In extensions (?...) the lookbehind assertion cannot choose between the beginning of string and a letter

[py-user]

BPO	14069
Nosy	@ezio-melotti, @py-user

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2014-07-03.16:47:35.434>
created_at = <Date 2012-02-21.02:55:58.429>
labels = ['expert-regex', 'invalid', 'type-bug', 'library']
title = 'In extensions (?...) the lookbehind assertion cannot choose between the beginning of string and a letter'
updated_at = <Date 2014-07-03.16:47:35.432>
user = 'https://github.com/py-user'

bugs.python.org fields:

activity = <Date 2014-07-03.16:47:35.432>
actor = 'ezio.melotti'
assignee = 'none'
closed = True
closed_date = <Date 2014-07-03.16:47:35.434>
closer = 'ezio.melotti'
components = ['Library (Lib)', 'Regular Expressions']
creation = <Date 2012-02-21.02:55:58.429>
creator = 'py.user'
dependencies = []
files = []
hgrepos = []
issue_num = 14069
keywords = []
message_count = 3.0
messages = ['153836', '222129', '222199']
nosy_count = 3.0
nosy_names = ['ezio.melotti', 'py.user', 'BreamoreBoy']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue14069'
versions = ['Python 3.2']

[py-user]

>>> import re
>>> re.search(r'(?<=(a|b))(\w+)', 'abc').groups()
('a', 'bc')
>>> re.search(r'(?<=(^))(\w+)', 'abc').groups()
('', 'abc')
>>> re.search(r'(?<=(^|$))(\w+)', 'abc').groups()
('', 'abc')
>>> re.search(r'(?<=($|^))(\w+)', 'abc').groups()
('', 'abc')
>>> re.search(r'(?<=(^|a))(\w+)', 'abc').groups()
Traceback (most recent call last):
  File "/usr/local/lib/python3.2/functools.py", line 176, in wrapper
    result = cache[key]
KeyError: (<class 'str'>, '(?<=(^|a))(\\w+)', 0)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.2/re.py", line 158, in search
    return _compile(pattern, flags).search(string)
  File "/usr/local/lib/python3.2/re.py", line 255, in _compile
    return _compile_typed(type(pattern), pattern, flags)
  File "/usr/local/lib/python3.2/functools.py", line 180, in wrapper
    result = user_function(*args, **kwds)
  File "/usr/local/lib/python3.2/re.py", line 267, in _compile_typed
    return sre_compile.compile(pattern, flags)
  File "/usr/local/lib/python3.2/sre_compile.py", line 495, in compile
    code = _code(p, flags)
  File "/usr/local/lib/python3.2/sre_compile.py", line 480, in _code
    _compile(code, p.data, flags)
  File "/usr/local/lib/python3.2/sre_compile.py", line 115, in _compile
    raise error("look-behind requires fixed-width pattern")
sre_constants.error: look-behind requires fixed-width pattern
>>>

[BreamoreBoy]

@py.user please accept our apologies for having missed this.

@ezio can you comment on this please.

[ezio-melotti]

I believe this is not a bug.
As the error says a look-behind requires the pattern to have a fixed length.

In re.search(r'(?<=(a|b))(\w+)', 'abc').groups() the two possible patterns in the look behind are "a" and "b", both with length 1.
In re.search(r'(?<=(^|$))(\w+)', 'abc').groups() the two possible patterns in the look behind are "^" and "$", both with length 0.

In re.search(r'(?<=(^|a))(\w+)', 'abc').groups() the two possible patterns in the look behind are "^" and "a", one with length 0 and the other with length 1. This is not allowed and raises an error.
Similarly, in re.search(r'(?<=(ab|a))(\w+)', 'abc').groups() the two patterns have length 2 and 1, and the same error is raised.

Closing as not a bug.