How to remove final space from string

Regex for this is overkill. There is no need for the performance hit for such a simple task. I would say the fastest is to check if there is a final space and then update;

private static bool CheckEndingBlank(ref string str)
{
    if (str[str.Length - 1] == ' ')
    {
        str = str.TrimEnd(' ');
        return true;
    }
    return false;
}

I am assuming here that you really do want to know if the last character is a space and want to remove the all trailing spaces.

As you already got answers, here is a few interesting links to help building and debugging RegEx.

Here is a link to RegEx documentation:
perlre - perldoc.perl.org[^]
Here is links to tools to help build RegEx and debug them:
.NET Regex Tester - Regex Storm[^]
Expresso Regular Expression Tool[^]
RegExr: Learn, Build, & Test RegEx[^]
This one show you the RegEx as a nice graph which is really helpful to understand what is doing a RegEx:
Debuggex: Online visual regex tester. JavaScript, Python, and PCRE.[^]

If you want to remove space at begin and end of a string, use the below

var str = "       Hello World!     ";
var trim = str.trim()

If you want to remove space only at the end of a string, use the below

var str = "       Hello World!     ";
var trim = str.replace(/(\s+$)/g, "")

Hope this helps you.