Maybe the shortest possible:
/([\da-f]{2}[:-]){5}[\da-f]{2}/i
Update: A better way exists to validate MAC addresses in PHP which supports for both hyphen-styled and colon-styled MAC addresses. Use filter_var():
// Returns $macAddress, if it's a valid MAC address
filter_var($macAddress, FILTER_VALIDATE_MAC);
As I know, it supports MAC addresses in these forms (x: a hexadecimal number):
xx:xx:xx:xx:xx:xx
xx-xx-xx-xx-xx-xx
xxxx.xxxx.xxxx