https://en.wikipedia.org/wiki/Regular_expression
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions
See this first about general regex: Regular Expressions - Enough to be Dangerous by Engineer Man. In the video he uses https://debuggex.com/ as visual tool.
See this next about JavaScript regex: Regular Expressions (Regex) in JavaScript - tutorial by freeCodeCamp.org. In the video he uses https://regex101.com as visual tool.