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Stemmer override token filter

Overrides stemming algorithms, by applying a custom mapping, then protecting these terms from being modified by stemmers. Must be placed before any stemming filters.

Rules are mappings in the form of token1[, ..., tokenN] => override.

Setting	Description
`rules`	A list of mapping rules to use.
`rules_path`	A path (either relative to `config` location, or absolute) to a list of mappings.

Here is an example:

PUT /my-index-000001
{
  "settings": {
    "analysis": {
      "analyzer": {
        "my_analyzer": {
          "tokenizer": "standard",
          "filter": [ "lowercase", "custom_stems", "porter_stem" ]
        }
      },
      "filter": {
        "custom_stems": {
          "type": "stemmer_override",
          "rules_path": "analysis/stemmer_override.txt"
        }
      }
    }
  }
}

Where the file looks like:

running, runs => run

stemmer => stemmer

You can also define the overrides rules inline:

PUT /my-index-000001
{
  "settings": {
    "analysis": {
      "analyzer": {
        "my_analyzer": {
          "tokenizer": "standard",
          "filter": [ "lowercase", "custom_stems", "porter_stem" ]
        }
      },
      "filter": {
        "custom_stems": {
          "type": "stemmer_override",
          "rules": [
            "running, runs => run",
            "stemmer => stemmer"
          ]
        }
      }
    }
  }
}

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https://www.elastic.co/guide/en/elasticsearch/reference/7.17/analysis-stemmer-override-tokenfilter.html