Regular Expressions: Actions, Pattern and Modifier

Written by Philip L Yuson

Who is this for
This is for those who would like to know the basic format of regular expressions.

Pre-Requisite Skills
Basic Perl Programming

Introduction

In the previous article, we showed how regular expressions can cut down on the code that you have to write. This article discusses the format of regular expressions.

Format

The regular expression format is like this:

action/pattern/modifier

The action can be any of the following:

Action	Description	Format
	Search once	/pattern/modifier
s	Substitute	s/oldpattern/newpattern/modifier
m	Match	m/pattern/modifier
y	Translate	y/replacecharacters/withthese/
q	Single Quote - similar to 'string'	q/string/
qq	Double quote - similar to "string"	qq/string/
qr	Compile the regular expression	qr/regexp/
qw	Compile as a list	qw/item1 item2 item3 item4 item5 .../

Pattern

The pattern is made up of characters that Perl recognizes: Perl uses metacharacters to denote something within the pattern. Listed below are the more common metacharacters.

Metacharacter	Descripion	Example	Explanation
\	Generally, treat the next character as is	\*\+	This represents the *+ characters as is
*	Previous character 0 or more times	A*	The letter A 0 or more times
+	Previous character 1 or more times	A+	The letter A 1 or more times
?	Previous character 0 or 1 time	N?	The letter N 0 or 1 time
^	String being matched should start with the pattern following the ^	^Concept	String should begin with the string Concept
.	Represents any character	...	Any 3 characters
$	String being matched should end with the pattern preceeding the $	Corporation$	String should end with the string Corporation
[ ]	Any characters enclosed between the two brackets	[1358]	Character should be 1, 3, 5 OR 8
{x, y}	Character should occur x minumum number of times and not more than y times	4{1, 3} g{3} L{4,}	Character 4 occurs 1 to 3 times Character g occurs 3 times Character L occurs at least 4 times

Modifiers

Modifiers define how a pattern is to be used. Below are the more common modifiers.

Modifier	Used in	Explanation
i	Match and substitute	Case insensitive
g	Match and substitute	Search all occurences of the pattern
e	Substitute	Evaluate the first pattern

Examples

$_ = 'Concept*Solutions*Corporation';

Regular Expression	Explanation	Result
/*/	Invalid regular expression because * should be preceded by a character	Script error
/\*/	Search for an asterisk (*)	1
/Solutions/	Search for a string 'Solutions'	1
/solutions/	Search for a string 'solutions' - not found because the search is case sensitive	0
/solutions/i	Search for a string 'solutions' - found because the search is NOT case sensitive	1
s/C/K/	Substitute the first upper case C with a K	Result is 1 $_ will have: KonceptSolutionsCorporation
s/C/K/g	Substitute all upper case C with a K	Result is 2 $_ will have: KonceptSolutionsKorporation
s/C/K/gi	Substitute all C with a K regardless of case	Result is 3 $_ will have: KonKeptSolutionsKorporation
y/con/KUD/	Change c to K, change o to U, change n to D	Result is 10 $_ will have: CUDKeptSUlutiUDsCUrpUratiUD
m/^[con]/gi	First character of string should be c, o or n, case is NOT sensitive	1
s/.n/\+\^/gi	If the substring has a character followed by an n, change the two characters to a + and @	Result is 3 $_ contains: C+@ceptSoluti+@sCorporati+@

These are just some of the basic regular expressions. The only way you can learn this is to try out combinations yourselves. To do this, you can write a simple perl script based on this:

$_ = 'Concept*Solutions*Corporation';
$a = your-regular-expression;
print "Result: $a: $_\n";

Have fun!

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