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.

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
        }
      }
    }
  }
}