Check if a string is a valid URL (very basic) -- see http://jsfiddle.net/nabil_kadimi/ftx9196o/

isURL.js

/**
 * @author Nabil Kadimi (nabil * at * kadimi * com)
 * @link http://jsfiddle.net/nabil_kadimi/ftx9196o/
 */
function isURL( url ) {
	var pattern = new RegExp('^(https?:\\/\\/)?'			// protocol
		+ '((([a-z\\d]([a-z\\d-]*[a-z\\d])*)\\.)+[a-z]{2,}|'	// domain name
		+ '((\\d{1,3}\\.){3}\\d{1,3}))'				// OR ip (v4) address
		+ '(\\:\\d+)?(\\/[-a-z\\d%_.~+]*)*'			// port and path
		+ '(\\?[;&a-z\\d%_.~+=-]*)?'				// query string
		+ '(\\#[-a-z\\d_]*)?$','i'				// fragment locator
	);
	return pattern.test( url );
}