  A product configurator which is complete, backtrack free and able to compute
the valid domains at any state of the configuration can be constructed by
building a Binary Decision Diagram (BDD). Despite the fact that the size of the
BDD is exponential in the number of variables in the worst case, BDDs have
proved to work very well in practice. Current BDD-based techniques can only
handle interactive configuration with small finite domains. In this paper we
extend the approach to handle string variables constrained by regular
expressions. The user is allowed to change the strings by adding letters at the
end of the string. We show how to make a data structure that can perform fast
valid domain computations given some assignment on the set of string variables.
  We first show how to do this by using one large DFA. Since this approach is
too space consuming to be of practical use, we construct a data structure that
simulates the large DFA and in most practical cases are much more space
efficient. As an example a configuration problem on $n$ string variables with
only one solution in which each string variable is assigned to a value of
length of $k$ the former structure will use $\Omega(k^n)$ space whereas the
latter only need $O(kn)$. We also show how this framework easily can be
combined with the recent BDD techniques to allow both boolean, integer and
string variables in the configuration problem.
