no-contradiction-with-assertion.md

pageClass	sidebarDepth	title	description	since
rule-details	0	regexp/no-contradiction-with-assertion	disallow elements that contradict assertions	v1.2.0

regexp/no-contradiction-with-assertion

💼 This rule is enabled in the ✅ plugin:regexp/recommended config.

💡 This rule is manually fixable by editor suggestions.

disallow elements that contradict assertions

📖 Rule Details

This rule reports elements that contradict an assertion. All elements reported by this rule fall into one of two categories:

1. An element/alternative that can never be entered.

   This means that the element is dead code and can be removed. Example:

   /* eslint regexp/no-contradiction-with-assertion: "error" */
   
   var foo = /(?=a)(\w|-)/;
   var foo = /(?=a)b*a/;

2. An element that is always entered.

   Right now, only quantifiers with a minimum of 0 are reported in this category. They are contradictory because the minimum of 0 is changed by the assertion to be effectively 1. Example:

   /* eslint regexp/no-contradiction-with-assertion: "error" */
   
   var foo = /a\b-?a/;
   var foo = /^foo$[\s\S]*?^end foo/m;

This rule is quite similar to regexp/no-useless-assertions. While regexp/no-useless-assertions tries to find assertions that contradict the pattern, this rule tries to find parts of the pattern that contradict assertions.

/* eslint regexp/no-contradiction-with-assertion: "error" */

/* ✓ GOOD */
var foo = /a\b-a/;
var foo = /a\ba/; // handled by regexp/no-useless-assertions

/* ✗ BAD */
var foo = /a\b-?a/;
var foo = /a\b(a|-)/;
var foo = /a\ba*-/;

🔧 Options

Nothing.

📚 Further reading

• regexp/no-useless-assertions

🚀 Version

This rule was introduced in eslint-plugin-regexp v1.2.0

🔍 Implementation

• Rule source
• Test source