diff --git a/50.md b/50.md
index 84ccab1e..29267e2a 100644
--- a/50.md
+++ b/50.md
@@ -53,4 +53,4 @@ Relay MAY support these extensions:
 - `language:<two letter ISO 639-1 language code>` - include only events of a specified language.
 - `sentiment:<negative/neutral/positive>` - include only events of a specific sentiment.
 - `nsfw:<true/false>` - include or exclude nsfw events. (default: true)
-- `regex:<regular-expression>` - A regex pattern to be searched.
+- `regex:<regular-expression>` - A regex pattern to be searched which must be compatible with the PCRE flavor.