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/*
  from: http://serverName.net:7777/foo/api/action/region
  to:   /foo/api/action/region
*/

//1
[''].concat(decodeURI('http://serverName.net:7777/foo/api/action/region').split('/').slice(4)).join('/');

//2 można użyć (.+:[0-9]+) - prosty regex gdy zawsze będzie numer portu
'http://serverName.net/foo/api/action/region'.replace(/^https*:\/\/[\w\.\:\d]+/, '');