Question

Is there a way to specify a regular expression to match every 2nd occurrence of a pattern in a string?

Examples

• searching for a against string abcdabcd should find one occurrence at position 5
• searching for ab against string abcdabcd should find one occurrence at position 5
• searching for dab against string abcdabcd should find no occurrences
• searching for a against string aaaa should find two occurrences at positions 2 and 4

Answer 1

Use capturing groups.

foo.*?(foo) 

Use a regex like this to match all occurrences in a string. Every returned match will contain a second occurrence as its first captured group.

Here’s an example that matches every second occurrence of \d+ in Python using findall:

import re  input = '10 is less than 20, 5 is less than 10' second_occurrences = re.findall(r'\d+.*?(\d+)', input)  print(second_occurrences) 

Output:

['20', '10']