regexp: allow escaping of any punctuation

More in line with other regexp packages
and egrep; accommodates overzealous escapers.

R=r
CC=golang-dev
https://golang.org/cl/1008041

src/pkg/regexp/all_test.go

@@ -28,6 +28,7 @@ var good_re = []string{
 	`[abc]`,
 	`[^1234]`,
 	`[^\n]`,
+	`\!\\`,
 }
 
 type stringError struct {
@@ -100,6 +101,14 @@ var matches = []tester{
 	// fixed bugs
 	tester{`ab$`, "cab", vec{1, 3}},
 	tester{`axxb$`, "axxcb", vec{}},
+
+	// can backslash-escape any punctuation
+	tester{`\!\"\#\$\%\&\'\(\)\*\+\,\-\.\/\:\;\<\=\>\?\@\[\\\]\^\_\{\|\}\~`,
+		`!"#$%&'()*+,-./:;<=>?@[\]^_{|}~`, vec{0, 31}},
+	tester{`[\!\"\#\$\%\&\'\(\)\*\+\,\-\.\/\:\;\<\=\>\?\@\[\\\]\^\_\{\|\}\~]+`,
+		`!"#$%&'()*+,-./:;<=>?@[\]^_{|}~`, vec{0, 31}},
+	tester{"\\`", "`", vec{0, 1}},
+	tester{"[\\`]+", "`", vec{0, 1}},
 }
 
 func compileTest(t *testing.T, expr string, error os.Error) *Regexp {

src/pkg/regexp/regexp.go

@@ -298,8 +298,8 @@ func special(c int) bool {
 	return false
 }
 
-func specialcclass(c int) bool {
+func ispunct(c int) bool {
-	for _, r := range `\-[]` {
+	for _, r := range "!\"#$%&'()*+,-./:;<=>?@[\\]^_`{|}~" {
 		if c == r {
 			return true
 		}
@@ -344,7 +344,7 @@ func (p *parser) charClass() instr {
 				p.error(ErrExtraneousBackslash)
 			case c == 'n':
 				c = '\n'
-			case specialcclass(c):
+			case ispunct(c):
 				// c is as delivered
 			default:
 				p.error(ErrBadBackslash)
@@ -439,7 +439,7 @@ func (p *parser) term() (start, end instr) {
 			p.error(ErrExtraneousBackslash)
 		case c == 'n':
 			c = '\n'
-		case special(c):
+		case ispunct(c):
 			// c is as delivered
 		default:
 			p.error(ErrBadBackslash)