为什么我的正则表达式没有正确返回 group(0)?

Question

我想从大量文件中查找日期。日期在一行上，格式为"21 September 2010"。每个文件中只有一个这样的日期。

以下代码仅返回月份，例如"September"。为什么 group(0( 没有像"21 September 2010"那样给我整个东西？ 这里缺少什么？ 谢谢！

months = ("January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December")
pattern = r"^d{2} +" + "|".join(months) + r" +d{4}$"
match = re.search(pattern, text)
if match:
fdate = match.group(0)

Answer 1

当您打印正则表达式时，您会看到它看起来像^d{2} +January|February|March|April|May|June|July|August|September|October|November|December +d{4}$.当您将其应用于21 September 2010时，您将看到它与September匹配，因为^d{2} +只能在字符串的开头与January匹配，因为月份备选方案未分组。

您需要对月份备选方案进行分组：

pattern = r"^d{{2}} +(?:{}) +d{{4}}$".format("|".join(months))

请参阅Python演示：

import re
text = "21 September 2010"
months = ("January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December")
pattern = r"^d{{2}} +(?:{}) +d{{4}}$".format("|".join(months))
match = re.search(pattern, text)
if match:
fdate = match.group(0)
print(fdate) # => 21 September 2010