[[analysis-kstem-tokenfilter]]
=== KStem token filter
++++
<titleabbrev>KStem</titleabbrev>
++++

Provides https://ciir.cs.umass.edu/pubfiles/ir-35.pdf[KStem]-based stemming for
the English language. The `kstem` filter combines
<<algorithmic-stemmers,algorithmic stemming>> with a built-in
<<dictionary-stemmers,dictionary>>.

The `kstem` filter tends to stem less aggressively than other English stemmer
filters, such as the <<analysis-porterstem-tokenfilter,`porter_stem`>> filter.

The `kstem` filter is equivalent to the
<<analysis-stemmer-tokenfilter,`stemmer`>> filter's
<<analysis-stemmer-tokenfilter-language-parm,`light_english`>> variant.

This filter uses Lucene's
{lucene-analysis-docs}/en/KStemFilter.html[KStemFilter].

[[analysis-kstem-tokenfilter-analyze-ex]]
==== Example

The following analyze API request uses the `kstem` filter to stem `the foxes
jumping quickly` to `the fox jump quick`:

[source,console]
----
GET /_analyze
{
  "tokenizer": "standard",
  "filter": [ "kstem" ],
  "text": "the foxes jumping quickly"
}
----

The filter produces the following tokens:

[source,text]
----
[ the, fox, jump, quick ]
----

////
[source,console-result]
----
{
  "tokens": [
    {
      "token": "the",
      "start_offset": 0,
      "end_offset": 3,
      "type": "<ALPHANUM>",
      "position": 0
    },
    {
      "token": "fox",
      "start_offset": 4,
      "end_offset": 9,
      "type": "<ALPHANUM>",
      "position": 1
    },
    {
      "token": "jump",
      "start_offset": 10,
      "end_offset": 17,
      "type": "<ALPHANUM>",
      "position": 2
    },
    {
      "token": "quick",
      "start_offset": 18,
      "end_offset": 25,
      "type": "<ALPHANUM>",
      "position": 3
    }
  ]
}
----
////

[[analysis-kstem-tokenfilter-analyzer-ex]]
==== Add to an analyzer

The following <<indices-create-index,create index API>> request uses the
`kstem` filter to configure a new <<analysis-custom-analyzer,custom
analyzer>>.

[IMPORTANT]
====
To work properly, the `kstem` filter requires lowercase tokens. To ensure tokens
are lowercased, add the <<analysis-lowercase-tokenfilter,`lowercase`>> filter
before the `kstem` filter in the analyzer configuration.
====

[source,console]
----
PUT /my-index-000001
{
  "settings": {
    "analysis": {
      "analyzer": {
        "my_analyzer": {
          "tokenizer": "whitespace",
          "filter": [
            "lowercase",
            "kstem"
          ]
        }
      }
    }
  }
}
----
