g1smd - 11:49 am on Jul 6, 2013 (gmt 0)
Since you're not reusing $2, the rule might even simplify to
RewriteRule ^play-([0-9a-zA-Z-]+)-online http://www.example.com/online-games/$1.html? [R=301,L]
as long as the new pattern doesn't match requests that it should not.
Can there be another hyphen between the play- and -online parts in the old style URLs? If not, omit "-" from the character group.