regexp: add Regexp.TextMarshaler/TextUnmarshaler

Fixes #46159

Change-Id: I51dc4e9e8915ab5a73f053690fb2395edbeb1151
Reviewed-on: https://go-review.googlesource.com/c/go/+/479401
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>

api/next/46159.txt

@@ -0,0 +1,2 @@
+pkg regexp, method (*Regexp) MarshalText() ([]uint8, error) #46159
+pkg regexp, method (*Regexp) UnmarshalText([]uint8) error #46159

src/regexp/all_test.go

@@ -947,3 +947,29 @@ func TestMinInputLen(t *testing.T) {
 		}
 	}
 }
+
+func TestUnmarshalText(t *testing.T) {
+	unmarshaled := new(Regexp)
+	for i := range goodRe {
+		re := compileTest(t, goodRe[i], "")
+		marshaled, err := re.MarshalText()
+		if err != nil {
+			t.Errorf("regexp %#q failed to marshal: %s", re, err)
+			continue
+		}
+		if err := unmarshaled.UnmarshalText(marshaled); err != nil {
+			t.Errorf("regexp %#q failed to unmarshal: %s", re, err)
+			continue
+		}
+		if unmarshaled.String() != goodRe[i] {
+			t.Errorf("UnmarshalText returned unexpected value: %s", unmarshaled.String())
+		}
+	}
+	t.Run("invalid pattern", func(t *testing.T) {
+		re := new(Regexp)
+		err := re.UnmarshalText([]byte(`\`))
+		if err == nil {
+			t.Error("unexpected success")
+		}
+	})
+}

src/regexp/regexp.go

@@ -1283,3 +1283,24 @@ func (re *Regexp) Split(s string, n int) []string {
 
 	return strings
 }
+
+// MarshalText implements the encoding.TextMarshaler interface. The output
+// matches that of calling the [Regexp.String] method.
+//
+// Note that the output is lossy in some cases: This method does not indicate
+// POSIX regular expressions (i.e. those compiled by calling CompilePOSIX), or
+// those for which the [Regexp.Longest] method has been called.
+func (re *Regexp) MarshalText() ([]byte, error) {
+	return []byte(re.String()), nil
+}
+
+// MarshalText implements the encoding.TextUnmarshaler interface by calling
+// Compile on the encoded value.
+func (re *Regexp) UnmarshalText(text []byte) error {
+	newRE, err := Compile(string(text))
+	if err != nil {
+		return err
+	}
+	*re = *newRE
+	return nil
+}