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re.Scanner groups #84440

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dchron mannequin opened this issue Apr 12, 2020 · 3 comments
Closed

re.Scanner groups #84440

dchron mannequin opened this issue Apr 12, 2020 · 3 comments
Labels
3.8 only security fixes topic-regex type-bug An unexpected behavior, bug, or error

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@dchron
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Mannequin

dchron mannequin commented Apr 12, 2020

BPO 40259
Nosy @ezio-melotti, @tirkarthi
Files
  • re.Scanner.txt
    		•

    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 = None
created_at = <Date 2020-04-12.07:52:00.286>
labels = ['expert-regex', 'type-bug', '3.8']
title = 're.Scanner groups'
updated_at = <Date 2020-04-12.14:06:10.839>
user = 'https://bugs.python.org/dchron'

    bugs.python.org fields:

    activity = <Date 2020-04-12.14:06:10.839>
actor = 'xtreak'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Regular Expressions']
creation = <Date 2020-04-12.07:52:00.286>
creator = 'dchron'
dependencies = []
files = ['49049']
hgrepos = []
issue_num = 40259
keywords = []
message_count = 2.0
messages = ['366226', '366249']
nosy_count = 4.0
nosy_names = ['ezio.melotti', 'mrabarnett', 'xtreak', 'dchron']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue40259'
versions = ['Python 3.8']
    @dchron dchron mannequin added 3.8 only security fixes topic-regex type-bug An unexpected behavior, bug, or error labels Apr 12, 2020
    @tirkarthi
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    Please add a description of the issue you are facing with a simple script of the behavior.

    @tirkarthi
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    Copy paste of the contents in the text file

    In the re module there is an experimental feature called Scanner.
    Some unexpected behavior was found while working with it.
    Here is an example:

    >>> re.Scanner([('\w+=(\d+);', lambda s,g: s.match.group(1))]).scan('x=5;')
(['5;'], '')

    The obvious error is the semicolon returned via capturing group 1.

    Adding a dummy rule at the beginning, seems to solve that issue:

    >>> re.Scanner([('z', None), ('\w+=(\d+);', lambda s,g: s.match.group(1))]).scan('x=5;')
(['5'], '')

    Adding a capturing group around \w+ also returns the correct answer:

    >>> re.Scanner([('z', None), ('(\w+)=(\d+);', lambda s,g: s.match.group(1))]).scan('x=5;')
(['x'], '')

    But then, if I ask for the second group, the problem appears again:

    >>> re.Scanner([('z', None), ('(\w+)=(\d+);', lambda s,g: s.match.group(2))]).scan('x=5;')
(['5;'], '')

    @ezio-melotti ezio-melotti transferred this issue from another repository Apr 10, 2022
    @serhiy-storchaka
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    re.Scanner uses groups internally. You should not refer to groups by number unless you know exactly how re.Scanner works internally. Use named groups instead.

    It could be a documentation issue, but re.Scanner is not officially documented, so nothing to do here.

    @TiagodePAlves TiagodePAlves mentioned this issue Apr 23, 2022
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