Positive and Negative Lookahead, regexp

Lookahead and Lookbehind Zero-Width Assertions
The negative lookahead construct is the pair of round brackets, opening bracket followed by a question mark and an exclamation point
<br />#a "q" not followed by a "u".<br /># 後面不是跟著u 的 q, 是要找q<br />q(?!u)<br /><br />
把 1234567 這樣的數字在印出加上逗號變成 1,234,567 這樣的格式
<br />num.to_s.gsub(/(\d)(?=\d{3}+(\.\d*)?$)/, '\1,')<br /><br />數字後面跟的數字(?=) 數目要是 3的倍數(\d{3}) 直到結尾 ($)<br />