⪢ NLP and tokenization

Data tokenization

Manticore doesn't store text as is for performing full-text searching on it. Instead, it extracts words and creates several structures that allow fast full-text searching. From the found words, a dictionary is built, which allows a quick look to discover if the word is present or not in the index. In addition, other structures record the documents and fields in which the word was found (as well as the position of it inside a field). All these are used when a full-text match is performed.

The process of demarcating and classifying words is called tokenization. The tokenization is applied at both indexing and searching, and it operates at the character and word level.

On the character level, the engine allows only certain characters to pass. This is defined by the charset_table. Anything else is replaced with a whitespace (which is considered the default word separator). The charset_table also allows mappings, such as lowercasing or simply replacing one character with another. Besides that, characters can be ignored, blended, defined as a phrase boundary.

At the word level, the base setting is the min_word_len which defines the minimum word length in characters to be accepted in the index. A common request is to match singular with plural forms of words. For this, morphology processors can be used.

Going further, we might want a word to be matched as another one because they are synonyms. For this, the word forms feature can be used, which allows one or more words to be mapped to another one.

Very common words can have some unwanted effects on searching, mostly because of their frequency they require lots of computing to process their doc/hit lists. They can be blacklisted with the stop words functionality. This helps not only in speeding up queries but also in decreasing the index size.

A more advanced blacklisting is bigrams, which allows creating a special token between a "bigram" (common) word and an uncommon word. This can speed up several times when common words are used in phrase searches.

In case of indexing HTML content, it's important not to index the HTML tags, as they can introduce a lot of "noise" in the index. HTML stripping can be used and can be configured to strip, but index certain tag attributes or completely ignore the content of certain HTML elements.

Supported languages

Manticore supports a wide range of languages, with basic support enabled for most languages via charset_table = non_cjk (which is the default value).

For many languages, Manticore provides a stopwords file that can be used to improve search relevance.

Additionally, advanced morphology is available for a few languages that can significantly improve search relevance by using dictionary-based lemmatization or stemming algorithms for better segmentation and normalization.

The table below lists all supported languages and indicates how to enable:

  • basic support (column "Supported")
  • stopwords (column "Stopwords file name")
  • advanced morphology (column "Advanced morphology")
Language Supported Stopwords file name Advanced morphology Notes
Afrikaans charset_table=non_cjk af -
Arabic charset_table=non_cjk ar morphology=stem_ar (Arabic stemmer); morphology=libstemmer_ar
Armenian charset_table=non_cjk hy -
Assamese specify charset_table specify charset_table manually - -
Basque charset_table=non_cjk eu -
Bengali charset_table=non_cjk bn -
Bishnupriya specify charset_table manually - -
Buhid specify charset_table manually - -
Bulgarian charset_table=non_cjk bg -
Catalan charset_table=non_cjk ca morphology=libstemmer_ca
Chinese charset_table=chinese or ngram_chars=chinese zh morphology=icu_chinese or ngram_chars=1 correspondingly ICU dictionary based segmentation is much more accurate than ngram-based
Croatian charset_table=non_cjk hr -
Kurdish charset_table=non_cjk ckb -
Czech charset_table=non_cjk cz morphology=stem_cz (Czech stemmer)
Danish charset_table=non_cjk da morphology=libstemmer_da
Dutch charset_table=non_cjk nl morphology=libstemmer_nl
English charset_table=non_cjk en morphology=lemmatize_en (single root form); morphology=lemmatize_en_all (all root forms); morphology=stem_en (Porter's English stemmer); morphology=stem_enru (Porter's English and Russian stemmers); morphology=libstemmer_en (English from libstemmer)
Esperanto charset_table=non_cjk eo -
Estonian charset_table=non_cjk et -
Finnish charset_table=non_cjk fi morphology=libstemmer_fi
French charset_table=non_cjk fr morphology=libstemmer_fr
Galician charset_table=non_cjk gl -
Garo specify charset_table manually - -
German charset_table=non_cjk de morphology=lemmatize_de (single root form); morphology=lemmatize_de_all (all root forms); morphology=libstemmer_de
Greek charset_table=non_cjk el morphology=libstemmer_el
Hebrew charset_table=non_cjk he -
Hindi charset_table=non_cjk hi morphology=libstemmer_hi
Hmong specify charset_table manually - -
Ho specify charset_table manually - -
Hungarian charset_table=non_cjk hu morphology=libstemmer_hu
Indonesian charset_table=non_cjk id morphology=libstemmer_id
Irish charset_table=non_cjk ga morphology=libstemmer_ga
Italian charset_table=non_cjk it morphology=libstemmer_it
Japanese ngram_chars=japanese - ngram_chars=japanese ngram_len=1 Requires ngram-based segmentation
Komi specify charset_table manually - -
Korean ngram_chars=korean - ngram_chars=korean ngram_len=1 Requires ngram-based segmentation
Large Flowery Miao specify charset_table manually - -
Latin charset_table=non_cjk la -
Latvian charset_table=non_cjk lv -
Lithuanian charset_table=non_cjk lt morphology=libstemmer_lt
Maba specify charset_table manually - -
Maithili specify charset_table manually - -
Marathi specify charset_table manually - -
Marathi charset_table=non_cjk mr -
Mende specify charset_table manually - -
Mru specify charset_table manually - -
Myene specify charset_table manually - -
Nepali specify charset_table manually - morphology=libstemmer_ne
Ngambay specify charset_table manually - -
Norwegian charset_table=non_cjk no morphology=libstemmer_no
Odia specify charset_table manually - -
Persian charset_table=non_cjk fa -
Polish charset_table=non_cjk pl -
Portuguese charset_table=non_cjk pt morphology=libstemmer_pt
Romanian charset_table=non_cjk ro morphology=libstemmer_ro
Russian charset_table=non_cjk ru morphology=lemmatize_ru (single root form); morphology=lemmatize_ru_all (all root forms); morphology=stem_ru (Porter's Russian stemmer); morphology=stem_enru (Porter's English and Russian stemmers); morphology=libstemmer_ru (from libstemmer)
Santali specify charset_table manually - -
Sindhi specify charset_table manually - -
Slovak charset_table=non_cjk sk -
Slovenian charset_table=non_cjk sl -
Somali charset_table=non_cjk so -
Sotho charset_table=non_cjk st -
Spanish charset_table=non_cjk es morphology=libstemmer_es
Swahili charset_table=non_cjk sw -
Swedish charset_table=non_cjk sv morphology=libstemmer_sv
Sylheti specify charset_table manually - -
Tamil specify charset_table manually - morphology=libstemmer_ta
Thai charset_table=thai th -
Turkish charset_table=non_cjk tr morphology=libstemmer_tr
Ukrainian charset_table=non_cjk,U+0406->U+0456,U+0456,U+0407->U+0457,U+0457,U+0490->U+0491,U+0491 - morphology=lemmatize_uk_all Requires installation of UK lemmatizer
Yoruba charset_table=non_cjk yo -
Zulu charset_table=non_cjk zu -