From 4e157f7a0e200c4ce304ff32844a53d65c210993 Mon Sep 17 00:00:00 2001
From: Brandon Dyck <brandon@dyck.us>
Date: Wed, 2 Apr 2025 14:06:12 -0600
Subject: [PATCH] Regexp: succeed on empty match at end of input

---
 bytes/regexp.go      | 2 +-
 bytes/regexp_test.go | 8 ++++++++
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/bytes/regexp.go b/bytes/regexp.go
index 66f6b65..44b2d0e 100644
--- a/bytes/regexp.go
+++ b/bytes/regexp.go
@@ -80,7 +80,7 @@ func Regexp(pattern string) gigaparsec.Parser[byte, string] {
 		// when searching a RuneReader.
 		dst := make([]byte, idx[1]-idx[0])
 		n, _, err := input.Read(dst)
-		if err != nil {
+		if err != nil && (!errors.Is(err, io.EOF) || n < uint64(len(dst))) {
 			return gigaparsec.Result[byte, string]{}, fmt.Errorf("Regexp: unexpected error: %w", err)
 		}
 		next := input.At(input.Pos() + n)
diff --git a/bytes/regexp_test.go b/bytes/regexp_test.go
index b8742fc..6e409f6 100644
--- a/bytes/regexp_test.go
+++ b/bytes/regexp_test.go
@@ -61,6 +61,14 @@ func TestRegexp(t *testing.T) {
 		must.NoError(t, err)
 		test.StrContains(t, result.Message().Got(), "hella")
 	})
+	t.Run("succeeds on empty matches", func(t *testing.T) {
+		p := pbytes.Regexp(".*")
+		result, err := p(gigaparsec.MakeState(strings.NewReader("")))
+		succeeded, value, _ := result.Status()
+		must.NoError(t, err)
+		must.True(t, succeeded)
+		must.EqOp(t, "", value)
+	})
 }
 
 func TestRuneReader(t *testing.T) {