Unexpected RegEx behavior in Delphi XE - regex

Question

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;

Answer 1

You need to escape the \ characters in your replacement string:
ReplaceWith = '{\\*\\bkmkstart BM0}\\plain\\f0\\fs24\\cf0 ^{\\*\\bkmkend BM0}\\plain\\f0\\fs24\\cf0 \\cell}';
When you make this change the output is:
{\*\bkmkstart BM0}\plain\f0\fs24\cf0 ^{\*\bkmkend BM0}\plain\f0\fs24\cf0 \cell}
In fact, for your replacement string, you only need to escape the backslash in \f0 which, as it happens, appears twice. Personally I think it's just easier to escape the backslash indiscriminately.
By combining regular expressions and RTF you've mixed your own special backslash soup — tread carefully. Just be thankful you aren't using C or older versions of C++ that do not support raw strings. That backslash soup would be completely unpalatable!