Function get_num_from_str 

pub fn get_num_from_str(usr_str: &str, numbers: &str) -> (String, String)

The characters of a string that are among numbers, and the rest, each in order: music21’s getNumFromStr, whose numbers are the ten digits.

use music21::common::stringtools::get_num_from_str;

let (numbers, rest) = get_num_from_str("23a954Hello", "0123456789");
assert_eq!((numbers.as_str(), rest.as_str()), ("23954", "aHello"));