How to write a function that will find the number in the address and print it as a result?

You will need to look at or enforce some kind of structure in your address string - because without that, you can't reliably automatically process it.

For example, how do you pick the house number out of this:

Flat 12, 23 Albert row, London L17 8DS

And also get the house number from this:

Helton House, 1 Laung Street, Bananatown, Hants GU32 8RT

And this:
Number One, LondonAll of which are valid UK addresses!

Without structure and a defined input format, your code will make more mistakes than correct extractions!

#include <regex>
#include <iterator>
#include <iostream>
#include <string>

using namespace std;

int House_number(string address) // warning, this may throw
{
  regex number_regex("[\\d]+");
  auto number_begin = sregex_iterator(address.begin(), address.end(), number_regex);

  return stoi( number_begin->str());
}



int main()
{
  const string sh = "Where is 221b Baker Street";
  const string tc = "Where is the cat?";
  try
  {
    cout << House_number(sh) << endl;
    cout << House_number(tc) << endl;
  }
  catch (exception & e)
  {
    cerr << "the following exception occurred: " << e.what() << endl;
  }
}

Anyway, as Richard correctly pointed out, this way you're not able to find Sherlock Holmes.