function StringReplace (const S, OldPattern, NewPattern: string; Flags: TReplaceFlags): string;

rfReplaceAll：全部替换
rfIgnoreCase：忽略大小写

For Example：

var
   aStr: String;
begin
   aStr := 'This is a book, not a pen!';
   ShowMessage(StringReplace (aStr, 'a', 'two', []));//This is two book, not a pen!只替换了第一个符合的字
   ShowMessage(StringReplace (aStr, 'a', 'two', [rfReplaceAll]));//This is two book, not two pen!替换了所有符合的字
   aStr := 'This is a book, not A pen!';
   ShowMessage(StringReplace (aStr, 'a', 'two', [rfReplaceAll]));//This is two book, not A pen!只替换了符合的字(小写a)
   ShowMessage(StringReplace (aStr, 'a', 'two', [rfReplaceAll, rfIgnoreCase]));//This is two book, not two pen!不管大小写替换了所有符合的字
end;