Vim to highlight only a submatch

Question

Folks, I'm writing a syntax highlighting script for vim, but I'm struggling on how to highlight only a submatch of a pattern.

For example, suppose that I want to highlight any number within #, like #42# to highlight only the 42 and not the #.

I don't fully understand what the \@= operator does, but I'm trying to use it like so:

syn match Number "\(#\)\@=\d\+\(#\)\@="

Edit: Just complementing Kent's answer, from the man page:

                            */\zs*
\zs Matches at any position, and sets the start of the match there: The
    next char is the first char of the whole match. |/zero-width|
    Example: >
        /^\s*\zsif
<   matches an "if" at the start of a line, ignoring white space.
    Can be used multiple times, the last one encountered in a matching
    branch is used.  Example: >
        /\(.\{-}\zsFab\)\{3}
<   Finds the third occurrence of "Fab".
    {not in Vi} {not available when compiled without the |+syntax| feature}
                            */\ze*
\ze Matches at any position, and sets the end of the match there: The
    previous char is the last char of the whole match. |/zero-width|
    Can be used multiple times, the last one encountered in a matching
    branch is used.
    Example: "end\ze\(if\|for\)" matches the "end" in "endif" and
    "endfor".
    {not in Vi} {not available when compiled without the |+syntax| feature}

Answer 1

You can use \zs and \ze:

syn match Number "#\zs\d\+\ze#"