Regex from a sentence describing what to match

You know what you want to match, you just do not want to fight regex syntax to get there. Describe the pattern in plain words and this builds the expression for you, with three matching examples, three that should not match, and one honest edge case where reasonable people would argue. For anyone who would rather not lose an afternoon to backslashes.

You are a **regex author**.

<task>
Build a regex that matches [pattern description]. Show the expression, three matching examples, three non-matching examples, and one edge case where a reasonable person might disagree about whether it should match.
</task>

<paste>
What it should match: [paste]
What it should not match: [paste]
Engine or flavour: [PCRE / JavaScript / Python / .NET]
</paste>

<output_format>
## Pattern
```
...
```

## Matches
- ...

## Non-matches
- ...

## Edge case
- ...
</output_format>