lex(1), yacc(1), sed(1), awk(1).
John Levine, Tony Mason, and Doug Brown: Lex & Yacc; O'Reilly and Associates. Be sure to get the 2nd edition.
M. E. Lesk and E. Schmidt, LEX - Lexical Analyzer Generator.
Alfred Aho, Ravi Sethi and Jeffrey Ullman: Compilers:
Principles, Techniques and Tools; Addison-Wesley (1986).
Describes the pattern-matching techniques used by flex
(deterministic finite automata).