regexp/syntax: fix handling of \Q...\E

It's not a group: must handle the inside as a sequence of literal chars,
not a single literal string.

That is, \Qab\E+ is the same as ab+, not (ab)+.

Fixes #11187.

Change-Id: I5406d05ccf7efff3a7f15395bdb0cfb2bd23a8ed
Reviewed-on: https://go-review.googlesource.com/17233Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/regexp/syntax/parse.go

@@ -830,7 +830,14 @@ func Parse(s string, flags Flags) (*Regexp, error) {
 						lit = t[2:i]
 						t = t[i+2:]
 					}
-					p.push(literalRegexp(lit, p.flags))
+					for lit != "" {
+						c, rest, err := nextRune(lit)
+						if err != nil {
+							return nil, err
+						}
+						p.literal(c)
+						lit = rest
+					}
 					break BigSwitch
 				case 'z':
 					p.op(OpEndText)

src/regexp/syntax/parse_test.go

@@ -144,6 +144,7 @@ var parseTests = []parseTest{
 	// Test Perl quoted literals
 	{`\Q+|*?{[\E`, `str{+|*?{[}`},
 	{`\Q+\E+`, `plus{lit{+}}`},
+	{`\Qab\E+`, `cat{lit{a}plus{lit{b}}}`},
 	{`\Q\\E`, `lit{\}`},
 	{`\Q\\\E`, `str{\\}`},
 
@@ -479,6 +480,7 @@ var invalidRegexps = []string{
 	`a{100000}`,
 	`a{100000,}`,
 	"((((((((((x{2}){2}){2}){2}){2}){2}){2}){2}){2}){2})",
+	`\Q\E*`,
 }
 
 var onlyPerl = []string{