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

the string class has the matches method, you can pass a regex there. String.matches(String)

for the regex you can see this: http://download.oracle.com/javase/1,5.0/docs/api/java/util/regex/Pattern.html
examples:

"song001.mp3".matches("song\\d{3}\\.mp3");

Answer 2

You can use Java regular expressions - please see http://www.vogella.de/articles/JavaRegularExpressions/article.html

Thanks...

Answer 3

Since you do not know the format in advance, you will have to write a method that converts a format string into a regexp. Not trivial, but possible. Here is a simple example for the 2 testcases you have given:

public static String getRegexpFromFormatString(String format)
{
    String toReturn = format;

    // escape some special regexp chars
    toReturn = toReturn.replaceAll("\\.", "\\\\.");
    toReturn = toReturn.replaceAll("\\!", "\\\\!");

    if (toReturn.indexOf("%") >= 0)
    {
        toReturn = toReturn.replaceAll("%s", "[\\\\w]+"); //accepts 0-9 A-Z a-z _

        while (toReturn.matches(".*%([0-9]+)[d]{1}.*"))
        {
            String digitStr = toReturn.replaceFirst(".*%([0-9]+)[d]{1}.*", "$1");
            int numDigits = Integer.parseInt(digitStr);
            toReturn = toReturn.replaceFirst("(.*)(%[0-9]+[d]{1})(.*)", "$1[0-9]{" + numDigits + "}$3");
        }
    }

    return "^" + toReturn + "$";
}

and some test code:

public static void main(String[] args) throws Exception
{
    String formats[] = {"hello %s!", "song%03d.mp3", "song%03d.mp3"};
    for (int i=0; i<formats.length; i++)
    {
        System.out.println("Format in [" + i + "]: " + formats[i]);
        System.out.println("Regexp out[" + i + "]: " + getRegexp(formats[i]));
    }

    String[] words = {"hello world!", "song001.mp3", "potato"};
    for (int i=0; i<formats.length; i++)
    {
        System.out.println("Word [" + i + "]: " + words[i] +
            " : matches=" + words[i].matches(getRegexpFromFormatString(formats[i])));
    }
}

Answer 4

There is not a simple way to do this. A straight-forward way would be to write some code that converts format strings (or a simpler subset of them) to regular expressions and then match those using the standard regular expression classes.

A better way is probably to rethink/refactor your code. Why do you want this?

Answer 5

You can use String.matches; although you'd need to use a regular expression then, rather then the format string.

It shouldn't be too hard to replace something like %03d with a \d{3} regex equivalent

Example:

"song001.mp3".matches("song\\d{3}\\.mp3") // True

"potato".matches("song\\d{3}\\.mp3") // False

If you really need the format string, you'll need to make a function that replaces the format with a regex equivalent, and escapes the regex reserved characters; then use the String.matches function.

Answer 6

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);
}