"For a while, I’ve been advocating the use of flexible inputs within Web forms. These input fields allow people to answer questions the way they want instead of the way a database requires them to.
For instance, there are basically five different ways a valid phone number could be specified in the United States. A simple script can check to see if one of these has been used and then submit the information in whatever format the back-end system requires. The chore of adhering to a specific format is taken off the person providing an answer and instead given to a small bit of code.
While most designers agree this is a good thing, I often get asked how people would understand that any valid format can be entered into a flexible input field. Because many forms still enforce specific formats through adjacent help text or strict input field affordances (as illustrated in the image above), can people answer questions correctly in the absence of these signals? The short answer is yes." (Continued via Functioning Form, LukeW) [Usability Resources]