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Regex validation

Minimum 3 characters, all small case, can use maximum of 2 numbers, no special characters allowed.
I tried using ^[a-zA-Z0-9]*$ but I'm unable to limit the numbers used
Can someone help me.

You could use a negative lookahead assertion to rule out more than 3 digits:
/^(?!(?:.*\d+){3,})[a-z0-9]{3,}$/
Here is an explanation of the pattern:
^ from the start of the string
(?!(?:.*\d+){3,}) assert that 3 or more digits do NOT occur
[a-z0-9]{3,} then match 3 or more lowercase letters or digits
$ end of the string
Here is a working demo.

You could use check if there are at least 3 allowed characters, and then match 0, 1 or 2 digits.
^(?=[A-Za-z\d]{3})[A-Za-z]*(?:\d[A-Za-z]*){0,2}$
Explanation
^ Start of string
(?=[A-Za-z\d]{3}) Positive lookahead, assert 3 allowed chars
[A-Za-z]* Match optional chars A-Za-z
(?:\d[A-Za-z]*){0,2} Repeat 0-2 times matching a single digit and optional chars A-Za-z
$ End of string
See the matches on regex101.

How to create Regex to filter out results with few complex conditions regarding length, case and classes of characters

I have the following filtered:
2 digits (?=..*\d)
2 uppercase characters (?=..*[a-z])
2 lowercase characters (?=..*[A-Z])
10 to 63 characters .{10,63}$
Which translates to:
(?=.{2,}\d)(?=..*[a-z])(?=..*[A-Z]).{10,63}
Then I want to exclude a word starting with the letter u, and ending with three to six digits:
([uU][0-9]{3,6})
However, how can I merge these two patterns to do the following:
It should not allow the following because it respectively:
# does not have the required combination of characters
aaaaaaaaaaaaaaa
# is too long
asadsfdfs12BDFsdfsdfdsfsdfsdfdsfdsfdfsdfsdfsdfsdsfdfsdfsdfssdfdfsdfssdfdfsdfssdfdfsdfsdfsdfsdfsfdsdfsdfsdfsdfsdfsdfsdfsdfsdfsdfsdfsdfsdfsdfsdfsdfsdfsdfsdfsdfsdfsdfs
# contains the pattern that shouldn't be allowed
U0000ABcd567890
ABcd56U00007890
D4gF3U432234
D4gF3u432234
ABcd567890U123456
should allow the following:
# it has the required combination of characters
ABcd5678990
ABcd567890
# does contain a part of the disallowed pattern (`([uU][0-9]{3,6})`), but does not fit that pattern entirely
ABcd567890U12
ABcd5U12abcdf
s3dU00sDdfgdg
ABcd56U007890
Created and example here: https://regex101.com/r/4b2Hu9/3

In your pattern you make use of a lookahead (?=..*\d) which has a different meaning than you assume.
It means if what is directly on the right is 2 or more times any char except a newline followed by a single digit and the same for the upper and lowercase variants.
You could update your pattern to:
^(?!.*[uU]\d{3,6})(?=(?:\D*\d){2})(?=(?:[^a-z]*[a-z]){2})(?=(?:[^A-Z]*[A-Z]){2}).{10,63}$
In parts
^ Start of string
(?!.*[uU]\d{3,6}) Negative lookahead, assert not u or U followed by 3-6 digits
(?=(?:\D*\d){2}) Assert 2 digits
(?=(?:[^a-z]*[a-z]){2}) Assert 2 lowercase chars
(?=(?:[^A-Z]*[A-Z]){2}) Assert 2 uppercase chars
.{10,63} Match any char except a newline 10-63 times
$ End of string
Regex demo

First, the way to ensure that the string contains, for example, two digits would be to use a positive lookahead:
(?=.*\d.*\d)
You can generalize this to your other filters.
To make sure the string contains 10 - 63 characters:
.{10,63}
You say you do not want the string to begin with u or U followed by 3 to 6 digits (presumbaly 7 digits is okay), use a negative lookahead:
(?![uU]\d{3,6}\D)
The \D is required to make sure that if there is a 7th digit, then the string will be accepted.
Putting it all together:
r'^(?=.*\d.*\d)(?=.*[a-z].*[a-z])(?=.*[A-Z].*[A-Z])(?![uU]\d{3,6}\D).{10,63}$'

Regex with lowercase, uppercase, alphanumeric, special characters and no more than 2 identical characters in a row with a minimum length of 8 chars [duplicate]

This question already has answers here:
Regex for password must contain at least eight characters, at least one number and both lower and uppercase letters and special characters
(42 answers)
Closed 3 years ago.
I'm trying to create a regex that allows the 4 main character types (lowercase, uppercase, alphanumeric, and special chars) with a minimum length of 8 and no more than 2 identical characters in a row.
I've tried searching for a potential solution and piecing together different regexes but no such luck! I was able to find this one on Owasp.org
^(?:(?=.*\d)(?=.*[A-Z])(?=.*[a-z])|(?=.*\d)(?=.*[^A-Za-z0-9])(?=.*[a-z])|(?=.*[^A-Za-z0-9])(?=.*[A-Z])(?=.*[a-z])|(?=.*\d)(?=.*[A-Z])(?=.*[^A-Za-z0-9]))(?!.*(.)\1{2,})[A-Za-z0-9!~<>,;:_=?*+#."&§%°()\|\[\]\-\$\^\#\/]{8,32}$
but it uses at least 3 out of the 4 different characters when I need all 4. I tried modifying it to require all 4 but I wasn't getting anywhere. If someone could please help me out I would greatly appreciate it!

Can you try the following?
var strongRegex = new RegExp("^(?=.*[a-z])(?=.*[A-Z])(?=.*[0-9])(?=.*[!##\$%\^&\*])(?=.{8,})");
Explanations
RegEx Description
(?=.*[a-z]) The string must contain at least 1 lowercase alphabetical character
(?=.*[A-Z]) The string must contain at least 1 uppercase alphabetical character
(?=.*[0-9]) The string must contain at least 1 numeric character
(?=.[!##\$%\^&]) The string must contain at least one special character, but we are escaping reserved RegEx characters to avoid conflict
(?=.{8,}) The string must be eight characters or longer
or try with
(?=.{8,100}$)(([a-z0-9])(?!\2))+$ The regex checks for lookahead and rejects if 2 chars are together
var strongerRegex = new RegExp("^(?=.*[a-z])(?=.*[A-Z])(?=.*[0-9])(?=.*[!##\$%\^&\*])(?=.{8,100}$)(([a-z0-9])(?!\2))+$");
reference

I think this might work from you (note: the approach was inspired by the solution to this SO question).
/^(?:([a-z0-9!~<>,;:_=?*+#."&§%°()|[\]$^#/-])(?!\1)){8,32}$/i
The regex basically breaks down like this:
// start the pattern at the beginning of the string
/^
// create a "non-capturing group" to run the check in groups of two
// characters
(?:
// start the capture the first character in the pair
(
// Make sure that it is *ONLY* one of the following:
// - a letter
// - a number
// - one of the following special characters:
// !~<>,;:_=?*+#."&§%°()|[\]$^#/-
[a-z0-9!~<>,;:_=?*+#."&§%°()|[\]$^#/-]
// end the capture the first character in the pair
)
// start a negative lookahead to be sure that the next character
// does not match whatever was captured by the first capture
// group
(?!\1)
// end the negative lookahead
)
// make sure that there are between 8 and 32 valid characters in the value
{8,32}
// end the pattern at the end of the string and make it case-insensitive
// with the "i" flag
$/i

You could use negative lookaheads based on contrast using a negated character class to match 0+ times not any of the listed, then match what is listed.
To match no more than 2 identical characters in a row, you could also use a negative lookahead with a capturing group and a backreference \1 to make sure there are not 3 of the same characters in a row.
^(?=[^a-z]*[a-z])(?=[^A-Z]*[A-Z])(?=[^0-9]*[0-9])(?=[^!~<>,;:_=?*+#."&§%°()|\[\]$^#\/-]*[!~<>,;:_=?*+#."&§%°()|\[\]$^#\/-])(?![a-zA-Z0-9!~<>,;:_=?*+#."&§%°()|\[\]$^#\/-]*([a-zA-Z0-9!~<>,;:_=?*+#."&§%°()|\[\]$^#\/-])\1\1)[a-zA-Z0-9!~<>,;:_=?*+#."&§%°()|\[\]$^#\/-]{8,}$
^ Start of string
(?=[^a-z]*[a-z]) Assert a-z
(?=[^A-Z]*[A-Z]) Assert A-Z
(?=[^0-9]*[0-9]) Assert 0-9
(?= Assert a char that you would consider special
[^!~<>,;:_=?*+#."&§%°()|\[\]$^#\/-]*
[!~<>,;:_=?*+#."&§%°()|\[\]$^#\/-]
)
(?! Assert not 3 times an identical char from the character class in a row
[a-zA-Z0-9!~<>,;:_=?*+#."&§%°()|\[\]$^#\/-]*
([a-zA-Z0-9!~<>,;:_=?*+#."&§%°()|\[\]$^#\/-])\1\1
)
[a-zA-Z0-9!~<>,;:_=?*+#."&§%°()|\[\]$^#\/-]{8,} Match any of the listed 8 or more times
$ End of string
Regex demo

Regex validation for mixed digits for a max of 6 characters

I need a regex validation for mixed length, a total length of 6 characters in that 4-6 characters in caps/numbers and 0-2 spaces.
I tried like
^[A-Z0-9]{4,6}+[\s]{0,2}$
but it results in a max length of 8 characters, but I need a max of 6 characters.

If the alphanumeric chars should only appear at the start of the string and the whitespaces can appear at the end (i.e. the order of the alphanumerics and whitespaces matters), you may use
/^(?=.{6}$)[A-Z0-9]{4,6}\s*$/
See the regex demo
Details
^ - start of string
(?=.{6}$) - the string length is restricted to exactly 6 non-line break chars
[A-Z0-9]{4,6} - 4, 5 or 6 uppercase ASCII letters or digits
\s* - 0+ whitespaces (but actually, only 0, 1 or 2 will be possible to add as the total length is already validated with the lookahead)
$ - end of string.
If you want to match the alphanumeric and whitespaces anywhere inside the string, you need a lookaround based regex like
^(?=(?:[^A-Z0-9]*[A-Z0-9]){4,6}[^A-Z0-9]*$)(?=(?:\S*\s){0,2}\S*$)[A-Z0-9\s]{6}$
See the regex demo
Details
^ - start of string
(?=(?:[^A-Z0-9]*[A-Z0-9]){4,6}[^A-Z0-9]*$) - a positive lookahead that requires the presence of 4 to 6 letters or digits anywhere inside the string
(?=(?:\S*\s){0,2}\S*$) - a positive lookahead that requires the presence of 0 to 2 whitespaces anywhere inside the string
[A-Z0-9\s]{6} - 6 ASCII uppercase letters, digits or whitespaces
$ - end of string.
To shorten the pattern, the second lookahead can be written as (?!(?:\S*\s){3}), it will fail the match if there are 3 whitespace chars anywhere inside the string. See the regex demo.

You can use | characters to accommodate several cases into one.
const regex = /(^[A-Z0-9]{4}\s{2}$)|(^[A-Z0-9]{5}\s$)|(^[A-Z0-9]{6}$)/g;
alert(regex.test(prompt('Enter input, including space(s)')));

If you want to match zero, one or two spaces at the end, you could use an alternation for those 3 cases.
^(?:[A-Z0-9]{4}[ ]{2}|[A-Z0-9]{5}[ ]|[A-Z0-9]{6})$
Regex demo
Explanation
^ Assert the start of the string
(?: Non capturing group
[A-Z0-9]{4}[ ]{2} Match uppercase or digit 4 times followed by 2 spaces
| Or
[A-Z0-9]{5} Match uppercase or digit 5 times followed by 1 space
| Or
[A-Z0-9]{6} Match uppercase or digit 6 times
) Close non capturing group
$ Assert the end of the string

Regex allowing no leading whitespace but allowing anywhere else and requiring at least one uppercase and one lowercase letters

I want regex code to validate usernames that have:
length between 6 and 30
contain at least one letter from A-Z
contain at least one digit from 0-9
not contain a space at the beginning but it might have at
the end or in the middle.
may contain special characters
So far I have tried this:
^[\S](?=.*\d)(?=.*[A-Z]).{6,30}$
It works quite good but when I choose an uppercase letter ONLY at the beginning it doesnt validate my password.
Test12 34 ----> Doesnt accept but should accept
TesT12 34 ----> Accept
tesT12 34 ----> Accept

The problem arises because the \S is at the pattern start before the lookaheads. That means, that the lookaheads that require an uppercase ASCII letter and a digit only check them after the first character in string.
Now, there can be two scenarios: 1) there may be any amount of whitespace characters in the string, but at its beginning, 2) only one whitespace char is allowed in the string, and not in the beginning
Scenario 1
Put \S after the lookaheads and decrement the limiting quantifier values to set the limits to 6-30 as \S already matches and consumes the first char:
^(?=.*\d)(?=.*[A-Z])\S.{5,29}$
^^ ^^^^
See the regex demo.
JS test:
var rx = /^(?=.*\d)(?=.*[A-Z])\S.{5,29}$/;
var vals = [ "Test12 34", "TesT12 34", "tesT12 34" ];
for (var s of vals) {
console.log(s, "=>", rx.test(s));
}
Scenario 2
Use
^(?=.{6,30}$)(?=.*\d)(?=.*[A-Z])\S+(?:\s\S*)?$
The length is restricted with the positive lookahead at the beginning ((?=.{6,30}$)) and the consuming \S+(?:\s\S*)? pattern will only allow a single \s whitespace (1 or 0 due to the last ? - one or zero occurrences quantifier) and it can be in the middle (as the first \S is quantified with +, one or more occurrences quantifier) or end (as the \S after \s is *-quantified, zero or more occurrences quantifier).
See the regex demo.