Hello,
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 master 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:

1) 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="JavaScript1.2">
function checkpostal(){
var re5digit=/^\d{5}$/ //regular expression defining a 5 digit number
if (document.myform.myinput.value.search(re5digit)==-1) //if match failed
alert("Please enter a valid 5 digit number inside form")
}
</script>

<form name="myform">
<input type="text" name="myinput" size=15>
<input type="button" onClick="checkpostal()" value="check">

</form>