bibtex-cite-matcher-alist is a variable defined in `bibtex.el'.
Its value is
]*{\\([^}]+\\)}" . 1))

Alist of rules to identify cited keys in a BibTeX entry.
Each rule should be of the form (REGEXP . SUBEXP), where SUBEXP
specifies which parenthesized expression in REGEXP is a cited key.
Case is significant.
Used by `bibtex-search-crossref' and for font-locking.
Set this variable before loading BibTeX mode.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 23.1 of Emacs.