In your case remove the g
modifier from the end, for example:
var dateRegex = /\/Date\((\d+)\)\//;
dateRegex.test("Date(1286445750000)"); // true
dateRegex.test("Date(1286445750000)"); // true
dateRegex.test("Date(1286445750000)"); // true
dateRegex.test("Date(1286445750000)"); // true
dateRegex.test("Date(1286445750000)"); // true
It's a bug with the way regexes are implemented in ECMAScript 3, there's a great post on the details here.