Support regexp inside the escaped part of moment.js pattern

We have come to the situation where it would be very helpful to be able to add some regexp support to the index name pattern. 

That pattern is actually passed to moment.js, and there's the possibility to escape a part of it (with square brackets). We need to be able to allow that text inside square brackets (therefore harmless for moment.js) to be a (rather limited) regexp. 

A code example works like this:
```
// output from moment.js
var possible = [ '.*tres 2001','.*tres 2002','.*tres 2003' ]; 

// actual indices available
var p = ['1dos 2001','dostres 2001','12345678tres 2001','holatres 2003','holatres 2004']; 

//indices that will be finnally taken into account
var indices = []; 

_.each(possible, function(patt){
  _.each(p, function(idx){
    var result = new RegExp(patt).exec(idx);
    if(result !== null)indices.push(idx);
  });
});

indices => ['dostres 2001','12345678tres 2001','holatres 2003']
```

Maybe there's something you would do different, so I'm totally up for comments and fixes. But I hope I made clear what our requirement is.

Thanks.