Write a regular expression and test it against real examples

Write a regular expression for the job below, for {{reader}} to paste into {{language}} and understand well enough to change later.

What it must do: {{goal}}

Strings it must match, with what to capture from each, if anything:
"""
{{should_match}}
"""

Strings it must not match:
"""
{{should_not_match}}
"""

Regex flavor and where it runs: {{flavor}}

Produce:
1. The pattern on its own line in a code block, exactly as it goes in the code, with any flags.
2. A breakdown: each part of the pattern on its own line, next to a plain description of what it matches.
3. A test table with the columns String, Should match, Does match, Captured, covering every string above plus up to 6 edge cases you add, such as an empty string, extra spaces, mixed case or a very long input. Mark the edge cases you added.
4. A short snippet in {{language}} that runs the pattern against all the test strings, so I can confirm the table myself.

Keep the pattern as simple as the examples allow, and prefer a readable pattern over a clever one. Say so if a plain string method or a real parser would do the job better, for example for HTML, nested brackets or full email address validation, where a regular expression is the wrong tool.

For example, the form of one breakdown line: "\d{4} matches exactly four digits, the year."

A good answer is one I can check line by line against the test table. Avoid nested quantifiers such as (a+)+ that can run for a very long time on some inputs, and point out any part that could be slow on long strings. Use only syntax that {{flavor}} supports; lookbehind, named groups and Unicode classes differ between flavors, so say which of these you used. If the examples conflict or leave a case open, such as whether a trailing period counts, list the question and the choice you made instead of deciding silently. Do not claim a string matches unless you have traced it through the pattern; if you cannot be sure, write "check" in the Does match column.