antlr/antlr4 · error · ArgumentException

tag delimiters out of order in pattern: {pattern}

Error message

tag delimiters out of order in pattern: {pattern}

What it means

While splitting a pattern, the i-th start delimiter position is not before the i-th stop delimiter position (starts[i] >= stops[i]). This happens with interleaved delimiters such as '<a><b>' where scanning pairs them out of order relative to the collected indexes, or when a stop appears before its start in the string. The pattern's tag structure is malformed.

Source

Thrown at runtime/CSharp/src/Tree/Pattern/ParseTreePatternMatcher.cs:645

                }
            }
            //		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
                string text = Sharpen.Runtime.Substring(pattern, 0, starts[0]);
                chunks.Add(new TextChunk(text));
            }
            for (int i_1 = 0; i_1 < ntags; i_1++)
            {
                // copy inside of <tag>
                string tag = Sharpen.Runtime.Substring(pattern, starts[i_1] + start.Length, stops[i_1]);

View on GitHub (pinned to 7d5770395b)

Solutions

1. Rewrite the pattern so each '<' immediately precedes its matching '>' with no crossing pairs.
2. Log the compiled starts/stops index lists (temporarily instrument the Chunkify/split path or count delimiters yourself) to see which pair is inverted.
3. Build patterns from small validated fragments ('text', '<tag>') joined in order instead of raw string surgery.

Example fix

// before
string pattern = fragA + "><" + fragB; // crossed delimiters
var p = parser.CompileParseTreePattern(pattern, RULE_r, null);

// after
string pattern = fragA + "<" + tag + ">" + fragB; // well-formed tag

// Verify each start precedes its paired stop before compiling
var starts = IndexesOfUnescaped(pattern, '<'); var stops = IndexesOfUnescaped(pattern, '>');
if (starts.Count == stops.Count)
    for (int i = 0; i < starts.Count; i++)
        if (starts[i] >= stops[i]) throw new ArgumentException($"tag delimiters crossed at {starts[i]}");

try { matcher.Compile(pattern); } catch (ArgumentException ex) when (ex.Message.Contains("out of order")) { /* rebuild the pattern from validated fragments */ }

When it happens

Trigger: Patterns with crossed or nested-looking delimiters (e.g. "><", "<a> ><") where the k-th stop occurs at or before the k-th start; malformed concatenation producing delimiter sequences like '>...<'.

Common situations: Programmatic pattern assembly that concatenates fragments each carrying their own delimiters in the wrong order; hand-editing patterns and swapping halves; escaped delimiters making the effective order cross.

Related errors

• unterminated tag in pattern: {pattern}
• missing start tag 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/1cb337edc1f5ed9a. Report an issue: GitHub.