Introduction to regex (PCRE)

Regex is a powerful tool, but it will take time and effort to learn. Once you have learned it, though, it will be useful in many situations, not only for UnrealIRCd spamfilters. UnrealIRCd uses the PCRE2 regex engine, this is the same regex engine as used in PHP.

An excellent introduction to regex, that explains regex step-by-step, is at http://regular-expressions.info/

There is also https://regex101.com/ which allows you test regex easily via their webinterface.