naiveRegexParser.swift

func parse(source: String) -> Person? {
    
    let lines = source.components(separatedBy: "\n")
    let age = matches(for: "(?<=age: )[0-9]+", in: lines[1]).flatMap({Int($0)})
    let name = matches(for: "(?<=name: )\\w+", in: lines[2])
    let city = matches(for: "(?<=city: )[\\w ]+", in: lines[3])
    
    if let a = age, let n = name, let c = city {
        return Person(name: n, age: a, city: c)
    } else {
        return nil
    }
}

let parsedPerson = parse(source: source)
print(parsedPerson)