Becoming a Regex Wizard with Komodo IDE's Rx Toolkit

Quick! Write a regular expression that matches an unbroken sequence of alphanumeric characters and underscores, but does not start with a digit.

Whoa, whoa, hold on. Wait a minute. If you are like me, you might be thinking "what an earth is a 'regular expression'?" I can think of a whole bunch of irregular expressions. What makes an expression "regular"?