them in parentheses.  The regex C<house(cat|keeper)> means matchC<house> followed by either C<cat> or C<keeper>.  Some more examplesare    /(a|b)b/;    # matches 'ab' or 'bb'    /(^a|b)c/;   # matches 'ac' at start of string or 'bc' anywhere    /house(cat|)/;  # matches either 'housecat' or 'house'    /house(cat(s|)|)/;  # matches either 'housecats' or 'housecat' or                        # 'house'.  Note groups can be nested.    "20" =~ /(19|20|)\d\d/;  # matches the null alternative '()\d\d',                             # because '20\d\d' can't match=head2 Extracting matchesThe grouping metacharacters C<()> also allow the extraction of theparts of a string that matched.  For each grouping, the part thatmatched inside goes into the special variables C<$1>, C<$2>, etc.They can be used just as ordinary variables:    # extract hours, minutes, seconds    $time =~ /(\d\d):(\d\d):(\d\d)/;  # match hh:mm:ss format    $hours = $1;    $minutes = $2;    $seconds = $3;In list context, a match C</regex/> with groupings will return thelist of matched values C<($1,$2,...)>.  So we could rewrite it as    ($hours, $minutes, $second) = ($time =~ /(\d\d):(\d\d):(\d\d)/);If the groupings in a regex are nested, C<$1> gets the group with theleftmost opening parenthesis, C<$2> the next opening parenthesis,etc.  For example, here is a complex regex and the matching variablesindicated below it:    /(ab(cd|ef)((gi)|j))/;     1  2      34Associated with the matching variables C<$1>, C<$2>, ... arethe B<backreferences> C<\1>, C<\2>, ...  Backreferences arematching variables that can be used I<inside> a regex:    /(\w\w\w)\s\1/; # find sequences like 'the the' in stringC<$1>, C<$2>, ... should only be used outside of a regex, and C<\1>,C<\2>, ... only inside a regex.=head2 Matching repetitionsThe B<quantifier> metacharacters C<?>, C<*>, C<+>, and C<{}> allow usto determine the number of repeats of a portion of a regex weconsider to be a match.  Quantifiers are put immediately after thecharacter, character class, or grouping that we want to specify.  Theyhave the following meanings:=over 4=item *C<a?> = match 'a' 1 or 0 times=item *C<a*> = match 'a' 0 or more times, i.e., any number of times=item *C<a+> = match 'a' 1 or more times, i.e., at least once=item *C<a{n,m}> = match at least C<n> times, but not more than C<m>times.=item *C<a{n,}> = match at least C<n> or more times=item *C<a{n}> = match exactly C<n> times=backHere are some examples:    /[a-z]+\s+\d*/;  # match a lowercase word, at least some space, and                     # any number of digits    /(\w+)\s+\1/;    # match doubled words of arbitrary length    $year =~ /\d{2,4}/;  # make sure year is at least 2 but not more                         # than 4 digits    $year =~ /\d{4}|\d{2}/;    # better match; throw out 3 digit datesThese quantifiers will try to match as much of the string as possible,while still allowing the regex to match.  So we have    $x = 'the cat in the hat';    $x =~ /^(.*)(at)(.*)$/; # matches,                            # $1 = 'the cat in the h'                            # $2 = 'at'                            # $3 = ''   (0 matches)The first quantifier C<.*> grabs as much of the string as possiblewhile still having the regex match. The second quantifier C<.*> hasno string left to it, so it matches 0 times.=head2 More matchingThere are a few more things you might want to know about matchingoperators.  In the code    $pattern = 'Seuss';    while (<>) {        print if /$pattern/;    }perl has to re-evaluate C<$pattern> each time through the loop.  IfC<$pattern> won't be changing, use the C<//o> modifier, to onlyperform variable substitutions once.  If you don't want anysubstitutions at all, use the special delimiter C<m''>:    @pattern = ('Seuss');    m/@pattern/; # matches 'Seuss'    m'@pattern'; # matches the literal string '@pattern'The global modifier C<//g> allows the matching operator to matchwithin a string as many times as possible.  In scalar context,successive matches against a string will have C<//g> jump from matchto match, keeping track of position in the string as it goes along.You can get or set the position with the C<pos()> function.For example,    $x = "cat dog house"; # 3 words    while ($x =~ /(\w+)/g) {        print "Word is $1, ends at position ", pos $x, "\n";    }prints    Word is cat, ends at position 3    Word is dog, ends at position 7    Word is house, ends at position 13A failed match or changing the target string resets the position.  Ifyou don't want the position reset after failure to match, add theC<//c>, as in C</regex/gc>.In list context, C<//g> returns a list of matched groupings, or ifthere are no groupings, a list of matches to the whole regex.  So    @words = ($x =~ /(\w+)/g);  # matches,                                # $word[0] = 'cat'                                # $word[1] = 'dog'                                # $word[2] = 'house'=head2 Search and replaceSearch and replace is performed using C<s/regex/replacement/modifiers>.The C<replacement> is a Perl double quoted string that replaces in thestring whatever is matched with the C<regex>.  The operator C<=~> isalso used here to associate a string with C<s///>.  If matchingagainst C<$_>, the S<C<$_ =~> > can be dropped.  If there is a match,C<s///> returns the number of substitutions made, otherwise it returnsfalse.  Here are a few examples:    $x = "Time to feed the cat!";    $x =~ s/cat/hacker/;   # $x contains "Time to feed the hacker!"    $y = "'quoted words'";    $y =~ s/^'(.*)'$/$1/;  # strip single quotes,                           # $y contains "quoted words"With the C<s///> operator, the matched variables C<$1>, C<$2>, etc.are immediately available for use in the replacement expression. Withthe global modifier, C<s///g> will search and replace all occurrencesof the regex in the string:    $x = "I batted 4 for 4";    $x =~ s/4/four/;   # $x contains "I batted four for 4"    $x = "I batted 4 for 4";    $x =~ s/4/four/g;  # $x contains "I batted four for four"The evaluation modifier C<s///e> wraps an C<eval{...}> around thereplacement string and the evaluated result is substituted for thematched substring.  Some examples:    # reverse all the words in a string    $x = "the cat in the hat";    $x =~ s/(\w+)/reverse $1/ge;   # $x contains "eht tac ni eht tah"    # convert percentage to decimal    $x = "A 39% hit rate";    $x =~ s!(\d+)%!$1/100!e;       # $x contains "A 0.39 hit rate"The last example shows that C<s///> can use other delimiters, such asC<s!!!> and C<s{}{}>, and even C<s{}//>.  If single quotes are usedC<s'''>, then the regex and replacement are treated as single quotedstrings.=head2 The split operatorC<split /regex/, string> splits C<string> into a list of substringsand returns that list.  The regex determines the character sequencethat C<string> is split with respect to.  For example, to split astring into words, use    $x = "Calvin and Hobbes";    @word = split /\s+/, $x;  # $word[0] = 'Calvin'                              # $word[1] = 'and'                              # $word[2] = 'Hobbes'To extract a comma-delimited list of numbers, use    $x = "1.618,2.718,   3.142";    @const = split /,\s*/, $x;  # $const[0] = '1.618'                                # $const[1] = '2.718'                                # $const[2] = '3.142'If the empty regex C<//> is used, the string is split into individualcharacters.  If the regex has groupings, then the list produced containsthe matched substrings from the groupings as well:    $x = "/usr/bin";    @parts = split m!(/)!, $x;  # $parts[0] = ''                                # $parts[1] = '/'                                # $parts[2] = 'usr'                                # $parts[3] = '/'                                # $parts[4] = 'bin'Since the first character of $x matched the regex, C<split> prependedan empty initial element to the list.=head1 BUGSNone.=head1 SEE ALSOThis is just a quick start guide.  For a more in-depth tutorial onregexes, see L<perlretut> and for the reference page, see L<perlre>.=head1 AUTHOR AND COPYRIGHTCopyright (c) 2000 Mark KvaleAll rights reserved.This document may be distributed under the same terms as Perl itself.=head2 AcknowledgmentsThe author would like to thank Mark-Jason Dominus, Tom Christiansen,Ilya Zakharevich, Brad Hughes, and Mike Giroux for all their helpfulcomments.=cut