The regular expression itself is very large in terms of a string (18,385 characters as of this writing). Thus it is quite cumbersome and somewhat annoying to manage in almost any file, be it schema, XSLT, or the list of error messages that helpfully say must match ….

However, in terms of actual storage space, the CSS3_selector_regex_generator.perl program, its RELAX NG output, and its XSLT output all together take up less than 1.7 MiB on my filesystem. That is, they could almost fit on a 1980s 3½ inch floppy disk.