Using Stamen map tiles in Leaflet without loading the tile.stamen.js file

gistfile1.js

var tonerUrl = "http://{S}tile.stamen.com/toner/{Z}/{X}/{Y}.png";

var url = tonerUrl.replace(/({[A-Z]})/g, function(s) {
    return s.toLowerCase();
});

var basemap = L.tileLayer(url, {
    subdomains: ['','a.','b.','c.','d.'],
    minZoom: 0,
    maxZoom: 20,
    type: 'png',
    attribution: 'Map tiles by <a href="http://stamen.com">Stamen Design</a>, under <a href="http://creativecommons.org/licenses/by/3.0">CC BY 3.0</a>. Data by <a href="http://openstreetmap.org">OpenStreetMap</a>, under <a href="http://creativecommons.org/licenses/by-sa/3.0">CC BY SA</a>'
});

basemap.addTo(map);

// Check out the tile.stamen.js file for which options you have to pass depending on the style and mapping library

I don't understand why you replace all '{S}', '{X}', etc. with lowercase instead of directly write them in lowercase in tonerUrl ?

Thank you very much for the code, very useful.