Whether src begins a URL scheme prefix (e.g. http://) at pos.
src
http://
pos
A prefix-only check, used for single-bracket links ([ftp://x Label]) whose URL portion is already delimited by the label, so the full URL body does not need to be validated here.
[ftp://x Label]
Whether
srcbegins a URL scheme prefix (e.g.http://) atpos.A prefix-only check, used for single-bracket links (
[ftp://x Label]) whose URL portion is already delimited by the label, so the full URL body does not need to be validated here.