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早期freebsd实现

This is a nearly-public-domain reimplementation of the V8 regexp(3) package.
It gives C programs the ability to use egrep-style regular expressions, and
does it in a much cleaner fashion than the analogous routines in SysV.

	Copyright (c) 1986 by University of Toronto.
	Written by Henry Spencer.  Not derived from licensed software.

	Permission is granted to anyone to use this software for any
	purpose on any computer system, and to redistribute it freely,
	subject to the following restrictions:

	1. The author is not responsible for the consequences of use of
		this software, no matter how awful, even if they arise
		from defects in it.

	2. The origin of this software must not be misrepresented, either
		by explicit claim or by omission.

	3. Altered versions must be plainly marked as such, and must not
		be misrepresented as being the original software.

Barring a couple of small items in the BUGS list, this implementation is
believed 100% compatible with V8.  It should even be binary-compatible,
sort of, since the only fields in a "struct regexp" that other people have
any business touching are declared in exactly the same way at the same
location in the struct (the beginning).

This implementation is *NOT* AT&T/Bell code, and is not derived from licensed
software.  Even though U of T is a V8 licensee.  This software is based on
a V8 manual page sent to me by Dennis Ritchie (the manual page enclosed
here is a complete rewrite and hence is not covered by AT&T copyright).
The software was nearly complete at the time of arrival of our V8 tape.
I haven't even looked at V8 yet, although a friend elsewhere at U of T has
been kind enough to run a few test programs using the V8 regexp(3) to resolve
a few fine points.  I admit to some familiarity with regular-expression
implementations of the past, but the only one that this code traces any
ancestry to is the one published in Kernighan & Plauger (from which this
one draws ideas but not code).

Simplistically:  put this stuff into a source directory, copy regexp.h into
/usr/include, inspect Makefile for compilation options that need changing
to suit your local environment, and then do "make r".  This compiles the
regexp(3) functions, compiles a test program, and runs a large set of
regression tests.  If there are no complaints, then put regexp.o, regsub.o,
and regerror.o into your C library, and regexp.3 into your manual-pages
directory.

Note that if you don't put regexp.h into /usr/include *before* compiling,
you'll have to add "-I." to CFLAGS before compiling.

The files are:

Makefile	instructions to make everything
regexp.3	manual page
regexp.h	header file, for /usr/include
regexp.c	source for regcomp() and regexec()
regsub.c	source for regsub()
regerror.c	source for default regerror()
regmagic.h	internal header file
try.c		source for test program
timer.c		source for timing program
tests		test list for try and timer

This implementation uses nondeterministic automata rather than the
deterministic ones found in some other implementations, which makes it
simpler, smaller, and faster at compiling regular expressions, but slower
at executing them.  In theory, anyway.  This implementation does employ
some special-case optimizations to make the simpler cases (which do make
up the bulk of regular expressions actually used) run quickly.  In general,
if you want blazing speed you're in the wrong place.  Replacing the insides
of egrep with this stuff is probably a mistake; if you want your own egrep
you're going to have to do a lot more work.  But if you want to use regular
expressions a little bit in something else, you're in luck.  Note that many
existing text editors use nondeterministic regular-expression implementations,
so you're in good company.

This stuff should be pretty portable, given appropriate option settings.
If your chars have less than 8 bits, you're going to have to change the
internal representation of the automaton, although knowledge of the details
of this is fairly localized.  There are no "reserved" char values except for
NUL, and no special significance is attached to the top bit of chars.
The string(3) functions are used a fair bit, on the grounds that they are
probably faster than coding the operations in line.  Some attempts at code
tuning have been made, but this is invariably a bit machine-specific.