$re = '/TOTALE EURO CONTANTI\s*\K\d+[.,]\d+/';
$str = 'Del burger Sr Via Carlo del Prete 106/d 50127 FIRENZE C.F. E P.IVA 08380120482 BRUNCH MAMMAMIA ONION RINGS SPICCHI PATATE ACQUA 0.5L TAVOLO 88 EURO 6,50 6,50 4,00 3,00 1,00 TOTALE EURO CONTANTI 21,00 21,00 NR.0057 23/05/18 20:21 MF U1 72005985
Del burger Sr Via Carlo del Prete 106/d 50127 FIRENZE C.F. E P.IVA 08380120482 BRUNCH MAMMAMIA ONION RINGS SPICCHI PATATE ACQUA 0.5L TAVOLO 88 EURO 6,50 6,50 4,00 3,00 1,00 TOTALE EURO CONTANTI 21.00 21,00 NR.0057 23/05/18 20:21 MF U1 72005985';
preg_match_all($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