› › ›

N-gram token filter

Forms n-grams of specified lengths from a token.

For example, you can use the ngram token filter to change fox to [ f, fo, o, ox, x ].

This filter uses Lucene’s NGramTokenFilter.

The ngram filter is similar to the edge_ngram token filter. However, the edge_ngram only outputs n-grams that start at the beginning of a token.

Example

The following analyze API request uses the ngram filter to convert Quick fox to 1-character and 2-character n-grams:

GET _analyze
{
  "tokenizer": "standard",
  "filter": [ "ngram" ],
  "text": "Quick fox"
}

The filter produces the following tokens:

[ Q, Qu, u, ui, i, ic, c, ck, k, f, fo, o, ox, x ]

Add to an analyzer

The following create index API request uses the ngram filter to configure a new custom analyzer.

PUT ngram_example
{
  "settings": {
    "analysis": {
      "analyzer": {
        "standard_ngram": {
          "tokenizer": "standard",
          "filter": [ "ngram" ]
        }
      }
    }
  }
}

Configurable parameters

max_gram: (Optional, integer) Maximum length of characters in a gram. Defaults to 2.
min_gram: (Optional, integer) Minimum length of characters in a gram. Defaults to 1.
preserve_original: (Optional, Boolean) Emits original token when set to true. Defaults to false.

You can use the index.max_ngram_diff index-level setting to control the maximum allowed difference between the max_gram and min_gram values.

Customize

To customize the ngram filter, duplicate it to create the basis for a new custom token filter. You can modify the filter using its configurable parameters.

For example, the following request creates a custom ngram filter that forms n-grams between 3-5 characters. The request also increases the index.max_ngram_diff setting to 2.

PUT ngram_custom_example
{
  "settings": {
    "index": {
      "max_ngram_diff": 2
    },
    "analysis": {
      "analyzer": {
        "default": {
          "tokenizer": "whitespace",
          "filter": [ "3_5_grams" ]
        }
      },
      "filter": {
        "3_5_grams": {
          "type": "ngram",
          "min_gram": 3,
          "max_gram": 5
        }
      }
    }
  }
}

© 2023-2025 Elasticsearch
As of September 2024, Elasticsearch is available under a choice of three licenses: the Server Side Public License (SSPL), the Elastic License, or the AGPLv3 (OSI approved).
Elasticsearch and the Elasticsearch logo are trademarks of Elasticsearch B.V., registered in the U.S. and in other countries.
https://www.elastic.co/guide/en/elasticsearch/reference/7.17/analysis-ngram-tokenfilter.html