For MDL-67767, an extra validation with a regular expression has been added because there is no PARAM with the requirements for validating the URL scheme format.

As Eloy has commented in MDL-68101:

I've already seen a couple of cases (recently) where we want to just enable {{[a-zA-Z0-9_] }} maybe we should consider adding some new type or adding some "extra" param to allow the regular expressions to be dynamically expanded when some more char is allowed.... note this is a side idea, it doesn't belong strictly to this issue.

So this issue is for adding this extra param. Something like: PARAM_ALPHANUM | PLUS_MINUS | PLUS_PLUS | PLUS_UNDERSCORE.