using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        string subjectString = "Lorem ipsum, Lorem ipsom Instagram: @instausername Lorem ipsum";

        string resultString = null;

        try {

            resultString = Regex.Replace(subjectString, "Instagram: (@[^ ]+)", "<a href='https://www.instagram.com/$1'> Instagram: $1<a/>", RegexOptions.IgnoreCase | RegexOptions.Multiline);

            Console.WriteLine(resultString);

        } catch (ArgumentException ex) {

            // Syntax error in the regular expression

        }

​

    }

}