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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.

Author Info:
By: Chrysanthus Forcha
Rating: 5 stars5 stars5 stars5 stars5 stars / 2
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 - Simple Word Matching
(Page 2 of 6 )

Consider the following code:


<html>


<head>

</head>


<body>

<script type="text/javascript">

var availableString = "Hello World";


var re = /World/ // the RegExp

if (re.test(availableString))

alert('Matched')

else

alert('Not Matched')

</script>

</body>


</html>


This is simple HTML. There is a BODY element. The Body element has only one tag, which is the JavaScript tag. If this page is opened, an alert box will appear showing the word "matched."

Let us look at the JavaScript. First, we have the statement


var availableString = "Hello World";


Here we have the string "Hello World" assigned to the availableString variable. The next line is,


var re = /World/; // the RegExp


Here we have the string "World", not enclosed by double quotes or single quotes, but enclosed by forward slashes. These two forward slashes make the string a RegExp, which is what to look for, in the available string. The regexp is assigned to the re variable.


The next line is,


if (re.test(availableString))


What we have as the if-condition expression is


re.test(availableString))


I want you to note the word "test" above. As you can see, re is the name of our regexp object. AvailableString is our string variable. test() is a method of all regexp objects (as I'll cover later). It returns true if the string in the regexp object (re) is seen in the available string (availableString). If the string is not seen, the method returns false.

So in the code, if the "if" condition is satisfied, the alert box displays "Matched" but otherwise it displays "Not Matched."

You can put the strings for the re and availableString variables in the conditional expression directly, as shown in the following code segment.


<script type="text/javascript">


if (/World/.test("Hello World"))

alert('Matched')

else

alert('Not Matched')


</script>


Note: instead of the variable re, we typed /World/ within two forward slashes, and instead of the variable availableString, we typed, "Hello World" in quotes.


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