Character folding

[[character-folding]] === Unicode Character Folding

In the same way as the lowercase token filter is a good starting point for many languages((("Unicode", "character folding")))((("tokens", "normalizing", "Unicode character folding"))) but falls short when exposed to the entire tower of Babel, so the <> requires a more effective Unicode character-folding counterpart((("character folding"))) for dealing with the many languages of the world.((("asciifolding token filter")))

The icu_folding token filter (provided by the <>) does the same job as the asciifolding filter, ((("icu_folding token filter")))but extends the transformation to scripts that are not ASCII-based, such as Greek, Hebrew, Han, conversion of numbers in other scripts into their Latin equivalents, plus various other numeric, symbolic, and punctuation transformations.

The icu_folding token filter applies Unicode normalization and case folding from nfkc_cf automatically,((("nfkc_cf normalization form"))) so the icu_normalizer is not required:

[source,js]

PUT /my_index { "settings": { "analysis": { "analyzer": { "my_folder": { "tokenizer": "icu_tokenizer", "filter": [ "icu_folding" ] } } } } }

GET /my_index/_analyze?analyzer=my_folder

١٢٣٤٥ <1>

<1> The Arabic numerals ١٢٣٤٥ are folded to their Latin equivalent: 12345.

If there are particular characters that you would like to protect from folding, you can use a http://icu-project.org/apiref/icu4j/com/ibm/icu/text/UnicodeSet.html[_UnicodeSet_] (much like a character class in regular expressions) to specify which Unicode characters may be folded. For instance, to exclude the Swedish letters å, ä, ö, ++Å++, Ä, and Ö from folding, you would specify a character class representing all Unicode characters, except for those letters: [^åäöÅÄÖ] (^ means everything except).((("swedish_folding filter")))((("swedish analyzer")))

[source,js]

PUT /my_index { "settings": { "analysis": { "filter": { "swedish_folding": { <1> "type": "icu_folding", "unicodeSetFilter": "åäöÅÄÖ" } }, "analyzer": { "swedish_analyzer": { <2> "tokenizer": "icu_tokenizer", "filter": [ "swedish_folding", "lowercase" ] } } } }

}

<1> The swedish_folding token filter customizes the icu_folding token filter to exclude Swedish letters, both uppercase and lowercase.

<2> The swedish analyzer first tokenizes words, then folds each token by using the swedish_folding filter, and then lowercases each token in case it includes some of the uppercase excluded letters: ++Å++, Ä, or Ö.