RethinkDB filters work on regular expressions, so you can simply match on the end of the line to filter a result:
r.db('test')
.table('users')
.filter( (doc) => doc('user_name').match("ieling$") )
If you want this to be case-insensitive, do the following:
r.db('test')
.table('users')
.filter( (doc) => doc('user_name').downcase().match("ieling$") )