Verify if String matches a format String

Question

In Java, how can you determine if a String matches a format string (ie: song%03d.mp3)?

In other words, how would you implement the following function?

Answer 1

I don't know of a library that does that. Here is an example how to convert a format pattern into a regex. Notice that Pattern.quote is important to handle accidental regexes in the format string.

// copied from java.util.Formatter
// %[argument_index$][flags][width][.precision][t]conversion
private static final String formatSpecifier
    = "%(\\d+\\$)?([-#+ 0,(\\<]*)?(\\d+)?(\\.\\d+)?([tT])?([a-zA-Z%])";

private static final Pattern formatToken = Pattern.compile(formatSpecifier);

public Pattern convert(final String format) {
    final StringBuilder regex = new StringBuilder();
    final Matcher matcher = formatToken.matcher(format);
    int lastIndex = 0;
    regex.append('^');
    while (matcher.find()) {
        regex.append(Pattern.quote(format.substring(lastIndex, matcher.start())));
        regex.append(convertToken(matcher.group(1), matcher.group(2), matcher.group(3), 
                                  matcher.group(4), matcher.group(5), matcher.group(6)));
        lastIndex = matcher.end();
    }
    regex.append(Pattern.quote(format.substring(lastIndex, format.length())));
    regex.append('$');
    return Pattern.compile(regex.toString());
}

Of course, implementing convertToken will be a challenge. Here is something to start with:

private static String convertToken(String index, String flags, String width, String precision, String temporal, String conversion) {
    if (conversion.equals("s")) {
        return "[\\w\\d]*";
    } else if (conversion.equals("d")) {
        return "[\\d]{" + width + "}";
    }
    throw new IllegalArgumentException("%" + index + flags + width + precision + temporal + conversion);
}