$re = '/(?i)(?:(?<=IBAN(?:[:\s]\s|\s[:\s]))NL\s?\d{2}\s?[A-Z]{4}[0 ]\s?\d{9,10})|(?:(?<=IBAN[:\s])NL\s?\d{2}\s?[A-Z]{4}[0 ]\s?\d{9,10})/m';
$str = 'Uw BTW nummer NL80
IBAN NL 11abna0317164300asdfasf234
iBAN NL21ABNA0417134300 22
Iban: NL 29 ABNA 401422366f sdf
IBAN :NL 39 ABNA 0822416395s
IBAN:NL 39 ABNA 0822416395s
';
preg_match_all($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