Verify if String matches a format String

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旧巷少年郎
旧巷少年郎 2021-02-13 09:15

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?

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  • 2021-02-13 09:33

    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");
    
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  • 2021-02-13 09:36

    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);
    }
    
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  • 2021-02-13 09:39

    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?

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  • 2021-02-13 09:46

    You can use the Pattern class to implement the method to do what you want. Take a look at the examples in the Pattern Java api page.

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  • 2021-02-13 09:47

    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.

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  • 2021-02-13 09:53

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

    Thanks...

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