Question

I would like to know how one could validate a password with regex.

I tried using lookahaed assertions but I somehow failed. I don’t know how I can match a certain length of a string.

My conditions are :

• At least 8 characters
• At least one digit
• At least one lower English letter
• At least one upper English letter
• At least one symbol of !@#$%-
• Only above characters are allowed

Thanks in advance!

Answer 1

you are in the right step with lookahead assertions.

Your requirements can be broken down to this :

• (?=.{8,})
• (?=.*\d)
• (?=.*[a-z])
• (?=.*[A-Z])
• (?=.*[!@#$%-])
• [\da-zA-Z!@#$%-]*

Putting it all together you end up with this :

foundMatch = Regex.IsMatch(subjectString, @"^(?=.{8,})(?=.*\d)(?=.*[a-z])(?=.*[A-Z])(?=.*[!@#$%-])[\da-zA-Z!@#$%-]*$");

Which can in turn be explained by this :

"
^                   # Assert position at the beginning of the string
(?=                 # Assert that the regex below can be matched, starting at this position (positive lookahead)
   .                   # Match any single character that is not a line break character
      {8,}                # Between 8 and unlimited times, as many times as possible, giving back as needed (greedy)
)
(?=                 # Assert that the regex below can be matched, starting at this position (positive lookahead)
   .                   # Match any single character that is not a line break character
      *                   # Between zero and unlimited times, as many times as possible, giving back as needed (greedy)
   \d                  # Match a single digit 0..9
)
(?=                 # Assert that the regex below can be matched, starting at this position (positive lookahead)
   .                   # Match any single character that is not a line break character
      *                   # Between zero and unlimited times, as many times as possible, giving back as needed (greedy)
   [a-z]               # Match a single character in the range between “a” and “z”
)
(?=                 # Assert that the regex below can be matched, starting at this position (positive lookahead)
   .                   # Match any single character that is not a line break character
      *                   # Between zero and unlimited times, as many times as possible, giving back as needed (greedy)
   [A-Z]               # Match a single character in the range between “A” and “Z”
)
(?=                 # Assert that the regex below can be matched, starting at this position (positive lookahead)
   .                   # Match any single character that is not a line break character
      *                   # Between zero and unlimited times, as many times as possible, giving back as needed (greedy)
   [!@#$%-]            # Match a single character present in the list below
                          # One of the characters “!@#$”
                          # The character “%”
                          # The character “-”
)
[\da-zA-Z!@#$%-]    # Match a single character present in the list below
                       # A single digit 0..9
                       # A character in the range between “a” and “z”
                       # A character in the range between “A” and “Z”
                       # One of the characters “!@#$”
                       # The character “%”
                       # The character “-”
   *                   # Between zero and unlimited times, as many times as possible, giving back as needed (greedy)
$                   # Assert position at the end of the string (or before the line break at the end of the string, if any)
"