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regexp.c

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/*
 * 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 "regexp.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)*(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( register 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( char *exp )
{
        register regexp *r;
        register char *scan;
        register char *longest;
        register 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 *)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 )
{
        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 == '\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 )
{
        register char *ret;
        register char *chain;
        register 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 )
{
        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);
}

/*
 - 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 )
{
        register 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 '[': {
                        register int classr;
                        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 {
                                                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;
                        register 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 )
{
        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( 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 )
{
        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(
        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(
        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( regexp *prog, 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(
        register regexp *prog,
        register char *string )
{
        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 = (char *)string;
                while ((s = strchr(s, prog->regmust[0])) != NULL) {
                        if (strncmp(s, prog->regmust, prog->regmlen) == 0)
                                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 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(
        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);
}

/*
 - 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( 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 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: {
                                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) == 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( 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 (*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( 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( regexp *r )
{
        register char *s;
        register char op = EXACTLY;     /* Arbitrary non-END op. */
        register char *next;


        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( 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;
        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 )
{
        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

---