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This is an interactive learning tool you can use to improve your understanding of regular expressions, and their different applications. The quiz is divided into a number of different tasks, which each have a short description of the problem for you to solve. Once you have solved each task, you will be presented with statistics over how well others have done in that task, and the next task will be made available to you. After you have finished a task you can continue improving upon your solution, making it shorter, and climb higher into the leader boards.
Do note that these tasks do not necessarily promote best practices in real life scenarios, but rather aim to teach you the fundamentals of writing powerful regular expressions. Parsing HTML with regex is seldom a good idea, despite there being a task in the quiz to do just that. The quiz is evaluated using the PCRE2 engine, which is also available in the main editor if you would like to do some testing. The tasks will get progressively harder, and you will receive less and less hand-holding the further you progress.
If you encounter any issues, quirky behavior or bugs, please open up an issue on github with all the relevant details: https://github.com/firasdib/Regex101/issues