Something like
select *
from foo
where regexp_like( col1, '[^[:alpha:]]' ) ;
should work
SQL> create table foo( col1 varchar2(100) );
Table created.
SQL> insert into foo values( 'abc' );
1 row created.
SQL> insert into foo values( 'abc123' );
1 row created.
SQL> insert into foo values( 'def' );
1 row created.
SQL> select *
2 from foo
3 where regexp_like( col1, '[^[:alpha:]]' ) ;
COL1
--------------------------------------------------------------------------------
abc123