"Capture groups" are a powerful feature of regular expressions that allow you to use the partial results of a regular expression match when creating the replacement string.
This is an extremely powerful new feature that allows considerable flexibility when doing Find and Replace.
One website I really like to both test and learn about regular expressions is:
https://regex101.com/
This looks like a good introductory tutorial:
https://regexone.com
One more:
https://coderpad.io/blog/development/...
I'm using the Javascript syntax for the regular expressions.
You don't need to wrap them with slashes or include any flags, I take care of all that for you behind-the-scenes.
Just enter the regular expression you want to match in the Find text area and the text to replace the matches in the Replace text area.
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