You can easily match those lines using a regular expression. Something like this:
fmt = sprintf('\\s*%c\\s*=\\s*(\\d+\\.\\d+)\\s*mm,','XYZ')
fmt = sprintf('^%s\\s*$',fmt)
str = fileread('values.txt');
spl = regexp(str,'[\n\r]+','split');
tmp = regexp(spl,fmt,'tokens','once');
idx = find(~cellfun('isempty',tmp));
for k = reshape(idx,1,[])
vec = str2double(tmp{k}) + 2; % add two as example data manipulation.
spl{k} = sprintf(' X = %.3f, Y = %.3f, Z = %.3f,',vec);
end
fid = fopen('newfile.txt','wt');
fprintf(fid,'%s\n',spl{:});
fclose(fid);
