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Token filters
Token filters receive the stream of tokens from the tokenizer and add, remove, or modify the tokens. For example, a token filter may lowercase the tokens so that Actions becomes action, remove stopwords like than, or add synonyms like talk for the word speak.
The following table lists all token filters that OpenSearch supports.
| Token filter | Underlying Lucene token filter | Description |
apostrophe | ApostropheFilter | In each token that contains an apostrophe, the apostrophe token filter removes the apostrophe itself and all characters following the apostrophe. |
asciifolding | ASCIIFoldingFilter | Converts alphabetic, numeric, and symbolic characters. |
cjk_bigram | CJKBigramFilter | Forms bigrams of Chinese, Japanese, and Korean (CJK) tokens. |
cjk_width | CJKWidthFilter | Normalizes Chinese, Japanese, and Korean (CJK) tokens according to the following rules: - Folds full-width ASCII character variants into the equivalent basic Latin characters. - Folds half-width Katakana character variants into the equivalent Kana characters. |
classic | ClassicFilter | Performs optional post-processing on the tokens generated by the classic tokenizer. Removes possessives ('s) and removes . from acronyms. |
common_grams | CommonGramsFilter | Generates bigrams for a list of frequently occurring terms. The output contains both single terms and bigrams. |
conditional | ConditionalTokenFilter | Applies an ordered list of token filters to tokens that match the conditions provided in a script. |
decimal_digit | DecimalDigitFilter | Converts all digits in the Unicode decimal number general category to basic Latin digits (0–9). |
delimited_payload | DelimitedPayloadTokenFilter | Separates a token stream into tokens with corresponding payloads, based on a provided delimiter. A token consists of all characters before the delimiter, and a payload consists of all characters after the delimiter. For example, if the delimiter is |, then for the string foo|bar, foo is the token and bar is the payload. |
delimited_term_freq | DelimitedTermFrequencyTokenFilter | Separates a token stream into tokens with corresponding term frequencies, based on a provided delimiter. A token consists of all characters before the delimiter, and a term frequency is the integer after the delimiter. For example, if the delimiter is |, then for the string foo|5, foo is the token and 5 is the term frequency. |
dictionary_decompounder | DictionaryCompoundWordTokenFilter | Decomposes compound words found in many Germanic languages. |
edge_ngram | EdgeNGramTokenFilter | Tokenizes the given token into edge n-grams (n-grams that start at the beginning of the token) of lengths between min_gram and max_gram. Optionally, keeps the original token. |
elision | ElisionFilter | Removes the specified elisions from the beginning of tokens. For example, changes l'avion (the plane) to avion (plane). |
fingerprint | FingerprintFilter | Sorts and deduplicates the token list and concatenates tokens into a single token. |
flatten_graph | FlattenGraphFilter | Flattens a token graph produced by a graph token filter, such as synonym_graph or word_delimiter_graph, making the graph suitable for indexing. |
hunspell | HunspellStemFilter | Uses Hunspell rules to stem tokens. Because Hunspell supports a word having multiple stems, this filter can emit multiple tokens for each consumed token. Requires you to configure one or more language-specific Hunspell dictionaries. |
hyphenation_decompounder | HyphenationCompoundWordTokenFilter | Uses XML-based hyphenation patterns to find potential subwords in compound words and checks the subwords against the specified word list. The token output contains only the subwords found in the word list. |
keep_types | TypeTokenFilter | Keeps or removes tokens of a specific type. |
keep_word | KeepWordFilter | Checks the tokens against the specified word list and keeps only those that are in the list. |
keyword_marker | KeywordMarkerFilter | Marks specified tokens as keywords, preventing them from being stemmed. |
keyword_repeat | KeywordRepeatFilter | Emits each incoming token twice: once as a keyword and once as a non-keyword. |
kstem | KStemFilter | Provides kstem-based stemming for the English language. Combines algorithmic stemming with a built-in dictionary. |
length | LengthFilter | Removes tokens whose lengths are shorter or longer than the length range specified by min and max. |
limit | LimitTokenCountFilter | Limits the number of output tokens. A common use case is to limit the size of document field values based on token count. |
lowercase | LowerCaseFilter | Converts tokens to lowercase. The default LowerCaseFilter is for the English language. You can set the language parameter to greek (uses GreekLowerCaseFilter), irish (uses IrishLowerCaseFilter), or turkish (uses TurkishLowerCaseFilter). |
min_hash | MinHashFilter | Uses the MinHash technique to estimate document similarity. Performs the following operations on a token stream sequentially: 1. Hashes each token in the stream. 2. Assigns the hashes to buckets, keeping only the smallest hashes of each bucket. 3. Outputs the smallest hash from each bucket as a token stream. |
multiplexer | N/A | Emits multiple tokens at the same position. Runs each token through each of the specified filter lists separately and outputs the results as separate tokens. |
ngram | NGramTokenFilter | Tokenizes the given token into n-grams of lengths between min_gram and max_gram. |
| Normalization | arabic_normalization: ArabicNormalizer german_normalization: GermanNormalizationFilter hindi_normalization: HindiNormalizer indic_normalization: IndicNormalizer sorani_normalization: SoraniNormalizer persian_normalization: PersianNormalizer scandinavian_normalization : ScandinavianNormalizationFilter scandinavian_folding: ScandinavianFoldingFilter serbian_normalization: SerbianNormalizationFilter | Normalizes the characters of one of the listed languages. |
pattern_capture | N/A | Generates a token for every capture group in the provided regular expression. Uses Java regular expression syntax. |
pattern_replace | N/A | Matches a pattern in the provided regular expression and replaces matching substrings. Uses Java regular expression syntax. |
phonetic | N/A | Uses a phonetic encoder to emit a metaphone token for each token in the token stream. Requires installing the analysis-phonetic plugin. |
porter_stem | PorterStemFilter | Uses the Porter stemming algorithm to perform algorithmic stemming for the English language. |
predicate_token_filter | N/A | Removes tokens that don’t match the specified predicate script. Supports inline Painless scripts only. |
remove_duplicates | RemoveDuplicatesTokenFilter | Removes duplicate tokens that are in the same position. |
reverse | ReverseStringFilter | Reverses the string corresponding to each token in the token stream. For example, the token dog becomes god. |
shingle | ShingleFilter | Generates shingles of lengths between min_shingle_size and max_shingle_size for tokens in the token stream. Shingles are similar to n-grams but apply to words instead of letters. For example, two-word shingles added to the list of unigrams [contribute, to, opensearch] are [contribute to, to opensearch]. |
snowball | N/A | Stems words using a Snowball-generated stemmer. You can use the snowball token filter with the following languages in the language field: Arabic, Armenian, Basque, Catalan, Danish, Dutch, English, Estonian, Finnish, French, German, German2, Hungarian, Irish, Italian, Kp, Lithuanian, Lovins, Norwegian, Porter, Portuguese, Romanian, Russian, Spanish, Swedish, Turkish. |
stemmer | N/A | Provides algorithmic stemming for the following languages in the language field: arabic, armenian, basque, bengali, brazilian, bulgarian, catalan, czech, danish, dutch, dutch_kp, english, light_english, lovins, minimal_english, porter2, possessive_english, estonian, finnish, light_finnish, french, light_french, minimal_french, galician, minimal_galician, german, german2, light_german, minimal_german, greek, hindi, hungarian, light_hungarian, indonesian, irish, italian, light_italian, latvian, Lithuanian, norwegian, light_norwegian, minimal_norwegian, light_nynorsk, minimal_nynorsk, portuguese, light_portuguese, minimal_portuguese, portuguese_rslp, romanian, russian, light_russian, sorani, spanish, light_spanish, swedish, light_swedish, turkish. |
stemmer_override | N/A | Overrides stemming algorithms by applying a custom mapping so that the provided terms are not stemmed. |
stop | StopFilter | Removes stop words from a token stream. |
synonym | N/A | Supplies a synonym list for the analysis process. The synonym list is provided using a configuration file. |
synonym_graph | N/A | Supplies a synonym list, including multiword synonyms, for the analysis process. |
trim | TrimFilter | Trims leading and trailing whitespace from each token in a stream. |
truncate | TruncateTokenFilter | Truncates tokens whose length exceeds the specified character limit. |
unique | N/A | Ensures each token is unique by removing duplicate tokens from a stream. |
uppercase | UpperCaseFilter | Converts tokens to uppercase. |
word_delimiter | WordDelimiterFilter | Splits tokens at non-alphanumeric characters and performs normalization based on the specified rules. |
word_delimiter_graph | WordDelimiterGraphFilter | Splits tokens at non-alphanumeric characters and performs normalization based on the specified rules. Assigns multi-position tokens a positionLength attribute. |