SubtitleEdit: SplitStartTags

public static string SplitStartTags(string line, ref string pre)
        {
            var l = 0;
            var len = line.Length;
            // ssa tag
            for (var r = l; r < len; r++)
            {
                if (line[r] == '{' && r + 1 < len && line[r + 1] == '\\')
                {
                    l = r;
                }
                else if (line[r] == '}' && line[l] == '{')
                {
                    l = r + 1;
                }
                else if (line[l] != '{')
                {
                    break;
                }
            }

            // html tags
            for (var r = l; r < len; r++)
            {
                if (line[r] == '<')
                {
                    l = r;
                }
                else if (line[r] == '>' && line[l] == '<')
                {
                    var tag = line.Substring(l, r - l + 1);
                    if (knownTags.Contains(tag))
                    {
                        l = r + 1;
                    }
                    else
                    {
                        // stop in invalid tag
                        break;
                    }
                }
                else if (line[l] != '<')
                {
                    break;
                }
            }

            pre = line.Substring(0, l);
            return line.Substring(l);
        }