The MATLAB function to retrieve the extension and filename is fileparts and to rebuild a new one is fullfile
But, in this case you can shortcut the operation by
filenames=strrep(meta,'csv','mat');
which will leave you with the original cellstr() array -- if you really want a string array, wrap it around the above.
As for the original Q? itself about the extra single quote, that must be in the initial data, there's nothing in the above code that would introduce such an artifact. So, I'd recomend going back and figuring out where it comes from and fix the problem there.
If there's some external part of the process that is doing it and you can't for some reason fix it, then
filenames=strrep(filenames,"'",'');
will fix up the present problem, but it would still be better to fix it before/where it happens instead.
The introduction of the string() class, while in a lot of ways redundant functionality of the cellstr() with not all that much advantage has the side benefit of being able to write the above to wrap the single quote inside a string instead of having to double-up the ticks to escape them in char() strings that gets to be very difficult to get right/read.
