languagetool-org/languagetool · error · RuntimeException

Error trying to synthesize POS tag " + posTag + "…

Error message

Error trying to synthesize POS tag " + posTag + " (posTagRegExp: true) from token " + token.getToken()

What it means

Wrapper exception in BaseSynthesizer.synthesize for the posTagRegExp=true code path: treating the POS tag as a regular expression and synthesizing matching word forms failed unexpectedly, and the error is rethrown with the offending tag and token attached. The inputs at fault are the POS-tag pattern and the analyzed token passed by the calling rule.

Solutions

1. Inspect the chained cause to find the underlying failure (often an invalid regex in the POS tag)
2. Simplify the POS-tag regular expression to isolate the failing construct
3. Test the tag pattern against the synthesizer's list of possible tags to confirm it matches something

When it happens

Trigger: Thrown at languagetool-core/src/main/java/org/languagetool/synthesis/BaseSynthesizer.java:261 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of languagetool-org/languagetool@2e990059ce (2026-09-06). Data as JSON: /api/errors/c5da20e32028f69c. Report an issue: GitHub.

Appendix: source

Thrown at languagetool-core/src/main/java/org/languagetool/synthesis/BaseSynthesizer.java:261

    }
    if (posTag.equals(SPELLNUMBER_FEMININE_TAG)) {
      return new String[] {getSpelledNumber("feminine " + token.getToken())};
    }
    if (posTag.equals(SPELLNUMBER_ROMAN_TAG)) {
      return new String[] {getRomanNumber(token.getToken())};
    }
    List<String> wordForms = lookup(token.getLemma(), posTag);
    return removeExceptions(wordForms.toArray(new String[0]));
  }

  @Override
  public String[] synthesize(AnalyzedToken token, String posTag, boolean posTagRegExp) throws IOException {
    if (posTagRegExp) {
      try {
        Pattern p = Pattern.compile(posTag);
        return synthesizeForPosTags(token.getLemma(), tag -> p.matcher(tag).matches());
      } catch (PatternSyntaxException e) {
        throw new RuntimeException("Error trying to synthesize POS tag " + posTag +
                " (posTagRegExp: true) from token " + token.getToken(), e);
      }
    }
    return removeExceptions(synthesize(token, posTag));
  }

  /**
   * Synthesize forms for the given lemma and for all POS tags satisfying the given predicate.
   * @since 5.3
   */
  public String[] synthesizeForPosTags(String lemma, Predicate<String> acceptTag) throws IOException {
    initPossibleTags();
    List<String> results = new ArrayList<>();
    for (String tag : possibleTags) {
      if (acceptTag.test(tag)) {
        results.addAll(lookup(lemma, tag));
      }
    }

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