A regular expression is a string of characters that define the pattern or patterns you are viewing. The syntax of regular expressions in Perl is very similar to what you will find within other regular expression.supporting programs, such as sed, grep, and awk.

The basic method for applying a regular expression is to use the pattern binding operators =~ and !~. The first operator is a test and assignment operator.

There are three regular expression operators within Perl

• Match Regular Expression - m//
• Substitute Regular Expression - s///
• Transliterate Regular Expression - tr///

The forward slashes in each case act as delimiters for the regular expression (regex) that you are specifying. If you are comfortable with any other delimiter then you can use in place of forward slash.

The Match Operator

The match operator, m//, is used to match a string or statement to a regular expression. For example, to match the character sequence "foo" against the scalar $bar, you might use a statement like this:

if ($bar =~ /foo/)

The m// actually works in the same fashion as the q// operator series.you can use any combination of naturally matching characters to act as delimiters for the expression. For example, m{}, m(), and m>< are all valid.

You can omit the m from m// if the delimiters are forward slashes, but for all other delimiters you must use the m prefix.

Note that the entire match expression.that is the expression on the left of =~ or !~ and the match operator, returns true (in a scalar context) if the expression matches. Therefore the statement:

$true = ($foo =~ m/foo/);

Will set $true to 1 if $foo matches the regex, or 0 if the match fails.

In a list context, the match returns the contents of any grouped expressions. For example, when extracting the hours, minutes, and seconds from a time string, we can use:

my ($hours, $minutes, $seconds) = ($time =~ m/(\d+):(\d+):(\d+)/);

Match Operator Modifiers

The match operator supports its own set of modifiers. The /g modifier allows for global matching. The /i modifier will make the match case insensitive. Here is the complete list of modifiers

Modifier	Description
i 	Makes the match case insensitive
m 	Specifies that if the string has newline or carriage
	return characters, the ^ and $ operators will now
	match against a newline boundary, instead of a
	string boundary
o 	Evaluates the expression only once
s 	Allows use of . to match a newline character
x 	Allows you to use white space in the expression for clarity
g 	Globally finds all matches
cg 	Allows the search to continue even after a global match fails

Matching Only Once

There is also a simpler version of the match operator - the ?PATTERN? operator. This is basically identical to the m// operator except that it only matches once within the string you are searching between each call to reset.

For example, you can use this to get the first and last elements within a list:

#!/usr/bin/perl

@list = qw/food foosball subeo footnote terfoot canic footbrdige/;

foreach (@list)
{
   $first = $1 if ?(foo.*)?;
   $last = $1 if /(foo.*)/;
}
print "First: $first, Last: $last\n";

This will produce following result
First: food, Last: footbrdige

The Substitution Operator

The substitution operator, s///, is really just an extension of the match operator that allows you to replace the text matched with some new text. The basic form of the operator is:

s/PATTERN/REPLACEMENT/;

The PATTERN is the regular expression for the text that we are looking for. The REPLACEMENT is a specification for the text or regular expression that we want to use to replace the found text with.

For example, we can replace all occurrences of .dog. with .cat. using

$string =~ s/dog/cat/;

Another example:

#/user/bin/perl

$string = 'The cat sat on the mat';
$string =~ s/cat/dog/;

print "Final Result is $string\n";

This will produce following result

The dog sat on the mat

Substitution Operator Modifiers

Here is the list of all modifiers used with substitution operator

Modifier	Description
i 	Makes the match case insensitive
m 	Specifies that if the string has newline or carriage
	return characters, the ^ and $ operators will now
	match against a newline boundary, instead of a
	string boundary
o 	Evaluates the expression only once
s 	Allows use of . to match a newline character
x 	Allows you to use white space in the expression
	for clarity
g 	Replaces all occurrences of the found expression
	with the replacement text
e 	Evaluates the replacement as if it were a Perl statement,
	and uses its return value as the replacement text

Translation

Translation is similar, but not identical, to the principles of substitution, but unlike substitution, translation (or transliteration) does not use regular expressions for its search on replacement values. The translation operators are:

tr/SEARCHLIST/REPLACEMENTLIST/cds
y/SEARCHLIST/REPLACEMENTLIST/cds

The translation replaces all occurrences of the characters in SEARCHLIST with the corresponding characters in REPLACEMENTLIST. For example, using the "The cat sat on the mat." string we have been using in this chapter:

#/user/bin/perl

$string = 'The cat sat on the mat';
$string =~ tr/a/o/;

print "$string\n";

This will produce following result

The cot sot on the mot.

$string =~ tr/a-z/A-Z/;

Translation Operator Modifiers

Following is the list of operators related to translation

Modifier 	Description
c 	Complement SEARCHLIST.
d 	Delete found but unreplaced characters.
s 	Squash duplicate replaced characters.

The /d modifier deletes the characters matching SEARCHLIST that do not have a corresponding entry in REPLACEMENTLIST. For example:

#!/usr/bin/perl 

$string = 'the cat sat on the mat.';
$string =~ tr/a-z/b/d;

print "$string\n";

This will produce following result
b b   b.

The last modifier, /s, removes the duplicate sequences of characters that were replaced, so:

#!/usr/bin/perl

$string = 'food';
$string = 'food';
$string =~ tr/a-z/a-z/s;

print $string;

This will produce following result
fod

More complex regular expressions

You don't just have to match on fixed strings. In fact, you can match on just about anything you could dream of by using more complex regular expressions. Here's a quick cheat sheet:

Character		Description
.              a single character
\s             a whitespace character (space, tab, newline)
\S             non-whitespace character
\d             a digit (0-9)
\D             a non-digit
\w             a word character (a-z, A-Z, 0-9, _)
\W             a non-word character
[aeiou]        matches a single character in the given set
[^aeiou]       matches a single character outside the given set
(foo|bar|baz)  matches any of the alternatives specified

Quantifiers can be used to specify how many of the previous thing you want to match on, where "thing" means either a literal character, one of the metacharacters listed above, or a group of characters or metacharacters in parentheses.

Character            Description
*              zero or more of the previous thing
+              one or more of the previous thing
?              zero or one of the previous thing
{3}            matches exactly 3 of the previous thing
{3,6}          matches between 3 and 6 of the previous thing
{3,}           matches 3 or more of the previous thing

The ^ metacharacter matches the beginning of the string and the $ metasymbol matches the end of the string.

Here are some brief examples

# nothing in the string (start and end are adjacent)
/^$/   

# a three digits, each followed by a whitespace
# character (eg "3 4 5 ")
/(\d\s) {3}/  

# matches a string in which every
# odd-numbered letter is a (eg "abacadaf")
/(a.)+/  

# string starts with one or more digits
/^\d+/

# string that ends with one or more digits
/\d+$/

Lets have alook at another example

#!/usr/bin/perl

$string = "Cats go Catatonic\nWhen given Catnip";
($start) = ($string =~ /\A(.*?) /);
@lines = $string =~ /^(.*?) /gm;
print "First word: $start\n","Line starts: @lines\n";


This will produce following result
First word: Cats
Line starts: Cats When

Matching Boundaries

The \b matches at any word boundary, as defined by the difference between the \w class and the \W class. Because \w includes the characters for a word, and \W the opposite, this normally means the termination of a word. The \B assertion matches any position that is not a word boundary. For example:

/\bcat\b/ # Matches 'the cat sat' but not 'cat on the mat'
/\Bcat\B/ # Matches 'verification' but not 'the cat on the mat'
/\bcat\B/ # Matches 'catatonic' but not 'polecat'
/\Bcat\b/ # Matches 'polecat' but not 'catatonic'

Selecting Alternatives

The | character is just like the standard or bitwise OR within Perl. It specifies alternate matches within a regular expression or group. For example, to match "cat" or "dog" in an expression, you might use this:

if ($string =~ /cat|dog/)

You can group individual elements of an expression together in order to support complex matches. Searching for two people.s names could be achieved with two separate tests, like this:

if (($string =~ /Martin Brown/) ||
   ($string =~ /Sharon Brown/))

This could be written as follows

if ($string =~ /(Martin|Sharon) Brown/)

Grouping Matching

From a regular-expression point of view, there is no difference between except, perhaps, that the former is slightly clearer.

$string =~ /(\S+)\s+(\S+)/;

and 

$string =~ /\S+\s+\S+/;

However, the benefit of grouping is that it allows us to extract a sequence from a regular expression. Groupings are returned as a list in the order in which they appear in the original. For example, in the following fragment we have pulled out the hours, minutes, and seconds from a string.

my ($hours, $minutes, $seconds) = ($time =~ m/(\d+):(\d+):(\d+)/);

As well as this direct method, matched groups are also available within the special $x variables, where x is the number of the group within the regular expression. We could therefore rewrite the preceding example as follows:

$time =~ m/(\d+):(\d+):(\d+)/;
my ($hours, $minutes, $seconds) = ($1, $2, $3);

When groups are used in substitution expressions, the $x syntax can be used in the replacement text. Thus, we could reformat a date string using this:

#!/usr/bin/perl

$date = '03/26/1999';
$date =~ s#(\d+)/(\d+)/(\d+)#$3/$1/$2#;

print "$date";

This will produce following result
1999/03/26

Using the \G Assertion

The \G assertion allows you to continue searching from the point where the last match occurred.

For example, in the following code we have used \G so that we can search to the correct position and then extract some information, without having to create a more complex, single regular expression:

#!/usr/bin/perl

$string = "The time is: 12:31:02 on 4/12/00";

$string =~ /:\s+/g;
($time) = ($string =~ /\G(\d+:\d+:\d+)/);
$string =~ /.+\s+/g;
($date) = ($string =~ m{\G(\d+/\d+/\d+)});

print "Time: $time, Date: $date\n";

This will produce following result
Time: 12:31:02, Date: 4/12/00

The \G assertion is actually just the metasymbol equivalent of the pos function, so between regular expression calls you can continue to use pos, and even modify the value of pos (and therefore \G) by using pos as an lvalue subroutine:

Regular Expression Variables

Regular expression variables include $, which contains whatever the last grouping match matched; $&, which contains the entire matched string; $`, which contains everything before the matched string; and $', which contains everything after the matched string.

The following code demonstrates the result:

#!/usr/bin/perl

$string = "The food is in the salad bar";
$string =~ m/foo/;
print "Before: $`\n";
print "Matched: $&\n";
print "After: $'\n";

This code prints the following when executed:
Before: The
Matched: foo
After: d is in the salad bar