Tips and Tricks

There are many simple but non-obvious ways how you can build error-checking rules for LanguageTool. Here are some tips. As a beginner, you should first read the overview.


Finding words in Context

For English, an easy way to get frequency data is the Google Web 1T 5-Gram Database

Also consider using the Corpus of Contemporary American English (COCA) to see the context of the words in your rule. On the COCA website, set display to KWIC and then search for a word. It will be displayed with its neighbour words. Note that the COCA website will ask you to register after some queries.

Ways to Use LanguageTool

Tagging a corpus using LanguageTool

LanguageTool has a POS tagger, sometimes it has a disambiguator or a chunker for a language, so you can use it to tag a big corpus. We added a special command-line switch --taggeronly or, for short, -t to disable rule checking and use only the tagger.

We didn’t go beyond 3 GB of pure text but you should experience no problem even with huge corpora.

java -jar languagetool-commandline.jar -l <language> -c <encoding> -t <corpus_file> > <tagged_corpus_file>

With some smart tooling, you can use such corpus to create grammar-checking rules semi-automatically (but this goes beyond a simple trick).

Checking text from standard input

LanguageTool will accept input text from the standard input, so you would be able to pipe the output from other tools to LT. To use STDIN, you must supply - as the filename.

Automatic application of suggestions

You can switch on automatic application of suggestions by using --apply (or -a for short) on the command-line. Note: only the first suggestion is applied if the rule has more than one suggestion. Make sure that you only select the rules which are reliable enough to allow automatic correction (use the switch -e to select the rules you want to use, or -d to disable some of the rules). The suggestions are applied in the order of the starting position of errors in the text. Try to make some tests to see if you have clashing or conflicting suggestions. LanguageTool checks if the error found by the rule is still there in the text before applying the suggestion, but that’s the only quality check. The text is checked only once, so if multiple suggestions create an error that another rule could correct, you can simply run LT on the output from the previous check.

The output from LanguageTool will be the corrected text (without any other messages). You can still get verbose output on STDERR (by specifying -v).

This mode of operation can be used to clean a corpus before running some information extraction on it, or to post-edit some automatically created text (for example, from machine translation).

Getting tags and rule matches

If you want to get all the matches for a given file, just run

java -jar languagetool-commandline.jar -l <language> -c <encoding> <file>

You can pipe this to a file (add > file at the end of the line). To get part-of-speech tags as assigned by LanguageTool, use:

java -jar languagetool-commandline.jar -l <language> -c <encoding> <file> >results.txt 2>tags.txt

You will get two files: results.txt with rule matches (and you can sort it using the stats.awk script mentioned below) and tags.txt that will contain all tags. Note: some operating systems allow pointing both std-err (2>) and std-in (>) to the same file.

Writing Error Detection Rules

For newcomers

Check the development overview page to learn basics of adding new XML rules.

Find the location of a subrule in XML

A <rulegroup> can contain many subrules. The subrule’s id will then be something like RULE_ID[n] with n being a number. It can be annoying to find the n’th rule in the XML file, as the subrules don’t have numbers. To find the line number of a subrule, run org.languagetool.commandline.Main with -v (verbose) as a parameter. The (sub)rule’s line number in the XML will then be printed like this:

1.) Line 1, column 29, Rule ID: ELLIPSIS[1] premium: false line=136645
Message: Consider using the typographical ellipsis character here instead.
Suggestion: …
This is just a test sentence...

Find which antipattern causes a rule not to match

If a rule with many antipatterns doesn’t match (but should), it can be difficult to find the antipattern that’s responsible for this behavior. In this case:

  1. Edit languagetool-commandline/src/main/resources/logback.xml so that it contains <root level="debug"> (instead of level="info")
  2. Write the affected sentence to a file and use org.languagetool.commandline.Main to check that file
  3. The output will then contain a line like this, pointing to the matching antipattern: DEBUG o.languagetool.AnalyzedTokenReadings 'wise' is immunized by antipattern in line 41901

Adding rules to an external file

Using grammar_custom.xml

Starting with LanguageTool 6.0, you can put a file grammar_custom.xml into the same directory that contains the grammar.xml file for your language. This file will be loaded in addition to grammar.xml. It can contain custom rules that you want to use now and with future versions of LanguageTool, without modifying existing files. The grammar_custom.xml needs to use the same XML syntax as grammar.xml and it must not introduce rule IDs that are in use by other rules already.

Using any other file

You can add your own rules to some external file. For that, you need to add a reference to that file to the grammar.xml. You need to add something like the following to the top of the file after <?xml-stylesheet...>:

    <!DOCTYPE rules [
        <!ENTITY UserRules SYSTEM "file:///path/to/user-rules.xml">

If your grammar.xml already has a <!DOCTYPE rules [...]> section, just add <!ENTITY UserRules SYSTEM "file:/path/to/user-rules.xml"> to it.

You will also need to add this entity to the <rules>...</rules> part of the file - make sure it is inside a <category>...</category>, but not in a category that’s turned off by default (<category ... default="off">):


LanguageTool will then, additionally, use the rules from /path/to/user-rules.xml. The URL doesn’t have to point to a local file, you can use http: as well. It’s also possible to access password-protected rule files like this: Note that this feature won’t work if your instance of LanguageTool is running with a Java security manager that forbids Authenticator.setDefault().

In the stand-alone GUI, to get details about an error, right-click on an error and click “More”. The message includes a link “Rule details on”. The link is not applicable to external rules. To remove the link from external rules, add external="yes" to each category.

Test a corpus and sort rule matches

It’s useful to run LanguageTool rules on a larger text and see the number of matches generated by the rules. To do it, run it from the command line (or straight from Eclipse by adding the following parameters)

java -jar languagetool-commandline.jar -l <language> -c <encoding> <filename.txt> > <output_file.txt>

Various forms of negation

There are three ways to negate the meaning of the token in XML rules:

  1. by negating the token
  2. by negating the part of speech
  3. by using exceptions

Exceptions are the easiest way and you can add many of them but adding a single negation via exception might be wordy. Remember, therefore, that even if you negate a token that has a part-of-speech specified, you’ll be matching tokens with this part of speech.

For example, this is correct:

    <token postag="SENT_START" negate_pos="yes"/>

This will match all tokens that appear in the middle or at the end of the sentence. It is equivalent to:

    <token><exception postag="SENT_START"/></token>

This, however, will match any end-of-sentence token (which is probably undesired):

    <token postag="SENT_END" negate="yes"/>

It is logically equivalent to:

    <token postag="SENT_END"><exception/></token>

Removing words

One of the most important forms of correction is a suggestion to remove a word. In LanguageTool, this is done implicitly in two ways:

In other words, you cannot correctly remove a single word preceded by a whitespace if you cannot specify a preceding or following token. This happens only if the word to be deleted appears at the end of the skipped block and at the end of the sentence at the same time.

Testing if the word is preceded with a whitespace

You can test whether a token is preceded with some whitespace characters (spaces, new lines etc.):

    <token spacebefore="yes" regexp="yes">"|'</token>
    <token spacebefore="no">a</token>
    <token spacebefore="no" regexp="yes">"|'</token>

The above tokens match only "A", 'A' or "a' (but not " A ").

Changing the case of matched word

When you want to change the case of a match, use

    <match no="1" case_conversion="startupper"/>


    <match no="1" case_conversion="startlower"/>

allupper and alllower are supported, too.

Note: LT automatically adjusts the case of suggestions if they are added as plain text. If you want to suppress the default behavior, you need to use the match element. If you’re not just changing the case but also changing the word, then you can use the following workaround: make match point at any token (it doesn’t matter as long it’s in the pattern) and use regular expression .* and replace it with the word you want to actually use. And add case_conversion:

    <pattern case_sensitive="yes">
    	<token negate="yes">foo<exception postag="SENT_START"/></token>
    <message>Change to <suggestion><match no="1" regexp_match=".*" regexp_replace="bar" case_conversion="alllower"/></suggestion></message>

LT automatically adjusts the case for tokens that are uppercase because they were at the beginning of the sentence. But there are also cases when you need a fairly complex process. For example, the incorrect Breton expression “Da Brest” should be changed to “vBrest” or “Vrest”. You can do this using the following code:

    <suggestion><match no="2" case_conversion="preserve" regexp_match="^"
     regexp_replace="v"/></suggestion> pe <suggestion><match no="2"
     regexp_match="^." regexp_replace="V"/></suggestion>?

Note that the regular expression in the first match element adds “v” to the beginning of the word, and to keep this character lowercase, we need to use startlower as the value of case_conversion. This is because we are not preserving the case of the original token but of our own suggestion.

Changing the matched word

When dealing with (partial) barbarisms e.g., one can replace strings in words. An example from the Dutch set:

    <match no="1" regexp_match="multiplechoice(.*)" regexp_replace="meerkeuze$1"/>

Including the skipped sequence

Sometimes it’s useful to include the skipped sequence in the pattern, for example to change both its beginning and end.

    <match no="1" include_skipped="all"/>

If you need to remove the beginning of the pattern, use include_skipped="following". The default setting is include_skipped="none", which means no inclusion of the skipped sequence of tokens.

Selective case sensitivity for a rule

Generally, a rule is either case-sensitive, or not. When needed, that can be changed on the token leven however. In the example below, case sensitivity has been switched of by making it regexp and adding (?iu) to instruct Java to ignore the case sensitivity.

    <pattern case_sensitive="yes">
      <token negate="yes" skip="1" regexp="yes">(?iu)ten</token>
      <token regexp="yes">one|two</token>

Testing suggestions

Run tests on your grammar rules. There are three ways:

For beginners, we recommend the method. It tests rules against examples and checks if they’re valid XML. During development of rules, you’ll find how many times there are stupid mistakes you wouldn’t see without tests. If you want to run the tests only for your language, simply give a two-letter code of your language as the command-line parameter, and tests for other languages will be skipped:

$ ./ ru

The above command runs only tests for Russian.

Testing corrections

If your suggestions are not explicit strings, but special codes or synthesized words, it’s recommended to test them. The easiest way is to use the correction attribute of the incorrect example (in correct examples, correction makes no sense).

For example:

 <example correction="back and forth" type="incorrect">How to move <marker>back and fourth</marker> from linux to xmb?</example>

If the rule doesn’t supply “back and forth” as a suggestion for this sentence, you will get an error during the standard rule test. Note: you don’t supply \1 or anything like that in the correction attribute: you supply the string that would be generated based on the suggestion element you supplied in the rule.

Multiple suggestions are joined using |:

 <example correction="back and forth|to and fro" type="incorrect">How to move <marker>back and fourth</marker>?</example>

Warn for misused word, except in correct context

Some (correct) words sound alike, but have a very different meaning. These are often confused. It is possible to warn for confusion at every occurrence of such a word, but it is better to suppress the warning when the context suggests the word is used properly.

Here is an example in Dutch, showing the words ‘aanvaart’ (= collides while sailing) and ‘aanvaardt’ (=accepts). Aanvaart is the most common mistake; the warning is suppressed however when typical ‘sailing’ words like boot (=boat) and haven (=harbour) are in the sentence.

    <rule id="ID2880" name="Aanvaart">
    		<token postag="SENT_START" skip="-1"><exception scope="next" regexp="yes">boot|haven</exception></token>
    			<token skip="-1">aanvaart<exception scope="next" regexp="yes">boot|haven</exception></token>
    		<token postag="SENT_END"><exception scope="current" regexp="yes">boot|haven</exception></token>
    	<message>Wanneer het botsing met boten is, is juist: <suggestion>aanvaardt</suggestion> of <suggestion>aanvaard</suggestion></message>
    	<short>aanvaardt? (ID2880)</short>
    	<example type="incorrect">Hij <marker>aanvaart</marker> de overeenkomst.</example>
    	<example>Als hij de boot toch weer <marker>aanvaart</marker>, heeft hij schade.</example>
    	<example>Als hij toch weer <marker>aanvaart</marker> tegen de boot, heeft hij schade.</example>

Using xxe for editing rules

It’s possible to edit rules using XML Mind XML Editor (xxe) in a WYSYWIM (what you see is what you mean) mode. The program is available in a free version for many platforms (as it’s developed in Java) though it’s not open source. It offers spell-checking and validation, and we have developed special CSS files to make editing files easier. Put these in the rules directory.

Simply open the file in xxe, and it should display in a form input mode. It is recommended that you uncheck Options -> Preferences -> Save -> Add open lines. Set a high value in Max line length.

Token unification

You can use unification to match several tokens with a similar feature. Combined with POS tagging it can be very useful for word agreement, e.g. if you need to match tokens which have the same gender.

Usage of part-of-speech tags

Testing if the POS tag is the only one

For some purposes, it’s useful to have rules that react on words that can only take one part-of-speech tag. The way you do it is the following:

    <token postag="postag1">
    	<exception negate_pos="yes" postag="postag1"/>

Why this works? The answer is simple, it first matches all tokens that have a POS tag postag1, and then checks if it has any other tag other than postag1, using the exception tag. The same method works for more POS tags - you can check if the token has only one of two POS tags (and not any other):

    <token postag="tag1|tag2" postag_regexp="yes">
    	<exception negate_pos="yes" postag="tag1|tag2" postag_regexp="yes"/>

To check if the token has exactly two tags, and only those, you can use the following notation using and:

      <token postag="tag1">
    	<exception negate_pos="yes" postag="tag1|tag2" postag_regexp="yes"/>
      <token postag="tag2"/>

It’s enough to set the exception on any of the tokens but they must be linked with and if both are supposed to appear. This can be useful for writing disambiguation rules, for example.

Adding words to the POS tagger

The LanguageTool part-of-speech tagger (POS tagger) uses a binary dictionary that comes with LanguageTool. With that, it can look up words, e.g. to detect that “children” is a plural noun in English. How to create and update that dictionary is described at Developing a tagger dictionary. An easier approach is to simply add words to org/languagetool/resource/XX/added_custom.txt (XX being a language code). Add words to that file in the form fullform lemma pos_tag, separated by tabs, for example: children child NNS (note than tabs are not displayed here). The words you’ve added will be recognized by LanguageTool after a restart.

Special POS tags

For all languages, there are special part-of-speech tags defined beside the tag set for the language. These are:

  1. SENT_START that matches the sentence start
  2. SENT_END that matches the sentence end
  3. UNKNOWN that matches a token that has no part-of-speech token assigned by the tagger

Suggesting the word with the same POS tag

For some languages (those that have a *_synth.dict file in their resources directory) it’s possible to inflect the words being suggested. However, a nice ability is to inflect the word just the way the matched token is inflected. Here’s how it’s done:

    <message>Here's another word: 
        <match no="1" postag="POS.*" postag_regexp="yes" >Wort</match>

Note the postag attribute - it allows to inflect “Wort” the same way “word” is inflected; without it, it’s impossible to choose the right inflection pattern for tokens with multiple readings. But even if there is only one reading, the default logic “replace the word with saving its grammatical form” works only when the postag is selected as above (not necessarily as a regular expression, but it makes things easier). “Wort” must be the lemma (the base form). Note that POS.* above is just a placeholder for a real POS tag.

However, if the POS tag specified in the postag attribute does not match the original POS tag, the same rule will generate all forms that match just this POS.*, disregarding the original POS tag. So the filtering action happens only when there is a match between the postag attribute value and the POS tag of the word matched.

Remember to test if the proposed corrections are what you meant by using the correction attribute in the example marked as incorrect.

Skipping and regular expressions over tokens

In some corpus query languages (for example, used in Poliqarp), you can have regular expression queries over tokens, like this:

[word="the"] [pos="jj"]{1,2} [pos="nn"]

Such a query would result in matching one or two words with POS tag equal to “jj”. There is an almost equivalent notation in LanguageTool:

    <token postag="jj" skip="1"><exception scope="next" negate_pos="yes" postag="jj|nn|SENT_END" postag_regexp="yes"/></token>
    <token postag="nn"/>

Note that you add negation, as exception already implies negation. This way, you get a positive condition (double-negation elimination). Another note: the POS tag is specified as a disjunction in the above example. This is because any token with a POS tag nn would be matched by the exception, so the end token of the pattern would never be found. To make sure that the negated POS exception is actually working properly you need to use the regular expression disjunction and include:

This regular expression:


is equivalent to

    <token postag="jj" skip="-1"><exception negate_pos="yes" scope="next" postag="jj"/></token>

The only missing operator is *. You cannot have a token that is matched zero times. In such a case, you have to write two instances of the rule: one without the asterisked token, and another with the notation equivalent to the +.