Finding the path from the filename

In a XSLT I receive a parameter with a full path and file name, like;

C:\Data\Test\File.xml

I have tried fumbling with substring and substring-before, but with no
luck.

What I need is the full path without the file name, like C:\Data\Test\

Sometimes parameter looks like this

..\Test\File.xml

And also here I need to find the path, like ..\Test\

Any suggestions in how to use the substring with some iteration?

/Thomas

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