/(http:\/\/.+?)[\s|\n]/ig creates a regular expression object with the pattern (http:\/\/.+?)[\s|\n] and flags i (case insensitive) and g (global).

The pattern itself searches for (and retains for later) the characters "http://" followed by one or more (+) single characters (.; 'single character' means any character except a newline). The question mark (?) makes the 'one or more single characters' what is called ungreedy (which is important but we can gloss over it right now). Next, [\s|\n] matches either a whitespace character (tab, space, newline), a vertical pipe character or a newline character (I think Wildhoney really meant just \s so that the match stops at whitespace (thanks to the ungreedy repetition earlier)).

Put simply, we match all instances of "http://" followed by one or more characters, followed by a whitespace character.