title: Remove special characters from data frame
body: <p>I have a matrix that contains the string "Energy per �m". 
  Before the 'm' is a diamond shaped symbol with a question mark in it - I don't know what it is. </p>
  
  <p>I have tried to get rid of it by using this on the column of the matrix:</p>
  
  <pre><code>a=gsub('Energy per �m','',a) 
  </code></pre>
  
  <p>[and using copy/paste for the first term of gsub], but it does not work.[unexpected symbol in "a=rep(5,Energy per"]. When I try to extract something from the original matrix with grepl I get:</p>
  
  <pre><code>46: In grepl("ref. value", raw$parameter) :
  input string 15318 is invalid in this locale
  </code></pre>
  
  <p>How can I get rid of all this sort of signs? I would like to have only 0-9, A-Z, a-z, / and '. The rest can be zapped.</p>