String Replace - changing invalid charactors to valid ones ?

If you have a string, say: string a = "This is a test."; then you do this: a.Replace("t","5"); and the result should be "This is a 5es5."

thanks mykonian :) so it would work something like this ? : String.Replace("a", "b"); but i get error : An object reference is required for the nonstatic field, method, or property 'string.Replace(string, string)' i am missing something here but im unsure what the reference should be ( being a newbie ;) any help would be greatful Thanks MrXs

in this case string.Replace might be better.

is regex.replace what im looking for ? if so how or is there a better way ? Thanks MrXs