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How to use RegEx regarding second instance and negation
string[] inputs =
{
    "<link href="6e18f2dc-1d66-4e7e-b6e7-4a68a4217feb#SectionId">",
    "<link href="6e18f2dc-1d66-4e7e-b6e7-4a68a4217feb">"
};

string pattern = @"(?<=<links+href="")[^#]+(#.+?)(?="">)";

foreach (var input in inputs)
{
    Console.WriteLine("Input: " + input);
    string result = Regex.Replace(input, pattern, "$1", RegexOptions.IgnoreCase);
    Console.WriteLine("Result: " + result);
}

string guidPattern = @"[a-f0-9]{8}-(?:[a-f0-9]{4}-){3}[a-f0-9]{12}";
string pattern = @"(?<=<links+href="")" + guidPattern + @"(#.+?)(?="">)";