Negative Lookahead (?! ... ) and Positive Lookahead(?= ... )

• Negative lookahead is indispensable if you want to match something not followed by something else.
• Positive lookahead is indispensable if you want to match something followed by something else.
• Positive/Negative Lookahaed は「結果マッチ」には含まれない。テストのみ行う
• マッチしなかったら、成功となり、カーソルは前に戻る。

/^a(?!xyz)b$/

a = false
axyzb = false
ab = true : ab