Solutions: 

^CME\.b?+[^/.]

[download]
or 
^CME\.(?:b[^/.]|[^b/.])

[download]

Note that you seem to think grep -P uses Perl, but it uses PCRE. There are some differences between the two.

In reply to Re: Strange negated character class behavior by ikegami
in thread Strange negated character class behavior by miguelnyc703

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