website/docs: improve docs for expressions

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

website/docs/expressions/_functions.md

@@ -0,0 +1,52 @@
+### `regex_match(value: Any, regex: str) -> bool`
+
+Check if `value` matches Regular Expression `regex`.
+
+Example:
+
+```python
+return regex_match(request.user.username, '.*admin.*')
+```
+
+### `regex_replace(value: Any, regex: str, repl: str) -> str`
+
+Replace anything matching `regex` within `value` with `repl` and return it.
+
+Example:
+
+```python
+user_email_local = regex_replace(request.user.email, '(.+)@.+', '')
+```
+
+### `ak_is_group_member(user: User, **group_filters) -> bool`
+
+Check if `user` is member of a group matching `**group_filters`.
+
+Example:
+
+```python
+return ak_is_group_member(request.user, name="test_group")
+```
+
+### `ak_user_by(**filters) -> Optional[User]`
+
+Fetch a user matching `**filters`.
+
+Returns "None" if no user was found, otherwise [User](/docs/expressions/reference/user-object)
+
+Example:
+
+```python
+other_user = ak_user_by(username="other_user")
+```
+
+## Comparing IP Addresses
+
+To compare IP Addresses or check if an IP Address is within a given subnet, you can use the functions `ip_address('192.0.2.1')` and `ip_network('192.0.2.0/24')`. With these objects you can do [arithmetic operations](https://docs.python.org/3/library/ipaddress.html#operators).
+
+You can also check if an IP Address is within a subnet by writing the following:
+
+```python
+ip_address('192.0.2.1') in ip_network('192.0.2.0/24')
+# evaluates to True
+```