Regular expression to validate username

Question

I\'m trying to create a regular expression to validate usernames against these criteria:

1. Only contains alphanumeric characters, underscore an

Answer 1

A slight modification to Phillip's answer fixes the latest requirement

^[a-zA-Z0-9]([._](?![._])|[a-zA-Z0-9]){6,18}[a-zA-Z0-9]$