diff --git a/regress/format-strings.sh b/regress/format-strings.sh
index 8d192ed2..bae66ab0 100644
--- a/regress/format-strings.sh
+++ b/regress/format-strings.sh
@@ -67,6 +67,7 @@ $TMUX set @true 1 || exit 1
 $TMUX set @false 0 || exit 1
 $TMUX set @warm Summer || exit 1
 $TMUX set @cold Winter || exit 1
+$TMUX set @v 'foo:bar' || exit 1
 
 # Plain string without substitutions et al
 test_format "abc xyz" "abc xyz"
@@ -74,6 +75,8 @@ test_format "abc xyz" "abc xyz"
 # Test basic escapes for "#", "{", "#{" "}", "#}", ","
 test_format "##" "#"
 test_format "#," ","
+test_format "#{s/#:/_/:@v}" "foo_bar"
+test_format "#{s/o/#:/:@v}" "f:::bar"
 test_format "{" "{"
 test_format "##{" "#{"
 test_format "#}" "}"