How to handle TextField validation in password in Flutter

Your regular expression should look like:

r'^(?=.*?[A-Z])(?=.*?[a-z])(?=.*?[0-9])(?=.*?[!@#\$&*~]).{8,}$

Here is an explanation:

r'^
  (?=.*[A-Z])       // should contain at least one upper case
  (?=.*[a-z])       // should contain at least one lower case
  (?=.*?[0-9])      // should contain at least one digit
  (?=.*?[!@#\$&*~]) // should contain at least one Special character
  .{8,}             // Must be at least 8 characters in length  
$

Match above expression with your password string. Using this method-

    String? validatePassword(String value) {
    RegExp regex =
        RegExp(r'^(?=.*?[A-Z])(?=.*?[a-z])(?=.*?[0-9])(?=.*?[!@#\$&*~]).{8,}$');
    if (value.isEmpty) {
      return 'Please enter password';
    } else {
      if (!regex.hasMatch(value)) {
        return 'Enter valid password';
      } else {
        return null;
      }
    }
  }