Modificando un poco una FAQ de atlante tenemos esto:
Código PHP:
<HTML>
<HEAD>
<TITLE></TITLE>
<script language="javascript">
<!--
function LP_data(){
var key=window.event.keyCode;
if (key<48 || (key>57 && key < 65) || (key > 90 && key<97) || key>122){
window.event.keyCode=0;
}}
</script>
</HEAD>
<basefont face=verdana size=2>
<BODY>
<form name="miForm">
<input type=text name="num" onKeypress="LP_data()"><br><br>
<input type="button" value="enviar" onClick="LP_box()">
</form>
</BODY>
</HTML>