A regex - or regular expression - is a sequence of characters that defines a specific search pattern. They can be used to find certain patterns of characters within a string, as well as replace a character or sequence of characters within said string. They are also frequently used to validate input, such as Matching an Email.
The following Matching an Email regex can be used to verify that a user has input a valid email address:
/^([a-z0-9_\.-]+)@([\da-z\.-]+)\.([a-z\.]{2,6})$/