[BUG] case_insensitive regexp query didn't return expected result on a wildcard field

[n9]

Describe the bug

Case insensitive regexp query on a wildcard field does not work.

Related component

Search

To Reproduce

Based on #15855

1. Create a simple index contains a wildcard field

   PUT case_test
   {
       "mappings": {
           "properties": {
               "wildcard": {
                   "type": "wildcard"
               }
           }
       }
   }
   

2. Bulk insert documents containing capital letters

   PUT _bulk?refresh=true
   {"index": {"_index": "case_test"}}
   {"wildcard": "TtAa"}
   {"index": {"_index": "case_test"}}
   {"wildcard": "ttaa"}
   {"index": {"_index": "case_test"}}
   {"wildcard": "TTAA"}
   

3. Perform regexp search on the wildcard field with case_insensitive set to true.

   POST case_test/_search
   {
       "query": {
           "regexp": {
               "wildcard": {
                   "value": "TtAa",
                   "case_insensitive": true
               }
           }
       }
   }
   

4. Check results

Expected behavior

All documents should be returned, but not.

Additional Details

Host/Environment (please complete the following information):

• Version 2.18.0

[msfroh]

@n9 -- Thank you! You are correct. This is a bug.

The problem here is this line, which should look more like this block. That is, if the case_insensitive flag is set, we should generate a case-insensitive term query using AutomatonQueries.

Are you willing to contribute a fix? Alternatively, would you be willing to contribute a failing test case to https://github.com/opensearch-project/OpenSearch/blob/aaa92aeeb75a745ee7b058cc4b30b734e861cfc4/rest-api-spec/src/main/resources/rest-api-spec/test/search/270_wildcard_fieldtype_queries.yml ?

[msfroh]

See also my comment over at #16754 (comment). I think we could address both of these issues with some refactoring to take everything down the regexp path and make sure that one path works well.