[issue35072] re.sub does not play nice with chr(92)

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

chr(92) is a single backspace character ('\\'). Since backspace has special 
meaning in the replacement string, it should be escaped.

re.sub(r'', r'\\', stringy_thingy)

--
nosy: +serhiy.storchaka
stage: test needed -> resolved
status: open -> closed

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[issue35072] re.sub does not play nice with chr(92)

[Matthew Barnett]

Matthew Barnett  added the comment:

@Ezio: the value of stringy_thingy is irrelevant because it never gets that 
far; it fails when it tries to parse the replacement, which occurs before 
attempting any matching.

I can't reproduce the difference either.

--
status: pending -> open

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[issue35072] re.sub does not play nice with chr(92)

[Ezio Melotti]

Ezio Melotti  added the comment:

I'm assuming you want to replace double backslashes with single backslashes in 
stringy_thing, so I defined stringy_thingy and tried both your snippets but 
they are both failing:
>>> stringy_thingy = r'foo\\bar\\baz'
>>> print(stringy_thingy)  # stringy_thingy contains double backslashes
foo\\bar\\baz
>>> re.sub(r'', chr(92), stringy_thingy)  # fails
Traceback (most recent call last):
  ...
  File "/usr/lib/python3.6/sre_parse.py", line 245, in __next
self.string, len(self.string) - 1) from None
sre_constants.error: bad escape (end of pattern) at position 0
>>>
>>> parser = re.compile(r'')
>>> parser.sub(chr(92), stringy_thingy)  # also fails
Traceback (most recent call last):
  ...
  File "/usr/lib/python3.6/sre_parse.py", line 245, in __next
self.string, len(self.string) - 1) from None
sre_constants.error: bad escape (end of pattern) at position 0

Replacing chr(92) with r'\\' works for both:
>>> print(re.sub(r'', r'\\', stringy_thingy))
foo\bar\baz
>>> print(parser.sub(r'\\', stringy_thingy))
foo\bar\baz

The docs[0] says: "repl can be a string or a function; if it is a string, any 
backslash escapes in it are processed."
So passing chr(92) (or '\\', which is equivalent) result in the above error 
("bad escape (end of pattern)") because it's seen as an incomplete escape 
sequence.  Passing r'\\' seems to work as intended.

ISTM there is no bug and re.sub works as documented.  Can you provide a 
stringy_thingy for which the first of your snippet fails but the second 
succeeds?

[0]: https://docs.python.org/3/library/re.html#re.sub

--
resolution:  -> not a bug
stage:  -> test needed
status: open -> pending
type:  -> behavior

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[issue35072] re.sub does not play nice with chr(92)

[Samuel Warfield]

New submission from Samuel Warfield :

Bug with regex substitutions. When calling the re.sub() method directly
char(92), the double backslash charecter as the replacement, throws an
exception. Whereas compiling a regex object then calling its own .sub()
method works completely fine. I did a quick look through the bug tracker 
search for similar issues and none were reported.

# Breaks
re.sub(r'', chr(92), stringy_thingy)

vs 

# Works
parser = re.compile(r'')
parser.sub(chr(92), stringy_thingy)

# Where stringy_thingy is a string that is being substituted

--
components: Regular Expressions
messages: 328509
nosy: Samuel Warfield, ezio.melotti, mrabarnett
priority: normal
severity: normal
status: open
title: re.sub does not play nice with chr(92)
versions: Python 3.7

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