code: function assertion(selector, attribute, regexp) {
  	var _this = this;
  
  	var msg = arguments.length > 3 && arguments[3] !== undefined ? arguments[3] : null;
  
  	this.message = msg;
  	if (!this.message) {
  		this.message = _util2.default.format('Testing if element <%s> has attribute <%s> that matches %s', selector, attribute, regexp.toString());
  	}
  	this.expected = regexp;
  
  	this.pass = function (value) {
  		return _this.expected.test(value);
  	};
  
  	this.value = function (result) {
  		if (result.value.error) {
  			console.error(result.value);
  			return "";
  		}
  
  		return result.value;
  	};
  
  	this.command = function (callback) {
  		return _this.api.getAttribute(selector, attribute, callback);
  	};
  }
docs: Assert that the element identified by the selector has an attribute that matches
  the provided regexp.
  
  h3 Examples:
  
  browser
  .url("http://www.github.com")
  .assert.attributeMatches("body", "class", /body-class/g)
  
  browser
  .url("http://www.github.com")
  .assert.attributeMatches("body", "class", new RegExp("body-class", "g")
  
  @author maxgalbu
  @param {String} selector - the element selector
  @param {String} attribute - the element attribute
  @param {RegExp} regexp - the regexp that should match the attribute
  @param {String} [msg] - output to identify the assertion