Re: Explicitly numbered subgroups in regular expressions

[Richard Stallman]

    What it does is add a new regexp syntax \(?<num>:<regexp>\) which is like
    \(<regexp>\) except that it specifies explicitly the number of the subgroup.
    E.g. (and (string-match "\\(?3:a\\)" "a") (match-data)) returns (0 1 nil
    nil nil nil 0 1).  There is no backward compatibility issue with this patch:
    such regexps are currently rejected as invalid.

The feature seems useful, but how does this affect the numbering of
groups that don't specify a number?  That has to be done right, then
documented in the two manuals.