f='Untitled.0001.jpg'
nf=regexprep(f,'(\S+\.)(\d+)(\.jpg)','${num2str(str2num($2))}$3')
Or
f='Untitled4.0201.jpg'
nf=regexp(f,'[1-9]\d+\>.jpg','match','once')
To rename the file
folder='C:\Users\you\Documents\MATLAB' % folder where your jpg are located
filetype=fullfile(folder,'*.jpg')
file=dir(filetype)
filename={file.name}
for k=1:numel(file)
f=filename{k}
nf=regexp(f,'[1-9]\d+\>.jpg','match','once')
move(fullfile(folder,f),fullfile(folder,nf))
end
