Regular Expression for password validation

Question

I currently use this regular expression to check if a string conforms to a few conditions   The conditions are  string must be between 8 and 15 characters long   string must contain at least one number   string must contain at least one uppercase letter   string must contain at least one lowercase letter         0-9         a-zA-Z       a-zA-Z0-9  8 15     It works for the most part  but it does not allow special character  Any help modifying this regex to allow special character is much appreciated

User · Answer

Is a regular expression an easier better way to enforce a simple constraint than the more obvious way   static bool ValidatePassword  string password       const int MIN LENGTH    8     const int MAX LENGTH   15      if   password    null   throw new ArgumentNullException        bool meetsLengthRequirements   password Length  gt   MIN LENGTH  amp  amp  password Length  lt   MAX LENGTH     bool hasUpperCaseLetter        false     bool hasLowerCaseLetter        false     bool hasDecimalDigit           false      if   meetsLengthRequirements           foreach  char c in password               if        char IsUpper c    hasUpperCaseLetter   true         else if   char IsLower c    hasLowerCaseLetter   true         else if   char IsDigit c    hasDecimalDigit      true                bool isValid   meetsLengthRequirements                amp  amp  hasUpperCaseLetter                amp  amp  hasLowerCaseLetter                amp  amp  hasDecimalDigit                   return isValid        Which do you think that maintenance programmer 3 years from now who needs to modify the constraint will have an easier time understanding

User · Answer

Update to Justin answer above  if you want to use it using Data Annotation in MVC you can do as follow  RegularExpression   quot        a-z        A-Z        d         da-zA-Z    8 15   quot   ErrorMessage    quot Password must be between 6 and 20 characters and contain one uppercase letter  one lowercase letter  one digit and one special character  quot

User · Answer

Long  and could maybe be shortened  Supports special characters   -     A    -     a-zA-Z0-9    A-Z      -     a-zA-Z0-9    a-z      -     a-zA-Z0-9    0-9   -     a-zA-Z0-9  8 15  z

User · Answer

Thanks Nicholas Carey  I was going to use regex first but what you wrote changed my mind  It is so much easier to maintain this way     You can set these from your custom service methods int minLen   8  int minDigit 2  int minSpChar 2   Boolean ErrorFlag   false    Check for password length if  model NewPassword Length  lt  minLen        ErrorFlag   true      ModelState AddModelError  NewPassword    Password must be at least     minLen     characters long          Check for Digits and Special Characters int digitCount   0  int splCharCount   0  foreach  char c in model NewPassword        if  char IsDigit c   digitCount        if  Regex IsMatch c ToString            amp       -    lt   gt                    splCharCount       if  digitCount  lt  minDigit        ErrorFlag   true      ModelState AddModelError  NewPassword    Password must have at least     minDigit     digit s        if  splCharCount  lt  minSpChar        ErrorFlag   true      ModelState AddModelError  NewPassword    Password must have at least     minSpChar     special character s         if  ErrorFlag      return View model

User · Answer

There seems to be a lot of confusion here  The answers I see so far don t correctly enforce the 1  number 1  lowercase 1  uppercase rule  meaning that passwords like abc123  123XYZ  or AB  amp    would still be accepted  Preventing all-lowercase  all-caps  or all-digits is not enough  you have to enforce the presence of at least one of each   Try the following          a-z        A-Z        d   8 15     If you also want to require at least one special character  which is probably a good idea   try this          a-z        A-Z        d         da-zA-Z    8 15     The   8 15  can be made more restrictive if you wish  for example  you could change it to  S 8 15  to disallow whitespace   but remember that doing so will reduce the strength of your password scheme   I ve tested this pattern and it works as expected  Tested on ReFiddle here  http   refiddle com 110    Edit  One small note  the easiest way to do this is with 3 separate regexes and the string s Length property  It s also easier to read and maintain  so do it that way if you have the option  If this is for validation rules in markup  though  you re probably stuck with a single regex

User · Answer

You may try this method       private bool ValidatePassword string password  out string ErrorMessage                        var input   password              ErrorMessage   string Empty               if  string IsNullOrWhiteSpace input                                 throw new Exception  Password should not be empty                               var hasNumber   new Regex    0-9                  var hasUpperChar   new Regex    A-Z                  var hasMiniMaxChars   new Regex     8 15                 var hasLowerChar   new Regex    a-z                  var hasSymbols   new Regex           amp                lt  gt      -                  if   hasLowerChar IsMatch input                                 ErrorMessage    Password should contain at least one lower case letter                    return false                            else if   hasUpperChar IsMatch input                                 ErrorMessage    Password should contain at least one upper case letter                    return false                            else if   hasMiniMaxChars IsMatch input                                 ErrorMessage    Password should not be lesser than 8 or greater than 15 characters                    return false                            else if   hasNumber IsMatch input                                 ErrorMessage    Password should contain at least one numeric value                    return false                             else if   hasSymbols IsMatch input                                 ErrorMessage    Password should contain at least one special case character                    return false                            else                               return true

User · Answer

Pattern satisfy  these below criteria    Password Length 8 and Maximum 15 character Require Unique Chars Require Digit Require Lower Case Require Upper Case           A-Za-z0-9          A-Z         a-z         0-9                 amp  -    8 15

User · Answer

Try this   also corrected check for upper case and lower case  it had a bug since you grouped them as  a-zA-Z  it only looks for atleast one lower or upper  So separated them out          0-9         a-z         A-Z        8 15      Update  I found that the regex doesn t really work as expected and this is not how it is supposed to be written too   Try something like this         8 15         d       A-Z        a-z        s        Between 8 and 15 inclusive  contains atleast one digit  atleast one upper case and atleast one lower case and no whitespace    And I think this is easier to understand as well

User · Answer

I would check them one-by-one  i e  look for a number  d   then if that fails you can tell the user they need to add a digit  This avoids returning an  Invalid  error without hinting to the user whats wrong with it

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