  We propose a new general BRST approach to string and string-like theories
which have a wider range of applicability than e g the conventional conformal
field theory method. The method involves a simple general regularization of all
basic commutators which makes all divergent sums to be expressible in terms of
zeta functions from which finite values then may be extracted in a rigorous
manner. The method is particular useful in order to investigate possible state
space representations to a given model. The method is applied to three string
models: The ordinary bosonic string, the tensionless string and the conformal
tensionless string. We also investigate different state spaces for these
models. The tensionless string models are treated in details. Although we
mostly rederive known results they appear in a new fashion which deepens our
understanding of these models. Furthermore, we believe that our treatment is
more rigorous than most of the previous ones. In the case of the conformal
tensionless string we find a new solution for d=4.
