Simplest way to allowing only given regular expressions in an input field. Uses jQuery for selecting the element though, you might want to change this.

regex_typing_validation.js

$(document).ready( function() {
	$("input.alphanumeric").keypress(function (e) {
		if (String.fromCharCode(e.keyCode).search(/[0-9A-Za-z]/) == -1) return false;
	});

	$("input.numeric").keypress(function (e) {
		if (String.fromCharCode(e.keyCode).search(/[0-9]/) == -1) return false;
	});
});