$re = '/\b
(?=
(?<result>
(?<consume>
(?> \w+ \h+ ){0,3}?
keyword \b
)
(?: \h+ (?! keyword \b ) \w+ ){0,3}
)
) \g{consume}
/ux';
$str = 'word1 word2 word3 keyword word4 word5 word6 keyword word7 word8 word9 word10
word2 keyword word4 word5 word6 keyword word7 word8
word2 word3 keyword word4 word5 word6 keyword word7 keyword word10';
preg_match_all($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