regexp.c

CMX990 demonstration board (DE9901)

/*
 * 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.
 *
 * 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. ***
 * *** The only change is to use ckalloc and ckfree instead of   ***
 * *** malloc and free.						 ***
 *
 * $Id: regexp.c,v 1.1.1.1 2001/04/29 20:34:09 karll Exp $
 */
#include "tclInt.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. */

/*
 * 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 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(char op);
STATIC char *regnext(char *p);
STATIC void regc(char 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(char *s1, char *s2);
#endif

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