Fix off-by-one string indexing in constraint checking

[al2me6]

Fixes #170, #172.

Also change string comparison type to StringComparison.Ordinal, which should be the correct type according to https://docs.microsoft.com/en-us/dotnet/standard/base-types/best-practices-strings.

[Bornholio]

This should also close #172

[al2me6]

I'm actually going to remove the string comparison change for now and send in a separate PR since there's more instances of culture-dependent comparisons.

[sarbian]

Thanks.