regexp/syntax: replace internal error on unexpected ) w/ ErrUnexpectedParen

Unbalanced extra right parenthesis produced an internal error instead of
a more descriptive one.

Fixes #3406.

R=r, rsc
CC=golang-dev
https://golang.org/cl/6201063

api/go1.txt

@@ -5484,6 +5484,7 @@ pkg regexp/syntax, const ErrMissingBracket ErrorCode
 pkg regexp/syntax, const ErrMissingParen ErrorCode
 pkg regexp/syntax, const ErrMissingRepeatArgument ErrorCode
 pkg regexp/syntax, const ErrTrailingBackslash ErrorCode
+pkg regexp/syntax, const ErrUnexpectedParen ErrorCode
 pkg regexp/syntax, const FoldCase Flags
 pkg regexp/syntax, const InstAlt InstOp
 pkg regexp/syntax, const InstAltMatch InstOp

src/pkg/regexp/syntax/parse.go

@@ -46,6 +46,7 @@ const (
 	ErrMissingParen          ErrorCode = "missing closing )"
 	ErrMissingRepeatArgument ErrorCode = "missing argument to repetition operator"
 	ErrTrailingBackslash     ErrorCode = "trailing backslash at end of expression"
+	ErrUnexpectedParen       ErrorCode = "unexpected )"
 )
 
 func (e ErrorCode) String() string {
@@ -1168,13 +1169,13 @@ func (p *parser) parseRightParen() error {
 
 	n := len(p.stack)
 	if n < 2 {
-		return &Error{ErrInternalError, ""}
+		return &Error{ErrUnexpectedParen, p.wholeRegexp}
 	}
 	re1 := p.stack[n-1]
 	re2 := p.stack[n-2]
 	p.stack = p.stack[:n-2]
 	if re2.Op != opLeftParen {
-		return &Error{ErrMissingParen, p.wholeRegexp}
+		return &Error{ErrUnexpectedParen, p.wholeRegexp}
 	}
 	// Restore flags at time of paren.
 	p.flags = re2.Flags

src/pkg/regexp/syntax/parse_test.go

@@ -441,10 +441,18 @@ var invalidRegexps = []string{
 	`(`,
 	`)`,
 	`(a`,
+	`a)`,
+	`(a))`,
 	`(a|b|`,
+	`a|b|)`,
+	`(a|b|))`,
 	`(a|b`,
+	`a|b)`,
+	`(a|b))`,
 	`[a-z`,
 	`([a-z)`,
+	`[a-z)`,
+	`([a-z]))`,
 	`x{1001}`,
 	`x{9876543210}`,
 	`x{2,1}`,