CODEWITHSUNDEEP
c#regexstring-matching
marcellorvalle
marcellorvalle

Reputation: 1679

Regex to match a pattern only if its length is bigger than a value

I have 2 strings and want to know

  1. if string 2 have a subset of the characters of string 1 on the same order

  2. this subset has a minimun size determined by me.

For example:

string 1 = stackoverflow
<br>minimun size = 5

string 2a = stack (MATCH)
string 2b = stac  (DO NOT MATCH)
string 2c = staov (MATCH)
string 2d = staZov (DO NOT MATCH)
string 2e = eflow (MATCH) 
string 2f = ewolf (DO NOT MATCH)
string 2g = somethingstacksomething (MATCH)

I am building the regex programatically, so the first part of the problem could be solved with the expression: (s)?(t)?(a)?(c)?(k)?(o)?(v)?(e)?(r)?(f)?(l)?(o)?(w)?

But i can't figure out how to apply the "minimun character" rule. Is it possible to do it using regex?

thanks in advance!

EDIT: Added another example to complete the problem specification. Also, if you want to know, this is part of a method to evaluate the password strength specified by a user. If he defines a password derived from some other information (login, born date, name, etc) we should warn him.

Upvotes: 3

Views: 1397

Answers (3)

ridgerunner
ridgerunner

Reputation: 34395

Interesting question! There is a way to do it, (but you aren't going to like it). It results in a potentially large (and possibly slow) regex. Assuming string1 is ABCD. Then here are the regexes for all the different length requirements:

length  = 4;
regex = @"
  ABCD
  ";

length  = 3;
regex = @"
    ABCD?
  | ABC?D
  | AB?CD
  | A?BCD
  ";

length  = 2;
regex = @"
    ABC?D?
  | AB?CD?
  | AB?C?D
  | A?BCD?
  | A?BC?D
  | A?B?CD
  ";

length  = 1
regex = @"
  [ABCD]
  ";

I haven't thought through the way you would go about assembling this programmatically, but I'm sure that it can be done. However, a large regex having lots of OR'ed alternatives may likely be slower that using the "match then check the length of the matched string" method you are currently using.

I scratched my head for quite a while on this one and I'm pretty sure there is no simple/clever regex construct that does what you are asking.

Upvotes: 1

Security Hound
Security Hound

Reputation: 2548

But i can't figure out how to apply the "minimun character" rule.

After you determine the possible matches. You need to determine the length of the result.

You can simply use String.Length to determine that.

Upvotes: 2

SLaks
SLaks

Reputation: 887807

You could add a lookahead that ensures that there are five characters: (?=.{5})s?t?a?c?k?o?v?e?r?f?l?o?w?

Upvotes: 2

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