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
你只需要做两个小改动。首先,使
END_PATTERN
不贪婪: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
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