```{index} single: Filter::m4 ``` # Filter::m4 M4 source filter ## SYNOPSIS ```perl use Filter::m4; use Filter::m4 'prefix'; ``` ## DESCRIPTION This source filter pipes the current source file through the M4 macro processor (`m4`) if it is available. As with all source filters its scope is limited to the current source file only. Every file you want to be processed by the filter must have the following line near the top. ```perl use Filter::m4; ``` ## EXAMPLE Here is a small example that shows how to define and use an M4 macro: ```perl use Filter::m4; define(`foo', `$1 =~ s/bar/baz/r') $a = "foobar"; print "a = " . foo(`$a') . "\n"; ``` The output of the above example: ```text a = foobaz ``` ## NOTES By default, M4 uses \` and ' as quotes; however, this is configurable using M4's `changequote` builtin. M4 uses `$1`, `$2`, etc., to indicate arguments in macros. To avoid clashes with Perl regex syntax it is recommended to use Perl's alternative forms `${1}`, `${1}`, etc. The following keywords in M4 and Perl are identical: ```text eval format index mkstemp shift substr ``` If you need such keywords in your Perl code you have to use one of the following three solutions. - Protect the keyword with M4 quotes, for example `` `shift' ``. - Redefine the problematic M4 builtin using `defn`, as outlined in section _Renaming macros_ of the M4 info manual. - Use the `prefix` option. This adds the prefix `m4_` to all M4 builtins (but not to user-defined macros). For example, you will have to use `m4_shift` instead of `shift`. ## AUTHOR Werner Lemberg ## DATE 17th March 2018.