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Thomas S. Christiansen (born February 13, 1963), nicknamed "tchrist" or occasionally "thoth", is a Unix developer and user known for his work with the Perl programming language.

Christiansen worked for several years at TSR Hobbies before attending the University of Wisconsin - Madison where he earned B.A.'s in Spanish and Computer Science, and an M.S. in Computer Science. He worked for five years at Convex Computer. In 1993, he established the Tom Christiansen Perl Consultancy, located in Lake Geneva, Wisconsin. In 2010, he joined the Biomedical Text Mining Group at the University of Colorado School of Medicine.

Christiansen was one of the early contributors to Perl after its public release in 1987. He was the author of much of the core Perl documentation, including the manual pages perlfaq and perltoot. In 1996, Christiansen wrote "Csh Programming Considered Harmful" about the limitations inherent in C Shell Programming. Books he co-authored include the second (1996) and third (2000) editions of Programming Perl, the second (1997) edition of Learning Perl (and its spin-off Learning Perl on Win32 Systems) and The Perl Cookbook (1998). In 1999, he was one of the original recipients of the White Camel Awards from Perl Mongers for his contribution to Perl's documentation.

The common phrase "Only perl can parse Perl" is attributed to Tom Christiansen, although it probably originated from "Only tex can understand TeX". Randal Schwartz also credits him with accidentally naming the Schwartzian Transform for optimizing some types of sorts. This happened after Schwartz used it in a Usenet message, and Christiansen replied to the message giving some corrections and in one place said "the Schwartzian transform" to refer to the transform that Schwartz used.

This author page uses material from the Wikipedia article "Tom Christiansen", which is released under the Creative Commons Attribution-Share-Alike License 3.0