Function change_case 

pub fn change_case(input: &str, options: Options) -> String

Core function to change case

use regex::Regex;
use change_case::{change_case, Options};
let options = Options::default()
    .split_regex(vec![Regex::new("([a-z])([A-Z0-9])").unwrap()]);
assert_eq!(change_case("camel2019", options), "camel 2019");
assert_eq!(change_case("camel2019", Options::default()), "camel2019");