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.

Answer 2

This may not be ideal, but at least it's something.

It looks like you could use a positive lookahead and PREG_OFFSET_CAPTURE to get all the string indexes for where a 3-digit number exists

$str = "123456";

preg_match_all("/\d(?=\d{2})/", $str, $matches, PREG_OFFSET_CAPTURE);

$numbers = array_map(function($m) use($str){
  return substr($str, $m[1], 3);
}, $matches[0]);

print_r($numbers);

Output

Array
(
    [0] => 123
    [1] => 234
    [2] => 345
    [3] => 456
)