Regular expressions are very powerful tools for performing pattern matches. PERL programmers and UNIX shell programmers
have enjoyed the benefits of regular expressions for years. Once you ****** the pattern language, most validation tasks become trivial. You can perform complex tasks that once required lengthy procedures with just a few lines of code using regular expressions.

So how are regular expressions implemented in JavaScript? There are two ways:

*) Using literal syntax.
2) When you need to dynamically construct the regular expression, via the RegExp() constructor.

The literal syntax looks something like:

var RegularExpression = /pattern/

while the RegExp() constructor method looks like

var RegularExpression = new RegExp("pattern");

The RegExp() method allows you to dynamically construct the search pattern as a string, and is useful when the pattern is not known ahead of time.

To use regular expressions to validate a string you need to define a pattern string that represents the search criteria, then use a relevant string method to denote the action (ie: search, replace etc). Patterns are defined using string literal characters and metacharacters. For example, the following regular expression determines whether a string contains a valid 5-digit US postal code (for sake or simplicity, other possibilities are not considered):

<script language="JavaScript*.2">
function checkpostal(){
var re5digit=/^\d{5}$/ //regular expression defining a 5 digit number
if (document.myform.myinput.value.search(re5digit)==-*) //if match failed
alert("Please enter a valid 5 digit number inside form")

<form name="myform">
<input type="text" name="myinput" size=*5>
<input type="button" onClick="checkpostal()" value="check">