I don't know what functions Informix has available but I would look for one that can locate a character string in another string.
In DB2 it would be something like:
WHERE LOCATE(COL1, COL2) > 0
This would search for the value of COL1 (A99999) in COL2(XXX0000/Zxy_A99999_1234) and return the starting position of that value. A 0 indicates it is not found so "> 0" indicates it was in the string. If you have that function it may be called something else.