preg_match_all how to get *all* combinations? Even overlapping ones

Question

Is there a way in the PHP regex functions to get all possible matches of a regex even if those matches overlap?

e.g. Get all the 3 digit substrings \'/[\\d

Answer 1

Look-ahead assertions to the rescue!

preg_match_all('/(?=(\d{3}))/', $str, $matches);
print_r($matches[1]);

It basically captures whatever the look-ahead assertion is matching. Since the assertion is zero width, $matches[0] will only contain empty strings, but $matches[1] will contain the expected captured patterns.