Open a file in cperl mode containing the following line:

  /(\d{4})(?: +){2}/;

e.g.: "emacs -Q file" then M-x cperl-mode <RET>

Put the cursor over the second opening brace, and type <DEL>
several times to delete what is before "{2}". When one types
<DEL> at this point:

  /(\d{4})(?:{2}/;
             ^cursor

Emacs freezes. One can type C-g to interrupt.

One can also reproduce the bug by typing: /(\d{4})(?{2
Emacs freezes before the "2" appears.

Note: This was the old Debian bug 734325. It is different from
bug 911573, which is fixed upstream (I could test), but this one
still occurs.