Expand environment variables in text (bash style)

/**
 * Expand environment variables in text (bash style)
 * @author gocha <http://twitter.com/gochaism> (I release it into public domain, though)
 */
import java.util.Map;
import java.util.regex.Pattern;
import java.util.regex.Matcher;
import java.util.regex.MatchResult;

public class VariablePath
{
    /**
     * Expand environment variables in text
     * @param path input path string
     * @param userVariableMap user-defined variable name/value map (null if not needed)
     * @param useEnvironmentVariables true if expand environment variables
     * @param throwException true if throw Exception if a variable is not defined
     * @return expanded path string
     * @throws IllegalArgumentException if a variable is not defined
     */
    private static String expand(String path, Map<String, String> userVariableMap, boolean useEnvironmentVariables, boolean throwException) throws IllegalArgumentException
    {
        Pattern variablePattern = Pattern.compile("\\$\\{(.*)\\}|\\$([0-9]+|[A-Za-z_][0-9A-Za-z_]*)");
        Matcher variableMatcher = variablePattern.matcher(path);
        StringBuffer pathBuffer = new StringBuffer();
        int lastMatchEnd = 0;
        while (variableMatcher.find())
        {
            // get variable name
            MatchResult matchResult = variableMatcher.toMatchResult();
            boolean variableHasPathensis = (matchResult.group(1) != null);
            String variableName = (variableHasPathensis ? matchResult.group(1) : matchResult.group(2));
            String variableContent = null;

            // get environment variable
            if (useEnvironmentVariables)
            {
                variableContent = System.getenv(variableName);
            }
            // if not available, get user-defined variable
            if (variableContent == null && userVariableMap != null)
            {
                variableContent = userVariableMap.get(variableName);
            }
            // if the variable is not defined
            if (variableContent == null)
            {
                if (throwException)
                {
                    // throw Exception if needed
                    throw new IllegalArgumentException(path.substring(matchResult.start(), matchResult.end()) + " is not defined.");
                }
                else
                {
                    // otherwise, use empty string
                    variableContent = "";
                }
            }

            // put the value, and a part of string just before the variable
            pathBuffer.append(path.substring(lastMatchEnd, matchResult.start()));
            pathBuffer.append(variableContent);
            lastMatchEnd = matchResult.end();
        }
        // put the rest, and return the result
        pathBuffer.append(path.substring(lastMatchEnd, path.length()));
        return pathBuffer.toString();
    }

    /**
     * Expand environment variables in text
     * @param path input path string
     * @return expanded path string
     */
    public static String expand(String path)
    {
        return expand(path, null, true, false);
    }

    /**
     * VariablePath test method
     * @param args path string
     */
    public static void main(String[] args)
    {
        if (args.length > 0)
        {
            try
            {
                String path = args[0];
                System.out.println("Input : " + path);
                System.out.println("Output: " + VariablePath.expand(path));
            }
            catch (Exception e)
            {
                e.printStackTrace();
            }
        }
        else
        {
            System.out.println("Usage: VariablePath PathString");
        }
    }
}