pwSimpleToValidRegExp

 In PottersWheel rule-based modeling a set of species is represented by
 a single pattern, e.g. 'R_@_S' for {'R_o_S', 'R_L_S'}. The pattern

 R_@_S

 is called a simple pattern, because it is not yet a valid regular
 expression but a shortcut provided by PottersWheel to simplify the
 creation of models. The function pwSimpleToValidRegExp
 converts the simple pattern into a valid regular expression,
 in this case to

 ^R_([a-z0-9]*[A-Z]+[a-zA-Z0-9]*|o)_S$