src/engine/regexp.cpp

/*
 * 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.
 *** THIS IS AN ALTERED VERSION.  It was altered by John Gilmore,
 *** hoptoad!gnu, on 27 Dec 1986, to add \n as an alternative to |
 *** to assist in implementing egrep.
 *** THIS IS AN ALTERED VERSION.  It was altered by John Gilmore,
 *** hoptoad!gnu, on 27 Dec 1986, to add \< and \> for word-matching
 *** as in BSD grep and ex.
 *** THIS IS AN ALTERED VERSION.  It was altered by John Gilmore,
 *** hoptoad!gnu, on 28 Dec 1986, to optimize characters quoted with \.
 *** THIS IS AN ALTERED VERSION.  It was altered by James A. Woods,
 *** ames!jaw, on 19 June 1987, to quash a regcomp() redundancy.
 *** THIS IS AN ALTERED VERSION.  It was altered by Christopher Seiwald
 *** seiwald@vix.com, on 28 August 1993, for use in jam.  Regmagic.h
 *** was moved into regexp.h, and the include of regexp.h now uses "'s
 *** to avoid conflicting with the system regexp.h.  Const, bless its
 *** soul, was removed so it can compile everywhere.  The declaration
 *** of strchr() was in conflict on AIX, so it was removed (as it is
 *** happily defined in string.h).
 *** THIS IS AN ALTERED VERSION.  It was altered by Christopher Seiwald
 *** seiwald@perforce.com, on 20 January 2000, to use function prototypes.
 *
 * 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.
 */


#include "jam.h"
#include "regexp.h"
#include "output.h"

#include <stdio.h>
#include <ctype.h>
#ifndef ultrix
# include <stdlib.h>
#endif
#include <string.h>


/*
 * 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 WORDA   12  /* no   Match "" at wordchar, where prev is nonword */
#define WORDZ   13  /* no   Match "" at nonwordchar, where prev is word */
#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 "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)*(const unsigned char *)(p))
#else
#define UCHARAT(p)  ((int)*(p)&CHARBITS)
#endif

#define FAIL(m) { regerror(m); return(NULL); }
#define ISMULT(c)   ((c) == '*' || (c) == '+' || (c) == '?')

/*
 * 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. */

/*
 * 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( int paren, int *flagp );
STATIC char *regbranch( int *flagp );
STATIC char *regpiece( int *flagp );
STATIC char *regatom( int *flagp );
STATIC char *regnode( int op );
STATIC char *regnext( char *p );
STATIC void regc( int b );
STATIC void reginsert( char op, char *opnd );
STATIC void regtail( char *p, char *val );
STATIC void regoptail( char *p, char *val );
#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( const char *exp )
{
    regexp *r;
    char *scan;
    char *longest;
    unsigned len;
    int flags;

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

    /* First pass: determine size, legality. */
#ifdef notdef
    if (exp[0] == '.' && exp[1] == '*') exp += 2;  /* aid grep */
#endif
    regparse = (char *)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("regexp too big");

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

    /* Second pass: emit code. */
    regparse = (char *)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(
    int paren,          /* Parenthesized? */
    int *flagp )
{
    char *ret;
    char *br;
    char *ender;
    int parno = 0;
    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 == '\n') {
        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( int *flagp )
{
    char *ret;
    char *chain;
    char *latest;
    int flags;

    *flagp = WORST;     /* Tentatively. */

    ret = regnode(BRANCH);
    chain = NULL;
    while (*regparse != '\0' && *regparse != ')' &&
           *regparse != '\n' && *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( int *flagp )
{
    char *ret;
    char op;
    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);
}

/*
 - 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( int *flagp )
{
    char *ret;
    int flags;

    *flagp = WORST;     /* Tentatively. */

    switch (*regparse++) {
    /* FIXME: these chars only have meaning at beg/end of pat? */
    case '^':
        ret = regnode(BOL);
        break;
    case '$':
        ret = regnode(EOL);
        break;
    case '.':
        ret = regnode(ANY);
        *flagp |= HASWIDTH|SIMPLE;
        break;
    case '[': {
            int classr;
            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 {
                        classr = UCHARAT(regparse-2)+1;
                        classend = UCHARAT(regparse);
                        if (classr > classend+1)
                            FAIL("invalid [] range");
                        for (; classr <= classend; classr++)
                            regc(classr);
                        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 '\n':
    case ')':
        FAIL("internal urp");   /* Supposed to be caught earlier. */
        break;
    case '?':
    case '+':
    case '*':
        FAIL("?+* follows nothing");
        break;
    case '\\':
        switch (*regparse++) {
        case '\0':
            FAIL("trailing \\");
            break;
        case '<':
            ret = regnode(WORDA);
            break;
        case '>':
            ret = regnode(WORDZ);
            break;
        /* FIXME: Someday handle \1, \2, ... */
        default:
            /* Handle general quoted chars in exact-match routine */
            goto de_fault;
        }
        break;
    de_fault:
    default:
        /*
         * Encode a string of characters to be matched exactly.
         *
         * This is a bit tricky due to quoted chars and due to
         * '*', '+', and '?' taking the SINGLE char previous
         * as their operand.
         *
         * On entry, the char at regparse[-1] is going to go
         * into the string, no matter what it is.  (It could be
         * following a \ if we are entered from the '\' case.)
         *
         * Basic idea is to pick up a good char in  ch  and
         * examine the next char.  If it's *+? then we twiddle.
         * If it's \ then we frozzle.  If it's other magic char
         * we push  ch  and terminate the string.  If none of the
         * above, we push  ch  on the string and go around again.
         *
         *  regprev  is used to remember where "the current char"
         * starts in the string, if due to a *+? we need to back
         * up and put the current char in a separate, 1-char, string.
         * When  regprev  is NULL,  ch  is the only char in the
         * string; this is used in *+? handling, and in setting
         * flags |= SIMPLE at the end.
         */
        {
            char *regprev;
            char ch;

            regparse--;         /* Look at cur char */
            ret = regnode(EXACTLY);
            for ( regprev = 0 ; ; ) {
                ch = *regparse++;   /* Get current char */
                switch (*regparse) {    /* look at next one */

                default:
                    regc(ch);   /* Add cur to string */
                    break;

                case '.': case '[': case '(':
                case ')': case '|': case '\n':
                case '$': case '^':
                case '\0':
                /* FIXME, $ and ^ should not always be magic */
                magic:
                    regc(ch);   /* dump cur char */
                    goto done;  /* and we are done */

                case '?': case '+': case '*':
                    if (!regprev)   /* If just ch in str, */
                        goto magic; /* use it */
                    /* End mult-char string one early */
                    regparse = regprev; /* Back up parse */
                    goto done;

                case '\\':
                    regc(ch);   /* Cur char OK */
                    switch (regparse[1]){ /* Look after \ */
                    case '\0':
                    case '<':
                    case '>':
                    /* FIXME: Someday handle \1, \2, ... */
                        goto done; /* Not quoted */
                    default:
                        /* Backup point is \, scan                           * point is after it. */
                        regprev = regparse;
                        regparse++;
                        continue;   /* NOT break; */
                    }
                }
                regprev = regparse; /* Set backup point */
            }
        done:
            regc('\0');
            *flagp |= HASWIDTH;
            if (!regprev)       /* One char? */
                *flagp |= SIMPLE;
        }
        break;
    }

    return(ret);
}

/*
 - regnode - emit a node
 */
static char *           /* Location. */
regnode( int op )
{
    char *ret;
    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( 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(
    char op,
    char *opnd )
{
    char *src;
    char *dst;
    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(
    char *p,
    char *val )
{
    char *scan;
    char *temp;
    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(
    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 const char *reginput;      /* String-input pointer. */
static const char *regbol;        /* Beginning of input, for ^ check. */
static const char **regstartp;    /* Pointer to startp array. */
static const char **regendp;      /* Ditto for endp. */

/*
 * Forwards.
 */
STATIC int regtry( regexp *prog, const char *string );
STATIC int regmatch( char *prog );
STATIC int regrepeat( char *p );

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

/*
 - regexec - match a regexp against a string
 */
int
regexec(
    regexp *prog,
    const char *string )
{
    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 = (char *)string;
        while ( ( s = strchr( s, prog->regmust[ 0 ] ) ) != NULL )
        {
            if ( !strncmp( s, prog->regmust, prog->regmlen ) )
                break;  /* Found it. */
            ++s;
        }
        if ( s == NULL )  /* Not present. */
            return 0;
    }

    /* Mark beginning of line for ^ . */
    regbol = (char *)string;

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

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


/*
 * 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 do not need to know
 * whether the rest of the match failed) by a loop instead of by recursion.
 */

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

    scan = prog;
#ifdef DEBUG
    if (scan != NULL && regnarrate)
        err_printf("%s(\n", regprop(scan));
#endif
    while (scan != NULL) {
#ifdef DEBUG
        if (regnarrate)
            err_printf("%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 WORDA:
            /* Must be looking at a letter, digit, or _ */
            if ((!isalnum(*reginput)) && *reginput != '_')
                return(0);
            /* Prev must be BOL or nonword */
            if (reginput > regbol &&
                (isalnum(reginput[-1]) || reginput[-1] == '_'))
                return(0);
            break;
        case WORDZ:
            /* Must be looking at non letter, digit, or _ */
            if (isalnum(*reginput) || *reginput == '_')
                return(0);
            /* We don't care what the previous char was */
            break;
        case ANY:
            if (*reginput == '\0')
                return(0);
            reginput++;
            break;
        case EXACTLY: {
                int len;
                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) == 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: {
                int no;
                const 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: {
                int no;
                const 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: {
                const 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: {
                char nextch;
                int no;
                const char *save;
                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( char *p )
{
    int count = 0;
    const char *scan;
    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) != 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( char *p )
{
    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( regexp *r )
{
    char *s;
    char op = EXACTLY; /* Arbitrary non-END op. */
    char *next;


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

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

/*
 - regprop - printable representation of opcode
 */
static char *
regprop( char *op )
{
    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;
    case WORDA:
        p = "WORDA";
        break;
    case WORDZ:
        p = "WORDZ";
        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(
    char *s1,
    char *s2 )
{
    char *scan1;
    char *scan2;
    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