Hacking BibTeX

I needed to use the Cambridge LaTeX style for books, which is very pretty and compact, but I wanted to produce PDF's where clicking on the title of articles or books would open a browser with that paper. See for example the blue text below, which links to the Physical Review A article

This is a trick that I used in my own thesis, as well as with other people's thesis as well. The trick is very simple. In this case I copied the cambridgeauthordate.bst file into cambridgeauthordateurl.bst and enlarged the list of allowed fields to include URL

ENTRY
  { address
    author
    booktitle
    chapter
    edition
    editor
    howpublished
    institution
    journal
    key
    month
    note
    number
    organization
    pages
    publisher
    school
    series
    title
    type
    volume
    year
    URL
  }
  {}
  { label extra.label sort.label }

and then I edited the section that starts with FUNCTION { format.title … so that it read

FUNCTION {format.title}
{ title empty$
    { "" }
    { URL empty$
      { title }
      { "\href{" URL * "}{" * title * "}" * }
      if$
    }
  if$
}

Save the file and include it with \bibliographystyle{cambridgeauthordateurl} Notice how the sixth line simply concatenates, using the postfix operator *, five different strings into the final text that forms the title.