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Introduction to Regular Expressions in JavaScript


What exactly is a regular expression, and how is it handled in JavaScript? These are the questions tackled by this five-part series. In this first part, we'll introduce the topic of regular expressions (often abbreviated RegExp), and delve into objects, patterns, and variables.

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By: Chrysanthus Forcha
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July 08, 2009
TABLE OF CONTENTS:
  1. · Introduction to Regular Expressions in JavaScript
  2. · Simple Word Matching
  3. · Meaning of Pattern
  4. · Regular Expression Object
  5. · Simple Usage of the Literal Text Format and the Constructor Function
  6. · The Flags

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Introduction to Regular Expressions in JavaScript - Meaning of Pattern
(Page 3 of 6 )

Consider the following string assigned to the availableString variable.


var availableString = "Examples of creatures are the bat, the cat and the rat.";


You may want to know if the words "bat," "cat" or "rat" exist in the string. Examining the string, we see that "bat," "cat" and "rat" each end in "at." The following regexp will be used to determine if "bat," "cat" or "rat" exist in the string:


var re = /[bcr]at/;


Note the square brackets around "bcr." Recall that b is the first letter in "bat," c is the first letter in "cat" and r is the first letter in "rat." These first letters are inside the square brackets. After the square brackets, you have the next two letters that are common to the three words and follow the different first letters.

The following script will produce (alert) a match:


<script type="text/javascript">

var availableString = "Examples of creatures are the bat, the cat and the rat.";

var re = /[bcr]at/;


if (re.test(availableString))

alert('Matched')

else

alert('Not Matched')

</script>


Now, the regular expression (regexp) is


[bcr]at


The two forward slashes added to the ends (as shown below) make the above expression a regular expression.


/[bcr]at/


What you have inside the two forward slashes is a pattern that describes a set of words (bat, cat and rat).

In this subject (Regular Expressions) the content inside the two forward slashes is called a pattern. So far, we have seen two types of patterns: one, /[bcr]at/,  describes a set of words, while the other, /World/, describes only one word. We shall see many more patterns in this series.


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