{"record":{"id":"f22501dd190ecee8","repo":"antlr/antlr4","slug":"unknown-rule-s-in-pattern-s","errorCode":null,"errorMessage":"Unknown rule %s in pattern: %s","messagePattern":"Unknown rule (.+?) in pattern: (.+?)","errorType":"exception","errorClass":"Exception","httpStatus":null,"severity":"error","filePath":"runtime/Python3/src/antlr4/tree/ParseTreePatternMatcher.py","lineNumber":292,"sourceCode":"\n    def tokenize(self, pattern:str):\n        # split pattern into chunks: sea (raw input) and islands (<ID>, <expr>)\n        chunks = self.split(pattern)\n\n        # create token stream from text and tags\n        tokens = list()\n        for chunk in chunks:\n            if isinstance( chunk, TagChunk ):\n                # add special rule token or conjure up new token from name\n                if chunk.tag[0].isupper():\n                    ttype = self.parser.getTokenType(chunk.tag)\n                    if ttype==Token.INVALID_TYPE:\n                        raise Exception(\"Unknown token \" + str(chunk.tag) + \" in pattern: \" + pattern)\n                    tokens.append(TokenTagToken(chunk.tag, ttype, chunk.label))\n                elif chunk.tag[0].islower():\n                    ruleIndex = self.parser.getRuleIndex(chunk.tag)\n                    if ruleIndex==-1:\n                        raise Exception(\"Unknown rule \" + str(chunk.tag) + \" in pattern: \" + pattern)\n                    ruleImaginaryTokenType = self.parser.getATNWithBypassAlts().ruleToTokenType[ruleIndex]\n                    tokens.append(RuleTagToken(chunk.tag, ruleImaginaryTokenType, chunk.label))\n                else:\n                    raise Exception(\"invalid tag: \" + str(chunk.tag) + \" in pattern: \" + pattern)\n            else:\n                self.lexer.setInputStream(InputStream(chunk.text))\n                t = self.lexer.nextToken()\n                while t.type!=Token.EOF:\n                    tokens.append(t)\n                    t = self.lexer.nextToken()\n        return tokens\n\n    # Split {@code <ID> = <e:expr> ;} into 4 chunks for tokenizing by {@link #tokenize}.#\n    def split(self, pattern:str):\n        p = 0\n        n = len(pattern)\n        chunks = list()\n        # find all start and stop indexes first, then collect","sourceCodeStart":274,"sourceCodeEnd":310,"githubUrl":"https://github.com/antlr/antlr4/blob/7d5770395bb7b02eb56e7c62662cb1d7c08f42a3/runtime/Python3/src/antlr4/tree/ParseTreePatternMatcher.py#L274-L310","documentation":"Raised by ParseTreePatternMatcher.tokenize() when a tag starting with a lowercase letter does not match any parser rule. Lowercase-first tags are rule tags (e.g. <expr>); the matcher resolves them via parser.getRuleIndex(), which returns -1 for unknown names.","triggerScenarios":"Compiling a pattern with a tag like <expression> when the grammar rule is named expr, or referencing a rule from a different grammar version.","commonSituations":"Grammar refactor renamed rules, pattern shared across grammars, or assuming a lexer rule name works as a parser rule tag.","solutions":["Match the tag exactly to a rule name in parser.ruleNames","Regenerate the parser from the current grammar and retry","If you meant a token, capitalize the first letter (<ID>)"],"exampleFix":"# before: rule is 'expr' in the grammar\npattern = parser.compilePattern('<expression>', exprRuleIndex)\n\n# after\npattern = parser.compilePattern('<expr>', exprRuleIndex)","handlingStrategy":"validation","validationCode":"assert 'expr' in parser.ruleNames, 'unknown rule tag'\npattern = parser.compilePattern('<expr>', exprIdx)","typeGuard":"def is_known_rule(tag: str, parser) -> bool:\n    return parser.getRuleIndex(tag) != -1","tryCatchPattern":"try:\n    p = matcher.compileTreePattern(pat, idx)\nexcept Exception as e:\n    if str(e).startswith('Unknown rule'):\n        # check parser.ruleNames for the correct name\n        ...","preventionTips":["Cross-check lowercase tags against parser.ruleNames","Regenerate parsers together with the patterns that target them"],"tags":["antlr","python","parse-tree-matching","pattern-compile"],"backgroundTag":null,"analyzedSha":"7d5770395bb7b02eb56e7c62662cb1d7c08f42a3","analyzedAt":"2026-08-14T14:47:56.354Z","schemaVersion":2},"datasetVersion":"2026-08-15T22:17:37.221Z"}