python - 为什么我的正则表达式不能正确返回group(0)？

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）不给我像"21 September 2010"这样的全部内容？
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)

最佳答案

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}$。将其应用于21 September 2010时，您将see that it matches September，因为^\d{2} +只能与字符串开头的January匹配，因为未对月份替代项进行分组。

您需要对月份替代方案进行分组：

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

请参见Python demo：

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