I've just looked at this, the default regex priority is
<regexlist>
<tvregex>[Ss]([\d]{1,2}).?[Ee]([\d]{1,2})</tvregex>
<tvregex>([\d]{1,2}) ?[xX] ?([\d]{1,2})</tvregex>
<tvregex>([0-9]+)([0-9][0-9])</tvregex>
</regexlist>
This is the regex file as created by MC when such a file does not exist.
MC loops through these until it finds a match, in the following order, unless the order is changed by the user
S01E01
Then
1x01
then
101