JamesNK/Newtonsoft.Json · error · JsonException

Unknown escape character: \

Error message

Unknown escape character: \

What it means

Thrown by ReadQuotedString when a backslash escape inside a single-quoted JSONPath string literal is followed by a character that is not one of the recognized escape codes (b, t, n, f, r, backslash, double-quote, single-quote, forward-slash). Only that fixed set of escapes is supported inside JSONPath quoted strings.

Source

Thrown at Src/Newtonsoft.Json/Linq/JsonPath/JPath.cs:688

                            resolvedChar = '\t';
                            break;
                        case 'n':
                            resolvedChar = '\n';
                            break;
                        case 'f':
                            resolvedChar = '\f';
                            break;
                        case 'r':
                            resolvedChar = '\r';
                            break;
                        case '\\':
                        case '"':
                        case '\'':
                        case '/':
                            resolvedChar = currentChar;
                            break;
                        default:
                            throw new JsonException(@"Unknown escape character: \" + currentChar);
                    }

                    sb.Append(resolvedChar);

                    _currentIndex++;
                }
                else if (currentChar == '\'')
                {
                    _currentIndex++;
                    return sb.ToString();
                }
                else
                {
                    _currentIndex++;
                    sb.Append(currentChar);
                }
            }

View on GitHub (pinned to 4f73e74372)

Solutions

1. Replace unsupported escapes: use \\uXXXX via JSON string rules outside JSONPath, or escape the raw characters differently.
2. For literal backslashes use the supported \\\\ escape (backslash-backslash).
3. Sanitize/escape user-supplied text before interpolating it into a JSONPath quoted string.

Example fix

// before
token.SelectTokens("$['C:\\new']");
// after
token.SelectTokens("$['C:\\\\new']");

static string EscapeForJsonPath(string value)
{
    return value.Replace("\\", "\\\\").Replace("'", "\\'");
}

try { token.SelectTokens($"$['{field}']").ToList(); }
catch (JsonException ex) when (ex.Message.StartsWith("Unknown escape character"))
{ /* field contained an unsupported escape; sanitize and retry */ }

When it happens

Trigger: A quoted field name or string value containing an unsupported escape. Example: token.SelectTokens("$['foo\\x']") or a query like "$..[?(@.name == 'a\\q')]" where \\x / \\q are not valid escapes.

Common situations: Forwarding user text containing Windows file paths or regex into a JSONPath without sanitization; assuming JSONPath supports the same escapes as C# or JSON verbatim strings (e.g. \uXXXX is not supported here); constructing paths from URIs that contain %2F style sequences.

Related errors

• Path ended with open query.
• Path ended with an open string.
• Path ended with an open regex.
• 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/da7f79189619e801. Report an issue: GitHub.