$re = '/^([A-Za-z][0-9]{6}|[0-9]{2}[A-Za-z][0-9]{6})$/m';
$str = 'Length 7 or 9 First character must be a letter a-z or A-Z if number of characters are 7 The remaining 6 must be numeric 0-9 example
a555444
B999999
Third character must be a letter a-z or A-Z if number of characters are 9 The remaining 8 must be numeric 0-9
example
12B456789
16K456745
';
preg_match_all($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