Finding all 1 digit and 2 digits numbers in a larger number using regex

I want to match all 1 digit and 2 digit numbers using regex.

Subject string: '12345'

Expected match: 1,2,3,4,5,12,23,34,45

I’m trying: d(d)? but as result i get 12,2,34,3,5

Answer

You can use

$s = "12345";
$res = [];
if (preg_match_all('~(?=((d)d?))~', $s, $m, PREG_SET_ORDER)) {
    $single = []; $double = [];
    foreach ($m as $v) {
        if ($v[1] != $v[2]) {
            array_push($single, $v[2]);
            array_push($double, $v[1]);
        } else {
            array_push($single, $v[1]);
        }
    }
    $res = array_merge($single, $double);
    print_r( $res );
}

See the PHP demo. Output:

Array
(
    [0] => 1
    [1] => 2
    [2] => 3
    [3] => 4
    [4] => 5
    [5] => 12
    [6] => 23
    [7] => 34
    [8] => 45
)

NOTES:

• (?=((d)d?)) – a regex that captures into Group 1 two or one digits, and into Group 2 the first digit of the previous sequence
• PREG_SET_ORDER groups the captures into the same parts of the match array
• If the match is a single digit, the $double array is not modified
• The final array is a combination of single digit array + double digit array.