Plugin for validate attribute values in xml

Hi Guys,

I'm started to look at plugin development because I want to create a pugin that can validate some attributed in xml files.
The case is following:
I have many xml files, these files have the same structure and tags may have attributes. The correct values of these attributes should be the names of the files in some project directory. So, this should validates dynamically. If there is a value in the attribute that don't matches with any file name in specified directory that it should be highlights as an error.

I read some developers articles but it is still not clear for me how to work with file content, how to highlighte some piece of code as an error, how to make complition and where can I find the necessary documentation.
Can somebody help me with the documentation or advise?