Updated for Minecraft 1.20.

Recompiled for 1.16

Fixed a bug where if you issued...

/REPLACE File1.txt A AA File2.txt

...it would go into an endless loop creating a string containing "AAAAAAAAAAA..." until the server crashed.

I rewrote it using my own custom string parser rather than use regular expressions or Java's string.replaceAll() (which goes into a loop if you replace "A" with "AA").

I even tried to do this the OOP way by extending String and adding my method as a method to my String sub object, but String won't allow itself to be extended.

Conclusion: sometimes the "best practices" aren't truly the best practices -- and you need to do things your own way to get it to work correctly.