are you sure? do you have leading or trailing spaces?? try alert(digit.test(frm.Mailing_StreetNO.value.replace(/^\s+|\s+$/g,"")))the following works for me, you can also use IsNaN function <form name="frm"> <input name="Mailing_StreetNO" value="123"> <input type=button value=check onclick="checkInput2()"> </form> <script language=javascript> function checkInput() { var digit=/^[0-9]{1,20}$/; alert(frm.Mailing_StreetNO.value); alert(digit.test(frm.Mailing_StreetNO.value)); alert(digit.test("123"));//true }function checkInput2() { alert (!isNaN(frm.Mailing_StreetNO.value));
<form name="frm">
<input name="Mailing_StreetNO" value="123">
<input type=button value=check onclick="checkInput2()">
</form>
<script language=javascript>
function checkInput()
{
var digit=/^[0-9]{1,20}$/;
alert(frm.Mailing_StreetNO.value);
alert(digit.test(frm.Mailing_StreetNO.value));
alert(digit.test("123"));//true
}function checkInput2()
{
alert (!isNaN(frm.Mailing_StreetNO.value));
}
</script>