The problem is `apropos' under OS X behaves differently from Linux.
1. The format: for command `apropos sudo', the output under Linux:
...
sudo (8)             - execute a command as another user
...
and the output under OS X:
...
sudo(8)                  - execute a command as another user
...
one-space difference.
2. For some items sharing a same manpage, e.g. for `apropos printf',
Linux:
...
printf (3)           - formatted output conversion
snprintf (3)         - formatted output conversion
sprintf (3)          - formatted output conversion
vprintf (3)          - formatted output conversion
vsnprintf (3)        - formatted output conversion
vsprintf (3)         - formatted output conversion
...
OS X:
...
printf(3), fprintf(3), sprintf(3), snprintf(3), asprintf(3), dprintf(3), vprintf(3), vfprintf(3), vsprintf(3), vsnprintf(3), vasprintf(3), vdprintf(3) - formatted output conversion
...
OS X puts them in one line.
So, we need special handling here, otherwise, completion with prefix
`man 3 printf' will give the result of "man 3 printf\(3\),"