[Python-Dev] Omission in re.sub?

Guido van Rossum guido at python.org
Sun Dec 11 22:04:56 CET 2011

I guess the current rule is that any escapes referring to characters
by a numeric value are not supported; this probably made some kind of
sense because \1 etc. are backreferences. But since we're discouraging
octal escapes anyway I think it's fine to improve over this.

On Sun, Dec 11, 2011 at 12:47 PM, MRAB <python at mrabarnett.plus.com> wrote:
> On 11/12/2011 20:27, Guido van Rossum wrote:
>> On Sun, Dec 11, 2011 at 12:12 PM, MRAB<python at mrabarnett.plus.com>
>> wrote:
>>> I've just come across an omission in re.sub which I hadn't noticed
>>> before.
>>> In re.sub the replacement string can contain escape sequences, for
>>> example:
>>>>>> repr(re.sub(r"x", r"\n", "axb"))
>>> "'a\\nb'"
>>> However:
>>>>>> repr(re.sub(r"x", r"\x0A", "axb"))
>>> "'a\\\\x0Ab'"
>>> Yes, it doesn't recognise "\xNN".
>>> Is there a reason for this?
>>> The regex module does the same, but is there any objection to me
>>> fixing it in the regex module? (I'm thinking about compatibility
>>> with re here.)
>> As long as there's a way to place a single backslash in the output
>> this seems fine to me, though I'm not sure it's important. Of course
>> it will likely break some test... the test will then have to be
>> fixed.
>> I can't remember why we did this -- is there a full list of all the
>> escapes that re.sub() interprets somewhere? I thought it was pretty
>> limited. Maybe it's the related list of escapes that are supported
>> in regular expressions?
> The documentation says: """That is, \n is converted to a single newline
> character, \r is converted to a linefeed, and so forth."""
> All of the other escape sequences work as expected, except for \uNNNN
> and \UNNNNNNNN which aren't supported at all in re.
> I should probably also add \N{...} to the list for completeness.
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--Guido van Rossum (python.org/~guido)

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