how to get keys which does not match a particular pattern in redis?

IMPORTANT: always use SCAN instead of (the evil) KEYS

Redis' pattern matching is somewhat functionally limited (see the implementation of stringmatchlen in util.c) and does not provide that which you seek ATM. That said, consider the following possible routes:

1. Extend stringmatchlen to match your requirements, possibly submitting it as a PR.
2. Consider what you're trying to do - fetching a subset of keys is always going to be inefficient unless you index them, consider tracking the names of all non-user keys (i.e.g. in a Redis Set) instead.
3. If you are really insistent on scanning the entire keyspace and match against negative patterns, one way to accomplish that is with a little bit of Lua magic.

Consider the following dataset and script:

127.0.0.1:6379> dbsize
(integer) 0
127.0.0.1:6379> set user:1 1
OK
127.0.0.1:6379> set use:the:force luke
OK
127.0.0.1:6379> set non:user a
OK

Lua (save this as scanregex.lua):

local re = ARGV[1]
local nt = ARGV[2]

local cur = 0
local rep = {}
local tmp

if not re then
  re = ".*"
end

repeat
  tmp = redis.call("SCAN", cur, "MATCH", "*")
  cur = tonumber(tmp[1])
  if tmp[2] then
    for k, v in pairs(tmp[2]) do
      local fi = v:find(re) 
      if (fi and not nt) or (not fi and nt) then
        rep[#rep+1] = v
      end
    end
  end
until cur == 0
return rep

Output - first time regular matching, 2nd time the complement:

foo@bar:~$ redis-cli --eval scanregex.lua , "^user"
1) "user:1"
foo@bar:~$ redis-cli --eval scanregex.lua , "^user" 1
1) "use:the:force"
2) "non:user"

@Karthikeyan Gopall you nailed it in your comment above and this saved me a bunch of time. Thanks!

Here's how you can use it in various combinations to get what you want:

redis.domain.com:6379[1]> set "hello" "foo"
OK
redis.domain.com:6379[1]> set "hillo" "bar"
OK
redis.domain.com:6379[1]> set "user" "baz"
OK
redis.domain.com:6379[1]> set "zillo" "bash"
OK
redis.domain.com:6379[1]> scan 0
1) "0"
2) 1) "zillo"
   2) "hello"
   3) "user"
   4) "hillo"
redis.domain.com:6379[1]> scan 0 match "[^u]*"
1) "0"
2) 1) "zillo"
   2) "hello"
   3) "hillo"
redis.domain.com:6379[1]> scan 0 match "[^u^z]*"
1) "0"
2) 1) "hello"
   2) "hillo"
redis.domain.com:6379[1]> scan 0 match "h[^i]*"
1) "0"
2) 1) "hello"