title: How to express a escape sequence in regex. ("" -> ")
body: <p>It's a question in Engineering a Compiler. 2.3.2</p>
  
  <p>In PL/I, we can insert a quotation mark into a string by writing two quotation marks in a row.</p>
  
  <p>For example, we write "This is a quotation mark, "", do you see that?" as a string and the output should be </p>
  
  <p>This is a quotation mark, ", do you see that?</p>
  
  <p>So, I'm trouble in writing a regex to express this sentence. </p>