Como substituir caracteres não numéricos no MySQL?

delimiter //

create function IF NOT EXISTS LeaveNumber(str varchar(50)) returns varchar(50)
no sql
begin
declare verification varchar(50);
declare result varchar(50) default '';
declare character varchar(2);
declare i integer default 1;

if char_length(str) > 0 then
    while(i <= char_length(str)) do
        set character = substring(str,i,1);
        set verification = find_in_set(character,'1,2,3,4,5,6,7,8,9,0');

        if verification > 0 then
            set result = concat(result,character);
        end if;

        set i = i + 1;

    end while;

return result;
else
return '';
end if;
end //


delimiter ;

select leaveNumber('fAX:-12abcDE234'); -- RESULT: 12234