regexp/syntax: exclude full range from String negation case

If the char class is 0x0-0x10ffff we mistakenly would String that to `[^]`,
which is not a valid regex.

Fixes #31807

Change-Id: I9ceeaddc28b67b8e1de12b6703bcb124cc784556
Reviewed-on: https://go-review.googlesource.com/c/go/+/175679Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/regexp/syntax/parse_test.go

@@ -185,6 +185,7 @@ var parseTests = []parseTest{
 	{`(?-s).`, `dnl{}`},
 	{`(?:(?:^).)`, `cat{bol{}dot{}}`},
 	{`(?-s)(?:(?:^).)`, `cat{bol{}dnl{}}`},
+	{`[\s\S]a`, `cat{cc{0x0-0x10ffff}lit{a}}`},
 
 	// RE2 prefix_tests
 	{`abc|abd`, `cat{str{ab}cc{0x63-0x64}}`},

src/regexp/syntax/regexp.go

@@ -139,7 +139,7 @@ func writeRegexp(b *strings.Builder, re *Regexp) {
 		b.WriteRune('[')
 		if len(re.Rune) == 0 {
 			b.WriteString(`^\x00-\x{10FFFF}`)
-		} else if re.Rune[0] == 0 && re.Rune[len(re.Rune)-1] == unicode.MaxRune {
+		} else if re.Rune[0] == 0 && re.Rune[len(re.Rune)-1] == unicode.MaxRune && len(re.Rune) > 2 {
 			// Contains 0 and MaxRune. Probably a negated class.
 			// Print the gaps.
 			b.WriteRune('^')