$re = '/(?ms)(?<=AUTHOR=\{)\{.*?\}(?:[^{}]*)(?=\},$)/';
$str = '@inproceedings{baz,
AUTHOR={{Baz}, {S}. and Bar, {G}. and
Foo, {M}},
year={2013}
}
@inproceedings{baz,
AUTHOR={{%Baz%}, {S!}. and Bar, {^G^}. and
Foo, {<M>}},
year={2013}
}
';
preg_match_all($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