bibtex-valid-entry is a compiled Lisp function in `bibtex.el'.

(bibtex-valid-entry &optional EMPTY-KEY)

Parse a valid BibTeX entry (maybe without key if EMPTY-KEY is t).
A valid entry is a syntactical correct one with type contained in
`bibtex-BibTeX-entry-alist'. Ignore @String and @Preamble entries.
Return a cons pair with buffer positions of beginning and end of entry
if a valid entry is found, nil otherwise. Do not move point.
After a call to this function `match-data' corresponds to the header
of the entry, see regexp `bibtex-entry-head'.