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N-gram token filter

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N-gram token filter

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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

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

resp = client.indices.analyze(
    tokenizer="standard",
    filter=[
        "ngram"
    ],
    text="Quick fox",
)
print(resp)

response = client.indices.analyze(
  body: {
    tokenizer: 'standard',
    filter: [
      'ngram'
    ],
    text: 'Quick fox'
  }
)
puts response

const response = await client.indices.analyze({
  tokenizer: "standard",
  filter: ["ngram"],
  text: "Quick fox",
});
console.log(response);

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

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

resp = client.indices.create(
    index="ngram_example",
    settings={
        "analysis": {
            "analyzer": {
                "standard_ngram": {
                    "tokenizer": "standard",
                    "filter": [
                        "ngram"
                    ]
                }
            }
        }
    },
)
print(resp)

response = client.indices.create(
  index: 'ngram_example',
  body: {
    settings: {
      analysis: {
        analyzer: {
          standard_ngram: {
            tokenizer: 'standard',
            filter: [
              'ngram'
            ]
          }
        }
      }
    }
  }
)
puts response

const response = await client.indices.create({
  index: "ngram_example",
  settings: {
    analysis: {
      analyzer: {
        standard_ngram: {
          tokenizer: "standard",
          filter: ["ngram"],
        },
      },
    },
  },
});
console.log(response);

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

Configurable parameters

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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

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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.

resp = client.indices.create(
    index="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
                }
            }
        }
    },
)
print(resp)

response = client.indices.create(
  index: 'ngram_custom_example',
  body: {
    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
          }
        }
      }
    }
  }
)
puts response

const response = await client.indices.create({
  index: "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,
        },
      },
    },
  },
});
console.log(response);

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
        }
      }
    }
  }
}

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