regex get by group.cs

 try {
                Regex regexObj = new Regex(@"library/(\d*)", RegexOptions.Multiline);
                resultString = regexObj.Match(subjectString).Groups[1].Value;
                Console.WriteLine(resultString);
            } catch (ArgumentException ex) {
                // Syntax error in the regular expression
                Console.WriteLine(ex.StackTrace);
            }

regexx

C# YouTube regex
var youtubeid = Regex.Match(videoUrl,
   @"^.*((youtu.be\/)|(v\/)|(\/u\/\w\/)|(embed\/)|(watch\?))\??v?=?([^#\&\?]*).*")
 .Groups[7].Value;

// remove any tags but not there content "<p>bob<span> johnson</span></p>" becomes "bob johnson
Regex.Replace(input, @"<(.|\n)*?>", string.Empty);
//remove tags with content "<p>bob<span> johnson</span></p>" becomes ""
<.*\s*/?>

//remove only specific tag
<%TAG%[^>]*>(.*?)</%TAG%>
<a[^>]*>(.*?)</a>


//remove all tag except br tag
(?!<\/?br\s*\/?>)<[^>]+>