HMCL-dev/HMCL · error · IllegalArgumentException

Illegal pattern (Bad escape):

Error message

Illegal pattern (Bad escape): 

What it means

replaceTokens scans a literal string from the Forge installer's install_profile.json data/args and interprets backslash as an escape character. If the string ends with a lone backslash there is no following character to escape, so the code throws IllegalArgumentException("Illegal pattern (Bad escape): ..."). This guards against a truncated or hand-mangled pattern in installer metadata.

Solutions

1. Re-download the Forge installer from the official maven (files.minecraftforge.net) to replace the corrupted/truncated jar
2. Check the value named in the message in install_profile.json inside the installer jar and fix/remove the trailing backslash
3. Try a different Forge version (older or newer) whose installer format is known-good
4. Update HMCL — parsing of installer data has been adjusted across versions

Example fix

// before (value in install_profile.json)
{"MAPPED_JAR": "C:\\temp\\"}
// after
{"MAPPED_JAR": "C:\\temp\\file.txt"}

// before installing, sanity-check installer data values
try (var zip = new java.util.zip.ZipFile(installer.toFile())) {
    String json = new String(zip.getInputStream(zip.getEntry("install_profile.json")).readAllBytes());
    if (json.matches(".*[^\\\\]\\\\\\"\\s*,\\s*")) throw new IllegalStateException("value ends with bare backslash");
}

try { installTask.execute(); } catch (IllegalArgumentException e) { if (e.getMessage().startsWith("Illegal pattern")) { reDownloadInstaller(); } else throw e; }

When it happens

Trigger: ForgeNewInstallTask.parseLiteral is called with a literal whose last character is '\' and which is not fully wrapped in {..} or '..' — i.e. a data/processor-arg value like "path\" ending in a trailing backslash.

Common situations: Corrupted or tampered forge installer downloads; manually edited install_profile.json; forge installer formats HMCL doesn't expect (very new/old Forge releases changing data value conventions); non-Windows paths with stray trailing backslashes injected into values.

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:
• Malformed forge installer file
• client_mappings download info not found
• Minecraft client JAR not found:

AI-assisted analysis of HMCL-dev/HMCL@24702dc5a0 (2026-09-10). Data as JSON: /api/errors/3942236e4c0bf5f2. Report an issue: GitHub.

Appendix: source

Thrown at HMCLCore/src/main/java/org/jackhuang/hmcl/download/forge/ForgeNewInstallTask.java:235

            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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