Regex: creating string dynamically

Question

Need regular expression to check the password policy

I have a model:

"password_settings": {
    "enable_password_max_length": false,
    "enable_password_min_length": false,
    "enable_min_number_of_upper": false,
    "enable_min_number_of_lower": false,
    "enable_min_number_of_numbers": false,
    "enable_min_number_of_special_characters": false,
    "password_max_length_value": 24,
    "password_min_length_value": 1,
    "min_number_of_upper_value": 1,
    "min_number_of_lower_value": 0,
    "min_num_of_numbers_value": 1,
    "min_number_of_special_characters_value": 1
},

And if some flags are "True" I need to create Regex dynamically.

for example:

enable_min_number_of_numbers= true
enable_min_number_of_upper = true
min_num_of_numbers_value= 3
min_number_of_upper_value = 2

Regex string will be

^(?=.*[A-Z]).{"min_number_of_upper_value",}(?=.*\d).{"min_num_of_numbers_value",}$

but it does not work for me.

What I need: Help me please to create Regex where I can disable or enable some parts of Regex and where I can set counts from my json.

Thank you.

Copy your json into any of the tool that generate a class for you stackoverflow.com/questions/21611674/…. And simply var my_object = JsonConvert.DeserializeObject<MyAutoGeneratedClass>(jsonText) and voila your object is populated with all your data. — Self
– Self, Commented Apr 26, 2021 at 7:54
It will be var pattern = $@"^(?=(?:[^A-Z]*[A-Z]){{{min_number_of_upper_value}}})(?=(?:\D*\d){{{min_num_of_numbers_value}}})"; — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Apr 26, 2021 at 9:47
And why doing this whole logic in one Regex? "Always program so that you could give your successor your home address" — Klamsi
– Klamsi, Commented Apr 26, 2021 at 9:48

Wiktor Stribiżew · Accepted Answer · 2021-04-26 10:51:16Z

1

Given your min_number_of_upper_value contains the min threshold of upper ASCII letters allowed in the string and min_num_of_numbers_value defines the min amount of digits in the string, you can use

var pattern = $@"^(?=(?:[^A-Z]*[A-Z]){{{min_number_of_upper_value}}})(?=(?:\D*\d){{{min_num_of_numbers_value}}})";

Do not use $, this anchor requires the end of string. If your code requires a full string match, then use

var pattern = $@"^(?=(?:[^A-Z]*[A-Z]){{{min_number_of_upper_value}}})(?=(?:\D*\d){{{min_num_of_numbers_value}}}).*";

where you may further adjust the consuming (.*) pattern in case your requirements are more specific.

answered Apr 26, 2021 at 10:51

Wiktor Stribiżew

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4 Comments

Igor Strekha Over a year ago

Does not work. I added 5 upper laters and 4 digitals regex101.com/r/YaCkLT/1

Wiktor Stribiżew Over a year ago

@IgorStrekha See the correct test: regex101.com/r/M2nSxf/1. In an interpolated string literal, a single literal { char is defined with double {. Same goes for the } char. See the C# snippet generating ^(?=(?:[^A-Z]*[A-Z]){5})(?=(?:\D*\d){4}).* correctly.

Igor Strekha Over a year ago

thank you. But could me help more. (?:[^a-z]*[a-z]){5}) - this part for lower letters, (?=(?:[^A-Z]*[A-Z]){5}) - for Upper letters, (?=(?:\D*\d){4}) - for digits. What will be for special symbols? and What will be for minimum lengh and + maximum? Thank you Wiktor...

Wiktor Stribiżew Over a year ago

@IgorStrekha See how to match special chars, I cannot help here without detailed specs. As for total min,max length, just use (?=.{{{min},{max}}}$) or (?=(?s:.){{{min},{max}}}$) (to allow linebreaks) after ^.

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