I was helping a developer convert some IIS rewrite rules over to use Undertow's predicate language and I found something interesting.  Here is an excerpt from an IIS rewrite.
 

<rule name="LowerCase Rule" stopProcessing="true">
  <match url="[A-Z]" ignoreCase="false" />
  <conditions logicalGrouping="MatchAll" trackAllCaptures="false">
    <add input="{URL}" pattern="^.*\.(css|js|jpg|jpeg|png|gif|pdf|csv|xml|ttf|eot|svg|woff|ico|woff2|json|swf|xap|cfm)$" ignoreCase="true" negate="true" />
  </conditions>
  <action type="Redirect" url="{ToLower:{URL}}" redirectType="Permanent" />
</rule>

 
The interesting part here being the {ToLower:{URL}}
 
Undertow's Predicate language doesn't appear to have any sort of basic string manipulation functions like lower or upper casing.
 
It would appear IIS's rewrite module has out-of-the-box support for the following "string functions"

• toLower
• UrlEncode
• UrlDecode

which is quite odd that I don't see a "toUpper" in there!  
 
https://docs.microsoft.com/en-us/iis/extensions/url-rewrite-module/url-rewrite-module-configuration-reference#string-functions
 
I recommend implementing these as exchange attributes themselves which can wrap another attribute (decorator pattern) in the format of

%{NameHere:attribute-to-wrap}

So an example would be:

%{toLower:%{METHOD}}

This would allow recursive nesting of transformations like so:

%{URLDecode:%{toLower:%{i,myHeader}}}

Please note, the exchange attribute parser needs a fix applied to allow parsing of an attribute that nests another attribute. https://issues.redhat.com/browse/UNDERTOW-2273