There are 2 matches in the string, so the regex must be written depending on the one you want
Local $str = 'Update for Microsoft Outlook Social Connector 2010 (KB2553308) KB234567 32-Bit Edition'
; this will get the first one
MsgBox(0,'', StringRegExpReplace($str, '(?i).*?(KB[[:digit:]]{5,}).*', "$1"))
; this will get the last one
MsgBox(0,'', StringRegExpReplace($str, '(?i).*(KB[[:digit:]]{5,}).*', "$1"))