Capturing group name can be used in a regular expression in two contexts: When introducing a group (?<name>...) or when referring it
\k<name>.
If the name is invalid (i.e. does not start with a Latin letter, or contains wrong chars) then we may see different error messages, some of
which look confusing.

Here are examples of the messages produced by the current JDK:
Unknown look-behind group near index 3
(?<>)
    ^
named capturing group is missing trailing '>' near index 4
\\k<>
     ^
Unknown look-behind group near index 4
(?<.>)
     ^
(named capturing group <.> does not exit near index 4
\\k<.>
     ^
named capturing group is missing trailing '>' near index 4
(?<a.>)
     ^
named capturing group is missing trailing '>' near index 4
\\k<a.>
     ^

In particular, this diversity is caused by that the internal Pattern.groupname() function lacks a check for the very first character
of the name.
So that when \k<name> is parsed, the first char is always accepted, no matter what it was.