# phpman > perldoc > Filter::m4

## NAME
    [Filter::m4](https://www.chedong.com/phpMan.php/perldoc/Filter%3A%3Am4/markdown) - M4 source filter

## SYNOPSIS
        use [Filter::m4](https://www.chedong.com/phpMan.php/perldoc/Filter%3A%3Am4/markdown);
        use [Filter::m4](https://www.chedong.com/phpMan.php/perldoc/Filter%3A%3Am4/markdown) '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.

        use [Filter::m4](https://www.chedong.com/phpMan.php/perldoc/Filter%3A%3Am4/markdown);

## EXAMPLE
    Here is a small example that shows how to define and use an M4 macro:

        use [Filter::m4](https://www.chedong.com/phpMan.php/perldoc/Filter%3A%3Am4/markdown);

        define(`foo', `$1 =~ s/bar/baz/r')

        $a = "foobar";
        print "a = " . foo(`$a') . "\n";

    The output of the above example:

        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:

        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.