xref: /src/lib/libc/regex/engine.c

/* $NetBSD: engine.c,v 1.30 2024/01/23 15:32:54 christos Exp $ */

/*-
 * SPDX-License-Identifier: BSD-3-Clause
 *
 * Copyright (c) 1992, 1993, 1994 Henry Spencer.
 * Copyright (c) 1992, 1993, 1994
 *	The Regents of the University of California.  All rights reserved.
 *
 * This code is derived from software contributed to Berkeley by
 * Henry Spencer.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 *	@(#)engine.c	8.5 (Berkeley) 3/20/94
 */

#include <sys/cdefs.h>
#ifdef __FBSDID
__FBSDID("$FreeBSD: head/lib/libc/regex/engine.c 368358 2020-12-05 03:16:05Z kevans $");
#endif
__RCSID("$NetBSD: engine.c,v 1.30 2024/01/23 15:32:54 christos Exp $");

#include <stdbool.h>

/*
 * The matching engine and friends.  This file is #included by regexec.c
 * after suitable #defines of a variety of macros used herein, so that
 * different state representations can be used without duplicating masses
 * of code.
 */

#ifdef SNAMES
#define	stepback sstepback
#define	matcher	smatcher
#define	walk	swalk
#define	dissect	sdissect
#define	backref	sbackref
#define	step	sstep
#define	print	sprint
#define	at	sat
#define	match	smat
#endif
#ifdef LNAMES
#define	stepback lstepback
#define	matcher	lmatcher
#define	walk	lwalk
#define	dissect	ldissect
#define	backref	lbackref
#define	step	lstep
#define	print	lprint
#define	at	lat
#define	match	lmat
#endif
#ifdef MNAMES
#define	stepback mstepback
#define	matcher	mmatcher
#define	walk	mwalk
#define	dissect	mdissect
#define	backref	mbackref
#define	step	mstep
#define	print	mprint
#define	at	mat
#define	match	mmat
#endif

/* another structure passed up and down to avoid zillions of parameters */
struct match {
	struct re_guts *g;
	int eflags;
	regmatch_t *pmatch;	/* [nsub+1] (0 element unused) */
	const char *offp;	/* offsets work from here */
	const char *beginp;	/* start of string -- virtual NUL precedes */
	const char *endp;	/* end of string -- virtual NUL here */
	const char *coldp;	/* can be no match starting before here */
	const char **lastpos;	/* [nplus+1] */
	STATEVARS;
	states st;		/* current states */
	states fresh;		/* states for a fresh start */
	states tmp;		/* temporary */
	states empty;		/* empty set of states */
	mbstate_t mbs;		/* multibyte conversion state */
};

/* ========= begin header generated by ./mkh ========= */
#ifdef __cplusplus
extern "C" {
#endif

/* === engine.c === */
static int matcher(struct re_guts *g, const char *string, size_t nmatch, regmatch_t pmatch[], int eflags);
static const char *dissect(struct match *m, const char *start, const char *stop, sopno startst, sopno stopst);
static const char *backref(struct match *m, const char *start, const char *stop, sopno startst, sopno stopst, sopno lev, int);
static const char *walk(struct match *m, const char *start, const char *stop, sopno startst, sopno stopst, bool fast);
static states step(struct re_guts *g, sopno start, sopno stop, states bef, wint_t ch, states aft, int sflags);
#define MAX_RECURSION	100
#define	BOL	(OUT-1)
#define	EOL	(BOL-1)
#define	BOLEOL	(BOL-2)
#define	NOTHING	(BOL-3)
#define	BOW	(BOL-4)
#define	EOW	(BOL-5)
#define	BADCHAR	(BOL-6)
#define	NWBND	(BOL-7)
#define	NONCHAR(c)	((c) <= OUT)
/* sflags */
#define	SBOS	0x0001
#define	SEOS	0x0002

#ifdef REDEBUG
static void print(struct match *m, const char *caption, states st, int ch, FILE *d);
#endif
#ifdef REDEBUG
static void at(struct match *m, const char *title, const char *start, const char *stop, sopno startst, sopno stopst);
#endif
#ifdef REDEBUG
static const char *pchar(int ch);
#endif

#ifdef __cplusplus
}
#endif
/* ========= end header generated by ./mkh ========= */

#ifdef REDEBUG
#define	SP(t, s, c)	print(m, t, s, c, stdout)
#define	AT(t, p1, p2, s1, s2)	at(m, t, p1, p2, s1, s2)
#define	NOTE(str)	{ if (m->eflags&REG_TRACE) printf("=%s\n", (str)); }
#else
#define	SP(t, s, c)	/* nothing */
#define	AT(t, p1, p2, s1, s2)	/* nothing */
#define	NOTE(s)	/* nothing */
#endif

/*
 * Given a multibyte string pointed to by start, step back nchar characters
 * from current position pointed to by cur.
 */
static const char *
stepback(const char *start, const char *cur, int nchar)
{
#ifdef NLS
	const char *ret;
	size_t wc, mbc;
	mbstate_t mbs;
	size_t clen;

	if (MB_CUR_MAX == 1)
		goto out;

	ret = cur;
	for (wc = nchar; wc > 0; wc--) {
		for (mbc = 1; mbc <= MB_CUR_MAX; mbc++) {
			if ((ret - mbc) < start)
				return (NULL);
			memset(&mbs, 0, sizeof(mbs));
			clen = mbrtowc(NULL, ret - mbc, mbc, &mbs);
			if (clen != (size_t)-1 && clen != (size_t)-2)
				break;
		}
		if (mbc > MB_CUR_MAX)
			return (NULL);
		ret -= mbc;
	}

	return (ret);
out:
#endif
	return (cur - nchar) > start ? cur - nchar : NULL;
}

/*
 - matcher - the actual matching engine
 == static int matcher(struct re_guts *g, const char *string, \
 ==	size_t nmatch, regmatch_t pmatch[], int eflags);
 */
static int			/* 0 success, REG_NOMATCH failure */
matcher(struct re_guts *g,
	const char *string,
	size_t nmatch,
	regmatch_t pmatch[],
	int eflags)
{
	const char *endp;
	size_t i;
	struct match mv;
	struct match *m = &mv;
	const char *dp = NULL;
	const sopno gf = g->firststate+1;	/* +1 for OEND */
	const sopno gl = g->laststate;
	const char *start;
	const char *stop;
	/* Boyer-Moore algorithms variables */
	const char *pp;
	size_t cj, mj;
	const char *mustfirst;
	const char *mustlast;
	size_t *matchjump;
	size_t *charjump;
	int error = 0;

	_DIAGASSERT(g != NULL);
	_DIAGASSERT(string != NULL);
	/* pmatch checked below */

	/* simplify the situation where possible */
	if (g->cflags&REG_NOSUB)
		nmatch = 0;
	if (eflags&REG_STARTEND) {
		_DIAGASSERT(pmatch != NULL);
		start = string + (size_t)pmatch[0].rm_so;
		stop = string + (size_t)pmatch[0].rm_eo;
	} else {
		start = string;
		stop = start + strlen(start);
	}
	if (stop < start)
		return(REG_INVARG);

	/* prescreening; this does wonders for this rather slow code */
	if (g->must != NULL) {
		if (g->charjump != NULL && g->matchjump != NULL) {
			mustfirst = g->must;
			mustlast = g->must + g->mlen - 1;
			charjump = g->charjump;
			matchjump = g->matchjump;
			pp = mustlast;
			for (dp = start+g->mlen-1; dp < stop;) {
				/* Fast skip non-matches */
				while (dp < stop && charjump[(int)*dp])
					dp += charjump[(int)*dp];

				if (dp >= stop)
					break;

				/* Greedy matcher */
				/* We depend on not being used for
				 * for strings of length 1
				 */
				while (*--dp == *--pp && pp != mustfirst);

				if (*dp == *pp)
					break;

				/* Jump to next possible match */
				mj = matchjump[pp - mustfirst];
				cj = charjump[(int)*dp];
				dp += (cj < mj ? mj : cj);
				pp = mustlast;
			}
			if (pp != mustfirst)
				return(REG_NOMATCH);
		} else {
			for (dp = start; dp < stop; dp++)
				if (*dp == g->must[0] &&
				    (size_t)(stop - dp) >= g->mlen &&
				    memcmp(dp, g->must, (size_t)g->mlen) == 0)
					break;
			if (dp == stop)		/* we didn't find g->must */
				return(REG_NOMATCH);
		}
	}

	/* match struct setup */
	m->g = g;
	m->eflags = eflags;
	m->pmatch = NULL;
	m->lastpos = NULL;
	m->offp = string;
	m->beginp = start;
	m->endp = stop;
	STATESETUP(m, 4);
	SETUP(m->st);
	SETUP(m->fresh);
	SETUP(m->tmp);
	SETUP(m->empty);
	CLEAR(m->empty);
	ZAPSTATE(&m->mbs);

	/* Adjust start according to moffset, to speed things up */
	if (dp != NULL && g->moffset > -1) {
		const char *nstart;

		nstart = stepback(start, dp, g->moffset);
		if (nstart != NULL)
			start = nstart;
	}

	SP("mloop", m->st, *start);

	/* this loop does only one repetition except for backrefs */
	for (;;) {
		endp = walk(m, start, stop, gf, gl, true);
		if (endp == NULL) {		/* a miss */
			error = REG_NOMATCH;
			goto done;
		}
		if (nmatch == 0 && !g->backrefs)
			break;		/* no further info needed */

		/* where? */
		assert(m->coldp != NULL);
		for (;;) {
			NOTE("finding start");
			endp = walk(m, m->coldp, stop, gf, gl, false);
			if (endp != NULL)
				break;
			assert(m->coldp < m->endp);
			m->coldp += XMBRTOWC(NULL, m->coldp,
			    (size_t)(m->endp - m->coldp), &m->mbs, 0);
		}
		if (nmatch == 1 && !g->backrefs)
			break;		/* no further info needed */

		/* oh my, he wants the subexpressions... */
		if (m->pmatch == NULL)
			m->pmatch = (regmatch_t *)malloc((m->g->nsub + 1) *
							sizeof(regmatch_t));
		if (m->pmatch == NULL) {
			error = REG_ESPACE;
			goto done;
		}
		for (i = 1; i <= m->g->nsub; i++)
			m->pmatch[i].rm_so = m->pmatch[i].rm_eo = -1;
		if (!g->backrefs && !(m->eflags&REG_BACKR)) {
			NOTE("dissecting");
			dp = dissect(m, m->coldp, endp, gf, gl);
		} else {
			if (g->nplus > 0 && m->lastpos == NULL)
				m->lastpos = malloc((g->nplus+1) *
						sizeof(const char *));
			if (g->nplus > 0 && m->lastpos == NULL) {
				error = REG_ESPACE;
				goto done;
			}
			NOTE("backref dissect");
			dp = backref(m, m->coldp, endp, gf, gl, (sopno)0, 0);
		}
		if (dp != NULL)
			break;

		/* uh-oh... we couldn't find a subexpression-level match */
		assert(g->backrefs);	/* must be back references doing it */
		assert(g->nplus == 0 || m->lastpos != NULL);
		for (;;) {
			if (dp != NULL || endp <= m->coldp)
				break;		/* defeat */
			NOTE("backoff");
			endp = walk(m, m->coldp, endp-1, gf, gl, false);
			if (endp == NULL)
				break;		/* defeat */
			/* try it on a shorter possibility */
#ifndef NDEBUG
			for (i = 1; i <= m->g->nsub; i++) {
				assert(m->pmatch[i].rm_so == (regoff_t)-1);
				assert(m->pmatch[i].rm_eo == (regoff_t)-1);
			}
#endif
			NOTE("backoff dissect");
			dp = backref(m, m->coldp, endp, gf, gl, (sopno)0, 0);
		}
		assert(dp == NULL || dp == endp);
		if (dp != NULL)		/* found a shorter one */
			break;

		/* despite initial appearances, there is no match here */
		NOTE("false alarm");
		/* recycle starting later */
		start = m->coldp + XMBRTOWC(NULL, m->coldp,
		    (size_t)(stop - m->coldp), &m->mbs, 0);
		assert(start <= stop);
	}

	/* fill in the details if requested */
	if (nmatch > 0) {
		_DIAGASSERT(pmatch != NULL);
		pmatch[0].rm_so = m->coldp - m->offp;
		pmatch[0].rm_eo = endp - m->offp;
	}
	if (nmatch > 1) {
		assert(m->pmatch != NULL);
		for (i = 1; i < nmatch; i++)
			if (i <= m->g->nsub)
				pmatch[i] = m->pmatch[i];
			else {
				pmatch[i].rm_so = (regoff_t)-1;
				pmatch[i].rm_eo = (regoff_t)-1;
			}
	}

done:
	if (m->pmatch != NULL) {
		free(m->pmatch);
		m->pmatch = NULL;
	}
	if (m->lastpos != NULL) {
		free(__UNCONST(m->lastpos));
		m->lastpos = NULL;
	}
	STATETEARDOWN(m);
	return error;
}

/*
 - dissect - figure out what matched what, no back references
 == static const char *dissect(struct match *m, const char *start, \
 ==	const char *stop, sopno startst, sopno stopst);
 */
static const char *		/* == stop (success) always */
dissect(
	struct match *m,
	const char *start,
	const char *stop,
	sopno startst,
	sopno stopst)
{
	int i;
	sopno ss;		/* start sop of current subRE */
	sopno es;		/* end sop of current subRE */
	const char *sp;		/* start of string matched by it */
	const char *stp;	/* string matched by it cannot pass here */
	const char *rest;	/* start of rest of string */
	const char *tail;	/* string unmatched by rest of RE */
	sopno ssub;		/* start sop of subsubRE */
	sopno esub;		/* end sop of subsubRE */
	const char *ssp;	/* start of string matched by subsubRE */
	const char *sep;	/* end of string matched by subsubRE */
	const char *oldssp;	/* previous ssp */
	const char *dp __unused;

	_DIAGASSERT(m != NULL);
	_DIAGASSERT(start != NULL);
	_DIAGASSERT(stop != NULL);

	AT("diss", start, stop, startst, stopst);
	sp = start;
	for (ss = startst; ss < stopst; ss = es) {
		/* identify end of subRE */
		es = ss;
		switch (OP(m->g->strip[es])) {
		case OPLUS_:
		case OQUEST_:
			es += OPND(m->g->strip[es]);
			break;
		case OCH_:
			while (OP(m->g->strip[es]) != O_CH)
				es += OPND(m->g->strip[es]);
			break;
		}
		es++;

		/* figure out what it matched */
		switch (OP(m->g->strip[ss])) {
		case OEND:
			assert(nope);
			break;
		case OCHAR:
			sp += XMBRTOWC(NULL, sp, (size_t)(stop - start),
			    &m->mbs, 0);
			break;
		case OBOL:
		case OEOL:
		case OBOW:
		case OEOW:
		case OBOS:
		case OEOS:
		case OWBND:
		case ONWBND:
			break;
		case OANY:
		case OANYOF:
			sp += XMBRTOWC(NULL, sp, (size_t)(stop - start),
			    &m->mbs, 0);
			break;
		case OBACK_:
		case O_BACK:
			assert(nope);
			break;
		/* cases where length of match is hard to find */
		case OQUEST_:
			stp = stop;
			for (;;) {
				/* how long could this one be? */
				rest = walk(m, sp, stp, ss, es, false);
				assert(rest != NULL);	/* it did match */
				/* could the rest match the rest? */
				tail = walk(m, rest, stop, es, stopst, false);
				if (tail == stop)
					break;		/* yes! */
				/* no -- try a shorter match for this one */
				stp = rest - 1;
				assert(stp >= sp);	/* it did work */
			}
			ssub = ss + 1;
			esub = es - 1;
			/* did innards match? */
			if (walk(m, sp, rest, ssub, esub, false) != NULL) {
				dp = dissect(m, sp, rest, ssub, esub);
				assert(dp == rest);
			} else		/* no */
				assert(sp == rest);
			sp = rest;
			break;
		case OPLUS_:
			stp = stop;
			for (;;) {
				/* how long could this one be? */
				rest = walk(m, sp, stp, ss, es, false);
				assert(rest != NULL);	/* it did match */
				/* could the rest match the rest? */
				tail = walk(m, rest, stop, es, stopst, false);
				if (tail == stop)
					break;		/* yes! */
				/* no -- try a shorter match for this one */
				stp = rest - 1;
				assert(stp >= sp);	/* it did work */
			}
			ssub = ss + 1;
			esub = es - 1;
			ssp = sp;
			oldssp = ssp;
			for (;;) {	/* find last match of innards */
				sep = walk(m, ssp, rest, ssub, esub, false);
				if (sep == NULL || sep == ssp)
					break;	/* failed or matched null */
				oldssp = ssp;	/* on to next try */
				ssp = sep;
			}
			if (sep == NULL) {
				/* last successful match */
				sep = ssp;
				ssp = oldssp;
			}
			assert(sep == rest);	/* must exhaust substring */
			assert(walk(m, ssp, sep, ssub, esub, false) == rest);
			dp = dissect(m, ssp, sep, ssub, esub);
			assert(dp == sep);
			sp = rest;
			break;
		case OCH_:
			stp = stop;
			for (;;) {
				/* how long could this one be? */
				rest = walk(m, sp, stp, ss, es, false);
				assert(rest != NULL);	/* it did match */
				/* could the rest match the rest? */
				tail = walk(m, rest, stop, es, stopst, false);
				if (tail == stop)
					break;		/* yes! */
				/* no -- try a shorter match for this one */
				stp = rest - 1;
				assert(stp >= sp);	/* it did work */
			}
			ssub = ss + 1;
			esub = ss + OPND(m->g->strip[ss]) - 1;
			assert(OP(m->g->strip[esub]) == OOR1);
			for (;;) {	/* find first matching branch */
				if (walk(m, sp, rest, ssub, esub, false) == rest)
					break;	/* it matched all of it */
				/* that one missed, try next one */
				assert(OP(m->g->strip[esub]) == OOR1);
				esub++;
				assert(OP(m->g->strip[esub]) == OOR2);
				ssub = esub + 1;
				esub += OPND(m->g->strip[esub]);
				if (OP(m->g->strip[esub]) == OOR2)
					esub--;
				else
					assert(OP(m->g->strip[esub]) == O_CH);
			}
			dp = dissect(m, sp, rest, ssub, esub);
			assert(dp == rest);
			sp = rest;
			break;
		case O_PLUS:
		case O_QUEST:
		case OOR1:
		case OOR2:
		case O_CH:
			assert(nope);
			break;
		case OLPAREN:
			i = OPND(m->g->strip[ss]);
			assert(0 < i && i <= m->g->nsub);
			m->pmatch[i].rm_so = sp - m->offp;
			break;
		case ORPAREN:
			i = OPND(m->g->strip[ss]);
			assert(0 < i && i <= m->g->nsub);
			m->pmatch[i].rm_eo = sp - m->offp;
			break;
		default:		/* uh oh */
			assert(nope);
			break;
		}
	}

	assert(sp == stop);
	return(sp);
}

#define	ISBOW(m, sp)					\
    (sp < m->endp && ISWORD(*sp) &&			\
    ((sp == m->beginp && !(m->eflags&REG_NOTBOL)) ||	\
    (sp > m->offp && !ISWORD(*(sp-1)))))
#define	ISEOW(m, sp)					\
    (((sp == m->endp && !(m->eflags&REG_NOTEOL)) ||	\
    (sp < m->endp && *sp == '\n' &&			\
    (m->g->cflags&REG_NEWLINE)) ||			\
    (sp < m->endp && !ISWORD(*sp)) ) &&			\
    (sp > m->beginp && ISWORD(*(sp-1))))		\

/*
 - backref - figure out what matched what, figuring in back references
 == static const char *backref(struct match *m, const char *start, \
 ==	const char *stop, sopno startst, sopno stopst, sopno lev);
 */
static const char *		/* == stop (success) or NULL (failure) */
backref(
	struct match *m,
	const char *start,
	const char *stop,
	sopno startst,
	sopno stopst,
	sopno lev,		/* PLUS nesting level */
	int rec)
{
	int i;
	sopno ss;		/* start sop of current subRE */
	const char *sp;		/* start of string matched by it */
	sopno ssub;		/* start sop of subsubRE */
	sopno esub;		/* end sop of subsubRE */
	const char *ssp;	/* start of string matched by subsubRE */
	const char *dp;
	size_t len;
	int hard;
	sop s;
	regoff_t offsave;
	cset *cs;
	wint_t wc;

	_DIAGASSERT(m != NULL);
	_DIAGASSERT(start != NULL);
	_DIAGASSERT(stop != NULL);

	AT("back", start, stop, startst, stopst);
	sp = start;

	/* get as far as we can with easy stuff */
	hard = 0;
	for (ss = startst; !hard && ss < stopst; ss++)
		switch (OP(s = m->g->strip[ss])) {
		case OCHAR:
			if (sp == stop)
				return(NULL);
			sp += XMBRTOWC(&wc, sp, (size_t)(stop - sp),
			    &m->mbs, BADCHAR);
			if (wc != (wint_t)OPND(s))
				return(NULL);
			break;
		case OANY:
			if (sp == stop)
				return(NULL);
			sp += XMBRTOWC(&wc, sp, (size_t)(stop - sp),
			    &m->mbs, BADCHAR);
			if (wc == BADCHAR)
				return (NULL);
			break;
		case OANYOF:
			if (sp == stop)
				return (NULL);
			cs = &m->g->sets[OPND(s)];
			sp += XMBRTOWC(&wc, sp, (size_t)(stop - sp),
			    &m->mbs, BADCHAR);
			if (wc == BADCHAR || !CHIN(cs, wc))
				return(NULL);
			break;
		case OBOS:
			if (sp == m->beginp && (m->eflags & REG_NOTBOL) == 0)
				{ /* yes */ }
			else
				return(NULL);
			break;
		case OEOS:
			if (sp == m->endp && (m->eflags & REG_NOTEOL) == 0)
				{ /* yes */ }
			else
				return(NULL);
			break;
		case OBOL:
			if ((sp == m->beginp && !(m->eflags&REG_NOTBOL)) ||
			    (sp > m->offp && sp < m->endp &&
			    *(sp-1) == '\n' && (m->g->cflags&REG_NEWLINE)))
				{ /* yes */ }
			else
				return(NULL);
			break;
		case OEOL:
			if ( (sp == m->endp && !(m->eflags&REG_NOTEOL)) ||
					(sp < m->endp && *sp == '\n' &&
						(m->g->cflags&REG_NEWLINE)) )
				{ /* yes */ }
			else
				return(NULL);
			break;
		case OWBND:
			if (ISBOW(m, sp) || ISEOW(m, sp))
				{ /* yes */ }
			else
				return(NULL);
			break;
		case ONWBND:
			if (((sp == m->beginp) && !ISWORD(*sp)) ||
			    (sp == m->endp && !ISWORD(*(sp - 1))))
				{ /* yes, beginning/end of subject */ }
			else if (ISWORD(*(sp - 1)) == ISWORD(*sp))
				{ /* yes, beginning/end of subject */ }
			else
				return(NULL);
			break;
		case OBOW:
			if (ISBOW(m, sp))
				{ /* yes */ }
			else
				return(NULL);
			break;
		case OEOW:
			if (ISEOW(m, sp))
				{ /* yes */ }
			else
				return(NULL);
			break;
		case O_QUEST:
			break;
		case OOR1:	/* matches null but needs to skip */
			ss++;
			s = m->g->strip[ss];
			do {
				assert(OP(s) == OOR2);
				ss += OPND(s);
			} while (OP(s = m->g->strip[ss]) != O_CH);
			/* note that the ss++ gets us past the O_CH */
			break;
		default:	/* have to make a choice */
			hard = 1;
			break;
		}
	if (!hard) {		/* that was it! */
		if (sp != stop)
			return(NULL);
		return(sp);
	}
	ss--;			/* adjust for the for's final increment */

	/* the hard stuff */
	AT("hard", sp, stop, ss, stopst);
	s = m->g->strip[ss];
	switch (OP(s)) {
	case OBACK_:		/* the vilest depths */
		i = OPND(s);
		assert(0 < i && i <= m->g->nsub);
		if (m->pmatch[i].rm_eo == -1)
			return(NULL);
		assert(m->pmatch[i].rm_so != -1);
		len = (size_t)(m->pmatch[i].rm_eo - m->pmatch[i].rm_so);
		if (len == 0 && rec++ > MAX_RECURSION)
			return(NULL);
		assert(stop - m->beginp >= len);
		if (sp > stop - len)
			return(NULL);	/* not enough left to match */
		ssp = m->offp + (size_t)m->pmatch[i].rm_so;
		if (memcmp(sp, ssp, len) != 0)
			return(NULL);
		while (m->g->strip[ss] != SOP(O_BACK, i))
			ss++;
		return(backref(m, sp+len, stop, ss+1, stopst, lev, rec));
	case OQUEST_:		/* to null or not */
		dp = backref(m, sp, stop, ss+1, stopst, lev, rec);
		if (dp != NULL)
			return(dp);	/* not */
		return(backref(m, sp, stop, ss+OPND(s)+1, stopst, lev, rec));
	case OPLUS_:
		assert(m->lastpos != NULL);
		assert(lev+1 <= m->g->nplus);
		m->lastpos[lev+1] = sp;
		return(backref(m, sp, stop, ss+1, stopst, lev+1, rec));
	case O_PLUS:
		if (sp == m->lastpos[lev])	/* last pass matched null */
			return(backref(m, sp, stop, ss+1, stopst, lev-1, rec));
		/* try another pass */
		m->lastpos[lev] = sp;
		dp = backref(m, sp, stop, ss-OPND(s)+1, stopst, lev, rec);
		if (dp == NULL)
			return(backref(m, sp, stop, ss+1, stopst, lev-1, rec));
		else
			return(dp);
	case OCH_:		/* find the right one, if any */
		ssub = ss + 1;
		esub = ss + OPND(s) - 1;
		assert(OP(m->g->strip[esub]) == OOR1);
		for (;;) {	/* find first matching branch */
			dp = backref(m, sp, stop, ssub, esub, lev, rec);
			if (dp != NULL)
				return(dp);
			/* that one missed, try next one */
			if (OP(m->g->strip[esub]) == O_CH)
				return(NULL);	/* there is none */
			esub++;
			assert(OP(m->g->strip[esub]) == OOR2);
			ssub = esub + 1;
			esub += OPND(m->g->strip[esub]);
			if (OP(m->g->strip[esub]) == OOR2)
				esub--;
			else
				assert(OP(m->g->strip[esub]) == O_CH);
		}
		/* NOTREACHED */
		break;
	case OLPAREN:		/* must undo assignment if rest fails */
		i = OPND(s);
		assert(0 < i && i <= m->g->nsub);
		offsave = m->pmatch[i].rm_so;
		m->pmatch[i].rm_so = sp - m->offp;
		dp = backref(m, sp, stop, ss+1, stopst, lev, rec);
		if (dp != NULL)
			return(dp);
		m->pmatch[i].rm_so = offsave;
		return(NULL);
	case ORPAREN:		/* must undo assignment if rest fails */
		i = OPND(s);
		assert(0 < i && i <= m->g->nsub);
		offsave = m->pmatch[i].rm_eo;
		m->pmatch[i].rm_eo = sp - m->offp;
		dp = backref(m, sp, stop, ss+1, stopst, lev, rec);
		if (dp != NULL)
			return(dp);
		m->pmatch[i].rm_eo = offsave;
		return(NULL);
	default:		/* uh oh */
		assert(nope);
		break;
	}

	/* "can't happen" */
	assert(nope);
	/* NOTREACHED */
	return "shut up gcc";
}

/*
 - walk - step through the string either quickly or slowly
 == static const char *walk(struct match *m, const char *start, \
 ==	const char *stop, sopno startst, sopno stopst, bool fast);
 */
static const char * /* where it ended, or NULL */
walk(struct match *m, const char *start, const char *stop, sopno startst,
	sopno stopst, bool fast)
{
	states st = m->st;
	states fresh = m->fresh;
	states empty = m->empty;
	states tmp = m->tmp;
	const char *p = start;
	wint_t c;
	wint_t lastc;		/* previous c */
	wint_t flagch;
	int sflags;
	const char *matchp;	/* last p at which a match ended */
	size_t i, clen;

	_DIAGASSERT(m != NULL);
	_DIAGASSERT(start != NULL);
	_DIAGASSERT(stop != NULL);

	sflags = 0;
	AT("walk", start, stop, startst, stopst);
	CLEAR(st);
	SET1(st, startst);
	SP("sstart", st, *p);
	st = step(m->g, startst, stopst, st, NOTHING, st, sflags);
	if (fast)
		ASSIGN(fresh, st);
	matchp = NULL;
	if (start == m->offp || (start == m->beginp && !(m->eflags&REG_NOTBOL)))
		c = OUT;
	else {
		/*
		 * XXX Wrong if the previous character was multi-byte.
		 * Newline never is (in encodings supported by FreeBSD),
		 * so this only breaks the ISWORD tests below.
		 */
		c = (uch)*(start - 1);
	}
	for (;;) {
		/* next character */
		lastc = c;
		sflags = 0;
		if (p == m->endp) {
			c = OUT;
			clen = 0;
		} else
			clen = XMBRTOWC(&c, p, (size_t)(m->endp - p),
			    &m->mbs, BADCHAR);

		if (fast && EQ(st, fresh))
			matchp = p;

		/* is there an EOL and/or BOL between lastc and c? */
		flagch = '\0';
		i = 0;
		if ( (lastc == '\n' && m->g->cflags&REG_NEWLINE) ||
				(lastc == OUT && !(m->eflags&REG_NOTBOL)) ) {
			flagch = BOL;
			i = m->g->nbol;
		}
		if ( (c == '\n' && m->g->cflags&REG_NEWLINE) ||
				(c == OUT && !(m->eflags&REG_NOTEOL)) ) {
			flagch = (flagch == BOL) ? BOLEOL : EOL;
			i += m->g->neol;
		}
		if (lastc == OUT && (m->eflags & REG_NOTBOL) == 0) {
			sflags |= SBOS;
			/* Step one more for BOS. */
			i++;
		}
		if (c == OUT && (m->eflags & REG_NOTEOL) == 0) {
			sflags |= SEOS;
			/* Step one more for EOS. */
			i++;
		}
		if (i != 0) {
			for (; i > 0; i--)
				st = step(m->g, startst, stopst, st, flagch, st,
				    sflags);
			SP("sboleol", st, c);
		}

		/* how about a word boundary? */
		if ( (flagch == BOL || (lastc != OUT && !ISWORD(lastc))) &&
					(c != OUT && ISWORD(c)) ) {
			flagch = BOW;
		}
		if ( (lastc != OUT && ISWORD(lastc)) &&
				(flagch == EOL || (c != OUT && !ISWORD(c))) ) {
			flagch = EOW;
		}
		if (flagch == BOW || flagch == EOW) {
			st = step(m->g, startst, stopst, st, flagch, st, sflags);
			SP("sboweow", st, c);
		}
		if (lastc != OUT && c != OUT &&
		    ISWORD(lastc) == ISWORD(c)) {
			flagch = NWBND;
		} else if ((lastc == OUT && !ISWORD(c)) ||
		    (c == OUT && !ISWORD(lastc))) {
			flagch = NWBND;
		}
		if (flagch == NWBND) {
			st = step(m->g, startst, stopst, st, flagch, st, sflags);
			SP("snwbnd", st, c);
		}

		/* are we done? */
		if (ISSET(st, stopst)) {
			if (fast)
				break;
			else
				matchp = p;
		}
		if (EQ(st, empty) || p == stop || clen > (size_t)(stop - p))
			break;		/* NOTE BREAK OUT */

		/* no, we must deal with this character */
		ASSIGN(tmp, st);
		if (fast)
			ASSIGN(st, fresh);
		else
			ASSIGN(st, empty);
		assert(c != OUT);
		st = step(m->g, startst, stopst, tmp, c, st, sflags);
		SP("saft", st, c);
		assert(EQ(step(m->g, startst, stopst, st, NOTHING, st, sflags),
		    st));
		p += clen;
	}

	if (fast) {
		assert(matchp != NULL);
		m->coldp = matchp;
		if (ISSET(st, stopst))
			return (p + XMBRTOWC(NULL, p, (size_t)(stop - p),
			    &m->mbs, 0));
		else
			return (NULL);
	} else
		return (matchp);
}

/*
 - step - map set of states reachable before char to set reachable after
 == static states step(struct re_guts *g, sopno start, sopno stop, \
 ==	states bef, int ch, states aft);
 == #define	BOL	(OUT-1)
 == #define	EOL	(BOL-1)
 == #define	BOLEOL	(BOL-2)
 == #define	NOTHING	(BOL-3)
 == #define	BOW	(BOL-4)
 == #define	EOW	(BOL-5)
 == #define	BADCHAR	(BOL-6)
 == #define	NONCHAR(c)	((c) <= OUT)
 */
static states
step(struct re_guts *g,
	sopno start,		/* start state within strip */
	sopno stop,		/* state after stop state within strip */
	states bef,		/* states reachable before */
	wint_t ch,		/* character or NONCHAR code */
	states aft,		/* states already known reachable after */
	int sflags)		/* state flags */
{
	cset *cs;
	sop s;
	sopno pc;
	onestate here;		/* note, macros know this name */
	sopno look;
	int i;

	_DIAGASSERT(g != NULL);

	for (pc = start, INIT(here, pc); pc != stop; pc++, INC(here)) {
		s = g->strip[pc];
		switch (OP(s)) {
		case OEND:
			assert(pc == stop-1);
			break;
		case OCHAR:
			/* only characters can match */
			assert(!NONCHAR(ch) || ch != OPND(s));
			if (ch == (wint_t)OPND(s))
				FWD(aft, bef, 1);
			break;
		case OBOS:
			if ((ch == BOL || ch == BOLEOL) && (sflags & SBOS) != 0)
				FWD(aft, bef, 1);
			break;
		case OEOS:
			if ((ch == EOL || ch == BOLEOL) && (sflags & SEOS) != 0)
				FWD(aft, bef, 1);
			break;
		case OBOL:
			if (ch == BOL || ch == BOLEOL)
				FWD(aft, bef, 1);
			break;
		case OEOL:
			if (ch == EOL || ch == BOLEOL)
				FWD(aft, bef, 1);
			break;
		case OBOW:
			if (ch == BOW)
				FWD(aft, bef, 1);
			break;
		case OEOW:
			if (ch == EOW)
				FWD(aft, bef, 1);
			break;
		case OWBND:
			if (ch == BOW || ch == EOW)
				FWD(aft, bef, 1);
			break;
		case ONWBND:
			if (ch == NWBND)
				FWD(aft, aft, 1);
			break;
		case OANY:
			if (!NONCHAR(ch))
				FWD(aft, bef, 1);
			break;
		case OANYOF:
			cs = &g->sets[OPND(s)];
			if (!NONCHAR(ch) && CHIN(cs, ch))
				FWD(aft, bef, 1);
			break;
		case OBACK_:		/* ignored here */
		case O_BACK:
			FWD(aft, aft, 1);
			break;
		case OPLUS_:		/* forward, this is just an empty */
			FWD(aft, aft, 1);
			break;
		case O_PLUS:		/* both forward and back */
			FWD(aft, aft, 1);
			i = ISSETBACK(aft, OPND(s));
			BACK(aft, aft, OPND(s));
			if (!i && ISSETBACK(aft, OPND(s))) {
				/* oho, must reconsider loop body */
				pc -= OPND(s) + 1;
				INIT(here, pc);
			}
			break;
		case OQUEST_:		/* two branches, both forward */
			FWD(aft, aft, 1);
			FWD(aft, aft, OPND(s));
			break;
		case O_QUEST:		/* just an empty */
			FWD(aft, aft, 1);
			break;
		case OLPAREN:		/* not significant here */
		case ORPAREN:
			FWD(aft, aft, 1);
			break;
		case OCH_:		/* mark the first two branches */
			FWD(aft, aft, 1);
			assert(OP(g->strip[pc+OPND(s)]) == OOR2);
			FWD(aft, aft, OPND(s));
			break;
		case OOR1:		/* done a branch, find the O_CH */
			if (ISSTATEIN(aft, here)) {
				for (look = 1;
				    OP(s = g->strip[pc+look]) != O_CH;
				    look += OPND(s))
					assert(OP(s) == OOR2);
				FWD(aft, aft, look + 1);
			}
			break;
		case OOR2:		/* propagate OCH_'s marking */
			FWD(aft, aft, 1);
			if (OP(g->strip[pc+OPND(s)]) != O_CH) {
				assert(OP(g->strip[pc+OPND(s)]) == OOR2);
				FWD(aft, aft, OPND(s));
			}
			break;
		case O_CH:		/* just empty */
			FWD(aft, aft, 1);
			break;
		default:		/* ooooops... */
			assert(nope);
			break;
		}
	}

	return(aft);
}

#ifdef REDEBUG
/*
 - print - print a set of states
 == #ifdef REDEBUG
 == static void print(struct match *m, const char *caption, states st, \
 ==	int ch, FILE *d);
 == #endif
 */
static void
print(struct match *m,
	const char *caption,
	states st,
	int ch,
	FILE *d)
{
	struct re_guts *g = m->g;
	sopno i;
	int first = 1;

	_DIAGASSERT(m != NULL);
	_DIAGASSERT(caption != NULL);

	if (!(m->eflags&REG_TRACE))
		return;

	_DIAGASSERT(d != NULL);

	fprintf(d, "%s", caption);
	if (ch != '\0')
		fprintf(d, " %s", pchar(ch));
	for (i = 0; i < g->nstates; i++)
		if (ISSET(st, i)) {
			fprintf(d, "%s%lu", (first) ? "\t" : ", ", i);
			first = 0;
		}
	fprintf(d, "\n");
}

/*
 - at - print current situation
 == #ifdef REDEBUG
 == static void at(struct match *m, const char *title, const char *start, \
 ==			 const char *stop, sopno startst, sopno stopst);
 == #endif
 */
static void
at(	struct match *m,
	const char *title,
	const char *start,
	const char *stop,
	sopno startst,
	sopno stopst)
{

	_DIAGASSERT(m != NULL);
	_DIAGASSERT(title != NULL);
	_DIAGASSERT(start != NULL);
	_DIAGASSERT(stop != NULL);

	if (!(m->eflags&REG_TRACE))
		return;

	printf("%s %s-", title, pchar(*start));
	printf("%s ", pchar(*stop));
	printf("%ld-%ld\n", (long)startst, (long)stopst);
}

#ifndef PCHARDONE
#define	PCHARDONE	/* never again */
/*
 - pchar - make a character printable
 == #ifdef REDEBUG
 == static const char *pchar(int ch);
 == #endif
 *
 * Is this identical to regchar() over in debug.c?  Well, yes.  But a
 * duplicate here avoids having a debugging-capable regexec.o tied to
 * a matching debug.o, and this is convenient.  It all disappears in
 * the non-debug compilation anyway, so it doesn't matter much.
 */
static const char *		/* -> representation */
pchar(int ch)
{
	static char pbuf[10];

	if (isprint((uch)ch) || ch == ' ')
		snprintf(pbuf, sizeof(pbuf), "%c", ch);
	else
		snprintf(pbuf, sizeof(pbuf), "\\%o", ch);
	return(pbuf);
}
#endif
#endif

#undef	stepback
#undef	matcher
#undef	walk
#undef	dissect
#undef	backref
#undef	step
#undef	print
#undef	at
#undef	match