I would try :
<asp:RegularExpressionValidator ... ValidationExpression="^FM-[a-z,A-Z]{4}\-[0-9]{2}\-[0-9]{2}\-[0-9]{5}$"/>
You don't need grouping function here (as you do when you enclose your expression in parenthesis) ; Matching is supposed to be on the entire textbox content. Thus the use of
^
(beginning of line or string) and
$
(end of line or string).
If that doesn't make the trick, then having the error message itself could help.