mongo get total population where the city starts with a digit

gistfile1.js

use test

db.zips.aggregate([
  { $project: {
    first_char: {$substr : ["$city",0,1]},
    pop: 1
  }
}, { $match: {
    first_char: /[0-9]/
  }
}, { $group: {
    _id: "Total",
    total: { $sum: '$pop' }
  }
}
]);