Sequence diagrams are a popular technique for describing interactions between software entities. However, because the OMG group's UML standard is not based on a rigorous mathematical structure, it is impossible to deduce a single interpretation for the notation's semantics, nor to understand precisely how its different fragments interact. While there are a lot of suggested semantics in the literature, they are too mathematically demanding for the majority of software engineers, and often incomplete, especially in dealing with the semantics of lifeline creation and deletion. In this work we describe a simple semantics based on the theory of regular languages, a mathematical theory that is a standard part of the curriculum in every computer science undergraduate degree and covers all the major compositional fragments, and the creation and deletion of lifelines.