[[analysis-edgengram-tokenfilter]]
=== Edge n-gram token filter
++++
<titleabbrev>Edge n-gram</titleabbrev>
++++

Forms an {wikipedia}/N-gram[n-gram] of a specified length from
the beginning of a token.

For example, you can use the `edge_ngram` token filter to change `quick` to
`qu`.

When not customized, the filter creates 1-character edge n-grams by default.

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

[NOTE]
====
The `edge_ngram` filter is similar to the <<analysis-ngram-tokenizer,`ngram`
token filter>>. However, the `edge_ngram` only outputs n-grams that start at the
beginning of a token. These edge n-grams are useful for
<<search-as-you-type,search-as-you-type>> queries.
====

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

The following <<indices-analyze,analyze API>> request uses the `edge_ngram`
filter to convert `the quick brown fox jumps` to 1-character and 2-character
edge n-grams:

[source,console]
--------------------------------------------------
GET _analyze
{
  "tokenizer": "standard",
  "filter": [
    { "type": "edge_ngram",
      "min_gram": 1,
      "max_gram": 2
    }
  ],
  "text": "the quick brown fox jumps"
}
--------------------------------------------------

The filter produces the following tokens:

[source,text]
--------------------------------------------------
[ t, th, q, qu, b, br, f, fo, j, ju ]
--------------------------------------------------

/////////////////////
[source,console-result]
--------------------------------------------------
{
  "tokens" : [
    {
      "token" : "t",
      "start_offset" : 0,
      "end_offset" : 3,
      "type" : "<ALPHANUM>",
      "position" : 0
    },
    {
      "token" : "th",
      "start_offset" : 0,
      "end_offset" : 3,
      "type" : "<ALPHANUM>",
      "position" : 0
    },
    {
      "token" : "q",
      "start_offset" : 4,
      "end_offset" : 9,
      "type" : "<ALPHANUM>",
      "position" : 1
    },
    {
      "token" : "qu",
      "start_offset" : 4,
      "end_offset" : 9,
      "type" : "<ALPHANUM>",
      "position" : 1
    },
    {
      "token" : "b",
      "start_offset" : 10,
      "end_offset" : 15,
      "type" : "<ALPHANUM>",
      "position" : 2
    },
    {
      "token" : "br",
      "start_offset" : 10,
      "end_offset" : 15,
      "type" : "<ALPHANUM>",
      "position" : 2
    },
    {
      "token" : "f",
      "start_offset" : 16,
      "end_offset" : 19,
      "type" : "<ALPHANUM>",
      "position" : 3
    },
    {
      "token" : "fo",
      "start_offset" : 16,
      "end_offset" : 19,
      "type" : "<ALPHANUM>",
      "position" : 3
    },
    {
      "token" : "j",
      "start_offset" : 20,
      "end_offset" : 25,
      "type" : "<ALPHANUM>",
      "position" : 4
    },
    {
      "token" : "ju",
      "start_offset" : 20,
      "end_offset" : 25,
      "type" : "<ALPHANUM>",
      "position" : 4
    }
  ]
}
--------------------------------------------------
/////////////////////

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

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

[source,console]
--------------------------------------------------
PUT edge_ngram_example
{
  "settings": {
    "analysis": {
      "analyzer": {
        "standard_edge_ngram": {
          "tokenizer": "standard",
          "filter": [ "edge_ngram" ]
        }
      }
    }
  }
}
--------------------------------------------------

[[analysis-edgengram-tokenfilter-configure-parms]]
==== Configurable parameters

`max_gram`::
+
--
(Optional, integer)
Maximum character length of a gram. For custom token filters, defaults to `2`.
For the built-in `edge_ngram` filter, defaults to `1`.

See <<analysis-edgengram-tokenfilter-max-gram-limits>>.
--

`min_gram`::
(Optional, integer)
Minimum character length of a gram. Defaults to `1`.

`preserve_original`::
(Optional, boolean)
Emits original token when set to `true`. Defaults to `false`.

`side`::
+
--
(Optional, string)
Deprecated. Indicates whether to truncate tokens from the `front` or `back`.
Defaults to `front`.

Instead of using the `back` value, you can use the
<<analysis-reverse-tokenfilter,`reverse`>> token filter before and after the
`edge_ngram` filter to achieve the same results.
--

[[analysis-edgengram-tokenfilter-customize]]
==== Customize

To customize the `edge_ngram` filter, duplicate it to create the basis
for a new custom token filter. You can modify the filter using its configurable
parameters.

For example, the following request creates a custom `edge_ngram`
filter that forms n-grams between 3-5 characters.

[source,console]
--------------------------------------------------
PUT edge_ngram_custom_example
{
  "settings": {
    "analysis": {
      "analyzer": {
        "default": {
          "tokenizer": "whitespace",
          "filter": [ "3_5_edgegrams" ]
        }
      },
      "filter": {
        "3_5_edgegrams": {
          "type": "edge_ngram",
          "min_gram": 3,
          "max_gram": 5
        }
      }
    }
  }
}
--------------------------------------------------

[[analysis-edgengram-tokenfilter-max-gram-limits]]
==== Limitations of the `max_gram` parameter

The `edge_ngram` filter's `max_gram` value limits the character length of
tokens. When the `edge_ngram` filter is used with an index analyzer, this
means search terms longer than the `max_gram` length may not match any indexed
terms.

For example, if the `max_gram` is `3`, searches for `apple` won't match the
indexed term `app`.

To account for this, you can use the
<<analysis-truncate-tokenfilter,`truncate`>> filter with a search analyzer
to shorten search terms to the `max_gram` character length. However, this could
return irrelevant results.

For example, if the `max_gram` is `3` and search terms are truncated to three
characters, the search term `apple` is shortened to `app`. This means searches
for `apple` return any indexed terms matching `app`, such as `apply`, `snapped`,
and `apple`.

We recommend testing both approaches to see which best fits your
use case and desired search experience.
