K does not accept ":"

[CharlesNepote]

Which is a problem if you want to use namespaces (like OpenStreetMap keys). Eg.

"recalled_product" => "yes"
"recalled_product:date" => "2021-05"
"recalled_product:link" => "https://rappel.conso.gouv.fr/fiche-rappel/383/Interne"

The regexp should be: ^[a-zA-Z0-9_]+:?[a-zA-Z0-9_]+?$ ( see https://regex101.com/r/2hegDH/1 ).

Maybe we have to decide to allow or not subnamespaces. It is possible in OpenStreetMap, eg. minspeed:lanes:forward=50 (source). Then the regexp should be: ^[a-zA-Z0-9_]+(:?[a-zA-Z0-9_]+?)+$.
It could be interesting for languages: namespace:property:fr=...

Do you see other chars that could be accepted?