String-Pattern Matching in a Visual Programming Language

Determining whether or not a pattern of characters is present within a body of text is such a fundamental problem that it has led to a number of notations for the specification of patterns. These include regular expressions, and notations found within string processing and other application-specific languages. Too often, these approaches are either overly simplistic, offering little in the way of pattern matching capabilities, or else they are extremely cryptic and terse. In the context of visual programming, the pattern matchingproblem is largely unaddressed; yet the benefits ascribed to such languages offer the potential for extremely powerful and intuitively meaningful pattern notations. In the CALVIN language, designed for the creation of courseware, the authors have used purely visual constructs throughout. In particular, the content of strings may be analysed by using a graphical and augmented version of regular expressions to specify patterns. This notation, and its realization within CALVIN, is described in some detail, and comparisons are made with the more conventional textual form.