exp/regexp/syntax: add Prog.NumCap

R=rsc, r
CC=golang-dev
https://golang.org/cl/4662083

src/pkg/exp/regexp/syntax/compile.go

@@ -185,6 +185,10 @@ func (c *compiler) cap(arg uint32) frag {
 	f := c.inst(InstCapture)
 	f.out = patchList(f.i << 1)
 	c.p.Inst[f.i].Arg = arg
+
+	if c.p.NumCap < int(arg)+1 {
+		c.p.NumCap = int(arg) + 1
+	}
 	return f
 }
 

src/pkg/exp/regexp/syntax/prog.go

@@ -10,8 +10,9 @@ import (
 
 // A Prog is a compiled regular expression program.
 type Prog struct {
-	Inst  []Inst
-	Start int // index of start instruction
+	Inst   []Inst
+	Start  int // index of start instruction
+	NumCap int // number of InstCapture insts in re
 }
 
 // An InstOp is an instruction opcode.