lowercase normalizer doubles backslashes in 8.19.x and 9.1.x

[MikePieperSer]

Elasticsearch Version

8.19.2

Installed Plugins

analysis-icu,analysis-smartcn

Java Version

bundled

OS Version

Linux edbae108e7d8 6.8.0-79-generic #79-Ubuntu SMP PREEMPT_DYNAMIC Tue Aug 12 14:42:46 UTC 2025 x86_64 x86_64 x86_64 GNU/Linux

Problem Description

I have a field defined as such:

"myfield": {
          "type": "keyword",
          "store": true,
         "fields": {
           "lowercase": {
              "type": "keyword",
              "normalizer": "lowercase"
            }
          }
        },

this stores the value in the original case and additionally a lower case variant. The lower case variant is created by Elasticsearch while storing/changing the document.

A value “AbC\\dEf” is stored as “abc\\def” in the lowercase sub-field (note: Strings are in JSON notation).

This worked till 8.18.5 and works in 9.0.5.

In 8.19.0 - 8.19.2 and 9.1.2 it stores the value as “abc\\\\def” and the value can’t be found anymore when I search with “abc\\def”.

Steps to Reproduce

scratch_49.sh

Logs (if relevant)

No response

[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)

[john-wagster]

fixed by: #134790

[elasticsearchmachine]

Pinging @elastic/es-storage-engine (Team:StorageEngine)

[yeikel]

We found this problem upgrading from 8.17.1 to 8.19.2 using copy_to with default configuration

Reproducer

PUT store
{
    "settings": {
        "number_of_replicas": 0,
        "number_of_shards": 1
    },
    "mappings": {
        "dynamic": "strict",
        "properties": {
            "target": {
                "type": "keyword",
                "doc_values": false
            },
            "source":{
              "type": "keyword",
              "index": false,
              "copy_to": [
                  "target"
              ],
              "doc_values": false
            }
        }
    }
}

PUT store/_doc/01
{
  "source": "hQ%m)zTCOH+MJ\\1l"
}

GET store/_search
{
  "query": {
    "term": {
      "target": {
        "value": "hQ%m)zTCOH+MJ\\1l"
      }
    }
  }
}

8.19.2 : does not return results
8.17.1: returns results

[jilldoty-elastic]

Closed by fix #134790. (and duplicate of #134770)