regexp.c

这是一个同样来自贝尔实验室的和UNIX有着渊源的操作系统, 其简洁的设计和实现易于我们学习和理解

/* $Source: /u/mark/src/pax/RCS/regexp.c,v $
 *
 * $Revision: 1.2 $
 *
 * regexp.c - regular expression matching
 *
 * DESCRIPTION
 *
 *	Underneath the reformatting and comment blocks which were added to 
 *	make it consistent with the rest of the code, you will find a
 *	modified version of Henry Specer's regular expression library.
 *	Henry's functions were modified to provide the minimal regular
 *	expression matching, as required by P1003.  Henry's code was
 *	copyrighted, and copy of the copyright message and restrictions
 *	are provided, verbatim, below:
 *
 *	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.
 *
 * AUTHORS
 *
 *     Mark H. Colburn, NAPS International ([e-mail])
 *     Henry Spencer, University of Torronto ([e-mail])
 *
 * Sponsored by The USENIX Association for public distribution. 
 *
 * $Log:	regexp.c,v $
 * Revision 1.2  89/02/12  10:05:39  mark
 * 1.2 release fixes
 * 
 * Revision 1.1  88/12/23  18:02:32  mark
 * Initial revision
 * 
 */

/* Headers */

#include "pax.h"

#ifndef lint
static char    *Ident = "$Id: regexp.c,v 1.2 89/02/12 10:05:39 mark Exp $";
#endif


/*
 * 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 "nxt" pointer, possibly plus an operand.  "Nxt" pointers of
 * all nodes except BRANCH implement concatenation; a "nxt" 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
				 * nxt... */
#define	BACK	7		/* no	Match "", "nxt" 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	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 "nxt" pointers, since precedence prevents
 *		anything being concatenated to any individual branch.  The
 *		"nxt" pointer of the last BRANCH in a choice points to the
 *		thing following the whole choice.  This is also where the
 *		final "nxt" pointer of each individual branch points; each
 *		branch starts with the operand node of a BRANCH node.
 *
 * BACK		Normal "nxt" pointers all implicitly point forward; BACK
 *		exists to make loop structures possible.
 *
 * STAR		complex '*', are implemented as circular BRANCH structures 
 *		using BACK.  Simple cases (one character per match) are 
 *		implemented with STAR 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 "nxt" pointer.
 * "Nxt" 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 "nxt" 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)

/*
 * Utility definitions.
 */

#define	FAIL(m)	{ regerror(m); return(NULL); }
#define	ISMULT(c)	((c) == '*')
#define	META	"^$.[()|*\\"
#ifndef CHARBITS
#define	UCHARAT(p)	((int)*(unsigned char *)(p))
#else
#define	UCHARAT(p)	((int)*(p)&CHARBITS)
#endif

/*
 * Flags to be passed up and down.
 */
#define	HASWIDTH	01	/* Known never to match null string. */
#define	SIMPLE		02	/* Simple enough to be STAR operand. */
#define	SPSTART		04	/* Starts with * */
#define	WORST		0	/* Worst case. */

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

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

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

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

    /* First pass: determine size, legality. */
    regparse = exp;
    regnpar = 1;
    regsize = 0L;
    regcode = &regdummy;
    regc(MAGIC);
    if (reg(0, &flags) == (char *)NULL)
	return ((regexp *)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 == (regexp *) NULL)
	FAIL("out of space");

    /* Second pass: emit code. */
    regparse = exp;
    regnpar = 1;
    regcode = r->program;
    regc(MAGIC);
    if (reg(0, &flags) == NULL)
	return ((regexp *) NULL);

    /* Dig out information for optimizations. */
    r->regstart = '\0';		/* Worst-case defaults. */
    r->reganch = 0;
    r->regmust = NULL;
    r->regmlen = 0;
    scan = r->program + 1;	/* First BRANCH. */
    if (OP(regnext(scan)) == END) {	/* Only one top-level choice. */
	scan = OPERAND(scan);

	/* Starting-point info. */
	if (OP(scan) == EXACTLY)
	    r->regstart = *OPERAND(scan);
	else if (OP(scan) == BOL)
	    r->reganch++;

	/*
	 * If there's something expensive in the r.e., find the longest
	 * literal string that must appear and make it the regmust.  Resolve
	 * ties in favor of later strings, since the regstart check works
	 * with the beginning of the r.e. and avoiding duplication
	 * strengthens checking.  Not a strong reason, but sufficient in the
	 * absence of others. 
	 */
	if (flags & SPSTART) {
	    longest = NULL;
	    len = 0;
	    for (; scan != NULL; scan = regnext(scan))
		if (OP(scan) == EXACTLY && strlen(OPERAND(scan)) >= len) {
		    longest = OPERAND(scan);
		    len = strlen(OPERAND(scan));
		}
	    r->regmust = longest;
	    r->regmlen = len;
	}
    }
    return (r);
}

/*
 - reg - regular expression, i.e. main body or parenthesized thing
 *
 * Caller must absorb opening parenthesis.
 *
 * Combining parenthesis handling with the base level of regular expression
 * is a trifle forced, but the need to tie the tails of the branches to what
 * follows makes it hard to avoid.
 */
static char *reg(paren, flagp)
int             paren;		/* Parenthesized? */
int            *flagp;
{
    register char  *ret;
    register char  *br;
    register char  *ender;
    register int    parno;
    int             flags;

    *flagp = HASWIDTH;		/* Tentatively. */

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

    /* Pick up the branches, linking them together. */
    br = regbranch(&flags);
    if (br == (char *)NULL)
	return ((char *)NULL);
    if (ret != (char *)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 == (char *)NULL)
	    return ((char *)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 != (char *)NULL; br = regnext(br))
	regoptail(br, ender);

    /* Check for proper termination. */
    if (paren && *regparse++ != ')') {
	FAIL("unmatched ()");
    } else if (!paren && *regparse != '\0') {
	if (*regparse == ')') {
	    FAIL("unmatched ()");
	} else
	    FAIL("junk on end");/* "Can't happen". */
	/* NOTREACHED */
    }
    return (ret);
}

/*
 - regbranch - one alternative of an | operator
 *
 * Implements the concatenation operator.
 */
static char  *regbranch(flagp)
int            *flagp;
{
    register char  *ret;
    register char  *chain;
    register char  *latest;
    int             flags;

    *flagp = WORST;		/* Tentatively. */

    ret = regnode(BRANCH);
    chain = (char *)NULL;
    while (*regparse != '\0' && *regparse != '|' && *regparse != ')') {
	latest = regpiece(&flags);
	if (latest == (char *)NULL)
	    return ((char *)NULL);
	*flagp |= flags & HASWIDTH;
	if (chain == (char *)NULL)	/* First piece. */
	    *flagp |= flags & SPSTART;
	else
	    regtail(chain, latest);
	chain = latest;
    }
    if (chain == (char *)NULL)		/* Loop ran zero times. */
	regnode(NOTHING);

    return (ret);
}

/*
 - regpiece - something followed by possible [*]
 *
 * Note that the branching code sequence used for * is somewhat optimized:  
 * they use the same NOTHING node as both the endmarker for their branch 
 * list and the body of the last branch.  It might seem that this node could 
 * be dispensed with entirely, but the endmarker role is not redundant.
 */
static char *regpiece(flagp)
int            *flagp;
{
    register char  *ret;
    register char   op;
    register char  *nxt;
    int             flags;

    ret = regatom(&flags);
    if (ret == (char *)NULL)
	return ((char *)NULL);

    op = *regparse;
    if (!ISMULT(op)) {
	*flagp = flags;
	return (ret);
    }
    if (!(flags & HASWIDTH))
	FAIL("* operand could be empty");
    *flagp = (WORST | SPSTART);

    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. */
    }