So is it alway an underscore after 2 characters?

Otherwise this should do: 

/^[a-z0-9_]+$/i

[download]
s$$([},&%#}/&/]+}%&{})*;#$&&s&&$^X.($'^"%]=\&(|?*{%
+.+=%;.#_}\&"^"-+%*).}%:##%}={~=~:.")&e&&s""`$''`"e

In reply to Re: Looking for a regular expression for the following pattern. by Skeeve
in thread Looking for a regular expression for the following pattern. by skooma

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