C# Online Compiler | .NET Fiddle

using System;
					
public static class Program
{
	const string ShareExtUNCFullPrefix = @"\\?\UNC\";  
	const string ShareExtPrefix = @"\\?\";      
	const string SharePrefix = @"\\";           
	const string DIR = @"\";
	const char  cDIR = '\\';
	const string DIRDOUBLE = @"\\";
	const string COLONDIR = @":\";
	const string COLON = @":";
	const char cCOLON = ':';
	const string DIRSPACE = @"\ ";
	const string SPACEDIR = @" \";
	const string SPACE = @" ";

	public static int IndexOfOccurence(this string s, char match, int occurence)
	{
		int i = 1;
		int index = -1;

		while (i <= occurence && (index = s.IndexOf(match, index + 1)) != -1)
		{
			if (i == occurence)
				return index;

			i++;
		}

		return -1;
	}
	
	/// <summary>
	/// Gets UNC Host and Share from a full UNC path. Cuts 4th backslash from start of path, must have least 1 char between.
	/// </summary>
	/// <param name="UNCFullPath">\\host\share returns empty, must end in a backslash</param>
	/// <returns>\\host\share from \\host\share\tail\, also \\?\UNC\ and \\?\ work</returns>
	/// <warn>//fails for one case when "\\?\UNC\share\tail\file.txt", UNC Server Name is named "UNC"</warn>
	/// <author>Metadataconsulting.blogspot.com</warn>
    public static string GetUNCHostShare(this string UNCFullPath)
	{
		if (UNCFullPath.StartsWith(DIRDOUBLE))
		{
			string p = UNCFullPath; //local working copy
			
			if (p.StartsWith(ShareExtPrefix))
			{
				p = p.Substring(3, p.Length - 3);
				
				if (p.StartsWith(@"\UNC\")) //fails for "\\?\UNC\share\tail\file.txt" one case
					p = DIRDOUBLE + p.Substring(5, p.Length - 5);
				else
					p = DIR + p;
			} 
			
			//starts with \\h\s is min 
			int idx3rd = (p.Length >= 5 ) ? p.IndexOf(cDIR, 3) : -1; //4th position zero index 

			//Console.WriteLine("p = " + p + " idx3rd=" + idx3rd); //DEBUG

			if (p[0]==cDIR && p[1]==cDIR && Char.IsLetter(p[2]) && p.Length >= 5 &&idx3rd > -1 && Char.IsLetter(p[idx3rd+1]) && p.IndexOf(cDIR, idx3rd + 2) > -1)
			{
				int i = p.IndexOfOccurence(cDIR, 4);

				if (i > -1)
					return p.Substring(0, i); //returns empty if i=0
				else
					return string.Empty;

			}
			else
				return string.Empty;
			
			

		}
		else
			return string.Empty;

	}

	public static void Main()
	{
		 Console.WriteLine(@"\\host\share\tail\dir1".GetUNCHostShare());
		 Console.WriteLine(@"\\host\share\tail\file.txt".GetUNCHostShare());
         Console.WriteLine(@"\\\?\UNC\share\tail\file.txt".GetUNCHostShare());
         Console.WriteLine(@"\\h\s\file.txt".GetUNCHostShare());
         Console.WriteLine(@"\\h\s\".GetUNCHostShare());
         Console.WriteLine(@"\\\s\".GetUNCHostShare());
         Console.WriteLine(@"\\h\\".GetUNCHostShare());
         Console.WriteLine(@"\\\\\\\\".GetUNCHostShare());
         Console.WriteLine(@"\\?\UNC\share\tail\file.txt".GetUNCHostShare() + ", 1 failed case - UNC is Server Name");
			
	}
}

            if (p[0]==cDIR && p[1]==cDIR && Char.IsLetter(p[2]) && p.Length >= 5 &&idx3rd > -1 && Char.IsLetter(p[idx3rd+1]) && p.IndexOf(cDIR, idx3rd + 2) > -1)
 
using System;
                    
public static class Program
{
    const string ShareExtUNCFullPrefix = @"\\?\UNC\";  
    const string ShareExtPrefix = @"\\?\";      
    const string SharePrefix = @"\\";           
    const string DIR = @"\";
    const char  cDIR = '\\';
    const string DIRDOUBLE = @"\\";
    const string COLONDIR = @":\";
    const string COLON = @":";
    const char cCOLON = ':';
    const string DIRSPACE = @"\ ";
    const string SPACEDIR = @" \";
    const string SPACE = @" ";
​
    public static int IndexOfOccurence(this string s, char match, int occurence)
    {
        int i = 1;
        int index = -1;
​
        while (i <= occurence && (index = s.IndexOf(match, index + 1)) != -1)
        {

Cached Result

\\host\share
\\host\share

\\h\s
\\h\s

\\share\tail, 1 failed case - UNC is Server Name