<i>sed &amp; awk</i> describes two text processing programs that are mainstays of the UNIX programmer's toolbox.<i>sed</i> is a "stream editor" for editing streams of text that might be too large to edit as a single file, or that might be generated on the fly as part of a larger data processing step. The most common operation done with <i>sed</i> is substitution, replacing one block of text with another.<i>awk</i> is a complete programming language. Unlike many conventional languages, <i>awk</i> is "data driven" -- you specify what kind of data you are interested in and the operations to be performed when that data is found. <i>awk</i> does many things for you, including automatically opening and closing data files, reading records, breaking the records up into fields, and counting the records. While <i>awk</i> provides the features of most conventional programming languages, it also includes some unconventional features, such as extended regular expression matching and associative arrays. <i>sed &amp; awk</i> describes both programs in detail and includes a chapter of example <i>sed</i> and <i>awk</i> scripts.This edition covers features of <i>sed</i> and <i>awk</i> that are mandated by the POSIX standard. This most notably affects <i>awk</i>, where POSIX standardized a new variable, CONVFMT, and new functions, <i>toupper</i>() and <i>tolower</i>(). The CONVFMT variable specifies the conversion format to use when converting numbers to strings (<i>awk</i> used to use OFMT for this purpose). The <i>toupper</i>() and <i>tolower</i>() functions each take a (presumably mixed case) string argument and return a new version of the string with all letters translated to the corresponding case.In addition, this edition covers GNU <i>sed</i>, newly available since the first edition. It also updates the first edition coverage of Bell Labs <i>nawk</i> and GNU <i>awk</i> (<i>gawk</i>), covers <i>mawk</i>, an additional freely available implementation of <i>awk</i>, and briefly discusses three commercial versions of <i>awk</i>, MKS <i>awk</i>, Thompson Automation <i>awk</i> (<i>tawk</i>), and Videosoft (VSAwk).