[BUG] Optional Characters in the end of strings lead to unexpected behavior

[ysz0507]

Describe the bug

I came across some weird behavior of the lucene regex matching.
My os database contains entries with the following values: abc, abc\, abc), abc&
In lucene regex have implicit anchors in the beginning and the and.

Regex	Result	Expected
ab	does not match abc	Expected
abc	matches abc, abc\, abc), abc&	Unexpected

So for abc is the anchor ignored for some characters?

Now let's say I want a query that only matches abc&:

Regex	Result	Expected
abc&	Invalid query	Expected since & is calculating a intersection
abc&&	Nothing is matched	?
abc\&	Nothing is matched	Unexpected

I did not found a way to match explicitly abc&.

Note that this behavior exists with and without the optional parameter being enabled.

Related component

Other

To Reproduce

1. Add entries with abc, abc\, abc) and abc& to the dataset.
2. Execute:

GET <index>
{"query": 
{"bool": 
{"must": 
  [
    {"regexp": 
      {"crash_componedsfnt.keyword": 
        {"value": "<regex>", "flags": "ALL"}
      }
    }
  ], 
"must_not": [], 
"filter": {
  "range": {"crash_timestamp": {"gte": "now-6000m", "lte": "now"}}}}
}, 
"aggs": {"aggregation_result": {"cardinality": {"field": "config_name.keyword"}}}, 
"size": 10}

3. See what entries are matched

Expected behavior

1. The regex abc should only match abc and not abc\, abc), abc&
2. The regex abc\& should match abc&

Additional Details

• this behavior exists with and without the optional parameter being enabled.
• OS: AL
• Version 2.11.0

Version retrieved from curl -X GET "http://localhost:9200"