Also set RXf_RTRIM if the pattern is /\s*$/u

It takes a bit more care to handle 0-or-more alternations, but this covers
more regex variantions used in the wild.

regcomp.c

@@ -8466,7 +8466,7 @@ Perl_re_op_compile(pTHX_ SV ** const patternp, int pat_count,
         /* It's safe to read through *next only if OP(first) is a regop of
          * the right type (not EXACT, for example).
          */
-        U8 nop = (fop == NOTHING || fop == MBOL || fop == SBOL || fop == PLUS)
+        U8 nop = (fop == NOTHING || fop == MBOL || fop == SBOL || fop == PLUS || fop == STAR)
                 ? OP(next) : 0;
 
         if (PL_regkind[fop] == NOTHING && nop == END)
@@ -8488,14 +8488,16 @@ Perl_re_op_compile(pTHX_ SV ** const patternp, int pat_count,
                   && *(STRING(first)) == ' '
                   && OP(regnext(first)) == END )
             RExC_rx->extflags |= (RXf_SKIPWHITE|RXf_WHITE);
-        else if (fop == PLUS
+        else if ((fop == PLUS || fop == STAR)
                  && nop == POSIXU && FLAGS(next) == _CC_SPACE) {
             regnode *second = regnext(first);
             regnode *third = (OP(second) == EOS || OP(second) == SEOL)
                 ? regnext(second) : NULL;
             if (third && OP(third) == END) {
                 /* /[[:space:]]+\z/u
-                 * /[[:space:]]+$/u */
+                 * /[[:space:]]+$/u
+                 * /[[:space:]]*$/u
+                 * etc */
                 RExC_rx->extflags |= RXf_RTRIM | RXf_CHECK_ALL;
             }
         }

regexec.c

@@ -923,10 +923,17 @@ Perl_re_intuit_start(pTHX_
 
     if(prog->extflags & RXf_RTRIM) {
         const char *s = strend;
+        if (strpos == strend && prog->minlen == 0) {
+            /* \s* and we are asked to match an empty string */
+            DEBUG_EXECUTE_r(Perl_re_printf( aTHX_
+                                            "Intuit: %sSuccessfully guessed:%s match at offset %ld\n",
+                                            PL_colors[4], PL_colors[5], (long)(s - strbeg)) );
+            return s;
+        }
         if (strpos >= strend) {
             /* This should be unreachable:
              * String shorter than min possible regex match (0 < 1)
-             * but in the future we might want to also handle *, ? and {0,...}
+             * but in the future we might want to also handle ? and {0,...}
              */
             DEBUG_EXECUTE_r(Perl_re_printf( aTHX_
                                             "  rtrim intuit on empty string ...\n"));
@@ -960,7 +967,7 @@ Perl_re_intuit_start(pTHX_
                 }
             }
         }
-        if (s < strend) {
+        if (s < strend || s == strend && prog->minlen == 0) {
             DEBUG_EXECUTE_r(Perl_re_printf( aTHX_
                                             "Intuit: %sSuccessfully guessed:%s match at offset %ld\n",
                                             PL_colors[4], PL_colors[5], (long)(s - strbeg)) );