JamesNK/Newtonsoft.Json · error · JsonException

Path ended with an open regex.

Error message

Path ended with an open regex.

What it means

Thrown by ReadRegexString when a JSONPath regex literal (/pattern/flags) is opened with '/' but the closing '/' is never found before end-of-input. The regex was started (e.g. via =~ operator) but never terminated.

Source

Thrown at Src/Newtonsoft.Json/Linq/JsonPath/JPath.cs:750

                        if (char.IsLetter(currentChar))
                        {
                            _currentIndex++;
                        }
                        else
                        {
                            break;
                        }
                    }

                    return _expression.Substring(startIndex, _currentIndex - startIndex);
                }
                else
                {
                    _currentIndex++;
                }
            }

            throw new JsonException("Path ended with an open regex.");
        }

        private bool Match(string s)
        {
            int currentPosition = _currentIndex;
            for (int i = 0; i < s.Length; i++)
            {
                if (currentPosition < _expression.Length && _expression[currentPosition] == s[i])
                {
                    currentPosition++;
                }
                else
                {
                    return false;
                }
            }

            _currentIndex = currentPosition;

View on GitHub (pinned to 4f73e74372)

Solutions

1. Terminate the regex with a second '/' and optional trailing flags (letters).
2. Escape literal '/' inside the pattern as \\/ so it is not treated as the terminator.
3. Test the regex query against a known token in a unit test.

Example fix

// before
token.SelectTokens("$..[?(@.name =~ /bob");
// after
token.SelectTokens("$..[?(@.name =~ /bob/i]");

static bool RegexClosed(string path)
{
    bool inRegex = false;
    for (int i = 0; i < path.Length; i++)
    {
        if (path[i] == '/' && (i == 0 || path[i-1] != '\\')) inRegex = !inRegex;
    }
    return !inRegex;
}

try { token.SelectTokens(path).ToList(); }
catch (JsonException ex) when (ex.Message == "Path ended with an open regex.")
{ /* regex literal missing closing slash */ }

When it happens

Trigger: A query using the =~ regex operator whose pattern is never closed. Example: token.SelectTokens("$..[?(@.name =~ /bob") — the regex opens but no second '/' appears, so the parser runs to the end of the string.

Common situations: Writing a regex query and forgetting the closing slash; constructing regex from user input that itself contains slashes consumed as part of the pattern; assuming flags must come before the closing slash.

Related errors

• Path ended with open query.
• Unknown escape character: \
• Path ended with an open string.
• Could not read query operator.
• Index {0} outside the bounds of JArray.

AI-assisted analysis of JamesNK/Newtonsoft.Json@4f73e74372 (2026-08-07). Data as JSON: /api/errors/dc52b15856698cc6. Report an issue: GitHub.