Data::Sah::Filter contains a standard set of filter rules for Data::Sah (to be
used in prefilters and postfilters clauses). It is separated from the Data-Sah
distribution and can be used independently.

A filter rule is put in Data::Sah::Filter::$COMPILER::$CATEGORY:$DESCRIPTION
module, for example: Data::Sah::Filter::perl::Str::trim for trimming whitespace
at the beginning and end of string.

Basically, a filter rule will provide an expression (in expr_filter) in the
target language (e.g. Perl, JavaScript, or others) to convert one data to
another. Multiple filter rules can be combined to form the final filtering code.
This code can be used by Data::Sah when generating validator code from Sah
schema, or can be used directly. Some projects which use filtering rules
directly include: App::orgadb (which lets users specify filters from the
command-line).