The backslash before “(“ and “)” is there to “escape” the opening and closing brackets, i.e. to make them recognized as ordinary characters. Otherwise, Word would consider that the brackets define a regex “group” for back-reference.

“http://” searches for itself as a string of characters.

“[!^13]*” matches anything that is not a paragraph end mark.

Note that “\(http://*\)” or “\(https://*\)” would have done the job more simply!