@roblourens , This was not touched upon when preserve case was implemented , I have changed it to make it work for group substitution also . The gist of the change is that instead of returning text.replace(this._regExp, this.pattern); we return text.replace(this._regExp, this.buildReplaceString(match, preserveCase));

NOTE : I have changed test methods to the standard used in other functions to make it look cleaner as this was getting pretty messy , and existing test methods would cover this fix test cases

#81729