You can do it without a regex with something like
Code:
$calc($len($1-) - $count($1-,a,b,c,d,e,f,g,h,i,j,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z,$chr(32)))
An equivalent regex solution is
Code:
$regex($1-,/[^ a-z]/gi)