The following example allows only number character in text field.

<script type=”text/javascript”>
function numberOnly(textField,e,isDecimal) {
// isDecimal = true for double
// isDecimal = false for integer

var key;
var keychar;
if (window.event) {
key = window.event.keyCode;
}
else if (e) {
key = e.which;
}
else {
return true;
}

// check double period,decimal,point
if ( ((textField.value).indexOf(‘.’) > -1) && key == 46){
alert(“Double period is not allow for this field.”);
return false;
}

var str = ‘0123456789′;
str +=isDecimal ? ‘.’:”;
keychar = String.fromCharCode(key);
if ((key==null) || (key==0) || (key==8) ||? (key==9) || (key==13) || (key==27) ) {
return true;
}
else if (((str).indexOf(keychar) > -1)) {
return true;
}

alert(“Enter number only in this field.”);
textField.focus();
//textField.select();
return false;
}
</script>

The html code could be like this:
<input type=”text” onkeypress=”return numberOnly(this,event,true);”/>