The viruses and worms are attacked the organizations in the world and the information are accessed by unauthorized users. These organizations preferred several mechanisms for intrusion detection and prevention systems to protect from these types of threats. Intrusion Detection System (IDS) is a embedded with system and intelligent subsystems. In this paper the signature based intrusion detection system is discussed. There are different pattern matching algorithms available to detect intrusion. Brute force and Knuth-Morris- Pratt are the single keyword pattern matching algorithms. If one or more occurrence of pattern present in the input text, then there is an intrusion and the intrusion alarm will be sent. In this paper the various real time Intrusion Detection Systems are discussed.