Regex issue in Angular with password pattern

Question

Having some problem with FormControl() Validators.pattern() with this regex expression for validate password with 1 char, 1 digit, min 8 chars, max 16 chars.

Tested passwords that pass the validators:

asdfqwer //that should not pass the validator

zxcvasdf //that should not pass the validator

asdfqwer2 //that's correct

Tested password that do not pass the validators:

programm //that's correct

Trying this pattern in server-side or using online regex test platforms there are no problems

// some code
password: new FormControl('',
    [
      Validators.minLength(8),
      Validators.maxLength(16),
      Validators.pattern('^$|^(?=.*[A-Za-z])(?=.*\d)[A-Za-z\d]{8,16}$')
    ]),
// some code

why you have ^$ at start ? ^(?=.*[A-Za-z])(?=.*\d)[A-Za-z\d]{8,16}$ this is enough you don't need to check length also — Code Maniac
– Code Maniac, Commented Aug 7, 2019 at 9:18

Wiktor Stribiżew · Accepted Answer · 2019-08-07 09:19:29Z

You may use

Validators.pattern('(?:(?=[^A-Za-z]*[A-Za-z])(?=\\D*\\d)[A-Za-z\\d]{8,16})?')

Angular will enclose the string pattern with ^ and $ automatically, making it '^(?:(?=[^A-Za-z]*[A-Za-z])(?=\\D*\\d)[A-Za-z\\d]{8,16})?$'.

Note the double backslashes: in a string literal, you must escape a backslash with another backslash to define a literal backslash, which is a regex escape char.

Since the whole pattern is enclosed with an optional non-capturing group, an empty string will also be matched with the pattern.

The (?=[^A-Za-z]*[A-Za-z]) and (?=\\D*\\d) now have no .* which is in line with the principle of contrast.

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