$re = '/(?<=[=,])\s*[A-Za-z2]+(?=\()/';
$str = '=VLOOKUP($A9,\'Summary\'!$A$10:$C$30,3,FALSE)
=IFERROR((C10/B10),"N/A")
=\'New Chart Data (Date)\'!L70
=IFERROR((C10/B10),Len(E30))';
preg_match_all($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