Development Overview

This page has everything you need to know to teach LanguageTool new error detection rules, plus more. You don't even have to be a programmer for that.

The three-minute introduction

This section tells you in a nutshell how to write your own LanguageTool rules for detecting errors (see this overview for how many rules LanguageTool already knows for your language):

  • Download the stand-alone version of LanguageTool from the homepage and unzip it.
  • Open LanguageTool-xx/org/languagetool/rules/en/grammar.xml in your preferred text editor or in an XML editor, where "xx" stands for the version number of the package you downloaded.
  • Search for name="Possible Typos" (it's quite at the top), add this odd rule just after that <category …> tag and save the file:
<rule id="EXAMPLE_RULE" name="My example rule">
    <message>Did you mean <suggestion>bicycle</suggestion>?</message>
    <example type="incorrect">My <marker>foo bar</marker> is broken.</example>
    <example type="correct">My bicycle is broken.</example>
  • Run languagetool.jar by clicking it or by calling java -jar languagetool.jar on the command line.
  • Select English as the text language and type something like A foo bar test, then start text checking.
  • LanguageTool will now check your text and suggest bicycle as a replacement for foo bar, because that's what the rule which we just added says.
  • Use our online rule editor to write rules if you don't want to edit the XML directly

That's it! You have just added a new rule. Keep on reading to get a grasp on what the elements of a rule mean and how to build more complex rules or use the rule creator to build simple rules. Check out the text analyzer to get an understanding of how LanguageTool analyzes text internally. Send us your rules so we can add them to the next release of LanguageTool.

Help wanted!

We're looking for people who support us writing new rules so LanguageTool can detect more errors. Also see the list of supported languages.

How can you help?

  1. Read this page (some features described here are quite advanced, so you won't need everything)
  2. Start writing rules for the error you'd like LanguageTool to detect
  3. See our wiki for more tips and tricks
  4. Post your rules to the forum or open an issue so we can include them in LanguageTool
  5. Subscribe to our forum

If your language isn't supported yet, you can add it by following the documentation in our wiki.

Source code checkout

If you are a Java developer and you want to extend LanguageTool or if you want to use the latest development version, check out LanguageTool from github:

git clone

Note that due to the embedded dictionaries our git repository is quite large (>300MB), so cloning might take some time. You can then build the code with mvn clean package or just run the tests with mvn clean test. You need at least Java 8 for building LT. Maven's default memory settings are often too low, so you will probably need to set your environment variable MAVEN_OPTS to:

-Xmx512m -XX:MaxPermSize=256m

After the build, the LibreOffice/OpenOffice extension can be found in languagetool-office-extension/target (named *.zip, rename it to *.oxt), the stand-alone version in languagetool-standalone/target (in a sub directory named e.g. LanguageTool-2.7-SNAPSHOT/, you cannot run the *.jar directly in the target directoy). See the Usage page for information on how to use those files. See Maven tips for hints on how to build faster.

Language checking process

This is what LanguageTool does when it analyzes a text for errors:

  1. The text is split into sentences
  2. Each sentence is split into words
  3. Each word is assigned its part-of-speech tag(s) (e.g. cars = plural noun, talked = simple past verb)
  4. The analyzed text is then matched against the built-in rules and against the rules loaded from the grammar.xml file

The most important thing you need to keep in mind is that LanguageTool's rules describe what errors look like, not what correct sentences look like (this is the opposite of how you learn a new language).

Adding new XML rules

Most rules are contained in rules/xx/grammar.xml, whereas xx is a language code like en or de. In the source code, this folder will be found under languagetool-language-modules/xx/src/main/resources/org/languagetool/; the standalone GUI version contains them under org/languagetool/.

A rule is basically a pattern which shows an error message to the user if the pattern matches. A pattern can address words or part-of-speech tags. Here are some examples of patterns that can be used in that file:

  • <token>think</token>
    matches the word think
  • <token>think</token> <token>about</token>
    Matches the phrase think about - as the text is split into words, you need to list each word separately as a token. This will not work: <token>think about</token>
  • <token regexp="yes">think|say</token>
    matches the regular expression think|say, i.e. the word think or the word say. You can write simple rules without knowing regular expressions, but if you want to learn more about them you can try this tutorial.
  • <token postag="VB" />
    matches a base form verb (like walk in we have to walk). See resource/en/tagset.txt for a list of possible English part-of-speech tags.
  • <token postag="V.*" postag_regexp="yes" />
    matches a word whose part-of-speech tag starts with V (in English, these are all verbs). See resource/en/tagset.txt for a list of possible English part-of-speech tags. Note that for tags with a special character like PRP$ you need to escape the $ with a backslash, as it has a special meaning in regular expressions: PRP\$.
  • <token>cause</token> <token regexp="yes" negate="yes">and|to</token>
    matches the word cause followed by any word that is not and or to
  • <token postag="SENT_START" /> <token>foobar</token>
    matches the word foobar only at the beginning of a sentence. The corresponding postag for the end of a sentence is SENT_END.

A pattern's tokens are matched case-insensitively by default. This can be changed for the whole pattern or for a single token by setting case_sensitive="yes".

Alternatively, case-sensitive matching can be turned on for single tokens by using (?-i) in regular expressions (ex: <token regexp="yes">(?-i)Bill</token> will match Bill but not bill).

A simple example

Here's an example of a complete rule that marks bed English, bat attitude etc as an error:

<rule id="BED_ENGLISH" name="Possible typo 'bed/bat(bad) English/...'">
        <token regexp="yes">bed|bat</token>
      <token regexp="yes">English|attitude</token>
    <message>Did you mean <suggestion>bad</suggestion>?</message>
    <example type="correct">Sorry for my <marker>bad</marker> English.</example>
    <example correction="bad" type="incorrect">Sorry for my <marker>bed</marker> English.</example>

The basic elements of a rule

A short description of the elements and their attributes:

  • element rule, attribute id: An internal identifier used to address this rule. This must be unique.
  • element rule, attribute name: A short text displayed in the configuration, describing the rule.
  • element antipattern (optional, may occur multiple times): A complex exception to the rule.
  • element pattern, sub element marker: What part of the original text should be marked as an error. If all tokens are part of the error you can omit this element.
  • element token, attribute regexp: if set to yes, interpret the given token as a regular expression
  • element message: The text displayed to the user if this rule matches. Use sub-element suggestion to suggest a possible replacement that corrects the error. It is possible to conditionally suppress parts of suggestions if they are misspelled (for this, you need to use element match with attribute suppress_misspelled set to yes). You can even suppress the whole rule from being matched if you use the same attribute for any suggestion element. Note: the tagger of the given language is used to make it work, so if you don't have a tagger yet, you cannot use this feature.
  • element url (optional): An URL to a page that explains the rule leading to the error in more detail. If it contains symbol &, then it needs to be escaped as &amp;.
  • element short (optional): A short description of the rule, displayed on the right-click menu in the GUI and in Libre/OpenOffice.
  • element example: At least two examples with one correct and one incorrect sentence. The sentence with the attribute type="incorrect" is supposed to be matched by this rule. The position of the error must be marked up with the sub-element marker. You can use the optional correction attribute to make the test also check whether the correction suggested by LanguageTool is what you expect. These sentences are used by the automatic test cases that can be run using sh (on Linux), testrules.bat (on Windows), or mvn clean test (for Java developers). The first correct and the first incorrect example will be shown to the user in the stand-alone version of LanguageTool if they look at the details of the error message.

Testing rules

The LanguageTool user interface (languagetool.jar) needs to be restarted if you have changed the grammar.xml file. Testing rules is faster with our embedded test case feature: just call sh en on Linux or testrules.bat en on Windows, using your language code instead of en.

This will test your rule with its example sentences: the incorrect sentence is supposed to be detected by your rule, while the correct sentence is not supposed to give an error. If that is not the case you will get a message. In that case, either your rule or your example sentences are not quite right yet.

Using is not only much faster than manually starting the user interface over and over again, it will always test all rules, so we recommend you use that during rule development.

Regular Expressions (requires LanguageTool 3.2 or later)

Alternatively to <pattern><token>…</token></pattern> it's sometimes easier to write a rule with a traditional regular expression:

<rule ...>
  <regexp>half an our</regexp>
  <message>Did you mean <suggestion>half an hour</suggestion>?</message>

It's important to note that this will not care about tokens, it just searches the regular expression per sentence. So this example would also match behalf an our and half an ourselves (which are probably also errors, but not the ones the rule is looking for). To avoid this, you'll need to specify boundaries with \b, for example <regexp>\bhalf an our\b</regexp>.


  • case_sensitive: Like <token>s, <regexp> supports the case_sensitive attribute, and like those it is case insensitive by default.
  • type: Supports the values smart (the default) and exact. smart interprets spaces in a way that <regexp>half an our</regexp> will also be found if the input contains multiple spaces or other whitespaces between the words. If your rule does any whitespace checking, you'll need to use type="exact".
  • mark: Specifies which parts of the match will be underlined. By default, the whole match is underlined. Use 1 to underline only the first group of the match etc.

You cannot use <regexp> to look for specific POS tags. For that, you can always use the regexp="yes" attribute on a <token> and combine it with the postag or postag_regexp attribute.


The inflected attribute of the token element is used to match not only the given word but also all of its inflected forms. For example <token inflected="yes">bicycle</token> will match bicycle, bicycles, bicycling etc.

Grouping rules

Sometimes it requires more than one rule to find all occurrences of an error. You can put all those rules in one rulegroup element. The rulegroup's id and name attribute will be used for all the rules of that group. Overlapping matches for rules in the same rulegroup are filtered out to avoid duplicate matches for the same error.


The rules are best put into categories that describe their purpose, and allow to enable or disable a number of rules at the same time. When creating a category, you can use the type attribute to describe the type of the error according to the Quality Issue Type from the W3 Internationalization Tag Set. This will make integration of LT with other tools easier.

Turning rules off by default

Some rules can be optional, useful only in specific registers, or very sensitive. You can turn them off by default by using an attribute default="off". The user can turn the rule on/off in the Options dialog box, and this setting is being saved in the configuration file.


Sometimes exceptions to rules require multiple tokens with complex interrelations. In such a case, one may use an antipattern:


This rule would match "This is a word1" but not "This is a word1 word2". You can use all subelements of pattern in antipattern but phrase and or. The text matched by the antipattern needs to overlap the text matched by the pattern for the antipattern to become active.

Antipatterns may be added to a group of rules (and then they are valid for all rules in the group) and for particular rules in a group.

Be aware that if there is or in the pattern this can also lead to problems with antipattern matching (as of LT 3.4).


To match a token optionally, use the min attribute with a value of 0. For example, to match a person or a nice person:

<token min="0">nice</token>

You can combine this with max to specify the maximum number of occurrences possible. For example, to match a person, a nice person, or a nice nice person:

<token min="0" max="2">nice</token>

<or>, <and>

or can be used to match a token if one or both of two conditions are matched. This is sometimes a more compact alternative to writing more than one rule. For example, this would match don't walk as well as do not walk:


and can be used to check that a token matches more than one condition. For example, this would only match if a token has both TAG_A and TAG_B:

  <token postag="TAG_A"/>
  <token postag="TAG_B"/>


The skip attribute of the token element is used in two situations:

Simulate a simple chunker for languages with flexible word order, e.g., for matching errors of rection; we could for example skip possible adverbs in some rule. skip="1" works exactly as two rules, i.e.

<token skip="1">A</token>

is equivalent to the pair of rules:

<token/>  <!-- this will match any word -->

Using negative value, we can match until the B is found, no matter how many tokens are skipped. This cannot be easily encoded using empty tokens as above because the sentence could be of any length.

Match coordinated words, for example to match both … as well as we could write:

<token skip="-1">both<exception scope="next">and</exception></token>

Here the exception is applied only to the skipped tokens.

The scope attribute of the exception is used to make the exception valid only for the token the exception is specified (scope="current") or for skipped tokens (scope="next"). Default behavior is scope="current". Using scopes is useful where several different exceptions should be applied to avoid false alarms. In some cases, it's useful to use scope="previous" in rules that already have skip="-1". This way, you can set an exception against a single token that immediately precedes the matched token. For example, we want to match tak after jak which is not preceded by a comma:

<token skip="-1">jak</token>
<token>tak<exception scope="previous">,</exception></token>

In this case, the rule excludes all sentences, where there is a comma before tak. Note that it's very hard to make such an exclusion otherwise.


In XML rules, you can refer to previously matched tokens in the pattern. For example:

 <token regexp="yes" skip="-1">ani|ni|i|lub|albo|czy|oraz<exception scope="next">,</exception></token>
 <token><match no="0"/></token>

This rule matches sequences like ani… ani, ni… ni, and i… i (with no comma in between) but you don't have to write all these cases explicitly. The first match (matches are numbered from zero, so it's <match no="0"/>) is automatically inserted into the second token. Note that this rule will match sentences like:

Nie kupiłem ani gruszek ani jabłek. Kupię to lub to lub tamto.

A similar mechanism can be used in suggestions, however there are more features, and tokens are numbered from 1 (for compatibility with the older notation \1 for the first matched token). For example:

<suggestion><match no="1"/></suggestion>

A more complicated example:

  <token regexp="yes">^(\p{Lu}{2}+[i]*\p{Lu}+[\p{L}&amp;&amp;[^\p{Lu}]]{1,4}+)</token>
<message>Prawdopodobny błąd zapisu odmiany;
  skrótowce odmieniamy z dywizem:
  <suggestion><match no="1"

This rule matches Polish inflected acronyms such as SMSem that should be written with a hyphen: SMS-em. So the acronym is matched with a complicated regular expression, and the match replaces the match using Java regular expression notation. Basically, the regular expression only shows two parts and inserts a hyphen between them.

For some languages (currently Polish, English, Catalan, Spanish, Galician, Dutch, Romanian, Slovak, Russian, Greek, and Ukrainian), element match can be used to insert an inflected matched token (or another word with a specified part of speech tag). For example:

 <token regexp="yes">has|have</token>
   <token postag="VBD|VBP|VB" postag_regexp="yes">
     <exception postag="VBN|NN:U.*|JJ.*|RB" postag_regexp="yes"/>
 <token><exception postag="VBG"/></token>
  Possible agreement error -- use past participle here:
  <suggestion><match no="2" postag="VBN"/></suggestion>.

The above rule takes the second verb with a POS tag VBN, VBP or VB and displays its form with a POS tag VBN in the suggestion. You can also specify POS tags using regular expressions (postag_regexp="yes") and replace POS tags – just like in the above example with acronyms. This is useful for large and complicated tagsets (for many examples, see the Polish rule file: rules/pl/grammar.xml).

Sometimes the rule should change the case of the matched word. For this purpose, you can use case_conversion attribute values: startlower, startupper, allupper and alllower.

Another useful thing is that match can refer to a token, but apply its POS to another word. This is useful for suggesting another word with the same part of speech. There is a special abbreviated syntax used for this purpose:

<match no="1" postag="verb:.*perf">kierować</match>

This syntax means: take the POS tag of the first matched token that matches the regular expression specified in the postag attribute, and then apply this POS tag to the verb kierować. This way the verb will be inflected just the way the matched verb was originally inflected. The reason why you need to specify the POS tag is that the matched token can have several POS tags (several readings).

Note that by default match element inside the token element inserts only a string – so it matches a string, and not part of speech tags. So even if it refers to a token with a POS tag, it copies the matched token, and not its POS token. However, you can use all above attributes to change the form of the token.

You can however use the match element to copy POS tags alone but to do so, you must use the attribute setpos="yes". All other attributes can be applied so that the POS could be converted appropriately. This can be useful for creating rules specifying grammatical agreement. Currently, such rules must be quite wordy, somewhat more terse syntax is in development.

You can use postag_replace to require the suggestion to have only some of the same POS tags as the matching word. As always with regular expressions, you put the relevant parts in parenthesis and then refer to them using $1, $2 etc:

<match no="1"


Starting with version 2.7, LanguageTool supports RuleFilters as a way to make rules more powerful by combining XML syntax and Java code. A RuleFilter takes a rule match from an XML pattern rule and filters it, i.e. it either keeps it as it is, modifies it, or discards it. In XML, the filter is used like this:

  <token regexp="yes">\d</token>
  <token regexp="yes">\d\d</token>
<filter class="org.languagetool.rules.en.MyFilter" args="myToken:\1 otherArg:\2"/>

In Java code, org.languagetool.rules.en.MyFilter implements the RuleFilter interface with its acceptRuleMatch(…) method. It gets the rule match and the arguments from the args attribute (with \1 etc being resolved to the matching token at that position) as a Map. If it returns null, the rule match will be discarded. If it returns a new rule match, that will be shown to the user. It can also simply return the rule match it gets as a parameter to accept the match.

Adding new Java rules

Rules that cannot be expressed with a pattern in grammar.xml can be written with Java code. As a developer, extend LanguageTool's Rule class and implement the match(AnalyzedSentence) method. If your rule doesn't work on the sentence level, implement TextLevelRule instead.

See for a simple example which you can use to develop your own rules. You will also need to add your rule class to the getRelevantRules() method in <YourLanguage>.java to activate it. If you're using the LanguageTool API, you can call JLanguageTool.addRule() instead.

Translating the user interface

We use Transifex to translate our property files. Updated translations are only copied to the LanguageTool source before a release, so if you need an early preview, say so on the LanguageTool forum and we'll update the files accordingly.

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