If you want readability you can write

    ^(?P<country_code>\+[0-9]{1,3}-?)?[0-9]{3}-[0-9]{3}-[0-9]{4}$

Most regex engines even have a verbose mode (typically the x flag) where this is a valid regex:

  ^
  (   
      \+
      [0-9]{1,3}
      -?
  )?   # optional country code
  [0-9]{3}
  -
  [0-9]{3}
  -
  [0-9]{4}
  $

I am not sure if everyone writes unreadable regex because the reputation of regex makes it seem pointless to try, or if being witty one-liners is a critical part of the value proposition of regex