regex / named-groups

regex named capture groups — (?P<name>) and (?<name>)

Quick Answer

# Python — (?P<name>...)
import re
m = re.match(r'(?P<year>\d{4})-(?P<month>\d{2})-(?P<day>\d{2})', '2024-01-15')
print(m.group('year'))   # 2024
print(m.groupdict())     # {'year': '2024', 'month': '01', 'day': '15'}

// JavaScript — (?<name>...)
const m = '2024-01-15'.match(/(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/);
console.log(m.groups.year);   // 2024

Usage

Named groups make regex matches self-documenting and let you access groups by name instead of index.

Other causes & fixes

Named backreference in pattern

# Python — \g<name> or (?P=name)
import re
# Match a word that appears twice
re.search(r'(?P<word>\w+) (?P=word)', 'the the fox')

# JavaScript — \k<name>
/(?<word>\w+) \k<word>/.test('the the fox')   // true

Named group in replacement

# Python — use \g<name> in sub()
import re
result = re.sub(
  r'(?P<year>\d{4})-(?P<month>\d{2})-(?P<day>\d{2})',
  r'\g<day>/\g<month>/\g<year>',
  '2024-01-15'
)
# result: '15/01/2024'

// JavaScript
'2024-01-15'.replace(
  /(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/,
  '$<day>/$<month>/$<year>'
)
// '15/01/2024'

PHP named groups

preg_match('/(?P<year>\d{4})-(?P<month>\d{2})/', '2024-01', $m);
echo $m['year'];   // 2024

Related