Added function to return the number of subexpressions in the group

regex/compile.go

@@ -18,6 +18,12 @@ type Reg struct {
 	numGroups int
 }
 
+// numSubexp eturns the number of sub-expressions in the given [Reg]. This is equivalent
+// to the number of capturing groups.
+func (r Reg) NumSubexp() int {
+	return r.numGroups
+}
+
 const concatRune rune = 0xF0001
 
 // Flags for shuntingYard - control its behavior