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# (?<=pattern)

A zero-width positive look-behind assertion. For example, /(?<=\t)\w+/ matches a word that follows a tab, without including the tab in $& . Works only for fixed-width look-behind.
# (?<!pattern)

A zero-width negative look-behind assertion. For example /(?<!bar)foo/ matches any occurrence of "foo" that does not follow "bar". Works only for fixed-width look-behind.

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(?<...)
A zero-width positive lookbehind assertion. For example, /(?<bad)boy/ matches the word boy that follows bad, without including bad in $&. This works only for fixed-width lookbehind.