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Elasticsearch provides dictionary-based stemming via the
{ref}/analysis-hunspell-tokenfilter.html[hunspell token filter].
Hunspell hunspell.github.io is the
spell checker used by Open Office, LibreOffice, Chrome, Firefox, Thunderbird, and many
other open and closed source projects.
Hunspell dictionaries can be obtained from the following:
extensions.openoffice.org: Download and
unzip the .oxt extension file.
addons.mozilla.org:
Download and unzip the .xpi addon file.
OpenOffice archive: Download and unzip the .zip file.
A Hunspell dictionary consists of two files with the same base name—such as
en_US—but with one of two extensions:
.dicContains all the root words, in alphabetical order, plus a code representing all possible suffixes and prefixes (which collectively are known as affixes)
.affContains the actual prefix or suffix transformation for each code listed
in the .dic file
The Hunspell token filter looks for dictionaries within a dedicated Hunspell
directory, which defaults to ./config/hunspell/. The .dic and .aff
files should be placed in a subdirectory whose name represents the language
or locale of the dictionaries. For instance, we could create a Hunspell
stemmer for American English with the following layout:
config/
└ hunspell/ (1)
└ en_US/ (2)
├ en_US.dic
├ en_US.aff
└ settings.yml (3)
The location of the Hunspell directory can be changed by setting
indices.analysis.hunspell.dictionary.location in the
config/elasticsearch.yml file.
en_US will be the name of the locale or language that we pass to the
hunspell token filter.
Per-language settings file, described in the following section.
The settings.yml file contains settings that apply to all of the
dictionaries within the language directory, such as these:
---
ignore_case: true
strict_affix_parsing: true
The meaning of these settings is as follows:
ignore_caseHunspell dictionaries are case sensitive by default: the surname Booker is a
different word from the noun booker, and so should be stemmed differently. It
may seem like a good idea to use the hunspell stemmer in case-sensitive
mode, but that can complicate things:
A word at the beginning of a sentence will be capitalized, and thus appear to be a proper noun.
The input text may be all uppercase, in which case almost no words will be found.
The user may search for names in all lowercase, in which case no capitalized words will be found.
As a general rule, it is a good idea to set ignore_case to true.
strict_affix_parsingThe quality of dictionaries varies greatly. Some dictionaries that are
available online have malformed rules in the .aff file. By default, Lucene
will throw an exception if it can’t parse an affix rule. If you need to deal
with a broken affix file, you can set strict_affix_parsing to false to tell
Lucene to ignore the broken rules.
If multiple dictionaries (.dic files) are placed in the same
directory, they will be merged together at load time. This allows you to
tailor the downloaded dictionaries with your own custom word lists:
config/
└ hunspell/
└ en_US/ (1)
├ en_US.dic
├ en_US.aff (2)
├ custom.dic
└ settings.yml
The custom and en_US dictionaries will be merged.
Multiple .aff files are not allowed, as they could use
conflicting rules.
The format of the .dic and .aff files is discussed in
Hunspell Dictionary Format.
Once your dictionaries are installed on all nodes, you can define a hunspell
token filter that uses them:
PUT /my_index
{
"settings": {
"analysis": {
"filter": {
"en_US": {
"type": "hunspell",
"language": "en_US" (1)
}
},
"analyzer": {
"en_US": {
"tokenizer": "standard",
"filter": [ "lowercase", "en_US" ]
}
}
}
}
}
The language has the same name as the directory where
the dictionary lives.
You can test the new analyzer with the analyze API,
and compare its output to that of the english analyzer:
GET /my_index/_analyze?analyzer=en_US (1)
reorganizes
GET /_analyze?analyzer=english (2)
reorganizes
Returns organize
Returns reorgan
An interesting property of the hunspell stemmer, as can be seen in the
preceding example, is that it can remove prefixes as well as as suffixes. Most
algorithmic stemmers remove suffixes only.
|
Tip
|
Hunspell dictionaries can consume a few megabytes of RAM. Fortunately, Elasticsearch creates only a single instance of a dictionary per node. All shards that use the same Hunspell analyzer share the same instance. |
While it is not necessary to understand the format of a Hunspell dictionary in
order to use the hunspell tokenizer, understanding the format will help you
write your own custom dictionaries. It is quite simple.
For instance, in the US English dictionary, the en_US.dic file contains an entry for
the word analyze, which looks like this:
analyze/ADSG
The en_US.aff file contains the prefix or suffix rules for the A, G,
D, and S flags. Each flag consists of a number of rules, only one of
which should match. Each rule has the following format:
[type] [flag] [letters to remove] [letters to add] [condition]
For instance, the following is suffix (SFX) rule D. It says that, when a
word ends in a consonant (anything but a, e, i, o, or u) followed by
a y, it can have the y removed and ied added (for example, ready →
readied).
SFX D y ied [^aeiou]y
The rules for the A, G, D, and S flags mentioned previously are as follows:
SFX D Y 4
SFX D 0 d e (1)
SFX D y ied [^aeiou]y
SFX D 0 ed [^ey]
SFX D 0 ed [aeiou]y
SFX S Y 4
SFX S y ies [^aeiou]y
SFX S 0 s [aeiou]y
SFX S 0 es [sxzh]
SFX S 0 s [^sxzhy] (2)
SFX G Y 2
SFX G e ing e (3)
SFX G 0 ing [^e]
PFX A Y 1
PFX A 0 re . (4)
analyze ends in an e, so it can become analyzed by adding a d.
analyze does not end in s, x, z, h, or y, so it can become
analyzes by adding an s.
analyze ends in an e, so it can become analyzing by removing the e
and adding ing.
The prefix re can be added to form reanalyze. This rule can be
combined with the suffix rules to form reanalyzes, reanalyzed,
reanalyzing.
More information about the Hunspell syntax can be found on the Hunspell documentation site.
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