antlr/antlr4 · error · IllegalArgumentException

unterminated tag in pattern: "+pattern

Error message

unterminated tag in pattern: "+pattern

What it means

Error "unterminated tag in pattern: "+pattern" thrown in antlr/antlr4.

Source

Thrown at runtime/Java/src/org/antlr/v4/runtime/tree/pattern/ParseTreePatternMatcher.java:444

			}
			else if ( p == pattern.indexOf(start,p) ) {
				starts.add(p);
				p += start.length();
			}
			else if ( p == pattern.indexOf(stop,p) ) {
				stops.add(p);
				p += stop.length();
			}
			else {
				p++;
			}
		}

//		System.out.println("");
//		System.out.println(starts);
//		System.out.println(stops);
		if ( starts.size() > stops.size() ) {
			throw new IllegalArgumentException("unterminated tag in pattern: "+pattern);
		}

		if ( starts.size() < stops.size() ) {
			throw new IllegalArgumentException("missing start tag in pattern: "+pattern);
		}

		int ntags = starts.size();
		for (int i=0; i<ntags; i++) {
			if ( starts.get(i)>=stops.get(i) ) {
				throw new IllegalArgumentException("tag delimiters out of order in pattern: "+pattern);
			}
		}

		// collect into chunks now
		if ( ntags==0 ) {
			String text = pattern.substring(0, n);
			chunks.add(new TextChunk(text));
		}

View on GitHub (pinned to 7d5770395b)

Solutions

1. Close every tag with the stop delimiter; scan the pattern for a '<' without a matching '>'.
2. If your pattern literally needs the start delimiter character, pick different delimiters with setDelimiters.

Example fix

// fix "<expr" to "<expr>"
ParseTreePattern p = matcher.compile("<expr> + <expr>", MyParser.RULE_add);

When it happens

Trigger: Pattern contains a start tag delimiter with no matching stop delimiter.

Common situations: Balance every start delimiter with a stop delimiter in the pattern string; escape literal delimiters if needed.

AI-assisted analysis of antlr/antlr4@7d5770395b (2026-08-14). Data as JSON: /api/errors/ee5190401a83cfbb. Report an issue: GitHub.