antlr/antlr4 · error · Exception

Unknown path element %s

Error message

Unknown path element %s

What it means

Raised by XPath element parsing in the Python3 runtime when a token appears that is none of TOKEN_REF, RULE_REF, WILDCARD, STRING, or EOF — i.e. lexically valid but not usable at that position in a path. Typically stray punctuation that the lexer accepted but the parser loop does not expect.

Source

Thrown at runtime/Python3/src/antlr4/xpath/XPath.py:125

                else:
                    el = next_el
            # Check for bangs
            if el.type == XPathLexer.BANG:
                invert = True
                next_el = next(tokens, None)
                if not next_el:
                    raise Exception('Missing element after %s' % el.getText())
                else:
                    el = next_el
            # Add searched element
            if el.type in [XPathLexer.TOKEN_REF, XPathLexer.RULE_REF, XPathLexer.WILDCARD, XPathLexer.STRING]:
                element = self.getXPathElement(el, anywhere)
                element.invert = invert
                elements.append(element)
            elif el.type==Token.EOF:
                break
            else:
                raise Exception("Unknown path element %s" % lexer.symbolicNames[el.type])
        return elements

    #
    # Convert word like {@code#} or {@code ID} or {@code expr} to a path
    # element. {@code anywhere} is {@code true} if {@code //} precedes the
    # word.
    #
    def getXPathElement(self, wordToken:Token, anywhere:bool):
        if wordToken.type==Token.EOF:
            raise Exception("Missing path element at end of path")

        word = wordToken.text
        if wordToken.type==XPathLexer.WILDCARD :
            return XPathWildcardAnywhereElement() if anywhere else XPathWildcardElement()

        elif wordToken.type in [XPathLexer.TOKEN_REF, XPathLexer.STRING]:
            tsource = self.parser.getTokenStream().tokenSource

View on GitHub (pinned to 7d5770395b)

Solutions

1. Simplify the path to strictly alternating separator/element form
2. Eliminate duplicated or misplaced separators and punctuation
3. Print the token types of the path (debug the token stream) to find which token fell through

Example fix

# before
nodes = XPath.findAll(tree, "/expr///*", parser)

# after
nodes = XPath.findAll(tree, "/expr/*", parser)

import re
if re.search(r'//{2,}|/\s*/(?!/)', xpath) or xpath.count('/') > 2 * len(re.findall(r'[A-Za-z_*\"]+', xpath)):
    raise ValueError('suspect separator placement in XPath')

try:
    nodes = XPath.findAll(tree, xpath, parser)
except Exception as e:
    if str(e).startswith('Unknown path element'):
        # simplify path to separator/element alternation
        ...

When it happens

Trigger: A path token stream containing an unexpected token type after separator/bang handling, e.g. an extra '/' in the middle producing a bare separator token where an element is required.

Common situations: Malformed paths like '/expr///*' or paths with repeated separators where an element was expected; also token streams from a mismatched XPathLexer version.

Related errors

• Missing element after %s
• Missing path element at end of path
• %s at index %d isn't a valid token name
• %s at index %d isn't a valid rule name
• The current recognizer does not provide a list of token name

AI-assisted analysis of antlr/antlr4@7d5770395b (2026-08-14). Data as JSON: /api/errors/4f3b5c2c983a3ee7. Report an issue: GitHub.