Regex Builder

Write a regex for me.

What it should match: [DESCRIBE THE PATTERN + PASTE 3-5 REAL STRINGS THAT MUST MATCH]
What it must NOT match: [PASTE 3-5 STRINGS THAT LOOK SIMILAR BUT MUST BE REJECTED: this is where most regexes fail]
Where it runs: [JAVASCRIPT / PYTHON / GO(RE2) / PCRE / grep: flavors differ on lookbehind, named groups, unicode]
The job: [VALIDATE FULL INPUT / EXTRACT PARTS / FIND-AND-REPLACE: changes anchoring and grouping]

Deliver:
1. **The regex**: with anchors explicitly decided (^$ for validation, none for search: state which and why), named groups for anything extracted, and the flavor's syntax verified.
2. **Piece-by-piece breakdown**: each component on its own line with what it matches, so I can maintain it in six months.
3. **Test table**: every string I provided plus 5 edge cases you add (empty string, unicode, maximal-length, the almost-valid trap, whitespace variants): | input | expected | why |. Walk the table and confirm the regex agrees. If any row fails, fix the regex before presenting it.
4. **ReDoS check**: state whether the pattern contains catastrophic-backtracking risk (nested quantifiers, overlapping alternations like (a+)+). If it does and my flavor isn't RE2, provide the safe rewrite: user-facing input + backtracking regex is a denial-of-service bug, not a style choice.
5. **The honesty clause**: if this job shouldn't be a regex, say so and show the better tool: a real parser for HTML/JSON/CSV, a date library for dates, a validator library for emails/URLs (the RFC-complete email regex is a famous trap; offer the pragmatic version and state what it deliberately rejects).

Rules: match the strings I gave, not the platonic ideal of the format. If my must-match examples conflict with the spec I described, flag the conflict and ask which wins.