So where are glibc regexps used in the most typical practical cases? At least grep does not use them (at least not in unmodified form) as a comment in the bug

So where are glibc regexps used in the most typical practical cases? At least grep does not use them (at least not in unmodified form) as a comment in the bug report notes. I think changing such behavior is unacceptable, regardless of what the spec says. Here I would to propose the API stability promise of the Linux kernel: We don't break existing programs. If somebody notes that the previous behavior is not correct according to the spec a new posixly correct mode can be introduced. But it should not magically become default.