This is called a negated lookbehind.
Matches x only if x is not preceded by y.This is called a negated lookbehind.
For example, /(?<!-)\d+/ matches a number only if it is not preceded by a minus sign.
/(?<!-)\d+/.exec('3') matches "3".
/(?<!-)\d+/.exec('-3') match is not found because the number is preceded by the minus sign.
Submitted by anonymous - 3 years ago