regex to remove NL and CR

I am extracting text from a text datatype field of a database. The target text is marked (beginning and end) by html comments that have a known--but not uniform--format. Unfortunately, the web-based interface used to maintain the data inserted hard line breaks in the text. I initially tried removing all NL and CR chars from the string, which has the unanticipated result of removing some spaces between words that had wrapped in the admin interface. Now, I need to remove all NL and CR that occur within a string starting with "<!-" and ending with "->". I have no idea where to start, because the NL and CR could also occur in the strings that mark the start and end of the comments. I am very new to regex, but starting to get it. If someone could just give me a push in the right direction. Nothing I have tested in regex coach comes close to matching what I need. Kathleen