There are two pre-existing questions on the site. One for Python, one for Java.
Just an idea: You have the text which was originally sent, so you can look for it and remove it and additional surrounding noise from the reply. It is not trivial, because additional line breaks, HTML elements, ">" characters are added by the mail client application.
The regex is definitely better if it works, because it is simple and it perfectly cuts the original text, but if you find that it frequently does not work then this can be a fallback method.