Regex Fun

Regex is one of powerful tool for pattern matching

today we are going to solve one of the regex problem for password matching

Question:
- The password must contain atleast one special character among # ! _ $ @
- The password must contain atleast two numbers
- The password must contain atleast one upper case alphabet and one lower case alphabet.
- The password must have a minimum length of 8.
- The password must have a maximum length of 25.

Example Input/Output:
Example 1:
Input:
kiC_3b0x3r
Output:
VALID
Example 2:
Input:
m@d31nindia
Output:
INVALID
Explanation:
No alphabet in uppercase.
Example 3:
Input:
M1kT!s0
Output:
INVALID
Explanation:
Minimum length  must be 8

Answer:
^(?=(?:[^a-z]*[a-z]){1})(?=(?:[^A-Z]*[A-Z]){1})(?=(?:[^0-9]*[0-9]){2})(?=(?:[^#!_$@]*[#!_$@]*){1}).{8,25}$

Explanation:
^(?=(?:[^a-z]*[a-z]){1})  check for atleast one a-z
(?=(?:[^A-Z]*[A-Z]){1})  check for atleast one a-z
(?=(?:[^0-9]*[0-9]){2})   check for atleast two numbers
(?=(?:[^#!_$@]*[#!_$@]*){1}).{8,25}$  check for atleast one special characters

{8,25} => minimum length is 8 and maximum length is 25

Things which are confusing are

?: -> is called non-capturing group which is bit confusing lets understand using a example

([0-9]+)(?:st|nd|rd|th)?

we have 1st,2nd,3rd,4th,.............

That will match numbers in the form 1, 2, 3... or in the form 1st, 2nd, 3rd,... but
it will only capture the numeric part.

?= -> lookheads there is good source to learn it easy try below link
          Lookahead

{} -> No of occurance

Resources:
http://www.regular-expressions.info/lookaround.html
https://stackoverflow.com/questions/2973436/regex-lookahead-lookbehind-and-atomic-groups
https://regex101.com/library/fR8728

any doubts drop in the comments