# Case insensitive regex query with character range **URL:** \ **Category:** Elasticsearch\ **Created:** [March 19, 2025, 5:08pm UTC](https://discuss.elastic.co/t/case-insensitive-regex-query-with-character-range/376136 "2025-03-19T17:08:14Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![petrsimon](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/petrsimon/32/142139_2.png) [@petrsimon](https://discuss.elastic.co/u/petrsimon)\ **Post date:** [March 19, 2025, 5:08pm UTC](https://discuss.elastic.co/t/case-insensitive-regex-query-with-character-range/376136/1 "2025-03-19T17:08:14Z")
Hi, I'm implementing regex search on ES 8.16 (using java client) against fields of type `keyword` and I came across strange issue when sending the `case_insensitive: true` option. 1. "[abc]+" -\> "abc", "ABC" 2. "[ABC]+" -\> "abc", "ABC" 3. "[a-c]+" -\> "abc" 4. "[A-C]+" -\> "ABC" Is there an explanation for this or is it a bug in lucene or ES? Thanks a lot for any insight! ---
**Author:** ![Mark\_Harwood1](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/mark_harwood1/32/101255_2.png) [@Mark\_Harwood1](https://discuss.elastic.co/u/Mark_Harwood1)\ **Post date:** [March 19, 2025, 11:14pm UTC](https://discuss.elastic.co/t/case-insensitive-regex-query-with-character-range/376136/2 "2025-03-19T23:14:10Z")
My guess is that the case sensitivity only applies to explicitly listed characters and because steps 3 and 4 have some unstated characters (b and B respectively) these are not interpreted as you'd hope. This is arguably a bug in Lucene. ---
**Author:** ![petrsimon](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/petrsimon/32/142139_2.png) [@petrsimon](https://discuss.elastic.co/u/petrsimon)\ **Post date:** [March 20, 2025, 9:03am UTC](https://discuss.elastic.co/t/case-insensitive-regex-query-with-character-range/376136/3 "2025-03-20T09:03:07Z")
I've tried reproducing in lucene 10, but there it only returns exactly what's in the the regex, i.e. regarless of the ASCII\_CASE\_INSENSITIVE flag. So apparently the 1. "[abc]+" -\> "abc", "ABC" 2. "[ABC]+" -\> "abc", "ABC" is implemented in ES. ---
**Author:** ![Christian\_Dahlqvist](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/christian_dahlqvist/32/4617_2.png) [@Christian\_Dahlqvist](https://discuss.elastic.co/u/Christian_Dahlqvist)\ **Post date:** [March 20, 2025, 9:11am UTC](https://discuss.elastic.co/t/case-insensitive-regex-query-with-character-range/376136/4 "2025-03-20T09:11:24Z")
> [@petrsimon](#): > > - "[a-c]+" -\> "abc" > - "[A-C]+" -\> "ABC" I believe the issue is that ranges where all characters are not explicitly stated are not working. Did you test the ones quoted above? ---
**Author:** ![petrsimon](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/petrsimon/32/142139_2.png) [@petrsimon](https://discuss.elastic.co/u/petrsimon)\ **Post date:** [March 20, 2025, 9:25am UTC](https://discuss.elastic.co/t/case-insensitive-regex-query-with-character-range/376136/5 "2025-03-20T09:25:04Z")
Yes, I've tested all these patterns, in Lucene with KeywordAnalyzer it's doing 1. "[abc]+" -\> "abc" 2. "[ABC]+" -\> "ABC" 3. "[a-c]+" -\> "abc" 4. "[A-C]+" -\> "ABC" ---
**Author:** ![petrsimon](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/petrsimon/32/142139_2.png) [@petrsimon](https://discuss.elastic.co/u/petrsimon)\ **Post date:** [March 20, 2025, 9:38am UTC](https://discuss.elastic.co/t/case-insensitive-regex-query-with-character-range/376136/6 "2025-03-20T09:38:40Z")
Lucene test [GitHub - petrsimon/lucene-regex](https://github.com/petrsimon/lucene-regex) ---
**Author:** ![dadoonet](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/dadoonet/32/137187_2.png) [@dadoonet](https://discuss.elastic.co/u/dadoonet)\ **Post date:** [March 20, 2025, 10:15am UTC](https://discuss.elastic.co/t/case-insensitive-regex-query-with-character-range/376136/7 "2025-03-20T10:15:24Z")
> [@Mark\_Harwood1](#): > > This is arguably a bug in Lucene. I agree and at least this should be documented I think. ---
**Author:** ![petrsimon](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/petrsimon/32/142139_2.png) [@petrsimon](https://discuss.elastic.co/u/petrsimon)\ **Post date:** [March 20, 2025, 12:33pm UTC](https://discuss.elastic.co/t/case-insensitive-regex-query-with-character-range/376136/8 "2025-03-20T12:33:10Z")
I've reported it at [Case insensitive regex query with character range · Issue #14378 · apache/lucene · GitHub](https://github.com/apache/lucene/issues/14378). Thanks all for your feedback. ---
**Author:** ![petrsimon](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/petrsimon/32/142139_2.png) [@petrsimon](https://discuss.elastic.co/u/petrsimon)\ **Post date:** [March 20, 2025, 3:42pm UTC](https://discuss.elastic.co/t/case-insensitive-regex-query-with-character-range/376136/9 "2025-03-20T15:42:33Z")
Seems like it's a known limitation of Lucene. Perhaps it would be nice to document it also in ES? ---
**Author:** ![RainTown](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/raintown/32/140206_2.png) [@RainTown](https://discuss.elastic.co/u/RainTown)\ **Post date:** [March 20, 2025, 4:47pm UTC](https://discuss.elastic.co/t/case-insensitive-regex-query-with-character-range/376136/10 "2025-03-20T16:47:54Z")
Good spot @petrsimon !! This is sort of thing that would infuriate me if I found that the regex `[abc]+` isn't treated exactly the same as `[a-c]+`. Really subtle and certainly counter-intuitive and therefore difficult to catch via testing in real world, creating really subtle user issues. I note that at > **[RegExp (Lucene 10.1.0 core API)](https://lucene.apache.org/core/10_1_0/core/org/apache/lucene/util/automaton/RegExp.html)** > > declaration: package: org.apache.lucene.util.automaton, class: RegExp it does not really give any detail on the scope of Lucene regex support - POSIX BRE / POSIX ERE / PCRE / ... It's clearly some subset, maybe documented elsewhere? Sadly the ES documentation at: > **[Regular expression syntax | Elasticsearch Guide \[8.17\] | Elastic](https://www.elastic.co/guide/en/elasticsearch/reference/current/regexp-syntax.html)** has exactly the (unfortunate) example above - > `[…]` > > Match one of the characters in the brackets. For example: > > [abc] # matches 'a', 'b', 'c' > > Inside the brackets, `-` indicates a range unless `-` is the first character or escaped. For example: > > [a-c] # matches 'a', 'b', or 'c' The strong implication is that those 2 things are functionally equivalent. But they are clearly not 100% equivalent. I can see the argument around character classes supporting different languages, where upper/lower case is more complex, so it's all more complicated than at first glance, .... But still it's hard to square that with `[abc]` meaning one thing `[a-c]` meaning another. Suggestion to try to add some note to the ES doc above, I note it already documents the lack of anchors. Maybe also on pages where the case\_insensitive: true/false toggle is documented. ---
**Author:** ![dadoonet](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/dadoonet/32/137187_2.png) [@dadoonet](https://discuss.elastic.co/u/dadoonet)\ **Post date:** [March 21, 2025, 3:07pm UTC](https://discuss.elastic.co/t/case-insensitive-regex-query-with-character-range/376136/11 "2025-03-21T15:07:26Z")
Thanks all for reporting this and helping on this topic. @Liam_Thompson created this PR to add some clarification in our documentation. > > > Flagged by @dadoonet in Slack: > > \> There's an interesting discussion \[on discu…ss\](https://discuss.elastic.co/t/case-insensitive-regex-query-with-character-range/376136/9?u=dadoonet) about \[Lucene regex\](https://lucene.apache.org/core/10\_1\_0/core/org/apache/lucene/util/automaton/RegExp.html) support. > \> Our \[documentation\](https://www.elastic.co/guide/en/elasticsearch/reference/current/regexp-syntax.html#regexp-standard-operators) says things like: > \> > \> - \`\[a-c\] # matches 'a', 'b', or 'c'\` > \> - \`\[^a-c\] # matches any character except 'a', 'b', or 'c'\` > \> > \> Which is correct unless you use as well \`case\_insensitive: true\` option. In which case you would expect \`B\` to match \`\[a-c\]\`. But it does not work that way and it's a known limitation as Robert answered in \[https://github.com/apache/lucene/issues/14378#issuecomment-2740658343\](https://github.com/apache/lucene/issues/14378#issuecomment-2740658343). > \>