chr = Regex.Replace(chr, @"\[url=(?<x>[^\]]*)\](?<y>[^\]]*)\[/url\]", @"<a href=$1 target=_blank>$2</a>", RegexOptions.IgnoreCase);
chr = Regex.Replace(chr, @"\[url\](?<x>[^\]]*)\[/url\]", @"<a href=$1 target=_blank>$1</a>", RegexOptions.IgnoreCase);