You can use _.groupBy
and Regular Expressions, to group the elements
var regExp = /\[(.*?)\]/g;
console.log(_.groupBy(arr, function(currentItem) {
var match, result = "";
while ((match = regExp.exec(currentItem)) !== null) {
result += match[1];
}
return result;
}));
Output
{ Dim1Mem1: [ '[Dim1].[Mem1].&1', '[Dim1].[Mem1].&5' ],
Dim2Mem1: [ '[Dim2].[Mem1].&2', '[Dim2].[Mem1].&77' ],
Dim3Mem1: [ '[Dim3].[Mem1].&1' ] }