switch nuke channel by expression node

set cut_paste_input [stack 0]
version 7.0 v10
ColorWheel {
 inputs 0
 gamma 0.45
 name ColorWheel2
 selected true
 xpos -68
 ypos -329
}
CheckerBoard2 {
 inputs 0
 name CheckerBoard3
 selected true
 xpos -191
 ypos -331
}
add_layer {JimmyChannel JimmyChannel.red JimmyChannel.green JimmyChannel.blue JimmyChannel.alpha}
ShuffleCopy {
 inputs 2
 alpha alpha2
 black red
 white green
 red2 blue
 green2 alpha
 out2 JimmyChannel
 name ShuffleCopy4
 label "模擬同一串流裡面有一組channel叫\nJimmyChannel"
 note_font_size 20
 selected true
 xpos -139
 ypos -217
}
Expression {
 channel0 {forward.u -forward.v -backward.u -backward.v}
 expr0 JimmyChannel.red
 channel1 {-forward.u forward.v -backward.u -backward.v}
 expr1 JimmyChannel.green
 channel2 {-forward.u -forward.v backward.u -backward.v}
 expr2 JimmyChannel.blue
 channel3 {-forward.u -forward.v -backward.u backward.v}
 expr3 JimmyChannel.alpha
 name Expression9
 label "將JimmyChannel切換到motion channel"
 note_font_size 20
 selected true
 xpos -139
 ypos -94
}

useful info about nuke expression node : http://www.nukepedia.com/written-tutorials/expressions-101
and thanks seadog's help!