$re = '/^((?\'phpvar\'[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*):([-+]?(?\'expr\'(\g<phpvar>|\d+(\.\d+)?|[-+]?\(\g<expr>\))([-+*\/]\g<expr>)?)))+(;\g<1>)*$/';
$str = 'header1:(a1+a2)*(a3-a1);header2:b1*(b2-b3)/b4;header3:c1*(c2);header4:((a1*(a2-b1))/(a3-a1))+b2';
preg_match_all($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