Regex: Any letters, digit, and 0 up to 3 special chars - regex

Question

It seems I'm stuck with a simple regex for a password check.
What I'd like:
8 up to 30 symbols (Total)
With any of these: [A-Za-z\d]
And 0 up to 3 of these: [ -/:-#[-`{-~À-ÿ] (Special list)
I took a look here and then I wrote something like:
(?=.{8,15}$)(?=.*[A-Za-z\d])(?!([ -\/:-#[-`{-~À-ÿ])\1{4}).*
But it doesn't work, one can put more than 3 of the special chars list.
Any tips?

Answer 1

After shuffling your regex around a bit, it works for the examples you provided (I think you made a mistake with the example "A#~` C:", it should not match as it has 6 special chars):
(?!.*(?:[ -\/:-#[-`{-~À-ÿ].*){4})^[A-Za-z\d -\/:-#[-`{-~À-ÿ]{8,30}$
It only needs one lookahead instead of two, because the length and character set check can be done without lookahead: ^[A-Za-z\d -/:-#[-`{-~À-ÿ]{8,30}$
I changed the negative lookahead a bit to be correct. Your mistake was to only check for consecutive special chars, and you inserted the wildcards .* in a way that made the lookahead never hit (because the wildcard allowed everything).

Answer 2

Will this work?
string characters = " -/:-#[-`{-~À-ÿ";
string letters = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
string[] inputs = {
"AABBCCDD",
"aaaaaaaa",
"11111111",
"a1a1a1a1",
"AA####AA",
"A1C EKFE",
"AADE F"
};
foreach (string input in inputs)
{
var counts = input.Cast<char>().Select(x => new { ch = characters.Contains(x.ToString()) ? 1 : 0, letter = letters.Contains(x.ToString()) ? 1 : 0, notmatch = (characters + letters).Contains(x) ? 0 : 1}).ToArray();
Boolean isMatch = (input.Length >= 8) && (input.Length <= 30) && (counts.Sum(x => x.notmatch) == 0) && (counts.Sum(x => x.ch) <= 3);
Console.WriteLine("Input : '{0}', Matches : '{1}'", input, isMatch ? "Match" : "No Match");
}
Console.ReadLine();

Answer 3

I would use: (if you want to stick to Regex)
var specialChars = #" -\/:-#[-`{-~À-ÿ";
var regularChars = #"A-Za-z\d";
if (Regex.Match(password,$"^(.[{regularChars}{specialChars}]{7,29})$").Success && Regex.Matches(password, $"[{specialChars}]").Count<=3))
{
//Password OK
}
If consists of:
Check Length and if password contains illegal characters
Check if ony contains 3 times special char
A litle faster:
var specialChars = #" -\/:-#[-`{-~À-ÿ";
var regularChars = #"A-Za-z\d";
var minChars = 8;
var maxChars = 30;
if (password.Length >= minChars && password.Length <= maxChars && Regex.Match(password,$"^[{regularChars}{specialChars}]+$").Success && Regex.Matches(password, $"[{specialChars}]").Count<=3))
{
//Password OK
}

Answer 4

Newbie here..I think I've managed to get what you need but one of the test cases you shared was kinda weird..
A#~` C:
OK -- Match (3 specials, it's okay)
Shouldn't this be failed because it has more than 3 specials?
Could you perhaps try this? If it works I'll type out the explanations for the regex.
https://regex101.com/r/KCL6R1/2
(?=^[A-Za-z\d -\/:-#[-`{-~À-ÿ]{8,30}$)^(?:[A-Za-z\d]*[ -\/:-#[-`{-~À-ÿ]){0,3}[A-Za-z\d]*$