- (?=pattern) 零宽正向先行断言(zero-width positive lookahead assertion)
- (?!pattern) 零宽负向先行断言(zero-width negative lookahead assertion)
- (?<=pattern) 零宽正向后行断言(zero-width positive lookbehind assertion)
- (?<!pattern) 零宽负向后行断言(zero-width negative lookbehind assertion)
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