bibtex-search-backward-field is a compiled Lisp function in `bibtex.el'.

(bibtex-search-backward-field NAME &optional BOUND)

Search backward to find a BibTeX field of name NAME.
If a syntactically correct field is found, return a pair containing
the boundaries of the name and text parts of the field. The search
is limited by the optional arg BOUND. If BOUND is t the search is
limited by the beginning of the current entry. Do not move point.