• If used at the very end of a pattern, \x matches a literal x rather than a ␀ character like it does at other positions.
• If used at the very end of a pattern, \u matches a literal u rather than throwing an error like it does at other positions.

These feel like bugs, since I'm not sure why they should behave differently only at the very end of a pattern.

^{Disclosure: I'm testing using Oniguruma 6.9.8 via vscode-oniguruma. However, the release notes for subsequent versions don't mention any related changes.}

More details:

Current behavior for \x: It's an escape for the ␀ character (equivalent to \0, \x00, etc.) if it's not followed by { or a hexadecimal digit.

• \x is an error if followed by a { that's followed by a hexadecimal digit but doesn't form a valid \x{…} code point escape. Ex: \x{F and \x{0,2} are errors.
• \x is an identity escape (matching a literal x) if followed by a { that isn't followed by a hexadecimal digit. Ex: \x{ matches x{, \x{G matches x{G, \x{} matches x{}, and \x{,2} matches 0–2 x characters, since {,2} is a quantifier with an implicit 0 min.
• \x is treated as an identity escape (matching a literal x) if it appears at the very end of a pattern.
  • This feels like a bug.

Current behavior for \u:

• Normally, any incomplete \uHHHH (including bare \u) throws an error.
• \u is treated as an identity escape (matching a literal u) if it appears at the very end of a pattern.
  • This feels like a bug.

Note: PCRE2 v10.45 recently removed the ability for \x to match a ␀ character, and recommends using \x00 instead.

* (Minor pattern syntax change) Parsing of the \x escape is stricter, and is
no longer parsed as an escape for the NUL character if not followed by '{' or
a hexadecimal digit. Use \x00 instead.