src/vs/editor/contrib/snippet/snippetParser.ts/Transform/resolve
class Transform:	resolve(value: string): string {
		const _this = this;
		let didMatch = false;
		let ret = value.replace(this.regexp, function () {
			didMatch = true;
			return _this._replace(Array.prototype.slice.call(arguments, 0, -2));
		});
		// when the regex didn't match and when the transform has
		// else branches, then run those
		if (!didMatch && this._children.some(child => child instanceof FormatString && Boolean(child.elseValue))) {
			ret = this._replace([]);
		}
		return ret;
	}