regex replace with capture group backreference

Quick Answer

// JavaScript — $1, $2... or $<name>
'John Smith'.replace(/(\w+) (\w+)/, '$2, $1')
// 'Smith, John'

# Python — \1, \2... or \g<name>
import re
re.sub(r'(\w+) (\w+)', r'\2, \1', 'John Smith')
# 'Smith, John'

# sed
echo "John Smith" | sed 's/\(\w\+\) \(\w\+\)/\2, \1/'

Usage

Backreferences let you reuse captured text in the replacement string without re-specifying it.

Other causes & fixes

Named group backreference in replacement

# Python — \g<name>
import re
re.sub(
  r'(?P<first>\w+) (?P<last>\w+)',
  r'\g<last>, \g<first>',
  'John Smith'
)
# 'Smith, John'

// JavaScript — $<name>
'John Smith'.replace(/(?<first>\w+) (?<last>\w+)/, '$<last>, $<first>')

Wrap matched text with backreference

// Wrap all numbers in <strong>
'Price: 42 dollars'.replace(/(\d+)/g, '<strong>$1</strong>')
// 'Price: <strong>42</strong> dollars'

sed extended regex

# Use -E for extended regex (no backslash for groups)
echo "2024-01-15" | sed -E 's/([0-9]{4})-([0-9]{2})-([0-9]{2})/\3\/\2\/\1/'
# 15/01/2024