Need Regular Expression for this(C#) - asp.net

Question

Updated::
Password strength:
Contain characters from three of the following four categories:
English uppercase characters (A through Z)
English lowercase characters (a through z)
Base 10 digits (0 through 9)
Non-alphabetic characters (for example, !, $, #, %
IS it possible to compare two fields value(entered) with regex...if yes then please add onr another condition to above list.
compare password with username entered they must be different

Answer 1

EDIT: This answer was written before the question was edited. It originally included the requirement to not include the user's account name, and be at least 8 characters long.
Given that you need to use the user's account name as part of it anyway, is there any reason you particularly want to do this as a regular expression? You may want to use regular expressions to express the patterns for the four categories (although there are other ways of doing it too) but I would write the rules out separately. For example:
// Categories is a list of regexes in this case. You could easily change
// it to anything else.
int categories = Categories.Count(regex => regex.IsMatch(password));
bool valid = password.IndexOf(name, StringComparison.OrdinalIgnoreCase) == -1
&& password.Length >= 8
&& categories >= 3;

Answer 2

If you need to do it in one expression it should be something like this:
^(?:(?=.*[a-z])(?=.*[A-Z])(?=.*[0-9])|(?=.*[a-z])(?=.*[A-Z])(?=.*[!%,.;:])|(?=.*[a-z])(?=.*[0-9])(?=.*[!%,.;:])|(?=.*[A-Z])(?=.*[0-9])(?=.*[!%,.;:])).{8,}$
See it here on Regexr
Positive lookaheads (the (?=.*[a-z])) are used to check if the string contains the character group you want.
The problem here is, you want 3 out of 4, that means you have to make an alternation with all the allowed combinations.
The last part .{8,} is then matching the string and checking for at least 8 characters.
^ and $ are anchors, that anchor the pattern to the start and the end of the string.
[!%,.;:] is a character class, here you can add all the characters you want to include. Maybe its simpler to use a Unicode script like \p{P} for all punctuation characters. For more details see here on regular-expresssions.info
Update
compare password with username entered they must be different
normally you should be able to build up your regular expression using string concatenation. I have no idea how it is in your case where you put the regex ...
Something like this (pseudo)
String Username = "FooBar";
regex = "^(?:(?=.*[a-z])(?=.*[A-Z])(?=.*[0-9])|(?=.*[a-z])(?=.*[A-Z])(?=.*[!%,.;:])|(?=.*[a-z])(?=.*[0-9])(?=.*[!%,.;:])|(?=.*[A-Z])(?=.*[0-9])(?=.*[!%,.;:]))(?i)(?!.*" + Username + ").+$";
I used here also an inline modifier (?i) to match it case independent. The (?!.* is the start of negative lookahead, meaning the string should not contain ...