<script language="javascript">
 // the string we are going to operate on
 var inputstr="A1234";
 // our Regular Expression
 var regex=/^[A-Z]\d{4}$/g;
 // do the comparison, if we have a match
 if (regex.test(inputstr))
 {
 alert("Matches");
 }
 else
 {
 alert("Does not match");
 }
 </script>