We define algebraic systems called concurrent regular expressions
which provide a modular description of languages of 
Petri nets. Concurrent regular expressions are
extension of regular expressions
with four operators - interleaving, interleaving closure, 
synchronous composition and renaming.
This alternative characterization of Petri net
languages gives us a flexible way of specifying concurrent
systems. Concurrent regular expressions are modular and
hence easier to use for specification. The proof of equivalence
also provides a natural decomposition method for Petri nets.