Diggit Drupalextras

Hello,
I have a content type where the article author can insert a youtube video link. In the teaser view i want to extract the from the link the video code and render the image only. Let say my video url field is called : "field_url_vid" (text field).
My youtube link is http://www.youtube.com/watch?v=XYZZZ. In this case i want to introduce a function in template.php that alters the output :
str_replace ('http://www.youtube.com/watch?v=', '', $field_url_vid');
to keep only the code.
I have been looking around and followed tutorials but all of them explain the way to alter existing preprocessing/processing functions. I did not get any explanation on how to use them for custom cck field.
Thanks in advance for any suggestion