A regular expression pattern is a template that matches a set of text strings.

A regular expression is a string comprised of normal or literal characters and special meta-characters. The normal characters match themselves, while the special characters define repetition, character classes, backreferences to prior matching fragments, etc…

While a complete discussion of regular expressions is beyond the scope of the EASA Help Pages, an in depth reference of EASA's regular expression pattern language, is found in the Java documentation below,

https://docs.oracle.com/javase/7/docs/api/java/util/regex/Pattern.html