Fixing the invalid xref check

[matentzn]

Resolves #1127

• [X] docs/ have been added/updated
• [x] tests have been added/updated
• [X] mvn verify says all tests pass
• [x] mvn site says all JavaDocs correct
• [X] CHANGELOG.md has been updated

This PR fixing the regular expression used to recognise XREFs. I tested it in the https://regex101.com/ sandbox:

• Match the prefix:
  • ^[A-Za-z_]: Starts with an alphabet letter (either uppercase or lowercase) or an underscore.
  • [A-Za-z0-9_.-]*: Followed by any number (including zero) of alphanumeric characters, underscores, periods, or hyphens.
  • [A-Za-z0-9_]: Ends the first part with an alphanumeric character or an underscore.
• :: Contains a colon separating the prefix from the reference.
• [^\\s]+$: Matches the reference, which comprises 1 or more characters that are not whitespace (including spaces, tabs, and newlines).

[matentzn]

I just updated the tests to illustrates that the new check works.