为什么我的正则表达式不能正确返回group（0）？

Question

I want to find the dates from a large number of files. The date is on a single line, and is in the format of "21 September 2010" . There is only one such date in each file.

The following codes return the month only, for example, "September" . Why group(0) does not give me the whole thing like "21 September 2010" ? What is missing here? Thank you!

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

When you print your regex, you will see it looks like ^\\d{2} +January|February|March|April|May|June|July|August|September|October|November|December +\\d{4}$ . When you apply it to 21 September 2010 , you will see that it matches September because the ^\\d{2} + can only be matched with January at the start of the string since the month alternatives are not grouped.

You need to group the month alternatives:

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

See the 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