bibtex-field-strings-alist is a variable defined in `bibtex.el'.
Its value is

Alist of regexps that M-x bibtex-clean-entry replaces by string constants.
Each element has the form (FIELDS REGEXP TO-STR), where FIELDS is a list
of BibTeX field names. In FIELDS search for REGEXP, which are replaced
by the BibTeX string constant TO-STR.
Space characters in REGEXP will be replaced by "[ \t\n]+".

You can customize this variable.