Regular expressions cannot be used for indent and unindent strings. Maybe following is enough - have not tested it:

/Indent Strings = "">"
/Unindent Strings = "</"

A small question: Are the tag names in double quotes in your XML files highlighted as strings?

Yes, then that's one of the reasons why the indent string does not work because the " is part of a string and indent strings are ignored in strings. Make following changes:

1. In the language definition line insert or modify the string character definition to String Chars = '. With this modification the simple indent string should work (not tested in lack of an example).
   
2. To highlight double quoted strings with the color of strings or a different color add following line above /C1"..":
   
   /Marker Characters = """"
   
   And create a color group for the marker characters like:
   
   /C8"Double quoted strings"
   ""
   
   Now you can specify the color for double quoted strings too.

But all these changes will not help for the indent string "">".

Reason: The double quote character is used in the wordfile as indent string delimiter. So UE reads "" and >" and ignores that because invalid syntax for indent strings.

I don't know if there is an escape character which can be used to tell UE that the following " character should be part of the indent string and is not a indent string delimiter. Ask IDM support by email.

But I think, there is none. If you always edit the same sort of XML files and there are not too many different name= and </name> tag pairs you can specify all that strings as indent and unindent strings.