From 82ed6b55467c94ead3b69cc12e5918c6dcc937f7 Mon Sep 17 00:00:00 2001
From: Brandon Dyck <brandon@dyck.us>
Date: Wed, 2 Apr 2025 14:24:21 -0600
Subject: [PATCH] Regexp: don't return error when match fails at EOF

---
 bytes/regexp.go      | 3 +++
 bytes/regexp_test.go | 7 +++++++
 2 files changed, 10 insertions(+)

diff --git a/bytes/regexp.go b/bytes/regexp.go
index 44b2d0e..539406e 100644
--- a/bytes/regexp.go
+++ b/bytes/regexp.go
@@ -69,6 +69,9 @@ func Regexp(pattern string) gigaparsec.Parser[byte, string] {
 			return gigaparsec.Result[byte, string]{}, fmt.Errorf("Regexp: reader error: %w", err)
 		}
 		if idx == nil {
+			if err == io.EOF {
+				return gigaparsec.Fail[byte, string](false, gigaparsec.MessageEnd(input.Pos())), nil
+			}
 			got := make([]byte, r.Count())
 			_, _, err = input.Read(got)
 			if err != nil {
diff --git a/bytes/regexp_test.go b/bytes/regexp_test.go
index 6e409f6..b91f61f 100644
--- a/bytes/regexp_test.go
+++ b/bytes/regexp_test.go
@@ -69,6 +69,13 @@ func TestRegexp(t *testing.T) {
 		must.True(t, succeeded)
 		must.EqOp(t, "", value)
 	})
+	t.Run("fails without an error at EOF", func(t *testing.T) {
+		p := pbytes.Regexp("a")
+		result, err := p(gigaparsec.MakeState(strings.NewReader("")))
+		succeeded, _, _ := result.Status()
+		must.NoError(t, err)
+		must.False(t, succeeded)
+	})
 }
 
 func TestRuneReader(t *testing.T) {