docs: anchor regex origin examples to avoid lookalike matches (#408)

README.md

@@ -223,8 +223,8 @@ app.listen(80, function () {
   - `String` - set `origin` to a specific origin. For example, if you set it to
     - `"http://example.com"` only requests from "http://example.com" will be allowed.
     - `"*"` for all domains to be allowed. 
-  - `RegExp` - set `origin` to a regular expression pattern which will be used to test the request origin. If it's a match, the request origin will be reflected. For example the pattern `/example\.com$/` will reflect any request that is coming from an origin ending with "example.com".
-  - `Array` - set `origin` to an array of valid origins. Each origin can be a `String` or a `RegExp`. For example `["http://example1.com", /\.example2\.com$/]` will accept any request from "http://example1.com" or from a subdomain of "example2.com".
+  - `RegExp` - set `origin` to a regular expression pattern which will be used to test the request origin. If it's a match, the request origin will be reflected. The origin header includes the scheme, so the pattern must anchor at the start of the origin to avoid unintended matches — for example `/^https?:\/\/(.+\.)?example\.com$/` will reflect `https://example.com` and any of its subdomains, but will not match attacker-controlled origins like `https://evil-example.com`. A trailing-only anchor such as `/example\.com$/` is **not safe** because it also matches domains that merely *end* with `example.com`.
+  - `Array` - set `origin` to an array of valid origins. Each origin can be a `String` or a `RegExp`. For example `["http://example1.com", /^https?:\/\/(.+\.)?example2\.com$/]` will accept any request from "http://example1.com" or from a subdomain of "example2.com". As with the single-`RegExp` form, anchor at the start of the origin so patterns don't accept lookalike domains.
   - `Function` - set `origin` to a function implementing some custom logic. The function takes the request origin as the first parameter and a callback (called as `callback(err, origin)`, where `origin` is a non-function value of the `origin` option) as the second.
 * `methods`: Configures the **Access-Control-Allow-Methods** CORS header. Expects a comma-delimited string (ex: 'GET,PUT,POST') or an array (ex: `['GET', 'PUT', 'POST']`).
 * `allowedHeaders`: Configures the **Access-Control-Allow-Headers** CORS header. Expects a comma-delimited string (ex: 'Content-Type,Authorization') or an array (ex: `['Content-Type', 'Authorization']`). If not specified, defaults to reflecting the headers specified in the request's **Access-Control-Request-Headers** header.