validateInput = function(inputEl, regexp, defaultValue, promptMessage) {
	// check element is not empty
	if(!inputEl.val()) {
		var promptResponse = prompt(promptMessage);
		// check response is returned and is a number
		if(promptResponse && promptResponse.match(regexp) && promptResponse != "" && promptResponse != defaultValue) {
			inputEl.val(promptResponse);
			inputEl.removeClass("not-validated");
			return true;
		} else {
			inputEl.addClass("not-validated");
			return false;
		}
	} else {
		// check if input matches all conditions
		if(inputEl.val().match(regexp) && inputEl.val() != null && inputEl.val() != defaultValue) {
			inputEl.removeClass("not-validated");
			return true;
		} else {
			var promptResponse = prompt(promptMessage);
			// check response is returned and is a number
			if(promptResponse && promptResponse.match(regexp) && promptResponse != "" && promptResponse != defaultValue) {
				inputEl.val(promptResponse);
				inputEl.removeClass("not-validated");
				return true;
			} else {
				inputEl.addClass("not-validated");
				return false;
			}
		}
	}
}