SQL & PL/SQL

I have 2 tables,say test1 and test2
test1 has a column col1 which is the primary key.
test2 has a column col2 which references test1(col1).

I tried dropping the foreign key constraint from test2 table but could'nt do that.
I used:

alter table test2 drop foreign key SYS_C00155513;
this yielded an oracle error

ERROR at line 1:
ORA-00905: missing keyword

While searching on google i came through another method:

alter table test2 DROP FOREIGN KEY(col2) references test1(col1);

but this also yielded the same error.