DdwrtParser.cs

public class DdwrtParser
{
    public JObject Read(ReadOnlySpan<char> str)
    {
        JObject result = new JObject();

        while (str.Length > 0)
        {
            if (str[0] != '{')
                throw new FormatException();

            str = str.Slice(1);
            string key = Until(ref str, "::").ToString();

            ReadOnlySpan<char> value = Until(ref str, "}");
            value = value.TrimStart().TrimStart(',');
            if (value.IsEmpty)
            {
                result.Add(key, JValue.CreateNull());
                continue;
            }

            if (value[value.Length - 1] == ';')
            {
                JObject item = new JObject();

                foreach (ReadOnlySpan<char> pair in value.Trim(';').Split(';'))
                {
                    ReadOnlySpan<char> itemValue = pair;
                    ReadOnlySpan<char> itemKey = Until(ref itemValue, "=");
                    item.Add(itemKey.ToString(), JValue.CreateString(itemValue.ToString()));
                }

                result.Add(key, item);
                continue;
            }

            if (value.StartsWith("'".AsSpan(), StringComparison.Ordinal))
            {
                JArray item = new JArray();

                foreach (ReadOnlySpan<char> arrayItem in value.Split(','))
                    item.Add(JValue.CreateString(arrayItem.Trim('\'').ToString()));

                result.Add(key, item);
                continue;
            }

            result.Add(key, JValue.CreateString(value.ToString()));
        }

        return result;
    }

    static ReadOnlySpan<char> Until(ref ReadOnlySpan<char> str, string lookup)
    {
        int index = str.IndexOf(lookup.AsSpan());
        if (index < 0)
            throw new FormatException();

        ReadOnlySpan<char> result = str.Slice(0, index);

        str = str.Slice(index + lookup.Length);

        return result;
    }
}