I didn't understand that you were wanting something that ALWAYS ended with the mom-word followed by exactly 2 numbers, you can match like:

if ( $regex($4,/(?:mom|mother|mum)\d\d$/i) > 0) msg #filters message

If that isn't matching what you want, or is matching things it should not, it helps to match regex if you can provide the types of things it should match, and very similar things it should not. If there can be text between the mom-word and the number, you can insert .* immediately preceding that 1st \d - though that can match when someone has a 3-digit number. If they switch to having a variable size number like mom123, then replace the \d\d with \d+