Add Cookbook S01-S22 (draft)

docs-src/pages/en/cookbook/s03-path-params.md

@@ -0,0 +1,53 @@
+---
+title: "S03. Use Path Parameters"
+order: 22
+status: "draft"
+---
+
+For dynamic URLs like `/users/:id` — the staple of REST APIs — just put `:name` in the path pattern. The matched values end up in `req.path_params`.
+
+## Basic usage
+
+```cpp
+svr.Get("/users/:id", [](const httplib::Request &req, httplib::Response &res) {
+  auto id = req.path_params.at("id");
+  res.set_content("user id: " + id, "text/plain");
+});
+```
+
+A request to `/users/42` fills `req.path_params["id"]` with `"42"`. `path_params` is a `std::unordered_map<std::string, std::string>`, so use `at()` to read it.
+
+## Multiple parameters
+
+You can have as many as you need.
+
+```cpp
+svr.Get("/orgs/:org/repos/:repo", [](const httplib::Request &req, httplib::Response &res) {
+  auto org = req.path_params.at("org");
+  auto repo = req.path_params.at("repo");
+  res.set_content(org + "/" + repo, "text/plain");
+});
+```
+
+This matches paths like `/orgs/anthropic/repos/cpp-httplib`.
+
+## Regex patterns
+
+For more flexible matching, use a `std::regex`-based pattern.
+
+```cpp
+svr.Get(R"(/users/(\d+))", [](const httplib::Request &req, httplib::Response &res) {
+  auto id = req.matches[1];
+  res.set_content("user id: " + std::string(id), "text/plain");
+});
+```
+
+Parentheses in the pattern become captures in `req.matches`. `req.matches[0]` is the full match; `req.matches[1]` onward are the captures.
+
+## Which to use
+
+- For plain IDs or slugs, `:name` is enough — readable, and the shape is obvious
+- Use regex when you want to constrain the URL to, say, numbers only or a UUID format
+- Mixing both can get confusing — stick with one style per project
+
+> **Note:** Path parameters come in as strings. If you need an integer, convert with `std::stoi()` and don't forget to handle conversion errors.