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Lowercase token filter

Changes token text to lowercase. For example, you can use the lowercase filter to change THE Lazy DoG to the lazy dog.

In addition to a default filter, the lowercase token filter provides access to Lucene’s language-specific lowercase filters for Greek, Irish, and Turkish.

Example

The following analyze API request uses the default lowercase filter to change the THE Quick FoX JUMPs to lowercase:

GET _analyze
{
  "tokenizer" : "standard",
  "filter" : ["lowercase"],
  "text" : "THE Quick FoX JUMPs"
}

The filter produces the following tokens:

[ the, quick, fox, jumps ]

Add to an analyzer

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

PUT lowercase_example
{
  "settings": {
    "analysis": {
      "analyzer": {
        "whitespace_lowercase": {
          "tokenizer": "whitespace",
          "filter": [ "lowercase" ]
        }
      }
    }
  }
}

Configurable parameters

language

(Optional, string) Language-specific lowercase token filter to use. Valid values include:

greek: Uses Lucene’s GreekLowerCaseFilter
irish: Uses Lucene’s IrishLowerCaseFilter
turkish: Uses Lucene’s TurkishLowerCaseFilter

If not specified, defaults to Lucene’s LowerCaseFilter.

Customize

To customize the lowercase 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 lowercase filter for the Greek language:

PUT custom_lowercase_example
{
  "settings": {
    "analysis": {
      "analyzer": {
        "greek_lowercase_example": {
          "type": "custom",
          "tokenizer": "standard",
          "filter": ["greek_lowercase"]
        }
      },
      "filter": {
        "greek_lowercase": {
          "type": "lowercase",
          "language": "greek"
        }
      }
    }
  }
}

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