Код:
var text = "some text1 and [link=http://google.com/]some_link[/link]";
var pattern:RegExp = new RegExp("\\[link=([^\\]]+)\\]([^\\[]+)\\[/link\\]", "ig")
//var pattern:RegExp = /\[link=([^\]]+)\]([^\[]+)\[\/link\]/gi;
trace(text.replace(pattern, "<a href='$1'>$2<\\a>"));
// some text1 and <a href='http://google.com/'>some_link<\a>