oxc-project/oxc · warning · OxcDiagnostic

Use uppercase characters for the value of the escape sequenc

Error message

Use uppercase characters for the value of the escape sequence.

What it means

Diagnostic from oxlint's `unicorn/escape-case` rule. It requires uppercase hex digits in escape sequences: `\xXX`, `\uXXXX`, and `\u{X...}` escapes in string literals, plus `\cX` control escapes in regular expression literals. The scanner walks each string's raw text (skipping `String.raw`-tagged templates) and each regex literal; any lowercase a-f in the escape value is flagged, and the autofix uppercases the value in place.

Source

Thrown at crates/oxc_linter/src/rules/unicorn/escape_case.rs:13

use std::str::Chars;

use oxc_ast::AstKind;
use oxc_diagnostics::OxcDiagnostic;
use oxc_macros::declare_oxc_lint;
use oxc_span::Span;

use crate::{
    AstNode, context::LintContext, rule::Rule, utils::is_string_raw_tagged_template_expression,
};

fn escape_case_diagnostic(span: Span) -> OxcDiagnostic {
    OxcDiagnostic::warn("Use uppercase characters for the value of the escape sequence.")
        .with_label(span)
}

#[derive(Debug, Default, Clone)]
pub struct EscapeCase;

declare_oxc_lint!(
    /// ### What it does
    ///
    /// Enforces defining escape sequence values with uppercase characters rather than lowercase ones.
    /// This promotes readability by making the escaped value more distinguishable from the identifier.
    ///
    /// ### Why is this bad?
    ///
    /// Using lowercase characters in escape sequences makes them less readable and harder to distinguish
    /// from surrounding code. Most style guides recommend uppercase for consistency and clarity.
    ///
    /// ### Examples

View on GitHub (pinned to e1e7af627c)

Solutions

1. Uppercase the hex digits: `\xA9`, `\uD834`, `\u{1D306}`, `/\cA/`.
2. Run `oxlint --fix` once to normalize every literal in the project.
3. Keep `String.raw` templates for text that must preserve raw escapes.

Example fix

// before
const foo = "\xa9";
// after
const foo = "\xA9";

When it happens

Trigger: `const foo = "\xa9";`, `"\ud834"`, `"\u{1d306}"`, or the regex `/\ca/` — any escape sequence whose hex digits include lowercase characters in a non-raw string or regex literal.

Common situations: Hand-typed color/unicode escapes in older code; ANSI escape strings (`\x1b` vs `\x1B`); enabling the unicorn pedantic preset and normalizing a legacy codebase.

Related errors

• Use a regular expression literal instead of the `RegExp` con
• Prefer `{} {}` over `{} {}` to check {}.
• Invalid escape sequence in template literal.
• No spaces inside empty pair of braces allowed
• Use Unicode escapes instead of hexadecimal escapes.

AI-assisted analysis of oxc-project/oxc@e1e7af627c (2026-08-20). Data as JSON: /api/errors/969e4fd3ecaa37dc. Report an issue: GitHub.