chore(placeholders): refactor matching capture and add regression test

framework/substitution.go

@@ -24,7 +24,7 @@ import (
 
 var (
 	// placeholderRegEx will search for ${...} with any sequence of characters between them.
-	placeholderRegEx = regexp.MustCompile(`\$(\$|{([^}]*)})`)
+	placeholderRegEx = regexp.MustCompile(`\$((?:\$?{([^}]*)})|\$)`)
 )
 
 func SplitRefParts(ref string) []string {
@@ -52,7 +52,7 @@ func SubstituteString(src string, inner func(string) (string, error)) (string, e
 		}
 
 		// support escaped dollars
-		if matches[1] == "$" {
+		if strings.HasPrefix(matches[1], "$") {
 			return matches[1]
 		}
 

framework/substitution_test.go

@@ -108,6 +108,9 @@ func TestSubstituteString(t *testing.T) {
 		{Input: "$abc", Expected: "$abc"},
 		{Input: "abc $$ abc", Expected: "abc $ abc"},
 		{Input: "$${abc}", Expected: "${abc}"},
+		{Input: "$$${abc}", ExpectedError: "invalid ref 'abc': unknown reference root, use $$ to escape the substitution"},
+		{Input: "$$$${abc}", Expected: "$${abc}"},
+		{Input: "$$$$${abc}", ExpectedError: "invalid ref 'abc': unknown reference root, use $$ to escape the substitution"},
 		{Input: "$${abc .4t3298y *(^&(*}", Expected: "${abc .4t3298y *(^&(*}"},
 		{Input: "my name is ${metadata.name}", Expected: "my name is test-name"},
 		{Input: "my name is ${metadata.thing\\.two}", ExpectedError: "invalid ref 'metadata.thing\\.two': key 'thing.two' not found"},