diff --git a/docs/basics.md b/docs/basics.md
index 79042790e..787604451 100644
--- a/docs/basics.md
+++ b/docs/basics.md
@@ -85,13 +85,17 @@ Frontends can be defined using the following rules:
 - `PathStrip`: Same as `Path` but strip the given prefix from the request URL's Path.
 - `PathPrefix`: PathPrefix adds a matcher for the URL path prefixes. This matches if the given template is a prefix of the full URL path.
 - `PathPrefixStrip`: Same as `PathPrefix` but strip the given prefix from the request URL's Path.
-- `AddPrefix` : Add prefix from the request URL's Path.
+- `AddPrefix`: Add prefix to the request URL's Path.
 
-You can use multlple values for a rule by separating them with `,`.
+You can use multiple values for a rule by separating them with `,`.
 You can use multiple rules by separating them by `;`.
 
 You can optionally enable `passHostHeader` to forward client `Host` header to the backend.
 
+In order to use path regular expressions, you must declare an arbitrarily named variable followed by the colon-separated regular expression, all enclosed in curly braces. Any pattern supported by [Go's regexp package](https://golang.org/pkg/regexp/) may be used. Example: `/posts/{id:[0-9]+}`.
+
+(Note that the variable has no special meaning; however, it is required by gorilla/mux which embeds the regular expression and defines the syntax.)
+
 Here is an example of frontends definition:
 
 ```toml