using System;
using System.Text.RegularExpressions;

public class Test
{
    public static void Main(string[] args)
    {
	    var longUrl = "http://stackoverflow.com/questions/20494457/limiting-the-number-of-characters-using-regular-expression";
	    var shortUrl = "http://stackoverflow.com/questions/20494457";
	    Console.WriteLine(GetHtml(longUrl)); // should truncate
		Console.WriteLine(GetHtml(shortUrl)); // should not truncate
	}
    
	public static string GetHtml(string url, int maxLength = 50)
	{
		var match = Regex.Match(url, String.Format("(https?://[^ ]{{{0}}})?[^ ]+", maxLength));
		var displayText = match.Groups[1].Length > 0 ? String.Format("{0}...", match.Groups[1]) : match.ToString();
		return String.Format("<a target=\"_blank\" href=\"{0}\">{1}</a>", match, displayText);
	}
}