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regexp.c

/*
 * 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.
 *** THIS IS AN ALTERED VERSION.  It was altered by Christopher Seiwald
 *** seiwald@perforce.com, on 05 November 2002, to const string literals.
 *
 * 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( const 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 const char *reginput;  /* String-input pointer. */
static char *regbol;          /* Beginning of input, for ^ check. */
static const char **regstartp;      /* Pointer to startp array. */
static const char **regendp;  /* Ditto for endp. */

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

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

/*
 - regexec - match a regexp against a string
 */
int
regexec(
      register regexp *prog,
      register const 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,
      const char *string )
{
      register int i;
      register const char **sp;
      register const char **ep;

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

      sp = prog->startp;
      ep = prog->endp;
      for (i = NSUBEXP; i > 0; i--) {
            *sp++ = NULL;
            *ep++ = NULL;
      }
      if (regmatch(prog->program + 1)) {
            prog->startp[0] = string;
            prog->endp[0] = reginput;
            return(1);
      } else
            return(0);
}

/*
 - regmatch - main matching routine
 *
 * Conceptually the strategy is simple:  check to see whether the current
 * node matches, call self recursively to see whether the rest matches,
 * and then act accordingly.  In practice we make some effort to avoid
 * recursion, in particular by going through "ordinary" nodes (that 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 const char *save;

                        no = OP(scan) - OPEN;
                        save = reginput;

                        if (regmatch(next)) {
                              /*
                               * Don't set startp if some later
                               * invocation of the same parentheses
                               * already has.
                               */
                              if (regstartp[no] == NULL)
                                    regstartp[no] = save;
                              return(1);
                        } else
                              return(0);
                  }
                  break;
            case CLOSE+1:
            case CLOSE+2:
            case CLOSE+3:
            case CLOSE+4:
            case CLOSE+5:
            case CLOSE+6:
            case CLOSE+7:
            case CLOSE+8:
            case CLOSE+9: {
                        register int no;
                        register const char *save;

                        no = OP(scan) - CLOSE;
                        save = reginput;

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

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

void
regerror( const char *s )
{
      printf( "re error %s\n", s );
}

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