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perlrequick



DESCRIPTION

       This page covers the very basics of understanding, creat­
       ing and using regular expressions ('regexes') in Perl.


The Guide

       Simple word matching

       The simplest regex is simply a word, or more generally, a
       string of characters.  A regex consisting of a word
       matches any string that contains that word:

           "Hello World" =~ /World/;  # matches

       In this statement, "World" is a regex and the "//" enclos­
       ing "/World/" tells perl to search a string for a match.
       The operator "=~" associates the string with the regex
       match and produces a true value if the regex matched, or
       false if the regex did not match.  In our case, "World"
       matches the second word in "Hello World", so the expres­
       sion is true.  This idea has several variations.

       Expressions like this are useful in conditionals:

           print "It matches\n" if "Hello World" =~ /World/;

       The sense of the match can be reversed by using "!~" oper­
       ator:

           print "It doesn't match\n" if "Hello World" !~ /World/;

       The literal string in the regex can be replaced by a vari­
       able:

           $greeting = "World";
           print "It matches\n" if "Hello World" =~ /$greeting/;

       If you're matching against $_, the "$_ =~" part can be
       omitted:

           $_ = "Hello World";
           print "It matches\n" if /World/;

       Finally, the "//" default delimiters for a match can be
       changed to arbitrary delimiters by putting an 'm' out
       front:

           "Hello World" =~ m!World!;   # matches, delimited by '!'
           "Hello World" =~ m{World};   # matches, note the matching '{}'
           "/usr/bin/perl" =~ m"/perl"; # matches after '/usr/bin',
                                        # '/' becomes an ordinary char

       Regexes must match a part of the string exactly in order
       regex notation.  The metacharacters are

           {}[]()^$.|*+?\

       A metacharacter can be matched by putting a backslash
       before it:

           "2+2=4" =~ /2+2/;    # doesn't match, + is a metacharacter
           "2+2=4" =~ /2\+2/;   # matches, \+ is treated like an ordinary +
           'C:\WIN32' =~ /C:\\WIN/;                       # matches
           "/usr/bin/perl" =~ /\/usr\/bin\/perl/;  # matches

       In the last regex, the forward slash '/' is also back­
       slashed, because it is used to delimit the regex.

       Non-printable ASCII characters are represented by escape
       sequences.  Common examples are "\t" for a tab, "\n" for a
       newline, and "\r" for a carriage return.  Arbitrary bytes
       are represented by octal escape sequences, e.g., "\033",
       or hexadecimal escape sequences, e.g., "\x1B":

           "1000\t2000" =~ m(0\t2)        # matches
           "cat"        =~ /\143\x61\x74/ # matches, but a weird way to spell cat

       Regexes are treated mostly as double quoted strings, so
       variable substitution works:

           $foo = 'house';
           'cathouse' =~ /cat$foo/;   # matches
           'housecat' =~ /${foo}cat/; # matches

       With all of the regexes above, if the regex matched any­
       where in the string, it was considered a match.  To spec­
       ify where it should match, we would use the anchor
       metacharacters "^" and "$".  The anchor "^" means match at
       the beginning of the string and the anchor "$" means match
       at the end of the string, or before a newline at the end
       of the string.  Some examples:

           "housekeeper" =~ /keeper/;         # matches
           "housekeeper" =~ /^keeper/;        # doesn't match
           "housekeeper" =~ /keeper$/;        # matches
           "housekeeper\n" =~ /keeper$/;      # matches
           "housekeeper" =~ /^housekeeper$/;  # matches

       Using character classes

       A character class allows a set of possible characters,
       rather than just a single character, to match at a partic­
       ular point in a regex.  Character classes are denoted by
       brackets "[...]", with the set of characters to be possi­
       bly matched inside.  Here are some examples:
       makes the match case-insensitive.

       Character classes also have ordinary and special charac­
       ters, but the sets of ordinary and special characters
       inside a character class are different than those outside
       a character class.  The special characters for a character
       class are "-]\^$" and are matched using an escape:

          /[\]c]def/; # matches ']def' or 'cdef'
          $x = 'bcr';
          /[$x]at/;   # matches 'bat, 'cat', or 'rat'
          /[\$x]at/;  # matches '$at' or 'xat'
          /[\\$x]at/; # matches '\at', 'bat, 'cat', or 'rat'

       The special character '-' acts as a range operator within
       character classes, so that the unwieldy "[0123456789]" and
       "[abc...xyz]" become the svelte "[0-9]" and "[a-z]":

           /item[0-9]/;  # matches 'item0' or ... or 'item9'
           /[0-9a-fA-F]/;  # matches a hexadecimal digit

       If '-' is the first or last character in a character
       class, it is treated as an ordinary character.

       The special character "^" in the first position of a char­
       acter class denotes a negated character class, which
       matches any character but those in the brackets.  Both
       "[...]" and "[^...]" must match a character, or the match
       fails.  Then

           /[^a]at/;  # doesn't match 'aat' or 'at', but matches
                      # all other 'bat', 'cat, '0at', '%at', etc.
           /[^0-9]/;  # matches a non-numeric character
           /[a^]at/;  # matches 'aat' or '^at'; here '^' is ordinary

       Perl has several abbreviations for common character
       classes:

       ·   \d is a digit and represents

               [0-9]

       ·   \s is a whitespace character and represents

               [\ \t\r\n\f]

       ·   \w is a word character (alphanumeric or _) and repre­
           sents

               [0-9a-zA-Z_]

       ·   \D is a negated \d; it represents any character but a
       ·   The period '.' matches any character but "\n"

       The "\d\s\w\D\S\W" abbreviations can be used both inside
       and outside of character classes.  Here are some in use:

           /\d\d:\d\d:\d\d/; # matches a hh:mm:ss time format
           /[\d\s]/;         # matches any digit or whitespace character
           /\w\W\w/;         # matches a word char, followed by a
                             # non-word char, followed by a word char
           /..rt/;           # matches any two chars, followed by 'rt'
           /end\./;          # matches 'end.'
           /end[.]/;         # same thing, matches 'end.'

       The word anchor  "\b" matches a boundary between a word
       character and a non-word character "\w\W" or "\W\w":

           $x = "Housecat catenates house and cat";
           $x =~ /\bcat/;  # matches cat in 'catenates'
           $x =~ /cat\b/;  # matches cat in 'housecat'
           $x =~ /\bcat\b/;  # matches 'cat' at end of string

       In the last example, the end of the string is considered a
       word boundary.

       Matching this or that

       We can match different character strings with the alterna­
       tion metacharacter '|'.  To match "dog" or "cat", we form
       the regex "dog|cat".  As before, perl will try to match
       the regex at the earliest possible point in the string.
       At each character position, perl will first try to match
       the first alternative, "dog".  If "dog" doesn't match,
       perl will then try the next alternative, "cat".  If "cat"
       doesn't match either, then the match fails and perl moves
       to the next position in the string.  Some examples:

           "cats and dogs" =~ /cat|dog|bird/;  # matches "cat"
           "cats and dogs" =~ /dog|cat|bird/;  # matches "cat"

       Even though "dog" is the first alternative in the second
       regex, "cat" is able to match earlier in the string.

           "cats"          =~ /c|ca|cat|cats/; # matches "c"
           "cats"          =~ /cats|cat|ca|c/; # matches "cats"

       At a given character position, the first alternative that
       allows the regex match to succeed will be the one that
       matches. Here, all the alternatives match at the first
       string position, so the first matches.

       Grouping things and hierarchical matching

           "20" =~ /(19|20|)\d\d/;  # matches the null alternative '()\d\d',
                                    # because '20\d\d' can't match

       Extracting matches

       The grouping metacharacters "()" also allow the extraction
       of the parts of a string that matched.  For each grouping,
       the part that matched inside goes into the special vari­
       ables $1, $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 "/regex/" with groupings will
       return the list of matched values "($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, $1 gets the group
       with the leftmost opening parenthesis, $2 the next opening
       parenthesis, etc.  For example, here is a complex regex
       and the matching variables indicated below it:

           /(ab(cd|ef)((gi)|j))/;
            1  2      34

       Associated with the matching variables $1, $2, ... are the
       backreferences "\1", "\2", ...  Backreferences are match­
       ing variables that can be used inside a regex:

           /(\w\w\w)\s\1/; # find sequences like 'the the' in string

       $1, $2, ... should only be used outside of a regex, and
       "\1", "\2", ... only inside a regex.

       Matching repetitions

       The quantifier metacharacters "?", "*", "+", and "{}"
       allow us to determine the number of repeats of a portion
       of a regex we consider to be a match.  Quantifiers are put
       immediately after the character, character class, or
       grouping that we want to specify.  They have the following
       meanings:

       ·   "a?" = match 'a' 1 or 0 times

       ·   "a*" = match 'a' 0 or more times, i.e., any number of
                            # 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 dates

       These 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 ".*" grabs as much of the string as
       possible while still having the regex match. The second
       quantifier ".*" has no string left to it, so it matches 0
       times.

       More matching

       There are a few more things you might want to know about
       matching operators.  In the code

           $pattern = 'Seuss';
           while (<>) {
               print if /$pattern/;
           }

       perl has to re-evaluate $pattern each time through the
       loop.  If $pattern won't be changing, use the "//o" modi­
       fier, to only perform variable substitutions once.  If you
       don't want any substitutions at all, use the special
       delimiter "m''":

           $pattern = 'Seuss';
           m'$pattern'; # matches '$pattern', not 'Seuss'

       The global modifier "//g" allows the matching operator to
       match within a string as many times as possible.  In
       scalar context, successive matches against a string will
       have "//g" jump from match to match, keeping track of
       position in the string as it goes along.  You can get or
       set the position with the "pos()" function.  For example,

           $x = "cat dog house"; # 3 words
           while ($x =~ /(\w+)/g) {
               print "Word is $1, ends at position ", pos $x, "\n";
           }

           @words = ($x =~ /(\w+)/g);  # matches,
                                       # $word[0] = 'cat'
                                       # $word[1] = 'dog'
                                       # $word[2] = 'house'

       Search and replace

       Search and replace is performed using "s/regex/replace­
       ment/modifiers".  The "replacement" is a Perl double
       quoted string that replaces in the string whatever is
       matched with the "regex".  The operator "=~" is also used
       here to associate a string with "s///".  If matching
       against $_, the "$_ =~"  can be dropped.  If there is a
       match, "s///" returns the number of substitutions made,
       otherwise it returns false.  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 "s///" operator, the matched variables $1, $2,
       etc.  are immediately available for use in the replacement
       expression. With the global modifier, "s///g" will search
       and replace all occurrences of 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 "s///e" wraps an "eval{...}"
       around the replacement string and the evaluated result is
       substituted for the matched 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 "s///" can use other delim­
       iters, such as "s!!!" and "s{}{}", and even "s{}//".  If
       single quotes are used "s'''", then the regex and replace­
       ment are treated as single quoted strings.

       The split operator

       "split /regex/, string" splits "string" into a list of
           $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 "//" is used, the string is split into
       individual characters.  If the regex has groupings, then
       the list produced contains the 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, "split"
       prepended an empty initial element to the list.


BUGS

       None.


SEE ALSO

       This is just a quick start guide.  For a more in-depth
       tutorial on regexes, see perlretut and for the reference
       page, see perlre.


AUTHOR AND COPYRIGHT

       Copyright (c) 2000 Mark Kvale All rights reserved.

       This document may be distributed under the same terms as
       Perl itself.

       Acknowledgments

       The author would like to thank Mark-Jason Dominus, Tom
       Christiansen, Ilya Zakharevich, Brad Hughes, and Mike
       Giroux for all their helpful comments.

perl v5.8.1                 2003-09-02             PERLREQUICK(1)
  
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