antlr/antlr4 · error · ArgumentException

missing start tag in pattern: {pattern}

Error message

missing start tag in pattern: {pattern}

What it means

While splitting a pattern, the scanner found more stop delimiters than start delimiters — a '>' (or custom stop) that appears without a preceding unmatched '<'. Like the unterminated-tag case, escaped stops (default '\>') are not counted, so an escape-character mismatch between pattern and matcher configuration also triggers this.

Source

Thrown at runtime/CSharp/src/Tree/Pattern/ParseTreePatternMatcher.cs:638

                            }
                            else
                            {
                                p++;
                            }
                        }
                    }
                }
            }
            //		System.out.println("");
            //		System.out.println(starts);
            //		System.out.println(stops);
            if (starts.Count > stops.Count)
            {
                throw new ArgumentException("unterminated tag in pattern: " + pattern);
            }
            if (starts.Count < stops.Count)
            {
                throw new ArgumentException("missing start tag in pattern: " + pattern);
            }
            int ntags = starts.Count;
            for (int i = 0; i < ntags; i++)
            {
                if (starts[i] >= stops[i])
                {
                    throw new ArgumentException("tag delimiters out of order in pattern: " + pattern);
                }
            }
            // collect into chunks now
            if (ntags == 0)
            {
                string text = Sharpen.Runtime.Substring(pattern, 0, n);
                chunks.Add(new TextChunk(text));
            }
            if (ntags > 0 && starts[0] > 0)
            {
                // copy text up to first tag into chunks

View on GitHub (pinned to 7d5770395b)

Solutions

1. Balance the pattern: remove the stray '>' or add the missing '<'.
2. Escape '>' that is meant as text: \>.
3. Pick delimiters that never occur in the target text via SetDelimiters.
4. Add a pre-compile lint for user patterns: unescaped starts must equal unescaped stops.

Example fix

// before
var p = parser.CompileParseTreePattern("List<int>> items", RULE_type, null);

// after
var p = parser.CompileParseTreePattern("List\<int\>\> items", RULE_type, null); // or use labels/tags for the type

int CountUnescaped(string s, char c) { int n = 0; for (int i = 0; i < s.Length; i++) { if (s[i] == '\\') i++; else if (s[i] == c) n++; } return n; }
if (CountUnescaped(pattern, '<') < CountUnescaped(pattern, '>')) throw new ArgumentException("missing start tag");

try { matcher.Compile(pattern); } catch (ArgumentException ex) when (ex.Message.Contains("missing start tag")) { /* remove or escape the stray '>' */ }

When it happens

Trigger: Pattern strings like "ID> = 3" or "a > b" (comparison operators as plain text) passed to Compile; switching escape characters via SetDelimiters while patterns still use the old escape.

Common situations: Matching source that contains '>' as text (generics like List<int>, arrow operators); unbalanced quotes/delimiters from string concatenation bugs; patterns assembled from templates that drop an opening delimiter.

Related errors

• unterminated tag in pattern: {pattern}
• tag delimiters out of order in pattern: {pattern}
• pattern cannot be null
• start cannot be null or empty
• stop cannot be null or empty

AI-assisted analysis of antlr/antlr4@7d5770395b (2026-08-14). Data as JSON: /api/errors/4dd5fc5bacf43807. Report an issue: GitHub.