harvey/sys/src/ape/cmd/pax/regexp.c

/* $Source: /u/mark/src/pax/RCS/regexp.c,v $
 *
 * $Revision: 1.2 $
 *
 * regexp.c - regular expression matching
 *
 * DESCRIPTION
 *
 *	Underneath the reformatting and comment blocks which were added to 
 *	make it consistent with the rest of the code, you will find a
 *	modified version of Henry Specer's regular expression library.
 *	Henry's functions were modified to provide the minimal regular
 *	expression matching, as required by P1003.  Henry's code was
 *	copyrighted, and copy of the copyright message and restrictions
 *	are provided, verbatim, below:
 *
 *	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.
 *
 * 	Beware that some of this code is subtly aware of the way operator
 * 	precedence is structured in regular expressions.  Serious changes in
 * 	regular-expression syntax might require a total rethink.
 *
 * AUTHORS
 *
 *     Mark H. Colburn, NAPS International (mark@jhereg.mn.org)
 *     Henry Spencer, University of Torronto (henry@utzoo.edu)
 *
 * Sponsored by The USENIX Association for public distribution. 
 *
 * $Log:	regexp.c,v $
 * Revision 1.2  89/02/12  10:05:39  mark
 * 1.2 release fixes
 * 
 * Revision 1.1  88/12/23  18:02:32  mark
 * Initial revision
 * 
 */

/* Headers */

#include "pax.h"

#ifndef lint
static char    *Ident = "$Id: regexp.c,v 1.2 89/02/12 10:05:39 mark Exp $";
#endif


/*
 * The "internal use only" fields in regexp.h are present to pass info from
 * compile to execute that permits the execute phase to run lots faster on
 * simple cases.  They are:
 *
 * regstart	char that must begin a match; '\0' if none obvious
 * reganch	is the match anchored (at beginning-of-line only)?
 * regmust	string (pointer into program) that match must include, or NULL
 * regmlen	length of regmust string
 *
 * Regstart and reganch permit very fast decisions on suitable starting points
 * for a match, cutting down the work a lot.  Regmust permits fast rejection
 * of lines that cannot possibly match.  The regmust tests are costly enough
 * that regcomp() supplies a regmust only if the r.e. contains something
 * potentially expensive (at present, the only such thing detected is * or +
 * at the start of the r.e., which can involve a lot of backup).  Regmlen is
 * supplied because the test in regexec() needs it and regcomp() is computing
 * it anyway.
 */

/*
 * Structure for regexp "program".  This is essentially a linear encoding
 * of a nondeterministic finite-state machine (aka syntax charts or
 * "railroad normal form" in parsing technology).  Each node is an opcode
 * plus a "nxt" pointer, possibly plus an operand.  "Nxt" pointers of
 * all nodes except BRANCH implement concatenation; a "nxt" pointer with
 * a BRANCH on both ends of it is connecting two alternatives.  (Here we
 * have one of the subtle syntax dependencies:  an individual BRANCH (as
 * opposed to a collection of them) is never concatenated with anything
 * because of operator precedence.)  The operand of some types of node is
 * a literal string; for others, it is a node leading into a sub-FSM.  In
 * particular, the operand of a BRANCH node is the first node of the branch.
 * (NB this is *not* a tree structure:  the tail of the branch connects
 * to the thing following the set of BRANCHes.)  The opcodes are:
 */

/* definition	number	opnd?	meaning */
#define	END	0		/* no	End of program. */
#define	BOL	1		/* no	Match "" at beginning of line. */
#define	EOL	2		/* no	Match "" at end of line. */
#define	ANY	3		/* no	Match any one character. */
#define	ANYOF	4		/* str	Match any character in this string. */
#define	ANYBUT	5		/* str	Match any character not in this
				 * string. */
#define	BRANCH	6		/* node	Match this alternative, or the
				 * nxt... */
#define	BACK	7		/* no	Match "", "nxt" ptr points backward. */
#define	EXACTLY	8		/* str	Match this string. */
#define	NOTHING	9		/* no	Match empty string. */
#define	STAR	10		/* node	Match this (simple) thing 0 or more
				 * times. */
#define	OPEN	20		/* no	Mark this point in input as start of
				 * #n. */
 /* OPEN+1 is number 1, etc. */
#define	CLOSE	30		/* no	Analogous to OPEN. */

/*
 * Opcode notes:
 *
 * BRANCH	The set of branches constituting a single choice are hooked
 *		together with their "nxt" pointers, since precedence prevents
 *		anything being concatenated to any individual branch.  The
 *		"nxt" pointer of the last BRANCH in a choice points to the
 *		thing following the whole choice.  This is also where the
 *		final "nxt" pointer of each individual branch points; each
 *		branch starts with the operand node of a BRANCH node.
 *
 * BACK		Normal "nxt" pointers all implicitly point forward; BACK
 *		exists to make loop structures possible.
 *
 * STAR		complex '*', are implemented as circular BRANCH structures 
 *		using BACK.  Simple cases (one character per match) are 
 *		implemented with STAR for speed and to minimize recursive 
 *		plunges.
 *
 * OPEN,CLOSE	...are numbered at compile time.
 */

/*
 * A node is one char of opcode followed by two chars of "nxt" pointer.
 * "Nxt" pointers are stored as two 8-bit pieces, high order first.  The
 * value is a positive offset from the opcode of the node containing it.
 * An operand, if any, simply follows the node.  (Note that much of the
 * code generation knows about this implicit relationship.)
 *
 * Using two bytes for the "nxt" pointer is vast overkill for most things,
 * but allows patterns to get big without disasters.
 */
#define	OP(p)	(*(p))
#define	NEXT(p)	(((*((p)+1)&0377)<<8) + (*((p)+2)&0377))
#define	OPERAND(p)	((p) + 3)

/*
 * Utility definitions.
 */

#define	FAIL(m)	{ regerror(m); return(NULL); }
#define	ISMULT(c)	((c) == '*')
#define	META	"^$.[()|*\\"
#ifndef CHARBITS
#define	UCHARAT(p)	((int)*(unsigned char *)(p))
#else
#define	UCHARAT(p)	((int)*(p)&CHARBITS)
#endif

/*
 * Flags to be passed up and down.
 */
#define	HASWIDTH	01	/* Known never to match null string. */
#define	SIMPLE		02	/* Simple enough to be STAR operand. */
#define	SPSTART		04	/* Starts with * */
#define	WORST		0	/* Worst case. */

/*
 * Global work variables for regcomp().
 */
static char    *regparse;	/* Input-scan pointer. */
static int      regnpar;	/* () count. */
static char     regdummy;
static char    *regcode;	/* Code-emit pointer; &regdummy = don't. */
static long     regsize;	/* Code size. */

/*
 * Forward declarations for regcomp()'s friends.
 */
#ifndef STATIC
#define	STATIC	static
#endif
STATIC char    *reg();
STATIC char    *regbranch();
STATIC char    *regpiece();
STATIC char    *regatom();
STATIC char    *regnode();
STATIC char    *regnext();
STATIC void     regc();
STATIC void     reginsert();
STATIC void     regtail();
STATIC void     regoptail();
#ifdef STRCSPN
STATIC int      strcspn();
#endif

/*
 - regcomp - compile a regular expression into internal code
 *
 * We can't allocate space until we know how big the compiled form will be,
 * but we can't compile it (and thus know how big it is) until we've got a
 * place to put the code.  So we cheat:  we compile it twice, once with code
 * generation turned off and size counting turned on, and once "for real".
 * This also means that we don't allocate space until we are sure that the
 * thing really will compile successfully, and we never have to move the
 * code and thus invalidate pointers into it.  (Note that it has to be in
 * one piece because free() must be able to free it all.)
 *
 * Beware that the optimization-preparation code in here knows about some
 * of the structure of the compiled regexp.
 */
regexp *regcomp(exp)
char           *exp;
{
    register regexp *r;
    register char  *scan;
    register char  *longest;
    register int    len;
    int             flags;
    extern char    *malloc();

    if (exp == (char *)NULL)
	FAIL("NULL argument");

    /* First pass: determine size, legality. */
    regparse = exp;
    regnpar = 1;
    regsize = 0L;
    regcode = &regdummy;
    regc(MAGIC);
    if (reg(0, &flags) == (char *)NULL)
	return ((regexp *)NULL);

    /* Small enough for pointer-storage convention? */
    if (regsize >= 32767L)	/* Probably could be 65535L. */
	FAIL("regexp too big");

    /* Allocate space. */
    r = (regexp *) malloc(sizeof(regexp) + (unsigned) regsize);
    if (r == (regexp *) NULL)
	FAIL("out of space");

    /* Second pass: emit code. */
    regparse = exp;
    regnpar = 1;
    regcode = r->program;
    regc(MAGIC);
    if (reg(0, &flags) == NULL)
	return ((regexp *) NULL);

    /* Dig out information for optimizations. */
    r->regstart = '\0';		/* Worst-case defaults. */
    r->reganch = 0;
    r->regmust = NULL;
    r->regmlen = 0;
    scan = r->program + 1;	/* First BRANCH. */
    if (OP(regnext(scan)) == END) {	/* Only one top-level choice. */
	scan = OPERAND(scan);

	/* Starting-point info. */
	if (OP(scan) == EXACTLY)
	    r->regstart = *OPERAND(scan);
	else if (OP(scan) == BOL)
	    r->reganch++;

	/*
	 * If there's something expensive in the r.e., find the longest
	 * literal string that must appear and make it the regmust.  Resolve
	 * ties in favor of later strings, since the regstart check works
	 * with the beginning of the r.e. and avoiding duplication
	 * strengthens checking.  Not a strong reason, but sufficient in the
	 * absence of others. 
	 */
	if (flags & SPSTART) {
	    longest = NULL;
	    len = 0;
	    for (; scan != NULL; scan = regnext(scan))
		if (OP(scan) == EXACTLY && strlen(OPERAND(scan)) >= len) {
		    longest = OPERAND(scan);
		    len = strlen(OPERAND(scan));
		}
	    r->regmust = longest;
	    r->regmlen = len;
	}
    }
    return (r);
}

/*
 - reg - regular expression, i.e. main body or parenthesized thing
 *
 * Caller must absorb opening parenthesis.
 *
 * Combining parenthesis handling with the base level of regular expression
 * is a trifle forced, but the need to tie the tails of the branches to what
 * follows makes it hard to avoid.
 */
static char *reg(paren, flagp)
int             paren;		/* Parenthesized? */
int            *flagp;
{
    register char  *ret;
    register char  *br;
    register char  *ender;
    register int    parno;
    int             flags;

    *flagp = HASWIDTH;		/* Tentatively. */

    /* Make an OPEN node, if parenthesized. */
    if (paren) {
	if (regnpar >= NSUBEXP)
	    FAIL("too many ()");
	parno = regnpar;
	regnpar++;
	ret = regnode(OPEN + parno);
    } else
	ret = (char *)NULL;

    /* Pick up the branches, linking them together. */
    br = regbranch(&flags);
    if (br == (char *)NULL)
	return ((char *)NULL);
    if (ret != (char *)NULL)
	regtail(ret, br);	/* OPEN -> first. */
    else
	ret = br;
    if (!(flags & HASWIDTH))
	*flagp &= ~HASWIDTH;
    *flagp |= flags & SPSTART;
    while (*regparse == '|') {
	regparse++;
	br = regbranch(&flags);
	if (br == (char *)NULL)
	    return ((char *)NULL);
	regtail(ret, br);	/* BRANCH -> BRANCH. */
	if (!(flags & HASWIDTH))
	    *flagp &= ~HASWIDTH;
	*flagp |= flags & SPSTART;
    }

    /* Make a closing node, and hook it on the end. */
    ender = regnode((paren) ? CLOSE + parno : END);
    regtail(ret, ender);

    /* Hook the tails of the branches to the closing node. */
    for (br = ret; br != (char *)NULL; br = regnext(br))
	regoptail(br, ender);

    /* Check for proper termination. */
    if (paren && *regparse++ != ')') {
	FAIL("unmatched ()");
    } else if (!paren && *regparse != '\0') {
	if (*regparse == ')') {
	    FAIL("unmatched ()");
	} else
	    FAIL("junk on end");/* "Can't happen". */
	/* NOTREACHED */
    }
    return (ret);
}

/*
 - regbranch - one alternative of an | operator
 *
 * Implements the concatenation operator.
 */
static char  *regbranch(flagp)
int            *flagp;
{
    register char  *ret;
    register char  *chain;
    register char  *latest;
    int             flags;

    *flagp = WORST;		/* Tentatively. */

    ret = regnode(BRANCH);
    chain = (char *)NULL;
    while (*regparse != '\0' && *regparse != '|' && *regparse != ')') {
	latest = regpiece(&flags);
	if (latest == (char *)NULL)
	    return ((char *)NULL);
	*flagp |= flags & HASWIDTH;
	if (chain == (char *)NULL)	/* First piece. */
	    *flagp |= flags & SPSTART;
	else
	    regtail(chain, latest);
	chain = latest;
    }
    if (chain == (char *)NULL)		/* Loop ran zero times. */
	regnode(NOTHING);

    return (ret);
}

/*
 - regpiece - something followed by possible [*]
 *
 * Note that the branching code sequence used for * is somewhat optimized:  
 * they use the same NOTHING node as both the endmarker for their branch 
 * list and the body of the last branch.  It might seem that this node could 
 * be dispensed with entirely, but the endmarker role is not redundant.
 */
static char *regpiece(flagp)
int            *flagp;
{
    register char  *ret;
    register char   op;
    register char  *nxt;
    int             flags;

    ret = regatom(&flags);
    if (ret == (char *)NULL)
	return ((char *)NULL);

    op = *regparse;
    if (!ISMULT(op)) {
	*flagp = flags;
	return (ret);
    }
    if (!(flags & HASWIDTH))
	FAIL("* operand could be empty");
    *flagp = (WORST | SPSTART);

    if (op == '*' && (flags & SIMPLE))
	reginsert(STAR, ret);
    else if (op == '*') {
	/* Emit x* as (x&|), where & means "self". */
	reginsert(BRANCH, ret);	/* Either x */
	regoptail(ret, regnode(BACK));	/* and loop */
	regoptail(ret, ret);	/* back */
	regtail(ret, regnode(BRANCH));	/* or */
	regtail(ret, regnode(NOTHING));	/* null. */
    } 
    regparse++;
    if (ISMULT(*regparse))
	FAIL("nested *");

    return (ret);
}

/*
 - regatom - the lowest level
 *
 * Optimization:  gobbles an entire sequence of ordinary characters so that
 * it can turn them into a single node, which is smaller to store and
 * faster to run.  Backslashed characters are exceptions, each becoming a
 * separate node; the code is simpler that way and it's not worth fixing.
 */
static char *regatom(flagp)
int            *flagp;
{
    register char  *ret;
    int             flags;

    *flagp = WORST;		/* Tentatively. */

    switch (*regparse++) {
    case '^':
	ret = regnode(BOL);
	break;
    case '$':
	ret = regnode(EOL);
	break;
    case '.':
	ret = regnode(ANY);
	*flagp |= HASWIDTH | SIMPLE;
	break;
    case '[':{
	    register int    class;
	    register int    classend;

	    if (*regparse == '^') {	/* Complement of range. */
		ret = regnode(ANYBUT);
		regparse++;
	    } else
		ret = regnode(ANYOF);
	    if (*regparse == ']' || *regparse == '-')
		regc(*regparse++);
	    while (*regparse != '\0' && *regparse != ']') {
		if (*regparse == '-') {
		    regparse++;
		    if (*regparse == ']' || *regparse == '\0')
			regc('-');
		    else {
			class = UCHARAT(regparse - 2) + 1;
			classend = UCHARAT(regparse);
			if (class > classend + 1)
			    FAIL("invalid [] range");
			for (; class <= classend; class++)
			    regc(class);
			regparse++;
		    }
		} else
		    regc(*regparse++);
	    }
	    regc('\0');
	    if (*regparse != ']')
		FAIL("unmatched []");
	    regparse++;
	    *flagp |= HASWIDTH | SIMPLE;
	}
	break;
    case '(':
	ret = reg(1, &flags);
	if (ret == (char *)NULL)
	    return ((char *)NULL);
	*flagp |= flags & (HASWIDTH | SPSTART);
	break;
    case '\0':
    case '|':
    case ')':
	FAIL("internal urp");	/* Supposed to be caught earlier. */
	break;
    case '*':
	FAIL("* follows nothing");
	break;
    case '\\':
	if (*regparse == '\0')
	    FAIL("trailing \\");
	ret = regnode(EXACTLY);
	regc(*regparse++);
	regc('\0');
	*flagp |= HASWIDTH | SIMPLE;
	break;
    default:{
	    register int    len;
	    register char   ender;

	    regparse--;
	    len = strcspn(regparse, META);
	    if (len <= 0)
		FAIL("internal disaster");
	    ender = *(regparse + len);
	    if (len > 1 && ISMULT(ender))
		len--;		/* Back off clear of * operand. */
	    *flagp |= HASWIDTH;
	    if (len == 1)
		*flagp |= SIMPLE;
	    ret = regnode(EXACTLY);
	    while (len > 0) {
		regc(*regparse++);
		len--;
	    }
	    regc('\0');
	}
	break;
    }

    return (ret);
}

/*
 - regnode - emit a node
 */
static char *regnode(op)
char            op;
{
    register char  *ret;
    register char  *ptr;

    ret = regcode;
    if (ret == &regdummy) {
	regsize += 3;
	return (ret);
    }
    ptr = ret;
    *ptr++ = op;
    *ptr++ = '\0';		/* Null "nxt" pointer. */
    *ptr++ = '\0';
    regcode = ptr;

    return (ret);
}

/*
 - regc - emit (if appropriate) a byte of code
 */
static void regc(b)
char            b;
{
    if (regcode != &regdummy)
	*regcode++ = b;
    else
	regsize++;
}

/*
 - reginsert - insert an operator in front of already-emitted operand
 *
 * Means relocating the operand.
 */
static void reginsert(op, opnd)
char            op;
char           *opnd;
{
    register char  *src;
    register char  *dst;
    register char  *place;

    if (regcode == &regdummy) {
	regsize += 3;
	return;
    }
    src = regcode;
    regcode += 3;
    dst = regcode;
    while (src > opnd)
	*--dst = *--src;

    place = opnd;		/* Op node, where operand used to be. */
    *place++ = op;
    *place++ = '\0';
    *place++ = '\0';
}

/*
 - regtail - set the next-pointer at the end of a node chain
 */
static void regtail(p, val)
char           *p;
char           *val;
{
    register char  *scan;
    register char  *temp;
    register int    offset;

    if (p == &regdummy)
	return;

    /* Find last node. */
    scan = p;
    for (;;) {
	temp = regnext(scan);
	if (temp == (char *)NULL)
	    break;
	scan = temp;
    }

    if (OP(scan) == BACK)
	offset = scan - val;
    else
	offset = val - scan;
    *(scan + 1) = (offset >> 8) & 0377;
    *(scan + 2) = offset & 0377;
}

/*
 - regoptail - regtail on operand of first argument; nop if operandless
 */
static void regoptail(p, val)
char           *p;
char           *val;
{
    /* "Operandless" and "op != BRANCH" are synonymous in practice. */
    if (p == (char *)NULL || p == &regdummy || OP(p) != BRANCH)
	return;
    regtail(OPERAND(p), val);
}

/*
 * regexec and friends
 */

/*
 * Global work variables for regexec().
 */
static char    *reginput;	/* String-input pointer. */
static char    *regbol;		/* Beginning of input, for ^ check. */
static char   **regstartp;	/* Pointer to startp array. */
static char   **regendp;	/* Ditto for endp. */

/*
 * Forwards.
 */
STATIC int      regtry();
STATIC int      regmatch();
STATIC int      regrepeat();

#ifdef DEBUG
int             regnarrate = 0;
void            regdump();
STATIC char    *regprop();
#endif

/*
 - regexec - match a regexp against a string
 */
int regexec(prog, string)
register regexp *prog;
register char  *string;
{
    register char  *s;

    /* Be paranoid... */
    if (prog == (regexp *)NULL || string == (char *)NULL) {
	regerror("NULL parameter");
	return (0);
    }
    /* Check validity of program. */
    if (UCHARAT(prog->program) != MAGIC) {
	regerror("corrupted program");
	return (0);
    }
    /* If there is a "must appear" string, look for it. */
    if (prog->regmust != (char *)NULL) {
	s = string;
	while ((s = strchr(s, prog->regmust[0])) != (char *)NULL) {
	    if (strncmp(s, prog->regmust, prog->regmlen) == 0)
		break;		/* Found it. */
	    s++;
	}
	if (s == (char *)NULL)		/* Not present. */
	    return (0);
    }
    /* Mark beginning of line for ^ . */
    regbol = string;

    /* Simplest case:  anchored match need be tried only once. */
    if (prog->reganch)
	return (regtry(prog, string));

    /* Messy cases:  unanchored match. */
    s = string;
    if (prog->regstart != '\0')
	/* We know what char it must start with. */
	while ((s = strchr(s, prog->regstart)) != (char *)NULL) {
	    if (regtry(prog, s))
		return (1);
	    s++;
	}
    else
	/* We don't -- general case. */
	do {
	    if (regtry(prog, s))
		return (1);
	} while (*s++ != '\0');

    /* Failure. */
    return (0);
}

/*
 - regtry - try match at specific point
 */
#ifdef __STDC__

static int regtry(regexp *prog, char *string)

#else

static int regtry(prog, string)
regexp         *prog;
char           *string;

#endif
{
    register int    i;
    register char **sp;
    register char **ep;

    reginput = string;
    regstartp = prog->startp;
    regendp = prog->endp;

    sp = prog->startp;
    ep = prog->endp;
    for (i = NSUBEXP; i > 0; i--) {
	*sp++ = (char *)NULL;
	*ep++ = (char *)NULL;
    }
    if (regmatch(prog->program + 1)) {
	prog->startp[0] = string;
	prog->endp[0] = reginput;
	return (1);
    } else
	return (0);
}

/*
 - regmatch - main matching routine
 *
 * Conceptually the strategy is simple:  check to see whether the current
 * node matches, call self recursively to see whether the rest matches,
 * and then act accordingly.  In practice we make some effort to avoid
 * recursion, in particular by going through "ordinary" nodes (that don't
 * need to know whether the rest of the match failed) by a loop instead of
 * by recursion.
 */
#ifdef __STDC__

static int regmatch(char *prog)

#else

static int regmatch(prog)
char           *prog;

#endif
{
    register char  *scan;	/* Current node. */
    char           *nxt;	/* nxt node. */

    scan = prog;
#ifdef DEBUG
    if (scan != (char *)NULL && regnarrate)
	fprintf(stderr, "%s(\n", regprop(scan));
#endif
    while (scan != (char *)NULL) {
#ifdef DEBUG
	if (regnarrate)
	    fprintf(stderr, "%s...\n", regprop(scan));
#endif
	nxt = regnext(scan);

	switch (OP(scan)) {
	case BOL:
	    if (reginput != regbol)
		return (0);
	    break;
	case EOL:
	    if (*reginput != '\0')
		return (0);
	    break;
	case ANY:
	    if (*reginput == '\0')
		return (0);
	    reginput++;
	    break;
	case EXACTLY:{
		register int    len;
		register char  *opnd;

		opnd = OPERAND(scan);
		/* Inline the first character, for speed. */
		if (*opnd != *reginput)
		    return (0);
		len = strlen(opnd);
		if (len > 1 && strncmp(opnd, reginput, len) != 0)
		    return (0);
		reginput += len;
	    }
	    break;
	case ANYOF:
	    if (*reginput == '\0' || 
		 strchr(OPERAND(scan), *reginput) == (char *)NULL)
		return (0);
	    reginput++;
	    break;
	case ANYBUT:
	    if (*reginput == '\0' || 
		 strchr(OPERAND(scan), *reginput) != (char *)NULL)
		return (0);
	    reginput++;
	    break;
	case NOTHING:
	    break;
	case BACK:
	    break;
	case OPEN + 1:
	case OPEN + 2:
	case OPEN + 3:
	case OPEN + 4:
	case OPEN + 5:
	case OPEN + 6:
	case OPEN + 7:
	case OPEN + 8:
	case OPEN + 9:{
		register int    no;
		register char  *save;

		no = OP(scan) - OPEN;
		save = reginput;

		if (regmatch(nxt)) {
		    /*
		     * Don't set startp if some later invocation of the same
		     * parentheses already has. 
		     */
		    if (regstartp[no] == (char *)NULL)
			regstartp[no] = save;
		    return (1);
		} else
		    return (0);
	    }
	    break;
	case CLOSE + 1:
	case CLOSE + 2:
	case CLOSE + 3:
	case CLOSE + 4:
	case CLOSE + 5:
	case CLOSE + 6:
	case CLOSE + 7:
	case CLOSE + 8:
	case CLOSE + 9:{
		register int    no;
		register char  *save;

		no = OP(scan) - CLOSE;
		save = reginput;

		if (regmatch(nxt)) {
		    /*
		     * Don't set endp if some later invocation of the same
		     * parentheses already has. 
		     */
		    if (regendp[no] == (char *)NULL)
			regendp[no] = save;
		    return (1);
		} else
		    return (0);
	    }
	    break;
	case BRANCH:{
		register char  *save;

		if (OP(nxt) != BRANCH)	/* No choice. */
		    nxt = OPERAND(scan);	/* Avoid recursion. */
		else {
		    do {
			save = reginput;
			if (regmatch(OPERAND(scan)))
			    return (1);
			reginput = save;
			scan = regnext(scan);
		    } while (scan != (char *)NULL && OP(scan) == BRANCH);
		    return (0);
		    /* NOTREACHED */
		}
	    }
	    break;
	case STAR:{
		register char   nextch;
		register int    no;
		register char  *save;
		register int    minimum;

		/*
		 * Lookahead to avoid useless match attempts when we know
		 * what character comes next. 
		 */
		nextch = '\0';
		if (OP(nxt) == EXACTLY)
		    nextch = *OPERAND(nxt);
		minimum = (OP(scan) == STAR) ? 0 : 1;
		save = reginput;
		no = regrepeat(OPERAND(scan));
		while (no >= minimum) {
		    /* If it could work, try it. */
		    if (nextch == '\0' || *reginput == nextch)
			if (regmatch(nxt))
			    return (1);
		    /* Couldn't or didn't -- back up. */
		    no--;
		    reginput = save + no;
		}
		return (0);
	    }
	    break;
	case END:
	    return (1);		/* Success! */
	    break;
	default:
	    regerror("memory corruption");
	    return (0);
	    break;
	}

	scan = nxt;
    }

    /*
     * We get here only if there's trouble -- normally "case END" is the
     * terminating point. 
     */
    regerror("corrupted pointers");
    return (0);
}

/*
 - regrepeat - repeatedly match something simple, report how many
 */
#ifdef __STDC__

static int regrepeat(char *p)

#else

static int regrepeat(p)
char           *p;

#endif
{
    register int    count = 0;
    register char  *scan;
    register char  *opnd;

    scan = reginput;
    opnd = OPERAND(p);
    switch (OP(p)) {
    case ANY:
	count = strlen(scan);
	scan += count;
	break;
    case EXACTLY:
	while (*opnd == *scan) {
	    count++;
	    scan++;
	}
	break;
    case ANYOF:
	while (*scan != '\0' && strchr(opnd, *scan) != (char *)NULL) {
	    count++;
	    scan++;
	}
	break;
    case ANYBUT:
	while (*scan != '\0' && strchr(opnd, *scan) == (char *)NULL) {
	    count++;
	    scan++;
	}
	break;
    default:			/* Oh dear.  Called inappropriately. */
	regerror("internal foulup");
	count = 0;		/* Best compromise. */
	break;
    }
    reginput = scan;

    return (count);
}


/*
 - regnext - dig the "nxt" pointer out of a node
 */
#ifdef __STDC__

static char *regnext(register char *p)

#else

static char *regnext(p)
register char  *p;

#endif
{
    register int    offset;

    if (p == &regdummy)
	return ((char *)NULL);

    offset = NEXT(p);
    if (offset == 0)
	return ((char *)NULL);

    if (OP(p) == BACK)
	return (p - offset);
    else
	return (p + offset);
}

#ifdef DEBUG

STATIC char    *regprop();

/*
 - regdump - dump a regexp onto stdout in vaguely comprehensible form
 */
#ifdef __STDC__

void regdump(regexp *r)

#else

void regdump(r)
regexp         *r;

#endif
{
    register char  *s;
    register char   op = EXACTLY;	/* Arbitrary non-END op. */
    register char  *nxt;
    extern char    *strchr();


    s = r->program + 1;
    while (op != END) {		/* While that wasn't END last time... */
	op = OP(s);
	printf("%2d%s", s - r->program, regprop(s));	/* Where, what. */
	nxt = regnext(s);
	if (nxt == (char *)NULL)	/* nxt ptr. */
	    printf("(0)");
	else
	    printf("(%d)", (s - r->program) + (nxt - s));
	s += 3;
	if (op == ANYOF || op == ANYBUT || op == EXACTLY) {
	    /* Literal string, where present. */
	    while (*s != '\0') {
		putchar(*s);
		s++;
	    }
	    s++;
	}
	putchar('\n');
    }

    /* Header fields of interest. */
    if (r->regstart != '\0')
	printf("start `%c' ", r->regstart);
    if (r->reganch)
	printf("anchored ");
    if (r->regmust != (char *)NULL)
	printf("must have \"%s\"", r->regmust);
    printf("\n");
}

/*
 - regprop - printable representation of opcode
 */
#ifdef __STDC__

static char *regprop(char *op)

#else

static char *regprop(op)
char           *op;

#endif
{
    register char  *p;
    static char     buf[50];

    strcpy(buf, ":");

    switch (OP(op)) {
    case BOL:
	p = "BOL";
	break;
    case EOL:
	p = "EOL";
	break;
    case ANY:
	p = "ANY";
	break;
    case ANYOF:
	p = "ANYOF";
	break;
    case ANYBUT:
	p = "ANYBUT";
	break;
    case BRANCH:
	p = "BRANCH";
	break;
    case EXACTLY:
	p = "EXACTLY";
	break;
    case NOTHING:
	p = "NOTHING";
	break;
    case BACK:
	p = "BACK";
	break;
    case END:
	p = "END";
	break;
    case OPEN + 1:
    case OPEN + 2:
    case OPEN + 3:
    case OPEN + 4:
    case OPEN + 5:
    case OPEN + 6:
    case OPEN + 7:
    case OPEN + 8:
    case OPEN + 9:
	sprintf(buf + strlen(buf), "OPEN%d", OP(op) - OPEN);
	p = (char *)NULL;
	break;
    case CLOSE + 1:
    case CLOSE + 2:
    case CLOSE + 3:
    case CLOSE + 4:
    case CLOSE + 5:
    case CLOSE + 6:
    case CLOSE + 7:
    case CLOSE + 8:
    case CLOSE + 9:
	sprintf(buf + strlen(buf), "CLOSE%d", OP(op) - CLOSE);
	p = (char *)NULL;
	break;
    case STAR:
	p = "STAR";
	break;
    default:
	regerror("corrupted opcode");
	break;
    }
    if (p != (char *)NULL)
	strcat(buf, p);
    return (buf);
}
#endif

/*
 * The following is provided for those people who do not have strcspn() in
 * their C libraries.  They should get off their butts and do something
 * about it; at least one public-domain implementation of those (highly
 * useful) string routines has been published on Usenet.
 */
#ifdef STRCSPN
/*
 * strcspn - find length of initial segment of s1 consisting entirely
 * of characters not from s2
 */

#ifdef __STDC__

static int strcspn(char *s1, char *s2)

#else

static int strcspn(s1, s2)
char           *s1;
char           *s2;

#endif
{
    register char  *scan1;
    register char  *scan2;
    register int    count;

    count = 0;
    for (scan1 = s1; *scan1 != '\0'; scan1++) {
	for (scan2 = s2; *scan2 != '\0';)	/* ++ moved down. */
	    if (*scan1 == *scan2++)
		return (count);
	count++;
    }
    return (count);
}
#endif


/*
 - regsub - perform substitutions after a regexp match
 */
#ifdef __STDC__

void regsub(regexp *prog, char *source, char *dest)

#else

void regsub(prog, source, dest)
regexp         *prog;
char           *source;
char           *dest;

#endif
{
    register char  *src;
    register char  *dst;
    register char   c;
    register int    no;
    register int    len;
    extern char    *strncpy();

    if (prog == (regexp *)NULL || 
	source == (char *)NULL || dest == (char *)NULL) {
	regerror("NULL parm to regsub");
	return;
    }
    if (UCHARAT(prog->program) != MAGIC) {
	regerror("damaged regexp fed to regsub");
	return;
    }
    src = source;
    dst = dest;
    while ((c = *src++) != '\0') {
	if (c == '&')
	    no = 0;
	else if (c == '\\' && '0' <= *src && *src <= '9')
	    no = *src++ - '0';
	else
	    no = -1;

	if (no < 0) {		/* Ordinary character. */
	    if (c == '\\' && (*src == '\\' || *src == '&'))
		c = *src++;
	    *dst++ = c;
	} else if (prog->startp[no] != (char *)NULL && 
		   prog->endp[no] != (char *)NULL) {
	    len = prog->endp[no] - prog->startp[no];
	    strncpy(dst, prog->startp[no], len);
	    dst += len;
	    if (len != 0 && *(dst - 1) == '\0') {	/* strncpy hit NUL. */
		regerror("damaged match string");
		return;
	    }
	}
    }
    *dst++ = '\0';
}


#ifdef __STDC__

void regerror(char *s)

#else

void regerror(s)
char           *s;

#endif
{
    fprintf(stderr, "regexp(3): %s", s);
    exit(1);
}