What is positive lookahead

Let's start with condition 1.After matching a the engine enters the positive lookahead and it notes that now it is going to match a positive lookahead.In positive lookbehind the regex engine searches for an element ( character, characters or a group) just before the item matched.The positive lookahead is similar in that it tries to match the pattern in the lookahead.A parameter of some parsing algorithms;Any valid regular expression can be used inside the lookahead.

@antario, i was confused about the negative look ahead/behind case in regex for a while and this site has a great explanation.So the string has to be exactly eight characters then it has to end, as evidenced by $.When it matches an a, which is after is in the sentence then the positive lookahead process starts.