[[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 <<asciifolding-token-filter,asciifolding token filter>> 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 <<icu-plugin,icu plug-in>>)
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:
PUT /my_index { "settings": { "analysis": { "analyzer": { "my_folder": { "tokenizer": "icu_tokenizer", "filter": [ "icu_folding" ] } } } } }
<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")))
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.