Don't know about "clean", but here are two solutions.
-- The hardcoded, bad performance. No transformation of your string though.
with patterns as (
select substr('ABCDEFGHIJ', 1, rownum) txt
from dual
connect by level <= length('ABCDEFGHIJ')
)
select d.txt, coalesce(max(length(p.txt)), 0)
from dummy d
left join patterns p
on instr(d.txt, p.txt) = 1
group by d.txt
order by 2 desc;
-- The cool one with regex.
-- Though transforming your input string,
-- this can also be done with ease making something that transorms it for you
-- like in the previous example, more complicated task than the previous,
-- as oracle sucks with string manipulation. You can however write it in java.
select d.txt, coalesce(LENGTH(REGEXP_SUBSTR(d.txt, '^A(B(C(D(E(F(G(H(I(J)?)?)?)?)?)?)?)?)')), 0)
from dummy d;
http://www.sqlfiddle.com/#!4/85ba6/23
UPDATE
with patterns as (
select substr('ABCDEFGHIJ', 1, rownum) txt
from dual
connect by level <= length('ABCDEFGHIJ')
)
select d.txt, coalesce(max(length(p.txt)), 0)
from dummy d
left join patterns p
on instr(d.txt, p.txt) = 1
where d.txt LIKE substr('ABCDEFGHIJ', 1, 1) || '%'
group by d.txt
order by 2 desc;
Updated fiddle: http://www.sqlfiddle.com/#!4/37400/6
Query plan produced tested on oracle 10g
SELECT STATEMENT, GOAL = ALL_ROWS
SORT ORDER BY
SORT GROUP BY NOSORT
NESTED LOOPS OUTER
INDEX RANGE SCAN I <<<< Uses the index.
VIEW
COUNT
CONNECT BY WITHOUT FILTERING
FAST DUAL
