antlr/antlr4 · error · ArgumentException

Unknowth path element ${el}

Error message

Unknowth path element ${el}

What it means

XPath.Split() accepts only the small XPath dialect understood by the ANTLR tree XPath support: '/', '//', '*', token references, rule references, and optional '!' inversion immediately after a separator. The default branch of the token switch is reached when the lexer produced a token that is not one of those expected top-level elements. This almost always means the path syntax is valid enough to lex but is not valid for this XPath subset.

Source

Thrown at runtime/CSharp/src/Tree/Xpath/XPath.cs:162

                    }

                    case XPathLexer.TokenRef:
                    case XPathLexer.RuleRef:
                    case XPathLexer.Wildcard:
                    {
                        elements.Add(GetXPathElement(el, false));
                        i++;
                        break;
                    }

                    case TokenConstants.EOF:
                    {
                        goto loop_break;
                    }

                    default:
                    {
                        throw new ArgumentException("Unknowth path element " + el);
                    }
                }
            }
loop_break: ;
            return elements.ToArray();
        }

        private sealed class _XPathLexer_87 : XPathLexer
        {
            public _XPathLexer_87(ICharStream baseArg1)
                : base(baseArg1)
            {
            }

            public override void Recover(LexerNoViableAltException e)
            {
                throw e;
            }

View on GitHub (pinned to 7d5770395b)

Solutions

1. Use only supported forms: '/rule', '//rule', '/TOKEN', '//TOKEN', '*', and '/!' or '//!' before the element for inversion.
2. Remove a leading or misplaced '!' and place inversion directly after a separator.
3. If quoting a token name, keep the leading separator, e.g. "/'TOKEN'" rather than "'TOKEN'".
4. Inspect the generated XPathLexer token types if the path still fails, and remove unsupported XPath constructs.

Example fix

// before
var p = new Antlr4.Runtime.Tree.Xpath.XPath(parser, "!ID");

// after
var p = new Antlr4.Runtime.Tree.Xpath.XPath(parser, "/!ID");

static bool IsSupportedXPathShape(string path) {
    if (string.IsNullOrWhiteSpace(path)) return false;
    return !path.StartsWith("!") && System.Text.RegularExpressions.Regex.IsMatch(
        path,
        "^(\/\/?!?[A-Za-z_\u00C0-\uFFFF][A-Za-z0-9_\u00C0-\uFFFF]*|\/\/?'?[^']*'?|\\*)*$");
}

try { var p = new Antlr4.Runtime.Tree.Xpath.XPath(parser, xpath); }
catch (ArgumentException ex) when (ex.Message.StartsWith("Unknowth path element")) { /* reject xpath */ }

When it happens

Trigger: A path begins with '!' without a preceding '/' or '//' (for example "!ID"); a quoted string is used as the first element (for example "'ID'" instead of "/ID" or "/'ID'"); or stray characters/tokens such as a top-level Bang appear after a word. Full XPath features such as predicates, '..', or axes are not supported and can also lead here or to the invalid-character error.

Common situations: Developers mistake Antlr4.Runtime.Tree.Xpath.XPath for a full XPath engine; hand-built path strings contain a trailing separator or misplaced '!'; paths are assembled by string concatenation; or documentation examples from a different runtime version are copied.

Related errors

• Missing path element at end of path
• Could not read path: ${path}
• Invalid tokens or characters at index ${pos} in path '${path
• ${word} at index ${startIndex} isn't a valid token name
• ${word} at index ${startIndex} isn't a valid rule name

AI-assisted analysis of antlr/antlr4@7d5770395b (2026-08-14). Data as JSON: /api/errors/0c9ee456d09a0dd3. Report an issue: GitHub.