oxc-project/oxc · warning · OxcDiagnostic

Use the 'v' flag.

Error message

Use the 'v' flag.

What it means

oxlint `eslint/require-unicode-regexp` with `{ "requireFlag": "v" }`: a regex literal or `RegExp` construction lacks the `v` flag. In `require_unicode_regexp_diagnostic` (require_unicode_regexp.rs:33) the V branch selects message 'Use the \'v\' flag.' and help 'Add the \'v\' flag.'. The `v` flag enables Unicode Sets mode (`\p{...}` classes, `--` subtraction) and stricter parsing; the config comment notes `u` may be preferred where `v` is unsupported.

Source

Thrown at crates/oxc_linter/src/rules/eslint/require_unicode_regexp.rs:33

    AstNode,
    ast_util::get_declaration_of_variable,
    ast_util::variable_declaration_kind,
    context::LintContext,
    rule::{DefaultRuleConfig, Rule},
    utils::is_regexp_callee,
};

fn require_unicode_regexp_diagnostic(
    span: Span,
    require_flag: Option<&RequireFlag>,
) -> OxcDiagnostic {
    let (msg, help_msg) = if matches!(require_flag, Some(RequireFlag::V)) {
        ("Use the 'v' flag.", "Add the 'v' flag.")
    } else {
        ("Use the 'u' flag.", "Add the 'u' flag.")
    };

    OxcDiagnostic::warn(msg)
        .with_note("The 'u' and 'v' flags enable Unicode-aware regular expression behavior and stricter pattern parsing.")
        .with_help(help_msg)
        .with_label(span)
}

#[derive(Debug, Default, Clone, Serialize, Deserialize, JsonSchema)]
#[serde(rename_all = "camelCase", default, deny_unknown_fields)]
struct RequireUnicodeRegexpConfig {
    /// The `u` flag may be preferred in environments that do not support the `v` flag.
    ///
    /// Examples of **incorrect** code for this rule with the `{ "requireFlag": "u" }` option:
    /// ```js
    /// const fooEmpty = /foo/;
    /// const fooEmptyRegexp = new RegExp('foo');
    /// const foo = /foo/v;
    /// const fooRegexp = new RegExp('foo', 'v');
    /// ```
    ///

View on GitHub (pinned to e1e7af627c)

Solutions

1. Add the flag to the literal: `/foo/` becomes `/foo/v` (note: `v` implies `u`, so `/foo/u` becomes `/foo/v`).
2. For constructed regexes pass the flag: `new RegExp("foo", "v")`.
3. If you must support runtimes without `v`, set `"requireFlag": "u"` in the rule config instead.

Example fix

// before
const re = /^\p{Letter}+$/u;

// after
const re = /^\p{Letter}+$/v;

# regex literals missing the v flag
rg -n --pcre2 '/[^\n]*/[dgimsuy]*' src/ | rg -v '/[^\n]*/[a-z]*v[a-z]*'

When it happens

Trigger: Rule configured with `requireFlag: "v"` and code containing `/foo/`, `/\d+/u`, or `new RegExp("foo")` — any regex without `v` in its flags.

Common situations: Shared config that mandates v-mode for consistent Unicode handling; upgrading a config from `u` to `v` and re-linting an older codebase; targeting Node/browsers that gained `v` support only recently.

Related errors

• Use the 'u' flag.
• Backreference '{back_reference}' will be ignored. It referen
• Backreference '{back_reference}' will be ignored. It referen
• Backreference '{back_reference}' will be ignored. It referen
• Unnecessary escape character {escape_char:?}

AI-assisted analysis of oxc-project/oxc@e1e7af627c (2026-08-20). Data as JSON: /api/errors/2b4b19629807e652. Report an issue: GitHub.