Question

我正在尝试为

等表达式编写解析器

“每周2017-11-03 15:00:00至2017-11-03 16:00:00至2017-12-03”

表示经常性的时间间隔。最后，我希望能够使用已解析的字段初始化dateutil.rrule对象。然而，大多数rrule参数是可选的，在字符串表示中对应于可能存在或不存在的模式。

但是，我无法阻止先前的模式“过于贪婪”。请考虑以下示例，其中包含两个测试用例：

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，我想在字符串中设置UNTIL_PATTERN可选项;因此我使用()?函数将其括在optional中。然而，问题是这使得第二次测试失败：

>       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 ===========================

问题在于，当我UNTIL_PATTERN可选时，END_PATTERN过于贪婪并消耗到字符串结尾。（如果我放弃它optional()，第二次测试通过，但第一次测试不会产生匹配。）

如何让两个测试通过？

Answer 1

您只需进行两项小修改即可。首先，使SimpleDateFormat format = new SimpleDateFormat("EEE MMM dd HH:mm:ss zzz yyyy", Locale.ENGLISH);非贪婪：

END_PATTERN

但是现在，因为它会尽可能少地匹配，所以你必须强制它匹配到字符串结尾，并带有一个字符串结束锚(?P<end>.+?)：

在Python中，如果后者是“可选的”，如何使正则表达式不会忽略下一个正则表达式

1 个答案: