antlr/antlr4 · error · IllegalArgumentException

Unknowth path element "+el

Error message

Unknowth path element "+el

What it means

After lexing, XPath.split switches on each token's type; the default branch throws IllegalArgumentException ('Unknowth path element ...' — typo is in the ANTLR source) when a token type is neither a separator, token/rule ref, wildcard, nor EOF. With the stock XPathLexer this is unreachable; it guards against lexer/runtime version mismatches or custom lexers.

Source

Thrown at runtime/Java/src/org/antlr/v4/runtime/tree/xpath/XPath.java:146

					}
					XPathElement pathElement = getXPathElement(next, anywhere);
					pathElement.invert = invert;
					elements.add(pathElement);
					i++;
					break;

				case XPathLexer.TOKEN_REF :
				case XPathLexer.RULE_REF :
				case XPathLexer.WILDCARD :
					elements.add( getXPathElement(el, false) );
					i++;
					break;

				case Token.EOF :
					break loop;

				default :
					throw new IllegalArgumentException("Unknowth path element "+el);
			}
		}
		return elements.toArray(new XPathElement[0]);
	}

	/**
	 * Convert word like {@code *} or {@code ID} or {@code expr} to a path
	 * element. {@code anywhere} is {@code true} if {@code //} precedes the
	 * word.
	 */
	protected XPathElement getXPathElement(Token wordToken, boolean anywhere) {
		if ( wordToken.getType()==Token.EOF ) {
			throw new IllegalArgumentException("Missing path element at end of path");
		}
		String word = wordToken.getText();
		int ttype = parser.getTokenType(word);
		int ruleIndex = parser.getRuleIndex(word);
		switch ( wordToken.getType() ) {

View on GitHub (pinned to 7d5770395b)

Solutions

1. Align antlr4 tool and runtime jar versions on the classpath
2. If subclassing XPathLexer, only emit token types the XPath splitter understands
3. Run dependency:tree / gradle dependencies to find duplicate antlr4 artifacts

try { xpath.split(path); } catch (IllegalArgumentException e) { /* suspect mixed ANTLR versions; fail fast with context */ }

When it happens

Trigger: Mixing an XPathLexer (or lexer token types) from a different ANTLR version than the XPath class; subclassing XPathLexer to emit custom token types.

Common situations: Conflicting antlr4 jar versions on the classpath (e.g. tool 4.x with runtime 4.y) so token type constants no longer line up.

Related errors

• Could not read path: "+path
• Invalid tokens or characters at index "+pos+" in path '"+pat
• Missing path element at end of 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/0f2cfe34480dad4d. Report an issue: GitHub.