If non-nil, Ido will do flexible string matching. Flexible
matching means that if the entered string does not match any item, any
item containing the entered characters in the given sequence will
match.
If ido-enable-flex-matching is non-nil, Ido will do a
more flexible matching (unless regexp matching is active) to find
possible matches among the available buffer or file names if no
matches are found using the normal prefix or substring matching.
The flexible matching implies that any item which simply contains all of the entered characters in the specified sequence will match.
For example, if you have four files alpha, beta, gamma, and delta, entering ‘aa’ will match alpha and gamma, while ‘ea’ matches beta and delta. If prefix matching is also active, ‘aa’ only matches alpha, while ‘ea’ does not match any files.