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# perlrequick

- NAME
- DESCRIPTION
- The Guide
  - Simple word matching
  - Using character classes
  - Matching this or that
  - Grouping things and hierarchical matching
  - Extracting matches
  - Matching repetitions
  - More matching
  - Search and replace
  - The split operator
- BUGS
- SEE ALSO
- AUTHOR AND COPYRIGHT
  - Acknowledgments

## NAME

perlrequick - Perl regular expressions quick start

## DESCRIPTION

This page covers the very basics of understanding, creating 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 `//` enclosing `/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 expression 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 !~ operator:

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

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

        $greeting = "World";
        print "It matches\n" if "Hello World" =~ /$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 for the
    statement to be true:

        "Hello World" =~ /world/;  # doesn't match, case sensitive
        "Hello World" =~ /o W/;    # matches, ' ' is an ordinary char
        "Hello World" =~ /World /; # doesn't match, no ' ' at end

    perl will always match at the earliest possible point in the string:

        "Hello World" =~ /o/;       # matches 'o' in 'Hello'
        "That hat is red" =~ /hat/; # matches 'hat' in 'That'

    Not all characters can be used 'as is' in a match.  Some characters,
    called metacharacters, are reserved for use in 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\/local\/bin\/perl/;  # matches

In the last regex, the forward slash `'/'` is also backslashed, 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 anywhere in the
    string, it was considered a match.  To specify 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 particular point in a regex.
    Character classes are denoted by brackets [...], with the set of
    characters to be possibly matched inside.  Here are some examples:

        /cat/;            # matches 'cat'
        /[bcr]at/;        # matches 'bat', 'cat', or 'rat'
        "abc" =~ /[cab]/; # matches 'a'

    In the last statement, even though 'c' is the first character in
    the class, the earliest point at which the regex can match is 'a'.

        /[yY][eE][sS]/; # match 'yes' in a case-insensitive way
                        # 'yes', 'Yes', 'YES', etc.
        /yes/i;         # also match 'yes' in a case-insensitive way

The last example shows a match with an `'i'` modifier, which makes the match case-insensitive.

    Character classes also have ordinary and special characters, 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 character 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 represents [0-9a-zA-Z_]
- \D is a negated \d; it represents any character but a digit [^0-9]
- \S is a negated \s; it represents any non-whitespace character [^\s]
- \W is a negated \w; it represents any non-word character [^\w]
- 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 match different character strings with the alternation
    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 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 wil be the one that matches. Here, all the alternatives match at the first string position, so th first matches.

### Grouping things and hierarchical matching

    The grouping metacharacters () allow a part of a regex to be
    treated as a single unit.  Parts of a regex are grouped by enclosing
    them in parentheses.  The regex house(cat|keeper) means match
    house followed by either cat or keeper.  Some more examples
    are

        /(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

### 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 variables $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
    matching 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 times
- `a+` = match 'a' 1 or more times, i.e., at least once
- `a{n,m}` = match at least `n` times, but not more than `m` times.
- `a{n,}` = match at least `n` or more times
- `a{n}` = match exactly `n` times

    Here 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 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 modifier, 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";
        }

    prints

        Word is cat, ends at position 3
        Word is dog, ends at position 7
        Word is house, ends at position 13

A failed match or changing the target string resets the position. If you don't want the position reset after failure to match, add the `//c`, as in `/regex/gc`.

    In list context, //g returns a list of matched groupings, or if
    there 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'

### Search and replace

    Search and replace is performed using s/regex/replacement/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 delimiters, such as `s!!!` and `s{}{}`, and even `s{}//`. If single quotes are used `s'''`, then the regex and replacement are treated as single quoted strings.

### The split operator

    split /regex/, string splits string into a list of substrings
    and returns that list.  The regex determines the character sequence
    that string is split with respect to.  For example, to split a
    string 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 // is used, the string is split into individual
    characters.  If the regex has groupings, then 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 the perlretut manpage and for the reference page, see the perlre manpage.

## 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.

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