| pageClass | sidebarDepth | title | description | since |
|---|---|---|---|---|
rule-details |
0 |
regexp/optimal-lookaround-quantifier |
disallow the alternatives of lookarounds that end with a non-constant quantifier |
v0.8.0 |
plugin:regexp/recommended config.
disallow the alternatives of lookarounds that end with a non-constant quantifier
Non-constant quantifiers are quantifiers that describe a range (e.g. ?, *,
+, {0,1}, {5,9}, {3,}). They have to match some number of times (the
minimum) after which further matches are optional until a certain maximum (potentially infinite) is reached.
It's obvious that /ba{2}/ and /ba{2,6}/ will match differently because of
the different quantifiers of a but that not the case if for lookarounds. Both
/b(?=a{2})/ and /b(?=a{2,6})/ will match strings the same way. I.e. for the
input string "baaa", both will create the same match arrays. The two regular
expressions are actually equivalent, meaning that (?=a{2}) is equivalent to
(?=a{2,6}).
More generally, if a non-constant quantifier is an end of the expression tree of a lookahead, that quantifier can be replaced with a constant quantifier that matched the element minimum-if-the-non-constant-quantifier many times. For lookbehinds, the non-constant quantifier has to be at the start of the expression tree, as lookbehinds are matched from right to left.
/* eslint regexp/optimal-lookaround-quantifier: "error" */
/* ✓ GOOD */
// lookaheads
var foo = /\w+(?=\s*:)/;
// lookbehinds
var foo = /(?<=ab+)/;
/* ✗ BAD */
// lookaheads
var foo = /(?=ab+)/; // == /(?=ab)/
var foo = /(?=ab*)/; // == /(?=a)/
var foo = /(?!ab?)/; // == /(?!a)/
var foo = /(?!ab{6,})/; // == /(?!ab{6})/
// lookbehinds
var foo = /(?<=a+b)/; // == /(?<=ab)/
var foo = /(?<!\w*\s*,)/; // == /(?<!,)/Nothing.
This rule was taken from eslint-plugin-clean-regex.
This rule is compatible with clean-regex/optimal-lookaround-quantifier rule.
This rule was introduced in eslint-plugin-regexp v0.8.0