JavaScript

Describes how recheck checks a regular expression of the javascript dialect, which is the default.

Engine

recheck follows the syntax and the semantics of ECMAScript 2026 RegExp, as V8 implements it.

Settings

The javascript dialect has no settings.

CLI

$ recheck --dialect=javascript '/^(a|a)*$/'

JavaScript

import { check } from "recheck";

await check("^(a|a)*$", "", { dialect: "javascript" });

Scala

import codes.quine.labs.recheck.ReDoS
import codes.quine.labs.recheck.common.Parameters
import codes.quine.labs.resyntax.dialect.Dialect

ReDoS.check("^(a|a)*$", "", Parameters(dialect = Dialect.JavaScript))

Flags and Units

The matching unit is utf16, or utf16-code-point with the u or v flag (see Matching Units).

recheck supports the following flags of RegExp.

Flag	RegExp property	Inline	Notes
d	hasIndices		No effect on matching.
g	global		No effect on matching, since recheck models a search from the start.
i	ignoreCase	(?i:...)
m	multiline	(?m:...)
s	dotAll	(?s:...)
u	unicode		An error with v.
v	unicodeSets		An error with u.
y	sticky		The match starts at the start of the input (lastIndex is 0).

Approximations

• With the y flag, the match starts only at the start of the input, as with lastIndex of 0. A match from a later lastIndex is not modeled, so recheck misses a pattern that is slow only there, where a look-behind, \b, \B or a multiline ^ sees the text before the starting position (e.g. /(?<=a)(?:b|b)*$/y with lastIndex of 1 on 'a' + 'b'.repeat(n) + '!').

Runtime-Attack Verification

• Command: node -.
• Matching: RegExp.prototype.exec.
• Engine limits: none.
• Engine errors: RangeError (e.g. V8 raises it for /^(?:a|b)*$/ on 10,000,000 characters).
• Environment variable: RECHECK_NODE.