First, let's see why we can use regular expressions and how to use them in the context of pentest.1. Regular expressions can greatly help in the process of exploration and data collection - in order not to collect any garbage, we will be able to select only that part of the content that will be really useful for us.2. In the process of developing auxiliary units for performing pentests (including even the stage of the intelligence).3. In case we got access to the target system, database, etc. - accordingly, we need to extract the data we are interested in, regulars can also be useful here.4. In the process of data processing - when you have large data arrays for processing and / or comparison, different kinds of tula with the ability to use regular expressions come to the rescue, as well as the ability to write an auxiliary script.5. If you worked with BurpSuite, you noticed in the same Intruder the ability to extract data from the page using regular expressions (also to the word about data parsing).For those who have never dealt with regular expressions, this may seem to be a real hell at first - some confusing meanings, parentheses, dots and all that, a natural question arises - where do you start?Not everyone likes and the option of reading a pile of books just fits; therefore, we will consider the least painful and most interesting options.1. Referring to the cheat sheet by regular expressions:RexEgg - https://www.rexegg.com/regex-quickstart.html2. After you have studied (or briefly examined) the basic information on regular expressions - let's try to solve an interesting exam:RegexOne - https://regexone.com/3. In the process of performing tasks, we can use some kind of auxiliary tool for testing:Regex 101 - https://regex101.com/4. After studying the cheat sheet and solving problems with regular expressions, it would probably be good to practice more real-life examples and the RegexBuddy will help us:Regex Buddy - https://www.regexbuddy.com/To work out RegexBuddy, try searching the Internet for live examples in the form of logs from various sources, or try parsing the pages and collecting data by criteria, and then fully understand them using regexbuddy (there is an opportunity to decompose everything by parameters).