Java regular expression: finding match errors

Question

I want to know the exact position of an error when a regular expression match has failed. But I couldn't find any class or method in Matcher neither in Pattern that can do that. Any help?

Pattern regExpPattern = Pattern.compile("My regular expression");
Matcher matcher = regExpPattern.matcher("Input string");
if (!matcher.matches()) {
    // Better error handling here
    throw new Exception("Invalid expression");
}

EDIT: Here is a working code excerpt:

public class FOPRequestParser {

private static Pattern regExpPattern;
private static String requestRegexp;
private static String regexpSeparators;

private static String[] INPUT_FORMATS = new String[] {"FO", "IFP"};
private static String[] OUTPUT_FORMATS = new String[] {"PDF", "PS", "AFP", "IFP"};

static {
    regexpSeparators = new String("(\\Q|@|\\E|=)");

    requestRegexp = new String("run" + regexpSeparators + "[a-zA-Z]+.*"
            + regexpSeparators + "inputFormat=(");
    for (int i = 0; i < INPUT_FORMATS.length; ++i) {
        requestRegexp += INPUT_FORMATS[i];
        if (i != INPUT_FORMATS.length - 1) {
            requestRegexp += "|";
        }
    }
    requestRegexp += ")" + regexpSeparators + "[a-zA-Z]+.*"
            + regexpSeparators + "outputFormat=(";
    for (int i = 0; i < OUTPUT_FORMATS.length; ++i) {
        requestRegexp += OUTPUT_FORMATS[i];
        if (i != OUTPUT_FORMATS.length - 1) {
            requestRegexp += "|";
        }
    }
    requestRegexp += ")";
}


public FOPRequestParser() {
    regExpPattern = Pattern.compile(requestRegexp);
}

public void processRequest(String request) throws Exception {
    Matcher matcher = regExpPattern.matcher(request);
    if (!matcher.matches()) {
        throw new Exception("Invalid expression");
    }
}

public static void main(String[] args) {

    FOPRequestParser conversion = new FOPRequestParser();
    try {
        String exp1 = new String("run|@|" +
                "c:\\input_file.fo|@|" +
                "inputFormat=FO|@|" +
                "c:\\output_file.pdf|@|" +
                "outputFormat=PDF");
        conversion.processRequest(exp1);
        System.out.println("Valid expression");
    } catch (Exception e) {
        e.printStackTrace();
    }
}

}

Answer 1

There are no API methods that tell you why there was not match.

If you use this for any sort of (input) validation you'd have to throw a generic exception saying something like "The provided value does not match the expected pattern <put-regex-or-human-readable-pattern-here>".

Answer 2

What is with this solution. It is not really nice but for my needs it worked:

int index = -1;
try {
     Field declaredField = Matcher.class.getDeclaredField("last");      
     declaredField.setAccessible(true); 
     index =Integer.parseInt(declaredField.get(matcher).toString()); } catch
(Exception e) { }