The error is Undefined function or variable 'files'. files is defined inside your for loop. So if it is undefined after the loop that means your loop didn't execute. The only reason for your loop not to execute is that length(file_data) is 0, i.e. no file was found.
Morale of the story: when interacting with the filesystem (or any data outside your program control, like user input), always check that you're getting what you're expecting. In this case, do check that there is at least one file.
Note: You don't need a loop to transform the structure returned by dir into a cell array of filenames:
files = {file_data.name};
works just as well. It would also have avoided the undefined variable error in regexp (which would just have returned an empty cell array. Most likely, another error would occur later because you haven't designed your code to cope with no file being found.
