I have 3 directories at current path.

$ls a_0db_data a_clean_0db_data a_clean_data $ls a_*_data a_0db_data: a_clean_0db_data: a_clean_data: $ls a_[a-z]*_data a_clean_0db_data: a_clean_data: 

I expected last ls command to match only a_clean_data. Why did it also match the one containing 0?

bash --version GNU bash, version 4.2.24(1)-release (i686-pc-linux-gnu)