MySQL locate or remove string suffix from a known set - mysql

Question

I want to write a MySQL stored procedure which will split a FQDN into host/authority/tld parts.
Let's say I have a list of known TLDs, and for the sake of illustration let's say it's the set
com
co.uk
uk
let's test it against these strings
input | output
----------------|-------
alpha.co.uk | alpha
mail.beta.uk | mail.beta
The output is the shortest substring of the input, starting from the beginning, such that CONCAT(output,'.',tld)=input for some tld which is a member of the given set.
Note that we need the shortest substring as the output, otherwise the output would be alpha.co in the first case, which is wrong.
I know how to write a MySQL function which tells me whether a given string is the suffix of another string, but here there are many possible such strings and any will do (provided no longer string is also a suffix of the input).
I know I could write a regex along the lines of co\.uk|uk|com but MySQL REGEX operator does not return the position of the match, just whether it matches or not.
Yes, I really do want a solution in SQL for this, not in the application language.
What's the best way to locate or remove the longest possible suffix, given a set of valid suffixes?

Answer 1

Here's one way to do that, relying on the fact that MIN() will yield the shortest of all the matches:
create table tld (tld varchar(100));
create table input (input varchar(100));
insert into tld values ('com'),('co.uk'),('uk');
insert into input values ('alpha.co.uk'),('mail.beta.com');
select
input.input as input,
min(substring(input.input, 1, length(input.input) - length(tld.tld) - 1)) as output
from input inner join tld
on input.input like concat('%.', tld.tld) group by input.input;
OR, if you only have a single value for input, then:
set #input = 'alpha.co.uk';
select min(substring(#input, 1, length(#input) - length(tld.tld) - 1)) as output
from tld
where #input like concat('%.', tld.tld);