var strA = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
var strB = "abcdefghijklmnopqrstuvwxyz";
var strC = "1234567890";
var strD = "-_";
var str = strA + strB + strC + strD;
var qfr = '/' + 'a' + '/';

var pattern = /a/;

// would like to use it as var pattern = / variable /;
// how would i do that;


var result = str.match(pattern);
for(i = 0; i < result.length; i++) {
  return true;
} else {
  return false;
}