diff --git a/website/usage/_linguistic-features/_rule-based-matching.jade b/website/usage/_linguistic-features/_rule-based-matching.jade
index bb75e7ad2..39062493a 100644
--- a/website/usage/_linguistic-features/_rule-based-matching.jade
+++ b/website/usage/_linguistic-features/_rule-based-matching.jade
@@ -339,7 +339,8 @@ p
     |  flag.
 
 +code.
-    IS_DEFINITELY = nlp.vocab.add_flag(re.compile(r'deff?in[ia]tely').match)
+    definitely_flag = lambda text: bool(re.compile(r'deff?in[ia]tely').match(text))
+    IS_DEFINITELY = nlp.vocab.add_flag(definitely_flag)
 
     matcher = Matcher(nlp.vocab)
     matcher.add('DEFINITELY', None, [{IS_DEFINITELY: True}])