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JavaScript regex with below rules

Need to create a regex for a string with below criteria
Allowable characters:
uppercase A to Z A-Z
lowercase a to z a-z
hyphen `
apostrophe '
single quote '
space
full stop .
numerals 0 to 9 0-9
Validations:
Must start with an alphabetic character a-zA-Z or apostrophe
Cannot have consecutive non-alpha characters except for a full stop followed by a space.
The regex I have from the previous question in this forum. Business came back and want to allow string starting with apostrophe along with [a-zA-Z]. This break some previous validations.
eg: a1rte is valid
'tyer4 is valid
'4rt is invalid
^(?!.*[0-9'`\.\s-]{2})[a-zA-Z][a-zA-Z0-9-`'.\s]+$
Please advise.

You might use
^(?=[a-zA-Z0-9`'. -]+$)(?!.*[0-9'` -]{2})[a-zA-Z'][^\r\n.]*(?:\.[ a-z][^\r\n.]*)*$
Explanation
^ Start of string
(?=[a-zA-Z0-9`'. -]+$) Assert only allowed characters
(?!.*[0-9'` -]{2}) Assert not 2 consecutive listed characters
[a-zA-Z'] Match either a char a-zA-Z or apostrophe
[^\r\n.]* Optionally match any char except a newline or a dot
(?:\.[ a-z][^\r\n.]*)* Optionally repeat matching a dot only followed by a space or char a-z
$ End of string
Regex demo

Regular expression to validate a string starts with a char and contains allowed chars

I need help with my regular expression written in javascript.
I have tried using the regularExpression generator online, and the best i can come up with is the this:
^[a-z.-]{0,50}$
The expression must validate the following
String first char MUST start with a-z (no alpha)
String can contain any char in range a-z (no alpha), 0-9 and the characters dash "-" and dot "."
String can be of max length 50 chars
Examples of success strings
username1
username.lastname
username-anotherstring1
this.is.also.ok
No good strings
1badusername
.verbad
-bad
also very bad has spaces
// Thanks

Almost (assuming "no alpha" means no uppercase letters)
https://regex101.com/r/O9hvLP/3
^[a-z]{1}[a-z0-9\.-]{0,49}$
The {1} is optional, I put it there for descriptive reasons

I think this should cover what you want
^[a-z][a-z0-9.-]{0,49}$
That is starts a-z but then has 0-49 of a-z, 0-9 or .-
Live example: https://regexr.com/5k8eu
Edit: Not sure if you intended to allow upper and lowercase, but if you did both character classes could add A-Z as well!

If the . and - can not be at the end, and there can not be consecutive ones, another option could be:
^[a-z](?=[a-z0-9.-]{0,49}$)[a-z0-9]*(?:[.-][a-z0-9]+)*$
Explanation
^ Start of string
[a-z] Match a single char a-z
(?=[a-z0-9.-]{0,49}$) Assert 0-49 chars to the right to the end of string
[a-z0-9]* Match optional chars a-z0-9
(?:[.-][a-z0-9]+)* Optionally match either . or - and 1+ times a char a-z0-9
$ End of string
Regex demo

Regex for String in React

How shall we write a regular expression to validate a string that should have a length of a minimum of 1 character and a maximum of 50 characters, have both upper case and lower case, alphanumeric, include space, and have mostly used special characters like #,._-&$#? The first character should be either alphabet or number then the rest can be as mentioned above.
*If it is only one character then it should be an alphanumeric one
I have tried a regex with my limited knowledge which looks like
^[a-zA-z]*[a-zA-Z\d\-_#&$%#\s]{1,50}$
But I am not able to match the string if there is only one character given, can anyone guide me to fix this

You can use
/^(?=[\p{L}0-9])[\p{L}\p{N}_#,.&$%#\s-]{1,50}$/u
See the regex demo
Details
^ - start of string
(?=[\p{L}0-9]) - the first char must be a Unicode letter (\p{L}) or an ASCII digit
[\p{L}\p{N}_#,.&$%#\s-]{1,50} - one to fifty
\p{L} - any Unicode letter
\p{N} - any Unicode digit
_#,.&$%#- - any of these chars
\s - any whitespace
$ - end of string.

Issues in password regular expression

Hi all I am making a password regular expression in javascript test() method, It will take the following inputs
solution
/^(?=.*\d)^(?=.*[!#$%'*+\-/=?^_{}|~])(?=.*[A-Z])(?=.*[a-z])\S{8,15}$/gm
May contains any letter except space
At least 8 characters long but not more the 15 character
Take at least one uppercase and one lowercase letter
Take at least one numeric and one special character
But I am not able to perform below task with (period, dot, fullStop)
(dot, period, full stop) provided that it is not the first or last character, and provided also that it does not appear two or more times consecutively.
Can anyone one help me to sort out this problem, Thanks in advance

You may move the \S{8,15} part with the $ anchor to the positive lookahead and place it as the first condition (to fail the whole string if it has spaces, or the length is less than 8 or more than 15) and replace that pattern with [^.]+(?:\.[^.]+)* consuming subpattern.
/^(?=\S{8,15}$)(?=.*\d)(?=.*[!#$%'*+\/=?^_{}|~-])(?=.*[A-Z])(?=.*[a-z])[^.]+(?:\.[^.]+)*$/
See the regex demo
Details:
^ - start of string
(?=\S{8,15}$) - the first condition that requires the string to have no whitespaces and be of 8 to 15 chars in length
(?=.*\d) - there must be a digit after any 0+ chars
(?=.*[!#$%'*+\/=?^_{}|~-]) - there must be one symbol from the defined set after any 0+ chars
(?=.*[A-Z]) - an uppercase ASCII letter is required
(?=.*[a-z]) - a lowercase ASCII letter is required
[^.]+(?:\.[^.]+)* - 1+ chars other than ., followed with 0 or more sequences of a . followed with 1 or more chars other than a dot (note that we do not have to add \s into these 2 negated character classes as the first lookahead already prevalidated the whole string, together with its length)
$ - end of string.

Javascript Password compliance

I need the password to fulfill these requirements
Password must contain at least 8 word characters
Must have at least 1 numeric digit e.g. 3
Must have at least 2 uppercase characters but not in one consecutive sequence
It doesn't seems to work with this
var pos = myPass.value.search(/^([\w.-]{8,})(?=.*\d)((.*?[A-Z]){2,})$/);
No.3 is the hardest.

You are mixing consuming and non-consuming patterns adding limiting quantifiers to the consuming patterns that match a sequence, while you need to just check if a string matches some restrictive patterns or not. To add those restrictions you need lookaheads. (?=.*\d) is a correct part of your regex, other aren't.
Also, a RegExp#test() is a better method to check if a string matches or not.
Use
/^(?=\D*\d)(?=(?:(?:^|[^A-Z]+)[A-Z]){2}).{8,}$/.test(my‌​Pass.value)
See the regex demo
Or, to allow only letters, digits, underscores, dots and hyphens in the password:
/^(?=\D*\d)(?=(?:(?:^|[^A-Z]+)[A-Z]){2})[\w.-]{8,}$/.test(my‌​Pass.value)
^^^^^^
Details:
^ - start of string
(?=\D*\d) - after 0+ non-digits (\D*) at the start of the string, there must be a digit (\d) (note that after this lookahead execution, the regex index is still at the beginning of the string)
(?=(?:(?:^|[^A-Z]+)[A-Z]){2}) - there must be 2 sequences ((?:...){2}) of:
(?:^|[^A-Z]+) - start of string or one or more chars other than uppercase letters
[A-Z] - an uppercase letter.
.{8,} - any 8 or more chars other than those used in a linebreak sequence
OR
[\w.-]{8,} - 8 or more ASCII letters and digits, underscores, dots or hyphens
$ - end of string.