Regex containing umlauts in square brackets not matching

[muety]

Description

I might be mistaken, but the following seems like a bug to me. Regexes that contain a German umlaut (or probably any other special character) wrapped between curly braces (like ^foo[ä]b) won't match as expected, e.g. won't match the word fooäbar.

Steps to Reproduce

Suppose you have the following user document in your database:

{
  "_id": "3f6b99deb2a9d8ba80ac3a8cf3000558",
  "_rev": "1-889fc2635b372fa8b0f436fe6c6d4393",
  "lastname": "Mütsch",
  "firstname": "Ferdinand",
  "created": 1702629477669
}

Try to run these queries against the /_find endpoint:

Query 1:

{
   "selector": {
      "lastname": {
         "$regex": "(?i)^müt"
      }
   }
}

Expected: returns above document
Actual: returns above document ✅

Query 2:

{
   "selector": {
      "lastname": {
         "$regex": "(?i)^mü[t]"
      }
   }
}

Expected: returns above document
Actual: returns above document ✅

Query 3:

{
   "selector": {
      "lastname": {
         "$regex": "(?i)^m[ü]t"
      }
   }
}

Expected: returns above document
Actual: returns empty result ❌

According to my tests on Regex101 with PCRE, the last query should match the above document's lastname field, but for some reason doesn't.

Your Environment

• CouchDB version used: 3.3.3 (inside Docker with latest 3-tagged image)
• Browser name and version: Firefox 120.0.1
• Operating system and version: Fedora 38 / whatever OS is used in the Docker image

[willholley]

The Erlang regex syntax can be a little unusual. In this case, I think the issue is that, as described in Square Brackets and Character Classes, you need to set UTF mode to get the behaviour you expect.

Changing the query to:

{
   "selector": {
      "lastname": {
         "$regex": "(*UTF8)(?i)^m[ü]t"
      }
   }
}

should give you the correct result.

[muety]

Awesome, thanks a lot 👍

[big-r81]

The Erlang regex syntax can be a little unusual. In this case, I think the issue is that, as described in Square Brackets and Character Classes, you need to set UTF mode to get the behaviour you expect.

Is this mentioned in the docs?

[willholley]

the docs layout isn't great but we do link to the Erlang syntax for reference in https://docs.couchdb.org/en/stable/api/database/find.html#condition-operators.