Java flat JSON object parser with clear errors

import java.util.LinkedHashMap;
import java.util.Map;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public final class FlatJson {
    private static final Pattern PAIR = Pattern.compile("\"([^\"]+)\"\\s*:\\s*(\"[^\"]*\"|[^,}\\s]+)");

    public static Map<String, String> parse(String s) {
        Map<String, String> out = new LinkedHashMap<>();
        String body = s.trim();
        if (!body.startsWith("{") || !body.endsWith("}"))
            throw new IllegalArgumentException("invalid JSON: expected an object, got: " + s);
        Matcher m = PAIR.matcher(body.substring(1, body.length() - 1));
        while (m.find()) {
            String v = m.group(2);
            out.put(m.group(1), v.startsWith("\"") ? v.substring(1, v.length() - 1) : v);
        }
        if (out.isEmpty() && !body.matches("\\{\\s*}"))
            throw new IllegalArgumentException("invalid JSON: no key:value pairs in " + s);
        return out;
    }
}