JavaScript: Invalid quantifier in regex

The regex is constructed on the fly, but I've output it to firebug:

(.{1,38})(+|$\n?)

the error is

invalid quantifier +|$\n?)

I'm not sure where to start.

The actual code is:

var re = top.RegExp;
var regex = new re("(.{1," + len + "})(+|$\\n?)", "gm");

UPDATE: Per Bennor McCarthy's instructions, I changed the code to this:

 var regex = new re("(.{1," + len + "})(\+|\$\\n?)", "gm");

Firebug still tells me this:

invalid quantifier +|$\n?)
[Break on this error] var regex = new re("(.{1," + len + "})(\+|\$\\n?)", "gm"); 

ANOTHER UPDATE Looks Like I had to double slash it and this solved the problem!

final code

var regex = new re("(.{1," + len + "})(\\+|\\$\\n?)", "gm");