I'm trying to understand the positive and negative lookahead, but I think I'm missing something.
q(?=u)
What I understand this regex to mean is: "match a q that is followed by a u", so I get a match with the string "quit", but getting only a group with 'q'.
But with the regex q(?=u)i, I don't get any result with the string "quit". Why does happen? Probably this regex doesn't have sense, but I would like to know what it means in order to understand the lookahead.
The lookahead doesn't consume its text. qu is different from q(?=u) -- the latter matches just a q, but requires it to be followed by u (which is however not captured or consumed).
And that's why q(?=u)i cannot match -- the q needs to be followed by u and i at the same time, which is impossible. In other words, it would find and capture qi but only if the q was immediately followed by u which is obviously not true if it is followed by i.
If you want to match qui, the regex for that is qui.
Negative lookahead is indispensable if you want to match something not followed by something else. When explaining character classes, this tutorial explained why you cannot use a negated character class to match a q not followed by a u. Negative lookahead provides the solution: q(?!u). The negative lookahead construct is the pair of parentheses, with the opening parenthesis followed by a question mark and an exclamation point. Inside the lookahead, we have the trivial regex u.
Positive lookahead works just the same. q(?=u) matches a q that is followed by a u, without making the u part of the match. The positive lookahead construct is a pair of parentheses, with the opening parenthesis followed by a question mark and an equals sign.
