15.1 INTRODUCTION

String matching is employed in several applications, such as packet classification, computational biology, spam blocking, and information retrieval. String search operates on a given alphabet set Σ of size |Σ|, a pattern P = p₀p₁ … p_m−1 of length m, and a text string T = t₀t₁ … t_n−1 of length n, with m ≤ n. The problem is to find all occurrences of the pattern P in the text string T. The average time complexity for implementing the string search problem on a single processor was proven to be O(n) [99]. We refer the reader to Reference 100 for a comprehensive review of the different hardware implementations of the string matching problem.

A hardware implementation for the search engine can be assumed to have the following characteristics:

• The text length n is typically big and variable.
• The pattern length m varies from a word of few characters to hundreds of characters (e.g., a URL address).
• The word length w is determined by the data storage organization and datapath bus width.
• Typically, the search engine is looking for the existence of the pattern P in the text T; that is, the search engine only locates the first occurrence of the P in T.
• The text string T is supplied to the hardware in word serial format.