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When you search for text, you can do so either with regular expressions or without. Replacing text in Emacs is no different, but with the added benefit of letting you leverage the power of elisp in the replace portion of search and replace.
In that sense, Emacs is different from other editors: you can use elisp and regexp capturing groups together — powerful, if you know elisp.
Emacs’s regular expression implementation is also different from PCRE. It follows the GNU standard for regular expressions with many additions (and quite a few omissions) to make it suitable for both package developers and Emacs users.