Lib\idlelib\colorizer.py, line 25, is:
  stringprefix = r"(?i:\br|u|f|fr|rf|b|br|rb)?"

The r prefix, but only the r prefix, must be preceded by a non-word character.

On pydev thread "IDLE colorizer", MRAB noted: "The \b will apply only to the first choice. Shouldn't it be more like:
  stringprefix = r"(?:\b(?i:r|u|f|fr|rf|b|br|rb))?"

I think the \b should be removed instead.  Currently uf'a' gets f'a' colored, while ur'a' only gets  'a' colored.  (Tim Peters pointed out the difference with a different example.) I prefer coloring the maximal legal string rather than the minimum.    I think the contrast between two chars legal by themselves, but differently colored when put together, makes the bug more obvious.