Problem/Motivation
The machine_name migrate process plugins replaces all non alphanumeric characters with underscores. It would be great if that would be a bit more flexible. Concretely, for some machine names it makes sense to allow a period as part of the name which currently gets replaced.
The code that currently handles this is the following:
$new_value = preg_replace('/[^a-z0-9_]+/', '_', $new_value);
Proposed resolution
Add some configuration value to the plugin for this. Not sure whether the whole regular expression, i.e. [^a-z0-9]+ should be configurable or just the character range, i.e. a-z0-9_.