From the replies :) > In CS theory, regular languages are a strict subset of context-free languages, but regular expression implementations in mainstream progr

From the replies :) > In CS theory, regular languages are a strict subset of context-free languages, but regular expression implementations in mainstream programming languages are more powerful. As noulakaz.net/weblog/2007/03/18/… describes, so-called "regular expressions" can check for prime numbers in unary, which is certainly something that a regular expression from CS theory can't accomplish. – Adam Mihalcin Commented Mar 19, 2012 at 23:50