title: Removing hidden characters from within strings
body: <p>My problem:  </p>
  
  <p>I have a .NET application that sends out newsletters via email.  When the newsletters are viewed in outlook, outlook displays a question mark in place of a hidden character it can’t recognize.  These hidden character(s) are coming from end users who copy and paste html that makes up the newsletters into a form and submits it.   A c# trim() removes these hidden chars if they occur at the end or beginning of the string. When the newsletter is viewed in gmail, gmail does a good job ignoring them.  When pasting these hidden characters in a word document and I turn on the “show paragraph marks and hidden symbols” option the symbols appear as one rectangle inside a bigger rectangle.   Also the text that makes up the newsletters can be in any language, so accepting Unicode chars is a must.  I've tried looping through the string to detect the character but the loop doesn't recognize it and passes over it.  Also asking the end user to paste the html into notepad first before submitting it is out of the question. </p>
  
  <p>My question:<br>
  How can I detect and eliminate these hidden characters using C#?  </p>