s_n_a_k_e_c_a_s_e_d

StringExtension.swift

import Foundation

extension String {
    func s_n_a_k_e_c_a_s_e_d() -> String { // OK, you can name it just “snakecased” if you prefer… :)
        var charactersToRemove = CharacterSet.alphanumerics.inverted
        charactersToRemove.remove(charactersIn: " ")
        let result = components(separatedBy: charactersToRemove).joined(separator: "")
        return result.replacingOccurrences(of: " ", with: "_").lowercased()
    }
}

"Hello world".s_n_a_k_e_c_a_s_e_d() // "hello_world"
"Hello! World!".s_n_a_k_e_c_a_s_e_d() // "hello_world"
"I am snake cased".s_n_a_k_e_c_a_s_e_d() // "i_am_snake_cased"
"s n a k e c a s e d".s_n_a_k_e_c_a_s_e_d() // "s_n_a_k_e_c_a_s_e_d"

// This implementation strips out everything but alphanumerics
// but can be modified for your desired behavior!