/g indicates the match should be performed repeatedly, so

/^(...)....(\d+)/g
[download]

is basically

/
    ^(...)....(\d+)
    (?: (?s:.*?) ^(...)....(\d+)
    (?: (?s:.*?) ^(...)....(\d+)
    (?: (?s:.*?) ^(...)....(\d+)
    etc
    )?)?)?
/xg
[download]

(But with less ability to backtrack)