# Assertions — Other assertions

> > [!NOTE] > The ? character may also be used as a quantifier. Characters Meaning x(?=y) Lookahead assertion: Matches "x" only if "x" is followed by "y". For example, /Jack(?=Sprat)/ matches "Jack" only if it is followed by "Sprat"./Jack(?=Sprat|Frost)/ matches "Jack" only if it is followed by "Sprat

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&gt; [!NOTE] &gt; The ? character may also be used as a quantifier.

Characters Meaning x(?=y) Lookahead assertion: Matches "x" only if "x" is followed by "y". For example, /Jack(?=Sprat)/ matches "Jack" only if it is followed by "Sprat"./Jack(?=Sprat|Frost)/ matches "Jack" only if it is followed by "Sprat" or "Frost". However, neither "Sprat" nor "Frost" is part of the match results. x(?!y) Negative lookahead assertion: Matches "x" only if "x" is not followed by "y". For example, /\d+(?!\.)/ matches a number only if it is not followed by a decimal point. /\d+(?!\.)/.exec('3.141') matches "141" but not "3". (?&amp;#x3C;=y)x Lookbehind assertion: Matches "x" only if "x" is preceded by "y". For example, /(?&amp;#x3C;=Jack)Sprat/ matches "Sprat" only if it is preceded by "Jack". /(?&amp;#x3C;=Jack|Tom)Sprat/ matches "Sprat" only if it is preceded by "Jack" or "Tom". However, neither "Jack" nor "Tom" is part of the match results. (?&amp;#x3C;!y)x Negative lookbehind assertion: Matches "x" only if "x" is not preceded by "y". For example, /(?&amp;#x3C;!-)\d+/ matches a number only if it is not preceded by a minus sign. /(?&amp;#x3C;!-)\d+/.exec('3') matches "3". /(?&amp;#x3C;!-)\d+/.exec('-3') match is not found because the number is preceded by the minus sign.

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