=head1 NAME
perlrequick - Perl regular expressions quick start
=head1 DESCRIPTION
This page covers the very basics of understanding, creating and
using regular expressions ('regexes') in Perl.
=head1 The Guide
=head2 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, C is a regex and the C enclosing
C tells perl to search a string for a match. The operator
C<=~> 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, C 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 regex 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:
$_ = "Hello World";
print "It matches\n" if /World/;
Finally, the C default delimiters for a match can be changed to
arbitrary delimiters by putting an C<'m'> out front:
"Hello World" =~ m!World!; # matches, delimited by '!'
"Hello World" =~ m{World}; # matches, note the matching '{}'
"/usr/bin/perl" =~ m"/perl"; # matches after '/usr/bin',
# '/' becomes an ordinary char
Regexes must match a part of the string I in order for the
statement to be true:
"Hello World" =~ /world/; # doesn't match, case sensitive
"Hello World" =~ /o W/; # matches, ' ' is an ordinary char
"Hello World" =~ /World /; # doesn't match, no ' ' at end
perl will always match at the earliest possible point in the string:
"Hello World" =~ /o/; # matches 'o' in 'Hello'
"That hat is red" =~ /hat/; # matches 'hat' in 'That'
Not all characters can be used 'as is' in a match. Some characters,
called B, are reserved for use in regex notation.
The metacharacters are
{}[]()^$.|*+?\
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\/bin\/perl/; # matches
In the last regex, the forward slash C<'/'> is also backslashed,
because it is used to delimit the regex.
Non-printable ASCII characters are represented by B.
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 in ASCII, 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' =~ /${foo}cat/; # matches
With all of the regexes above, if the regex matched anywhere in the
string, it was considered a match. To specify I it should
match, we would use the B metacharacters C<^> and C<$>. 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$/; # matches
"housekeeper" =~ /^housekeeper$/; # matches
=head2 Using character classes
A B 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 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 regex 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, 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, which matches any character but
those in the brackets. 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 > 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 different character strings with the B
metacharacter C<'|'>. To match C or C, we form the regex
C. 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, C. If
C doesn't match, perl will then try the next alternative, C.
If C 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 is the first alternative in the second regex,
C 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.
=head2 Grouping things and hierarchical matching
The B metacharacters C<()> 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 C means match
C followed by either C or C. 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 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 regex 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 regex 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 C<\1>, C<\2>, ... Backreferences are
matching variables that can be used I a regex:
/(\w\w\w)\s\1/; # find sequences like 'the the' in string
C<$1>, C<$2>, ... should only be used outside of a regex, and C<\1>,
C<\2>, ... only inside a regex.
=head2 Matching repetitions
The B metacharacters C, C<*>, C<+>, and C<{}> 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:
=over 4
=item *
C = match 'a' 1 or 0 times
=item *
C = match 'a' 0 or more times, i.e., any number of times
=item *
C = match 'a' 1 or more times, i.e., at least once
=item *
C = match at least C times, but not more than C
times.
=item *
C = match at least C or more times
=item *
C = match exactly C 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 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 possible
while still having the regex 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 modifier, to only
perform variable substitutions once. If you don't want any
substitutions at all, use the special delimiter C:
@pattern = ('Seuss');
m/@pattern/; # matches 'Seuss'
m'@pattern'; # matches the literal string '@pattern'
The global modifier C 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 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 function.
For example,
$x = "cat dog house"; # 3 words
while ($x =~ /(\w+)/g) {
print "Word is $1, ends at position ", pos $x, "\n";
}
prints
Word is cat, ends at position 3
Word is dog, ends at position 7
Word is house, ends at position 13
A failed match or changing the target string resets the position. If
you don't want the position reset after failure to match, add the
C, as in C.
In list context, C returns a list of matched groupings, or if
there are no groupings, a list of matches to the whole regex. So
@words = ($x =~ /(\w+)/g); # matches,
# $word[0] = 'cat'
# $word[1] = 'dog'
# $word[2] = 'house'
=head2 Search and replace
Search and replace is performed using C.
The C is a Perl double quoted string that replaces in the
string whatever is matched with the C. The operator C<=~> is
also used here to associate a string with C 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 C wraps an C 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 C