How to get all combinations (ordered sampling without replacement) in regex - sql

Question

I'm trying to match a comma-separated string of numbers to a certain pattern within an sql query. I used regular expressions for similar problems in the past successfully, so I'm trying to get them working here as well. The problem is as follows:
The string may contain any number in a range (e.g. 1-4) exactly 0-1 times.
Two numbers are comma-separated
The numbers have to be in ascending order
(I think this is kind of a case of ordered sampling without replacement)
Sticking with the example of 1-4, the following entries should match:
1
1,2
1,3
1,4
1,2,3
1,2,4
1,3,4
1,2,3,4
2
2,3
2,4
3
3,4
4
and these should not:
q dawda 323123 a3 a1 1aa,1234 4321 a4,32,1a 1112222334411
1,,2,33,444, 11,12,a 234 2,2,3 33 3,3,3 3,34 34 123 1,4,4,4a 1,444
The best try I currently have is:
\b[1-4][\,]?[2-4]?[\,]?[3-4]?[\,]?[4]?\b
This still has two major drawbacks:
It delivers quite a lot of false positives. Numbers are not eliminated after they occurred once.
It will get rather long, when the range of numbers increases, e.g. 1-18 is already possible as well, bigger ranges are thinkable of.
I used regexpal for testing purposes.
Side notes:
As I'm using sql it would be possible to implement some algorithm in another language to generate all the possible combinations and save them in a table that can be used for joining, see e.g. How to get all possible combinations of a list’s elements?. I would like to only rely on that as a last resort, as the creation of new tables will be involved and these will contain a lot of entries.
The resulting sql statement that uses the regex should run on both Postgres and Oracle.
The set of positive examples is also referred to as "powerset".
Edit: Clarified the list of positive examples

Answer 1

I wouldn't use Regex for this, as e.g. the requirements "have to be unique" and "have to be in ascending order" can't really be expressed with a regular expression (at least I can't think of a way to do that).
As you also need to have an expression that is identical in Postgres and Oracle, I would create a function that checks such a list and then hide the DBMS specific implementation in that function.
For Postgres I would use its array handling features to implement that function:
create or replace function is_valid(p_input text)
returns boolean
as
$$
select coalesce(array_agg(x order by x) = string_to_array(p_input, ','), false)
from (
select distinct x
from unnest(string_to_array(p_input,',')) as t(x)
where x ~ '^[0-9]+$' -- only numbers
) t
where x::int between 1 and 4 -- the cast is safe as the inner query only returns valid numbers
$$
language sql;
The inner query returns all (distinct) elements from the input list as individual numbers. The outer query then aggregates that back for values in the desired range and numeric order. If that result isn't the same as the input, the input isn't valid.
Then with the following sample data:
with sample_data (input) as (
values
('1'),
('1,2'),
('1,3'),
('1,4'),
('1,2,3'),
('1,2,4'),
('foo'),
('1aa,1234'),
('1,,2,33,444,')
)
select input, is_valid(input)
from sample_data;
It will return:
input | is_valid
-------------+---------
1 | true
1,2 | true
1,3 | true
1,4 | true
1,2,3 | true
1,2,4 | true
foo | false
1aa,1234 | false
1,,2,33,444, | false
If you want to use the same function in Postgres and Oracle you probably need to use returns integer in Postgres as Oracle still doesn't support a boolean data type in SQL
Oracle's string processing functions are less powerful than Postgres' functions (e.g. no string_to_array or unnest), but you can probably implement a similar logic in PL/SQL as well (albeit more complicated)