匹配2个正则表达式图案，正面或背面有1个图案

Question

I have a regex that matches 2 patterns with 1 pattern in the front or back, but the first array return 2 empty indexes. Why is it doing that, and how would I stop it from doing that?

$text = "i did";
preg_match("~(?:(did) (.+)|(.+) (did))~", $text, $match);
print_r($match);

echo "<br>";

$text = "did i";
preg_match("~(?:(did) (.+)|(.+) (did))~", $text, $match);
print_r($match);

Result:

Array ( [0] => i did [1] => [2] => [3] => i [4] => did ) 
Array ( [0] => did i [1] => did [2] => i )

Want Result:

Array ( [0] => i did [1] => i [2] => did ) 
Array ( [0] => did i [1] => did [2] => i )

Answer 1

You can use a branch reset (?|...):

Alternatives inside a branch reset group share the same capturing groups. The syntax is (?|regex) where (?| opens the group and regex is any regular expression.

Your preg_match will look like:

preg_match("~(?|(did) (.+)|(.+) (did))~", $text, $match);

See IDEONE demo

Results:

Array
(
    [0] => i did
    [1] => i
    [2] => did
)

I guess your regex is a sample one. If you need to match a word after or before did, use the \w shorthand class:

preg_match("~(?|(did) (\w+)|(\w+) (did))~", $text, $match);

See another demo