This patch implements a regular expression feature, which allows   
some interesting patterns, in the same way as implemented in perl.   
For example, (?(1)yes|no) matches with "yes" if group "1" exists, and   
with "no", if it doesn't. Without this feature, the regular expression   
must be duplicated to get the same results. In addition to perl's 
feature, it will also accept a Python named group as argument. 
   
Here's an example:   
   
(<)?\w+@\w+(\.\w+)+(?(1)>)   
  
This is a poor email matching regular expression, which will match   
with or without the "<>" symbols.