Then what is a real lack of freedom? I could use the regex code under an invariant license;
If the license for the code did not allow modification, you could not make it implement different behavior. You would substantively lack the ability to change the functionality. That is a lack of real freedom. However, we do accept the license of TeX. This license says that you cannot modify the source file at all, but you can use a change file when you build it. Use of a change file give you the ability to change the functionality as you wish, so you have freedom to change the substantive behavior of TeX. The fact that the original TeX source is invariant is a considerable inconvenience, but only an inconvenience. And given a big, sometimes nasty, world, he might very well not be free to use it. He may be in a country, or working for someone, that will not tolerate him espousing the message in that invariant section in any way. That is possible. In the same way, he could be in a country that prohibits the functionality of the program, or issued a patent on it, or have an employer who dislikes the license conditions or the license preamble, and will not tolerate its use. Those things can happen, and they are real problems; but if the possibility were a reasons to say that the program is not free, then no program would be free. There's a difference. The GPL is full of technical requirements. But when Amnesty International releases its freedom fighting robot code under the GPL, I'm free to reuse it for my baby mulcher. You would likewise be free the GFDL-covered manual for the robot to document your baby mulcher. Inclusion of invariant sections, if any, would not stop you from making it a useful and accurate manual for the mulcher.