toke.c: Change S_scan_str parameter meaning

The 'keep_bracketed' parameter is used to tell this static function to
not delete the escaping backslash if the string's delimiter character is
also found within the interior of the string.

This parameter is always currently false, except when compiled with
PERL_MAD.  However it has been Okayed to remove this compilation option,
so the parameter can be changed to whatever is desired.

A future commit will change this parameter to be true sometimes.  In
doing so, it sets up a conflict with the fairly new 're_reparse'
parameter (this conflict also exists with PERL_MAD, but clearly nobody
has tried to compile it that way for a while)

However the new conflict only happens when the opening delimiter of the
string is the same as the closing one, and happily the new commits only
need to have this parameter be true when the opening and closing are
different, so things can be set up so that there is actually no
conflict, given that MAD is being removed.

At the time that scan_str is called, what the delimiters are is not
necessarily known, so this commit refactors the meaning of the parameter
to be TRUE iff the delimiters that scan_str finds actually differ.  It
renames the parameter to indicate this.

toke.c

@@ -10498,7 +10498,8 @@ S_scan_inputsymbol(pTHX_ char *start)
 /* scan_str
    takes:
 	start			position in buffer
-	keep_quoted		preserve \ on the embedded delimiter(s)
+        keep_bracketed_quoted   preserve \ quoting of embedded delimiters, but
+                                only if they are of the open/close form
 	keep_delims		preserve the delimiters around the string
 	re_reparse		compiling a run-time /(?{})/:
 				   collapse // to /,  and skip encoding src
@@ -10550,7 +10551,7 @@ S_scan_inputsymbol(pTHX_ char *start)
 */
 
 STATIC char *
-S_scan_str(pTHX_ char *start, int keep_quoted, int keep_delims, int re_reparse,
+S_scan_str(pTHX_ char *start, int keep_bracketed_quoted, int keep_delims, int re_reparse,
 		 bool deprecate_escaped_meta, char **delimp
     )
 {
@@ -10625,6 +10626,13 @@ S_scan_str(pTHX_ char *start, int keep_quoted, int keep_delims, int re_reparse,
             || ! ckWARN_d(WARN_DEPRECATED)))
     {
         deprecate_escaped_meta = FALSE;
+
+    /* By only preserving quoting of open/close delimiters, we avoid a conflict
+     * with 're_reparse', which in one place below is looked at only if
+     * 'keep_bracketed_quoted' is FALSE, but also only if the opening and
+     * closing delimiters are different */
+    if (PL_multi_open == PL_multi_close) {
+        keep_bracketed_quoted = FALSE;
     }
 
     /* create a new SV to hold the contents.  79 is the SV's initial length.
@@ -10710,7 +10718,7 @@ S_scan_str(pTHX_ char *start, int keep_quoted, int keep_delims, int re_reparse,
 			for (t = svlast-2; t >= SvPVX_const(sv) && *t == '\\';)
 			    t--;
 			if ((svlast-1 - t) % 2) {
-			    if (!keep_quoted) {
+			    if (!keep_bracketed_quoted) {
 				*(svlast-1) = term;
 				*svlast = '\0';
 				SvCUR_set(sv, SvCUR(sv) - 1);
@@ -10728,7 +10736,7 @@ S_scan_str(pTHX_ char *start, int keep_quoted, int keep_delims, int re_reparse,
 			    /* At here, all closes are "was quoted" one,
 			       so we don't check PL_multi_close. */
 			    if (*t == '\\') {
-				if (!keep_quoted && *(t+1) == PL_multi_open)
+				if (!keep_bracketed_quoted && *(t+1) == PL_multi_open)
 				    t++;
 				else
 				    *w++ = *t++;
@@ -10769,13 +10777,12 @@ S_scan_str(pTHX_ char *start, int keep_quoted, int keep_delims, int re_reparse,
 		    COPLINE_INC_WITH_HERELINES;
 		/* handle quoted delimiters */
 		if (*s == '\\' && s+1 < PL_bufend && term != '\\') {
-		    if (!keep_quoted
+		    if (!keep_bracketed_quoted
 		        && (s[1] == term
 			    || (re_reparse && s[1] == '\\'))
 		    )
 			s++;
-		    /* any other quotes are simply copied straight through */
-		    else
+		    else /* any other quotes are simply copied straight through */
 			*to++ = *s++;
 		}
 		/* terminate when run out of buffer (the for() condition), or
@@ -10804,7 +10811,7 @@ S_scan_str(pTHX_ char *start, int keep_quoted, int keep_delims, int re_reparse,
 		    COPLINE_INC_WITH_HERELINES;
 		/* backslashes can escape the open or closing characters */
 		if (*s == '\\' && s+1 < PL_bufend) {
-		    if (!keep_quoted &&
+		    if (!keep_bracketed_quoted &&
 			((s[1] == PL_multi_open) || (s[1] == PL_multi_close)))
                     {
 			s++;