Regex for matching string, contains only number and count must be 2 min values

Question

Is there any regex for validating length of a number string for 2 consecutive digits or number

Example :

"12541256442545245215" = Count (20)
"125412564425452452"= Count (18)

Need a regex to check a string which contains only number and count must be 18 or 20.

I tried using the below regex but it allows length 19 also.

^[0-9.]{18,20}$

Answer 1

You can use this regex:

^[0-9.]{18}(?:[0-9.]{2})?$

About this regex:

^            # start
[0-9.]{18}   # match digit or DOT 18 times
(?:          # start non-capturing group
   [0-9.]{2} # match digit or DOT 2 times
)?           # end non-capturing group, ? makes this group *optional*
$            # end

If you don't want to allow DOT then use:

^[0-9]{18}(?:[0-9]{2})?$