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regexp.c

/* 
   Copyright (c) 1995-2000 by Cisco systems, Inc.

   Permission to use, copy, modify, and distribute modified and
   unmodified copies of this software for any purpose and without fee is
   hereby granted, provided that (a) this copyright and permission notice
   appear on all copies of the software and supporting documentation, (b)
   the name of Cisco Systems, Inc. not be used in advertising or
   publicity pertaining to distribution of the program without specific
   prior permission, and (c) notice be given in supporting documentation
   that use, modification, copying and distribution is by permission of
   Cisco Systems, Inc.

   Cisco Systems, Inc. makes no representations about the suitability
   of this software for any purpose.  THIS SOFTWARE IS PROVIDED ``AS
   IS'' AND WITHOUT ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING,
   WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
   FITNESS FOR A PARTICULAR PURPOSE.
*/

/*
 * regcomp and regexec -- regsub and regerror are elsewhere
 * @(#)regexp.c   1.3 of 18 April 87
 *
 *    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.
 */
#include <stdio.h>
#include "regexp.h"
#include "regmagic.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     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) == '?')
#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. */

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

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

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

      if (exp == 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("regexp too big");

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

      /* 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);
}

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

      *flagp = WORST;         /* Tentatively. */

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

                  if (*regparse == '^') { /* Complement of range. */
                        ret = regnode(ANYBUT);
                        regparse++;
                  } else
                        ret = regnode(ANYOF);
                  if (*regparse == ']' || *regparse == '-')
                        regc(*regparse++);
                  while (*regparse != '\0' && *regparse != ']') {
                        if (*regparse == '-') {
                              regparse++;
                              if (*regparse == ']' || *regparse == '\0')
                                    regc('-');
                              else {
                                    class = UCHARAT(regparse-2)+1;
                                    classend = UCHARAT(regparse);
                                    if (class > classend+1)
                                          FAIL("invalid [] range");
                                    for (; class <= classend; class++)
                                          regc(class);
                                    regparse++;
                              }
                        } else
                              regc(*regparse++);
                  }
                  regc('\0');
                  if (*regparse != ']')
                        FAIL("unmatched []");
                  regparse++;
                  *flagp |= HASWIDTH|SIMPLE;
            }
            break;
      case '(':
            ret = reg(1, &flags);
            if (ret == 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 '\\':
            if (*regparse == '\0')
                  FAIL("trailing \\");
            ret = regnode(EXACTLY);
            regc(*regparse++);
            regc('\0');
            *flagp |= HASWIDTH|SIMPLE;
            break;
      default: {
                  register int len;
                  register char ender;

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

      return(ret);
}

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

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

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

      return(ret);
}

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

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

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

      src = regcode;
      regcode += 3;
      dst = regcode;
      while (src > opnd)
            *--dst = *--src;

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

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

      if (p == &regdummy)
            return;

      /* Find last node. */
      scan = p;
      for (;;) {
            temp = regnext(scan);
            if (temp == 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();
STATIC int regmatch();
STATIC int regrepeat();

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

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

      /* 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 = 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 = string;

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

      /* Messy cases:  unanchored match. */
      s = string;
      if (prog->regstart != '\0')
            /* We know what char it must start with. */
            while ((s = strchr(s, prog->regstart)) != 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);
}

/*
 - 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. */
      extern char *strchr();

      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 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(p)
char *p;
{
      register int count = 0;
      register char *scan;
      register char *opnd;
      extern char *strchr();

      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(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

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