A quick refresher on the syntax of named groups (also known as named subpatterns) in regular expressions.

There are three alternative notations of named groups in PCRE:

  • (?P<name>...)
  • (?<name>...)
  • (?'name'...)

The first one originates in Python and is the only one supported by the re standard library module:

import re

# match date in format YYYY/M/D

regex = re.compile(
    '(?P<year>[0-9]{4})'
    '/'
    '(?P<month>[0-9]{1,2})'
    '/'
    '(?P<day>[0-9]{1,2})'
)

match = regex.match('1969/7/20')

>>> match['month']
'7'
>>> match.groupdict()
{'year': '1969', 'month': '7', 'day': '20'}

The other two notations are not recognized and raise an re.error: unknown extension error.

You can further experiment with the above example by going to https://regex101.com/r/UE4Zex/1 and switching between the PCRE and Python regex flavor.