Remove all whitespace from C# string with regex

Instead of a RegEx use Replace for something that simple:

LastName = LastName.Replace(" ", String.Empty);

Regex.Replace does not modify its first argument (recall that strings are immutable in .NET) so the call

Regex.Replace(LastName, @"\s+", "");

leaves the LastName string unchanged. You need to call it like this:

LastName = Regex.Replace(LastName, @"\s+", "");

All three of your regular expressions would have worked. However, the first regex would remove all plus characters as well, which I imagine would be unintentional.

No need for regex. This will also remove tabs, newlines etc

var newstr = String.Join("",str.Where(c=>!char.IsWhiteSpace(c)));

WhiteSpace chars : 0009 , 000a , 000b , 000c , 000d , 0020 , 0085 , 00a0 , 1680 , 180e , 2000 , 2001 , 2002 , 2003 , 2004 , 2005 , 2006 , 2007 , 2008 , 2009 , 200a , 2028 , 2029 , 202f , 205f , 3000.