Delete the first known character in a string with sed

There's no need to capture and replace.

sed 's/^@//'

This replaces the character @ when it's first ^ in the string, with nothing. Thus, deleting it.

sed 's/^@\(.*\)/\1/'

^ means beginning of the string

@ your known char

(.*) the rest, captured

then captured block will be substituted to output Sorry, can't test it at the moment, but should be something like that