Redmine - Help: RE: Custom Fields - how to style URLs?

Oh yeah now I see it. Sorry, I didn't read your post word for word.
Your problem gets a bit tricky there. You cannot influence the form the input values of the custom field are displayed. The regex is mainly used to check the format of the input value and can be used to extract parts used by the URL. It does not affect the display of the value itself.
What you could do is to use the 'long text' field instead of the normal one. The advantage of the 'long-text' field is that you can add formatting like in the wiki pages. Then you can create the link with whatever text you like to display.