title: I have some trouble with "grep"and collating symbols
body: <p>Here is my problem.A existed file named data.f,I use collating symbol "48",I want to match "48"in my file with Collating symbols in bracket expressions.</p>
  
  <pre><code>grep '[[.48.]]' data.f
  </code></pre>
  
  <p>but there is some error tip:</p>
  
  <pre><code>grep: Invalid collation character
  </code></pre>
  
  <p>but, there is no problem with character classes in bracket expressions.</p>
  
  <pre><code>grep "[[:alpha:]]" data.f
  </code></pre>