One RegExp to match urls, email addresses, Twitter usernames and hashtags.

rx.js

  function combine() {
    var ret = '', flags = '';
    [ ].forEach.call(arguments, function (exp) {
      exp = exp.toString();
      if (exp[0] !== '/') {
        flags = exp;
      } else {
        ret += exp.substr(1, exp.length - 2);
      }
    });

    return new RegExp(ret, flags);
  }

  var rxALL = combine(/(?=\w)/,
  /*Email    *//([a-z0-9_\-\.]+@(?:[a-z0-9\-]+\.)+[a-z\-]+)|/,
  /*URL      *//((?:(?:https?\:\/\/)?(?:[a-z0-9\-]+\.)+[a-z]{2,3}(?!\w))(?:[\S]*[a-z0-9\/=&])?)|/,
  /*Username *//(@[a-z0-9_]+)|/,
  /*Tag      *//(#[a-z0-9_]{2,})/,
            /(?!\w)/, 'gi');

rx.min.js

var rxALL = /(?=\w)([a-z0-9_\-\.]+@(?:[a-z0-9\-]+\.)+[a-z\-]+)|((?:(?:https?\:\/\/)?(?:[a-z0-9\-]+\.)+[a-z]{2,3}(?!\w))(?:[\S]*[a-z0-9\/=&])?)|(@[a-z0-9_]+)|(#[a-z0-9_]{2,})(?!\w)/gi;