fix: wrong message in cases with no index

#877

internal/check/substitution.go

@@ -189,6 +189,14 @@ func subMsg(s Substitution, index int, observed string) (string, error) {
 		expected = core.CapFirst(expected)
 	}
 
+	// TODO: Why do we need to check for this?
+	//
+	// This feels like a bug in `regexp2`.
+	hasIndex := regexp2.MustCompileStd(`\$\d+`)
+	if !hasIndex.MatchStringStd(expected) {
+		return expected, nil
+	}
+
 	msg := s.msgMap[index]
 	if s.Ignorecase {
 		msg = `(?i)` + msg

testdata/features/checks.feature

@@ -100,6 +100,7 @@ Feature: Checks
             test2.md:3:1:demo.CapSub:Use 'Change to the `/etc` directory' instead of 'Change into the `/etc` directory'.
             test2.md:7:1:demo.CapSub:Use 'Change to the `/home/user` directory' instead of 'Change into the `/home/user` directory'.
             test2.md:9:1:demo.CapSub:Use 'Change to the `/etc/X11` directory' instead of 'Change into the `/etc/X11` directory'.
+            test2.md:11:6:Bugs.WrongExp:Use 'versus' instead of 'vs'.
             """
 
     Scenario: Sequence

testdata/fixtures/checks/Substitution/.vale.ini

@@ -12,3 +12,4 @@ Bugs.SameCase = YES
 
 [test2.md]
 demo.CapSub = YES
+Bugs.WrongExp = YES

testdata/fixtures/checks/Substitution/test2.md

@@ -6,4 +6,6 @@ Within the home directory.
 
 Change into the `/home/user` directory.
 
-Change into the `/etc/X11` directory.
+Change into the `/etc/X11` directory.
+
+Blue vs red.

testdata/styles/Bugs/WrongExp.yml

@@ -0,0 +1,14 @@
+---
+extends: substitution
+message: "Use '%s' instead of '%s'."
+ignorecase: false
+level: error
+nonword: true
+action:
+  name: replace
+swap:
+  '\b(?:[Ee]\.[Gg]\.)': for example
+  '\b(?:[Ii]\.[Ee]\.)': that is
+  '\b(?:[Ee][Tt][Cc]\.)': and so on
+  '\b(?:[Vv][Ss]\.)': versus
+  '\b(?:[Vv][Ss]\s)(?!Code)': versus