title: regex group reference error
body: <pre><code>p = r'([\,|\.]\d{1}$)'
  re.sub(p, r"\1", v)
  </code></pre>
  
  <p>works, but I want to add a zero to the capture group, not replace with capture group '10', how can I do this?</p>
  
  <pre><code>re.sub(p, r"\10", v)
  </code></pre>
  
  <p>fails:</p>
  
  <pre><code>Traceback (most recent call last):
    File "&lt;stdin&gt;", line 1, in &lt;module&gt;
    File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/re.py", line 151, in sub
      return _compile(pattern, flags).sub(repl, string, count)
    File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/re.py", line 275, in filter
      return sre_parse.expand_template(template, match)
    File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/sre_parse.py", line 802, in expand_template
      raise error, "invalid group reference"
  sre_constants.error: invalid group reference
  </code></pre>