jquery.validate.js

(function($) {
	var rules = {
		body: function(val) {
			if (val.length == 0) return 'requires body';
			if (val.length > 140) return 'body has a 140 character limit';
			return true;
		},

		username: function(val) {
			if (val.length > 20) return 'username has a 20 character limit';
			return true;
		},

		password: function(val) {
			if (val.length < 8 && val.length > 16) return 'passward has a 8-16 character limit';
			return true;
		},

		email: function(val) {
			// RegExp: http://blog.livedoor.jp/dankogai/archives/50954045.html
			if (!/^(?:(?:(?:(?:[a-zA-Z0-9_!#\$\%&'*+/=?\^`{}~|\-]+)(?:\.(?:[a-zA-Z0-9_!#\$\%&'*+/=?\^`{}~|\-]+))*)|(?:"(?:\\[^\r\n]|[^\\"])*")))\@(?:(?:(?:(?:[a-zA-Z0-9_!#\$\%&'*+/=?\^`{}~|\-]+)(?:\.(?:[a-zA-Z0-9_!#\$\%&'*+/=?\^`{}~|\-]+))*)|(?:\[(?:\\\S|[\x21-\x5a\x5e-\x7e])*\])))$/.test(val))
				return 'invalid Email address';
			return true;
		}
	};

	$.fn.validate = function(type) {
		var validator = rules[type];
		return validator ?
			this.blur(function() {
				var input = $(this);
				$('+ .error', input).remove();

				var r = validator(input.val());
				if (r === true)
					input.removeClass('error');
				else
					input.addClass('error').after('<p class="error">'+r+'</p>');
			}) :
			this;
	};
})(jQuery);