Alfred Aho

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Alfred Vaino Aho (born August 9, 1941 in Timmins, Ontario) is a Canadian computer scientist.



Aho received a B.A.Sc. in Engineering Physics from the University of Toronto and a Ph.D. in Electrical Engineering/Computer Science from Princeton University. He conducted research at Bell Labs from 1967 to 1991, and again from 1997 to 2002 as Vice President of the Computing Sciences Research Center. He is currently the Lawrence Gussman Professor of Computer Science at Columbia University. He served as chair of the department from 1995 to 1997, and again in the spring of 2003.

In his PhD thesis, Aho created indexed grammars and the nested stack automaton as vehicles for extending the power of context-free languages but retaining many of their decidability and closure properties. Indexed grammars have been used to model parallel rewriting systems particularly in biological applications.

After graduating from Princeton, Aho joined the Computing Sciences Research Center at Bell Labs where he devised efficient regular expression and string pattern matching algorithms which he implemented in the first versions of the Unix tools egrep and fgrep. The fgrep algorithm has become known as the Aho-Corasick algorithm and is widely used in a number of bibliographic search systems, including the one developed by Margaret J. Corasick, and other string searching applications.

At Bell Labs, Aho worked closely with Steve Johnson and Jeffrey Ullman to develop efficient algorithms for analyzing and translating programming languages. Steve Johnson used the bottom-up LALR parsing algorithms to create the syntax-analyzer generator yacc, and Michael E. Lesk and Eric E. Schmidt used Aho's regular expression pattern matching algorithms to create the lexical-analyzer generator lex. The lex and yacc tools and their derivatives have been used to develop the front ends of many of today's programming language compilers.

Aho and Ullman wrote a series of textbooks on compiling techniques that codified the theory relevant to compiler design. Their 1977 textbook Principles of Compiler Design had a green dragon on the front cover and became known as "the green dragon book." In 1986 Aho and Ullman were joined by Ravi Sethi to create a new edition, "the red dragon book" (which was briefly shown in the 1995 movie "Hackers"), and in 2007 also by Monica Lam to create "the purple dragon book." The dragon books have been the most widely used compiler texbooks throughout the world.

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