$re = '/^(.*?)\s*#flashcard\n([^:]+):.*?^(\S[^\n]+)\s+(?:---|<!--)/ms';
$str = 'How do we achieve A? #flashcard
To achieve A we need to do the following:
Do B
Do C
Finally do D
---
How do we achieve W? #flashcard
To achieve W we need to do the following:
Do Y
Do Z
<!--
';
preg_match_all($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