diff --git a/docs/configuration/entrypoints.md b/docs/configuration/entrypoints.md
index 28e5cc049..99d4f0028 100644
--- a/docs/configuration/entrypoints.md
+++ b/docs/configuration/entrypoints.md
@@ -150,7 +150,11 @@ To redirect an entrypoint rewriting the URL.
 ```
 
 !!! note
-    Please note that `regex` and `replacement` do not have to be set in the `redirect` structure if an entrypoint is defined for the redirection (they will not be used in this case).
+    Please note that `regex` and `replacement` do not have to be set in the `redirect` structure if an `entrypoint` is defined for the redirection (they will not be used in this case).
+
+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.
+
+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).
 
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