monline

"This is a tale of two approaches to regular expression matching. One of them is in widespread use in the standard interpreters for many languages, including Perl. The other is used only in a few places, notably most implementations of awk and grep. The two approaches have wildly different performance characteristics ..."
A very interesting article on regular expression matching algorithms.

The tutorial at regular-expressions.info can help a lot in understanding these concepts, while the description at php.net is not really focusing on them.

There're a few things about conditions in procmail recipes that are not quite trivial from the procmailrc manpage, but you should know if you want to understand how things work.

The use of backslashes in Java string literals can be a bit confusing at first, so be sure to read this short intro on the topic.