<?php
$string = '2005-03-IT-31';
if (preg_match ('/^[\d]{4}-[\d]{2}-[a-z]{2}-[\d]{2}$/i',$string)) {
 echo "Right format";
 }
else {
 echo "Wrong format";
 }
 
?>

<?php
$string = '2005-03-IT-31';
if (preg_match('/^\\d{4}-\\d{2}-[a-z]{2}-\\d{2}$/i',$string)) {
  echo "Right format";
} else {
  echo "Wrong format";
}
?>

 ... [\d]{4}...

 ... \\d{4} ...