It looks like you're attempting to create the following structure, correct me if I'm wrong;
The following should do the trick:
RewriteRule ^classifieds/([0-9]+)_([^.]+)\.html$ classifieds.php?do=viewitem&itemid=$1&title=$2 [L]
The first capture looks for a set of numbers, which you have accomplished on your own. The second capture after the underscore is looking for any group of characters up until it finds a period. Since it's enclosed in a character range, you don't have to escape it as a control character. Outside of the range it does need to be escaped, as shown here.
Edit: This allows a lot of freedom in how the title is presented. If you want to ensure that it's only alphabetical, or alphanumerical, replace [^.] with [A-Za-z] or [A-Za-z0-9] or finally [a-z] with NC added at the end [NC,L]