func regexUpdate() {
    struct Item {
        let source: String
        let target: String
    }
    let values = [Item(source:"ab123451", target: "ab12345"),
                  Item(source:"abc12345", target: "ab12345"),
                  Item(source:"abc12", target: "ab12"),
                  Item(source:"a12", target: "a")]
    let pattern = "([a-zA-z]{1,2})[a-zA-Z]*([1-9]{1,5})[1-9]*"
    values.forEach { aValue in
        let modified = aValue.source.replacingOccurrences(of: pattern, with: "$1$2", options: .regularExpression)
        print("Initial: \(aValue.source) - (modified) \(modified) \(modified == aValue.target ? "==" : "!=") \(aValue.target) (target) ")
    }
}


Output:

Initial: ab123451 - (modified) ab12345 == ab12345 (target)
Initial: abc12345 - (modified) ab12345 == ab12345 (target)
Initial: abc12 - (modified) ab12 == ab12 (target)
Initial: a12 - (modified) a12 != a (target)