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Actually, an Amazon product URL can also include a product title and have extra params after the ASIN. Example:

```none
https://www.amazon.com/Hitchhikers-Guide-Galaxy/dp/B0009JKV9W/ref=sr_1_1?crid=13ECW_etc.....
                       ^^^^^^^^^^^^^^^^^^^^^^^^               ^^^^^^^^^^^^^^^
                         product description                  extra stuff
```

But that part is optional. For example, the same product above can be accessed by the URL `https://www.amazon.com/dp/B0009JKV9W`.

---

Anyway, the regex could be like this:

```none
https://www\.amazon\.com(?:/[^/]+)?/[dg]p/([a-zA-Z0-9]{10})
```

The `(?:/[^/]+)?` part matches the optional product title:

- `[^/]` means "any character that's not a slash"
- `+` means "one or more occurrences"

Therefore, `/[^/]+` is a slash followed by one or more characters that are not a slash. And it's all inside parenthesis, followed by `?` (which means "zero or one occurrence", AKA "optional").

Parenthesis normally create a capturing group, but the `(?:` syntax makes it a non-capturing group. I prefer to use this to indicate that I'm not interested in whatever matches that part of the regex, and it prevents the engine from creating groups that I don't want to look at.

Then `[dg]` matches either `d` or `g`, and it's followed by `p`.

And finally, `[a-zA-Z0-9]` matches any lowercase or uppercase ASCII letter, or any digit between 0 and 9. `{10}` means "exactly 10 occurrences". And everything is inside parenthesis, to create a capturing group, whose value you can easily get by using any engine/library.

For example, in Python:

```python
import re

url = ... # some URL

if match := re.match(r'https://www\.amazon\.com(?:/[^/]+)?/[dg]p/([a-zA-Z0-9]{10})', url):
    # get capturing group 1
    asin = match[1]
```

As groups are indexed in the order they appear in the expression, the information you want to extract will be in group 1 (as this is the first capturing group of the regex).

You can also check [here](https://regex101.com/r/5p72uY/1) an example of this regex working.

---

The [other answer](/posts/289889/289892#answer-289892) used `A-z` to match ASCII letters, but this interval also matches the characters `[`, `\`, `]`, `^`, `_` and ``` ` ```. Using `a-zA-Z`, you guarantee that only ASCII letters are matched.


---

That said, regex isn't exactly the best tool for URL parsing. Of course for specific, controlled cases (such as "_I know it's always an Amazon product URL, which has a well defined structure_"), it works fine. But for more general cases, there are speciliazed libraries/modules/functions that can parse, validate, handle tons of corner cases, and so on (things that can be harder to solve with a regular expression).