MAC input validation in SharePoint 2010 list

I would like to validate the input of a MAC address in a list and have gotten the formula 90% there. The final step that I'd like to accomplish is to set any upper-case characters to lower. I know this is achievable through the LOWER() function, but when used in conjunction with the rest of the formula it breaks. Here's what I'm working with:

=(LEN(MAC)=17)
+(MID(MAC,3,1)=":")
+(MID(MAC,6,1)=":")
+(MID(MAC,9,1)=":")
+(MID(MAC,12,1)=":")
+(MID(MAC,15,1)=":")
=6

It works quite simply of just checking for the colon-delimiter between each octet in the MAC. Is there a way to use LOWER() to enforce lower-case characters?