Update the middleware documentation

docs/content/middlewares/replacepathregex.md

@@ -1,4 +1,69 @@
-# TODO -- ReplacePathRegex
+# ReplacePathRegex
 
 Updating the Path Before Forwarding the Request (Using a Regex)
-{: .subtitle }
+{: .subtitle }
+
+`TODO: add schema`
+
+The ReplaceRegex replace a path from an url to another with regex matching and replacement.
+
+## Configuration Examples
+
+```yaml tab="Docker"
+# Replace path with regex
+labels:
+- "traefik.http.middlewares.test-replacepathregex.replacepathregex.regex=^/foo/(.*)"
+- "traefik.http.middlewares.test-replacepathregex.replacepathregex.replacement=/bar/$1"
+```
+
+```yaml tab="Kubernetes"
+# Replace path with regex
+apiVersion: traefik.containo.us/v1alpha1
+kind: Middleware
+metadata:
+  name: test-replacepathregex
+spec:
+  replacePathRegex:
+    regex: ^/foo/(.*)
+    replacement: /bar/$1
+```
+
+```yaml tab="Rancher"
+# Replace path with regex
+labels:
+- "traefik.http.middlewares.test-replacepathregex.replacepathregex.regex=^/foo/(.*)"
+- "traefik.http.middlewares.test-replacepathregex.replacepathregex.replacement=/bar/$1"
+```
+
+```toml tab="File"
+# Redirect with domain replacement
+[http.middlewares]
+  [http.middlewares.test-replacepathregex.replacePathRegex]
+    regex = "^/foo/(.*)"
+    replacement = "/bar/$1"
+```
+
+## Configuration Options
+
+### General
+
+The ReplacePathRegex middleware will:
+
+- replace the matching path by the specified one.
+- store the original path in a `X-Replaced-Path` header.
+
+### `regex`
+
+The `Regex` option is the regular expression to match and capture the path from the request URL.
+
+!!! warning
+
+    Care should be taken when defining replacement expand variables: `$1x` is equivalent to `${1x}`, not `${1}x` (see [Regexp.Expand](https://golang.org/pkg/regexp/#Regexp.Expand)), so use `${1}` syntax.
+
+!!! tip
+
+    Regular expressions and replacements can be tested using online tools such as [Go Playground](https://play.golang.org/p/mWU9p-wk2ru) or the [Regex101](https://regex101.com/r/58sIgx/2).
+    
+### `replacement`
+
+The `replacement` option defines how to modify the path to have the new target path.