$re = '/((?:\<[^<>]+\>\s?)?)([:]\w+(?!\w*[;]))((?:\s?\<\/[^<>]+\>)?)((?:(?:.(?!\1\2\3))*.){2})\1\2\3/';
$str = 'testing this string <b>that should</b> <lin>:match</lin> very with a string, but the <lin>:match</lin> isn\'t what so simple, it <b>needs</b> to replace a certain <lin>:match</lin> (can be the first, seconds... anyone) in the whole <b>text</b>';
$subst = "$1$2$3$4$1REPLACE$3";
$result = preg_replace($re, $subst, $str, 1);
echo "The result of the substitution is ".$result;
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