$re = '/^ISA(?:[^*]*[*][^*]*){8}\*(\d+)\* /';
$str = 'ISA *00* *00* *ZZ*SOME STRING *ZZ*99999999 *130605* 2239*|*00501*000000001*0*P*>
REF*TJ*12345677*
REF*PQ*23432211
LX*1';
preg_match($re, $str, $matches, PREG_OFFSET_CAPTURE, 0);
// Print the entire match result
var_dump($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