Aleksey Cheusov:

Hi.
As you may know, mawk's internal regular expression engine
doesn't support character classes and therefore is not conformant
to POSIX regexp.

I've written patch which builds mawk with external regexp engine.
There are lots of different regexp engines world-wide.
You can use any of them
On most modern systems system-wide regexp engine also conforms to POSIX.
As a result mawk conforms to POSIX regexp, if you build
it with appropriate regexp engine like this.

# regexec/regcomp functions from libc will be used
./configure && make

# TRE implementation will be used
CFLAGS='-O3 -I/usr/include/tre' LDFLAGS='-ltre' ./configure && make

Whether UTF-8 support is provided or not
also depends on external regexp library.

If you find bugs in it, let me know.

The patch is here.

Do not forget to run antoconf
to create new 'configure' script.