JsonApiSpec::isValidMemberName does not match its documentation

I introduced that code in #2829328: Clean up Drupal\jsonapi\Access\CustomParameterNames and make it follow the spec more closely.

Overall, this is an especially hard to read bit of code regardless.

Happy to make it more legible. But code that checks spec conformity is bound to be hard to read. That's why it references http://jsonapi.org/format/#document-member-names!

Specifically, {1} only matches a single occurrence of a character, + is for "one or more".

So you're referring to that first line of code that you quoted, right?

Yep, and it's totally possible that I'm misreading the regular expression because of rules that come later in the expression.

That's exactly what I think is happening. To be fair, it is confusing:

         // First character must be "globally allowed". Length must be >=1.
         self::MEMBER_NAME_GLOBALLY_ALLOWED_CHARACTER_CLASS . '{1}' .

This comment applies to the ENTIRE regexp. Note this is indented less than what follows:

           // As many non-globally allowed characters as desired.
           self::MEMBER_NAME_INNER_ALLOWED_CHARACTERS . '*' .
           // If length > 1, then it must end in a "globally allowed" character.
           self::MEMBER_NAME_GLOBALLY_ALLOWED_CHARACTER_CLASS . '{1}' .

All thee rules together comply with what the spec says:

• Member names MUST contain at least one character.
• Member names MUST contain only the allowed characters listed below.
• Member names MUST start and end with a “globally allowed character”, as defined below.

So AFAICT this is correct. But if we can clarify the comments/code, then great. OTOH, I think there's little value in doing so for something like this, precisely because it's the codifying of a spec, and comparing the code with the spec should always lead to readers ending up concluding it is correct. Hence, agreed with Minor priority.