Write and Explain a Regular Expression

Write a regular expression for the task below, built from the examples I give you.

Language or tool and regex flavor: {{language or tool}}
What the pattern must do: {{task}}
Strings that must match:
{{should match}}

Strings that must NOT match:
{{should not match}}

Extra rules (anchors, case sensitivity, Unicode, multiline, maximum length): none
Is this for validation, extraction or search-and-replace? extraction

Give me:
1. The pattern, in a code block, written for {{language or tool}} including how to escape it in a string literal in that language.
2. A breakdown of the pattern, one piece per line, saying what each part matches and why it is there.
3. A table of every example I gave you with the result you expect (match or no match, and the captured groups if any). Walk through at least two of the tricky ones step by step to show you did not just assume.
4. Edge cases the pattern does not handle or handles differently from what I might expect, and a stricter or looser version if relevant.
5. Any risk of catastrophic backtracking or very slow matching on long input, and how to avoid it.
6. A short test snippet in {{language or tool}} that runs all my examples and prints the result.

Rules: only use features supported in the flavor I named. If the examples are ambiguous or conflict with each other, ask me which is right before writing the pattern. If a regular expression is the wrong tool for the task, such as parsing HTML or fully validating an email address, say so and suggest the right approach.

4 required fields are empty and will stay as a {{placeholder}} in the copied text.