Version
5.4.0 of GNU awk
(gawk) has been released. This is a major release with a change in
gawk's default regular-expression matcher: it now uses MinRX
as the default regular-expression engine.

This matcher is fully POSIX compliant, which the current GNU matchers
are not. In particular it follows POSIX rules for finding the longest
leftmost submatches.  It is also more strict as to regular expression
syntax, but primarily in a few corner cases that normal, correct,
regular expression usage should not encounter.

Because regular expression matching is such a fundamental part of
awk/gawk, the original GNU matchers are still included in gawk.  In order
to use them, give a value to the GAWK_GNU_MATCHERS environment variable
before invoking gawk.

[...] The original GNU matchers will eventually be removed from
gawk.  So, please take the time to notice and report any issues in the
MinRX matcher, so that they can be ironed out sooner rather than later.

See the release announcement for additional changes.

https://lwn.net/Articles/1060050/
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