Clarify handling of regex-based parsing

[Ms2ger]

We've got text like

1. Let valid be true if dateTime matches the "datetime" regular expression in the polyfill and false otherwise.
2. If valid is false, then
   1. Throw a RangeError exception.
3. Let year, month, day, hour, minute, second, millisecond, microsecond, nanosecond be the respective parts of dateTime interpreted as decimal numbers.

That's not great. I don't know what the best solution is, though - perhaps creating actual JS RegExp objects? Thoughts welcome.

[littledan]

In some other places, we reference a grammar and productions within the grammar. Can we do that here?

[ptomato]

Unfortunately, you have to purchase ISO 8601 to get the full text, so if we referenced something in ISO, our document would become less accessible.

From this openly available draft of ISO 8601-2 it seems that there is a grammar, but only for the extensions, not the main date/time format in ISO 8601-1. So we'd probably end up writing a grammar anyway.

[ptomato]

Ah, there's a grammar in https://www.ietf.org/rfc/rfc3339.txt appendix A.

[littledan]

Yeah, we can cite RFC 3339 normatively here, rather than ISO 8601.

[Ms2ger]

Time zones are probably the hardest case here, since they have a couple of alternative syntaxes.