Custom Regex Validation Rule

I need a single, well-crafted regular expression that will serve purely for data validation in a production system. I will share the exact input format and edge-case examples once we begin; for now, assume the pattern must be tight enough to reject malformed entries while staying performant.

Please deliver:
• The final regex, ready to drop into my codebase
• A short explanation of the logic so future developers can maintain it
• A minimal set of test strings that prove it works (valid and invalid cases)

If you prefer look-ahead, look-behind, or other advanced constructs, that’s fine as long as they’re supported in PCRE, JavaScript, and Python engines—I’ll confirm the runtime before we start.