Regular expression confusion

[York]

I have two backslash - a. and I want to replace them with one backslash, 
but I failed:

 >>> import re
 >>> a = '\\\\'
 >>> re.sub(r'\\\\', '\\', '\\\\')
Traceback (most recent call last):
   File "<stdin>", line 1, in ?
   File "/usr/lib/python2.3/sre.py", line 143, in sub
     return _compile(pattern, 0).sub(repl, string, count)
   File "/usr/lib/python2.3/sre.py", line 258, in _subx
     template = _compile_repl(template, pattern)
   File "/usr/lib/python2.3/sre.py", line 245, in _compile_repl
     raise error, v # invalid expression
sre_constants.error: bogus escape (end of line)
 >>>

anybody knows why?

Thanks,

York