In essence, software may substitute human to execute some certain actions. The description and analysis for software behavior is always research focus. For the effective expression of the software interactions and the improvement of analysis efficiency, presents a novel descriptive language for software behavior pattern—BPL based on regular expressions, which describes software behavior as a combination of traces and constraints marked with lowercase letters after abstracting and generalizing the features of recurrent event sequences of the observable software behaviors at runtime. Finally, BPL is used to represent the process of electronic transaction.