karatelabs/karate · error · io.karatelabs.parser.ParserException

invalid escape sequence in template literal: \

Error message

invalid escape sequence in template literal: \<esc>

What it means

In a template literal (backtick string), a backslash followed by digit 1-9 is an invalid legacy octal escape; the ES spec makes malformed escapes early errors in untagged templates, so JsLexer rejects them at lex time. Fix the escape or double-escape if you intended a literal backslash.

Solutions

1. Remove the invalid escape or correct it (use \0 for NUL, or \x01 / \u0001 for codepoint 1)
2. Escape the backslash itself: write \\1 in the template literal to mean backslash followed by 1
3. Move regex-replacement patterns into actual RegExp context instead of a template literal
4. Use a plain single-quoted string with proper escaping if no interpolation is needed

Example fix

// before
var s = `match group: \1`;
// after
var s = `match group: \\1`; // or use a real regex replacement context

// reject invalid template escapes before handing the string to eval
function hasBadTemplateEscapes(tl) {
  return /\\[1-9]/.test(tl) || /\\0[0-9]/.test(tl);
}
if (hasBadTemplateEscapes(tpl)) throw new Error('invalid legacy octal escape in template literal');

null

try {
  karate.eval('var s = `' + tpl + '`;');
} catch (e) {
  if (String(e.message).indexOf('invalid escape sequence in template literal') >= 0) {
    tpl = tpl.replace(/\\/g, '\\\\'); // escape all backslashes and retry
  } else throw e;
}

When it happens

Trigger: A non-tagged template literal in JS evaluated by karate-js contains an escape like \1 through \9, e.g. `col \1` or a regex fragment pasted into a template string; hit in scanTemplateContent (JsLexer.java:391).

Common situations: Copy-pasting regex replacement patterns ($1, \1) into template literals, Windows path fragments like `C:\temp\2`, porting old octal escapes from other languages, string-building code that interpolates numbers right after a backslash.

Related errors

• invalid escape sequence in template literal: \0
• invalid hex escape sequence in template literal
• invalid unicode escape sequence in template literal
• unicode escape sequence out of range in template literal
• invalid numeric separator

AI-assisted analysis of karatelabs/karate@a22eb90246 (2026-09-12). Data as JSON: /api/errors/a6e7d5aae64ac516. Report an issue: GitHub.

Appendix: source

Thrown at karate-js/src/main/java/io/karatelabs/parser/JsLexer.java:391

            c = peek();
            if (c == '`') {
                break;
            }
            if (c == '$' && peek(1) == '{') {
                break;
            }
            if (c == '\\') {
                advance();
                if (isAtEnd()) {
                    break;
                }
                char esc = peek();
                // Spec: in non-tagged template literals, malformed escape sequences are
                // early errors. We have no way to tell tag vs no-tag at lex time, so we
                // reject at lex time; tagged templates are not yet supported anyway.
                // Invalid legacy octal: \1..\9 or \0 followed by a decimal digit.
                if (esc >= '1' && esc <= '9') {
                    throw new ParserException("invalid escape sequence in template literal: \\" + esc);
                }
                if (esc == '0' && peek(1) >= '0' && peek(1) <= '9') {
                    throw new ParserException("invalid escape sequence in template literal: \\0" + peek(1));
                }
                // Hex escape backslash-x HH — require exactly two hex digits.
                if (esc == 'x') {
                    if (!isHexDigit(peek(1)) || !isHexDigit(peek(2))) {
                        throw new ParserException("invalid hex escape sequence in template literal");
                    }
                }
                // Unicode escape backslash-u HHHH or backslash-u {H...} — require proper form.
                if (esc == 'u') {
                    char next = peek(1);
                    if (next == '{') {
                        // backslash-u { codepoint } — one or more hex digits, closed by }
                        int j = 2;
                        if (!isHexDigit(peek(j))) {
                            throw new ParserException("invalid unicode escape sequence in template literal");

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