If you don't specify any options, grep regards a pattern as BRE.
http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap09.html#tag_09_03
(', `)' and `?' are treated as normal character in BRE. If you expect
that they are treated as special character, you must escape them with
backslash.
BTW, I see you want to use ERE.
http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap09.html#tag_09_04
If so, you must specify -E option. I test below on Fedora 20.
$ cd /usr/include/c++/4.8.2
$ grep -E "#include <(bits/)?random(\.h)?>" * 2>/dev/null
$ grep "#include <(bits/)?random(\.h)?>" * 2>/dev/null
random:#include <bits/random.h>
