a tool for visualizing how regular expression engines use 'finite-state automata' to match regular expression patterns against text. a finite state automaton is model of behavior composed of states, transitions & actions. a state stores information about the past, i.e. it reflects the input changes from the system start to the present moment.
the graphs are based on a 'pattern', which shows the regular expression. the 'input' is the string that is visually matched. as users type into the input string. the color of this string indicates whether it’s a complete match (green), a partial match (blue), or a non-match (red). [osteele.com|thnkx Angus]