So you want to match: C1234_.IDX, C1234_45.IDX and C1234.IDX but not C1234ABC.IDX. Is this correct?

If so, the pattern could be something like this :

i.e. start of string, one of the five letters cljso, 4 digits, an optional ("_" followed by any number of digits), followed by a dot and an extension chosen among dat and idx at the end of the string.

If this is not what you need, please be clearer on your requirement.