#ifndef lint
static char *sccsid = "@(#)regexp.c	2.1 7/29/92";
static char *copyright = "Copyright (c) 1986 by University of Toronto.";
#endif

/***

* program name:
    xvi
* function:
    PD version of UNIX "vi" editor, with extensions.
* module name:
    regexp.c
* module function:
    Regular expression routines.
* history:
    Regular expression routines by Henry Spencer.
    Modfied for use with STEVIE (ST Editor for VI Enthusiasts,
     Version 3.10) by Tony Andrews.
    Adapted for use with Xvi by Chris & John Downey.
    Original copyright notice appears below.
    Please note that this is a modified version.
***/

/*
 * regcomp and regexec -- regsub and regerror are elsewhere
 *
 *	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.
 *
 * $Log:	regexp.c,v $
 * Revision 1.2	 88/04/28  08:09:45  tony
 * First modification of the regexp library. Added an external variable
 * 'reg_ic' which can be set to indicate that case should be ignored.
 * Added a new parameter to regexec() to indicate that the given string
 * comes from the beginning of a line and is thus eligible to match
 * 'beginning-of-line'.
 *
 * xvi, version 1.7:
 *
 * Pb(P_ignorecase) replaces reg_ic.
 *
 * BWORD (beginning of word) & EWORD (end of word) implemented.
 *
 * Some strings passed to regerror() are altered slightly, for
 * consistency with other error messages in xvi.
 */

#include "xvi.h"
#include "regexp.h"
#include "regmagic.h"

#ifdef	MEGAMAX
overlay "regexp"
#endif

/*
 * Exported functions.
 */
int	cstrncmp P((char *s1, char *s2, int n));
char	*cstrchr P((char *s, int c));

/*
 * 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 "next" pointer, possibly plus an operand.  "Next" pointers of
 * all nodes except BRANCH implement concatenation; a "next" 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 next... */
#define BACK	7	/* no	Match "", "next" 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 PLUS	11	/* node Match this (simple) thing 1 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. */
#define BWORD	64	/* no	Beginning of word. */
#define EWORD	65	/* no	End of word. */

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

/*
 * See regmagic.h for one further detail of program structure.
 */


/*
 * Utility definitions.
 */
#ifndef CHARBITS
#define UCHARAT(p)	((int)*(unsigned char *)(p))
#else
#define UCHARAT(p)	((int)*(p)&CHARBITS)
#endif

#define FAIL(m) 	{ regerror(m); return(NULL); }
#define ISMULT(c)	((c) == '*' || (c) == '+' || (c) == '?')
#define META		"^$.[()|?+*\\"

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

/*
 * mkup - convert to upper case IF we're doing caseless compares
 */
#define mkup(c)		((Pb(P_ignorecase) && is_lower(c)) ? \
						to_upper(c) : (c))

/*
 * 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 P((int paren, int *flagp));
STATIC	char	*regbranch P((int *flagp));
STATIC	char	*regpiece P((int *flagp));
STATIC	char	*regatom P((int *flagp));
STATIC	char	*regnode P((int op));
STATIC	char	*regnext P((char *p));
STATIC	void	regc P((int b));
STATIC	void	reginsert P((int op, char *opnd));
STATIC	void	regtail P((char *p, char *val));
STATIC	void	regoptail P((char *p, char *val));

#ifdef STRCSPN
    static	int	strcspn P((char *s1, char *s2));
#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;

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

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

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

    /* Allocate space. */
    r = (regexp *) alloc(sizeof(regexp) + (unsigned)regsize);
    if (r == NULL)
	return NULL;

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

    /* Pick up the branches, linking them together. */
    br = regbranch(&flags);
    if (br == NULL)
	return(NULL);
    if (ret != 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 == NULL)
	    return(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 != 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 = NULL;
    while (*regparse != '\0' && *regparse != '|' && *regparse != ')') {
	latest = regpiece(&flags);
	if (latest == NULL)
	    return(NULL);
	*flagp |= flags&HASWIDTH;
	if (chain == NULL)	/* First piece. */
	    *flagp |= flags&SPSTART;
	else
	    regtail(chain, latest);
	chain = latest;
    }
    if (chain == NULL)	/* Loop ran zero times. */
	(void) regnode(NOTHING);

    return(ret);
}

/*
 - regpiece - something followed by possible [*+?]
 *
 * Note that the branching code sequences used for ? and the general cases
 * of * and + are 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 *next;
    int flags;

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

    op = *regparse;
    if (!ISMULT(op)) {
	*flagp = flags;
	return(ret);
    }

    if (!(flags&HASWIDTH) && op != '?')
	FAIL("*+ operand could be empty");
    *flagp = (op != '+') ? (WORST|SPSTART) : (WORST|HASWIDTH);

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

    return(ret);
}

/*
 * This is called by regatom() for characters with no special meaning.
 */
static char *
regdefault(flagp)
int *flagp;
{
    register int len;
    register char ender;
    register char *ret;

    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');
    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. */

#if 0
    if (Pn(P_regextype) == rt_TAGS)
    {
	switch (*regparse)
	{
	    case '^':
	    case '$':
		break;
	    case '\\':
		switch (*++regparse)
		{
		    case '^':
		    case '$':
			ret = regnode(EXACTLY);
			regc(*regparse);
			regc('\0');
			*flagp |= HASWIDTH|SIMPLE;
			regparse++;
			return ret;
		}
		break;
	    default:
		return regdefault(flagp);
	}
    }
#endif
    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 == NULL)
	    return(NULL);
	*flagp |= flags&(HASWIDTH|SPSTART);
	break;
    case '\0':
    case '|':
    case ')':
	FAIL("Internal urp");	/* Supposed to be caught earlier. */
	break;
    case '?':
    case '+':
    case '*':
	FAIL("?+* follows nothing");
	break;
    case '\\':
	switch (*regparse)
	{
	    case '\0':
		FAIL("Trailing \\");
	    case '<':
		ret = regnode(BWORD);
		break;
	    case '>':
		ret = regnode(EWORD);
		break;
	    default:
		ret = regnode(EXACTLY);
		regc(*regparse);
		regc('\0');
		*flagp |= HASWIDTH|SIMPLE;
	}
	regparse++;
	break;
    default:
	regparse--;
	ret = regdefault(flagp);
    }

    return(ret);
}

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

    ret = regcode;
    if (ret == &regdummy) {
	regsize += 3;
	return(ret);
    }

    ptr = ret;
    *ptr++ = op;
    *ptr++ = '\0';		/* Null "next" pointer. */
    *ptr++ = '\0';
    regcode = ptr;

    return(ret);
}

/*
 - regc - emit (if appropriate) a byte of code
 */
static void
regc(b)
int 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)
int 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 == 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 == 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 P((regexp *prog, char *string));
STATIC	int	regmatch P((char *prog));
STATIC	int	regrepeat P((char *p));

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

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

    /* Be paranoid... */
    if (prog == NULL || string == 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 != NULL) {
	s = string;
	while ((s = cstrchr(s, prog->regmust[0])) != NULL) {
	    if (cstrncmp(s, prog->regmust, prog->regmlen) == 0)
		break;	/* Found it. */
	    s++;
	}
	if (s == NULL)	/* Not present. */
	    return(0);
    }

    /* Mark beginning of line for ^ . */
    if (at_bol)
	regbol = string;	/* is possible to match bol */
    else
	regbol = NULL;		/* we aren't there, so don't match it */

    /* 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 = cstrchr(s, prog->regstart)) != 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
 */
static int			/* 0 failure, 1 success */
regtry(prog, string)
regexp *prog;
char *string;
{
    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++ = NULL;
	*ep++ = NULL;
    }
    if (regmatch(prog->program + 1)) {
	prog->startp[0] = string;
	prog->endp[0] = reginput;
	return(1);
    } else
	return(0);
}

/*
 * A word is defined, for BWORD & EWORD, as any sequence of
 * alphanumeric characters and/or underscores.
 */
#define inword(c)	(is_alnum(c) || (c) == '_')

/*
 - 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.
 */
static int			/* 0 failure, 1 success */
regmatch(prog)
char *prog;
{
    register char *scan;	/* Current node. */
    char *next;		/* Next node. */

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

	switch (OP(scan)) {
	case BOL:
	    if (reginput != regbol)
		return(0);
	    break;
	case EOL:
	    if (*reginput != '\0')
		return(0);
	    break;
	case BWORD:
	{
	    register int	c;

	    /*
	     * Test for beginning of word.
	     */
	    if (
		(c = *reginput) == '\0'
		||
		!inword(c)
		||
		(
		    reginput != regbol
		    &&
		    inword(reginput[-1])
		)
	    )
		return 0;
	    break;
	}
	case EWORD:
	{
	    register int	c;

	    /*
	     * Test for end of word.
	     */
	    if (
		(
		    (c = *reginput) != '\0'
		    &&
		    inword(c)
		)
		||
		reginput == regbol
		||
		!inword(reginput[-1])
	    )
		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 (mkup(*opnd) != mkup(*reginput))
		return(0);
	    len = strlen(opnd);
	    if (len > 1
		&& cstrncmp(opnd, reginput, len) != 0)
		return(0);
	    reginput += len;
	    break;
	}
	case ANYOF:
	    if (*reginput == '\0'
		|| strchr(OPERAND(scan), *reginput) == NULL)
		return(0);
	    reginput++;
	    break;
	case ANYBUT:
	    if (*reginput == '\0'
		|| strchr(OPERAND(scan), *reginput) != 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(next)) {
		/*
		 * Don't set startp if some later
		 * invocation of the same parentheses
		 * already has.
		 */
		if (regstartp[no] == 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(next)) {
		/*
		 * Don't set endp if some later
		 * invocation of the same parentheses
		 * already has.
		 */
		if (regendp[no] == NULL)
		    regendp[no] = save;
		return(1);
	    } else
		return(0);
	    break;
	}
	case BRANCH:
	{
	    register char *save;

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

	    /*
	     * Lookahead to avoid useless match attempts
	     * when we know what character comes next.
	     */
	    nextch = '\0';
	    if (OP(next) == EXACTLY)
		nextch = *OPERAND(next);
	    min = (OP(scan) == STAR) ? 0 : 1;
	    save = reginput;
	    no = regrepeat(OPERAND(scan));
	    while (no >= min) {
		/* If it could work, try it. */
		if (nextch == '\0' || *reginput == nextch)
		    if (regmatch(next))
			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 = next;
    }

    /*
     * 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
 */
static int
regrepeat(p)
char *p;
{
    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 (mkup(*opnd) == mkup(*scan)) {
	    count++;
	    scan++;
	}
	break;
    case ANYOF:
	while (*scan != '\0' && strchr(opnd, *scan) != NULL) {
	    count++;
	    scan++;
	}
	break;
    case ANYBUT:
	while (*scan != '\0' && strchr(opnd, *scan) == NULL) {
	    count++;
	    scan++;
	}
	break;
    default:		/* Oh dear.  Called inappropriately. */
	regerror("Internal foulup");
	count = 0;	/* Best compromise. */
	break;
    }
    reginput = scan;

    return(count);
}

/*
 - regnext - dig the "next" pointer out of a node
 */
static char *
regnext(p)
register char *p;
{
    register int offset;

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

    offset = NEXT(p);
    if (offset == 0)
	return(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
 */
void
regdump(r)
regexp *r;
{
    register char *s;
    register char op = EXACTLY;	/* Arbitrary non-END op. */
    register char *next;
    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. */
	next = regnext(s);
	if (next == NULL)		/* Next ptr. */
	    printf("(0)");
	else
	    printf("(%d)", (s-r->program)+(next-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 != NULL)
	printf("must have \"%s\"", r->regmust);
    printf("\n");
}

/*
 - regprop - printable representation of opcode
 */
static char *
regprop(op)
char *op;
{
    register char *p;
    static char buf[50];

    (void) 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 = 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 = NULL;
	break;
    case STAR:
	p = "STAR";
	break;
    case PLUS:
	p = "PLUS";
	break;
    default:
	regerror("Corrupted opcode");
	break;
    }
    if (p != NULL)
	(void) 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
 */

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

int
cstrncmp(s1, s2, n)
register char	*s1, *s2;
register int	n;
{
    if (!Pb(P_ignorecase)) {
	return(strncmp(s1, s2, n));
    }

    while (
	    n > 0
	    &&
	    *s1 != '\0'
	    &&
	    *s2 != '\0'
	    &&
	    mkup(*s1) == mkup(*s2)
    ) {
	s1++;
	s2++;
	n--;
    }
    if (n == 0) {
	return(0);
    } else {
	return(mkup(*s1) - mkup(*s2));
    }
}

char *
cstrchr(s, c)
char	*s;
int	c;
{
    register char	*p;
    register int	uc;

    uc = mkup(c);
    for (p = s; *p != '\0'; p++) {
	if (mkup(*p) == uc)
	    return(p);
    }
    return(NULL);
}