9.17. RE Group¶
Match particular parts of a string
Possible to name matches
9.17.1. Syntax¶
Define:
(...)
- Positional group
(?P<name>...)
- Named group
(?:...)
- Non-matching group
(?#...)
- Comment
Backreference:
(?P=name)
- Backreferencing by group name
\number
- Backreferencing by group number
Example:
(?P<tag><.*?>)text(?P=tag)
(?P<tag><.*?>)text\1
(.+) \1
matchesthe the
or55 55
(.+) \1
not matchesthethe
(note the space after the group)
9.17.2. SetUp¶
>>> import re
>>> TEXT = 'Mark Watney of Ares 3 landed on Mars on: Nov 7th, 2035 at 1:37 pm'
9.17.3. Positional Groups¶
>>> result = re.search(r'(\w+) (\w+)', TEXT)
>>>
>>> result
<re.Match object; span=(0, 11), match='Mark Watney'>
>>>
>>> result.group()
'Mark Watney'
>>>
>>> result.group(1)
'Mark'
>>>
>>> result.group(2)
'Watney'
>>>
>>> result.groups()
('Mark', 'Watney')
9.17.4. Named Groups¶
Usage of group in
re.match()
>>> result = re.match(r'(?P<firstname>\w+) (?P<lastname>\w+)', TEXT)
>>>
>>> result.group('firstname')
'Mark'
>>>
>>> result.group('lastname')
'Watney'
>>>
>>> result.group(1)
'Mark'
>>>
>>> result.group(2)
'Watney'
>>>
>>> result.groups()
('Mark', 'Watney')
>>>
>>> result.groupdict()
{'firstname': 'Mark', 'lastname': 'Watney'}
9.17.5. Non-Capturing Groups¶
>>> year = r'(?P<year>\d{4})'
>>> month = r'(?P<month>\w+)'
>>> day = r'(?P<day>\d{1,2})'
>>> re.findall(f'{month} {day}th, {year}', TEXT)
[('Nov', '7', '2035')]
>>>
>>> re.findall(f'{month} {day}st|nd|rd|th, {year}', TEXT)
[('', '', ''), ('', '', '2035')]
>>>
>>> re.findall(f'{month} {day}[stndrdth], {year}', TEXT)
[]
>>>
>>> re.findall(f'{month} {day}[st]|[nd]|[rd]|[th], {year}', TEXT)
[('', '', ''), ('', '', ''), ('', '', ''), ('', '', ''), ('', '', ''), ('', '', ''), ('', '', ''), ('', '', ''), ('', '', ''), ('Nov', '7', ''), ('', '', '2035')]
>>>
>>> re.findall(f'{month} {day}(st|nd|rd|th), {year}', TEXT)
[('Nov', '7', 'th', '2035')]
>>>
>>> re.findall(f'{month} {day}(?:st|nd|rd|th), {year}', TEXT)
[('Nov', '7', '2035')]
9.17.6. Use Case - 0x01¶
>>> line = 'value=123'
>>>
>>> re.findall(r'(\w+)\s?=\s?(\d+)', line)
[('value', '123')]
>>> line = 'value = 123'
>>>
>>> re.findall(r'(\w+)\s?=\s?(\d+)', line)
[('value', '123')]
9.17.7. Use Case - 0x02¶
>>> TEXT = 'Mark Watney of Ares 3 landed on Mars on: Nov 7th, 2035 at 1:37 pm'
>>>
>>> year = r'(?P<year>\d{4})'
>>> month = r'(?P<month>\w+)'
>>> day = r'(?P<day>\d{1,2})'
Positional Groups:
>>> re.findall('Ares \d', TEXT)
['Ares 3']
>>>
>>> re.findall('Ares (\d)', TEXT)
['3']
>>> re.findall(r'Nov [0-9]th', TEXT)
['Nov 7th']
>>>
>>> re.findall(r'Nov ([0-9])th', TEXT)
['7']
>>>
>>> re.findall(r'Nov [0-9](st|nd|th|rd)', TEXT)
['th']
Named Groups:
>>> re.findall(f'{month} {day}th, {year}', TEXT)
[('Nov', '7', '2035')]
>>>
>>> result = re.search(f'{month} {day}th, {year}', TEXT)
>>>
>>> result.span()
(41, 54)
>>>
>>> result.group()
'Nov 7th, 2035'
>>> result.group(1)
'Nov'
>>> result.group(2)
'7'
>>> result.group(3)
'2035'
>>>
>>> result.group('month')
'Nov'
>>> result.group('day')
'7'
>>> result.group('year')
'2035'
>>>
>>> result.groups()
('Nov', '7', '2035')
>>>
>>> result.groupdict()
{'month': 'Nov', 'day': '7', 'year': '2035'}
Non-capturing Groups:
>>> re.findall(f'{month} {day}(st|nd|rd|th), {year}', TEXT)
[('Nov', '7', 'th', '2035')]
>>>
>>> re.findall(f'{month} {day}(?:st|nd|rd|th), {year}', TEXT)
[('Nov', '7', '2035')]
Comments:
>>> re.findall(f'{month} {day}th(?#ordinal), {year}', TEXT)
[('Nov', '7', '2035')]