Pattern: Quantifier that implies incorrect matching behavior
Issue: -
Confusing quantifiers are ones which imply one thing but don't deliver on that.
An example of this is (?:a?b*|c+){4}. The group is quantified with {4} which implies that at least 4 characters will be matched, but this is not the case. The whole pattern will match the empty string. It does that because in the a?b* alternative, it's possible to choose 0 many a and b. So rather than {4}, {0,4} should be used to reflect the fact that the empty string can be matched.
/* eslint regexp/confusing-quantifier: "error" */
/* ✓ GOOD */
var foo = /a*/;
var foo = /(a|b|c)+/;
var foo = /a?/;
/* ✗ BAD */
var foo = /(a?){4}/; // warns about `{4}`
var foo = /(a?b*)+/; // warns about `+`