Fixing decode error of + strings (plus sign)

[TBeijen]

Fix for #13

The first regex OR (now removed) caused regex to pass even if none of the succeeding oct|dec|hex definitions was present.

Quite confident this fixes it:

• Testsuite contains tests for oct|dec|hex (decode_test.go)
• No unexpected side-effects in testsuite

[apparentlymart]

Hi @TBeijen! Thanks for working on this and sorry for the very slow response.

I agree that this seems like a more accurate read of the rules in Tag Resolution: it requires there to be at least one digit after the + or - to count as an tag:yaml.org,2002:int, but this was previously allowing zero digits.

Looking closely at that specification text, I think there's also another inconsistency: the [-+]? part is included only for the decimal form, and not for the octal and hex forms. Therefore I think the [-+]? pattern ought to be inside the disjunction directly before the [0-9_]+ pattern.

In other words, I think the correct pattern is (0o[0-7_]+|[-+]?[0-9_]+|0x[0-9a-fA-F_]+). Would you agree?

(The spec doesn't mention allowing underscores in any of these, so I'm not really sure where that came from in #6 now that I look at it in retrospect. Perhaps we should drop those too, because otherwise _ would be interpreted as an integer and that doesn't seem right. 🤔 )