From 54419202fe641775651c11d97f1a1397e84dbc0c Mon Sep 17 00:00:00 2001
From: Ben Moskovitz <ben.m@buildkite.com>
Date: Wed, 5 Jun 2024 11:48:10 +1000
Subject: [PATCH] Fix literal dollar sign escapes

---
 interpolate_test.go |  1 +
 parser.go           | 26 +++++++++++++++++---------
 parser_test.go      |  8 ++++++++
 3 files changed, 26 insertions(+), 9 deletions(-)

diff --git a/interpolate_test.go b/interpolate_test.go
index 9c996d3..d33deda 100644
--- a/interpolate_test.go
+++ b/interpolate_test.go
@@ -290,6 +290,7 @@ func TestEscapingVariables(t *testing.T) {
 		{`Do this \${SUCH_ESCAPE}`, `Do this ${SUCH_ESCAPE}`},
 		{`Do this $${SUCH_ESCAPE:-$OTHERWISE}`, `Do this ${SUCH_ESCAPE:-$OTHERWISE}`},
 		{`Do this \${SUCH_ESCAPE:-$OTHERWISE}`, `Do this ${SUCH_ESCAPE:-$OTHERWISE}`},
+		{`echo "my favourite mountain is cotopaxi" | grep 'xi$$'`, `echo "my favourite mountain is cotopaxi" | grep 'xi$'`},
 	} {
 		result, err := interpolate.Interpolate(nil, tc.Str)
 		if err != nil {
diff --git a/parser.go b/parser.go
index 52954c2..c69b8bf 100644
--- a/parser.go
+++ b/parser.go
@@ -123,20 +123,28 @@ func (p *Parser) parseExpression(stop ...rune) (Expression, error) {
 }
 
 func (p *Parser) parseEscapedExpansion() (Expansion, error) {
-	// if it's an escaped brace expansion, (eg $${MY_COOL_VAR:-5}) consume text until the close brace
-	if c := p.peekRune(); c == '{' {
+	next := p.peekRune()
+	switch {
+	case next == '{':
+		// if it's an escaped brace expansion, (eg $${MY_COOL_VAR:-5}) consume text until the close brace
 		id := p.scanUntil(func(r rune) bool { return r == '}' })
 		id = id + string(p.nextRune()) // we know that the next rune is a close brace, chuck it on the end
 		return EscapedExpansion{Identifier: id}, nil
-	}
 
-	// otherwise, it's an escaped identifier (eg $$MY_COOL_VAR)
-	id, err := p.scanIdentifier()
-	if err != nil {
-		return nil, err
-	}
+	case unicode.IsLetter(next):
+		// it's an escaped identifier (eg $$MY_COOL_VAR)
+		id, err := p.scanIdentifier()
+		if err != nil {
+			return nil, err
+		}
 
-	return EscapedExpansion{Identifier: id}, nil
+		return EscapedExpansion{Identifier: id}, nil
+
+	default:
+		// there's no identifier or brace afterward, so it's probably a literal escaped dollar sign, so return an empty identifier
+		// that will be expanded to a single dollar sign
+		return EscapedExpansion{Identifier: ""}, nil
+	}
 }
 
 func (p *Parser) parseExpansion() (Expansion, error) {
diff --git a/parser_test.go b/parser_test.go
index 747ccdf..a223f93 100644
--- a/parser_test.go
+++ b/parser_test.go
@@ -171,6 +171,14 @@ func TestParser(t *testing.T) {
 			String:   "$$MOUNTAIN",
 			Expected: []interpolate.ExpressionItem{{Expansion: interpolate.EscapedExpansion{Identifier: "MOUNTAIN"}}},
 		},
+		{
+			String: "this is a regex! /^start.*end$$/", // the dollar sign at the end of the regex has to be escaped to be treated as a literal dollar sign by this library
+			Expected: []interpolate.ExpressionItem{
+				{Text: "this is a regex! /^start.*end"},
+				{Expansion: interpolate.EscapedExpansion{Identifier: ""}},
+				{Text: "/"},
+			},
+		},
 	}
 
 	for _, tc := range testCases {