regexp/syntax: unexport ErrUnexpectedParen

This new error is the only API change in the current draft of
Go 1.0.2 CLs. I'd like to include the CL that introduced it,
because it replaces a mysterious 'internal error' with a
useful error message, but I don't want any API changes,
so unexport the error constant for now. It can be
re-exported for Go 1.1.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/6294055

src/pkg/regexp/syntax/parse.go

@@ -46,9 +46,11 @@ const (
 	ErrMissingParen          ErrorCode = "missing closing )"
 	ErrMissingRepeatArgument ErrorCode = "missing argument to repetition operator"
 	ErrTrailingBackslash     ErrorCode = "trailing backslash at end of expression"
-	ErrUnexpectedParen       ErrorCode = "unexpected )"
 )
 
+// TODO: Export for Go 1.1.
+const errUnexpectedParen ErrorCode = "unexpected )"
+
 func (e ErrorCode) String() string {
 	return string(e)
 }
@@ -1169,13 +1171,13 @@ func (p *parser) parseRightParen() error {
 
 	n := len(p.stack)
 	if n < 2 {
-		return &Error{ErrUnexpectedParen, p.wholeRegexp}
+		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{ErrUnexpectedParen, p.wholeRegexp}
+		return &Error{errUnexpectedParen, p.wholeRegexp}
 	}
 	// Restore flags at time of paren.
 	p.flags = re2.Flags