No doubt I should know this regex point but ..

[Gustavo Niemeyer]

> I am writing a class that walks through a file tree and (among other
> things) filters out which files it considers.  So I want to have
> __init__( .. ,filterExpression='..') For the default I want to filter
> nothing.  So I need a Python regular expression that never matches.
> What is (the best) one?

There are many.. "(?=a)b" is just one of them. OTOH, this is awful. :-)

Do as Aahz suggested and use None as default.

-- 
Gustavo Niemeyer
http://niemeyer.net