A `@propertyWrapper` that validates a string using a regular expression. If the string matches then it is assigned to the property if it does not then the property is net to nil. Also included is an example of validating email addresses.

RegularExpressionValidation.swift

import Foundation

@propertyWrapper
public struct RegularExpressionValidation {

    private var _verifiedString: String?
    private let _regexPattern: String
    private lazy var _regex: NSRegularExpression? = try? NSRegularExpression(pattern: _regexPattern, options: [.caseInsensitive])
    
    public init(_ regexString: String) {
        _regexPattern = regexString
    }
    
    public var wrappedValue: String? {
        get { _verifiedString }
        set {
            guard let searchString = newValue else { _verifiedString = nil; return }
            let range = NSRange(searchString.startIndex..<searchString.endIndex, in: searchString)
            if _regex?.firstMatch(in: searchString, options: [], range: range) != nil {
                _verifiedString = searchString
            }
            else {
                _verifiedString = nil
            }
        }
    }

}
    
@propertyWrapper
public struct EmailValidation {
    
    @RegularExpressionValidation("^(?:[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*|\"(?:[\\x01-\\x08\\x0b\\x0c\\x0e-\\x1f\\x21\\x23-\\x5b\\x5d-\\x7f]|\\[\\x01-\\x09\\x0b\\x0c\\x0e-\\x7f])*\")@(?:(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?|\\[(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?|[a-z0-9-]*[a-z0-9]:(?:[\\x01-\\x08\\x0b\\x0c\\x0e-\\x1f\\x21-\\x5a\\x53-\\x7f]|\\[\\x01-\\x09\\x0b\\x0c\\x0e-\\x7f])+)\\])$")
    private var _stringEmail: String?
    
    public init(wrappedValue value: String?) {
        _stringEmail = value
    }
    
    public var wrappedValue: String? {
        get { _stringEmail }
        set { _stringEmail = newValue }
    }
}