PHP regex for allowing specific usernames

PHP regex for allowing specific usernames

This is my first question here. Currently I'm developing a php forum
script. I want let people register username there abide by my rules.
The rules are simple:
• First character must be a letter • They can also use numbers but it
isn't the first • They can use fullstop/underscore once but not at the end
of a username
Please help me by putting down a function. I read some tutorial but didn't
get regex well. And for this reason I'm stucked there. Thanks in advance
for your answer.