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This README documents GNU e?grep version 1.6. All bugs reported for
previous versions have been fixed.
See the file INSTALL for compilation and installation instructions.
Send bug reports to bug-gnu-utils@prep.ai.mit.edu.
GNU e?grep is provided "as is" with no warranty. The exact terms
under which you may use and (re)distribute this program are detailed
in the GNU General Public License, in the file COPYING.
GNU e?grep is based on a fast lazy-state deterministic matcher (about
twice as fast as stock Unix egrep) hybridized with a Boyer-Moore-Gosper
search for a fixed string that eliminates impossible text from being
considered by the full regexp matcher without necessarily having to
look at every character. The result is typically many times faster
than Unix grep or egrep. (Regular expressions containing backreferencing
may run more slowly, however.)
GNU e?grep is brought to you by the efforts of several people:
Mike Haertel wrote the deterministic regexp code and the bulk
of the program.
James A. Woods is responsible for the hybridized search strategy
of using Boyer-Moore-Gosper fixed-string search as a filter
before calling the general regexp matcher.
Arthur David Olson contributed code that finds fixed strings for
the aforementioned BMG search for a large class of regexps.
Richard Stallman wrote the backtracking regexp matcher that is
used for \<digit> backreferences, as well as the getopt that
is provided for 4.2BSD sites. The backtracking matcher was
originally written for GNU Emacs.
D. A. Gwyn wrote the C alloca emulation that is provided so
System V machines can run this program. (Alloca is used only
by RMS' backtracking matcher, and then only rarely, so there
is no loss if your machine doesn't have a "real" alloca.)
Scott Anderson and Henry Spencer designed the regression tests
used in the "regress" script.
Paul Placeway wrote the manual page, based on this README.
If you are interested in improving this program, you may wish to try
any of the following:
1. Replace the fast search loop with a faster search loop.
There are several things that could be improved, the most notable
of which would be to calculate a minimal delta2 to use.
2. Make backreferencing \<digit> faster. Right now, backreferencing is
handled by calling the Emacs backtracking matcher to verify the partial
match. This is slow; if the DFA routines could handle backreferencing
themselves a speedup on the order of three to four times might occur
in those cases where the backtracking matcher is called to verify nearly
every line. Also, some portability problems due to the inclusion of the
emacs matcher would be solved because it could then be eliminated.
Note that expressions with backreferencing are not true regular
expressions, and thus are not equivalent to any DFA. So this is hard.
3. Handle POSIX style regexps. I'm not sure if this could be called an
improvement; some of the things on regexps in the POSIX draft I have
seen are pretty sickening. But it would be useful in the interests of
conforming to the standard.
4. Replace the main driver program grep.c with the much cleaner main driver
program used in GNU fgrep.
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