gistfile1.as

package
{
	import flash.display.Sprite;
 
	public class Main extends Sprite
	{
		public function Main()
		{
			const ELEMENTS:String = "Na Sb"; // 元素記号を半角スペース区切りで並べる
			var str:String = "[Na(cryptand 222)]3Sb7";
			str = str.replace(/(?<=])(\d+)/g, "<sub>$1</sub>");
			for each (var element:String in ELEMENTS.split(" "))
			{
				str = str.replace(new RegExp("(?<=" + element + ")(\\d+)", "g"), "<sub>$1</sub>");
			}
			trace(str);
		}
	}
}