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g1smd - 9:28 pm on Sep 30, 2011 (gmt 0)
This would work,
^(Moz|Mozilla|Mozilla/5\.0)$
but why force the parser to search for "Moz" in the input string three times.
Once you found it the first time, look for more stuff on the end, or not:
^(Moz(illa(/5\.0)?)?)$
There's no reason why this should fail, unless there's more characters on the end of the original input string.
This would also match ^Moz[^\ ]*$ but would also match all sorts of other stuff.