db.stuff.find_one({"foo": /bar/i});

username = '/' + str(username) + '/i'
response = request.db['user'].find_one({"username":username},{"username":1})

db.stuff.find_one({'name': re.compile(username, re.IGNORECASE)})

db.stuff.find_one({'name_lower': username.lower()})

import re
db.stuff.find_one({'name': re.compile('^' + username + '$', re.IGNORECASE)})

import re
db.stuff.find_one({'name': re.compile('^' + re.escape(username) + '$', re.IGNORECASE)})