See http://xkcd.com/859/ for why.
Various releases of the 5.13.x series have added new regular expression
modifiers, C</a>, C</d>, C</l>, and C</u>. They were only available in
-infix form (e.g., C<(?a:...)> until this release; now they are usable
+infix form (e.g., C<(?a:...)>) until this release; now they are usable
in suffix form. This change was made too late to change all the
affected documentation, so there are a number of places that erroneously
say these must be used in infix form.