regex question mark - make the previous element optional

Will let the previous element to exist precisely 0 or 1 times - basically makes is optional.

Example:

egrep -r 'disabled?' /etc/

Output:

/etc/exim4/exim4.conf.template:  # This is disabled by default so that DNSless systems don't break. If
/etc/exim4/exim4.conf.template:  # MTAs, you may want to disable this. If you are handling both relaying fro
/etc/sysctl.conf:#  Enabling this option disables Stateless Address Autoconfiguration

So, last letter d in disabled is optional.
Note: this also matches the word disables