Here's a simple regular expression function that will return a Boolean indicating if the specified string is an Email Address.
// is the string an email address
def isEmail(String emailAddress) {
def emailRegEx = /^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\\.[a-zA-Z]{2}$/
def isEmail = !emailAddress.matches(emailRegEx)
return isEmail
}