This is to fix a regression introduced by

I just realized that private-use units (pu_single_unit) in the draft 46.1 references ‘single_unit’, which has have validity constraints in v46.1. That negates the whole purpose. Something like xxx-meter is valid, but xxx-smoot is not (it doesn’t match any existing unit). Moreover, they cannot have multiple parts, so the example given of xxx-square-knuts is illegal on two counts (square-xxx-knuts would be ok).

To fix this, we should change the pu_single_unit to the following:

pu_single_unit
:= ("xxx-" | "x-") [a-z0-9]{3,8}

• Examples:

  • square-xxx-knuts (a Harry Potter unit)

• Notes:

  • "x-" is only for backwards compatibility; it is deprecated and should not be generated

  • See Private-Use Units

Background

Here is the current status in the draft v46.1, where single_unit ends up referencing base_component (where the validity constraint is — I bolded that)

pu_single_unit
:= "xxx-" single_unit
   | "x-" single_unit

• Examples:

  • xxx-square-knuts (a Harry Potter unit)

• Notes:

  • "x-" is only for backwards compatibility; it is deprecated and should not be generated

  • See Private-Use Units

…

base_component
:= [a-z]{3,}

• [ wfc: must not have a prefix as an initial segment. ]

• [ wfc: must not be a value in <unitIdComponent type="X"> for X in {prefix, suffix, power, and, per} ]

• [ vc: Must be an attribute value of the source in: <convertUnit source='…' …> or the type in <unitAlias type="…" replacement="…" …> ]