Question

For example I have the text

a1aabca2aa3adefa4a

I want to extract 2 and 3 with a regex between abc and def, so 1 and 4 should be not included in the result.

I tried this

if(preg_match_all('#abc(?:a(\d)a)+def#is', file_get_contents('test.txt'), $m, PREG_SET_ORDER))
  print_r($m);

I get this

> Array
(
    [0] => Array
        (
            [0] => abca1aa2adef
            [1] => 3
        )

)

But I want this

Array
(
    [0] => Array
        (
            [0] => abca1aa2adef
            [1] => 2
            [2] => 3
        )

)

Is this possible with one preg_match_all call? How can I do it?

Thanks

Answer 1

preg_match_all(
    '/\d       # match a digit
    (?=.*def)  # only if followed by <anything> + def
    (?!.*abc)  # and not followed by <anything> + abc
    /x', 
    $subject, $result, PREG_PATTERN_ORDER);
$result = $result[0];

works on your example. It assumes that there is exactly one instance of abc and def per line in your string.

The reason why your attempt didn’t work is that your capturing group (\d) that matches the digit is within another, repeated group (?:a(\d)a)+. With every repetition, the result of the capture is overwritten. This is how regular expressions work.

In other words – see what’s happening during the match:

Current position    Current part of regex    Capturing group 1
--------------------------------------------------------------
a1a                 no match, advancing...   undefined
   abc              abc                      undefined
      a2a           (?:a(\d)a)               2
         a3a        (?:a(\d)a) (repeated)    3 (overwrites 2)
            def     def                      3