This is so easy. You can do it in one line with strrep(). See full demo:
% This is what you're starting with.
currentFolder = 'C:\Users\donald\Documents\MATLAB\RD2\A2\RD'
% Append a trailing slash so we don't convert RD2 as well as RD.
% Simply change \RD\ to \PE\ using strrep().
% HERE IS THE ONE SINGLE LINE OF CODE YOU WANT:
desiredFolder = strrep(upper([currentFolder '\']), '\RD\', '\PE\')
% If the folder doesn't exist, create it.
if ~exist(desiredFolder, 'dir')
% mkdir(desiredFolder);
end
% Build the full file name with fullfile.
fullFileName = fullfile(desiredFolder, 'pe1.mat')
Alternatively, you can just get the parent folder of RD and then append the folder name you want and the file name:
lastSlashPosition = find(currentFolder == '\', 1, 'last')
parentFolder = currentFolder(1:lastSlashPosition-1)
fullFileName = sprintf('%s/PE/pe1.mat', parentFolder)
% Note: forward slashes work just fine in Windows.
