Is there a Java utility to verify if a string is a valid HTML escape character?

Question

I want a method in the following format:

public boolean isValidHtmlEscapeCode(String string);

Usage would be:

isValidHtmlEscapeCode("A") == false
isValidHtmlEscapeCode("ש") == true // Valid unicode character
isValidHtmlEscapeCode("ש") == true // same as 1513 but in HEX
isValidHtmlEscapeCode("�") == false // Invalid unicode character

I wasn't able to find anything that does that - is there any utility that does that? If not, is there any smart way to do it?

Answer 1

public static boolean isValidHtmlEscapeCode(String string) {
    if (string == null) {
        return false;
    }
    Pattern p = Pattern
            .compile("&(?:#x([0-9a-fA-F]+)|#([0-9]+)|([0-9A-Za-z]+));");
    Matcher m = p.matcher(string);

    if (m.find()) {
        int codePoint = -1;
        String entity = null;
        try {
            if ((entity = m.group(1)) != null) {
                if (entity.length() > 6) {
                    return false;
                }
                codePoint = Integer.parseInt(entity, 16);
            } else if ((entity = m.group(2)) != null) {
                if (entity.length() > 7) {
                    return false;
                }
                codePoint = Integer.parseInt(entity, 10);
            } else if ((entity = m.group(3)) != null) {
                return namedEntities.contains(entity);
            }
            return 0x00 <= codePoint && codePoint < 0xd800
                    || 0xdfff < codePoint && codePoint <= 0x10FFFF;
        } catch (NumberFormatException e) {
            return false;
        }
    } else {
        return false;
    }
}

Here's the set of named entities http://pastebin.com/XzzMYDjF

Answer 2

you might want to have a look at Apache commons StringUtils: http://commons.apache.org/lang/api-2.3/org/apache/commons/lang/StringEscapeUtils.html#unescapeHtml(java.lang.String)

with the unescapeHtml you could do sth. like:

String input = "A";
String unescaped = StringEscapeUtils.unescapeHtml(input);
boolean containsValidEscape = !input.equals(a);

Answer 3

Not sure if this is a perfect solution, but you can use Apache Commons Lang:

try {
    return StringEscapeUtils.unescapeHtml4(code).length() < code.length();
} catch (IllegalArgumentException e) {
    return false;
}

Answer 4

This should be the method you wanted:

public static boolean isValidHtmlEscapeCode(String string) {
String temp = "";
try {
    temp = StringEscapeUtils.unescapeHtml3(string);
} catch (IllegalArgumentException e) {
    return false;
}
return !string.equals(temp);
}