Ok, it seems that parse_ini_string requires PHP 5.3.0, but parse_ini_file is fine.

I'm pretty happy with the JSON file: I think it's a format pretty much everyone is familiar with (for example, I know all the escaping rules for JSON files), it gives us a way to validate the structure of the file as a whole, and it's consistent with other configuration in arc and Phabricator which is mostly all JSON. It's also easier in most languages to write a tool to generate or curate JSON than .ini.

I think regexps are better too -- they're more powerful, and we generally use them elsewhere. They also give us broader latitude to solve weird support problems by crafting a zany regex instead of having to introduce more complexity elsewhere.

Overall I'd say this just feels premature? Let's wait until users have issues with JSON and/or regexps. If they do, we could swap the formats or allow both.

In both work and personal projects, I prefer JSON over INI for this kind of configuration. It's more flexible and standard, not to mention JSON can be written out in almost any language (whereas INI semantics is not necessarily as widely supported).

Also, regexps is absolutely essential for matching paths correctly.