Say you want to stop users from entering non-aphanumeric characters or white spaces.

Using regular expressions would be the easiest method:

Here is my javascript code:

<script language="Javascript">
function alphaNumericCheck(){
var regex=/^[0-9A-Za-z]+$/; //^[a-zA-z]+$/
if(regex.test(document.add_data.password.value)){
alert("Good")
return true;
} else {
alert("Please fix: password")
return false;
}
}
</script>
For numbers only use /^[0-9]+$/

For mixed text and numbers, with spaces /^[0-9a-zA-Zs]+$/

Here are more useful regular expressions:

[a-zA-Z] any letter
d any number; same as [0-9]
D any NOT number; same as [^0-9]
w any alphanumeric character; same as [a-zA-Z-0-9_]
W any NON-alphanumeric character; same as [^a-zA-Z0-9_]
s any whitespace (tab, space, newline, etc...)
S any NON-whitespace
n newline
t tab

To view a full html sample of the above code click the links below using regular expressions in JavaScript