We Keep Coding

How to write regex to match password with rules

I'm trying to write regex to match password with following rule
Minimum length - 8 characters, if it includes a number and a lowercase letter OR 15 characters with any combination of characters
Here's my regex
^(?=.*\d[a-z]).{8,}|([a-zA-Z0-9]{15,})$
Expected result:
2232ddds - correct
ddds2222 - correct
Actual result:
2232ddds - correct
ddds2222 - incorrect
Could you help me to find a problem?

The problems are two:
The anchors only affect the alternatives they stand next to
The (?=.*\d[a-z]) lookahead requires a digit and a lowercase to appear in this order only, if there is a letter and then a digit, it won't work.
You may use
/^(?:(?=.*\d)(?=.*[a-z]).{8,}|[a-zA-Z0-9]{15,})$/
^^^|-----------------| ^ ^
See the regex demo.
If you want to make it more efficient and follow best practices use
/^(?:(?=\D*\d)(?=[^a-z]*[a-z]).{8,}|[a-zA-Z0-9]{15,})$/
Details
^ - start of string
(?: - a non-capturing group start
(?=\D*\d) - at least 1 digit
(?=[^a-z]*[a-z]) - at least 1 lowercase letter
.{8,} - any 8 or more chars other than line break chars
| - or
[a-zA-Z0-9]{15,} - 15 or more alphanumeric chars only
) - end of non-capturing group
$ - end of string.

Here a few options you can choose to build your regex:
String contains at least one:
Lower case (?=.*[a-z])
Upper case (?=.*[A-Z])
Numbers (?=.*\d)
Symbols (?=.[!##\$%\^&])
Minimal characters of string: (?=.{8,})
Your regex would be: (?=.*[a-z])(?=.*\d)(?=.{8,})|(?=.{15,})
Hope this helps!

Issue with javascript regex not matching less than 3 characters

I have the following javascript regex:
/^[^\s][a-z0-9 ]+[^\s]$/i
I need to allow any alphanumeric character as well as spaces inside the string but not at the beginning nor at the end.
Oddly enough, the above regex will not accept less than 3 characters, e.g. aa will not match but aaa will.
I am not sure why. Can anyone please help ?

You have: [^\s] (requires matching at least one non-whitespace character), [a-z0-9 ]+ (requires matching at least one alphanumeric or space character), and [^\s] again (requires matching at least one non-whitespace character). So, in total, you need at least 3 characters in the string.
Use word boundaries at the beginning and end instead:
/^\b[a-z0-9 ]+\b$/i
https://regex101.com/r/2GhH3N/1

Try the following regex:
^(?! )[a-z0-9 ]*[a-z0-9]$
Details:
^(?! ) - Start of the string and no space after it (so here we exclude the
initial space).
[a-z0-9 ]* - A sequence of letters, digits and spaces, possibly empty
(the content before the last letter(see below).
[a-z0-9]$ - The last letter and the end of string (so here we exclude the
terminal space).

You should re-write the expression as
/^[a-z0-9]+(?:\s+[a-z0-9]+)*$/i
See the regex demo.
NOTE: If only one whitespace is allowed between the alphanumeric chars use
/^[a-z0-9]+(?:\s[a-z0-9]+)*$/i
^^
Details
^ - start of string
[a-z0-9]+ - 1+ letters/digits
(?:\s+[a-z0-9]+)* - 0 or more repetitions of 1+ whitespaces (\s+) and 1+ digit/letters
$ - end of string.
See the regex graph:

Email validation expression allows underscore

Email validation expression /^(?!_)\w+([\.-]?\w+)*#(?!_)\w+([\.-]?\w+)*(\.\w{2,3})+$/ allows underscore for some cases, but otherwise works perfectly.
It does not fail the following email address:
tets_name#gmail.com
test____name#gmail.com
Here is the pattern :
var pattern =/^(?!_)\w+([\.-]?\w+)*#(?!_)\w+([\.-]?\w+)*(\.\w{2,3})+$/;
if (pattern.test(Email)) {
return false;
}
How can I restrict this to not allow underscore?

Note that \w matches ASCII letters ([A-Za-z]), digits ([0-9]) and an underscore.
To make sure your regex does not match underscores replace all \w with [a-zA-Z0-9] and the last \w{2,3} can be replaced with [a-zA-Z]{2,3}:
/^[a-zA-Z0-9]+(?:[.-][a-zA-Z0-9]+)*#[a-zA-Z0-9]+(?:[.-][a-zA-Z0-9]+)*\.[a-zA-Z]{2,3}$/
If you plan to match emails that only contain single underscores between letters/digits and not at the start/end use
/^[a-zA-Z0-9]+(?:[_.-][a-zA-Z0-9]+)*#[a-zA-Z0-9]+(?:[_.-][a-zA-Z0-9]+)*\.[a-zA-Z]{2,3}$/
See this regex and another regex here.
Details
^ - start of string
[a-zA-Z0-9]+ - 1 or more ASCII letters/digits
(?:[.-][a-zA-Z0-9]+)* - zero or more sequences of
[.-] - a dot or - (no need to escape a dot inside a character class)
[a-zA-Z0-9]+ - 1 or more ASCII letters/digits
# - a # char
[a-zA-Z0-9]+ - 1 or more ASCII letters/digits
(?:[.-][a-zA-Z0-9]+)* - zero or more sequences of
[.-] - a dot or - (no need to escape a dot inside a character class)
[a-zA-Z0-9]+ - 1 or more ASCII letters/digits
\. - a dot
[a-zA-Z]{2,3} - 2 or 3 ASCII letters
$ - end of string.

You may try this, i just added .* in the negative look ahead , you were only looking for single _ at the start of your string but - can be at other postitions
^(?!.*_)\w+([\.-]?\w+)*#\w+([\.-]?\w+)*(\.\w{2,3})+$
try demo here
see explanation on the link

domain name validation with custom rules

I am working on regular expression with own custom rule.
rules are the hostname must be 3-63 characters,
whole name must be 256 characters,
no special characters except dot(.), hyphen(-)
I tried this var regx = /^([A-Za-z0-9-]{3,63}?\.)+[a-zA-Z]{2,6}$/;
but the problem is the pattern is applicable to next string after dot(.). What I mean to say is
for example : "qwerty.abcde.com"
in the above "qwerty" should be 3-63 characters , but "abcde" can be any no.of characters .My pattern is applicable to next string after dot.that 3-63 rule should be applicable to only "qwerty" not to "abcde". can any one help me out.
Thanks in advance

You may use the following regex:
/^(?!.{257})[A-Za-z0-9-]{3,63}\.(?:[A-Za-z0-9-]+\.)*[a-zA-Z]{2,6}$/
See the regex demo
Details:
^ - start of string
(?!.{257}) - a negative lookahead that fails the match if the string contains 257 or more chars (other than line break chars)
[A-Za-z0-9-]{3,63} - 3 to 63 alphanumeric and - chars
\. - a dot
(?:[A-Za-z0-9-]+\.)* - zero or more sequences of
[A-Za-z0-9-]+ - 1 or more alphanumeric and - chars
\. - a dot
[a-zA-Z]{2,6} - 2 to 6 ASCII letters
$ - end of string.
So, the negative lookahead checks for the whole string length, and the {3,63} limiting quantifier is only applied to the chunk of chars before the first ..

split javascript string using a regexp to separate numbers from other characters

so for example:
"10.cm" ...becomes... [10,".cm"] or ["10",".cm"], either will do as I can work with a string once it's split up.
i tried
"10.cm".split(/[0-9]/|/[abc]/)
but it seems that i don't have such a great understanding of using regexp's
thanks

You may tokenize the string into digits and non-digits with /\d+|\D+/g regex:
var s = "10.cm";
console.log(s.match(/\d+|\D+/g));
Details:
\d+ - matches 1 or more digits
| - or
\D+ - matches 1 or more characters other than digits.

/\W/ Matches any non-word character. This includes spaces and punctuation, but not underscores. In this solution can be used /\W/ with split and join methods. You can separate numbers from other characters.
let s = "10.cm";
console.log(s.split(/\W/).join(" "));
output = 10 cm