match-beginning

match-beginning is a built-in function in `C source code'.

(match-beginning SUBEXP)

Return position of start of text matched by last search.
SUBEXP, a number, specifies which parenthesized expression in the last
regexp.
Value is nil if SUBEXPth pair didn't match, or there were less than
SUBEXP pairs.
Zero means the entire text matched by the whole regexp or whole string.