mail-header-parse

mail-header-parse is a compiled Lisp function in `mailheader.el'.

(mail-header-parse PARSING-RULES HEADERS)

Apply PARSING-RULES to HEADERS.
PARSING-RULES is an alist whose keys are header names (symbols) and whose
value is a parsing function. The function takes one argument, a string,
and return a list of values, which will destructively replace the value
associated with the key in HEADERS, after being prepended with the original
value.