regex - Match at every second occurrence

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 grouping.

foo.*?(foo)