procedure TForm1.Button1Click(Sender: TObject);
var
str,str1:string;
begin
str:='abcdefg';
str1:=copy(str,3,length(str)-5);//先得到第三和第四字符串
Edit1.Text:=StringReplace(str,str1,'', [rfReplaceAll]);//将原字符串中减去第三和第四字符串去
end;