antlr/antlr4 · error · ArgumentException

stop cannot be null or empty

Error message

stop cannot be null or empty

What it means

ParseTreePatternMatcher.SetDelimiters rejects a null or empty stop delimiter. The stop delimiter closes a tag in a pattern string; without it the tag scanner cannot find tag boundaries, so the call fails fast. Note the escape (third) argument is not validated — only start and stop are.

Source

Thrown at runtime/CSharp/src/Tree/Pattern/ParseTreePatternMatcher.cs:222

        /// or empty.
        /// </exception>
        /// <exception>
        /// IllegalArgumentException
        /// if
        /// <paramref name="stop"/>
        /// is
        /// <see langword="null"/>
        /// or empty.
        /// </exception>
        public virtual void SetDelimiters(string start, string stop, string escapeLeft)
        {
            if (string.IsNullOrEmpty(start))
            {
                throw new ArgumentException("start cannot be null or empty");
            }
            if (string.IsNullOrEmpty(stop))
            {
                throw new ArgumentException("stop cannot be null or empty");
            }
            this.start = start;
            this.stop = stop;
            this.escape = escapeLeft;
        }

        /// <summary>
        /// Does
        /// <paramref name="pattern"/>
        /// matched as rule
        /// <paramref name="patternRuleIndex"/>
        /// match
        /// <paramref name="tree"/>
        /// ?
        /// </summary>
        public virtual bool Matches(IParseTree tree, string pattern, int patternRuleIndex)
        {
            ParseTreePattern p = Compile(pattern, patternRuleIndex);

View on GitHub (pinned to 7d5770395b)

Solutions

1. Pass a non-empty stop delimiter, e.g. SetDelimiters("[", "]", "\\").
2. Check both delimiter values with string.IsNullOrEmpty before calling and report which one is missing.
3. Keep the default delimiters ('<', '>') unless there is a concrete grammar conflict with angle brackets.

Example fix

// before
matcher.SetDelimiters("[", "", "\\");

// after
matcher.SetDelimiters("[", "]", "\\");

if (string.IsNullOrEmpty(start) || string.IsNullOrEmpty(stop)) throw new ConfigurationErrorsException("tag delimiters must be non-empty");
matcher.SetDelimiters(start, stop, escape);

static bool IsValidDelimiter(string s) => !string.IsNullOrEmpty(s);

When it happens

Trigger: Calling matcher.SetDelimiters("[", "", "\\") or SetDelimiters("[", null, ...) — e.g. reading delimiters from user input where only the opening one was provided.

Common situations: Delimiter pairs configured from a single config value where the closing one is derived incorrectly; interactive tooling that lets users set tag delimiters; typos in the argument order (start/stop swapped or escape passed in the stop slot).

Related errors

• start cannot be null or empty
• Invalid state number.
• The ATN must be a lexer ATN.
• tree cannot be null
• patternTree cannot be null

AI-assisted analysis of antlr/antlr4@7d5770395b (2026-08-14). Data as JSON: /api/errors/589e213980072213. Report an issue: GitHub.