Regular expressions (RegExp) are string of commands for searching and extracting substrings. To look at, the search patterns look like indecipherable gobbledygook, but each character has a purpose in its context.
They are not suited to every purpose, for example transforming HTML is full of pitfalls, and a compiler is better suited (Which may still use RegExp to tokenise the input). But for extracting text from strings that have a known or expected format, it is perfectly suited.
It is very different to a programming language, but once you get the basics you can only admire it's power. They take a while to master though!