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Length token filter

Removes tokens shorter or longer than specified character lengths. For example, you can use the length filter to exclude tokens shorter than 2 characters and tokens longer than 5 characters.

This filter uses Lucene’s LengthFilter.

The length filter removes entire tokens. If you’d prefer to shorten tokens to a specific length, use the truncate filter.

Example

The following analyze API request uses the length filter to remove tokens longer than 4 characters:

GET _analyze
{
  "tokenizer": "whitespace",
  "filter": [
    {
      "type": "length",
      "min": 0,
      "max": 4
    }
  ],
  "text": "the quick brown fox jumps over the lazy dog"
}

The filter produces the following tokens:

[ the, fox, over, the, lazy, dog ]

Add to an analyzer

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

PUT length_example
{
  "settings": {
    "analysis": {
      "analyzer": {
        "standard_length": {
          "tokenizer": "standard",
          "filter": [ "length" ]
        }
      }
    }
  }
}

Configurable parameters

min: (Optional, integer) Minimum character length of a token. Shorter tokens are excluded from the output. Defaults to 0.
max: (Optional, integer) Maximum character length of a token. Longer tokens are excluded from the output. Defaults to Integer.MAX_VALUE, which is 2^31-1 or 2147483647.

Customize

To customize the length 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 length filter that removes tokens shorter than 2 characters and tokens longer than 10 characters:

PUT length_custom_example
{
  "settings": {
    "analysis": {
      "analyzer": {
        "whitespace_length_2_to_10_char": {
          "tokenizer": "whitespace",
          "filter": [ "length_2_to_10_char" ]
        }
      },
      "filter": {
        "length_2_to_10_char": {
          "type": "length",
          "min": 2,
          "max": 10
        }
      }
    }
  }
}

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