title: Perl regexp to match random character/number combination?
body: <p>I'm trying to match patterns like these with perl regexp:</p>
  
  <pre><code>_b04it4_
  _bg4n5p_
  _qp9bp_
  _hp32z7_
  </code></pre>
  
  <p>...that is, underscore followed by some combination of characters and numbers.
  I guess the "rule" is that there are >=1 [a-z] characters and >=1 [0-9] character/number, and no spaces, "mixed in any combination", between two underscore-characters.</p>
  
  <p>And want to replace this with something, eg. "_X_". 
  I'd appreciate some help with this .. My own attempts are looking horrible and don't work very well :)</p>