How to check if a Regex expression matches an entire string in c#?

I am new to regex expressions so sorry if this is a really noob question.

I have a regex expression... What I want to do is check if a string matches the regex expression in its entirety without the regex expression matching any subsets of the string.

For example...

If my regex expression is looking for a match of \sA\s*, it should return a match if the string it is comparing it to is " A " but if it compares to the string " A B" it should not return a match.

Any help would be appreciated? I code in C#.