regexp: fix LiteralPrefix for certain onepass progs

The prefix computation for onepass was incorrectly returning
complete=true when it encountered a beginning-of-text empty width match
(^) in the middle of an expression.

Fix by returning complete only when the prefix is followed by $ and then
an accepting state.

Fixes #11175.

Change-Id: Ie9c4cf5f76c1d2c904a6fb2f016cedb265c19fde
Reviewed-on: https://go-review.googlesource.com/16200
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/regexp/all_test.go

@@ -359,6 +359,19 @@ var metaTests = []MetaTest{
 	{`!@#$%^&*()_+-=[{]}\|,<.>/?~`, `!@#\$%\^&\*\(\)_\+-=\[\{\]\}\\\|,<\.>/\?~`, `!@#`, false},
 }
 
+var literalPrefixTests = []MetaTest{
+	// See golang.org/issue/11175.
+	// output is unused.
+	{`^0^0$`, ``, `0`, false},
+	{`^0^`, ``, ``, false},
+	{`^0$`, ``, `0`, true},
+	{`$0^`, ``, ``, false},
+	{`$0$`, ``, ``, false},
+	{`^^0$$`, ``, ``, false},
+	{`^$^$`, ``, ``, false},
+	{`$$0^^`, ``, ``, false},
+}
+
 func TestQuoteMeta(t *testing.T) {
 	for _, tc := range metaTests {
 		// Verify that QuoteMeta returns the expected string.
@@ -390,7 +403,7 @@ func TestQuoteMeta(t *testing.T) {
 }
 
 func TestLiteralPrefix(t *testing.T) {
-	for _, tc := range metaTests {
+	for _, tc := range append(metaTests, literalPrefixTests...) {
 		// Literal method needs to scan the pattern.
 		re := MustCompile(tc.pattern)
 		str, complete := re.LiteralPrefix()

src/regexp/onepass.go

@@ -59,7 +59,12 @@ func onePassPrefix(p *syntax.Prog) (prefix string, complete bool, pc uint32) {
 		buf.WriteRune(i.Rune[0])
 		pc, i = i.Out, &p.Inst[i.Out]
 	}
-	return buf.String(), i.Op == syntax.InstEmptyWidth && (syntax.EmptyOp(i.Arg))&syntax.EmptyBeginText != 0, pc
+	if i.Op == syntax.InstEmptyWidth &&
+		syntax.EmptyOp(i.Arg)&syntax.EmptyEndText != 0 &&
+		p.Inst[i.Out].Op == syntax.InstMatch {
+		complete = true
+	}
+	return buf.String(), complete, pc
 }
 
 // OnePassNext selects the next actionable state of the prog, based on the input character.