gnulib/doc/posix-headers/regex.texi

@node regex.h
@section @file{regex.h}

POSIX specification:@* @url{https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/regex.h.html}

Gnulib module: regex
@mindex regex

Portability problems fixed by Gnulib:
@itemize
@item
This header file is missing on some platforms:
mingw, MSVC 14.

@item
On some systems that have this header file,
GNU extensions like @code{re_set_syntax} and @code{RE_SYNTAX_EMACS}
are not declared or supported:
FreeBSD 14.2, OpenBSD 7.6, NetBSD 10.1, macOS 15,
Minix 3.3.0, AIX 7.3, HP-UX 11, Solaris 11.4.

@item
On some systems that support GNU extensions, @code{RE_SYNTAX_EMACS} is 0
even though it should be @code{(RE_CHAR_CLASSES | RE_INTERVALS)}
to be compatible with Emacs 21 (2001) and later:
glibc 2.42, Cygwin 2.6.x.
@end itemize

Portability problems not fixed by Gnulib:
@itemize
@item
This header file is not self-contained on some platforms: it requires
@code{<sys/types.h>} to be included first.
@end itemize