<script language="Javascript">

// Returns true if the string only contains alpha characters (empty string = true)
function isAlpha(txt)
{
	return ValidString(txt,'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz');
}

// Returns true if the string only contains numeric characters (empty string = true)
function isNumeric(txt)
{
	return ValidString(txt,'0123456789');
}

// Returns true if the string only contains alpha numeric characters (empty string = true)
function isAlphaNumeric(txt)
{
	return ValidString(txt,'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789');
}

// Returns true if the CheckString only contains characters passed in ValidString (empty string = true)
function ValidString(ChkString,ValidString)
{
	for (i=0; i<ChkString.length; i++)
	{
		if (ValidString.indexOf(ChkString.substring(i,i+1)) == -1) return false;
	}
	return true;
}


function SubmitForm()
{
	if (!isAlpha(document.Test.Alpha.value)) { alert('Alpha Only must only contain A-Z or a-z');return }
	if (!isAlphaNumeric(document.Test.AlphaNumeric.value)) { alert('Alpha Numeric must only contain A-Z, a-z, or 0-9'); return }
	if (!isNumeric(document.Test.Numeric.value)) { alert('Number must only contain 0-9'); return }
	alert('All is okay!');
}
</script>

<form name="Test">
Alpha Only: <input type="Text" name="Alpha" value=""><br>
Alpha Numeric Only: <input type="Text" name="AlphaNumeric" value=""><br>
Numeric Only: <input type="Text" name="Numeric" value=""><br>
<input type="Button" value="Go!" onClick="SubmitForm()">
</form>