I have trouble with validate_username for registration when i limit characters to USERNAME_LETTER_NUM_SPACERS.

The check with mb_ereg_search fails also for valid user names that should be allowed...eg "test_test" or "name lastname"

I have validated that only with 3.0.0 but that code it hasnt changed in 3.0.1

MyMultibyte Settings are:
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Multibyte Support enabled
Multibyte string engine libmbfl
Multibyte (japanese) regex support enabled
Multibyte regex (oniguruma) version 4.4.4
Multibyte regex (oniguruma) backtrack check On