The re_graph.pl program graphs regular expressions. The guts of the regular
expression engine is a simple state machine. The various states and
operations in the regular expression parser can be displayed using a
surprisingly simple diagram.

A few notes on what you are looking at:
 * The nodes Start and Stop denote the beginning and end of the regular
   expression.
 * The solid squares denote atoms. Lines indicate the next state. When a
   line splits, the state machine will take the top line first. If it's
   path is blocked it will backup and take the next lower line. This is
   repeated until it finds a path to the end or all paths are exhausted.
 * Brown boxes indicate a grouping operation, i.e. ().
 * Green boxes indicate a zero with test. The state machine will perform
   the test inside the box before moving ahead.