Bugfix: stricter regex; disallow trailing newline

[tvdijen]

To prevent false positives, we can make use of the PCRE_DOLLAR_ENDONLY flag (D) that will constraint the $ anchor to match the end of the string only, not a newline character.

[baszoetekouw]

Looks good to me. Maybe for completeness, you could add a test that starts with a newline (should work with the existing regex, if the docs are right about the behaviour of \A).