If you want to ignore *all* special characters, the you could just
switch to using [string first ...]
if you really want to use regexp, but want to match literal strings
you could follow the opthers advice to pre-process and escape all
special characters or use one of the following:
regexp "***=$a" $b
or
regexp "(?q)$a" $b
If you wnt to allow *some* special characters but not others, then
you will need to do the escaping yourself.
Bruce