title: listing files in the directory using grep
body: <p>List all the files in <code>/usr/bin</code> whose <strong>filenames</strong> contain <strong>lowercase English alphabet
  characters</strong> only and also contain the word <strong>file</strong> as a (contiguous) substring. </p>
  
  <p>For example, <strong>file</strong> and <strong>profiles</strong> are such files, but <strong>git-ls-files</strong> is not.</p>
  
  <p>This is the exact question I have and I can only use <code>grep</code>, <code>ls</code>, <code>cat</code> and <code>wc</code> for it.</p>
  
  <pre><code>ls /usr/bin/ | grep '[^-]*file'
  </code></pre>
  
  <p>This is what I got so far and output is below. I dont know how to display for example just <code>file</code> since <code>* is zero or more occurences</code>. And no idea how to put lowercase thing in the regex as well..</p>
  
  <pre><code>check-binary-files
  clean-binary-files
  desktop-file-install
  desktop-file-validate
  ecryptfs-rewrite-file
  file
  filep
  git-cat-file
  git-diff-files
  git-ls-files
  git-merge-file
  git-merge-one-file
  git-unpack-file
  lockfile
  nsrfile
  pamfile
  pcprofiledump
  pnmfile
  ppufiles
  profiles
  </code></pre>