print(), etc.) or any unary operator (chdir(), etc.) | |
73 | is followed by a left parenthesis as the next token, the operator and | |
74 | arguments within parentheses are taken to be of highest precedence, | |
75 | just like a normal function call. | |
76 | ||
77 | In the absence of parentheses, the precedence of list operators such as | |
78 | C<print>, C<sort>, or C<chmod> is either very high or very low depending on | |
54310121 | 79 | whether you are looking at the left side or the right side of the operator. |
a0d0e21e LW |
80 | For example, in |
81 | ||
82 | @ary = (1, 3, sort 4, 2); | |
83 | print @ary; # prints 1324 | |
84 | ||
19799a22 GS |
85 | the commas on the right of the sort are evaluated before the sort, |
86 | but the commas on the left are evaluated after. In other words, | |
87 | list operators tend to gobble up all arguments that follow, and | |
a0d0e21e | 88 | then act like a simple TERM with regard to the preceding expression. |
19799a22 | 89 | Be careful with parentheses: |
a0d0e21e LW |
90 | |
91 | # These evaluate exit before doing the print: | |
92 | print($foo, exit); # Obviously not what you want. | |
93 | print $foo, exit; # Nor is this. | |
94 | ||
95 | # These do the print before evaluating exit: | |
96 | (print $foo), exit; # This is what you want. | |
97 | print($foo), exit; # Or this. | |
98 | print ($foo), exit; # Or even this. | |
99 | ||
100 | Also note that | |
101 | ||
102 | print ($foo & 255) + 1, "\n"; | |
103 | ||
d042e63d MS |
104 | probably doesn't do what you expect at first glance. The parentheses |
105 | enclose the argument list for C<print> which is evaluated (printing | |
106 | the result of C<$foo & 255>). Then one is added to the return value | |
107 | of C<print> (usually 1). The result is something like this: | |
108 | ||
109 | 1 + 1, "\n"; # Obviously not what you meant. | |
110 | ||
111 | To do what you meant properly, you must write: | |
112 | ||
113 | print(($foo & 255) + 1, "\n"); | |
114 | ||
115 | See L<Named Unary Operators> for more discussion of this. | |
a0d0e21e LW |
116 | |
117 | Also parsed as terms are the C<do {}> and C<eval {}> constructs, as | |
54310121 | 118 | well as subroutine and method calls, and the anonymous |
a0d0e21e LW |
119 | constructors C<[]> and C<{}>. |
120 | ||
2ae324a7 | 121 | See also L<Quote and Quote-like Operators> toward the end of this section, |
da87341d | 122 | as well as L</"I/O Operators">. |
a0d0e21e LW |
123 | |
124 | =head2 The Arrow Operator | |
d74e8afc | 125 | X<arrow> X<dereference> X<< -> >> |
a0d0e21e | 126 | |
35f2feb0 | 127 | "C<< -> >>" is an infix dereference operator, just as it is in C |
19799a22 GS |
128 | and C++. If the right side is either a C<[...]>, C<{...}>, or a |
129 | C<(...)> subscript, then the left side must be either a hard or | |
130 | symbolic reference to an array, a hash, or a subroutine respectively. | |
131 | (Or technically speaking, a location capable of holding a hard | |
132 | reference, if it's an array or hash reference being used for | |
133 | assignment.) See L<perlreftut> and L<perlref>. | |
a0d0e21e | 134 | |
19799a22 GS |
135 | Otherwise, the right side is a method name or a simple scalar |
136 | variable containing either the method name or a subroutine reference, | |
137 | and the left side must be either an object (a blessed reference) | |
138 | or a class name (that is, a package name). See L<perlobj>. | |
a0d0e21e | 139 | |
5f05dabc | 140 | =head2 Auto-increment and Auto-decrement |
d74e8afc | 141 | X<increment> X<auto-increment> X<++> X<decrement> X<auto-decrement> X<--> |
a0d0e21e | 142 | |
d042e63d MS |
143 | "++" and "--" work as in C. That is, if placed before a variable, |
144 | they increment or decrement the variable by one before returning the | |
145 | value, and if placed after, increment or decrement after returning the | |
146 | value. | |
147 | ||
148 | $i = 0; $j = 0; | |
149 | print $i++; # prints 0 | |
150 | print ++$j; # prints 1 | |
a0d0e21e | 151 | |
b033823e | 152 | Note that just as in C, Perl doesn't define B<when> the variable is |
89d205f2 | 153 | incremented or decremented. You just know it will be done sometime |
b033823e | 154 | before or after the value is returned. This also means that modifying |
c543c01b | 155 | a variable twice in the same statement will lead to undefined behavior. |
b033823e A |
156 | Avoid statements like: |
157 | ||
158 | $i = $i ++; | |
159 | print ++ $i + $i ++; | |
160 | ||
161 | Perl will not guarantee what the result of the above statements is. | |
162 | ||
54310121 | 163 | The auto-increment operator has a little extra builtin magic to it. If |
a0d0e21e LW |
164 | you increment a variable that is numeric, or that has ever been used in |
165 | a numeric context, you get a normal increment. If, however, the | |
5f05dabc | 166 | variable has been used in only string contexts since it was set, and |
5a964f20 | 167 | has a value that is not the empty string and matches the pattern |
9c0670e1 | 168 | C</^[a-zA-Z]*[0-9]*\z/>, the increment is done as a string, preserving each |
a0d0e21e LW |
169 | character within its range, with carry: |
170 | ||
c543c01b TC |
171 | print ++($foo = "99"); # prints "100" |
172 | print ++($foo = "a0"); # prints "a1" | |
173 | print ++($foo = "Az"); # prints "Ba" | |
174 | print ++($foo = "zz"); # prints "aaa" | |
a0d0e21e | 175 | |
6a61d433 HS |
176 | C<undef> is always treated as numeric, and in particular is changed |
177 | to C<0> before incrementing (so that a post-increment of an undef value | |
178 | will return C<0> rather than C<undef>). | |
179 | ||
5f05dabc | 180 | The auto-decrement operator is not magical. |
a0d0e21e LW |
181 | |
182 | =head2 Exponentiation | |
d74e8afc | 183 | X<**> X<exponentiation> X<power> |
a0d0e21e | 184 | |
19799a22 | 185 | Binary "**" is the exponentiation operator. It binds even more |
cb1a09d0 AD |
186 | tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is |
187 | implemented using C's pow(3) function, which actually works on doubles | |
188 | internally.) | |
a0d0e21e LW |
189 | |
190 | =head2 Symbolic Unary Operators | |
d74e8afc | 191 | X<unary operator> X<operator, unary> |
a0d0e21e | 192 | |
5f05dabc | 193 | Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower |
a0d0e21e | 194 | precedence version of this. |
d74e8afc | 195 | X<!> |
a0d0e21e | 196 | |
da2f94c5 FC |
197 | Unary "-" performs arithmetic negation if the operand is numeric, |
198 | including any string that looks like a number. If the operand is | |
199 | an identifier, a string consisting of a minus sign concatenated | |
200 | with the identifier is returned. Otherwise, if the string starts | |
201 | with a plus or minus, a string starting with the opposite sign is | |
202 | returned. One effect of these rules is that -bareword is equivalent | |
8705167b | 203 | to the string "-bareword". If, however, the string begins with a |
353c6505 | 204 | non-alphabetic character (excluding "+" or "-"), Perl will attempt to convert |
06705523 SP |
205 | the string to a numeric and the arithmetic negation is performed. If the |
206 | string cannot be cleanly converted to a numeric, Perl will give the warning | |
207 | B<Argument "the string" isn't numeric in negation (-) at ...>. | |
d74e8afc | 208 | X<-> X<negation, arithmetic> |
a0d0e21e | 209 | |
972b05a9 JH |
210 | Unary "~" performs bitwise negation, i.e., 1's complement. For |
211 | example, C<0666 & ~027> is 0640. (See also L<Integer Arithmetic> and | |
212 | L<Bitwise String Operators>.) Note that the width of the result is | |
213 | platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64 | |
214 | bits wide on a 64-bit platform, so if you are expecting a certain bit | |
f113cf86 | 215 | width, remember to use the "&" operator to mask off the excess bits. |
d74e8afc | 216 | X<~> X<negation, binary> |
a0d0e21e | 217 | |
f113cf86 TC |
218 | When complementing strings, if all characters have ordinal values under |
219 | 256, then their complements will, also. But if they do not, all | |
220 | characters will be in either 32- or 64-bit complements, depending on your | |
221 | architecture. So for example, C<~"\x{3B1}"> is C<"\x{FFFF_FC4E}"> on | |
222 | 32-bit machines and C<"\x{FFFF_FFFF_FFFF_FC4E}"> on 64-bit machines. | |
223 | ||
a0d0e21e LW |
224 | Unary "+" has no effect whatsoever, even on strings. It is useful |
225 | syntactically for separating a function name from a parenthesized expression | |
226 | that would otherwise be interpreted as the complete list of function | |
5ba421f6 | 227 | arguments. (See examples above under L<Terms and List Operators (Leftward)>.) |
d74e8afc | 228 | X<+> |
a0d0e21e | 229 | |
19799a22 GS |
230 | Unary "\" creates a reference to whatever follows it. See L<perlreftut> |
231 | and L<perlref>. Do not confuse this behavior with the behavior of | |
232 | backslash within a string, although both forms do convey the notion | |
233 | of protecting the next thing from interpolation. | |
d74e8afc | 234 | X<\> X<reference> X<backslash> |
a0d0e21e LW |
235 | |
236 | =head2 Binding Operators | |
d74e8afc | 237 | X<binding> X<operator, binding> X<=~> X<!~> |
a0d0e21e | 238 | |
c07a80fd | 239 | Binary "=~" binds a scalar expression to a pattern match. Certain operations |
cb1a09d0 AD |
240 | search or modify the string $_ by default. This operator makes that kind |
241 | of operation work on some other string. The right argument is a search | |
2c268ad5 TP |
242 | pattern, substitution, or transliteration. The left argument is what is |
243 | supposed to be searched, substituted, or transliterated instead of the default | |
f8bab1e9 | 244 | $_. When used in scalar context, the return value generally indicates the |
8ff32507 FC |
245 | success of the operation. The exceptions are substitution (s///) |
246 | and transliteration (y///) with the C</r> (non-destructive) option, | |
247 | which cause the B<r>eturn value to be the result of the substitution. | |
248 | Behavior in list context depends on the particular operator. | |
000c65fc DG |
249 | See L</"Regexp Quote-Like Operators"> for details and L<perlretut> for |
250 | examples using these operators. | |
f8bab1e9 GS |
251 | |
252 | If the right argument is an expression rather than a search pattern, | |
2c268ad5 | 253 | substitution, or transliteration, it is interpreted as a search pattern at run |
89d205f2 YO |
254 | time. Note that this means that its contents will be interpolated twice, so |
255 | ||
256 | '\\' =~ q'\\'; | |
257 | ||
258 | is not ok, as the regex engine will end up trying to compile the | |
259 | pattern C<\>, which it will consider a syntax error. | |
a0d0e21e LW |
260 | |
261 | Binary "!~" is just like "=~" except the return value is negated in | |
262 | the logical sense. | |
263 | ||
8ff32507 FC |
264 | Binary "!~" with a non-destructive substitution (s///r) or transliteration |
265 | (y///r) is a syntax error. | |
4f4d7508 | 266 | |
a0d0e21e | 267 | =head2 Multiplicative Operators |
d74e8afc | 268 | X<operator, multiplicative> |
a0d0e21e LW |
269 | |
270 | Binary "*" multiplies two numbers. | |
d74e8afc | 271 | X<*> |
a0d0e21e LW |
272 | |
273 | Binary "/" divides two numbers. | |
d74e8afc | 274 | X</> X<slash> |
a0d0e21e | 275 | |
f7918450 KW |
276 | Binary "%" is the modulo operator, which computes the division |
277 | remainder of its first argument with respect to its second argument. | |
278 | Given integer | |
54310121 | 279 | operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is |
f7918450 | 280 | C<$a> minus the largest multiple of C<$b> less than or equal to |
54310121 | 281 | C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the |
282 | smallest multiple of C<$b> that is not less than C<$a> (i.e. the | |
89b4f0ad | 283 | result will be less than or equal to zero). If the operands |
4848a83b TS |
284 | C<$a> and C<$b> are floating point values and the absolute value of |
285 | C<$b> (that is C<abs($b)>) is less than C<(UV_MAX + 1)>, only | |
286 | the integer portion of C<$a> and C<$b> will be used in the operation | |
287 | (Note: here C<UV_MAX> means the maximum of the unsigned integer type). | |
288 | If the absolute value of the right operand (C<abs($b)>) is greater than | |
289 | or equal to C<(UV_MAX + 1)>, "%" computes the floating-point remainder | |
290 | C<$r> in the equation C<($r = $a - $i*$b)> where C<$i> is a certain | |
f7918450 | 291 | integer that makes C<$r> have the same sign as the right operand |
4848a83b TS |
292 | C<$b> (B<not> as the left operand C<$a> like C function C<fmod()>) |
293 | and the absolute value less than that of C<$b>. | |
0412d526 | 294 | Note that when C<use integer> is in scope, "%" gives you direct access |
f7918450 | 295 | to the modulo operator as implemented by your C compiler. This |
55d729e4 GS |
296 | operator is not as well defined for negative operands, but it will |
297 | execute faster. | |
f7918450 | 298 | X<%> X<remainder> X<modulo> X<mod> |
55d729e4 | 299 | |
62d10b70 GS |
300 | Binary "x" is the repetition operator. In scalar context or if the left |
301 | operand is not enclosed in parentheses, it returns a string consisting | |
302 | of the left operand repeated the number of times specified by the right | |
303 | operand. In list context, if the left operand is enclosed in | |
3585017f YST |
304 | parentheses or is a list formed by C<qw/STRING/>, it repeats the list. |
305 | If the right operand is zero or negative, it returns an empty string | |
306 | or an empty list, depending on the context. | |
d74e8afc | 307 | X<x> |
a0d0e21e LW |
308 | |
309 | print '-' x 80; # print row of dashes | |
310 | ||
311 | print "\t" x ($tab/8), ' ' x ($tab%8); # tab over | |
312 | ||
313 | @ones = (1) x 80; # a list of 80 1's | |
314 | @ones = (5) x @ones; # set all elements to 5 | |
315 | ||
316 | ||
317 | =head2 Additive Operators | |
d74e8afc | 318 | X<operator, additive> |
a0d0e21e LW |
319 | |
320 | Binary "+" returns the sum of two numbers. | |
d74e8afc | 321 | X<+> |
a0d0e21e LW |
322 | |
323 | Binary "-" returns the difference of two numbers. | |
d74e8afc | 324 | X<-> |
a0d0e21e LW |
325 | |
326 | Binary "." concatenates two strings. | |
d74e8afc ITB |
327 | X<string, concatenation> X<concatenation> |
328 | X<cat> X<concat> X<concatenate> X<.> | |
a0d0e21e LW |
329 | |
330 | =head2 Shift Operators | |
d74e8afc ITB |
331 | X<shift operator> X<operator, shift> X<<< << >>> |
332 | X<<< >> >>> X<right shift> X<left shift> X<bitwise shift> | |
333 | X<shl> X<shr> X<shift, right> X<shift, left> | |
a0d0e21e | 334 | |
55497cff | 335 | Binary "<<" returns the value of its left argument shifted left by the |
336 | number of bits specified by the right argument. Arguments should be | |
982ce180 | 337 | integers. (See also L<Integer Arithmetic>.) |
a0d0e21e | 338 | |
55497cff | 339 | Binary ">>" returns the value of its left argument shifted right by |
340 | the number of bits specified by the right argument. Arguments should | |
982ce180 | 341 | be integers. (See also L<Integer Arithmetic>.) |
a0d0e21e | 342 | |
b16cf6df JH |
343 | Note that both "<<" and ">>" in Perl are implemented directly using |
344 | "<<" and ">>" in C. If C<use integer> (see L<Integer Arithmetic>) is | |
345 | in force then signed C integers are used, else unsigned C integers are | |
346 | used. Either way, the implementation isn't going to generate results | |
347 | larger than the size of the integer type Perl was built with (32 bits | |
348 | or 64 bits). | |
349 | ||
350 | The result of overflowing the range of the integers is undefined | |
351 | because it is undefined also in C. In other words, using 32-bit | |
352 | integers, C<< 1 << 32 >> is undefined. Shifting by a negative number | |
353 | of bits is also undefined. | |
354 | ||
a0d0e21e | 355 | =head2 Named Unary Operators |
d74e8afc | 356 | X<operator, named unary> |
a0d0e21e LW |
357 | |
358 | The various named unary operators are treated as functions with one | |
568e6d8b | 359 | argument, with optional parentheses. |
a0d0e21e LW |
360 | |
361 | If any list operator (print(), etc.) or any unary operator (chdir(), etc.) | |
362 | is followed by a left parenthesis as the next token, the operator and | |
363 | arguments within parentheses are taken to be of highest precedence, | |
3981b0eb JA |
364 | just like a normal function call. For example, |
365 | because named unary operators are higher precedence than ||: | |
a0d0e21e LW |
366 | |
367 | chdir $foo || die; # (chdir $foo) || die | |
368 | chdir($foo) || die; # (chdir $foo) || die | |
369 | chdir ($foo) || die; # (chdir $foo) || die | |
370 | chdir +($foo) || die; # (chdir $foo) || die | |
371 | ||
3981b0eb | 372 | but, because * is higher precedence than named operators: |
a0d0e21e LW |
373 | |
374 | chdir $foo * 20; # chdir ($foo * 20) | |
375 | chdir($foo) * 20; # (chdir $foo) * 20 | |
376 | chdir ($foo) * 20; # (chdir $foo) * 20 | |
377 | chdir +($foo) * 20; # chdir ($foo * 20) | |
378 | ||
379 | rand 10 * 20; # rand (10 * 20) | |
380 | rand(10) * 20; # (rand 10) * 20 | |
381 | rand (10) * 20; # (rand 10) * 20 | |
382 | rand +(10) * 20; # rand (10 * 20) | |
383 | ||
568e6d8b RGS |
384 | Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are |
385 | treated like named unary operators, but they don't follow this functional | |
386 | parenthesis rule. That means, for example, that C<-f($file).".bak"> is | |
387 | equivalent to C<-f "$file.bak">. | |
d74e8afc | 388 | X<-X> X<filetest> X<operator, filetest> |
568e6d8b | 389 | |
5ba421f6 | 390 | See also L<"Terms and List Operators (Leftward)">. |
a0d0e21e LW |
391 | |
392 | =head2 Relational Operators | |
d74e8afc | 393 | X<relational operator> X<operator, relational> |
a0d0e21e | 394 | |
35f2feb0 | 395 | Binary "<" returns true if the left argument is numerically less than |
a0d0e21e | 396 | the right argument. |
d74e8afc | 397 | X<< < >> |
a0d0e21e | 398 | |
35f2feb0 | 399 | Binary ">" returns true if the left argument is numerically greater |
a0d0e21e | 400 | than the right argument. |
d74e8afc | 401 | X<< > >> |
a0d0e21e | 402 | |
35f2feb0 | 403 | Binary "<=" returns true if the left argument is numerically less than |
a0d0e21e | 404 | or equal to the right argument. |
d74e8afc | 405 | X<< <= >> |
a0d0e21e | 406 | |
35f2feb0 | 407 | Binary ">=" returns true if the left argument is numerically greater |
a0d0e21e | 408 | than or equal to the right argument. |
d74e8afc | 409 | X<< >= >> |
a0d0e21e LW |
410 | |
411 | Binary "lt" returns true if the left argument is stringwise less than | |
412 | the right argument. | |
d74e8afc | 413 | X<< lt >> |
a0d0e21e LW |
414 | |
415 | Binary "gt" returns true if the left argument is stringwise greater | |
416 | than the right argument. | |
d74e8afc | 417 | X<< gt >> |
a0d0e21e LW |
418 | |
419 | Binary "le" returns true if the left argument is stringwise less than | |
420 | or equal to the right argument. | |
d74e8afc | 421 | X<< le >> |
a0d0e21e LW |
422 | |
423 | Binary "ge" returns true if the left argument is stringwise greater | |
424 | than or equal to the right argument. | |
d74e8afc | 425 | X<< ge >> |
a0d0e21e LW |
426 | |
427 | =head2 Equality Operators | |
d74e8afc | 428 | X<equality> X<equal> X<equals> X<operator, equality> |
a0d0e21e LW |
429 | |
430 | Binary "==" returns true if the left argument is numerically equal to | |
431 | the right argument. | |
d74e8afc | 432 | X<==> |
a0d0e21e LW |
433 | |
434 | Binary "!=" returns true if the left argument is numerically not equal | |
435 | to the right argument. | |
d74e8afc | 436 | X<!=> |
a0d0e21e | 437 | |
35f2feb0 | 438 | Binary "<=>" returns -1, 0, or 1 depending on whether the left |
6ee5d4e7 | 439 | argument is numerically less than, equal to, or greater than the right |
d4ad863d | 440 | argument. If your platform supports NaNs (not-a-numbers) as numeric |
7d3a9d88 NC |
441 | values, using them with "<=>" returns undef. NaN is not "<", "==", ">", |
442 | "<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN | |
443 | returns true, as does NaN != anything else. If your platform doesn't | |
444 | support NaNs then NaN is just a string with numeric value 0. | |
d74e8afc | 445 | X<< <=> >> X<spaceship> |
7d3a9d88 | 446 | |
2b54f59f YST |
447 | perl -le '$a = "NaN"; print "No NaN support here" if $a == $a' |
448 | perl -le '$a = "NaN"; print "NaN support here" if $a != $a' | |
a0d0e21e LW |
449 | |
450 | Binary "eq" returns true if the left argument is stringwise equal to | |
451 | the right argument. | |
d74e8afc | 452 | X<eq> |
a0d0e21e LW |
453 | |
454 | Binary "ne" returns true if the left argument is stringwise not equal | |
455 | to the right argument. | |
d74e8afc | 456 | X<ne> |
a0d0e21e | 457 | |
d4ad863d JH |
458 | Binary "cmp" returns -1, 0, or 1 depending on whether the left |
459 | argument is stringwise less than, equal to, or greater than the right | |
460 | argument. | |
d74e8afc | 461 | X<cmp> |
a0d0e21e | 462 | |
0d863452 | 463 | Binary "~~" does a smart match between its arguments. Smart matching |
0f7107a0 | 464 | is described in L<perlsyn/"Smart matching in detail">. |
0d863452 RH |
465 | X<~~> |
466 | ||
a034a98d DD |
467 | "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified |
468 | by the current locale if C<use locale> is in effect. See L<perllocale>. | |
469 | ||
a0d0e21e | 470 | =head2 Bitwise And |
d74e8afc | 471 | X<operator, bitwise, and> X<bitwise and> X<&> |
a0d0e21e | 472 | |
2cdc098b | 473 | Binary "&" returns its operands ANDed together bit by bit. |
2c268ad5 | 474 | (See also L<Integer Arithmetic> and L<Bitwise String Operators>.) |
a0d0e21e | 475 | |
2cdc098b MG |
476 | Note that "&" has lower priority than relational operators, so for example |
477 | the brackets are essential in a test like | |
478 | ||
479 | print "Even\n" if ($x & 1) == 0; | |
480 | ||
a0d0e21e | 481 | =head2 Bitwise Or and Exclusive Or |
d74e8afc ITB |
482 | X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor> |
483 | X<bitwise xor> X<^> | |
a0d0e21e | 484 | |
2cdc098b | 485 | Binary "|" returns its operands ORed together bit by bit. |
2c268ad5 | 486 | (See also L<Integer Arithmetic> and L<Bitwise String Operators>.) |
a0d0e21e | 487 | |
2cdc098b | 488 | Binary "^" returns its operands XORed together bit by bit. |
2c268ad5 | 489 | (See also L<Integer Arithmetic> and L<Bitwise String Operators>.) |
a0d0e21e | 490 | |
2cdc098b MG |
491 | Note that "|" and "^" have lower priority than relational operators, so |
492 | for example the brackets are essential in a test like | |
493 | ||
494 | print "false\n" if (8 | 2) != 10; | |
495 | ||
a0d0e21e | 496 | =head2 C-style Logical And |
d74e8afc | 497 | X<&&> X<logical and> X<operator, logical, and> |
a0d0e21e LW |
498 | |
499 | Binary "&&" performs a short-circuit logical AND operation. That is, | |
500 | if the left operand is false, the right operand is not even evaluated. | |
501 | Scalar or list context propagates down to the right operand if it | |
502 | is evaluated. | |
503 | ||
504 | =head2 C-style Logical Or | |
d74e8afc | 505 | X<||> X<operator, logical, or> |
a0d0e21e LW |
506 | |
507 | Binary "||" performs a short-circuit logical OR operation. That is, | |
508 | if the left operand is true, the right operand is not even evaluated. | |
509 | Scalar or list context propagates down to the right operand if it | |
510 | is evaluated. | |
511 | ||
c963b151 | 512 | =head2 C-style Logical Defined-Or |
d74e8afc | 513 | X<//> X<operator, logical, defined-or> |
c963b151 BD |
514 | |
515 | Although it has no direct equivalent in C, Perl's C<//> operator is related | |
89d205f2 | 516 | to its C-style or. In fact, it's exactly the same as C<||>, except that it |
c963b151 | 517 | tests the left hand side's definedness instead of its truth. Thus, C<$a // $b> |
89d205f2 | 518 | is similar to C<defined($a) || $b> (except that it returns the value of C<$a> |
d90d5a38 | 519 | rather than the value of C<defined($a)>) and yields the same result as |
bdc7923b RGS |
520 | C<defined($a) ? $a : $b> (except that the ternary-operator form can be |
521 | used as a lvalue, while C<$a // $b> cannot). This is very useful for | |
522 | providing default values for variables. If you actually want to test if | |
523 | at least one of C<$a> and C<$b> is defined, use C<defined($a // $b)>. | |
c963b151 | 524 | |
d042e63d MS |
525 | The C<||>, C<//> and C<&&> operators return the last value evaluated |
526 | (unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably | |
527 | portable way to find out the home directory might be: | |
a0d0e21e | 528 | |
c543c01b TC |
529 | $home = $ENV{HOME} |
530 | // $ENV{LOGDIR} | |
531 | // (getpwuid($<))[7] | |
532 | // die "You're homeless!\n"; | |
a0d0e21e | 533 | |
5a964f20 TC |
534 | In particular, this means that you shouldn't use this |
535 | for selecting between two aggregates for assignment: | |
536 | ||
537 | @a = @b || @c; # this is wrong | |
538 | @a = scalar(@b) || @c; # really meant this | |
539 | @a = @b ? @b : @c; # this works fine, though | |
540 | ||
f23102e2 RGS |
541 | As more readable alternatives to C<&&> and C<||> when used for |
542 | control flow, Perl provides the C<and> and C<or> operators (see below). | |
543 | The short-circuit behavior is identical. The precedence of "and" | |
c963b151 | 544 | and "or" is much lower, however, so that you can safely use them after a |
5a964f20 | 545 | list operator without the need for parentheses: |
a0d0e21e LW |
546 | |
547 | unlink "alpha", "beta", "gamma" | |
548 | or gripe(), next LINE; | |
549 | ||
550 | With the C-style operators that would have been written like this: | |
551 | ||
552 | unlink("alpha", "beta", "gamma") | |
553 | || (gripe(), next LINE); | |
554 | ||
eeb6a2c9 | 555 | Using "or" for assignment is unlikely to do what you want; see below. |
5a964f20 TC |
556 | |
557 | =head2 Range Operators | |
d74e8afc | 558 | X<operator, range> X<range> X<..> X<...> |
a0d0e21e LW |
559 | |
560 | Binary ".." is the range operator, which is really two different | |
fb53bbb2 | 561 | operators depending on the context. In list context, it returns a |
54ae734e | 562 | list of values counting (up by ones) from the left value to the right |
2cdbc966 | 563 | value. If the left value is greater than the right value then it |
fb53bbb2 | 564 | returns the empty list. The range operator is useful for writing |
54ae734e | 565 | C<foreach (1..10)> loops and for doing slice operations on arrays. In |
2cdbc966 JD |
566 | the current implementation, no temporary array is created when the |
567 | range operator is used as the expression in C<foreach> loops, but older | |
568 | versions of Perl might burn a lot of memory when you write something | |
569 | like this: | |
a0d0e21e LW |
570 | |
571 | for (1 .. 1_000_000) { | |
572 | # code | |
54310121 | 573 | } |
a0d0e21e | 574 | |
8f0f46f8 | 575 | The range operator also works on strings, using the magical |
576 | auto-increment, see below. | |
54ae734e | 577 | |
5a964f20 | 578 | In scalar context, ".." returns a boolean value. The operator is |
8f0f46f8 | 579 | bistable, like a flip-flop, and emulates the line-range (comma) |
580 | operator of B<sed>, B<awk>, and various editors. Each ".." operator | |
581 | maintains its own boolean state, even across calls to a subroutine | |
582 | that contains it. It is false as long as its left operand is false. | |
a0d0e21e LW |
583 | Once the left operand is true, the range operator stays true until the |
584 | right operand is true, I<AFTER> which the range operator becomes false | |
8f0f46f8 | 585 | again. It doesn't become false till the next time the range operator |
586 | is evaluated. It can test the right operand and become false on the | |
587 | same evaluation it became true (as in B<awk>), but it still returns | |
588 | true once. If you don't want it to test the right operand until the | |
589 | next evaluation, as in B<sed>, just use three dots ("...") instead of | |
19799a22 GS |
590 | two. In all other regards, "..." behaves just like ".." does. |
591 | ||
592 | The right operand is not evaluated while the operator is in the | |
593 | "false" state, and the left operand is not evaluated while the | |
594 | operator is in the "true" state. The precedence is a little lower | |
595 | than || and &&. The value returned is either the empty string for | |
8f0f46f8 | 596 | false, or a sequence number (beginning with 1) for true. The sequence |
597 | number is reset for each range encountered. The final sequence number | |
598 | in a range has the string "E0" appended to it, which doesn't affect | |
599 | its numeric value, but gives you something to search for if you want | |
600 | to exclude the endpoint. You can exclude the beginning point by | |
601 | waiting for the sequence number to be greater than 1. | |
df5f8116 CW |
602 | |
603 | If either operand of scalar ".." is a constant expression, | |
604 | that operand is considered true if it is equal (C<==>) to the current | |
605 | input line number (the C<$.> variable). | |
606 | ||
607 | To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>, | |
608 | but that is only an issue if you use a floating point expression; when | |
609 | implicitly using C<$.> as described in the previous paragraph, the | |
610 | comparison is C<int(EXPR) == int($.)> which is only an issue when C<$.> | |
611 | is set to a floating point value and you are not reading from a file. | |
612 | Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what | |
613 | you want in scalar context because each of the operands are evaluated | |
614 | using their integer representation. | |
615 | ||
616 | Examples: | |
a0d0e21e LW |
617 | |
618 | As a scalar operator: | |
619 | ||
df5f8116 | 620 | if (101 .. 200) { print; } # print 2nd hundred lines, short for |
950b09ed | 621 | # if ($. == 101 .. $. == 200) { print; } |
9f10b797 RGS |
622 | |
623 | next LINE if (1 .. /^$/); # skip header lines, short for | |
f343f960 | 624 | # next LINE if ($. == 1 .. /^$/); |
9f10b797 RGS |
625 | # (typically in a loop labeled LINE) |
626 | ||
627 | s/^/> / if (/^$/ .. eof()); # quote body | |
a0d0e21e | 628 | |
5a964f20 TC |
629 | # parse mail messages |
630 | while (<>) { | |
631 | $in_header = 1 .. /^$/; | |
df5f8116 CW |
632 | $in_body = /^$/ .. eof; |
633 | if ($in_header) { | |
f343f960 | 634 | # do something |
df5f8116 | 635 | } else { # in body |
f343f960 | 636 | # do something else |
df5f8116 | 637 | } |
5a964f20 | 638 | } continue { |
df5f8116 | 639 | close ARGV if eof; # reset $. each file |
5a964f20 TC |
640 | } |
641 | ||
acf31ca5 SF |
642 | Here's a simple example to illustrate the difference between |
643 | the two range operators: | |
644 | ||
645 | @lines = (" - Foo", | |
646 | "01 - Bar", | |
647 | "1 - Baz", | |
648 | " - Quux"); | |
649 | ||
9f10b797 RGS |
650 | foreach (@lines) { |
651 | if (/0/ .. /1/) { | |
acf31ca5 SF |
652 | print "$_\n"; |
653 | } | |
654 | } | |
655 | ||
9f10b797 RGS |
656 | This program will print only the line containing "Bar". If |
657 | the range operator is changed to C<...>, it will also print the | |
acf31ca5 SF |
658 | "Baz" line. |
659 | ||
660 | And now some examples as a list operator: | |
a0d0e21e LW |
661 | |
662 | for (101 .. 200) { print; } # print $_ 100 times | |
3e3baf6d | 663 | @foo = @foo[0 .. $#foo]; # an expensive no-op |
a0d0e21e LW |
664 | @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items |
665 | ||
5a964f20 | 666 | The range operator (in list context) makes use of the magical |
5f05dabc | 667 | auto-increment algorithm if the operands are strings. You |
a0d0e21e LW |
668 | can say |
669 | ||
c543c01b | 670 | @alphabet = ("A" .. "Z"); |
a0d0e21e | 671 | |
54ae734e | 672 | to get all normal letters of the English alphabet, or |
a0d0e21e | 673 | |
c543c01b | 674 | $hexdigit = (0 .. 9, "a" .. "f")[$num & 15]; |
a0d0e21e LW |
675 | |
676 | to get a hexadecimal digit, or | |
677 | ||
c543c01b | 678 | @z2 = ("01" .. "31"); print $z2[$mday]; |
a0d0e21e | 679 | |
ea4f5703 YST |
680 | to get dates with leading zeros. |
681 | ||
682 | If the final value specified is not in the sequence that the magical | |
683 | increment would produce, the sequence goes until the next value would | |
684 | be longer than the final value specified. | |
685 | ||
686 | If the initial value specified isn't part of a magical increment | |
c543c01b | 687 | sequence (that is, a non-empty string matching C</^[a-zA-Z]*[0-9]*\z/>), |
ea4f5703 YST |
688 | only the initial value will be returned. So the following will only |
689 | return an alpha: | |
690 | ||
c543c01b | 691 | use charnames "greek"; |
ea4f5703 YST |
692 | my @greek_small = ("\N{alpha}" .. "\N{omega}"); |
693 | ||
c543c01b TC |
694 | To get the 25 traditional lowercase Greek letters, including both sigmas, |
695 | you could use this instead: | |
ea4f5703 | 696 | |
c543c01b TC |
697 | use charnames "greek"; |
698 | my @greek_small = map { chr } | |
699 | ord "\N{alpha}" .. ord "\N{omega}"; | |
700 | ||
701 | However, because there are I<many> other lowercase Greek characters than | |
702 | just those, to match lowercase Greek characters in a regular expression, | |
703 | you would use the pattern C</(?:(?=\p{Greek})\p{Lower})+/>. | |
a0d0e21e | 704 | |
df5f8116 CW |
705 | Because each operand is evaluated in integer form, C<2.18 .. 3.14> will |
706 | return two elements in list context. | |
707 | ||
708 | @list = (2.18 .. 3.14); # same as @list = (2 .. 3); | |
709 | ||
a0d0e21e | 710 | =head2 Conditional Operator |
d74e8afc | 711 | X<operator, conditional> X<operator, ternary> X<ternary> X<?:> |
a0d0e21e LW |
712 | |
713 | Ternary "?:" is the conditional operator, just as in C. It works much | |
714 | like an if-then-else. If the argument before the ? is true, the | |
715 | argument before the : is returned, otherwise the argument after the : | |
cb1a09d0 AD |
716 | is returned. For example: |
717 | ||
54310121 | 718 | printf "I have %d dog%s.\n", $n, |
c543c01b | 719 | ($n == 1) ? "" : "s"; |
cb1a09d0 AD |
720 | |
721 | Scalar or list context propagates downward into the 2nd | |
54310121 | 722 | or 3rd argument, whichever is selected. |
cb1a09d0 AD |
723 | |
724 | $a = $ok ? $b : $c; # get a scalar | |
725 | @a = $ok ? @b : @c; # get an array | |
726 | $a = $ok ? @b : @c; # oops, that's just a count! | |
727 | ||
728 | The operator may be assigned to if both the 2nd and 3rd arguments are | |
729 | legal lvalues (meaning that you can assign to them): | |
a0d0e21e LW |
730 | |
731 | ($a_or_b ? $a : $b) = $c; | |
732 | ||
5a964f20 TC |
733 | Because this operator produces an assignable result, using assignments |
734 | without parentheses will get you in trouble. For example, this: | |
735 | ||
736 | $a % 2 ? $a += 10 : $a += 2 | |
737 | ||
738 | Really means this: | |
739 | ||
740 | (($a % 2) ? ($a += 10) : $a) += 2 | |
741 | ||
742 | Rather than this: | |
743 | ||
744 | ($a % 2) ? ($a += 10) : ($a += 2) | |
745 | ||
19799a22 GS |
746 | That should probably be written more simply as: |
747 | ||
748 | $a += ($a % 2) ? 10 : 2; | |
749 | ||
4633a7c4 | 750 | =head2 Assignment Operators |
d74e8afc | 751 | X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=> |
5ac3b81c | 752 | X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=> |
d74e8afc | 753 | X<%=> X<^=> X<x=> |
a0d0e21e LW |
754 | |
755 | "=" is the ordinary assignment operator. | |
756 | ||
757 | Assignment operators work as in C. That is, | |
758 | ||
759 | $a += 2; | |
760 | ||
761 | is equivalent to | |
762 | ||
763 | $a = $a + 2; | |
764 | ||
765 | although without duplicating any side effects that dereferencing the lvalue | |
54310121 | 766 | might trigger, such as from tie(). Other assignment operators work similarly. |
767 | The following are recognized: | |
a0d0e21e LW |
768 | |
769 | **= += *= &= <<= &&= | |
9f10b797 RGS |
770 | -= /= |= >>= ||= |
771 | .= %= ^= //= | |
772 | x= | |
a0d0e21e | 773 | |
19799a22 | 774 | Although these are grouped by family, they all have the precedence |
a0d0e21e LW |
775 | of assignment. |
776 | ||
b350dd2f GS |
777 | Unlike in C, the scalar assignment operator produces a valid lvalue. |
778 | Modifying an assignment is equivalent to doing the assignment and | |
779 | then modifying the variable that was assigned to. This is useful | |
780 | for modifying a copy of something, like this: | |
a0d0e21e | 781 | |
c543c01b | 782 | ($tmp = $global) =~ tr [0-9] [a-j]; |
a0d0e21e LW |
783 | |
784 | Likewise, | |
785 | ||
786 | ($a += 2) *= 3; | |
787 | ||
788 | is equivalent to | |
789 | ||
790 | $a += 2; | |
791 | $a *= 3; | |
792 | ||
b350dd2f GS |
793 | Similarly, a list assignment in list context produces the list of |
794 | lvalues assigned to, and a list assignment in scalar context returns | |
795 | the number of elements produced by the expression on the right hand | |
796 | side of the assignment. | |
797 | ||
c543c01b TC |
798 | =head2 The Triple-Dot Operator |
799 | X<...> X<... operator> X<yada-yada operator> X<whatever operator> | |
800 | X<triple-dot operator> | |
801 | ||
802 | The triple-dot operator, C<...>, sometimes called the "whatever operator", the | |
803 | "yada-yada operator", or the "I<et cetera>" operator, is a placeholder for | |
804 | code. Perl parses it without error, but when you try to execute a whatever, | |
805 | it throws an exception with the text C<Unimplemented>: | |
806 | ||
807 | sub unimplemented { ... } | |
808 | ||
809 | eval { unimplemented() }; | |
810 | if ($@ eq "Unimplemented" ) { | |
811 | say "Oh look, an exception--whatever."; | |
812 | } | |
813 | ||
814 | You can only use the triple-dot operator to stand in for a complete statement. | |
815 | These examples of the triple-dot work: | |
816 | ||
817 | { ... } | |
818 | ||
819 | sub foo { ... } | |
820 | ||
821 | ...; | |
822 | ||
823 | eval { ... }; | |
824 | ||
825 | sub foo { | |
826 | my ($self) = shift; | |
827 | ...; | |
828 | } | |
829 | ||
830 | do { | |
831 | my $variable; | |
832 | ...; | |
833 | say "Hurrah!"; | |
834 | } while $cheering; | |
835 | ||
836 | The yada-yada--or whatever--cannot stand in for an expression that is | |
837 | part of a larger statement since the C<...> is also the three-dot version | |
838 | of the binary range operator (see L<Range Operators>). These examples of | |
839 | the whatever operator are still syntax errors: | |
840 | ||
841 | print ...; | |
842 | ||
843 | open(PASSWD, ">", "/dev/passwd") or ...; | |
844 | ||
845 | if ($condition && ...) { say "Hello" } | |
846 | ||
847 | There are some cases where Perl can't immediately tell the difference | |
848 | between an expression and a statement. For instance, the syntax for a | |
849 | block and an anonymous hash reference constructor look the same unless | |
850 | there's something in the braces that give Perl a hint. The whatever | |
851 | is a syntax error if Perl doesn't guess that the C<{ ... }> is a | |
852 | block. In that case, it doesn't think the C<...> is the whatever | |
853 | because it's expecting an expression instead of a statement: | |
854 | ||
855 | my @transformed = map { ... } @input; # syntax error | |
856 | ||
857 | You can use a C<;> inside your block to denote that the C<{ ... }> is | |
858 | a block and not a hash reference constructor. Now the whatever works: | |
859 | ||
860 | my @transformed = map {; ... } @input; # ; disambiguates | |
861 | ||
862 | my @transformed = map { ...; } @input; # ; disambiguates | |
863 | ||
748a9306 | 864 | =head2 Comma Operator |
d74e8afc | 865 | X<comma> X<operator, comma> X<,> |
a0d0e21e | 866 | |
5a964f20 | 867 | Binary "," is the comma operator. In scalar context it evaluates |
a0d0e21e LW |
868 | its left argument, throws that value away, then evaluates its right |
869 | argument and returns that value. This is just like C's comma operator. | |
870 | ||
5a964f20 | 871 | In list context, it's just the list argument separator, and inserts |
ed5c6d31 PJ |
872 | both its arguments into the list. These arguments are also evaluated |
873 | from left to right. | |
a0d0e21e | 874 | |
344f2c40 IG |
875 | The C<< => >> operator is a synonym for the comma except that it causes |
876 | its left operand to be interpreted as a string if it begins with a letter | |
877 | or underscore and is composed only of letters, digits and underscores. | |
878 | This includes operands that might otherwise be interpreted as operators, | |
879 | constants, single number v-strings or function calls. If in doubt about | |
c543c01b | 880 | this behavior, the left operand can be quoted explicitly. |
344f2c40 IG |
881 | |
882 | Otherwise, the C<< => >> operator behaves exactly as the comma operator | |
883 | or list argument separator, according to context. | |
884 | ||
885 | For example: | |
a44e5664 MS |
886 | |
887 | use constant FOO => "something"; | |
888 | ||
889 | my %h = ( FOO => 23 ); | |
890 | ||
891 | is equivalent to: | |
892 | ||
893 | my %h = ("FOO", 23); | |
894 | ||
895 | It is I<NOT>: | |
896 | ||
897 | my %h = ("something", 23); | |
898 | ||
719b43e8 RGS |
899 | The C<< => >> operator is helpful in documenting the correspondence |
900 | between keys and values in hashes, and other paired elements in lists. | |
748a9306 | 901 | |
a44e5664 MS |
902 | %hash = ( $key => $value ); |
903 | login( $username => $password ); | |
904 | ||
a0d0e21e | 905 | =head2 List Operators (Rightward) |
d74e8afc | 906 | X<operator, list, rightward> X<list operator> |
a0d0e21e | 907 | |
c543c01b | 908 | On the right side of a list operator, the comma has very low precedence, |
a0d0e21e LW |
909 | such that it controls all comma-separated expressions found there. |
910 | The only operators with lower precedence are the logical operators | |
911 | "and", "or", and "not", which may be used to evaluate calls to list | |
912 | operators without the need for extra parentheses: | |
913 | ||
c543c01b TC |
914 | open HANDLE, "< $file" |
915 | or die "Can't open $file: $!\n"; | |
a0d0e21e | 916 | |
5ba421f6 | 917 | See also discussion of list operators in L<Terms and List Operators (Leftward)>. |
a0d0e21e LW |
918 | |
919 | =head2 Logical Not | |
d74e8afc | 920 | X<operator, logical, not> X<not> |
a0d0e21e LW |
921 | |
922 | Unary "not" returns the logical negation of the expression to its right. | |
923 | It's the equivalent of "!" except for the very low precedence. | |
924 | ||
925 | =head2 Logical And | |
d74e8afc | 926 | X<operator, logical, and> X<and> |
a0d0e21e LW |
927 | |
928 | Binary "and" returns the logical conjunction of the two surrounding | |
c543c01b TC |
929 | expressions. It's equivalent to C<&&> except for the very low |
930 | precedence. This means that it short-circuits: the right | |
a0d0e21e LW |
931 | expression is evaluated only if the left expression is true. |
932 | ||
c963b151 | 933 | =head2 Logical or, Defined or, and Exclusive Or |
f23102e2 | 934 | X<operator, logical, or> X<operator, logical, xor> |
d74e8afc | 935 | X<operator, logical, defined or> X<operator, logical, exclusive or> |
f23102e2 | 936 | X<or> X<xor> |
a0d0e21e LW |
937 | |
938 | Binary "or" returns the logical disjunction of the two surrounding | |
c543c01b TC |
939 | expressions. It's equivalent to C<||> except for the very low precedence. |
940 | This makes it useful for control flow: | |
5a964f20 TC |
941 | |
942 | print FH $data or die "Can't write to FH: $!"; | |
943 | ||
c543c01b TC |
944 | This means that it short-circuits: the right expression is evaluated |
945 | only if the left expression is false. Due to its precedence, you must | |
946 | be careful to avoid using it as replacement for the C<||> operator. | |
947 | It usually works out better for flow control than in assignments: | |
5a964f20 TC |
948 | |
949 | $a = $b or $c; # bug: this is wrong | |
950 | ($a = $b) or $c; # really means this | |
951 | $a = $b || $c; # better written this way | |
952 | ||
19799a22 | 953 | However, when it's a list-context assignment and you're trying to use |
c543c01b | 954 | C<||> for control flow, you probably need "or" so that the assignment |
5a964f20 TC |
955 | takes higher precedence. |
956 | ||
957 | @info = stat($file) || die; # oops, scalar sense of stat! | |
958 | @info = stat($file) or die; # better, now @info gets its due | |
959 | ||
c963b151 BD |
960 | Then again, you could always use parentheses. |
961 | ||
a0d0e21e | 962 | Binary "xor" returns the exclusive-OR of the two surrounding expressions. |
c543c01b | 963 | It cannot short-circuit (of course). |
a0d0e21e LW |
964 | |
965 | =head2 C Operators Missing From Perl | |
d74e8afc ITB |
966 | X<operator, missing from perl> X<&> X<*> |
967 | X<typecasting> X<(TYPE)> | |
a0d0e21e LW |
968 | |
969 | Here is what C has that Perl doesn't: | |
970 | ||
971 | =over 8 | |
972 | ||
973 | =item unary & | |
974 | ||
975 | Address-of operator. (But see the "\" operator for taking a reference.) | |
976 | ||
977 | =item unary * | |
978 | ||
54310121 | 979 | Dereference-address operator. (Perl's prefix dereferencing |
a0d0e21e LW |
980 | operators are typed: $, @, %, and &.) |
981 | ||
982 | =item (TYPE) | |
983 | ||
19799a22 | 984 | Type-casting operator. |
a0d0e21e LW |
985 | |
986 | =back | |
987 | ||
5f05dabc | 988 | =head2 Quote and Quote-like Operators |
89d205f2 | 989 | X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m> |
d74e8afc ITB |
990 | X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>> |
991 | X<escape sequence> X<escape> | |
992 | ||
a0d0e21e LW |
993 | While we usually think of quotes as literal values, in Perl they |
994 | function as operators, providing various kinds of interpolating and | |
995 | pattern matching capabilities. Perl provides customary quote characters | |
996 | for these behaviors, but also provides a way for you to choose your | |
997 | quote character for any of them. In the following table, a C<{}> represents | |
9f10b797 | 998 | any pair of delimiters you choose. |
a0d0e21e | 999 | |
2c268ad5 TP |
1000 | Customary Generic Meaning Interpolates |
1001 | '' q{} Literal no | |
1002 | "" qq{} Literal yes | |
af9219ee | 1003 | `` qx{} Command yes* |
2c268ad5 | 1004 | qw{} Word list no |
af9219ee MG |
1005 | // m{} Pattern match yes* |
1006 | qr{} Pattern yes* | |
1007 | s{}{} Substitution yes* | |
2c268ad5 | 1008 | tr{}{} Transliteration no (but see below) |
c543c01b | 1009 | y{}{} Transliteration no (but see below) |
7e3b091d | 1010 | <<EOF here-doc yes* |
a0d0e21e | 1011 | |
af9219ee MG |
1012 | * unless the delimiter is ''. |
1013 | ||
87275199 | 1014 | Non-bracketing delimiters use the same character fore and aft, but the four |
c543c01b | 1015 | sorts of ASCII brackets (round, angle, square, curly) all nest, which means |
9f10b797 | 1016 | that |
87275199 | 1017 | |
c543c01b | 1018 | q{foo{bar}baz} |
35f2feb0 | 1019 | |
9f10b797 | 1020 | is the same as |
87275199 | 1021 | |
c543c01b | 1022 | 'foo{bar}baz' |
87275199 GS |
1023 | |
1024 | Note, however, that this does not always work for quoting Perl code: | |
1025 | ||
c543c01b | 1026 | $s = q{ if($a eq "}") ... }; # WRONG |
87275199 | 1027 | |
c543c01b TC |
1028 | is a syntax error. The C<Text::Balanced> module (standard as of v5.8, |
1029 | and from CPAN before then) is able to do this properly. | |
87275199 | 1030 | |
19799a22 | 1031 | There can be whitespace between the operator and the quoting |
fb73857a | 1032 | characters, except when C<#> is being used as the quoting character. |
19799a22 GS |
1033 | C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the |
1034 | operator C<q> followed by a comment. Its argument will be taken | |
1035 | from the next line. This allows you to write: | |
fb73857a | 1036 | |
1037 | s {foo} # Replace foo | |
1038 | {bar} # with bar. | |
1039 | ||
c543c01b TC |
1040 | The following escape sequences are available in constructs that interpolate, |
1041 | and in transliterations: | |
5691ca5f | 1042 | X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N> X<\N{}> |
04341565 | 1043 | X<\o{}> |
5691ca5f | 1044 | |
2c4c1ff2 KW |
1045 | Sequence Note Description |
1046 | \t tab (HT, TAB) | |
1047 | \n newline (NL) | |
1048 | \r return (CR) | |
1049 | \f form feed (FF) | |
1050 | \b backspace (BS) | |
1051 | \a alarm (bell) (BEL) | |
1052 | \e escape (ESC) | |
c543c01b | 1053 | \x{263A} [1,8] hex char (example: SMILEY) |
2c4c1ff2 | 1054 | \x1b [2,8] restricted range hex char (example: ESC) |
fb121860 | 1055 | \N{name} [3] named Unicode character or character sequence |
2c4c1ff2 KW |
1056 | \N{U+263D} [4,8] Unicode character (example: FIRST QUARTER MOON) |
1057 | \c[ [5] control char (example: chr(27)) | |
1058 | \o{23072} [6,8] octal char (example: SMILEY) | |
1059 | \033 [7,8] restricted range octal char (example: ESC) | |
5691ca5f KW |
1060 | |
1061 | =over 4 | |
1062 | ||
1063 | =item [1] | |
1064 | ||
2c4c1ff2 KW |
1065 | The result is the character specified by the hexadecimal number between |
1066 | the braces. See L</[8]> below for details on which character. | |
96448467 DG |
1067 | |
1068 | Only hexadecimal digits are valid between the braces. If an invalid | |
1069 | character is encountered, a warning will be issued and the invalid | |
1070 | character and all subsequent characters (valid or invalid) within the | |
1071 | braces will be discarded. | |
1072 | ||
1073 | If there are no valid digits between the braces, the generated character is | |
1074 | the NULL character (C<\x{00}>). However, an explicit empty brace (C<\x{}>) | |
c543c01b | 1075 | will not cause a warning (currently). |
40687185 KW |
1076 | |
1077 | =item [2] | |
1078 | ||
2c4c1ff2 KW |
1079 | The result is the character specified by the hexadecimal number in the range |
1080 | 0x00 to 0xFF. See L</[8]> below for details on which character. | |
96448467 DG |
1081 | |
1082 | Only hexadecimal digits are valid following C<\x>. When C<\x> is followed | |
2c4c1ff2 | 1083 | by fewer than two valid digits, any valid digits will be zero-padded. This |
c543c01b | 1084 | means that C<\x7> will be interpreted as C<\x07>, and a lone <\x> will be |
2c4c1ff2 | 1085 | interpreted as C<\x00>. Except at the end of a string, having fewer than |
c543c01b | 1086 | two valid digits will result in a warning. Note that although the warning |
96448467 DG |
1087 | says the illegal character is ignored, it is only ignored as part of the |
1088 | escape and will still be used as the subsequent character in the string. | |
1089 | For example: | |
1090 | ||
1091 | Original Result Warns? | |
1092 | "\x7" "\x07" no | |
1093 | "\x" "\x00" no | |
1094 | "\x7q" "\x07q" yes | |
1095 | "\xq" "\x00q" yes | |
1096 | ||
40687185 KW |
1097 | =item [3] |
1098 | ||
fb121860 | 1099 | The result is the Unicode character or character sequence given by I<name>. |
2c4c1ff2 | 1100 | See L<charnames>. |
40687185 KW |
1101 | |
1102 | =item [4] | |
1103 | ||
2c4c1ff2 KW |
1104 | C<\N{U+I<hexadecimal number>}> means the Unicode character whose Unicode code |
1105 | point is I<hexadecimal number>. | |
40687185 KW |
1106 | |
1107 | =item [5] | |
1108 | ||
5691ca5f KW |
1109 | The character following C<\c> is mapped to some other character as shown in the |
1110 | table: | |
1111 | ||
1112 | Sequence Value | |
1113 | \c@ chr(0) | |
1114 | \cA chr(1) | |
1115 | \ca chr(1) | |
1116 | \cB chr(2) | |
1117 | \cb chr(2) | |
1118 | ... | |
1119 | \cZ chr(26) | |
1120 | \cz chr(26) | |
1121 | \c[ chr(27) | |
1122 | \c] chr(29) | |
1123 | \c^ chr(30) | |
1124 | \c? chr(127) | |
1125 | ||
d813941f KW |
1126 | In other words, it's the character whose code point has had 64 xor'd with |
1127 | its uppercase. C<\c?> is DELETE because C<ord("@") ^ 64> is 127, and | |
1128 | C<\c@> is NULL because the ord of "@" is 64, so xor'ing 64 itself produces 0. | |
1129 | ||
5691ca5f KW |
1130 | Also, C<\c\I<X>> yields C< chr(28) . "I<X>"> for any I<X>, but cannot come at the |
1131 | end of a string, because the backslash would be parsed as escaping the end | |
1132 | quote. | |
1133 | ||
1134 | On ASCII platforms, the resulting characters from the list above are the | |
1135 | complete set of ASCII controls. This isn't the case on EBCDIC platforms; see | |
1136 | L<perlebcdic/OPERATOR DIFFERENCES> for the complete list of what these | |
1137 | sequences mean on both ASCII and EBCDIC platforms. | |
1138 | ||
1139 | Use of any other character following the "c" besides those listed above is | |
17a3df4c KW |
1140 | discouraged, and some are deprecated with the intention of removing |
1141 | those in Perl 5.16. What happens for any of these | |
d813941f KW |
1142 | other characters currently though, is that the value is derived by xor'ing |
1143 | with the seventh bit, which is 64. | |
5691ca5f KW |
1144 | |
1145 | To get platform independent controls, you can use C<\N{...}>. | |
1146 | ||
40687185 KW |
1147 | =item [6] |
1148 | ||
2c4c1ff2 KW |
1149 | The result is the character specified by the octal number between the braces. |
1150 | See L</[8]> below for details on which character. | |
04341565 DG |
1151 | |
1152 | If a character that isn't an octal digit is encountered, a warning is raised, | |
1153 | and the value is based on the octal digits before it, discarding it and all | |
1154 | following characters up to the closing brace. It is a fatal error if there are | |
1155 | no octal digits at all. | |
1156 | ||
1157 | =item [7] | |
1158 | ||
c543c01b | 1159 | The result is the character specified by the three-digit octal number in the |
2c4c1ff2 KW |
1160 | range 000 to 777 (but best to not use above 077, see next paragraph). See |
1161 | L</[8]> below for details on which character. | |
1162 | ||
1163 | Some contexts allow 2 or even 1 digit, but any usage without exactly | |
40687185 | 1164 | three digits, the first being a zero, may give unintended results. (For |
04341565 | 1165 | example, see L<perlrebackslash/Octal escapes>.) Starting in Perl 5.14, you may |
c543c01b | 1166 | use C<\o{}> instead, which avoids all these problems. Otherwise, it is best to |
04341565 DG |
1167 | use this construct only for ordinals C<\077> and below, remembering to pad to |
1168 | the left with zeros to make three digits. For larger ordinals, either use | |
d90d5a38 | 1169 | C<\o{}> , or convert to something else, such as to hex and use C<\x{}> |
04341565 | 1170 | instead. |
40687185 | 1171 | |
40687185 KW |
1172 | Having fewer than 3 digits may lead to a misleading warning message that says |
1173 | that what follows is ignored. For example, C<"\128"> in the ASCII character set | |
1174 | is equivalent to the two characters C<"\n8">, but the warning C<Illegal octal | |
1175 | digit '8' ignored> will be thrown. To avoid this warning, make sure to pad | |
f6050459 | 1176 | your octal number with C<0>'s: C<"\0128">. |
5691ca5f | 1177 | |
2c4c1ff2 KW |
1178 | =item [8] |
1179 | ||
c543c01b | 1180 | Several constructs above specify a character by a number. That number |
2c4c1ff2 | 1181 | gives the character's position in the character set encoding (indexed from 0). |
c543c01b | 1182 | This is called synonymously its ordinal, code position, or code point. Perl |
2c4c1ff2 KW |
1183 | works on platforms that have a native encoding currently of either ASCII/Latin1 |
1184 | or EBCDIC, each of which allow specification of 256 characters. In general, if | |
1185 | the number is 255 (0xFF, 0377) or below, Perl interprets this in the platform's | |
1186 | native encoding. If the number is 256 (0x100, 0400) or above, Perl interprets | |
c543c01b | 1187 | it as a Unicode code point and the result is the corresponding Unicode |
2c4c1ff2 KW |
1188 | character. For example C<\x{50}> and C<\o{120}> both are the number 80 in |
1189 | decimal, which is less than 256, so the number is interpreted in the native | |
1190 | character set encoding. In ASCII the character in the 80th position (indexed | |
1191 | from 0) is the letter "P", and in EBCDIC it is the ampersand symbol "&". | |
1192 | C<\x{100}> and C<\o{400}> are both 256 in decimal, so the number is interpreted | |
1193 | as a Unicode code point no matter what the native encoding is. The name of the | |
1194 | character in the 100th position (indexed by 0) in Unicode is | |
1195 | C<LATIN CAPITAL LETTER A WITH MACRON>. | |
1196 | ||
1197 | There are a couple of exceptions to the above rule. C<\N{U+I<hex number>}> is | |
1198 | always interpreted as a Unicode code point, so that C<\N{U+0050}> is "P" even | |
1199 | on EBCDIC platforms. And if L<C<S<use encoding>>|encoding> is in effect, the | |
1200 | number is considered to be in that encoding, and is translated from that into | |
1201 | the platform's native encoding if there is a corresponding native character; | |
1202 | otherwise to Unicode. | |
1203 | ||
5691ca5f | 1204 | =back |
4c77eaa2 | 1205 | |
e526e8bb KW |
1206 | B<NOTE>: Unlike C and other languages, Perl has no C<\v> escape sequence for |
1207 | the vertical tab (VT - ASCII 11), but you may use C<\ck> or C<\x0b>. (C<\v> | |
1208 | does have meaning in regular expression patterns in Perl, see L<perlre>.) | |
1209 | ||
1210 | The following escape sequences are available in constructs that interpolate, | |
904501ec | 1211 | but not in transliterations. |
d74e8afc | 1212 | X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q> |
904501ec | 1213 | |
c543c01b TC |
1214 | \l lowercase next character only |
1215 | \u titlecase (not uppercase!) next character only | |
1216 | \L lowercase all characters till \E seen | |
1217 | \U uppercase all characters till \E seen | |
1d2dff63 | 1218 | \Q quote non-word characters till \E |
7e31b643 | 1219 | \E end either case modification or quoted section |
c543c01b TC |
1220 | (whichever was last seen) |
1221 | ||
1222 | C<\L>, C<\U>, and C<\Q> can stack, in which case you need one | |
1223 | C<\E> for each. For example: | |
1224 | ||
1225 | say "This \Qquoting \ubusiness \Uhere isn't quite\E done yet,\E is it?"; | |
1226 | This quoting\ Business\ HERE\ ISN\'T\ QUITE\ done\ yet\, is it? | |
a0d0e21e | 1227 | |
95cc3e0c | 1228 | If C<use locale> is in effect, the case map used by C<\l>, C<\L>, |
c543c01b | 1229 | C<\u>, and C<\U> is taken from the current locale. See L<perllocale>. |
b6538e4f | 1230 | If Unicode (for example, C<\N{}> or code points of 0x100 or |
c543c01b TC |
1231 | beyond) is being used, the case map used by C<\l>, C<\L>, C<\u>, and |
1232 | C<\U> is as defined by Unicode. That means that case-mapping | |
1233 | a single character can sometimes produce several characters. | |
a034a98d | 1234 | |
5a964f20 TC |
1235 | All systems use the virtual C<"\n"> to represent a line terminator, |
1236 | called a "newline". There is no such thing as an unvarying, physical | |
19799a22 | 1237 | newline character. It is only an illusion that the operating system, |
5a964f20 TC |
1238 | device drivers, C libraries, and Perl all conspire to preserve. Not all |
1239 | systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example, | |
c543c01b TC |
1240 | on the ancient Macs (pre-MacOS X) of yesteryear, these used to be reversed, |
1241 | and on systems without line terminator, | |
1242 | printing C<"\n"> might emit no actual data. In general, use C<"\n"> when | |
5a964f20 TC |
1243 | you mean a "newline" for your system, but use the literal ASCII when you |
1244 | need an exact character. For example, most networking protocols expect | |
2a380090 | 1245 | and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators, |
5a964f20 TC |
1246 | and although they often accept just C<"\012">, they seldom tolerate just |
1247 | C<"\015">. If you get in the habit of using C<"\n"> for networking, | |
1248 | you may be burned some day. | |
d74e8afc ITB |
1249 | X<newline> X<line terminator> X<eol> X<end of line> |
1250 | X<\n> X<\r> X<\r\n> | |
5a964f20 | 1251 | |
904501ec MG |
1252 | For constructs that do interpolate, variables beginning with "C<$>" |
1253 | or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or | |
ad0f383a A |
1254 | C<< $href->{key}[0] >> are also interpolated, as are array and hash slices. |
1255 | But method calls such as C<< $obj->meth >> are not. | |
af9219ee MG |
1256 | |
1257 | Interpolating an array or slice interpolates the elements in order, | |
1258 | separated by the value of C<$">, so is equivalent to interpolating | |
c543c01b TC |
1259 | C<join $", @array>. "Punctuation" arrays such as C<@*> are usually |
1260 | interpolated only if the name is enclosed in braces C<@{*}>, but the | |
1261 | arrays C<@_>, C<@+>, and C<@-> are interpolated even without braces. | |
af9219ee | 1262 | |
bc7b91c6 EB |
1263 | For double-quoted strings, the quoting from C<\Q> is applied after |
1264 | interpolation and escapes are processed. | |
1265 | ||
1266 | "abc\Qfoo\tbar$s\Exyz" | |
1267 | ||
1268 | is equivalent to | |
1269 | ||
1270 | "abc" . quotemeta("foo\tbar$s") . "xyz" | |
1271 | ||
1272 | For the pattern of regex operators (C<qr//>, C<m//> and C<s///>), | |
1273 | the quoting from C<\Q> is applied after interpolation is processed, | |
1274 | but before escapes are processed. This allows the pattern to match | |
1275 | literally (except for C<$> and C<@>). For example, the following matches: | |
1276 | ||
1277 | '\s\t' =~ /\Q\s\t/ | |
1278 | ||
1279 | Because C<$> or C<@> trigger interpolation, you'll need to use something | |
1280 | like C</\Quser\E\@\Qhost/> to match them literally. | |
1d2dff63 | 1281 | |
a0d0e21e LW |
1282 | Patterns are subject to an additional level of interpretation as a |
1283 | regular expression. This is done as a second pass, after variables are | |
1284 | interpolated, so that regular expressions may be incorporated into the | |
1285 | pattern from the variables. If this is not what you want, use C<\Q> to | |
1286 | interpolate a variable literally. | |
1287 | ||
19799a22 GS |
1288 | Apart from the behavior described above, Perl does not expand |
1289 | multiple levels of interpolation. In particular, contrary to the | |
1290 | expectations of shell programmers, back-quotes do I<NOT> interpolate | |
1291 | within double quotes, nor do single quotes impede evaluation of | |
1292 | variables when used within double quotes. | |
a0d0e21e | 1293 | |
5f05dabc | 1294 | =head2 Regexp Quote-Like Operators |
d74e8afc | 1295 | X<operator, regexp> |
cb1a09d0 | 1296 | |
5f05dabc | 1297 | Here are the quote-like operators that apply to pattern |
cb1a09d0 AD |
1298 | matching and related activities. |
1299 | ||
a0d0e21e LW |
1300 | =over 8 |
1301 | ||
b6fa137b | 1302 | =item qr/STRING/msixpodual |
01c6f5f4 | 1303 | X<qr> X</i> X</m> X</o> X</s> X</x> X</p> |
a0d0e21e | 1304 | |
87e95b7f YO |
1305 | This operator quotes (and possibly compiles) its I<STRING> as a regular |
1306 | expression. I<STRING> is interpolated the same way as I<PATTERN> | |
1307 | in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation | |
1308 | is done. Returns a Perl value which may be used instead of the | |
f6050459 | 1309 | corresponding C</STRING/msixpodual> expression. The returned value is a |
85dd5c8b | 1310 | normalized version of the original pattern. It magically differs from |
64c5a566 | 1311 | a string containing the same characters: C<ref(qr/x/)> returns "Regexp", |
85dd5c8b | 1312 | even though dereferencing the result returns undef. |
a0d0e21e | 1313 | |
87e95b7f YO |
1314 | For example, |
1315 | ||
1316 | $rex = qr/my.STRING/is; | |
85dd5c8b | 1317 | print $rex; # prints (?si-xm:my.STRING) |
87e95b7f YO |
1318 | s/$rex/foo/; |
1319 | ||
1320 | is equivalent to | |
1321 | ||
1322 | s/my.STRING/foo/is; | |
1323 | ||
1324 | The result may be used as a subpattern in a match: | |
1325 | ||
1326 | $re = qr/$pattern/; | |
1327 | $string =~ /foo${re}bar/; # can be interpolated in other patterns | |
1328 | $string =~ $re; # or used standalone | |
1329 | $string =~ /$re/; # or this way | |
1330 | ||
f6050459 | 1331 | Since Perl may compile the pattern at the moment of execution of the qr() |
87e95b7f YO |
1332 | operator, using qr() may have speed advantages in some situations, |
1333 | notably if the result of qr() is used standalone: | |
1334 | ||
1335 | sub match { | |
1336 | my $patterns = shift; | |
1337 | my @compiled = map qr/$_/i, @$patterns; | |
1338 | grep { | |
1339 | my $success = 0; | |
1340 | foreach my $pat (@compiled) { | |
1341 | $success = 1, last if /$pat/; | |
1342 | } | |
1343 | $success; | |
1344 | } @_; | |
5a964f20 TC |
1345 | } |
1346 | ||
87e95b7f YO |
1347 | Precompilation of the pattern into an internal representation at |
1348 | the moment of qr() avoids a need to recompile the pattern every | |
1349 | time a match C</$pat/> is attempted. (Perl has many other internal | |
1350 | optimizations, but none would be triggered in the above example if | |
1351 | we did not use qr() operator.) | |
1352 | ||
765fa144 | 1353 | Options (specified by the following modifiers) are: |
87e95b7f YO |
1354 | |
1355 | m Treat string as multiple lines. | |
1356 | s Treat string as single line. (Make . match a newline) | |
1357 | i Do case-insensitive pattern matching. | |
1358 | x Use extended regular expressions. | |
1359 | p When matching preserve a copy of the matched string so | |
1360 | that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be defined. | |
1361 | o Compile pattern only once. | |
b6fa137b | 1362 | l Use the locale |
5e2aa8f5 KW |
1363 | u Use Unicode rules |
1364 | a Use ASCII for \d, \s, \w; specifying two a's further restricts | |
1365 | /i matching so that no ASCII character will match a non-ASCII | |
1366 | one | |
b6fa137b | 1367 | d Use Unicode or native charset, as in 5.12 and earlier |
87e95b7f YO |
1368 | |
1369 | If a precompiled pattern is embedded in a larger pattern then the effect | |
c543c01b | 1370 | of "msixpluad" will be propagated appropriately. The effect the "o" |
87e95b7f YO |
1371 | modifier has is not propagated, being restricted to those patterns |
1372 | explicitly using it. | |
1373 | ||
b6fa137b | 1374 | The last four modifiers listed above, added in Perl 5.14, |
5e2aa8f5 | 1375 | control the character set semantics. |
da392a17 | 1376 | |
87e95b7f | 1377 | See L<perlre> for additional information on valid syntax for STRING, and |
5e2aa8f5 | 1378 | for a detailed look at the semantics of regular expressions. In |
f6050459 KW |
1379 | particular, all the modifiers execpt C</o> are further explained in |
1380 | L<perlre/Modifiers>. C</o> is described in the next section. | |
a0d0e21e | 1381 | |
b6fa137b | 1382 | =item m/PATTERN/msixpodualgc |
89d205f2 YO |
1383 | X<m> X<operator, match> |
1384 | X<regexp, options> X<regexp> X<regex, options> X<regex> | |
01c6f5f4 | 1385 | X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> |
a0d0e21e | 1386 | |
b6fa137b | 1387 | =item /PATTERN/msixpodualgc |
a0d0e21e | 1388 | |
5a964f20 | 1389 | Searches a string for a pattern match, and in scalar context returns |
19799a22 GS |
1390 | true if it succeeds, false if it fails. If no string is specified |
1391 | via the C<=~> or C<!~> operator, the $_ string is searched. (The | |
1392 | string specified with C<=~> need not be an lvalue--it may be the | |
1393 | result of an expression evaluation, but remember the C<=~> binds | |
006671a6 | 1394 | rather tightly.) See also L<perlre>. |
a0d0e21e | 1395 | |
f6050459 | 1396 | Options are as described in C<qr//> above; in addition, the following match |
01c6f5f4 | 1397 | process modifiers are available: |
a0d0e21e | 1398 | |
950b09ed KW |
1399 | g Match globally, i.e., find all occurrences. |
1400 | c Do not reset search position on a failed match when /g is in effect. | |
a0d0e21e | 1401 | |
725a61d7 | 1402 | If "/" is the delimiter then the initial C<m> is optional. With the C<m> |
c543c01b | 1403 | you can use any pair of non-whitespace (ASCII) characters |
725a61d7 Z |
1404 | as delimiters. This is particularly useful for matching path names |
1405 | that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is | |
1406 | the delimiter, then a match-only-once rule applies, | |
1407 | described in C<m?PATTERN?> below. | |
19799a22 | 1408 | If "'" is the delimiter, no interpolation is performed on the PATTERN. |
ed02a3bf DN |
1409 | When using a character valid in an identifier, whitespace is required |
1410 | after the C<m>. | |
a0d0e21e | 1411 | |
532c9e80 KW |
1412 | PATTERN may contain variables, which will be interpolated |
1413 | every time the pattern search is evaluated, except | |
1f247705 GS |
1414 | for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and |
1415 | C<$|> are not interpolated because they look like end-of-string tests.) | |
532c9e80 KW |
1416 | Perl will not recompile the pattern unless an interpolated |
1417 | variable that it contains changes. You can force Perl to skip the | |
1418 | test and never recompile by adding a C</o> (which stands for "once") | |
1419 | after the trailing delimiter. | |
1420 | Once upon a time, Perl would recompile regular expressions | |
1421 | unnecessarily, and this modifier was useful to tell it not to do so, in the | |
1422 | interests of speed. But now, the only reasons to use C</o> are either: | |
1423 | ||
1424 | =over | |
1425 | ||
1426 | =item 1 | |
1427 | ||
1428 | The variables are thousands of characters long and you know that they | |
1429 | don't change, and you need to wring out the last little bit of speed by | |
1430 | having Perl skip testing for that. (There is a maintenance penalty for | |
1431 | doing this, as mentioning C</o> constitutes a promise that you won't | |
1432 | change the variables in the pattern. If you change them, Perl won't | |
1433 | even notice.) | |
1434 | ||
1435 | =item 2 | |
1436 | ||
1437 | you want the pattern to use the initial values of the variables | |
1438 | regardless of whether they change or not. (But there are saner ways | |
1439 | of accomplishing this than using C</o>.) | |
1440 | ||
1441 | =back | |
a0d0e21e | 1442 | |
e9d89077 DN |
1443 | =item The empty pattern // |
1444 | ||
5a964f20 | 1445 | If the PATTERN evaluates to the empty string, the last |
d65afb4b | 1446 | I<successfully> matched regular expression is used instead. In this |
c543c01b | 1447 | case, only the C<g> and C<c> flags on the empty pattern are honored; |
d65afb4b HS |
1448 | the other flags are taken from the original pattern. If no match has |
1449 | previously succeeded, this will (silently) act instead as a genuine | |
1450 | empty pattern (which will always match). | |
a0d0e21e | 1451 | |
89d205f2 YO |
1452 | Note that it's possible to confuse Perl into thinking C<//> (the empty |
1453 | regex) is really C<//> (the defined-or operator). Perl is usually pretty | |
1454 | good about this, but some pathological cases might trigger this, such as | |
1455 | C<$a///> (is that C<($a) / (//)> or C<$a // />?) and C<print $fh //> | |
1456 | (C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl | |
1457 | will assume you meant defined-or. If you meant the empty regex, just | |
1458 | use parentheses or spaces to disambiguate, or even prefix the empty | |
c963b151 BD |
1459 | regex with an C<m> (so C<//> becomes C<m//>). |
1460 | ||
e9d89077 DN |
1461 | =item Matching in list context |
1462 | ||
19799a22 | 1463 | If the C</g> option is not used, C<m//> in list context returns a |
a0d0e21e | 1464 | list consisting of the subexpressions matched by the parentheses in the |
f7e33566 GS |
1465 | pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are |
1466 | also set, and that this differs from Perl 4's behavior.) When there are | |
1467 | no parentheses in the pattern, the return value is the list C<(1)> for | |
1468 | success. With or without parentheses, an empty list is returned upon | |
1469 | failure. | |
a0d0e21e LW |
1470 | |
1471 | Examples: | |
1472 | ||
c543c01b TC |
1473 | open(TTY, "+>/dev/tty") |
1474 | || die "can't access /dev/tty: $!"; | |
1475 | ||
a0d0e21e LW |
1476 | <TTY> =~ /^y/i && foo(); # do foo if desired |
1477 | ||
1478 | if (/Version: *([0-9.]*)/) { $version = $1; } | |
1479 | ||
1480 | next if m#^/usr/spool/uucp#; | |
1481 | ||
1482 | # poor man's grep | |
1483 | $arg = shift; | |
1484 | while (<>) { | |
c543c01b | 1485 | print if /$arg/o; # compile only once (no longer needed!) |
a0d0e21e LW |
1486 | } |
1487 | ||
1488 | if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/)) | |
1489 | ||
1490 | This last example splits $foo into the first two words and the | |
5f05dabc | 1491 | remainder of the line, and assigns those three fields to $F1, $F2, and |
c543c01b TC |
1492 | $Etc. The conditional is true if any variables were assigned; that is, |
1493 | if the pattern matched. | |
a0d0e21e | 1494 | |
19799a22 | 1495 | The C</g> modifier specifies global pattern matching--that is, |
3dd93342 | 1496 | matching as many times as possible within the string. How it behaves |
1497 | depends on the context. In list context, it returns a list of the | |
19799a22 | 1498 | substrings matched by any capturing parentheses in the regular |
3dd93342 | 1499 | expression. If there are no parentheses, it returns a list of all |
19799a22 GS |
1500 | the matched strings, as if there were parentheses around the whole |
1501 | pattern. | |
a0d0e21e | 1502 | |
7e86de3e | 1503 | In scalar context, each execution of C<m//g> finds the next match, |
19799a22 | 1504 | returning true if it matches, and false if there is no further match. |
3dd93342 | 1505 | The position after the last match can be read or set using the C<pos()> |
1506 | function; see L<perlfunc/pos>. A failed match normally resets the | |
7e86de3e | 1507 | search position to the beginning of the string, but you can avoid that |
3dd93342 | 1508 | by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target |
7e86de3e | 1509 | string also resets the search position. |
c90c0ff4 | 1510 | |
e9d89077 DN |
1511 | =item \G assertion |
1512 | ||
c90c0ff4 | 1513 | You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a |
3dd93342 | 1514 | zero-width assertion that matches the exact position where the |
1515 | previous C<m//g>, if any, left off. Without the C</g> modifier, the | |
1516 | C<\G> assertion still anchors at C<pos()> as it was at the start of | |
1517 | the operation (see L<perlfunc/pos>), but the match is of course only | |
1518 | attempted once. Using C<\G> without C</g> on a target string that has | |
1519 | not previously had a C</g> match applied to it is the same as using | |
1520 | the C<\A> assertion to match the beginning of the string. Note also | |
1521 | that, currently, C<\G> is only properly supported when anchored at the | |
1522 | very beginning of the pattern. | |
c90c0ff4 | 1523 | |
1524 | Examples: | |
a0d0e21e LW |
1525 | |
1526 | # list context | |
1527 | ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g); | |
1528 | ||
1529 | # scalar context | |
c543c01b TC |
1530 | local $/ = ""; |
1531 | while ($paragraph = <>) { | |
1532 | while ($paragraph =~ /\p{Ll}['")]*[.!?]+['")]*\s/g) { | |
19799a22 | 1533 | $sentences++; |
a0d0e21e LW |
1534 | } |
1535 | } | |
c543c01b TC |
1536 | say $sentences; |
1537 | ||
1538 | Here's another way to check for sentences in a paragraph: | |
1539 | ||
1540 | my $sentence_rx = qr{ | |
1541 | (?: (?<= ^ ) | (?<= \s ) ) # after start-of-string or whitespace | |
1542 | \p{Lu} # capital letter | |
1543 | .*? # a bunch of anything | |
1544 | (?<= \S ) # that ends in non-whitespace | |
1545 | (?<! \b [DMS]r ) # but isn't a common abbreviation | |
1546 | (?<! \b Mrs ) | |
1547 | (?<! \b Sra ) | |
1548 | (?<! \b St ) | |
1549 | [.?!] # followed by a sentence ender | |
1550 | (?= $ | \s ) # in front of end-of-string or whitespace | |
1551 | }sx; | |
1552 | local $/ = ""; | |
1553 | while (my $paragraph = <>) { | |
1554 | say "NEW PARAGRAPH"; | |
1555 | my $count = 0; | |
1556 | while ($paragraph =~ /($sentence_rx)/g) { | |
1557 | printf "\tgot sentence %d: <%s>\n", ++$count, $1; | |
1558 | } | |
1559 | } | |
1560 | ||
1561 | Here's how to use C<m//gc> with C<\G>: | |
a0d0e21e | 1562 | |
137443ea | 1563 | $_ = "ppooqppqq"; |
44a8e56a | 1564 | while ($i++ < 2) { |
1565 | print "1: '"; | |
c90c0ff4 | 1566 | print $1 while /(o)/gc; print "', pos=", pos, "\n"; |
44a8e56a | 1567 | print "2: '"; |
c90c0ff4 | 1568 | print $1 if /\G(q)/gc; print "', pos=", pos, "\n"; |
44a8e56a | 1569 | print "3: '"; |
c90c0ff4 | 1570 | print $1 while /(p)/gc; print "', pos=", pos, "\n"; |
44a8e56a | 1571 | } |
5d43e42d | 1572 | print "Final: '$1', pos=",pos,"\n" if /\G(.)/; |
44a8e56a | 1573 | |
1574 | The last example should print: | |
1575 | ||
1576 | 1: 'oo', pos=4 | |
137443ea | 1577 | 2: 'q', pos=5 |
44a8e56a | 1578 | 3: 'pp', pos=7 |
1579 | 1: '', pos=7 | |
137443ea | 1580 | 2: 'q', pos=8 |
1581 | 3: '', pos=8 | |
5d43e42d DC |
1582 | Final: 'q', pos=8 |
1583 | ||
1584 | Notice that the final match matched C<q> instead of C<p>, which a match | |
1585 | without the C<\G> anchor would have done. Also note that the final match | |
ac036724 | 1586 | did not update C<pos>. C<pos> is only updated on a C</g> match. If the |
c543c01b TC |
1587 | final match did indeed match C<p>, it's a good bet that you're running a |
1588 | very old (pre-5.6.0) version of Perl. | |
44a8e56a | 1589 | |
c90c0ff4 | 1590 | A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can |
e7ea3e70 | 1591 | combine several regexps like this to process a string part-by-part, |
c90c0ff4 | 1592 | doing different actions depending on which regexp matched. Each |
1593 | regexp tries to match where the previous one leaves off. | |
e7ea3e70 | 1594 | |
3fe9a6f1 | 1595 | $_ = <<'EOL'; |
950b09ed | 1596 | $url = URI::URL->new( "http://example.com/" ); die if $url eq "xXx"; |
3fe9a6f1 | 1597 | EOL |
c543c01b TC |
1598 | |
1599 | LOOP: { | |
950b09ed | 1600 | print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc; |
c543c01b TC |
1601 | print(" lowercase"), redo LOOP if /\G\p{Ll}+\b[,.;]?\s*/gc; |
1602 | print(" UPPERCASE"), redo LOOP if /\G\p{Lu}+\b[,.;]?\s*/gc; | |
1603 | print(" Capitalized"), redo LOOP if /\G\p{Lu}\p{Ll}+\b[,.;]?\s*/gc; | |
1604 | print(" MiXeD"), redo LOOP if /\G\pL+\b[,.;]?\s*/gc; | |
1605 | print(" alphanumeric"), redo LOOP if /\G[\p{Alpha}\pN]+\b[,.;]?\s*/gc; | |
1606 | print(" line-noise"), redo LOOP if /\G\W+/gc; | |
950b09ed | 1607 | print ". That's all!\n"; |
c543c01b | 1608 | } |
e7ea3e70 IZ |
1609 | |
1610 | Here is the output (split into several lines): | |
1611 | ||
c543c01b TC |
1612 | line-noise lowercase line-noise UPPERCASE line-noise UPPERCASE |
1613 | line-noise lowercase line-noise lowercase line-noise lowercase | |
1614 | lowercase line-noise lowercase lowercase line-noise lowercase | |
1615 | lowercase line-noise MiXeD line-noise. That's all! | |
44a8e56a | 1616 | |
c543c01b | 1617 | =item m?PATTERN?msixpodualgc |
725a61d7 | 1618 | X<?> X<operator, match-once> |
87e95b7f | 1619 | |
c543c01b | 1620 | =item ?PATTERN?msixpodualgc |
55d389e7 | 1621 | |
725a61d7 Z |
1622 | This is just like the C<m/PATTERN/> search, except that it matches |
1623 | only once between calls to the reset() operator. This is a useful | |
87e95b7f | 1624 | optimization when you want to see only the first occurrence of |
ceb131e8 | 1625 | something in each file of a set of files, for instance. Only C<m??> |
87e95b7f YO |
1626 | patterns local to the current package are reset. |
1627 | ||
1628 | while (<>) { | |
ceb131e8 | 1629 | if (m?^$?) { |
87e95b7f YO |
1630 | # blank line between header and body |
1631 | } | |
1632 | } continue { | |
725a61d7 | 1633 | reset if eof; # clear m?? status for next file |
87e95b7f YO |
1634 | } |
1635 | ||
c543c01b TC |
1636 | Another example switched the first "latin1" encoding it finds |
1637 | to "utf8" in a pod file: | |
1638 | ||
1639 | s//utf8/ if m? ^ =encoding \h+ \K latin1 ?x; | |
1640 | ||
1641 | The match-once behavior is controlled by the match delimiter being | |
725a61d7 Z |
1642 | C<?>; with any other delimiter this is the normal C<m//> operator. |
1643 | ||
1644 | For historical reasons, the leading C<m> in C<m?PATTERN?> is optional, | |
1645 | but the resulting C<?PATTERN?> syntax is deprecated, will warn on | |
c543c01b TC |
1646 | usage and might be removed from a future stable release of Perl (without |
1647 | further notice!). | |
87e95b7f | 1648 | |
b6fa137b | 1649 | =item s/PATTERN/REPLACEMENT/msixpodualgcer |
87e95b7f | 1650 | X<substitute> X<substitution> X<replace> X<regexp, replace> |
4f4d7508 | 1651 | X<regexp, substitute> X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> X</e> X</r> |
87e95b7f YO |
1652 | |
1653 | Searches a string for a pattern, and if found, replaces that pattern | |
1654 | with the replacement text and returns the number of substitutions | |
1655 | made. Otherwise it returns false (specifically, the empty string). | |
1656 | ||
c543c01b | 1657 | If the C</r> (non-destructive) option is used then it runs the |
679563bb KW |
1658 | substitution on a copy of the string and instead of returning the |
1659 | number of substitutions, it returns the copy whether or not a | |
c543c01b TC |
1660 | substitution occurred. The original string is never changed when |
1661 | C</r> is used. The copy will always be a plain string, even if the | |
1662 | input is an object or a tied variable. | |
4f4d7508 | 1663 | |
87e95b7f | 1664 | If no string is specified via the C<=~> or C<!~> operator, the C<$_> |
c543c01b TC |
1665 | variable is searched and modified. Unless the C</r> option is used, |
1666 | the string specified must be a scalar variable, an array element, a | |
1667 | hash element, or an assignment to one of those; that is, some sort of | |
1668 | scalar lvalue. | |
87e95b7f YO |
1669 | |
1670 | If the delimiter chosen is a single quote, no interpolation is | |
1671 | done on either the PATTERN or the REPLACEMENT. Otherwise, if the | |
1672 | PATTERN contains a $ that looks like a variable rather than an | |
1673 | end-of-string test, the variable will be interpolated into the pattern | |
1674 | at run-time. If you want the pattern compiled only once the first time | |
1675 | the variable is interpolated, use the C</o> option. If the pattern | |
1676 | evaluates to the empty string, the last successfully executed regular | |
1677 | expression is used instead. See L<perlre> for further explanation on these. | |
87e95b7f YO |
1678 | |
1679 | Options are as with m// with the addition of the following replacement | |
1680 | specific options: | |
1681 | ||
1682 | e Evaluate the right side as an expression. | |
4f4d7508 DC |
1683 | ee Evaluate the right side as a string then eval the result. |
1684 | r Return substitution and leave the original string untouched. | |
87e95b7f | 1685 | |
ed02a3bf DN |
1686 | Any non-whitespace delimiter may replace the slashes. Add space after |
1687 | the C<s> when using a character allowed in identifiers. If single quotes | |
1688 | are used, no interpretation is done on the replacement string (the C</e> | |
1689 | modifier overrides this, however). Unlike Perl 4, Perl 5 treats backticks | |
1690 | as normal delimiters; the replacement text is not evaluated as a command. | |
1691 | If the PATTERN is delimited by bracketing quotes, the REPLACEMENT has | |
1692 | its own pair of quotes, which may or may not be bracketing quotes, e.g., | |
87e95b7f YO |
1693 | C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the |
1694 | replacement portion to be treated as a full-fledged Perl expression | |
1695 | and evaluated right then and there. It is, however, syntax checked at | |
1696 | compile-time. A second C<e> modifier will cause the replacement portion | |
1697 | to be C<eval>ed before being run as a Perl expression. | |
1698 | ||
1699 | Examples: | |
1700 | ||
1701 | s/\bgreen\b/mauve/g; # don't change wintergreen | |
1702 | ||
1703 | $path =~ s|/usr/bin|/usr/local/bin|; | |
1704 | ||
1705 | s/Login: $foo/Login: $bar/; # run-time pattern | |
1706 | ||
1707 | ($foo = $bar) =~ s/this/that/; # copy first, then change | |
4f4d7508 DC |
1708 | ($foo = "$bar") =~ s/this/that/; # convert to string, copy, then change |
1709 | $foo = $bar =~ s/this/that/r; # Same as above using /r | |
1710 | $foo = $bar =~ s/this/that/r | |
1711 | =~ s/that/the other/r; # Chained substitutes using /r | |
1712 | @foo = map { s/this/that/r } @bar # /r is very useful in maps | |
87e95b7f YO |
1713 | |
1714 | $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count | |
1715 | ||
1716 | $_ = 'abc123xyz'; | |
1717 | s/\d+/$&*2/e; # yields 'abc246xyz' | |
1718 | s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz' | |
1719 | s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz' | |
1720 | ||
1721 | s/%(.)/$percent{$1}/g; # change percent escapes; no /e | |
1722 | s/%(.)/$percent{$1} || $&/ge; # expr now, so /e | |
1723 | s/^=(\w+)/pod($1)/ge; # use function call | |
1724 | ||
4f4d7508 DC |
1725 | $_ = 'abc123xyz'; |
1726 | $a = s/abc/def/r; # $a is 'def123xyz' and | |
1727 | # $_ remains 'abc123xyz'. | |
1728 | ||
87e95b7f YO |
1729 | # expand variables in $_, but dynamics only, using |
1730 | # symbolic dereferencing | |
1731 | s/\$(\w+)/${$1}/g; | |
1732 | ||
1733 | # Add one to the value of any numbers in the string | |
1734 | s/(\d+)/1 + $1/eg; | |
1735 | ||
c543c01b TC |
1736 | # Titlecase words in the last 30 characters only |
1737 | substr($str, -30) =~ s/\b(\p{Alpha}+)\b/\u\L$1/g; | |
1738 | ||
87e95b7f YO |
1739 | # This will expand any embedded scalar variable |
1740 | # (including lexicals) in $_ : First $1 is interpolated | |
1741 | # to the variable name, and then evaluated | |
1742 | s/(\$\w+)/$1/eeg; | |
1743 | ||
1744 | # Delete (most) C comments. | |
1745 | $program =~ s { | |
1746 | /\* # Match the opening delimiter. | |
1747 | .*? # Match a minimal number of characters. | |
1748 | \*/ # Match the closing delimiter. | |
1749 | } []gsx; | |
1750 | ||
1751 | s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_, expensively | |
1752 | ||
1753 | for ($variable) { # trim whitespace in $variable, cheap | |
1754 | s/^\s+//; | |
1755 | s/\s+$//; | |
1756 | } | |
1757 | ||
1758 | s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields | |
1759 | ||
1760 | Note the use of $ instead of \ in the last example. Unlike | |
1761 | B<sed>, we use the \<I<digit>> form in only the left hand side. | |
1762 | Anywhere else it's $<I<digit>>. | |
1763 | ||
1764 | Occasionally, you can't use just a C</g> to get all the changes | |
1765 | to occur that you might want. Here are two common cases: | |
1766 | ||
1767 | # put commas in the right places in an integer | |
1768 | 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g; | |
1769 | ||
1770 | # expand tabs to 8-column spacing | |
1771 | 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e; | |
1772 | ||
1c424184 FC |
1773 | C<s///le> is treated as a substitution followed by the C<le> operator, not |
1774 | the C</le> flags. This may change in a future version of Perl. It | |
1775 | produces a warning if warnings are enabled. To disambiguate, use a space | |
1776 | or change the order of the flags: | |
1777 | ||
1778 | s/foo/bar/ le 5; # "le" infix operator | |
1779 | s/foo/bar/el; # "e" and "l" flags | |
1780 | ||
87e95b7f YO |
1781 | =back |
1782 | ||
1783 | =head2 Quote-Like Operators | |
1784 | X<operator, quote-like> | |
1785 | ||
01c6f5f4 RGS |
1786 | =over 4 |
1787 | ||
a0d0e21e | 1788 | =item q/STRING/ |
5d44bfff | 1789 | X<q> X<quote, single> X<'> X<''> |
a0d0e21e | 1790 | |
5d44bfff | 1791 | =item 'STRING' |
a0d0e21e | 1792 | |
19799a22 | 1793 | A single-quoted, literal string. A backslash represents a backslash |
68dc0745 | 1794 | unless followed by the delimiter or another backslash, in which case |
1795 | the delimiter or backslash is interpolated. | |
a0d0e21e LW |
1796 | |
1797 | $foo = q!I said, "You said, 'She said it.'"!; | |
1798 | $bar = q('This is it.'); | |
68dc0745 | 1799 | $baz = '\n'; # a two-character string |
a0d0e21e LW |
1800 | |
1801 | =item qq/STRING/ | |
d74e8afc | 1802 | X<qq> X<quote, double> X<"> X<""> |
a0d0e21e LW |
1803 | |
1804 | =item "STRING" | |
1805 | ||
1806 | A double-quoted, interpolated string. | |
1807 | ||
1808 | $_ .= qq | |
1809 | (*** The previous line contains the naughty word "$1".\n) | |
19799a22 | 1810 | if /\b(tcl|java|python)\b/i; # :-) |
68dc0745 | 1811 | $baz = "\n"; # a one-character string |
a0d0e21e LW |
1812 | |
1813 | =item qx/STRING/ | |
d74e8afc | 1814 | X<qx> X<`> X<``> X<backtick> |
a0d0e21e LW |
1815 | |
1816 | =item `STRING` | |
1817 | ||
43dd4d21 JH |
1818 | A string which is (possibly) interpolated and then executed as a |
1819 | system command with C</bin/sh> or its equivalent. Shell wildcards, | |
1820 | pipes, and redirections will be honored. The collected standard | |
1821 | output of the command is returned; standard error is unaffected. In | |
1822 | scalar context, it comes back as a single (potentially multi-line) | |
1823 | string, or undef if the command failed. In list context, returns a | |
1824 | list of lines (however you've defined lines with $/ or | |
1825 | $INPUT_RECORD_SEPARATOR), or an empty list if the command failed. | |
5a964f20 TC |
1826 | |
1827 | Because backticks do not affect standard error, use shell file descriptor | |
1828 | syntax (assuming the shell supports this) if you care to address this. | |
1829 | To capture a command's STDERR and STDOUT together: | |
a0d0e21e | 1830 | |
5a964f20 TC |
1831 | $output = `cmd 2>&1`; |
1832 | ||
1833 | To capture a command's STDOUT but discard its STDERR: | |
1834 | ||
1835 | $output = `cmd 2>/dev/null`; | |
1836 | ||
1837 | To capture a command's STDERR but discard its STDOUT (ordering is | |
1838 | important here): | |
1839 | ||
1840 | $output = `cmd 2>&1 1>/dev/null`; | |
1841 | ||
1842 | To exchange a command's STDOUT and STDERR in order to capture the STDERR | |
1843 | but leave its STDOUT to come out the old STDERR: | |
1844 | ||
1845 | $output = `cmd 3>&1 1>&2 2>&3 3>&-`; | |
1846 | ||
1847 | To read both a command's STDOUT and its STDERR separately, it's easiest | |
2359510d SD |
1848 | to redirect them separately to files, and then read from those files |
1849 | when the program is done: | |
5a964f20 | 1850 | |
2359510d | 1851 | system("program args 1>program.stdout 2>program.stderr"); |
5a964f20 | 1852 | |
30398227 SP |
1853 | The STDIN filehandle used by the command is inherited from Perl's STDIN. |
1854 | For example: | |
1855 | ||
c543c01b TC |
1856 | open(SPLAT, "stuff") || die "can't open stuff: $!"; |
1857 | open(STDIN, "<&SPLAT") || die "can't dupe SPLAT: $!"; | |
40bbb707 | 1858 | print STDOUT `sort`; |
30398227 | 1859 | |
40bbb707 | 1860 | will print the sorted contents of the file named F<"stuff">. |
30398227 | 1861 | |
5a964f20 TC |
1862 | Using single-quote as a delimiter protects the command from Perl's |
1863 | double-quote interpolation, passing it on to the shell instead: | |
1864 | ||
1865 | $perl_info = qx(ps $$); # that's Perl's $$ | |
1866 | $shell_info = qx'ps $$'; # that's the new shell's $$ | |
1867 | ||
19799a22 | 1868 | How that string gets evaluated is entirely subject to the command |
5a964f20 TC |
1869 | interpreter on your system. On most platforms, you will have to protect |
1870 | shell metacharacters if you want them treated literally. This is in | |
1871 | practice difficult to do, as it's unclear how to escape which characters. | |
1872 | See L<perlsec> for a clean and safe example of a manual fork() and exec() | |
1873 | to emulate backticks safely. | |
a0d0e21e | 1874 | |
bb32b41a GS |
1875 | On some platforms (notably DOS-like ones), the shell may not be |
1876 | capable of dealing with multiline commands, so putting newlines in | |
1877 | the string may not get you what you want. You may be able to evaluate | |
1878 | multiple commands in a single line by separating them with the command | |
1879 | separator character, if your shell supports that (e.g. C<;> on many Unix | |
1880 | shells; C<&> on the Windows NT C<cmd> shell). | |
1881 | ||
0f897271 GS |
1882 | Beginning with v5.6.0, Perl will attempt to flush all files opened for |
1883 | output before starting the child process, but this may not be supported | |
1884 | on some platforms (see L<perlport>). To be safe, you may need to set | |
1885 | C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of | |
1886 | C<IO::Handle> on any open handles. | |
1887 | ||
bb32b41a GS |
1888 | Beware that some command shells may place restrictions on the length |
1889 | of the command line. You must ensure your strings don't exceed this | |
1890 | limit after any necessary interpolations. See the platform-specific | |
1891 | release notes for more details about your particular environment. | |
1892 | ||
5a964f20 TC |
1893 | Using this operator can lead to programs that are difficult to port, |
1894 | because the shell commands called vary between systems, and may in | |
1895 | fact not be present at all. As one example, the C<type> command under | |
1896 | the POSIX shell is very different from the C<type> command under DOS. | |
1897 | That doesn't mean you should go out of your way to avoid backticks | |
1898 | when they're the right way to get something done. Perl was made to be | |
1899 | a glue language, and one of the things it glues together is commands. | |
1900 | Just understand what you're getting yourself into. | |
bb32b41a | 1901 | |
da87341d | 1902 | See L</"I/O Operators"> for more discussion. |
a0d0e21e | 1903 | |
945c54fd | 1904 | =item qw/STRING/ |
d74e8afc | 1905 | X<qw> X<quote, list> X<quote, words> |
945c54fd JH |
1906 | |
1907 | Evaluates to a list of the words extracted out of STRING, using embedded | |
1908 | whitespace as the word delimiters. It can be understood as being roughly | |
1909 | equivalent to: | |
1910 | ||
c543c01b | 1911 | split(" ", q/STRING/); |
945c54fd | 1912 | |
efb1e162 CW |
1913 | the differences being that it generates a real list at compile time, and |
1914 | in scalar context it returns the last element in the list. So | |
945c54fd JH |
1915 | this expression: |
1916 | ||
1917 | qw(foo bar baz) | |
1918 | ||
1919 | is semantically equivalent to the list: | |
1920 | ||
c543c01b | 1921 | "foo", "bar", "baz" |
945c54fd JH |
1922 | |
1923 | Some frequently seen examples: | |
1924 | ||
1925 | use POSIX qw( setlocale localeconv ) | |
1926 | @EXPORT = qw( foo bar baz ); | |
1927 | ||
1928 | A common mistake is to try to separate the words with comma or to | |
1929 | put comments into a multi-line C<qw>-string. For this reason, the | |
89d205f2 | 1930 | C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable) |
945c54fd JH |
1931 | produces warnings if the STRING contains the "," or the "#" character. |
1932 | ||
8ff32507 | 1933 | =item tr/SEARCHLIST/REPLACEMENTLIST/cdsr |
d74e8afc | 1934 | X<tr> X<y> X<transliterate> X</c> X</d> X</s> |
a0d0e21e | 1935 | |
8ff32507 | 1936 | =item y/SEARCHLIST/REPLACEMENTLIST/cdsr |
a0d0e21e | 1937 | |
2c268ad5 | 1938 | Transliterates all occurrences of the characters found in the search list |
a0d0e21e LW |
1939 | with the corresponding character in the replacement list. It returns |
1940 | the number of characters replaced or deleted. If no string is | |
c543c01b TC |
1941 | specified via the C<=~> or C<!~> operator, the $_ string is transliterated. |
1942 | ||
1943 | If the C</r> (non-destructive) option is present, a new copy of the string | |
1944 | is made and its characters transliterated, and this copy is returned no | |
1945 | matter whether it was modified or not: the original string is always | |
1946 | left unchanged. The new copy is always a plain string, even if the input | |
1947 | string is an object or a tied variable. | |
8ada0baa | 1948 | |
c543c01b TC |
1949 | Unless the C</r> option is used, the string specified with C<=~> must be a |
1950 | scalar variable, an array element, a hash element, or an assignment to one | |
1951 | of those; in other words, an lvalue. | |
8ff32507 | 1952 | |
89d205f2 | 1953 | A character range may be specified with a hyphen, so C<tr/A-J/0-9/> |
2c268ad5 | 1954 | does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>. |
54310121 | 1955 | For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the |
1956 | SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has | |
c543c01b TC |
1957 | its own pair of quotes, which may or may not be bracketing quotes; |
1958 | for example, C<tr[aeiouy][yuoiea]> or C<tr(+\-*/)/ABCD/>. | |
1959 | ||
1960 | Note that C<tr> does B<not> do regular expression character classes such as | |
1961 | C<\d> or C<\pL>. The C<tr> operator is not equivalent to the tr(1) | |
1962 | utility. If you want to map strings between lower/upper cases, see | |
1963 | L<perlfunc/lc> and L<perlfunc/uc>, and in general consider using the C<s> | |
1964 | operator if you need regular expressions. The C<\U>, C<\u>, C<\L>, and | |
1965 | C<\l> string-interpolation escapes on the right side of a substitution | |
1966 | operator will perform correct case-mappings, but C<tr[a-z][A-Z]> will not | |
1967 | (except sometimes on legacy 7-bit data). | |
cc255d5f | 1968 | |
8ada0baa JH |
1969 | Note also that the whole range idea is rather unportable between |
1970 | character sets--and even within character sets they may cause results | |
1971 | you probably didn't expect. A sound principle is to use only ranges | |
1972 | that begin from and end at either alphabets of equal case (a-e, A-E), | |
1973 | or digits (0-4). Anything else is unsafe. If in doubt, spell out the | |
1974 | character sets in full. | |
1975 | ||
a0d0e21e LW |
1976 | Options: |
1977 | ||
1978 | c Complement the SEARCHLIST. | |
1979 | d Delete found but unreplaced characters. | |
1980 | s Squash duplicate replaced characters. | |
8ff32507 FC |
1981 | r Return the modified string and leave the original string |
1982 | untouched. | |
a0d0e21e | 1983 | |
19799a22 GS |
1984 | If the C</c> modifier is specified, the SEARCHLIST character set |
1985 | is complemented. If the C</d> modifier is specified, any characters | |
1986 | specified by SEARCHLIST not found in REPLACEMENTLIST are deleted. | |
1987 | (Note that this is slightly more flexible than the behavior of some | |
1988 | B<tr> programs, which delete anything they find in the SEARCHLIST, | |
1989 | period.) If the C</s> modifier is specified, sequences of characters | |
1990 | that were transliterated to the same character are squashed down | |
1991 | to a single instance of the character. | |
a0d0e21e LW |
1992 | |
1993 | If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted | |
1994 | exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter | |
1995 | than the SEARCHLIST, the final character is replicated till it is long | |
5a964f20 | 1996 | enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated. |
a0d0e21e LW |
1997 | This latter is useful for counting characters in a class or for |
1998 | squashing character sequences in a class. | |
1999 | ||
2000 | Examples: | |
2001 | ||
c543c01b | 2002 | $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case ASCII |
a0d0e21e LW |
2003 | |
2004 | $cnt = tr/*/*/; # count the stars in $_ | |
2005 | ||
2006 | $cnt = $sky =~ tr/*/*/; # count the stars in $sky | |
2007 | ||
2008 | $cnt = tr/0-9//; # count the digits in $_ | |
2009 | ||
2010 | tr/a-zA-Z//s; # bookkeeper -> bokeper | |
2011 | ||
2012 | ($HOST = $host) =~ tr/a-z/A-Z/; | |
c543c01b | 2013 | $HOST = $host =~ tr/a-z/A-Z/r; # same thing |
8ff32507 | 2014 | |
c543c01b | 2015 | $HOST = $host =~ tr/a-z/A-Z/r # chained with s///r |
8ff32507 | 2016 | =~ s/:/ -p/r; |
a0d0e21e LW |
2017 | |
2018 | tr/a-zA-Z/ /cs; # change non-alphas to single space | |
2019 | ||
8ff32507 FC |
2020 | @stripped = map tr/a-zA-Z/ /csr, @original; |
2021 | # /r with map | |
2022 | ||
a0d0e21e | 2023 | tr [\200-\377] |
c543c01b | 2024 | [\000-\177]; # wickedly delete 8th bit |
a0d0e21e | 2025 | |
19799a22 GS |
2026 | If multiple transliterations are given for a character, only the |
2027 | first one is used: | |
748a9306 LW |
2028 | |
2029 | tr/AAA/XYZ/ | |
2030 | ||
2c268ad5 | 2031 | will transliterate any A to X. |
748a9306 | 2032 | |
19799a22 | 2033 | Because the transliteration table is built at compile time, neither |
a0d0e21e | 2034 | the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote |
19799a22 GS |
2035 | interpolation. That means that if you want to use variables, you |
2036 | must use an eval(): | |
a0d0e21e LW |
2037 | |
2038 | eval "tr/$oldlist/$newlist/"; | |
2039 | die $@ if $@; | |
2040 | ||
2041 | eval "tr/$oldlist/$newlist/, 1" or die $@; | |
2042 | ||
7e3b091d | 2043 | =item <<EOF |
d74e8afc | 2044 | X<here-doc> X<heredoc> X<here-document> X<<< << >>> |
7e3b091d DA |
2045 | |
2046 | A line-oriented form of quoting is based on the shell "here-document" | |
2047 | syntax. Following a C<< << >> you specify a string to terminate | |
2048 | the quoted material, and all lines following the current line down to | |
89d205f2 YO |
2049 | the terminating string are the value of the item. |
2050 | ||
2051 | The terminating string may be either an identifier (a word), or some | |
2052 | quoted text. An unquoted identifier works like double quotes. | |
2053 | There may not be a space between the C<< << >> and the identifier, | |
2054 | unless the identifier is explicitly quoted. (If you put a space it | |
2055 | will be treated as a null identifier, which is valid, and matches the | |
2056 | first empty line.) The terminating string must appear by itself | |
2057 | (unquoted and with no surrounding whitespace) on the terminating line. | |
2058 | ||
2059 | If the terminating string is quoted, the type of quotes used determine | |
2060 | the treatment of the text. | |
2061 | ||
2062 | =over 4 | |
2063 | ||
2064 | =item Double Quotes | |
2065 | ||
2066 | Double quotes indicate that the text will be interpolated using exactly | |
2067 | the same rules as normal double quoted strings. | |
7e3b091d DA |
2068 | |
2069 | print <<EOF; | |
2070 | The price is $Price. | |
2071 | EOF | |
2072 | ||
2073 | print << "EOF"; # same as above | |
2074 | The price is $Price. | |
2075 | EOF | |
2076 | ||
89d205f2 YO |
2077 | |
2078 | =item Single Quotes | |
2079 | ||
2080 | Single quotes indicate the text is to be treated literally with no | |
2081 | interpolation of its content. This is similar to single quoted | |
2082 | strings except that backslashes have no special meaning, with C<\\> | |
2083 | being treated as two backslashes and not one as they would in every | |
2084 | other quoting construct. | |
2085 | ||
c543c01b TC |
2086 | Just as in the shell, a backslashed bareword following the C<<< << >>> |
2087 | means the same thing as a single-quoted string does: | |
2088 | ||
2089 | $cost = <<'VISTA'; # hasta la ... | |
2090 | That'll be $10 please, ma'am. | |
2091 | VISTA | |
2092 | ||
2093 | $cost = <<\VISTA; # Same thing! | |
2094 | That'll be $10 please, ma'am. | |
2095 | VISTA | |
2096 | ||
89d205f2 YO |
2097 | This is the only form of quoting in perl where there is no need |
2098 | to worry about escaping content, something that code generators | |
2099 | can and do make good use of. | |
2100 | ||
2101 | =item Backticks | |
2102 | ||
2103 | The content of the here doc is treated just as it would be if the | |
2104 | string were embedded in backticks. Thus the content is interpolated | |
2105 | as though it were double quoted and then executed via the shell, with | |
2106 | the results of the execution returned. | |
2107 | ||
2108 | print << `EOC`; # execute command and get results | |
7e3b091d | 2109 | echo hi there |
7e3b091d DA |
2110 | EOC |
2111 | ||
89d205f2 YO |
2112 | =back |
2113 | ||
2114 | It is possible to stack multiple here-docs in a row: | |
2115 | ||
7e3b091d DA |
2116 | print <<"foo", <<"bar"; # you can stack them |
2117 | I said foo. | |
2118 | foo | |
2119 | I said bar. | |
2120 | bar | |
2121 | ||
2122 | myfunc(<< "THIS", 23, <<'THAT'); | |
2123 | Here's a line | |
2124 | or two. | |
2125 | THIS | |
2126 | and here's another. | |
2127 | THAT | |
2128 | ||
2129 | Just don't forget that you have to put a semicolon on the end | |
2130 | to finish the statement, as Perl doesn't know you're not going to | |
2131 | try to do this: | |
2132 | ||
2133 | print <<ABC | |
2134 | 179231 | |
2135 | ABC | |
2136 | + 20; | |
2137 | ||
872d7e53 TS |
2138 | If you want to remove the line terminator from your here-docs, |
2139 | use C<chomp()>. | |
2140 | ||
2141 | chomp($string = <<'END'); | |
2142 | This is a string. | |
2143 | END | |
2144 | ||
2145 | If you want your here-docs to be indented with the rest of the code, | |
2146 | you'll need to remove leading whitespace from each line manually: | |
7e3b091d DA |
2147 | |
2148 | ($quote = <<'FINIS') =~ s/^\s+//gm; | |
89d205f2 | 2149 | The Road goes ever on and on, |
7e3b091d DA |
2150 | down from the door where it began. |
2151 | FINIS | |
2152 | ||
2153 | If you use a here-doc within a delimited construct, such as in C<s///eg>, | |
2154 | the quoted material must come on the lines following the final delimiter. | |
2155 | So instead of | |
2156 | ||
2157 | s/this/<<E . 'that' | |
2158 | the other | |
2159 | E | |
2160 | . 'more '/eg; | |
2161 | ||
2162 | you have to write | |
2163 | ||
89d205f2 YO |
2164 | s/this/<<E . 'that' |
2165 | . 'more '/eg; | |
2166 | the other | |
2167 | E | |
7e3b091d DA |
2168 | |
2169 | If the terminating identifier is on the last line of the program, you | |
2170 | must be sure there is a newline after it; otherwise, Perl will give the | |
2171 | warning B<Can't find string terminator "END" anywhere before EOF...>. | |
2172 | ||
c543c01b TC |
2173 | Additionally, quoting rules for the end-of-string identifier are |
2174 | unrelated to Perl's quoting rules. C<q()>, C<qq()>, and the like are not | |
89d205f2 YO |
2175 | supported in place of C<''> and C<"">, and the only interpolation is for |
2176 | backslashing the quoting character: | |
7e3b091d DA |
2177 | |
2178 | print << "abc\"def"; | |
2179 | testing... | |
2180 | abc"def | |
2181 | ||
2182 | Finally, quoted strings cannot span multiple lines. The general rule is | |
2183 | that the identifier must be a string literal. Stick with that, and you | |
2184 | should be safe. | |
2185 | ||
a0d0e21e LW |
2186 | =back |
2187 | ||
75e14d17 | 2188 | =head2 Gory details of parsing quoted constructs |
d74e8afc | 2189 | X<quote, gory details> |
75e14d17 | 2190 | |
19799a22 GS |
2191 | When presented with something that might have several different |
2192 | interpretations, Perl uses the B<DWIM> (that's "Do What I Mean") | |
2193 | principle to pick the most probable interpretation. This strategy | |
2194 | is so successful that Perl programmers often do not suspect the | |
2195 | ambivalence of what they write. But from time to time, Perl's | |
2196 | notions differ substantially from what the author honestly meant. | |
2197 | ||
2198 | This section hopes to clarify how Perl handles quoted constructs. | |
2199 | Although the most common reason to learn this is to unravel labyrinthine | |
2200 | regular expressions, because the initial steps of parsing are the | |
2201 | same for all quoting operators, they are all discussed together. | |
2202 | ||
2203 | The most important Perl parsing rule is the first one discussed | |
2204 | below: when processing a quoted construct, Perl first finds the end | |
2205 | of that construct, then interprets its contents. If you understand | |
2206 | this rule, you may skip the rest of this section on the first | |
2207 | reading. The other rules are likely to contradict the user's | |
2208 | expectations much less frequently than this first one. | |
2209 | ||
2210 | Some passes discussed below are performed concurrently, but because | |
2211 | their results are the same, we consider them individually. For different | |
2212 | quoting constructs, Perl performs different numbers of passes, from | |
6deea57f | 2213 | one to four, but these passes are always performed in the same order. |
75e14d17 | 2214 | |
13a2d996 | 2215 | =over 4 |
75e14d17 IZ |
2216 | |
2217 | =item Finding the end | |
2218 | ||
6deea57f TS |
2219 | The first pass is finding the end of the quoted construct, where |
2220 | the information about the delimiters is used in parsing. | |
2221 | During this search, text between the starting and ending delimiters | |
2222 | is copied to a safe location. The text copied gets delimiter-independent. | |
2223 | ||
2224 | If the construct is a here-doc, the ending delimiter is a line | |
2225 | that has a terminating string as the content. Therefore C<<<EOF> is | |
2226 | terminated by C<EOF> immediately followed by C<"\n"> and starting | |
2227 | from the first column of the terminating line. | |
2228 | When searching for the terminating line of a here-doc, nothing | |
2229 | is skipped. In other words, lines after the here-doc syntax | |
2230 | are compared with the terminating string line by line. | |
2231 | ||
2232 | For the constructs except here-docs, single characters are used as starting | |
2233 | and ending delimiters. If the starting delimiter is an opening punctuation | |
2234 | (that is C<(>, C<[>, C<{>, or C<< < >>), the ending delimiter is the | |
2235 | corresponding closing punctuation (that is C<)>, C<]>, C<}>, or C<< > >>). | |
2236 | If the starting delimiter is an unpaired character like C</> or a closing | |
2237 | punctuation, the ending delimiter is same as the starting delimiter. | |
2238 | Therefore a C</> terminates a C<qq//> construct, while a C<]> terminates | |
2239 | C<qq[]> and C<qq]]> constructs. | |
2240 | ||
2241 | When searching for single-character delimiters, escaped delimiters | |
2242 | and C<\\> are skipped. For example, while searching for terminating C</>, | |
2243 | combinations of C<\\> and C<\/> are skipped. If the delimiters are | |
2244 | bracketing, nested pairs are also skipped. For example, while searching | |
2245 | for closing C<]> paired with the opening C<[>, combinations of C<\\>, C<\]>, | |
2246 | and C<\[> are all skipped, and nested C<[> and C<]> are skipped as well. | |
2247 | However, when backslashes are used as the delimiters (like C<qq\\> and | |
2248 | C<tr\\\>), nothing is skipped. | |
2249 | During the search for the end, backslashes that escape delimiters | |
2250 | are removed (exactly speaking, they are not copied to the safe location). | |
75e14d17 | 2251 | |
19799a22 GS |
2252 | For constructs with three-part delimiters (C<s///>, C<y///>, and |
2253 | C<tr///>), the search is repeated once more. | |
6deea57f TS |
2254 | If the first delimiter is not an opening punctuation, three delimiters must |
2255 | be same such as C<s!!!> and C<tr)))>, in which case the second delimiter | |
2256 | terminates the left part and starts the right part at once. | |
b6538e4f | 2257 | If the left part is delimited by bracketing punctuation (that is C<()>, |
6deea57f | 2258 | C<[]>, C<{}>, or C<< <> >>), the right part needs another pair of |
b6538e4f | 2259 | delimiters such as C<s(){}> and C<tr[]//>. In these cases, whitespace |
6deea57f | 2260 | and comments are allowed between both parts, though the comment must follow |
b6538e4f TC |
2261 | at least one whitespace character; otherwise a character expected as the |
2262 | start of the comment may be regarded as the starting delimiter of the right part. | |
75e14d17 | 2263 | |
19799a22 GS |
2264 | During this search no attention is paid to the semantics of the construct. |
2265 | Thus: | |
75e14d17 IZ |
2266 | |
2267 | "$hash{"$foo/$bar"}" | |
2268 | ||
2a94b7ce | 2269 | or: |
75e14d17 | 2270 | |
89d205f2 | 2271 | m/ |
2a94b7ce | 2272 | bar # NOT a comment, this slash / terminated m//! |
75e14d17 IZ |
2273 | /x |
2274 | ||
19799a22 GS |
2275 | do not form legal quoted expressions. The quoted part ends on the |
2276 | first C<"> and C</>, and the rest happens to be a syntax error. | |
2277 | Because the slash that terminated C<m//> was followed by a C<SPACE>, | |
2278 | the example above is not C<m//x>, but rather C<m//> with no C</x> | |
2279 | modifier. So the embedded C<#> is interpreted as a literal C<#>. | |
75e14d17 | 2280 | |
89d205f2 YO |
2281 | Also no attention is paid to C<\c\> (multichar control char syntax) during |
2282 | this search. Thus the second C<\> in C<qq/\c\/> is interpreted as a part | |
2283 | of C<\/>, and the following C</> is not recognized as a delimiter. | |
0d594e51 TS |
2284 | Instead, use C<\034> or C<\x1c> at the end of quoted constructs. |
2285 | ||
75e14d17 | 2286 | =item Interpolation |
d74e8afc | 2287 | X<interpolation> |
75e14d17 | 2288 | |
19799a22 | 2289 | The next step is interpolation in the text obtained, which is now |
89d205f2 | 2290 | delimiter-independent. There are multiple cases. |
75e14d17 | 2291 | |
13a2d996 | 2292 | =over 4 |
75e14d17 | 2293 | |
89d205f2 | 2294 | =item C<<<'EOF'> |
75e14d17 IZ |
2295 | |
2296 | No interpolation is performed. | |
6deea57f TS |
2297 | Note that the combination C<\\> is left intact, since escaped delimiters |
2298 | are not available for here-docs. | |
75e14d17 | 2299 | |
6deea57f | 2300 | =item C<m''>, the pattern of C<s'''> |
89d205f2 | 2301 | |
6deea57f TS |
2302 | No interpolation is performed at this stage. |
2303 | Any backslashed sequences including C<\\> are treated at the stage | |
2304 | to L</"parsing regular expressions">. | |
89d205f2 | 2305 | |
6deea57f | 2306 | =item C<''>, C<q//>, C<tr'''>, C<y'''>, the replacement of C<s'''> |
75e14d17 | 2307 | |
89d205f2 | 2308 | The only interpolation is removal of C<\> from pairs of C<\\>. |
6deea57f TS |
2309 | Therefore C<-> in C<tr'''> and C<y'''> is treated literally |
2310 | as a hyphen and no character range is available. | |
2311 | C<\1> in the replacement of C<s'''> does not work as C<$1>. | |
89d205f2 YO |
2312 | |
2313 | =item C<tr///>, C<y///> | |
2314 | ||
6deea57f TS |
2315 | No variable interpolation occurs. String modifying combinations for |
2316 | case and quoting such as C<\Q>, C<\U>, and C<\E> are not recognized. | |
2317 | The other escape sequences such as C<\200> and C<\t> and backslashed | |
2318 | characters such as C<\\> and C<\-> are converted to appropriate literals. | |
89d205f2 YO |
2319 | The character C<-> is treated specially and therefore C<\-> is treated |
2320 | as a literal C<->. | |
75e14d17 | 2321 | |
89d205f2 | 2322 | =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF"> |
75e14d17 | 2323 | |
19799a22 GS |
2324 | C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are |
2325 | converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar"> | |
2326 | is converted to C<$foo . (quotemeta("baz" . $bar))> internally. | |
6deea57f TS |
2327 | The other escape sequences such as C<\200> and C<\t> and backslashed |
2328 | characters such as C<\\> and C<\-> are replaced with appropriate | |
2329 | expansions. | |
2a94b7ce | 2330 | |
19799a22 GS |
2331 | Let it be stressed that I<whatever falls between C<\Q> and C<\E>> |
2332 | is interpolated in the usual way. Something like C<"\Q\\E"> has | |
2333 | no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the | |
2334 | result is the same as for C<"\\\\E">. As a general rule, backslashes | |
2335 | between C<\Q> and C<\E> may lead to counterintuitive results. So, | |
2336 | C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same | |
2337 | as C<"\\\t"> (since TAB is not alphanumeric). Note also that: | |
2a94b7ce IZ |
2338 | |
2339 | $str = '\t'; | |
2340 | return "\Q$str"; | |
2341 | ||
2342 | may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">. | |
2343 | ||
19799a22 | 2344 | Interpolated scalars and arrays are converted internally to the C<join> and |
92d29cee | 2345 | C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes: |
75e14d17 | 2346 | |
19799a22 | 2347 | $foo . " XXX '" . (join $", @arr) . "'"; |
75e14d17 | 2348 | |
19799a22 | 2349 | All operations above are performed simultaneously, left to right. |
75e14d17 | 2350 | |
19799a22 GS |
2351 | Because the result of C<"\Q STRING \E"> has all metacharacters |
2352 | quoted, there is no way to insert a literal C<$> or C<@> inside a | |
2353 | C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became | |
2354 | C<"\\\$">; if not, it is interpreted as the start of an interpolated | |
2355 | scalar. | |
75e14d17 | 2356 | |
19799a22 | 2357 | Note also that the interpolation code needs to make a decision on |
89d205f2 | 2358 | where the interpolated scalar ends. For instance, whether |
35f2feb0 | 2359 | C<< "a $b -> {c}" >> really means: |
75e14d17 IZ |
2360 | |
2361 | "a " . $b . " -> {c}"; | |
2362 | ||
2a94b7ce | 2363 | or: |
75e14d17 IZ |
2364 | |
2365 | "a " . $b -> {c}; | |
2366 | ||
19799a22 GS |
2367 | Most of the time, the longest possible text that does not include |
2368 | spaces between components and which contains matching braces or | |
2369 | brackets. because the outcome may be determined by voting based | |
2370 | on heuristic estimators, the result is not strictly predictable. | |
2371 | Fortunately, it's usually correct for ambiguous cases. | |
75e14d17 | 2372 | |
6deea57f | 2373 | =item the replacement of C<s///> |
75e14d17 | 2374 | |
19799a22 | 2375 | Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation |
6deea57f TS |
2376 | happens as with C<qq//> constructs. |
2377 | ||
2378 | It is at this step that C<\1> is begrudgingly converted to C<$1> in | |
2379 | the replacement text of C<s///>, in order to correct the incorrigible | |
2380 | I<sed> hackers who haven't picked up the saner idiom yet. A warning | |
2381 | is emitted if the C<use warnings> pragma or the B<-w> command-line flag | |
2382 | (that is, the C<$^W> variable) was set. | |
2383 | ||
2384 | =item C<RE> in C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>, | |
2385 | ||
cc74c5bd TS |
2386 | Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\E>, |
2387 | and interpolation happens (almost) as with C<qq//> constructs. | |
2388 | ||
5d03b57c KW |
2389 | Processing of C<\N{...}> is also done here, and compiled into an intermediate |
2390 | form for the regex compiler. (This is because, as mentioned below, the regex | |
2391 | compilation may be done at execution time, and C<\N{...}> is a compile-time | |
2392 | construct.) | |
2393 | ||
cc74c5bd TS |
2394 | However any other combinations of C<\> followed by a character |
2395 | are not substituted but only skipped, in order to parse them | |
2396 | as regular expressions at the following step. | |
6deea57f | 2397 | As C<\c> is skipped at this step, C<@> of C<\c@> in RE is possibly |
1749ea0d | 2398 | treated as an array symbol (for example C<@foo>), |
6deea57f | 2399 | even though the same text in C<qq//> gives interpolation of C<\c@>. |
6deea57f TS |
2400 | |
2401 | Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and | |
19799a22 GS |
2402 | a C<#>-comment in a C<//x>-regular expression, no processing is |
2403 | performed whatsoever. This is the first step at which the presence | |
2404 | of the C<//x> modifier is relevant. | |
2405 | ||
1749ea0d TS |
2406 | Interpolation in patterns has several quirks: C<$|>, C<$(>, C<$)>, C<@+> |
2407 | and C<@-> are not interpolated, and constructs C<$var[SOMETHING]> are | |
2408 | voted (by several different estimators) to be either an array element | |
2409 | or C<$var> followed by an RE alternative. This is where the notation | |
19799a22 GS |
2410 | C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as |
2411 | array element C<-9>, not as a regular expression from the variable | |
2412 | C<$arr> followed by a digit, which would be the interpretation of | |
2413 | C</$arr[0-9]/>. Since voting among different estimators may occur, | |
2414 | the result is not predictable. | |
2415 | ||
19799a22 GS |
2416 | The lack of processing of C<\\> creates specific restrictions on |
2417 | the post-processed text. If the delimiter is C</>, one cannot get | |
2418 | the combination C<\/> into the result of this step. C</> will | |
2419 | finish the regular expression, C<\/> will be stripped to C</> on | |
2420 | the previous step, and C<\\/> will be left as is. Because C</> is | |
2421 | equivalent to C<\/> inside a regular expression, this does not | |
2422 | matter unless the delimiter happens to be character special to the | |
2423 | RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an | |
2424 | alphanumeric char, as in: | |
2a94b7ce IZ |
2425 | |
2426 | m m ^ a \s* b mmx; | |
2427 | ||
19799a22 | 2428 | In the RE above, which is intentionally obfuscated for illustration, the |
6deea57f | 2429 | delimiter is C<m>, the modifier is C<mx>, and after delimiter-removal the |
89d205f2 | 2430 | RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one |
19799a22 GS |
2431 | reason you're encouraged to restrict your delimiters to non-alphanumeric, |
2432 | non-whitespace choices. | |
75e14d17 IZ |
2433 | |
2434 | =back | |
2435 | ||
19799a22 | 2436 | This step is the last one for all constructs except regular expressions, |
75e14d17 IZ |
2437 | which are processed further. |
2438 | ||
6deea57f TS |
2439 | =item parsing regular expressions |
2440 | X<regexp, parse> | |
75e14d17 | 2441 | |
19799a22 | 2442 | Previous steps were performed during the compilation of Perl code, |
ac036724 | 2443 | but this one happens at run time, although it may be optimized to |
19799a22 | 2444 | be calculated at compile time if appropriate. After preprocessing |
6deea57f | 2445 | described above, and possibly after evaluation if concatenation, |
19799a22 GS |
2446 | joining, casing translation, or metaquoting are involved, the |
2447 | resulting I<string> is passed to the RE engine for compilation. | |
2448 | ||
2449 | Whatever happens in the RE engine might be better discussed in L<perlre>, | |
2450 | but for the sake of continuity, we shall do so here. | |
2451 | ||
2452 | This is another step where the presence of the C<//x> modifier is | |
2453 | relevant. The RE engine scans the string from left to right and | |
2454 | converts it to a finite automaton. | |
2455 | ||
2456 | Backslashed characters are either replaced with corresponding | |
2457 | literal strings (as with C<\{>), or else they generate special nodes | |
2458 | in the finite automaton (as with C<\b>). Characters special to the | |
2459 | RE engine (such as C<|>) generate corresponding nodes or groups of | |
2460 | nodes. C<(?#...)> comments are ignored. All the rest is either | |
2461 | converted to literal strings to match, or else is ignored (as is | |
2462 | whitespace and C<#>-style comments if C<//x> is present). | |
2463 | ||
2464 | Parsing of the bracketed character class construct, C<[...]>, is | |
2465 | rather different than the rule used for the rest of the pattern. | |
2466 | The terminator of this construct is found using the same rules as | |
2467 | for finding the terminator of a C<{}>-delimited construct, the only | |
2468 | exception being that C<]> immediately following C<[> is treated as | |
2469 | though preceded by a backslash. Similarly, the terminator of | |
2470 | C<(?{...})> is found using the same rules as for finding the | |
2471 | terminator of a C<{}>-delimited construct. | |
2472 | ||
2473 | It is possible to inspect both the string given to RE engine and the | |
2474 | resulting finite automaton. See the arguments C<debug>/C<debugcolor> | |
2475 | in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line | |
4a4eefd0 | 2476 | switch documented in L<perlrun/"Command Switches">. |
75e14d17 IZ |
2477 | |
2478 | =item Optimization of regular expressions | |
d74e8afc | 2479 | X<regexp, optimization> |
75e14d17 | 2480 | |
7522fed5 | 2481 | This step is listed for completeness only. Since it does not change |
75e14d17 | 2482 | semantics, details of this step are not documented and are subject |
19799a22 GS |
2483 | to change without notice. This step is performed over the finite |
2484 | automaton that was generated during the previous pass. | |
2a94b7ce | 2485 | |
19799a22 GS |
2486 | It is at this stage that C<split()> silently optimizes C</^/> to |
2487 | mean C</^/m>. | |
75e14d17 IZ |
2488 | |
2489 | =back | |
2490 | ||
a0d0e21e | 2491 | =head2 I/O Operators |
d74e8afc ITB |
2492 | X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle> |
2493 | X<< <> >> X<@ARGV> | |
a0d0e21e | 2494 | |
54310121 | 2495 | There are several I/O operators you should know about. |
fbad3eb5 | 2496 | |
7b8d334a | 2497 | A string enclosed by backticks (grave accents) first undergoes |
19799a22 GS |
2498 | double-quote interpolation. It is then interpreted as an external |
2499 | command, and the output of that command is the value of the | |
e9c56f9b JH |
2500 | backtick string, like in a shell. In scalar context, a single string |
2501 | consisting of all output is returned. In list context, a list of | |
2502 | values is returned, one per line of output. (You can set C<$/> to use | |
2503 | a different line terminator.) The command is executed each time the | |
2504 | pseudo-literal is evaluated. The status value of the command is | |
2505 | returned in C<$?> (see L<perlvar> for the interpretation of C<$?>). | |
2506 | Unlike in B<csh>, no translation is done on the return data--newlines | |
2507 | remain newlines. Unlike in any of the shells, single quotes do not | |
2508 | hide variable names in the command from interpretation. To pass a | |
2509 | literal dollar-sign through to the shell you need to hide it with a | |
2510 | backslash. The generalized form of backticks is C<qx//>. (Because | |
2511 | backticks always undergo shell expansion as well, see L<perlsec> for | |
2512 | security concerns.) | |
d74e8afc | 2513 | X<qx> X<`> X<``> X<backtick> X<glob> |
19799a22 GS |
2514 | |
2515 | In scalar context, evaluating a filehandle in angle brackets yields | |
2516 | the next line from that file (the newline, if any, included), or | |
2517 | C<undef> at end-of-file or on error. When C<$/> is set to C<undef> | |
2518 | (sometimes known as file-slurp mode) and the file is empty, it | |
2519 | returns C<''> the first time, followed by C<undef> subsequently. | |
2520 | ||
2521 | Ordinarily you must assign the returned value to a variable, but | |
2522 | there is one situation where an automatic assignment happens. If | |
2523 | and only if the input symbol is the only thing inside the conditional | |
2524 | of a C<while> statement (even if disguised as a C<for(;;)> loop), | |
2525 | the value is automatically assigned to the global variable $_, | |
2526 | destroying whatever was there previously. (This may seem like an | |
2527 | odd thing to you, but you'll use the construct in almost every Perl | |
17b829fa | 2528 | script you write.) The $_ variable is not implicitly localized. |
19799a22 GS |
2529 | You'll have to put a C<local $_;> before the loop if you want that |
2530 | to happen. | |
2531 | ||
2532 | The following lines are equivalent: | |
a0d0e21e | 2533 | |
748a9306 | 2534 | while (defined($_ = <STDIN>)) { print; } |
7b8d334a | 2535 | while ($_ = <STDIN>) { print; } |
a0d0e21e LW |
2536 | while (<STDIN>) { print; } |
2537 | for (;<STDIN>;) { print; } | |
748a9306 | 2538 | print while defined($_ = <STDIN>); |
7b8d334a | 2539 | print while ($_ = <STDIN>); |
a0d0e21e LW |
2540 | print while <STDIN>; |
2541 | ||
19799a22 | 2542 | This also behaves similarly, but avoids $_ : |
7b8d334a | 2543 | |
89d205f2 | 2544 | while (my $line = <STDIN>) { print $line } |
7b8d334a | 2545 | |
19799a22 GS |
2546 | In these loop constructs, the assigned value (whether assignment |
2547 | is automatic or explicit) is then tested to see whether it is | |
2548 | defined. The defined test avoids problems where line has a string | |
2549 | value that would be treated as false by Perl, for example a "" or | |
2550 | a "0" with no trailing newline. If you really mean for such values | |
2551 | to terminate the loop, they should be tested for explicitly: | |
7b8d334a GS |
2552 | |
2553 | while (($_ = <STDIN>) ne '0') { ... } | |
2554 | while (<STDIN>) { last unless $_; ... } | |
2555 | ||
5ef4d93e | 2556 | In other boolean contexts, C<< <filehandle> >> without an |
2557 | explicit C<defined> test or comparison elicits a warning if the | |
9f1b1f2d | 2558 | C<use warnings> pragma or the B<-w> |
19799a22 | 2559 | command-line switch (the C<$^W> variable) is in effect. |
7b8d334a | 2560 | |
5f05dabc | 2561 | The filehandles STDIN, STDOUT, and STDERR are predefined. (The |
19799a22 GS |
2562 | filehandles C<stdin>, C<stdout>, and C<stderr> will also work except |
2563 | in packages, where they would be interpreted as local identifiers | |
2564 | rather than global.) Additional filehandles may be created with | |
2565 | the open() function, amongst others. See L<perlopentut> and | |
2566 | L<perlfunc/open> for details on this. | |
d74e8afc | 2567 | X<stdin> X<stdout> X<sterr> |
a0d0e21e | 2568 | |
35f2feb0 | 2569 | If a <FILEHANDLE> is used in a context that is looking for |
19799a22 GS |
2570 | a list, a list comprising all input lines is returned, one line per |
2571 | list element. It's easy to grow to a rather large data space this | |
2572 | way, so use with care. | |
a0d0e21e | 2573 | |
35f2feb0 | 2574 | <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>. |
19799a22 | 2575 | See L<perlfunc/readline>. |
fbad3eb5 | 2576 | |
35f2feb0 GS |
2577 | The null filehandle <> is special: it can be used to emulate the |
2578 | behavior of B<sed> and B<awk>. Input from <> comes either from | |
a0d0e21e | 2579 | standard input, or from each file listed on the command line. Here's |
35f2feb0 | 2580 | how it works: the first time <> is evaluated, the @ARGV array is |
5a964f20 | 2581 | checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened |
a0d0e21e LW |
2582 | gives you standard input. The @ARGV array is then processed as a list |
2583 | of filenames. The loop | |
2584 | ||
2585 | while (<>) { | |
2586 | ... # code for each line | |
2587 | } | |
2588 | ||
2589 | is equivalent to the following Perl-like pseudo code: | |
2590 | ||
3e3baf6d | 2591 | unshift(@ARGV, '-') unless @ARGV; |
a0d0e21e LW |
2592 | while ($ARGV = shift) { |
2593 | open(ARGV, $ARGV); | |
2594 | while (<ARGV>) { | |
2595 | ... # code for each line | |
2596 | } | |
2597 | } | |
2598 | ||
19799a22 GS |
2599 | except that it isn't so cumbersome to say, and will actually work. |
2600 | It really does shift the @ARGV array and put the current filename | |
2601 | into the $ARGV variable. It also uses filehandle I<ARGV> | |
ac036724 | 2602 | internally. <> is just a synonym for <ARGV>, which |
19799a22 | 2603 | is magical. (The pseudo code above doesn't work because it treats |
35f2feb0 | 2604 | <ARGV> as non-magical.) |
a0d0e21e | 2605 | |
48ab5743 ML |
2606 | Since the null filehandle uses the two argument form of L<perlfunc/open> |
2607 | it interprets special characters, so if you have a script like this: | |
2608 | ||
2609 | while (<>) { | |
2610 | print; | |
2611 | } | |
2612 | ||
2613 | and call it with C<perl dangerous.pl 'rm -rfv *|'>, it actually opens a | |
2614 | pipe, executes the C<rm> command and reads C<rm>'s output from that pipe. | |
2615 | If you want all items in C<@ARGV> to be interpreted as file names, you | |
2616 | can use the module C<ARGV::readonly> from CPAN. | |
2617 | ||
35f2feb0 | 2618 | You can modify @ARGV before the first <> as long as the array ends up |
a0d0e21e | 2619 | containing the list of filenames you really want. Line numbers (C<$.>) |
19799a22 GS |
2620 | continue as though the input were one big happy file. See the example |
2621 | in L<perlfunc/eof> for how to reset line numbers on each file. | |
5a964f20 | 2622 | |
89d205f2 | 2623 | If you want to set @ARGV to your own list of files, go right ahead. |
5a964f20 TC |
2624 | This sets @ARGV to all plain text files if no @ARGV was given: |
2625 | ||
2626 | @ARGV = grep { -f && -T } glob('*') unless @ARGV; | |
a0d0e21e | 2627 | |
5a964f20 TC |
2628 | You can even set them to pipe commands. For example, this automatically |
2629 | filters compressed arguments through B<gzip>: | |
2630 | ||
2631 | @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV; | |
2632 | ||
2633 | If you want to pass switches into your script, you can use one of the | |
a0d0e21e LW |
2634 | Getopts modules or put a loop on the front like this: |
2635 | ||
2636 | while ($_ = $ARGV[0], /^-/) { | |
2637 | shift; | |
2638 | last if /^--$/; | |
2639 | if (/^-D(.*)/) { $debug = $1 } | |
2640 | if (/^-v/) { $verbose++ } | |
5a964f20 | 2641 | # ... # other switches |
a0d0e21e | 2642 | } |
5a964f20 | 2643 | |
a0d0e21e | 2644 | while (<>) { |
5a964f20 | 2645 | # ... # code for each line |
a0d0e21e LW |
2646 | } |
2647 | ||
89d205f2 YO |
2648 | The <> symbol will return C<undef> for end-of-file only once. |
2649 | If you call it again after this, it will assume you are processing another | |
19799a22 | 2650 | @ARGV list, and if you haven't set @ARGV, will read input from STDIN. |
a0d0e21e | 2651 | |
b159ebd3 | 2652 | If what the angle brackets contain is a simple scalar variable (e.g., |
35f2feb0 | 2653 | <$foo>), then that variable contains the name of the |
19799a22 GS |
2654 | filehandle to input from, or its typeglob, or a reference to the |
2655 | same. For example: | |
cb1a09d0 AD |
2656 | |
2657 | $fh = \*STDIN; | |
2658 | $line = <$fh>; | |
a0d0e21e | 2659 | |
5a964f20 TC |
2660 | If what's within the angle brackets is neither a filehandle nor a simple |
2661 | scalar variable containing a filehandle name, typeglob, or typeglob | |
2662 | reference, it is interpreted as a filename pattern to be globbed, and | |
2663 | either a list of filenames or the next filename in the list is returned, | |
19799a22 | 2664 | depending on context. This distinction is determined on syntactic |
35f2feb0 GS |
2665 | grounds alone. That means C<< <$x> >> is always a readline() from |
2666 | an indirect handle, but C<< <$hash{key}> >> is always a glob(). | |
5a964f20 | 2667 | That's because $x is a simple scalar variable, but C<$hash{key}> is |
ef191992 YST |
2668 | not--it's a hash element. Even C<< <$x > >> (note the extra space) |
2669 | is treated as C<glob("$x ")>, not C<readline($x)>. | |
5a964f20 TC |
2670 | |
2671 | One level of double-quote interpretation is done first, but you can't | |
35f2feb0 | 2672 | say C<< <$foo> >> because that's an indirect filehandle as explained |
5a964f20 TC |
2673 | in the previous paragraph. (In older versions of Perl, programmers |
2674 | would insert curly brackets to force interpretation as a filename glob: | |
35f2feb0 | 2675 | C<< <${foo}> >>. These days, it's considered cleaner to call the |
5a964f20 | 2676 | internal function directly as C<glob($foo)>, which is probably the right |
19799a22 | 2677 | way to have done it in the first place.) For example: |
a0d0e21e LW |
2678 | |
2679 | while (<*.c>) { | |
2680 | chmod 0644, $_; | |
2681 | } | |
2682 | ||
3a4b19e4 | 2683 | is roughly equivalent to: |
a0d0e21e LW |
2684 | |
2685 | open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|"); | |
2686 | while (<FOO>) { | |
5b3eff12 | 2687 | chomp; |
a0d0e21e LW |
2688 | chmod 0644, $_; |
2689 | } | |
2690 | ||
3a4b19e4 GS |
2691 | except that the globbing is actually done internally using the standard |
2692 | C<File::Glob> extension. Of course, the shortest way to do the above is: | |
a0d0e21e LW |
2693 | |
2694 | chmod 0644, <*.c>; | |
2695 | ||
19799a22 GS |
2696 | A (file)glob evaluates its (embedded) argument only when it is |
2697 | starting a new list. All values must be read before it will start | |
2698 | over. In list context, this isn't important because you automatically | |
2699 | get them all anyway. However, in scalar context the operator returns | |
069e01df | 2700 | the next value each time it's called, or C<undef> when the list has |
19799a22 GS |
2701 | run out. As with filehandle reads, an automatic C<defined> is |
2702 | generated when the glob occurs in the test part of a C<while>, | |
2703 | because legal glob returns (e.g. a file called F<0>) would otherwise | |
2704 | terminate the loop. Again, C<undef> is returned only once. So if | |
2705 | you're expecting a single value from a glob, it is much better to | |
2706 | say | |
4633a7c4 LW |
2707 | |
2708 | ($file) = <blurch*>; | |
2709 | ||
2710 | than | |
2711 | ||
2712 | $file = <blurch*>; | |
2713 | ||
2714 | because the latter will alternate between returning a filename and | |
19799a22 | 2715 | returning false. |
4633a7c4 | 2716 | |
b159ebd3 | 2717 | If you're trying to do variable interpolation, it's definitely better |
4633a7c4 | 2718 | to use the glob() function, because the older notation can cause people |
e37d713d | 2719 | to become confused with the indirect filehandle notation. |
4633a7c4 LW |
2720 | |
2721 | @files = glob("$dir/*.[ch]"); | |
2722 | @files = glob($files[$i]); | |
2723 | ||
a0d0e21e | 2724 | =head2 Constant Folding |
d74e8afc | 2725 | X<constant folding> X<folding> |
a0d0e21e LW |
2726 | |
2727 | Like C, Perl does a certain amount of expression evaluation at | |
19799a22 | 2728 | compile time whenever it determines that all arguments to an |
a0d0e21e LW |
2729 | operator are static and have no side effects. In particular, string |
2730 | concatenation happens at compile time between literals that don't do | |
19799a22 | 2731 | variable substitution. Backslash interpolation also happens at |
a0d0e21e LW |
2732 | compile time. You can say |
2733 | ||
2734 | 'Now is the time for all' . "\n" . | |
2735 | 'good men to come to.' | |
2736 | ||
54310121 | 2737 | and this all reduces to one string internally. Likewise, if |
a0d0e21e LW |
2738 | you say |
2739 | ||
2740 | foreach $file (@filenames) { | |
5a964f20 | 2741 | if (-s $file > 5 + 100 * 2**16) { } |
54310121 | 2742 | } |
a0d0e21e | 2743 | |
19799a22 GS |
2744 | the compiler will precompute the number which that expression |
2745 | represents so that the interpreter won't have to. | |
a0d0e21e | 2746 | |
fd1abbef | 2747 | =head2 No-ops |
d74e8afc | 2748 | X<no-op> X<nop> |
fd1abbef DN |
2749 | |
2750 | Perl doesn't officially have a no-op operator, but the bare constants | |
2751 | C<0> and C<1> are special-cased to not produce a warning in a void | |
2752 | context, so you can for example safely do | |
2753 | ||
2754 | 1 while foo(); | |
2755 | ||
2c268ad5 | 2756 | =head2 Bitwise String Operators |
d74e8afc | 2757 | X<operator, bitwise, string> |
2c268ad5 TP |
2758 | |
2759 | Bitstrings of any size may be manipulated by the bitwise operators | |
2760 | (C<~ | & ^>). | |
2761 | ||
19799a22 GS |
2762 | If the operands to a binary bitwise op are strings of different |
2763 | sizes, B<|> and B<^> ops act as though the shorter operand had | |
2764 | additional zero bits on the right, while the B<&> op acts as though | |
2765 | the longer operand were truncated to the length of the shorter. | |
2766 | The granularity for such extension or truncation is one or more | |
2767 | bytes. | |
2c268ad5 | 2768 | |
89d205f2 | 2769 | # ASCII-based examples |
2c268ad5 TP |
2770 | print "j p \n" ^ " a h"; # prints "JAPH\n" |
2771 | print "JA" | " ph\n"; # prints "japh\n" | |
2772 | print "japh\nJunk" & '_____'; # prints "JAPH\n"; | |
2773 | print 'p N$' ^ " E<H\n"; # prints "Perl\n"; | |
2774 | ||
19799a22 | 2775 | If you are intending to manipulate bitstrings, be certain that |
2c268ad5 | 2776 | you're supplying bitstrings: If an operand is a number, that will imply |
19799a22 | 2777 | a B<numeric> bitwise operation. You may explicitly show which type of |
2c268ad5 TP |
2778 | operation you intend by using C<""> or C<0+>, as in the examples below. |
2779 | ||
4358a253 SS |
2780 | $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF) |
2781 | $foo = '150' | 105; # yields 255 | |
2c268ad5 TP |
2782 | $foo = 150 | '105'; # yields 255 |
2783 | $foo = '150' | '105'; # yields string '155' (under ASCII) | |
2784 | ||
2785 | $baz = 0+$foo & 0+$bar; # both ops explicitly numeric | |
2786 | $biz = "$foo" ^ "$bar"; # both ops explicitly stringy | |
a0d0e21e | 2787 | |
1ae175c8 GS |
2788 | See L<perlfunc/vec> for information on how to manipulate individual bits |
2789 | in a bit vector. | |
2790 | ||
55497cff | 2791 | =head2 Integer Arithmetic |
d74e8afc | 2792 | X<integer> |
a0d0e21e | 2793 | |
19799a22 | 2794 | By default, Perl assumes that it must do most of its arithmetic in |
a0d0e21e LW |
2795 | floating point. But by saying |
2796 | ||
2797 | use integer; | |
2798 | ||
3eab78e3 CW |
2799 | you may tell the compiler to use integer operations |
2800 | (see L<integer> for a detailed explanation) from here to the end of | |
2801 | the enclosing BLOCK. An inner BLOCK may countermand this by saying | |
a0d0e21e LW |
2802 | |
2803 | no integer; | |
2804 | ||
19799a22 | 2805 | which lasts until the end of that BLOCK. Note that this doesn't |
3eab78e3 CW |
2806 | mean everything is an integer, merely that Perl will use integer |
2807 | operations for arithmetic, comparison, and bitwise operators. For | |
2808 | example, even under C<use integer>, if you take the C<sqrt(2)>, you'll | |
2809 | still get C<1.4142135623731> or so. | |
19799a22 GS |
2810 | |
2811 | Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<", | |
89d205f2 | 2812 | and ">>") always produce integral results. (But see also |
13a2d996 | 2813 | L<Bitwise String Operators>.) However, C<use integer> still has meaning for |
19799a22 GS |
2814 | them. By default, their results are interpreted as unsigned integers, but |
2815 | if C<use integer> is in effect, their results are interpreted | |
2816 | as signed integers. For example, C<~0> usually evaluates to a large | |
0be96356 | 2817 | integral value. However, C<use integer; ~0> is C<-1> on two's-complement |
19799a22 | 2818 | machines. |
68dc0745 | 2819 | |
2820 | =head2 Floating-point Arithmetic | |
06ce2fa3 | 2821 | |
d74e8afc | 2822 | X<floating-point> X<floating point> X<float> X<real> |
68dc0745 | 2823 | |
2824 | While C<use integer> provides integer-only arithmetic, there is no | |
19799a22 GS |
2825 | analogous mechanism to provide automatic rounding or truncation to a |
2826 | certain number of decimal places. For rounding to a certain number | |
2827 | of digits, sprintf() or printf() is usually the easiest route. | |
2828 | See L<perlfaq4>. | |
68dc0745 | 2829 | |
5a964f20 TC |
2830 | Floating-point numbers are only approximations to what a mathematician |
2831 | would call real numbers. There are infinitely more reals than floats, | |
2832 | so some corners must be cut. For example: | |
2833 | ||
2834 | printf "%.20g\n", 123456789123456789; | |
2835 | # produces 123456789123456784 | |
2836 | ||
8548cb57 RGS |
2837 | Testing for exact floating-point equality or inequality is not a |
2838 | good idea. Here's a (relatively expensive) work-around to compare | |
5a964f20 TC |
2839 | whether two floating-point numbers are equal to a particular number of |
2840 | decimal places. See Knuth, volume II, for a more robust treatment of | |
2841 | this topic. | |
2842 | ||
2843 | sub fp_equal { | |
2844 | my ($X, $Y, $POINTS) = @_; | |
2845 | my ($tX, $tY); | |
2846 | $tX = sprintf("%.${POINTS}g", $X); | |
2847 | $tY = sprintf("%.${POINTS}g", $Y); | |
2848 | return $tX eq $tY; | |
2849 | } | |
2850 | ||
68dc0745 | 2851 | The POSIX module (part of the standard perl distribution) implements |
19799a22 GS |
2852 | ceil(), floor(), and other mathematical and trigonometric functions. |
2853 | The Math::Complex module (part of the standard perl distribution) | |
2854 | defines mathematical functions that work on both the reals and the | |
2855 | imaginary numbers. Math::Complex not as efficient as POSIX, but | |
68dc0745 | 2856 | POSIX can't work with complex numbers. |
2857 | ||
2858 | Rounding in financial applications can have serious implications, and | |
2859 | the rounding method used should be specified precisely. In these | |
2860 | cases, it probably pays not to trust whichever system rounding is | |
2861 | being used by Perl, but to instead implement the rounding function you | |
2862 | need yourself. | |
5a964f20 TC |
2863 | |
2864 | =head2 Bigger Numbers | |
d74e8afc | 2865 | X<number, arbitrary precision> |
5a964f20 | 2866 | |
c543c01b TC |
2867 | The standard C<Math::BigInt>, C<Math::BigRat>, and C<Math::BigFloat> modules, |
2868 | along with the C<bigint>, C<bigrat>, and C<bitfloat> pragmas, provide | |
19799a22 | 2869 | variable-precision arithmetic and overloaded operators, although |
cd5c4fce | 2870 | they're currently pretty slow. At the cost of some space and |
19799a22 GS |
2871 | considerable speed, they avoid the normal pitfalls associated with |
2872 | limited-precision representations. | |
5a964f20 | 2873 | |
c543c01b TC |
2874 | use 5.010; |
2875 | use bigint; # easy interface to Math::BigInt | |
2876 | $x = 123456789123456789; | |
2877 | say $x * $x; | |
2878 | +15241578780673678515622620750190521 | |
2879 | ||
2880 | Or with rationals: | |
2881 | ||
2882 | use 5.010; | |
2883 | use bigrat; | |
2884 | $a = 3/22; | |
2885 | $b = 4/6; | |
2886 | say "a/b is ", $a/$b; | |
2887 | say "a*b is ", $a*$b; | |
2888 | a/b is 9/44 | |
2889 | a*b is 1/11 | |
2890 | ||
2891 | Several modules let you calculate with (bound only by memory and CPU time) | |
2892 | unlimited or fixed precision. There are also some non-standard modules that | |
2893 | provide faster implementations via external C libraries. | |
cd5c4fce T |
2894 | |
2895 | Here is a short, but incomplete summary: | |
2896 | ||
950b09ed KW |
2897 | Math::Fraction big, unlimited fractions like 9973 / 12967 |
2898 | Math::String treat string sequences like numbers | |
2899 | Math::FixedPrecision calculate with a fixed precision | |
2900 | Math::Currency for currency calculations | |
2901 | Bit::Vector manipulate bit vectors fast (uses C) | |
2902 | Math::BigIntFast Bit::Vector wrapper for big numbers | |
2903 | Math::Pari provides access to the Pari C library | |
2904 | Math::BigInteger uses an external C library | |
2905 | Math::Cephes uses external Cephes C library (no big numbers) | |
2906 | Math::Cephes::Fraction fractions via the Cephes library | |
2907 | Math::GMP another one using an external C library | |
cd5c4fce T |
2908 | |
2909 | Choose wisely. | |
16070b82 GS |
2910 | |
2911 | =cut |