I'm creating a web site that requires users to log in using either their email address OR mobile number.
What is the best way from a UX point of view to indicate that only one of the fields should be filled in. I'm thinking either an option to select which one the user will enter, having two text boxes and disabling one when text is in the other, or having one text box with some fancy regexing to work out if what is entered is an email or password. However, I'm not a UX designer and I'm open to any better ideas.