$re = '/^commits:[ ]*([\[.*?\]\(]?\b[0-9a-f]{5,40}\b\.\.\.\b[0-9a-f]{5,40}\b[\)]?)/im';
$str = 'commits:56af25a...d3fead4
Commits: [56af25a...d3fead4](https://github.com/Glimpse/Semantic-Release-Notes/compare/56af25a...d3fead4)
Commits: [56af25a...d3fead4](https://github.com/Glimpse/Semantic-Release-Notes/compare/56af25a...d3fead4) Commits:
Commits: test[56af25a...d3fead4](https://github.com/Glimpse/Semantic-Release-Notes/compare/56af25a...d3fead4)
Commits: test [56af25a...d3fead4](https://github.com/Glimpse/Semantic-Release-Notes/compare/56af25a...d3fead4)';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php