1=head1 NAME 2 3perlrequick - Perl regular expressions quick start 4 5=head1 DESCRIPTION 6 7This page covers the very basics of understanding, creating and 8using regular expressions ('regexes') in Perl. 9 10 11=head1 The Guide 12 13=head2 Simple word matching 14 15The simplest regex is simply a word, or more generally, a string of 16characters. A regex consisting of a word matches any string that 17contains that word: 18 19 "Hello World" =~ /World/; # matches 20 21In this statement, C<World> is a regex and the C<//> enclosing 22C</World/> tells Perl to search a string for a match. The operator 23C<=~> associates the string with the regex match and produces a true 24value if the regex matched, or false if the regex did not match. In 25our case, C<World> matches the second word in C<"Hello World">, so the 26expression is true. This idea has several variations. 27 28Expressions like this are useful in conditionals: 29 30 print "It matches\n" if "Hello World" =~ /World/; 31 32The sense of the match can be reversed by using C<!~> operator: 33 34 print "It doesn't match\n" if "Hello World" !~ /World/; 35 36The literal string in the regex can be replaced by a variable: 37 38 $greeting = "World"; 39 print "It matches\n" if "Hello World" =~ /$greeting/; 40 41If you're matching against C<$_>, the C<$_ =~> part can be omitted: 42 43 $_ = "Hello World"; 44 print "It matches\n" if /World/; 45 46Finally, the C<//> default delimiters for a match can be changed to 47arbitrary delimiters by putting an C<'m'> out front: 48 49 "Hello World" =~ m!World!; # matches, delimited by '!' 50 "Hello World" =~ m{World}; # matches, note the matching '{}' 51 "/usr/bin/perl" =~ m"/perl"; # matches after '/usr/bin', 52 # '/' becomes an ordinary char 53 54Regexes must match a part of the string I<exactly> in order for the 55statement to be true: 56 57 "Hello World" =~ /world/; # doesn't match, case sensitive 58 "Hello World" =~ /o W/; # matches, ' ' is an ordinary char 59 "Hello World" =~ /World /; # doesn't match, no ' ' at end 60 61Perl will always match at the earliest possible point in the string: 62 63 "Hello World" =~ /o/; # matches 'o' in 'Hello' 64 "That hat is red" =~ /hat/; # matches 'hat' in 'That' 65 66Not all characters can be used 'as is' in a match. Some characters, 67called B<metacharacters>, are reserved for use in regex notation. 68The metacharacters are 69 70 {}[]()^$.|*+?\ 71 72A metacharacter can be matched by putting a backslash before it: 73 74 "2+2=4" =~ /2+2/; # doesn't match, + is a metacharacter 75 "2+2=4" =~ /2\+2/; # matches, \+ is treated like an ordinary + 76 'C:\WIN32' =~ /C:\\WIN/; # matches 77 "/usr/bin/perl" =~ /\/usr\/bin\/perl/; # matches 78 79In the last regex, the forward slash C<'/'> is also backslashed, 80because it is used to delimit the regex. 81 82Non-printable ASCII characters are represented by B<escape sequences>. 83Common examples are C<\t> for a tab, C<\n> for a newline, and C<\r> 84for a carriage return. Arbitrary bytes are represented by octal 85escape sequences, e.g., C<\033>, or hexadecimal escape sequences, 86e.g., C<\x1B>: 87 88 "1000\t2000" =~ m(0\t2) # matches 89 "cat" =~ /\143\x61\x74/ # matches in ASCII, but a weird way to spell cat 90 91Regexes are treated mostly as double-quoted strings, so variable 92substitution works: 93 94 $foo = 'house'; 95 'cathouse' =~ /cat$foo/; # matches 96 'housecat' =~ /${foo}cat/; # matches 97 98With all of the regexes above, if the regex matched anywhere in the 99string, it was considered a match. To specify I<where> it should 100match, we would use the B<anchor> metacharacters C<^> and C<$>. The 101anchor C<^> means match at the beginning of the string and the anchor 102C<$> means match at the end of the string, or before a newline at the 103end of the string. Some examples: 104 105 "housekeeper" =~ /keeper/; # matches 106 "housekeeper" =~ /^keeper/; # doesn't match 107 "housekeeper" =~ /keeper$/; # matches 108 "housekeeper\n" =~ /keeper$/; # matches 109 "housekeeper" =~ /^housekeeper$/; # matches 110 111=head2 Using character classes 112 113A B<character class> allows a set of possible characters, rather than 114just a single character, to match at a particular point in a regex. 115Character classes are denoted by brackets C<[...]>, with the set of 116characters to be possibly matched inside. Here are some examples: 117 118 /cat/; # matches 'cat' 119 /[bcr]at/; # matches 'bat', 'cat', or 'rat' 120 "abc" =~ /[cab]/; # matches 'a' 121 122In the last statement, even though C<'c'> is the first character in 123the class, the earliest point at which the regex can match is C<'a'>. 124 125 /[yY][eE][sS]/; # match 'yes' in a case-insensitive way 126 # 'yes', 'Yes', 'YES', etc. 127 /yes/i; # also match 'yes' in a case-insensitive way 128 129The last example shows a match with an C<'i'> B<modifier>, which makes 130the match case-insensitive. 131 132Character classes also have ordinary and special characters, but the 133sets of ordinary and special characters inside a character class are 134different than those outside a character class. The special 135characters for a character class are C<-]\^$> and are matched using an 136escape: 137 138 /[\]c]def/; # matches ']def' or 'cdef' 139 $x = 'bcr'; 140 /[$x]at/; # matches 'bat, 'cat', or 'rat' 141 /[\$x]at/; # matches '$at' or 'xat' 142 /[\\$x]at/; # matches '\at', 'bat, 'cat', or 'rat' 143 144The special character C<'-'> acts as a range operator within character 145classes, so that the unwieldy C<[0123456789]> and C<[abc...xyz]> 146become the svelte C<[0-9]> and C<[a-z]>: 147 148 /item[0-9]/; # matches 'item0' or ... or 'item9' 149 /[0-9a-fA-F]/; # matches a hexadecimal digit 150 151If C<'-'> is the first or last character in a character class, it is 152treated as an ordinary character. 153 154The special character C<^> in the first position of a character class 155denotes a B<negated character class>, which matches any character but 156those in the brackets. Both C<[...]> and C<[^...]> must match a 157character, or the match fails. Then 158 159 /[^a]at/; # doesn't match 'aat' or 'at', but matches 160 # all other 'bat', 'cat, '0at', '%at', etc. 161 /[^0-9]/; # matches a non-numeric character 162 /[a^]at/; # matches 'aat' or '^at'; here '^' is ordinary 163 164Perl has several abbreviations for common character classes. (These 165definitions are those that Perl uses in ASCII-safe mode with the C</a> modifier. 166Otherwise they could match many more non-ASCII Unicode characters as 167well. See L<perlrecharclass/Backslash sequences> for details.) 168 169=over 4 170 171=item * 172 173\d is a digit and represents 174 175 [0-9] 176 177=item * 178 179\s is a whitespace character and represents 180 181 [\ \t\r\n\f] 182 183=item * 184 185\w is a word character (alphanumeric or _) and represents 186 187 [0-9a-zA-Z_] 188 189=item * 190 191\D is a negated \d; it represents any character but a digit 192 193 [^0-9] 194 195=item * 196 197\S is a negated \s; it represents any non-whitespace character 198 199 [^\s] 200 201=item * 202 203\W is a negated \w; it represents any non-word character 204 205 [^\w] 206 207=item * 208 209The period '.' matches any character but "\n" 210 211=back 212 213The C<\d\s\w\D\S\W> abbreviations can be used both inside and outside 214of character classes. Here are some in use: 215 216 /\d\d:\d\d:\d\d/; # matches a hh:mm:ss time format 217 /[\d\s]/; # matches any digit or whitespace character 218 /\w\W\w/; # matches a word char, followed by a 219 # non-word char, followed by a word char 220 /..rt/; # matches any two chars, followed by 'rt' 221 /end\./; # matches 'end.' 222 /end[.]/; # same thing, matches 'end.' 223 224The S<B<word anchor> > C<\b> matches a boundary between a word 225character and a non-word character C<\w\W> or C<\W\w>: 226 227 $x = "Housecat catenates house and cat"; 228 $x =~ /\bcat/; # matches cat in 'catenates' 229 $x =~ /cat\b/; # matches cat in 'housecat' 230 $x =~ /\bcat\b/; # matches 'cat' at end of string 231 232In the last example, the end of the string is considered a word 233boundary. 234 235=head2 Matching this or that 236 237We can match different character strings with the B<alternation> 238metacharacter C<'|'>. To match C<dog> or C<cat>, we form the regex 239C<dog|cat>. As before, Perl will try to match the regex at the 240earliest possible point in the string. At each character position, 241Perl will first try to match the first alternative, C<dog>. If 242C<dog> doesn't match, Perl will then try the next alternative, C<cat>. 243If C<cat> doesn't match either, then the match fails and Perl moves to 244the next position in the string. Some examples: 245 246 "cats and dogs" =~ /cat|dog|bird/; # matches "cat" 247 "cats and dogs" =~ /dog|cat|bird/; # matches "cat" 248 249Even though C<dog> is the first alternative in the second regex, 250C<cat> is able to match earlier in the string. 251 252 "cats" =~ /c|ca|cat|cats/; # matches "c" 253 "cats" =~ /cats|cat|ca|c/; # matches "cats" 254 255At a given character position, the first alternative that allows the 256regex match to succeed will be the one that matches. Here, all the 257alternatives match at the first string position, so the first matches. 258 259=head2 Grouping things and hierarchical matching 260 261The B<grouping> metacharacters C<()> allow a part of a regex to be 262treated as a single unit. Parts of a regex are grouped by enclosing 263them in parentheses. The regex C<house(cat|keeper)> means match 264C<house> followed by either C<cat> or C<keeper>. Some more examples 265are 266 267 /(a|b)b/; # matches 'ab' or 'bb' 268 /(^a|b)c/; # matches 'ac' at start of string or 'bc' anywhere 269 270 /house(cat|)/; # matches either 'housecat' or 'house' 271 /house(cat(s|)|)/; # matches either 'housecats' or 'housecat' or 272 # 'house'. Note groups can be nested. 273 274 "20" =~ /(19|20|)\d\d/; # matches the null alternative '()\d\d', 275 # because '20\d\d' can't match 276 277=head2 Extracting matches 278 279The grouping metacharacters C<()> also allow the extraction of the 280parts of a string that matched. For each grouping, the part that 281matched inside goes into the special variables C<$1>, C<$2>, etc. 282They can be used just as ordinary variables: 283 284 # extract hours, minutes, seconds 285 $time =~ /(\d\d):(\d\d):(\d\d)/; # match hh:mm:ss format 286 $hours = $1; 287 $minutes = $2; 288 $seconds = $3; 289 290In list context, a match C</regex/> with groupings will return the 291list of matched values C<($1,$2,...)>. So we could rewrite it as 292 293 ($hours, $minutes, $second) = ($time =~ /(\d\d):(\d\d):(\d\d)/); 294 295If the groupings in a regex are nested, C<$1> gets the group with the 296leftmost opening parenthesis, C<$2> the next opening parenthesis, 297etc. For example, here is a complex regex and the matching variables 298indicated below it: 299 300 /(ab(cd|ef)((gi)|j))/; 301 1 2 34 302 303Associated with the matching variables C<$1>, C<$2>, ... are 304the B<backreferences> C<\g1>, C<\g2>, ... Backreferences are 305matching variables that can be used I<inside> a regex: 306 307 /(\w\w\w)\s\g1/; # find sequences like 'the the' in string 308 309C<$1>, C<$2>, ... should only be used outside of a regex, and C<\g1>, 310C<\g2>, ... only inside a regex. 311 312=head2 Matching repetitions 313 314The B<quantifier> metacharacters C<?>, C<*>, C<+>, and C<{}> allow us 315to determine the number of repeats of a portion of a regex we 316consider to be a match. Quantifiers are put immediately after the 317character, character class, or grouping that we want to specify. They 318have the following meanings: 319 320=over 4 321 322=item * 323 324C<a?> = match 'a' 1 or 0 times 325 326=item * 327 328C<a*> = match 'a' 0 or more times, i.e., any number of times 329 330=item * 331 332C<a+> = match 'a' 1 or more times, i.e., at least once 333 334=item * 335 336C<a{n,m}> = match at least C<n> times, but not more than C<m> 337times. 338 339=item * 340 341C<a{n,}> = match at least C<n> or more times 342 343=item * 344 345C<a{n}> = match exactly C<n> times 346 347=back 348 349Here are some examples: 350 351 /[a-z]+\s+\d*/; # match a lowercase word, at least some space, and 352 # any number of digits 353 /(\w+)\s+\g1/; # match doubled words of arbitrary length 354 $year =~ /^\d{2,4}$/; # make sure year is at least 2 but not more 355 # than 4 digits 356 $year =~ /^\d{4}$|^\d{2}$/; # better match; throw out 3 digit dates 357 358These quantifiers will try to match as much of the string as possible, 359while still allowing the regex to match. So we have 360 361 $x = 'the cat in the hat'; 362 $x =~ /^(.*)(at)(.*)$/; # matches, 363 # $1 = 'the cat in the h' 364 # $2 = 'at' 365 # $3 = '' (0 matches) 366 367The first quantifier C<.*> grabs as much of the string as possible 368while still having the regex match. The second quantifier C<.*> has 369no string left to it, so it matches 0 times. 370 371=head2 More matching 372 373There are a few more things you might want to know about matching 374operators. 375The global modifier C<//g> allows the matching operator to match 376within a string as many times as possible. In scalar context, 377successive matches against a string will have C<//g> jump from match 378to match, keeping track of position in the string as it goes along. 379You can get or set the position with the C<pos()> function. 380For example, 381 382 $x = "cat dog house"; # 3 words 383 while ($x =~ /(\w+)/g) { 384 print "Word is $1, ends at position ", pos $x, "\n"; 385 } 386 387prints 388 389 Word is cat, ends at position 3 390 Word is dog, ends at position 7 391 Word is house, ends at position 13 392 393A failed match or changing the target string resets the position. If 394you don't want the position reset after failure to match, add the 395C<//c>, as in C</regex/gc>. 396 397In list context, C<//g> returns a list of matched groupings, or if 398there are no groupings, a list of matches to the whole regex. So 399 400 @words = ($x =~ /(\w+)/g); # matches, 401 # $word[0] = 'cat' 402 # $word[1] = 'dog' 403 # $word[2] = 'house' 404 405=head2 Search and replace 406 407Search and replace is performed using C<s/regex/replacement/modifiers>. 408The C<replacement> is a Perl double-quoted string that replaces in the 409string whatever is matched with the C<regex>. The operator C<=~> is 410also used here to associate a string with C<s///>. If matching 411against C<$_>, the S<C<$_ =~>> can be dropped. If there is a match, 412C<s///> returns the number of substitutions made; otherwise it returns 413false. Here are a few examples: 414 415 $x = "Time to feed the cat!"; 416 $x =~ s/cat/hacker/; # $x contains "Time to feed the hacker!" 417 $y = "'quoted words'"; 418 $y =~ s/^'(.*)'$/$1/; # strip single quotes, 419 # $y contains "quoted words" 420 421With the C<s///> operator, the matched variables C<$1>, C<$2>, etc. 422are immediately available for use in the replacement expression. With 423the global modifier, C<s///g> will search and replace all occurrences 424of the regex in the string: 425 426 $x = "I batted 4 for 4"; 427 $x =~ s/4/four/; # $x contains "I batted four for 4" 428 $x = "I batted 4 for 4"; 429 $x =~ s/4/four/g; # $x contains "I batted four for four" 430 431The non-destructive modifier C<s///r> causes the result of the substitution 432to be returned instead of modifying C<$_> (or whatever variable the 433substitute was bound to with C<=~>): 434 435 $x = "I like dogs."; 436 $y = $x =~ s/dogs/cats/r; 437 print "$x $y\n"; # prints "I like dogs. I like cats." 438 439 $x = "Cats are great."; 440 print $x =~ s/Cats/Dogs/r =~ s/Dogs/Frogs/r =~ s/Frogs/Hedgehogs/r, "\n"; 441 # prints "Hedgehogs are great." 442 443 @foo = map { s/[a-z]/X/r } qw(a b c 1 2 3); 444 # @foo is now qw(X X X 1 2 3) 445 446The evaluation modifier C<s///e> wraps an C<eval{...}> around the 447replacement string and the evaluated result is substituted for the 448matched substring. Some examples: 449 450 # reverse all the words in a string 451 $x = "the cat in the hat"; 452 $x =~ s/(\w+)/reverse $1/ge; # $x contains "eht tac ni eht tah" 453 454 # convert percentage to decimal 455 $x = "A 39% hit rate"; 456 $x =~ s!(\d+)%!$1/100!e; # $x contains "A 0.39 hit rate" 457 458The last example shows that C<s///> can use other delimiters, such as 459C<s!!!> and C<s{}{}>, and even C<s{}//>. If single quotes are used 460C<s'''>, then the regex and replacement are treated as single-quoted 461strings. 462 463=head2 The split operator 464 465C<split /regex/, string> splits C<string> into a list of substrings 466and returns that list. The regex determines the character sequence 467that C<string> is split with respect to. For example, to split a 468string into words, use 469 470 $x = "Calvin and Hobbes"; 471 @word = split /\s+/, $x; # $word[0] = 'Calvin' 472 # $word[1] = 'and' 473 # $word[2] = 'Hobbes' 474 475To extract a comma-delimited list of numbers, use 476 477 $x = "1.618,2.718, 3.142"; 478 @const = split /,\s*/, $x; # $const[0] = '1.618' 479 # $const[1] = '2.718' 480 # $const[2] = '3.142' 481 482If the empty regex C<//> is used, the string is split into individual 483characters. If the regex has groupings, then the list produced contains 484the matched substrings from the groupings as well: 485 486 $x = "/usr/bin"; 487 @parts = split m!(/)!, $x; # $parts[0] = '' 488 # $parts[1] = '/' 489 # $parts[2] = 'usr' 490 # $parts[3] = '/' 491 # $parts[4] = 'bin' 492 493Since the first character of $x matched the regex, C<split> prepended 494an empty initial element to the list. 495 496=head1 BUGS 497 498None. 499 500=head1 SEE ALSO 501 502This is just a quick start guide. For a more in-depth tutorial on 503regexes, see L<perlretut> and for the reference page, see L<perlre>. 504 505=head1 AUTHOR AND COPYRIGHT 506 507Copyright (c) 2000 Mark Kvale 508All rights reserved. 509 510This document may be distributed under the same terms as Perl itself. 511 512=head2 Acknowledgments 513 514The author would like to thank Mark-Jason Dominus, Tom Christiansen, 515Ilya Zakharevich, Brad Hughes, and Mike Giroux for all their helpful 516comments. 517 518=cut 519 520