Next try...

/^(([a-zA-Z0-9]{2,}_)*[a-zA-Z0-9]{2,})$/ && print "$_\n" 
    for "1", "a\n", "aa\n", "a_", "aa", "aa_bb_cc_dd", "a1_11_1c_11", 
+"aaa", "aa_";
[download]

Still matches a trailing newline. It does not match "aa_" on purpose as this is not a sequence of two or more alphanumeric characters separated by underscores which was my claim if not the original question.