[[algorithmic-stemmers]] === Algorithmic Stemmers

Most of the stemmers available in Elasticsearch are algorithmic((("stemming words", "algorithmic stemmers"))) in that they apply a series of rules to a word in order to reduce it to its root form, such as stripping the final s or es from plurals. They don't have to know anything about individual words in order to stem them.

These algorithmic stemmers have the advantage that they are available out of the box, are fast, use little memory, and work well for regular words. The downside is that they don't cope well with irregular words like be, are, and am, or mice and mouse.

One of the earliest stemming algorithms((("English", "stemmers for")))((("Porter stemmer for English"))) is the Porter stemmer for English, which is still the recommended English stemmer today. Martin Porter subsequently went on to create the http://snowball.tartarus.org/[Snowball language] for creating stemming algorithms, and a number((("Snowball langauge (stemmers)"))) of the stemmers available in Elasticsearch are written in Snowball.


The http://bit.ly/1IObUjZ[`kstem` token filter] is a stemmer for English which((("kstem token filter"))) combines the algorithmic approach with a built-in dictionary. The dictionary contains a list of root words and exceptions in order to avoid conflating words incorrectly. kstem tends to stem less aggressively than the Porter stemmer.


==== Using an Algorithmic Stemmer

While you ((("stemming words", "algorithmic stemmers", "using")))can use the http://bit.ly/17LseXy[`porter_stem`] or http://bit.ly/1IObUjZ[`kstem`] token filter directly, or create a language-specific Snowball stemmer with the http://bit.ly/1Cr4tNI[`snowball`] token filter, all of the algorithmic stemmers are exposed via a single unified interface: the http://bit.ly/1AUfpDN[`stemmer` token filter], which accepts the language parameter.

For instance, perhaps you find the default stemmer used by the english analyzer to be too aggressive and ((("english analyzer", "default stemmer, examining")))you want to make it less aggressive. The first step is to look up the configuration for the english analyzer in the http://bit.ly/1xtdoJV[language analyzers] documentation, which shows the following:


{ "settings": { "analysis": { "filter": { "englishstop": { "type": "stop", "stopwords": "_english" }, "english_keywords": { "type": "keyword_marker", <1> "keywords": [] }, "english_stemmer": { "type": "stemmer", "language": "english" <2> }, "english_possessive_stemmer": { "type": "stemmer", "language": "possessive_english" <2> } }, "analyzer": { "english": { "tokenizer": "standard", "filter": [ "english_possessive_stemmer", "lowercase", "english_stop", "english_keywords", "english_stemmer" ] } } } }


<1> The keyword_marker token filter lists words that should not be stemmed.((("keyword_marker token filter"))) This defaults to the empty list.

<2> The english analyzer uses two stemmers: the possessive_english and the english stemmer. The ((("english stemmer")))((("possessive_english stemmer")))possessive stemmer removes 's from any words before passing them on to the english_stop, english_keywords, and english_stemmer.

Having reviewed the current configuration, we can use it as the basis for a new analyzer, with((("english analyzer", "customizing the stemmer"))) the following changes:

  • Change the english_stemmer from english (which maps to the http://bit.ly/17LseXy[`porter_stem`] token filter) to light_english (which maps to the less aggressive http://bit.ly/1IObUjZ[`kstem`] token filter).

  • Add the <> token filter to remove any diacritics from foreign words.((("asciifolding token filter")))

  • Remove the keyword_marker token filter, as we don't need it. (We discuss this in more detail in <>.)

Our new custom analyzer would look like this:


PUT /myindex { "settings": { "analysis": { "filter": { "english_stop": { "type": "stop", "stopwords": "_english" }, "light_english_stemmer": { "type": "stemmer", "language": "light_english" <1> }, "english_possessive_stemmer": { "type": "stemmer", "language": "possessive_english" } }, "analyzer": { "english": { "tokenizer": "standard", "filter": [ "english_possessive_stemmer", "lowercase", "english_stop", "light_english_stemmer", <1> "asciifolding" <2> ] } } } }


<1> Replaced the english stemmer with the less aggressive light_english stemmer

<2> Added the asciifolding token filter