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A quick study guide for Perl regexps

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From:
Mark Kvale
Date:
April 11, 2000 17:41
Subject:
A quick study guide for Perl regexps
Message ID:
200004120041.e3C0fZb24423@phy.ucsf.edu
Mark-Jason Dominus wrote:

> I'd suggest that you write a 2000-word brief introduction that
> discusses the very basics, and has a reference to your longer guide at
> the end.

The following is a quick study guide to the very basics of regexps. It
weighs in at about 2600 words.

Should both the longer tutorial and the quick study guide be included
in the core documentation? One or the other? Neither?

        -Mark



=head1 NAME

perlrequick - Perl regular expressions quick start

=head1 DESCRIPTION

This page covers the very basics of understanding, creating and
using regular expressions ('regexps') in Perl.

=head1 The Guide

=head2 Simple word matching

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

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

In this statement, C<World> is a regexp and the C<//> enclosing
C</World/> tells perl to search a string for a match.  The operator
C<=~> associates the string with the regexp match and produces a true
value if the regexp matched, or false if the regexp did not match.  In
our case, C<World> matches the second word in C<"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 C<!~> operator:

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

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

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

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

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

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

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

Regexps must match a part of the string I<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 B<metacharacters>, are reserved for use in regexp 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 regexp, the forward slash C<'/'> is also backslashed,
because it is used to delimit the regexp.

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

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

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

    $foo = 'house';
    'cathouse' =~ /cat$foo/;   # matches
    'housecat' =~ /$foocat/;   # doesn't match, there is no $foocat
    'housecat' =~ /${foo}cat/; # matches

With all of the regexps above, if the regexp matched anywhere in the
string, it was considered a match.  To specify I<where> it should
match, we would use the B<anchor> metacharacters C<^> and C<$>.  The
anchor C<^> means match at the beginning of the string and the anchor
C<$> 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

=head2 Using character classes

A B<character class> allows a set of possible characters, rather than
just a single character, to match at a particular point in a regexp.
Character classes are denoted by brackets C<[...]>, 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<'c'> is the first character in
the class, the earliest point at which the regexp can match is C<'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 C<'i'> B<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 C<-]\^$> 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 C<'-'> acts as a range operator within character
classes, so that the unwieldy C<[0123456789]> and C<[abc...xyz]>
become the svelte C<[0-9]> and C<[a-z]>:

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

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

The special character C<^> in the first position of a character class
denotes a B<negated character class>, which matches any character but
those in the bracket.  Both C<[...]> and C<[^...]> 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:

=over 4

=item *
\d is a digit and represents [0-9]

=item *
\s is a whitespace character and represents [\ \t\r\n\f]

=item *
\w is a word character (alphanumeric or _) and represents [0-9a-zA-Z_]

=item *
\D is a negated \d; it represents any character but a digit [^0-9]

=item *
\S is a negated \s; it represents any non-whitespace character [^\s]

=item *
\W is a negated \w; it represents any non-word character [^\w]

=item *
The period '.' matches any character but "\n"

=back

The C<\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 S<B<word anchor> > C<\b> matches a boundary between a word
character and a non-word character C<\w\W> or C<\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.

=head2 Matching this or that

We can match match different character strings with the B<alternation>
metacharacter C<'|'>.  To match C<dog> or C<cat>, we form the regexp
C<dog|cat>.  As before, perl will try to match the regexp at the
earliest possible point in the string.  At each character position,
perl will first try to match the the first alternative, C<dog>.  If
C<dog> doesn't match, perl will then try the next alternative, C<cat>.
If C<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 C<dog> is the first alternative in the second regexp,
C<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
regexp match to succeed wil be the one that matches. Here, all the
alternatives match at the first string position, so th first matches.

=head2 Grouping things and hierarchical matching

The B<grouping> metacharacters C<()> allow a part of a regexp to be
treated as a single unit.  Parts of a regexp are grouped by enclosing
them in parentheses.  The regexp C<house(cat|keeper)> means match
C<house> followed by either C<cat> or C<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

=head2 Extracting matches

The grouping metacharacters C<()> 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 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</regexp/ with groupings will return the
list 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 regexp are nested, C<$1> gets the group with the
leftmost opening parenthesis, C<$2> the next opening parenthesis,
etc.  For example, here is a complex regexp and the matching variables
indicated below it:

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

Associated with the matching variables C<$1>, C<$2>, ... are
the B<backreferences> C<\1>, C<\2>, ...  Backreferences are
matching variables that can be used I<inside> a regexp:

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

C<$1>, C<$2>, ... should only be used outside of a regexp, and C<\1>,
C<\2>, ... only inside a regexp.

=head2 Matching repetitions

The B<quantifier> metacharacters C<?>, C<*>, C<+>, and C<{}> allow us
to determine the number of repeats of a portion of a regexp 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:

=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

=back

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 regexp to match.  So we have

    $x =~ /^(.*)(at)(.*)$/; # matches,
                            # $1 = 'the cat in the h'
                            # $2 = 'at'
                            # $3 = ''   (0 matches)

The first quantifier C<.*> grabs as much of the string as possible
while still having the regexp match. The second quantifier C<.*> has
no string left to it, so it matches 0 times.

=head2 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 C<$pattern> each time through the loop.  If
C<$pattern> won't be changing, use the C<//o> modifier, to only
perform variable substitutions once.  If you don't want any
substitutions at all, use the special delimiter C<m''>:

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

The global modifier C<//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 C<//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 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 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<//c>, as in C</regexp/gc>.

In list context, C<//g> returns a list of matched groupings, or if
there are no groupings, a list of matches to the whole regexp.  So

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

=head2 Search and replace

Search and replace is perform using C<s/regexp/replacement/modifiers>.
The C<replacement> is a Perl double quoted string that replaces in the
string whatever is matched with the C<regexp>.  The operator C<=~> is
also used here to associate a string with C<s///>.  If matching
against C<$_>, the S<C<$_ =~> > can be dropped.  If there is a match,
C<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 C<s///> operator, the matched variables C<$1>, C<$2>, etc.
are immediately available for use in the replacement expression. With
the global modifier, C<s///g> will search and replace all occurrences
of the regexp 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 the
replacement string and the evaluated result is substituted for the
matched substring.  This counts character frequencies in a line:

    $x = "the cat";
    $x =~ s/(.)/$chars{$1}++;$1/eg;  # final $1 replaces char with itself
    print "frequency of '$_' is $chars{$_}\n"
        foreach (sort {$chars{$b} <=> $chars{$a}} keys %chars);

This prints

    frequency of 't' is 2
    frequency of 'e' is 1
    frequency of ' ' is 1
    frequency of 'h' is 1
    frequency of 'a' is 1
    frequency of 'c' is 1

C<s///> can use other delimiters, such as C<s!!!> and C<s{}{}>, and
even C<s{}//>.  If single quotes are used C<s'''>, then the regexp and
replacement are treated as single quoted strings.

=head2 The split operator

C<split /regexp/, string> splits C<string> into a list of substrings
and returns that list.  The regexp determines the character sequence
that C<string> is split with respect to.  For example, to split a
string into words, use

    $x = "Calvin and Hobbes";
    @words = split /\s+/, $x;  # $word[0] = 'Calvin'
                               # $word[1] = 'and'
                               # $word[2] = 'Hobbes'

If the empty regexp C<//> is used, the string is split into individual
characters.  If the regexp 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 regexp, C<split> prepended
an empty initial element to the list.

=head1 BUGS

None.

=head1 SEE ALSO

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

=head1 AUTHOR AND COPYRIGHT

Copyright (c) 2000 Mark Kvale 
All rights reserved.

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

=cut



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