Unexpected RegEx behavior in Delphi XE

Delphi XE, using Delphi's own RegularExpressions unit.

I'm attempting to correct some bad RTF code, where 'bookmark' tags cross the boundaries of a table cell. Seems simple enough. The code I'm using is below. Here's the general idea.

Given this text

{\*\bkmkstart BM0}\plain\f0\fs24\cf0 ^\cell}

Look for a match to this pattern (there should be exactly one in the given text):

{\\\*\\bkmkstart BM0}\\plain\\f[0-9]\\fs[0-9]+\\cf[0-9] \^\\cell}

When found, replace it with this (non-RegEx) string:

{\*\bkmkstart BM0}\plain\f0\fs24\cf0 ^{\*\bkmkend BM0}\plain\f0\fs24\cf0 \cell}

The expected results are that the first string should be replaced with the last string, eg:

{\*\bkmkstart BM0}\plain\f0\fs24\cf0 ^\cell} *becomes*
{\*\bkmkstart BM0}\plain\f0\fs24\cf0 ^{\*\bkmkend BM0}\plain\f0\fs24\cf0 \cell}

However, the result I'm actually getting is this:

{\*\bkmkstart BM0}\plain{\*\bkmkstart bm0}\plain\f0\fs24\cf0 ^\cell}\fs24\cf0 ^{\*\bkmkend BM0}\plain{\*\bkmkstart bm0}\plain\f0\fs24\cf0 ^\cell}\fs24\cf0 \cell}

It looks as if the RegEx parser is getting horribly confused somehow, but I can't even characterize what is happening. It's not a mere double replacement, or an insertion instead of replacement. The 'ReplaceWith' string does seem to be the source of the confusion, though. If I use a nice simple 'XXXX' for the ReplaceWith string, instead of the RTF, it works exactly as it should.

So, any ideas how/why the RegEx search/replace is breaking so strangely here?

Here is the code I'm using:

procedure TfrmMain.btnProcessClick(Sender: TObject);
const
  SourceString = '{\*\bkmkstart BM0}\plain\f0\fs24\cf0 ^\cell}';
  RegExFind = '{\\\*\\bkmkstart BM0}\\plain\\f[0-9]\\fs[0-9]+\\cf[0-9] \^\\cell}';
  ReplaceWith = '{\*\bkmkstart BM0}\plain\f0\fs24\cf0 ^{\*\bkmkend BM0}\plain\f0\fs24\cf0 \cell}';
var
  ResultStr: string;
  MyRegEx: TRegEx;
begin
  MyRegEx := TRegEx.Create (RegExFind);
  ResultStr := MyRegEx.Replace (SourceString, ReplaceWith);
  ShowMessage (ResultStr);
end;