The GNU extension bits in the base system are old, no longer faithful
to upstream, and surprising in some regards. Switch to documenting
WITH_GNU_GREP_COMPAT and default GNU_GREP_COMPAT to OFF in the name of
good behavior.
According to http://www.regular-expressions.info, GNU extensions:
- Add missing quantifiers to BREs: \?, \+
- Add branching to BREs: \|
- Add backreferences (\1 through \9) to EREs
- Add \w, \W, \s, and \S corresponding to :alnum:, [^[:alnum:]],
:space:, and [^[:space:]] respectively
- Add word boundaries and anchors:
\b: word boundary
\B: not word boundary
\<: Strt of word
\>: End of word
\`: Start of subject string
\': End of subject string
These extensions are still available in /usr/bin/grep by default today,
as it is still GNU grep. As part of the bsdgrep migration plan these
extensions may be added to bsdgrep's regex support if necessary.
Submitted by: Kyle Evans <kevans91 at ksu.edu>
Reviewed by: cem
Differential Revision: https://reviews.freebsd.org/D10114
