From: Jeffrey Schwab <jeff@...>
Date: 2005-12-03T17:37:32+09:00
Subject: Re: Regex: except when

basi wrote:
> Hello,
> I haven't grokked how to include an exception to a condition in a
> regular expression. For example, I can't express this correctly:
> 
> aString matches if it has a sequence of 3 or more consonants, except
> when the consonants [ng] is in the sequence (in that order, n followed
> by g).
> 
> Thus: plants will match, but bangking will not match.

You might find negative look-ahead helpful.  For example:

consonant = '[bcdfghjklmnpqrstvwxyz]'
r = /#{consonant}{3,}(?!.*ng)/

print "plants: ", r =~ "plants", "\n"
print "bangking: ", r =~ "bangking", "\n"