We present lambda-re, a minimal functional calculus with _regular
expression types_ for strings, in order to establish a theoretical
foundation for using regular expressions as types of strings in text
processing languages.  The major technical novelties in this paper
(with respect to other work such as XDuce) are (1) the use of regular
expression _effects_ to statically analyze the shape of the output of
an even diverging program and (2) the treatment of as-patterns in
non-tail positions.  We expect that our approach will be of help for
bringing the merits of static typing into scripting languages such as
Perl, Python, and Ruby without sacrificing too much of their
expressiveness and flexibility.