What is a regex for Twitter-like names?

I have been coding for a while but never had the need for regular expressions until recently. I need to do a regular expression that accepts usernames as Twitter does. Basically, I want to allow one underscore at a time. There can be more than one underscore in a name but these should not be consecutive characters. Alphanumeric characters are also allowed. But numbers cannot start a name.

Names such as

• _myname67
• myname67
• my_name
• _my_67_name_

are valid but

• 94myname
• __myname
• my__name
• my name

are not valid.

I have played with Rubular and come up with a couple regexes:

• /^[^0-9\s+](_?[a-z0-9]+_?)+$/i
• /^([a-z_?])+$/i

The problem I keep running into is that these match more than one underscores.