Editing M4 (computer language) (section)

==Features==
m4 offers these facilities:

* a [[Free-form language|free-form syntax]], rather than [[line-based syntax]]
* a high degree of macro expansion (arguments get expanded during scan and again during interpretation)
* text replacement
* parameter substitution
* file inclusion
* string manipulation
* conditional evaluation
* arithmetic expressions
* system interface
* programmer diagnostics
* programming language independent
* human language independent
* provides programming language capabilities

Unlike most earlier macro processors, m4 does not target any particular computer or human language; historically, however, its development originated for supporting the [[Ratfor]] dialect of [[Fortran]]. Unlike some other macro processors, m4 is [[Turing-complete]] as well as a practical programming language.

Unquoted identifiers which match defined macros are replaced with their definitions. Placing identifiers in quotes suppresses expansion until possibly later, such as when a quoted string is expanded as part of macro replacement. Unlike most languages, strings in m4 are quoted using the [[backtick]] (`) as the starting [[delimiter]], and [[apostrophe]] (') as the ending delimiter. Separate starting and ending delimiters allows the arbitrary nesting of quotation marks in strings to be used, allowing a fine degree of control of how and when macro expansion takes place in different parts of a string.