title: Python Regex to detect underscore between letters
body: <p>How do I make a regex in python that returns a string with all underscores between lowercase letters?
  For example, it should detect and return: <code>'aa_bb_cc' ,  'swd_qq' , 'hello_there_friend'</code></p>
  <p>But it should not return these: <code>'aA_bb' , 'aa_'  ,  '_ddQ' , 'aa_baa_2cs'</code></p>
  <p>My code is <code>([a-z]+_[a-z]+)+</code>    ,  but it returns only one underscore. It should return all underscores seperated by lowercase letters.</p>
  <p>For example, when I pass the string <code>&quot;aab_cbbbc_vv&quot;</code>, it returns only <code>'aab_cbbbc'</code> instead of <code>'aab_cbbbc_vv'</code></p>
  <p>Thank you</p>