I have seen two example of this.
The first example come from "恋する星座" part 9 where the girl said a sentences with words "行かなきゃ"
The second example come from the song "Catch A Break" sung by Eguchi Takuya. The lyrics goes "買いに行かなきゃ"
Why does they use "-なきゃ" in said examples? Is it used only in daily conversation? What are the other example of words using "-なきゃ"?
[Sorry if I ask too much I'm really curious]