stanfordnlp/CoreNLP · error · IllegalArgumentException

Invalid captureGroupId=

Error message

Invalid captureGroupId=

What it means

BackRefPatternExpr matches a previously captured group; its constructor requires captureGroupId to be a positive group index (>= 1). A non-positive value throws IllegalArgumentException 'Invalid captureGroupId='. Group ids are 1-based, mirroring regex back-reference semantics.

Solutions

1. Ensure the referenced capture group exists and its id is >= 1 before creating the back reference
2. Create groups first, capture their assigned ids, then build BackRefPatternExpr with that id
3. Validate user-provided group indices: reject <= 0 early with a clear message
4. Use \1-style back references only after at least one group in the pattern

Example fix

// before
new BackRefPatternExpr(matcher, 0); // invalid group id
// after
new BackRefPatternExpr(matcher, 1); // first captured group

if (captureGroupId <= 0) {
  throw new IllegalArgumentException("captureGroupId must be >= 1, got: " + captureGroupId);
}

boolean isValidCaptureGroupId(int id) {
  return id >= 1;
}

try {
  BackRefPatternExpr expr = new BackRefPatternExpr(matcher, groupId);
} catch (IllegalArgumentException e) {
  log.error("Bad capture group id", e);
}

When it happens

Trigger: Constructing a BackRefPatternExpr (directly or via a parser/builder) with captureGroupId <= 0, e.g. passing 0 as a default or computing a group index that was never assigned.

Common situations: Programmatic pattern building where the back-reference is emitted before any group was defined, or off-by-one handling of 1-based capture group numbering.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• Invalid nextBranchIndex=
• Error compiling
• Invalid minMatch=
• Cannot put a child trie with no keys
• Value cannot be null

AI-assisted analysis of stanfordnlp/CoreNLP@1b7edd19c4 (2026-09-10). Data as JSON: /api/errors/9cadd6e8e5e8ad9e. Report an issue: GitHub.

Appendix: source

Thrown at src/edu/stanford/nlp/ling/tokensregex/SequencePattern.java:556

      return new SequencePatternExpr(newPatterns);
    }

    public String toString() {
      return StringUtils.join(patterns, " ");
    }
  }

  // Expression that indicates a back reference
  // Need to match a previously matched group somehow
  public static class BackRefPatternExpr extends PatternExpr {

    private static final long serialVersionUID = -4649629486266561619L;

    private final NodesMatchChecker matcher; // How a match is determined
    private final int captureGroupId;  // Indicates the previously matched group this need to match

    public BackRefPatternExpr(NodesMatchChecker matcher, int captureGroupId) {
      if (captureGroupId <= 0) { throw new IllegalArgumentException("Invalid captureGroupId=" + captureGroupId); }
      this.captureGroupId = captureGroupId;
      this.matcher = matcher;
    }

    @Override
    protected Frag build() {
      State s = new BackRefState(matcher, captureGroupId);
      return new Frag(s);
    }

    @Override
    protected int assignGroupIds(int start) {
      return start;
    }
    @Override
    protected void updateBindings(VarGroupBindings bindings) {}

    @Override

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