Correct the regex strings in the master readme

[shaun-wilson]

The readme says the regex strings are the source of truth, however some of the regex patterns do not properly convey the characters that are allowed at particular positions.

For example, the pattern to match a nested primitive label is currently <([^>]+)>, but it is known that the characters <>%{}| cannot be present within a label, therefore the pattern should be something like <([^<>%{}\|]+)>