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regexp.c

/*
 * TclRegComp and TclRegExec -- TclRegSub is elsewhere
 *
 *    Copyright (c) 1986 by University of Toronto.
 *    Written by Henry Spencer.  Not derived from licensed software.
 *
 *    Permission is granted to anyone to use this software for any
 *    purpose on any computer system, and to redistribute it freely,
 *    subject to the following restrictions:
 *
 *    1. The author is not responsible for the consequences of use of
 *          this software, no matter how awful, even if they arise
 *          from defects in it.
 *
 *    2. The origin of this software must not be misrepresented, either
 *          by explicit claim or by omission.
 *
 *    3. Altered versions must be plainly marked as such, and must not
 *          be misrepresented as being the original software.
 *
 * Beware that some of this code is subtly aware of the way operator
 * precedence is structured in regular expressions.  Serious changes in
 * regular-expression syntax might require a total rethink.
 *
 * *** NOTE: this code has been altered slightly for use in Tcl: ***
 * *** 1. Use ckalloc and ckfree instead of  malloc and free.      ***
 * *** 2. Add extra argument to regexp to specify the real   ***
 * ***    start of the string separately from the start of the     ***
 * ***    current search. This is needed to search for multiple    ***
 * ***    matches within a string.                     ***
 * *** 3. Names have been changed, e.g. from regcomp to            ***
 * ***    TclRegComp, to avoid clashes with other            ***
 * ***    regexp implementations used by applications.             ***
 * *** 4. Added errMsg declaration and TclRegError procedure       ***
 * *** 5. Various lint-like things, such as casting arguments      ***
 * ***        in procedure calls.                            ***
 *
 * *** NOTE: This code has been altered for use in MT-Sturdy Tcl ***
 * *** 1. All use of static variables has been changed to access ***
 * ***    fields of a structure.                                 ***
 * *** 2. This in addition to changes to TclRegError makes the   ***
 * ***    code multi-thread safe.                                ***
 *
 * RCS: @(#) $Id: regexp.c,v 1.2 1998/09/14 18:39:57 stanton Exp $
 */

#include "tclInt.h"
#include "tclPort.h"

/*
 * The variable below is set to NULL before invoking regexp functions
 * and checked after those functions.  If an error occurred then TclRegError
 * will set the variable to point to a (static) error message.  This
 * mechanism unfortunately does not support multi-threading, but the
 * procedures TclRegError and TclGetRegError can be modified to use
 * thread-specific storage for the variable and thereby make the code
 * thread-safe.
 */

static char *errMsg = NULL;

/*
 * 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 TclRegComp() 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 TclRegExec() needs it and TclRegComp() 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 (OPEN+NSUBEXP)    /* 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)     { TclRegError(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 TclRegComp().
 */
struct regcomp_state  {
    char *regparse;           /* Input-scan pointer. */
    int regnpar;        /* () count. */
    char *regcode;            /* Code-emit pointer; &regdummy = don't. */
    long regsize;       /* Code size. */
};

static char regdummy;

/*
 * The first byte of the regexp internal "program" is actually this magic
 * number; the start node begins in the second byte.
 */
#define     MAGIC 0234


/*
 * Forward declarations for TclRegComp()'s friends.
 */

static char *           reg _ANSI_ARGS_((int paren, int *flagp,
                      struct regcomp_state *rcstate));
static char *           regatom _ANSI_ARGS_((int *flagp,
                      struct regcomp_state *rcstate));
static char *           regbranch _ANSI_ARGS_((int *flagp,
                      struct regcomp_state *rcstate));
static void       regc _ANSI_ARGS_((int b,
                      struct regcomp_state *rcstate));
static void       reginsert _ANSI_ARGS_((int op, char *opnd,
                      struct regcomp_state *rcstate));
static char *           regnext _ANSI_ARGS_((char *p));
static char *           regnode _ANSI_ARGS_((int op,
                      struct regcomp_state *rcstate));
static void             regoptail _ANSI_ARGS_((char *p, char *val));
static char *           regpiece _ANSI_ARGS_((int *flagp,
                      struct regcomp_state *rcstate));
static void             regtail _ANSI_ARGS_((char *p, char *val));

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

/*
 - TclRegComp - 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 *
TclRegComp(exp)
char *exp;
{
      register regexp *r;
      register char *scan;
      register char *longest;
      register int len;
      int flags;
      struct regcomp_state state;
      struct regcomp_state *rcstate= &state;

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

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

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

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

      /* Second pass: emit code. */
      rcstate->regparse = exp;
      rcstate->regnpar = 1;
      rcstate->regcode = r->program;
      regc(MAGIC, rcstate);
      if (reg(0, &flags, rcstate) == 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 && ((int) 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, rcstate)
int paren;              /* Parenthesized? */
int *flagp;
struct regcomp_state *rcstate;
{
      register char *ret;
      register char *br;
      register char *ender;
      register int parno = 0;
      int flags;

      *flagp = HASWIDTH;      /* Tentatively. */

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

      /* Pick up the branches, linking them together. */
      br = regbranch(&flags,rcstate);
      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 (*rcstate->regparse == '|') {
            rcstate->regparse++;
            br = regbranch(&flags,rcstate);
            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,rcstate); 
      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 && *rcstate->regparse++ != ')') {
            FAIL("unmatched ()");
      } else if (!paren && *rcstate->regparse != '\0') {
            if (*rcstate->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, rcstate)
int *flagp;
struct regcomp_state *rcstate;
{
      register char *ret;
      register char *chain;
      register char *latest;
      int flags;

      *flagp = WORST;         /* Tentatively. */

      ret = regnode(BRANCH,rcstate);
      chain = NULL;
      while (*rcstate->regparse != '\0' && *rcstate->regparse != '|' &&
                        *rcstate->regparse != ')') {
            latest = regpiece(&flags, rcstate);
            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,rcstate);

      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, rcstate)
int *flagp;
struct regcomp_state *rcstate;
{
      register char *ret;
      register char op;
      register char *next;
      int flags;

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

      op = *rcstate->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, rcstate);
      else if (op == '*') {
            /* Emit x* as (x&|), where & means "self". */
            reginsert(BRANCH, ret, rcstate);                /* Either x */
            regoptail(ret, regnode(BACK,rcstate));          /* and loop */
            regoptail(ret, ret);                /* back */
            regtail(ret, regnode(BRANCH,rcstate));          /* or */
            regtail(ret, regnode(NOTHING,rcstate));         /* null. */
      } else if (op == '+' && (flags&SIMPLE))
            reginsert(PLUS, ret, rcstate);
      else if (op == '+') {
            /* Emit x+ as x(&|), where & means "self". */
            next = regnode(BRANCH,rcstate);                 /* Either */
            regtail(ret, next);
            regtail(regnode(BACK,rcstate), ret);            /* loop back */
            regtail(next, regnode(BRANCH,rcstate));         /* or */
            regtail(ret, regnode(NOTHING,rcstate));         /* null. */
      } else if (op == '?') {
            /* Emit x? as (x|) */
            reginsert(BRANCH, ret, rcstate);                /* Either x */
            regtail(ret, regnode(BRANCH,rcstate));          /* or */
            next = regnode(NOTHING,rcstate);          /* null. */
            regtail(ret, next);
            regoptail(ret, next);
      }
      rcstate->regparse++;
      if (ISMULT(*rcstate->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, rcstate)
int *flagp;
struct regcomp_state *rcstate;
{
      register char *ret;
      int flags;

      *flagp = WORST;         /* Tentatively. */

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

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

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

      return(ret);
}

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

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

      ptr = ret;
      *ptr++ = (char)op;
      *ptr++ = '\0';          /* Null "next" pointer. */
      *ptr++ = '\0';
      rcstate->regcode = ptr;

      return(ret);
}

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

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

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

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

      place = opnd;           /* Op node, where operand used to be. */
      *place++ = (char)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) = (char)((offset>>8)&0377);
      *(scan+2) = (char)(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);
}

/*
 * TclRegExec and friends
 */

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

/*
 * Forwards.
 */
static int        regtry _ANSI_ARGS_((regexp *prog, char *string,
                      struct regexec_state *restate));
static int        regmatch _ANSI_ARGS_((char *prog,
                      struct regexec_state *restate));
static int        regrepeat _ANSI_ARGS_((char *p,
                      struct regexec_state *restate));

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

/*
 - TclRegExec - match a regexp against a string
 */
int
TclRegExec(prog, string, start)
register regexp *prog;
register char *string;
char *start;
{
      register char *s;
      struct regexec_state state;
      struct regexec_state *restate= &state;

      /* Be paranoid... */
      if (prog == NULL || string == NULL) {
            TclRegError("NULL parameter");
            return(0);
      }

      /* Check validity of program. */
      if (UCHARAT(prog->program) != MAGIC) {
            TclRegError("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, (size_t) prog->regmlen)
                      == 0)
                        break;      /* Found it. */
                  s++;
            }
            if (s == NULL)    /* Not present. */
                  return(0);
      }

      /* Mark beginning of line for ^ . */
      restate->regbol = start;

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

      /* 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, restate))
                        return(1);
                  s++;
            }
      else
            /* We don't -- general case. */
            do {
                  if (regtry(prog, s, restate))
                        return(1);
            } while (*s++ != '\0');

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

/*
 - regtry - try match at specific point
 */
static int              /* 0 failure, 1 success */
regtry(prog, string, restate)
regexp *prog;
char *string;
struct regexec_state *restate;
{
      register int i;
      register char **sp;
      register char **ep;

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

      sp = prog->startp;
      ep = prog->endp;
      for (i = NSUBEXP; i > 0; i--) {
            *sp++ = NULL;
            *ep++ = NULL;
      }
      if (regmatch(prog->program + 1,restate)) {
            prog->startp[0] = string;
            prog->endp[0] = restate->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, restate)
char *prog;
struct regexec_state *restate;
{
    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 (restate->reginput != restate->regbol) {
                return 0;
            }
            break;
          case EOL:
            if (*restate->reginput != '\0') {
                return 0;
            }
            break;
          case ANY:
            if (*restate->reginput == '\0') {
                return 0;
            }
            restate->reginput++;
            break;
          case EXACTLY: {
            register int len;
            register char *opnd;

            opnd = OPERAND(scan);
            /* Inline the first character, for speed. */
            if (*opnd != *restate->reginput) {
                return 0 ;
            }
            len = strlen(opnd);
            if (len > 1 && strncmp(opnd, restate->reginput, (size_t) len)
                  != 0) {
                return 0;
            }
            restate->reginput += len;
            break;
          }
          case ANYOF:
            if (*restate->reginput == '\0'
                  || strchr(OPERAND(scan), *restate->reginput) == NULL) {
                return 0;
            }
            restate->reginput++;
            break;
          case ANYBUT:
            if (*restate->reginput == '\0'
                  || strchr(OPERAND(scan), *restate->reginput) != NULL) {
                return 0;
            }
            restate->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;

      doOpen:
            no = OP(scan) - OPEN;
            save = restate->reginput;

            if (regmatch(next,restate)) {
                /*
                 * Don't set startp if some later invocation of the
                 * same parentheses already has.
                 */
                if (restate->regstartp[no] == NULL) {
                  restate->regstartp[no] = save;
                }
                return 1;
            } else {
                return 0;
            }
          }
          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;

      doClose:
            no = OP(scan) - CLOSE;
            save = restate->reginput;

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

            if (OP(next) != BRANCH) { /* No choice. */
                next = OPERAND(scan); /* Avoid recursion. */
            } else {
                do {
                  save = restate->reginput;
                  if (regmatch(OPERAND(scan),restate))
                      return(1);
                  restate->reginput = save;
                  scan = regnext(scan);
                } while (scan != NULL && OP(scan) == BRANCH);
                return 0;
            }
            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 = restate->reginput;
            no = regrepeat(OPERAND(scan),restate);
            while (no >= min) {
                /* If it could work, try it. */
                if (nextch == '\0' || *restate->reginput == nextch)
                  if (regmatch(next,restate))
                      return(1);
                /* Couldn't or didn't -- back up. */
                no--;
                restate->reginput = save + no;
            }
            return(0);
          }
          case END:
            return(1);  /* Success! */
          default:
            if (OP(scan) > OPEN && OP(scan) < OPEN+NSUBEXP) {
                goto doOpen;
            } else if (OP(scan) > CLOSE && OP(scan) < CLOSE+NSUBEXP) {
                goto doClose;
            }
            TclRegError("memory corruption");
            return 0;
      }

      scan = next;
    }

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

/*
 - regrepeat - repeatedly match something simple, report how many
 */
static int
regrepeat(p, restate)
char *p;
struct regexec_state *restate;
{
      register int count = 0;
      register char *scan;
      register char *opnd;

      scan = restate->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. */
            TclRegError("internal foulup");
            count = 0;  /* Best compromise. */
            break;
      }
      restate->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;


      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:
            if (OP(op) > OPEN && OP(op) < OPEN+NSUBEXP) {
                sprintf(buf+strlen(buf), "OPEN%d", OP(op)-OPEN);
                p = NULL;
                break;
            } else if (OP(op) > CLOSE && OP(op) < CLOSE+NSUBEXP) {
                sprintf(buf+strlen(buf), "CLOSE%d", OP(op)-CLOSE);
                p = NULL;
            } else {
                TclRegError("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

/*
 *----------------------------------------------------------------------
 *
 * TclRegError --
 *
 *    This procedure is invoked by the regexp code when an error
 *    occurs.  It saves the error message so it can be seen by the
 *    code that called Spencer's code.
 *
 * Results:
 *    None.
 *
 * Side effects:
 *    The value of "string" is saved in "errMsg".
 *
 *----------------------------------------------------------------------
 */

void
TclRegError(string)
    char *string;             /* Error message. */
{
    errMsg = string;
}

char *
TclGetRegError()
{
    return errMsg;
}

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