-#define RXf_SPLIT (1<<(RXf_BASE_SHIFT-1))
-#if RXf_SPLIT != RXf_PMf_SPLIT
-# error "RXf_SPLIT does not match RXf_PMf_SPLIT"
-#endif
-
-/* Manually decorate this function with gcc-style attributes just to
- * avoid having to restructure the header files and their called order,
- * as proto.h would have to be included before this file, and isn't */
-
-PERL_STATIC_INLINE const char *
-get_regex_charset_name(const U32 flags, STRLEN* const lenp)
- __attribute__warn_unused_result__;
-
-#define MAX_CHARSET_NAME_LENGTH 2
+#define RXf_SPLIT RXf_PMf_SPLIT
+
+/* Currently the regex flags occupy a single 32-bit word. Not all bits are
+ * currently used. The lower bits are shared with their corresponding PMf flag
+ * bits, up to but not including _RXf_PMf_SHIFT_NEXT. The unused bits
+ * immediately follow; finally the used RXf-only (unshared) bits, so that the
+ * highest bit in the word is used. This gathers all the unused bits as a pool
+ * in the middle, like so: 11111111111111110000001111111111
+ * where the '1's represent used bits, and the '0's unused. This design allows
+ * us to allocate off one end of the pool if we need to add a shared bit, and
+ * off the other end if we need a non-shared bit, without disturbing the other
+ * bits. This maximizes the likelihood of being able to change things without
+ * breaking binary compatibility.
+ *
+ * To add shared bits, do so in op_reg_common.h. This should change
+ * _RXf_PMf_SHIFT_NEXT so that things won't compile. Then come to regexp.h and
+ * op.h and adjust the constant adders in the definitions of RXf_BASE_SHIFT and
+ * Pmf_BASE_SHIFT down by the number of shared bits you added. That's it.
+ * Things should be binary compatible. But if either of these gets to having
+ * to subtract rather than add, leave at 0 and instead adjust all the entries
+ * that are in terms of it. But if the first one of those is already
+ * RXf_BASE_SHIFT+0, there are no bits left, and a redesign is in order.
+ *
+ * To remove unshared bits, just delete its entry. If you're where breaking
+ * binary compatibility is ok to do, you might want to adjust things to move
+ * the newly opened space so that it gets absorbed into the common pool.
+ *
+ * To add unshared bits, first use up any gaps in the middle. Otherwise,
+ * allocate off the low end until you get to RXf_BASE_SHIFT+0. If that isn't
+ * enough, move RXf_BASE_SHIFT down (if possible) and add the new bit at the
+ * other end instead; this preserves binary compatibility.
+ *
+ * For the regexp bits, PL_reg_extflags_name[] in regnodes.h has a comment
+ * giving which bits are used/unused */