makes C<split //> much faster than it would otherwise be. It's even
faster than C<unpack>.
-=item RXf_MODIFIES_VARS
+=item RXf_NO_INPLACE_SUBST
Added in perl 5.18.0, this flag indicates that a regular expression might
-assign to non-magical variables (such as $REGMARK and $REGERROR) during
-matching. C<s///> will skip certain optimisations when this is set.
+perform an operation that would interfere with inplace substituion. For
+instance it might contain lookbehind, or assign to non-magical variables
+(such as $REGMARK and $REGERROR) during matching. C<s///> will skip
+certain optimisations when this is set.
=back