I would like to validate cells according to the following rule ...

The cell can contain the following:
A single zero "0" or
a single period "." or
a string of any length consisting of the letters R, W, B, L

Eg all the following are valid
0
..
rrr
bbbbb
bbrrl
l
wrbblllrrrrbbbbbb

The following are invalid
00
0r
rrrbbbh
..rbl

Is this possible using a custom validation?

Thanks in advance.
Neil