Firebird 3.0

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create table tablename(col varchar(100));

insert into tablename(col) values('aaaaaaaaaaa');
1 rows affected

insert into tablename(col) values('11111122222');
1 rows affected

insert into tablename(col) values('1111112222244');
1 rows affected

insert into tablename(col) values('11111xx2222');
1 rows affected

select * from tablename
where (col similar to '[0-9]+') and character_length(col) = 11

COL

11111122222

select * from tablename
where col similar to '[0-9]{11}'

COL

11111122222