We give a class of timed regular expressions that involve the use of colored parentheses for specifying timing constraints. The expressions are given in a metrical format, and their semantics is based upon an "overlapping concatenation" of timed words. We then give a calculus for emptiness checking of a regular expression, that does not go through translating expressions into timed automata. To this end we use the class of 2n-automata, studied in a parallel paper [Dim02] in connection with the problem of representing timing constraints.
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