[regex] What is a regular expression for a MAC Address?

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