C# Regex – Add a . after every letter in a string

I am trying to add a . (a dot) after every letter in a string via means of a regex but I am struggling a little.

What I have tried so far:

public string convert(string s)
{

    s =  Regex.Replace(s, ".{1}", "$0.");

    return s;
}

The intput string contains only letters, each seperated by a space except for the last one.
Eg, the input could be "R B" or "A S"
If I use my above code the string becomes "R. .B." or "A. .S" .

I need to remove the . before each character.
The output string needs to become "R. B." or "A. S." .
How would I go about removing a . that appears before each letter? (i think it adds the extra . because of the space). Also, the string can be longer or shorter, 2 letters isnt a fixed number.

Thanks in advance.

>Solution :

Try matching on [A-Za-z], which means any single letter:

public string convert(string s)
{
    s =  Regex.Replace(s, "[A-Za-z]", "$0.");

    return s;
}