在Python中,如果下一个正则表达式是“可选的”,如何使正则表达式不忽略下一个正则表达式

2024-03-28 11:43:31 发布

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我正在尝试为表达式编写一个解析器,例如

"Weekly from 2017-11-03 15:00:00 till 2017-11-03 16:00:00 until 2017-12-03"

代表一个周期性的时间间隔。最后,我希望能够用解析的字段初始化dateutil.rrule对象。但是,大多数rrule参数是可选的,在字符串表示中,这些参数对应于可能存在或不存在的模式。你知道吗

然而,我很难防止前面的模式“过于贪婪”。考虑以下带有两个测试用例的示例:

import re
import pytest

from dateutil.rrule import FREQNAMES

def match_pattern(string):
    SPACES = r'\s*'

    freq_names = [freq.lower() for freq in FREQNAMES] + [freq.title() for freq in FREQNAMES]
    FREQ_PATTERN = '(?P<freq>{})?'.format("|".join(freq_names))

    START_PATTERN = 'from' + SPACES + r'(?P<start>.+)'
    END_PATTERN = 'till' + SPACES + r'(?P<end>.+)'

    UNTIL_PATTERN = optional('until' + SPACES + r'(?P<until>.+)')
    # UNTIL_PATTERN = 'until' + SPACES + r'(?P<until>.+)'

    PATTERN = SPACES + FREQ_PATTERN \
            + SPACES + START_PATTERN \
            + SPACES + END_PATTERN \
            + SPACES + UNTIL_PATTERN + SPACES

    return re.match(PATTERN, string).groupdict()

def optional(pattern):
    '''Encloses the given regular expression in an optional group (i.e., one that matches 0 or 1 repetitions of the original regular expression).'''
    return '({})?'.format(pattern)

'''Tests'''
def test_match_pattern():
    string = "Weekly from 2017-11-03 15:00:00 till 2017-11-03 16:00:00"

    groups = match_pattern(string)
    assert groups['freq'] == "Weekly"
    assert groups['start'].strip() == "2017-11-03 15:00:00"
    assert groups['end'].strip() == "2017-11-03 16:00:00"

def test_match_pattern_with_until():
    string = "Weekly from 2017-11-03 15:00:00 till 2017-11-03 16:00:00 until 2017-12-03"

    groups = match_pattern(string)
    assert groups['freq'] == "Weekly"
    assert groups['start'].strip() == "2017-11-03 15:00:00"
    assert groups['end'].strip() == "2017-11-03 16:00:00"
    assert groups['until'].strip() == "2017-12-03"

if __name__ == "__main__":
    # pytest.main([__file__])
    pytest.main([__file__+"::test_match_pattern", "-s"])
    # pytest.main([__file__+"::test_match_pattern_with_until", "-s"])

在这里,我想在字符串中使UNTIL_PATTERN成为可选的;因此我使用optional函数将它括在()?中。但问题是,这会导致第二次测试失败:

>       assert groups['end'].strip() == "2017-11-03 16:00:00"
E       assert '2017-11-03 1...il 2017-12-03' == '2017-11-03 16:00:00'
E         - 2017-11-03 16:00:00 until 2017-12-03
E         + 2017-11-03 16:00:00

parse_date.py:44: AssertionError
=========================== 1 failed in 0.07 seconds ===========================

问题是,当我将UNTIL_PATTERN设为可选时,END_PATTERN过于贪婪,一直消耗到字符串的末尾。(如果我省去了optional(),第二个测试通过了,但是第一个测试没有产生匹配)。你知道吗

我怎样才能让这两个测试都通过?你知道吗


Tags: fromstringmatchassertoptionalpatterngroupsspaces
1条回答
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1楼 · 发布于 2024-03-28 11:43:31

你只需要做两个小改动。首先,使END_PATTERN不贪婪:

(?P<end>.+?)

但是现在,因为它将尽可能少地匹配,所以您必须强制它匹配,直到字符串末尾有一个字符串结束锚点$

PATTERN = SPACES + FREQ_PATTERN \
        + SPACES + START_PATTERN \
        + SPACES + END_PATTERN \
        + SPACES + UNTIL_PATTERN + SPACES + '$'

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