title: perl regexp minimal match the word before underscore
body: <p>may i know how to write the regexp for this case?</p>
  
  <pre><code>$aword = abc_def_ghi regexp substitute to abc_def_xxx
  $bword = jkl_mno_pqr regexp substitute to jkl_mno_xxx
  $cword = abc_def     regexp substitute to abc_xxx
  </code></pre>
  
  <p>Zh</p>