Test was performed on 5.2.7 with no plugins. It was done only against the summary field.
Test issue summary was: "Test_1234"
1. summary ~ 'test_1234' - Full match with underscore WORKS.
2. summary ~ 'test_123' - Partial match with underscore FAILS.
3. summary ~ 'test_123*' - Partial match with underscore and wildcard WORKS.
4. summary ~ 'test_' - Partial match with no trailing characters after the underscore returns many issues which do not contain an underscore, one that does "Test Test _ Test Test", but not the one we want. FAILS.
5. summary ~ 'test_*' - Partial match with a wildcard WORKS.
6. summary ~ 'est_1234'- Partial match with missing leading characters. FAILS.
7. summary ~ 'est' - Partial match with missing leading characters and no underscore. FAILS.
8. summary ~ '*est' - Leading wildcards are prohibited by the system.
I kind of understand 6 and 7 not working because we only claim to perform "simple derivatives of that word" in https://confluence.atlassian.com/display/JIRA/Advanced+Searching#AdvancedSearching-CONTAINS:~ but in concert with test 8, it means that you can not do any kind of mid word matching.
Tests 2 and 4 seem to meet the requirement of "simple derivatives of that word" in that it is a string with a number of values removed from the end. How do we distinguish between test 4 failing, vs test 5 working?
Test 4 also shows that the underscore "_" character is not being correctly handler by Lucene as it brings up a number of issues where "test" is mentioned in the summary but no underscore is present.
Consistency in the behavior of this operator should be improved.