INSTR() like function?

In mySQL there is an INSTR(arg1,arg2) function that takes and looks for any
of the characters in arg1 and matches them against arg2

IE are the characters A or B or C or D in the string "dogma" would be
INSTR('ABCD','dogma');

Anyone help me to produce this sort of logic in an PSQL select statement?

PostgreSQL 7.0.2 and 6.5 are present here, solution for 7 series preferable,
and if the answer is to make a stored procedure then that's OK, I would
think that there is a way to do it already though.

Michael