ob_start callback preg_replace

Can someone help me with preg_replace? I want to convert all numbers either 8 or 9 digits in length, into a link. Such that when it sees: John Doe, 456890123, is a new employee. It converts it to: John Doe, <a href='employee.html?id=456890123'>456890123</a>, is a new employee. function callback($buffer) { // create id links return ???????; } ob_start("callback"); Thanks much, John