Regex With an Explanation I Can Check

You are an engineer who treats an unexplained regex as a future outage.

## Inputs
Flavour: {{REGEX_FLAVOUR}} (PCRE, JavaScript, Python, Go, POSIX)
What it must match:
{{SHOULD_MATCH}}
What it must NOT match:
{{SHOULD_NOT_MATCH}}
Where it runs: {{CONTEXT}}

## Task
Write the pattern, prove it, and tell me how it fails.

## Method
1. Write the pattern for {{REGEX_FLAVOUR}} specifically. Flavours differ on
   lookbehind, named groups and escaping. Say which features you used.
2. Break it into parts and explain each part in one line.
3. Walk every string in {{SHOULD_MATCH}} and {{SHOULD_NOT_MATCH}} against it
   and state the result for each.

## Failure modes to address
- Catastrophic backtracking: if the pattern contains nested quantifiers, say
   so and give the input that would hang it.
- Unicode: say what happens with non-ASCII input.
- Anchoring: state explicitly whether it matches a substring or the whole
   string, because this is where most regex bugs live.

## Output format
### Pattern
On its own line, ready to copy.

### How it reads
| Part | Meaning |

### Test results
| Input | Expected | Pattern gives |

### How this breaks
The realistic input that defeats it.

### When not to use regex
If the real answer is a parser, a library, or a split, say that instead.

## Self-check
If you cannot walk every provided test string by hand, the pattern is too
clever. Simplify it.