Given such qualities, it isn't a surprise that email has become a defacto tool of business for communicating with its clientele.
These days, users are expected to supply a valid email for just about everything, provided in exchange for the privilege of downloading software, learning more about the latest sales, and even for simply offering a comment to a favorite blogger's most recent entry.
Finally, the dollar-sign located at the conclusion of the component signals that the string cannot contain any additional characters following this two-to-four character alphabetical string.
So for example, this regular expression would validate "jason", "go49ers" and "999999", but not ".ilovespam", ":-)hi" or "%$^abc".
This second component operates exactly like the first, save for two important differences.
These easily implementable procedures will go a very long way towards eliminating future asdf's, blah's, and other invalid entries from your user database.
As a byproduct, you'll also learn a bit more about regular expressions, and PHP's regular expression and networking functions.
I'll provide the regular expression in its entirety here, and then offer a thorough explanation of its components.