Here are several approaches to match a digit n times
Learn Regex (Regular Expressions) by examples with beginner's guide. You will find how they work and how to use them in a range of common scenarios - validations for - numbers, dates, capital letters, groups, cheat sheets. You can find how to use regex in programming languages and also in text editors.
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This post cover advanced techniques for regex extraction from text.
In this post: python regex match python regex search python
You can check other example here: Notepad++ regex replace capture
you can check also: java regex matcher example Java regular