From 08fd0c6fb44e4ee001bf268f7d7d26533bf10369 Mon Sep 17 00:00:00 2001
From: Arpit Jain <arpitjain099@gmail.com>
Date: Fri, 26 Jun 2026 11:58:11 +0900
Subject: [PATCH] Fix envsubst parse of empty replace pattern

The replace parser rejected an empty pattern such as ${VAR/#/prefix-}
or ${VAR/%/-suffix} with ErrParseFuncSubstitution because the arg[1]
scan hit the delimiter immediately and produced an illegal token.

Peek for the delimiter before scanning the pattern and emit an empty
pattern node, mirroring the existing empty-replacement handling. This
keeps the two-argument shape the eval side already supports and matches
bash, where an empty prefix or suffix pattern prepends or appends the
replacement.

Signed-off-by: Arpit Jain <arpitjain099@gmail.com>
Assisted-by: Claude Code
---
 envsubst/eval_test.go        | 12 ++++++++++++
 envsubst/parse/parse.go      | 15 +++++++++++----
 envsubst/parse/parse_test.go | 22 ++++++++++++++++++++++
 3 files changed, 45 insertions(+), 4 deletions(-)

diff --git a/envsubst/eval_test.go b/envsubst/eval_test.go
index 0702c985e..0236f97ba 100644
--- a/envsubst/eval_test.go
+++ b/envsubst/eval_test.go
@@ -126,6 +126,18 @@ func TestExpand(t *testing.T) {
 			input:  "${stringZ/#abc/XYZ}",
 			output: "XYZABC123ABCabc",
 		},
+		// replace prefix with empty pattern (prepend)
+		{
+			params: map[string]string{"stringZ": "somevalue"},
+			input:  "${stringZ/#/prefix-}",
+			output: "prefix-somevalue",
+		},
+		// replace suffix with empty pattern (append)
+		{
+			params: map[string]string{"stringZ": "somevalue"},
+			input:  "${stringZ/%/-suffix}",
+			output: "somevalue-suffix",
+		},
 		// replace all
 		{
 			params: map[string]string{"stringZ": "abcABC123ABCabc"},
diff --git a/envsubst/parse/parse.go b/envsubst/parse/parse.go
index 39c72009f..1570cc73f 100644
--- a/envsubst/parse/parse.go
+++ b/envsubst/parse/parse.go
@@ -283,11 +283,18 @@ func (t *Tree) parseReplaceFunc(name string) (Node, error) {
 
 	// scan arg[1]
 	{
-		param, err := t.parseParam(acceptNotSlash, scanIdent|scanEscape)
-		if err != nil {
-			return nil, err
+		// An empty pattern (e.g. ${param/#/string}) means the next rune is the
+		// delimiter, so there is nothing to scan. Record an empty pattern node
+		// to keep the two-argument shape the eval side expects.
+		if t.scanner.peek() == '/' {
+			node.Args = append(node.Args, newTextNode(""))
+		} else {
+			param, err := t.parseParam(acceptNotSlash, scanIdent|scanEscape)
+			if err != nil {
+				return nil, err
+			}
+			node.Args = append(node.Args, param)
 		}
-		node.Args = append(node.Args, param)
 	}
 
 	// expect delimiter
diff --git a/envsubst/parse/parse_test.go b/envsubst/parse/parse_test.go
index d51e6dbaf..171081d56 100644
--- a/envsubst/parse/parse_test.go
+++ b/envsubst/parse/parse_test.go
@@ -219,6 +219,28 @@ var tests = []struct {
 			},
 		},
 	},
+	{
+		Text: "${string/#/replacement}",
+		Node: &FuncNode{
+			Param: "string",
+			Name:  "/#",
+			Args: []Node{
+				&TextNode{Value: ""},
+				&TextNode{Value: "replacement"},
+			},
+		},
+	},
+	{
+		Text: "${string/%/replacement}",
+		Node: &FuncNode{
+			Param: "string",
+			Name:  "/%",
+			Args: []Node{
+				&TextNode{Value: ""},
+				&TextNode{Value: "replacement"},
+			},
+		},
+	},
 
 	//
 	// default value functions