java - Expand environment variables in text

Question

I'm trying to write a function to perform substitutions of environment variables in java. So if I had a string that looked like this:

User ${USERNAME}'s APPDATA path is ${APPDATA}.

I want the result to be:

User msmith's APPDATA path is C:UsersmsmithAppDataRoaming.

So far my broken implementation looks like this:

public static String expandEnvVars(String text) {        
    Map<String, String> envMap = System.getenv();
    String pattern = "\$\{([A-Za-z0-9]+)\}";
    Pattern expr = Pattern.compile(pattern);
    Matcher matcher = expr.matcher(text);
    if (matcher.matches()) {
        for (int i = 1; i <= matcher.groupCount(); i++) {
            String envValue = envMap.get(matcher.group(i).toUpperCase());
            if (envValue == null) {
                envValue = "";
            } else {
                envValue = envValue.replace("", "");
            }
            Pattern subexpr = Pattern.compile("\$\{" + matcher.group(i) + "\}");
            text = subexpr.matcher(text).replaceAll(envValue);
        }
    }
    return text;
}

Using the above sample text, matcher.matches() returns false. However if my sample text, is ${APPDATA} it works.

Can anyone help?

Answer 1

You don't want to use matches(). Matches will try to match the entire input string.

Attempts to match the entire region against the pattern.

What you want is while(matcher.find()) {. That will match each instance of your pattern. Check out the documentation for find().

Within each match, group 0 will be the entire matched string (${appdata}) and group 1 will be the appdata part.

Your end result should look something like:

String pattern = "\$\{([A-Za-z0-9]+)\}";
Pattern expr = Pattern.compile(pattern);
Matcher matcher = expr.matcher(text);
while (matcher.find()) {
    String envValue = envMap.get(matcher.group(1).toUpperCase());
    if (envValue == null) {
        envValue = "";
    } else {
        envValue = envValue.replace("", "");
    }
    Pattern subexpr = Pattern.compile(Pattern.quote(matcher.group(0)));
    text = subexpr.matcher(text).replaceAll(envValue);
}