general php problem - ereg question

[eluser]skattabrain[/eluser]
OK, so i upgraded php and looks like ereg and ereg replace are gone! so i have some old code in there, in regards to checking credit cards. I don't know what 2nd condition is doing ... the !ereg('5[1-5]', $ccnum)

Code:

if ($type == 'mastercard')
{
if (strlen($ccnum) != 16 || !ereg('5[1-5]', $ccnum)) return false;
}