regexp: improve Regexp.ReplaceAll documentation and tests related to …

…Expand part

Fixes #40329

src/regexp/example_test.go

@@ -228,11 +228,18 @@ func ExampleRegexp_ReplaceAll() {
 	fmt.Printf("%s\n", re.ReplaceAll([]byte("-ab-axxb-"), []byte("$1")))
 	fmt.Printf("%s\n", re.ReplaceAll([]byte("-ab-axxb-"), []byte("$1W")))
 	fmt.Printf("%s\n", re.ReplaceAll([]byte("-ab-axxb-"), []byte("${1}W")))
+
+	re2 := regexp.MustCompile(`a(?P<1W>x*)b`)
+	fmt.Printf("%s\n", re2.ReplaceAll([]byte("-ab-axxb-"), []byte("$1W")))
+	fmt.Printf("%s\n", re2.ReplaceAll([]byte("-ab-axxb-"), []byte("${1}W")))
+
 	// Output:
 	// -T-T-
 	// --xx-
 	// ---
 	// -W-xxW-
+	// --xx-
+	// -W-xxW-
 }
 
 func ExampleRegexp_ReplaceAllLiteralString() {
@@ -252,11 +259,18 @@ func ExampleRegexp_ReplaceAllString() {
 	fmt.Println(re.ReplaceAllString("-ab-axxb-", "$1"))
 	fmt.Println(re.ReplaceAllString("-ab-axxb-", "$1W"))
 	fmt.Println(re.ReplaceAllString("-ab-axxb-", "${1}W"))
+
+	re2 := regexp.MustCompile(`a(?P<1W>x*)b`)
+	fmt.Printf("%s\n", re2.ReplaceAllString("-ab-axxb-", "$1W"))
+	fmt.Println(re.ReplaceAllString("-ab-axxb-", "${1}W"))
+
 	// Output:
 	// -T-T-
 	// --xx-
 	// ---
 	// -W-xxW-
+	// --xx-
+	// -W-xxW-
 }
 
 func ExampleRegexp_ReplaceAllStringFunc() {

src/regexp/regexp.go

@@ -573,8 +573,8 @@ func Match(pattern string, b []byte) (matched bool, err error) {
 }
 
 // ReplaceAllString returns a copy of src, replacing matches of the Regexp
-// with the replacement string repl. Inside repl, $ signs are interpreted as
-// in Expand, so for instance $1 represents the text of the first submatch.
+// with the replacement string repl.
+// Inside repl, $ signs are interpreted as in Expand.
 func (re *Regexp) ReplaceAllString(src, repl string) string {
 	n := 2
 	if strings.Contains(repl, "$") {
@@ -672,8 +672,8 @@ func (re *Regexp) replaceAll(bsrc []byte, src string, nmatch int, repl func(dst
 }
 
 // ReplaceAll returns a copy of src, replacing matches of the Regexp
-// with the replacement text repl. Inside repl, $ signs are interpreted as
-// in Expand, so for instance $1 represents the text of the first submatch.
+// with the replacement text repl.
+// Inside repl, $ signs are interpreted as in Expand.
 func (re *Regexp) ReplaceAll(src, repl []byte) []byte {
 	n := 2
 	if bytes.IndexByte(repl, '$') >= 0 {