fix ugly bugs in TRE regex parser

1. * in BRE is not special at the beginning of the regex or a
subexpression. this broke ncurses' build scripts.

2. \\( in BRE is a literal \ followed by a literal (, not a literal \
followed by a subexpression opener.

3. the ^ in \\(^ in BRE is a literal ^ only at the beginning of the
entire BRE. POSIX allows treating it as an anchor at the beginning of
a subexpression, but TRE's code for checking if it was at the
beginning of a subexpression was wrong, and fixing it for the sake of
supporting a non-portable usage was too much trouble when just
removing this non-portable behavior was much easier.

this patch also moved lots of the ugly logic for empty atom checking
out of the default/literal case and into new cases for the relevant
characters. this should make parsing faster and make the code smaller.
if nothing else it's a lot more readable/logical.

at some point i'd like to revisit and overhaul lots of this code...

src/regex/regcomp.c

@@ -961,6 +961,8 @@ tre_parse(tre_parse_ctx_t *ctx)
   tre_stack_t *stack = ctx->stack;
   int bottom = tre_stack_num_objects(stack);
   int depth = 0;
+  wchar_t wc;
+  int clen;
 
   if (!ctx->nofirstsub)
     {
@@ -1155,10 +1157,9 @@ tre_parse(tre_parse_ctx_t *ctx)
 	    {
 	    case CHAR_LPAREN:  /* parenthesized subexpression */
 
-	      if (ctx->cflags & REG_EXTENDED
-		  || (ctx->re > ctx->re_start
-		      && *(ctx->re - 1) == CHAR_BACKSLASH))
+	      if (ctx->cflags & REG_EXTENDED)
 		{
+		lparen:
 		  depth++;
 		    {
 		      ctx->re++;
@@ -1174,25 +1175,6 @@ tre_parse(tre_parse_ctx_t *ctx)
 		goto parse_literal;
 	      break;
 
-	    case CHAR_RPAREN:  /* end of current subexpression */
-	      if ((ctx->cflags & REG_EXTENDED && depth > 0)
-		  || (ctx->re > ctx->re_start
-		      && *(ctx->re - 1) == CHAR_BACKSLASH))
-		{
-		  /* We were expecting an atom, but instead the current
-		     subexpression was closed.	POSIX leaves the meaning of
-		     this to be implementation-defined.	 We interpret this as
-		     an empty expression (which matches an empty string).  */
-		  result = tre_ast_new_literal(ctx->mem, EMPTY, -1, -1);
-		  if (result == NULL)
-		    return REG_ESPACE;
-		  if (!(ctx->cflags & REG_EXTENDED))
-		    ctx->re--;
-		}
-	      else
-		goto parse_literal;
-	      break;
-
 	    case CHAR_LBRACKET: /* bracket expression */
 	      ctx->re++;
 	      status = tre_parse_bracket(ctx, &result);
@@ -1203,13 +1185,14 @@ tre_parse(tre_parse_ctx_t *ctx)
 	    case CHAR_BACKSLASH:
 	      /* If this is "\(" or "\)" chew off the backslash and
 		 try again. */
-	      if (!(ctx->cflags & REG_EXTENDED)
-		  && (*(ctx->re + 1) == CHAR_LPAREN
-		      || *(ctx->re + 1) == CHAR_RPAREN))
+	      if (!(ctx->cflags & REG_EXTENDED) && *(ctx->re + 1) == CHAR_LPAREN)
 		{
 		  ctx->re++;
-		  STACK_PUSHX(stack, int, PARSE_ATOM);
-		  break;
+		  goto lparen;
+		}
+	      if (!(ctx->cflags & REG_EXTENDED) && *(ctx->re + 1) == CHAR_LPAREN)
+		{
+		  goto empty_atom;
 		}
 
 	      /* If a macro is used, parse the expanded macro recursively. */
@@ -1369,12 +1352,9 @@ tre_parse(tre_parse_ctx_t *ctx)
 	      break;
 
 	    case CHAR_CARET:	 /* beginning of line assertion */
-	      /* '^' has a special meaning everywhere in EREs, and in the
-		 beginning of the RE and after \( is BREs. */
+	      /* '^' has a special meaning everywhere in EREs, and at
+		 beginning of BRE. */
 	      if (ctx->cflags & REG_EXTENDED
-		  || (ctx->re - 2 >= ctx->re_start
-		      && *(ctx->re - 2) == CHAR_BACKSLASH
-		      && *(ctx->re - 1) == CHAR_LPAREN)
 		  || ctx->re == ctx->re_start)
 		{
 		  result = tre_ast_new_literal(ctx->mem, ASSERTION,
@@ -1389,10 +1369,8 @@ tre_parse(tre_parse_ctx_t *ctx)
 
 	    case CHAR_DOLLAR:	 /* end of line assertion. */
 	      /* '$' is special everywhere in EREs, and in the end of the
-		 string and before \) is BREs. */
+		 string in BREs. */
 	      if (ctx->cflags & REG_EXTENDED
-		  || (*(ctx->re + 1) == CHAR_BACKSLASH
-		      && *(ctx->re + 2) == CHAR_RPAREN)
 		  || !*(ctx->re + 1))
 		{
 		  result = tre_ast_new_literal(ctx->mem, ASSERTION,
@@ -1405,34 +1383,27 @@ tre_parse(tre_parse_ctx_t *ctx)
 		goto parse_literal;
 	      break;
 
+	    case CHAR_RPAREN:
+	      if (!depth)
+	        goto parse_literal;
+	    case CHAR_STAR:
+	    case CHAR_PIPE:
+	    case CHAR_LBRACE:
+	    case CHAR_PLUS:
+	    case CHAR_QUESTIONMARK:
+	      if (!(ctx->cflags & REG_EXTENDED))
+	        goto parse_literal;
+
+	    empty_atom:
+	      result = tre_ast_new_literal(ctx->mem, EMPTY, -1, -1);
+	      if (!result)
+		return REG_ESPACE;
+	      break;
+
 	    default:
 	    parse_literal:
 
-	      /* We are expecting an atom.  If the subexpression (or the whole
-		 regexp ends here, we interpret it as an empty expression
-		 (which matches an empty string).  */
-	      if (
-		  (!*ctx->re
-		   || *ctx->re == CHAR_STAR
-		   || (ctx->cflags & REG_EXTENDED
-		       && (*ctx->re == CHAR_PIPE
-			   || *ctx->re == CHAR_LBRACE
-			   || *ctx->re == CHAR_PLUS
-			   || *ctx->re == CHAR_QUESTIONMARK))
-		   /* Test for "\)" in BRE mode. */
-		   || (!(ctx->cflags & REG_EXTENDED)
-		       && !*(ctx->re + 1)
-		       && *ctx->re == CHAR_BACKSLASH
-		       && *(ctx->re + 1) == CHAR_LBRACE)))
-		{
-		  result = tre_ast_new_literal(ctx->mem, EMPTY, -1, -1);
-		  if (!result)
-		    return REG_ESPACE;
-		  break;
-		}
-
-	      wchar_t wc;
-	      int clen = mbtowc(&wc, ctx->re, -1);
+	      clen = mbtowc(&wc, ctx->re, -1);
 	      if (clen<0) clen=1, wc=WEOF;
 
 	      /* Note that we can't use an tre_isalpha() test here, since there