pleae provide validations code

^(?=.*[A-Z])(?=.*\d)(?!.*(.)\1\1)[a-zA-Z0-9@]{6,12}$

Special Characters - Not Allowed
Spaces - Not Allowed
Minimum and Maximum Length of field - 6 to 12 Characters
Met by [a-zA-Z0-9@]{6,12}
Numeric Character - At least one character
Met by positive lookahead (?=.*\d)
At least one Capital Letter
Met by positive lookahead (?=.*[A-Z])
Repetitive Characters - Allowed only two repetitive characters
I am not sure what you mean by this. The negative lookahead (?!.*(.)\1\1) makes sure that no character is allowed to appear more than two times in a row. Substring aa is okay, aaa is not.
Make it (?!.*(.+)\1\1) to reject repeated substrings of length more than one (like ababab) or add .* before \1 to reject non-continuous repeated appearances too.

refer: http://stackoverflow.com/questions/12899876/checking-strings-for-a-strong-enough-password[^]


OR

You can do these checks using positive look ahead assertions:

<pre lang="xml">^(?=.*[A-Z].*[A-Z])(?=.*[!@#$&amp;*])(?=.*[0-9].*[0-9])(?=.*[a-z].*[a-z].*[a-z]).{8}$
<a href="http://www.rubular.com/r/UAwoaPM0Ji">Rubular link</a>[<a href="http://www.rubular.com/r/UAwoaPM0Ji" target="_blank" title="New Window">^</a>]

Explanation:

^                         Start anchor
(?=.*[A-Z].*[A-Z])        Ensure string has two uppercase letters.
(?=.*[!@#$&amp;*])            Ensure string has one special case letter.
(?=.*[0-9].*[0-9])        Ensure string has two digits.
(?=.*[a-z].*[a-z].*[a-z]) Ensure string has three lowercase letters.
.{8}                      Ensure string is of length 8.
$                         End anchor.</pre>

refer: regex-for-password-strength[^]


OR

public static bool IsStrongPassword(string password)
{
    // Minimum and Maximum Length of field - 6 to 12 Characters
    if (password.Length < 6 || password.Length > 12)
        return false;

    // Special Characters - Not Allowed
    // Spaces - Not Allowed
    if (!(password.All(c => char.IsLetter(c) || char.IsDigit(c))))  
        return false;

    // Numeric Character - At least one character
    if (!password.Any(c => char.IsDigit(c)))
        return false;

    // At least one Capital Letter
    if (!password.Any(c => char.IsUpper(c)))
        return false;

    // Repetitive Characters - Allowed only two repetitive characters
    var repeatCount = 0;
    var lastChar = '\0';
    foreach(var c in password)
    {
        if (c == lastChar)
            repeatCount++;
        else
            repeatCount = 0;
        if (repeatCount == 2)
            return false;
        lastChar = c;
    }

    return true;
}

refer: strong-password-regex[^]

RETURNS IF IT IS VALID PASSWORD OR NOT


ALTER FUNCTION [dbo].[IsValidPassword]
(
@LoginID NVARCHAR(50),
@inputPassword NVARCHAR(50)
)
RETURNS int
AS
BEGIN

DECLARE @LoginCompId int;
DECLARE @MinLength int=6;
DECLARE @Result BIT = 0
DECLARE @Counter INT = 1
DECLARE @IsValidPassword INT = 0
DECLARE @Count int=0
DECLARE @outputvalue varchar(200)
DECLARE @isValid Char(1);

SELECT @LoginCompId = comp_id FROM mst_user_info with(nolock) WHERE login_id = @LoginID
SELECT @MinLength = para_val FROM sys_param with(nolock) WHERE comp_id = @LoginCompId and para_name='PASSWORD_MINLENGTH'

select @isValid=dbo.CheckValidString(@inputPassword)
--WHILE (@Counter <= LEN(@inputPassword) - 1)
--BEGIN
--IF(ASCII((SELECT SUBSTRING(@inputPassword, @Counter, 1))) = ASCII((SELECT SUBSTRING(@inputPassword, @Counter + 2, 1))))
-- BEGIN
-- SET @Result = 1
-- BREAK
-- END
-- SET @Counter = @Counter + 1
--END

IF(@isValid='Y')
BEGIN
IF( @inputPassword COLLATE Latin1_General_BIN LIKE '%[a-z]%' )
BEGIN
SET @Count=@Count+1
END

IF(@inputPassword COLLATE Latin1_General_BIN LIKE '%[A-Z]%' )
BEGIN
SET @Count=@Count+1
END

IF(@inputPassword LIKE '%[0-9]%')
BEGIN
SET @Count=@Count+1
END

IF(@inputPassword LIKE '%[~!@#$%^&]%')
BEGIN
SET @Count=@Count+1
END

SELECT @IsValidPassword=CASE WHEN
LEN(@inputPassword) >= @MinLength AND
@Count >= 3
THEN 1 ELSE 0 END
END
ELSE
BEGIN
SET @IsValidPassword=0
END


RETURN @IsValidPassword
END

----------------

CheckValidString

ALTER FUNCTION [dbo].[CheckValidString]
(
@input varchar(50)

)
RETURNS varchar
AS
BEGIN

declare @inputLength integer = datalength(@input);
declare @intIndex integer = 1;
declare @intMaxCount integer = 3;
declare @singleChar Char(1);
declare @isValid Char(1);
while (1 = 1)
begin
if @intIndex > @inputLength
begin
select @isValid = 'Y';
break;
end

select @singleChar = substring(@input, @intIndex, 1)

if(@inputLength - datalength(replace(@input, @singleChar, '')) > @intMaxCount)
begin
select @isValid = 'N';
break;
end
select @intIndex += 1;
end

RETURN @isValid
END
-------------
sp-----[UspValidatePassword]
-----------------------------------


ALTER PROCEDURE [dbo].[UspValidatePassword]
@LoginID NVARCHAR(50),
@inputPassword NVARCHAR(50),
@ErrorMsg varchar(250) OUTPUT

AS
BEGIN

DECLARE @IsValidPassword int
DECLARE @CheckPassWithUserid int
DECLARE @MinLength varchar=6;
DECLARE @LoginCompId int;
SET @ERRORMSG='';

SELECT @LoginCompId = comp_id FROM mst_user_info with(nolock) WHERE login_id = @LoginID
SELECT @MinLength = para_val FROM sys_param with(nolock) WHERE comp_id = @LoginCompId and para_name='PASSWORD_MINLENGTH'

if(@LoginID<>@inputPassword)
BEGIN
SELECT @ISVALIDPASSWORD=[dbo].[IsValidPassword](@LOGINID,@INPUTPASSWORD)
print(@ISVALIDPASSWORD)
IF(@ISVALIDPASSWORD=0)
BEGIN
SET @ERRORMSG='Incorrect Password ! Password must be of minimum '+@MinLength+' characters and contain any 3 combination of Digit, Upper Case, Lower Case and Special Character.';
print @ERRORMSG
END

END
ELSE
BEGIN
SET @ERRORMSG='Login ID and Password should not be same !';
END
END