title: I have a text field, which will accept only the NEXT TO NEXT ALPHANUMERIC that is single alphabet next is single numeric FOR EXAMPLE:- s3f4G5A2s3
body: <p>html:</p>
  
  <p>input type="text" id="custom_expression"</p>
  
  <p>javascript:</p>
  
  <p>script src="http://ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js"</p>
  
  <p></p>
  
  <p>$(document).ready(function()  </p>
  
  <p>{</p>
  
  <p>$(function()</p>
  
  <p>{</p>
  
  <p>$('#custom_Expression').keypress(function(e)</p>
  
  <p>{</p>
  
  <p>var txt = String.fromCharCode(e.which);</p>
  
  <p>console.log(txt + ' : ' + e.which);</p>
  
  <p>if(!txt.match(/^([a-zA-Z][0-9]){3}$/g)</p>
  
  <p>{</p>
  
  <p>return false;</p>
  
  <p>}</p>
  
  <p>});</p>
  
  <p>});</p>