Regexp: succeed on empty match at end of input

TODO.txt

@@ -4,4 +4,6 @@ Rename Seq2 to Seq
 Document Seq
 Should MakeState be private now that there's Run?
 What's Megaparsec got that we ain't got?
-Add and benchmark naïve Seq
+Add and benchmark naïve Seq
+chainl
+whitespace handling

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)

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) {