regexp.c

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/*
 * 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.                                ***
 *
 * SCCS: @(#) regexp.c 1.13 97/04/29 17:49:17
 */

#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;