This regex tutorial will explain how to validate phone numbers. Applying this aspect of regex to forms is a great way to make sure users are giving you correct data.
\(?(\d{3})[-.)]\-?\.?\d{3}[-.]\d{4}
The above REGEX code allows the developer to find any American phone number as well as obscure the phone number to keep the user data private using Quantifiers, Character Classes, Bracket Expressions, Groups, and Lazy/Greedy Matching.