'[.]' is the same as '\.' - it means match one of the characters inside (or one not inside) the square brackets, ie [xyzX] will match any of x, y, x, or X, while [^xyzX] will match any that is NOT x, y, z, or X. as usual you can use *+? etc to specify repetition.
anyway you do this because '.' means 'any character'
edit: a better way would be
which will match any of
rename -n 's|.*([a-zA-Z][0-9][0-9][a-zA-Z][0-9][0-9]\.[a-zA-Z0-9]*)|\L$1|' fileOrFiles
and turn them into
really long filename.with.extra.stuff.A12B33.extension