Editing Eqn (software) (section)

{{Short description|Preprocessor that formats equations for drawing}}
{{lowercase title}}
{{Infobox software
| name = eqn
| logo = 
| screenshot = 
| screenshot size = 
| caption = 
| author = [[Brian Kernighan]],<br />[[Lorinda Cherry]]<br />([[AT&T Bell Laboratories]])
| developer = Various [[open-source software|open-source]] and [[commercial software|commercial]] developers
| released = {{Start date and age|1974|6}}
| latest release version = 
| latest release date = 
| programming language = [[C (programming language)|C]], [[Yacc]]<ref name="Kernighan_Brailsford" />
| operating system = [[Unix]], [[Unix-like]], [[Plan 9 from Bell Labs|Plan 9]]
| platform = [[Cross-platform]]
| genre = [[Command (computing)|Command]]
| license = Plan 9: [[MIT License]]
| website = 
}}
Part of the [[troff]] suite of [[Unix]] document layout tools, '''eqn''' is a [[preprocessor]] that formats equations for printing.  A similar program, '''neqn''', accepted the same input as eqn, but produced output tuned to look better in [[nroff]]. The eqn program was created in 1974 by [[Brian Kernighan]] and [[Lorinda Cherry]].
It was implemented using [[yacc]] compiler-compiler.<ref name="Kernighan_Brailsford" />

The input language used by eqn allows the user to write mathematical expressions in much the same way as they would be spoken aloud.  The language is defined by a [[context-free grammar]], together with [[operator precedence]] and [[operator associativity]] rules.<ref name="Kernighan_Cherry_1975" />  The eqn language is similar to the mathematical component of [[TeX]], which appeared several years later, but is simpler and less complete.

An independent compatible implementation of the eqn preprocessor has been developed by [[GNU]] as part of [[groff (software)|groff]], the GNU version of troff.  The GNU implementation extends the original language by adding a number of new [[keyword (computer programming)|keywords]] such as ''smallover'' and ''accent''. [[mandoc]], a specialised compiler for UNIX [[man pages]], also contains a standalone eqn parser/formatter.