javascript - Phone number validation problems -

i have written javascript validate phone number field if entered (it optional field) doesn't seem working, if enter wrong values still submits. here code:

<script> function validatephone() {  var num1 = document.getelementbyid('workno');  if (num1 !== null)     {     regex = /\(\d{2}\)\d{8}/;     } if (!num1.match(regex))     {     alert('that not correct telephone number format');     return false;     } }  </script>  <form name="eoiform" form method="post" action="<?php echo $_server["php_self"];?>" id="eoi" onsubmit="return validatephone();">  <input type="text" id="workno" name="workno">  <input type="submit" name="submit" id="submit" value="submit">  </form> 

can spot mistake?

your num1 variable contains element object, not value can't tested regexp stands.

it ever null if element not exist; "" if got no value.

function validatephone() {    var num1 = document.getelementbyid('workno').value;   if (num1 !== "" && !num1.match(/\(\d{2}\)\d{8}/))   {     alert('that not correct telephone number format');     return false;   } } 

you have invalid form method="post" in html.