Restricting a Field to Alphanumeric Only

JavaScript can be used to guide the user in entering correct data. This page describes a method for restricting entered data to just letters and numbers -- no spaces, commas or other types of characters are allowed. This technique is useful if you are asking for information such as a User ID which has only alphanumeric characters.

First, copy this JavaScript code into the <HEAD> section of your document:

<SCRIPT TYPE="text/javascript">
<!--
// copyright 1999 Idocs, Inc. https://www.idocs.com
// Distribute this script freely but keep this notice in place
function letternumber(e)
{
var key;
var keychar;

if (window.event)
   key = window.event.keyCode;
else if (e)
   key = e.which;
else
   return true;
keychar = String.fromCharCode(key);
keychar = keychar.toLowerCase();

// control keys
if ((key==null) || (key==0) || (key==8) || 
    (key==9) || (key==13) || (key==27) )
   return true;

// alphas and numbers
else if ((("abcdefghijklmnopqrstuvwxyz0123456789").indexOf(keychar) > -1))
   return true;
else
   return false;
}
//-->
</SCRIPT>

Using this script we can set a field as alphanumeric using the onKeyPress attribute. Use the exact onKeyPress attribute highlighted in this example. It is important that you use return in the attribute or the technique won't work..

<FORM ACTION="../cgi-bin/mycgi.pl" METHOD=POST>
User ID: <INPUT NAME=ID MAXLENGTH=8 onKeyPress="return letternumber(event)">
<INPUT TYPE=SUBMIT VALUE="go">
</FORM>

This creates this form:

The next page shows a similar technique which allows you to restrict a field to just numbers.