How do you get all matches in a repeated backreference? [duplicate]

Question

With the test string:

This is a string1 string2 string3 string4 string5 string6 string7 so it is.

Is it possible to get all the stringXs as backreferences? Currently what I've tried ends up overwriting each time so I end up with one match - the last one.

For instance using a regex like this:

/This is a (?:(string\d) )+so it is./

Would always end up with a match of string7 at the end.

The best I've found so far is to remove the ?: above and explode on the spaces, but I'd be interested in whether there's a way to do it purely with regex.

update specifically to PHP, what would be the best way to achieve this?

Answer 1

You may use this regex using \G:

(?:This is a|\G(?!\A))\h+((?=.*so it is\.)string\d+)

• \G asserts position at the end of the previous match or the start of the string for the first match
• (?!\A) is negative lookahead to assert that we don't match \G at the line start
• (?=.*so it is\.) is positive lookahead to assert that we have so it is. ahead of current position in input

RegEx Demo

Code:

$re = '/(?:This is a|\G(?!\A))\h+((?=.*so it is\.)string\d+)/m';
$str = 'This is a string1 string2 string3 string4 string5 string6 string7 so it is.';

preg_match_all($re, $str, $matches);

// Print the entire match result
print_r($matches[1]);