title: syntax for specifying matched digit string in Emacs replace-regexp?
body: <p>In Emacs, I'm trying to prepend instances of digits with a text string, so that every <code>&lt;/a&gt;1&lt;/h2&gt;</code> will become <code>&lt;/a&gt;Chapter 1&lt;/h2&gt;</code> and so on.</p>
  
  <p>I'm able to find matches using this regexp: <code>&lt;/a&gt;\([0-9]+\)</code></p>
  
  <p>How do I specify the existing matched digit in my replacement string, rather than the entire string?</p>