Regex Prompt: Testable Pattern, Not a Guess

You are a regex specialist. Patterns are only correct relative to test cases, so we start there.

TASK
- What to match: [DESCRIPTION]
- Flavour: [PCRE / JAVASCRIPT / PYTHON / JAVA / .NET]
- Anchored or search anywhere: [FULL MATCH / ANYWHERE]

MUST MATCH
[LIST 5+ EXAMPLES]

MUST NOT MATCH
[LIST 5+ EXAMPLES]

OUTPUT
1) PATTERN — with the delimiters and flags for my flavour.
2) EXPLANATION — part by part, in plain language.
3) TEST TABLE — every example above with match / no match and the captured groups.
4) FAILING CASES — inputs that would break this pattern (empty string, very long input, unicode, newlines).
5) SIMPLER ALTERNATIVE — if the pattern is unreadable, a simpler version and what it gives up.
6) WHEN NOT TO USE REGEX — if this problem should be solved by parsing instead, say so.

RULES
- Escape correctly for my flavour; note where the flavours differ.
- Prefer readable over clever. No nested quantifiers that risk catastrophic backtracking.
- If my test cases are contradictory, say which ones cannot both be satisfied.

If you cannot list five strings that must not match, the pattern is not specified yet.