This would be a script thing preferably, not UI. For the letters, first create a mapping table of valid 2 letter combinations. The 2nd field of the mapping table would be just a 1. For example:
MAPPING LOAD * INLINE [
Instead of inline you can also do this from any data source you like (e.g. Excel). Assuming your field is called "myField" the load would then look as follows:
fabs(applymap('letter_combo_map',left(myField,2),0) * isnum(num#(right(myField,6)))) as field_valid
... //specify your data source, etc
The new field "field_valid" will be 1 if it matches the format, otherwise 0.