text: Title: Update various regex escape sequences
  Body: The latest versions of Python are more strict wrt. escape in regex.
  For instance with 3.6.8, there are 10+ warnings like this one:
  ```
  ...
  lib/python3.6/site-packages/nltk/featstruct.py:2092: DeprecationWarning: invalid escape sequence \d
      RANGE_RE = re.compile('(-?\d+):(-?\d+)')
  ```
  
  The regex(es) should be updated to silence these warnings.