How to build such a regular expression?

Question

C++ / C#

You want to write a function that returns a regular expression to check if a string is an unsigned integer (with arbitrary indentation).

std::regex getRegex(unsigned length, unsigned minLength)
{
    std::assert(minLength <= length);
    // ...
}

Valid examples:

12345
 2345
  345
   45
    5

It is possible directly through OR, but then the number of ORs will be equal to length - minLength, which is not good if there is a large difference between them (the regular expression will be very long).

Answer 1

(?= *\d+).{5}
https://www.debuggex.com/r/zavW3W9gxmoFKzJQ