Info-get-token is a compiled Lisp function in `info.el'.

(Info-get-token POS START ALL &optional ERRORSTRING)

Return the token around POS.
POS must be somewhere inside the token.
START is a regular expression which will match the
beginning of the tokens delimited string.
ALL is a regular expression with a single
parenthesized subpattern which is the token to be
returned. E.g. '{(.*)}' would return any string
enclosed in braces around POS.
ERRORSTRING optional fourth argument, controls action on no match:
nil: return nil
t: beep
a string: signal an error, using that string.