-#define REXEC_COPY_STR 0x01 /* Need to copy the string. */
-#define REXEC_CHECKED 0x02 /* check_substr already checked. */
-#define REXEC_SCREAM 0x04 /* use scream table. */
-#define REXEC_IGNOREPOS 0x08 /* \G matches at start. */
-#define REXEC_NOT_FIRST 0x10 /* This is another iteration of //g. */
- /* under REXEC_COPY_STR, it's ok for the
- * engine (modulo PL_sawamperand etc)
- * to skip copying ... */
-#define REXEC_COPY_SKIP_PRE 0x20 /* ...the $` part of the string, or */
-#define REXEC_COPY_SKIP_POST 0x40 /* ...the $' part of the string */
+
+/* bits in flags arg of Perl_regexec_flags() */
+
+#define REXEC_COPY_STR 0x01 /* Need to copy the string for captures. */
+#define REXEC_CHECKED 0x02 /* re_intuit_start() already called. */
+#define REXEC_SCREAM 0x04 /* currently unused. */
+#define REXEC_IGNOREPOS 0x08 /* use stringarg, not pos(), for \G match */
+#define REXEC_NOT_FIRST 0x10 /* This is another iteration of //g:
+ no need to copy string again */
+
+ /* under REXEC_COPY_STR, it's ok for the
+ engine (modulo PL_sawamperand etc)
+ to skip copying: ... */
+#define REXEC_COPY_SKIP_PRE 0x20 /* ...the $` part of the string, or */
+#define REXEC_COPY_SKIP_POST 0x40 /* ...the $' part of the string */
+#define REXEC_FAIL_ON_UNDERFLOW 0x80 /* fail the match if $& would start before
+ the start pos (so s/.\G// would fail
+ on second iteration */