For a language as rich as RTF, regular expressions are not going to get
you all that far: they are probably only suitable for writing the
lexical analyzer (or tokenizer).

assuming the following RTF:
{\rtf1\ansi{\fonttbl\f0\fswiss Helvetica;}\f0\pard
This is some {\b bold} text.\par
}

it can be easily converted w/ regular expressions to something like:
<g><rtf>1</rtf><ansi/><g><fonttbl/><f>0</f><fswiss/>Helvetica<sc/></g><f>0</f><pard/>
This is some <g><b/>bold</g> text.<par/>
</g>