GPT-5.3-Codex vs Grok 4.5 for coding

A practical (but **not perfect**) regex is:

```regex
^(?:[A-Z]{1,2}\d[A-Z\d]?\s?\d[A-Z]{2})$
```

One-line breakdown of each part:

- `^` — start of string (so nothing can appear before the postcode).  
- `(?: ... )` — non-capturing group for the whole postcode pattern.  
- `[A-Z]{1,2}` — 1 or 2 letters for the postcode area.  
- `\d` — one digit for the district.  
- `[A-Z\d]?` — optional extra district character (letter or digit).  
- `\s?` — optional space between outward and inward parts.  
- `\d` — one digit for the sector.  
- `[A-Z]{2}`…

**Regex:** `/^[A-Z]{1,2}\d[A-Z\d]? ?\d[A-Z]{2}$/i`

- `^` — start of string  
- `[A-Z]{1,2}` — 1–2 letters (area)  
- `\d` — one digit (district start)  
- `[A-Z\d]?` — optional letter or digit (rest of district)  
- ` ?` — optional space  
- `\d` — one digit (sector)  
- `[A-Z]{2}` — two letters (unit)  
- `$` — end of string  
- `i` flag — case-insensitive  

Two valid UK postcodes this would reject: `EC1A 1BB` (the `A` after the digit in the outward code is only partially covered by the optional class in some edge positions), `W1A 0AX` (same…