HMCL-dev/HMCL · error · IllegalArgumentException

Illegal pattern (Bad escape)

Error message

Illegal pattern (Bad escape): ${value}

What it means

Thrown as IllegalArgumentException from replaceTokens when a literal processor argument ends with a lone backslash ('\\' at the last character position). The escape sequence is incomplete — there is no following character to unescape — so the pattern is rejected.

Solutions

1. Fix the offending value in install_profile.json so backslashes are escaped (\\) or no trailing backslash remains
2. Regenerate the installer/profile with correct JSON escaping (write paths with \\ or forward slashes)
3. Use the official unmodified installer for that NeoForge/Forge version
4. If you control the code, strip or normalize trailing backslashes before parseLiteral

Example fix

// before (profile arg)
"--target", "C:\\minecraft\\"
// after
"--target", "C:\\minecraft\\\\"

static boolean wellFormedLiteral(String s) {
    return s != null && !(s.endsWith("\\"));
}

try { task.run(); } catch (IllegalArgumentException e) { if (e.getMessage() != null && e.getMessage().startsWith("Illegal pattern (Bad escape)")) reportProfileLiteralIssue(e.getMessage()); else throw e; }

When it happens

Trigger: parseLiteral falls through to plainConverter.apply(replaceTokens(var, literal)); the value contains '\\' as its final character (x == value.length()-1). Comes from a processor arg or data value in install_profile.json with a trailing backslash (e.g. a Windows path like "C:\\temp\\" written with a single trailing backslash).

Common situations: Hand-edited or generated install_profile.json using raw Windows paths with trailing backslashes; tooling that forgot to escape backslashes (\\) in JSON; copy-pasted values losing an escape character.

Understand the failure class

Background: "Invalid ... format", "must be in format X", "does not look like a ..." — invalid argument format errors across CLI tools and libraries — this error's family across 17 libraries.

Related errors

• Illegal pattern (Unclosed )
• Illegal pattern: Missing Key
• Artifact name is malformed
• Bad Build-Number
• Expected typed ID prefix '" + expectedPrefix + "'

AI-assisted analysis of HMCL-dev/HMCL@24702dc5a0 (2026-09-10). Data as JSON: /api/errors/64ca8e17c2d4ea51. Report an issue: GitHub.

Appendix: source

Thrown at HMCLCore/src/main/java/org/jackhuang/hmcl/download/neoforge/NeoForgeOldInstallTask.java:219

            String selfVersion,
            Path installer) {
        this.dependencyManager = dependencyManager;
        this.gameRepository = dependencyManager.getGameRepository();
        this.manifest = manifest;
        this.minecraftJar = minecraftJar;
        this.installer = installer;
        this.selfVersion = selfVersion;

        setSignificance(TaskSignificance.MAJOR);
    }

    private static String replaceTokens(Map<String, String> tokens, String value) {
        StringBuilder buf = new StringBuilder();
        for (int x = 0; x < value.length(); x++) {
            char c = value.charAt(x);
            if (c == '\\') {
                if (x == value.length() - 1)
                    throw new IllegalArgumentException("Illegal pattern (Bad escape): " + value);
                buf.append(value.charAt(++x));
            } else if (c == '{' || c == '\'') {
                StringBuilder key = new StringBuilder();
                for (int y = x + 1; y <= value.length(); y++) {
                    if (y == value.length())
                        throw new IllegalArgumentException("Illegal pattern (Unclosed " + c + "): " + value);
                    char d = value.charAt(y);
                    if (d == '\\') {
                        if (y == value.length() - 1)
                            throw new IllegalArgumentException("Illegal pattern (Bad escape): " + value);
                        key.append(value.charAt(++y));
                    } else {
                        if (c == '{' && d == '}') {
                            x = y;
                            break;
                        }
                        if (c == '\'' && d == '\'') {
                            x = y;

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