Android : Allowing specific characters using input filter for edittext in android

I have an edittext for which i have set an input filter as follows:

filter_username = new InputFilter() { 
         public CharSequence filter(CharSequence source, int start, int end, Spanned dest, int dstart, int dend) {
                boolean keepOriginal = true;
                StringBuilder sb = new StringBuilder(end - start);
                for (int i = start; i < end; i++) {

                    char c = source.charAt(i);

                    if (isCharAllowed2(c)) // put your condition here
                        sb.append(c);
                    else
                        keepOriginal = false;
                }
                if (keepOriginal)
                    return null;
                else {
                    if (source instanceof Spanned) {
                        SpannableString sp = new SpannableString(sb);
                        TextUtils.copySpansFrom((Spanned) source, start, sb.length(), null, sp, 0);
                        return sp;
                    } else {
                        return sb;
                    }           
                }
            }

            private boolean isCharAllowed2(char c) {
                return Character.isLetterOrDigit(c);

            }

}; 

txtusername.setFilters(new InputFilter[]{filter_username});

The issue is i want to do the following changes to the above filter:

1) The first character should not be a digit 2) Underscore and dot are the only characters allowed

Can you tell me how to modify the above filter in order to suit my requirements?

EDIT: I figured out the special characters part by the following changes:

private boolean isCharAllowed2(char c) {
                return Character.isLetterOrDigit(c)||c=='_'||c=='.';

            }

How do i prevent the first character from being a digit or a period?