regexp: add (*Regexp).Longest

Fixes #3696.

R=rsc
CC=golang-dev
https://golang.org/cl/7133051

src/pkg/regexp/regexp.go

@@ -130,6 +130,14 @@ func CompilePOSIX(expr string) (*Regexp, error) {
 	return compile(expr, syntax.POSIX, true)
 }
 
+// Longest sets the match semantics of the regexp to leftmost-longest.
+// That is, when matching against text, the regexp returns a match that
+// begins as early as possible in the input (leftmost), and among those
+// it chooses a match that is as long as possible.
+func (re *Regexp) Longest() {
+	re.longest = true
+}
+
 func compile(expr string, mode syntax.Flags, longest bool) (*Regexp, error) {
 	re, err := syntax.Parse(expr, mode)
 	if err != nil {