Java function to return if string contains illegal characters

Question

I have the following characters that I would like to be considered "illegal":

~, #, @, *, +, %, {, }, <, >, [, ], |, “, ”, \, _, ^

I'd like to write a method that inspects a string and determines (true/false) if that string contains these illegals:

public boolean containsIllegals(String toExamine) {
    return toExamine.matches("^.*[~#@*+%{}<>[]|\"\\_^].*$");
}

However, a simple matches(...) check isn't feasible for this. I need the method to scan every character in the string and make sure it's not one of these characters. Of course, I could do something horrible like:

public boolean containsIllegals(String toExamine) {
    for(int i = 0; i < toExamine.length(); i++) {
        char c = toExamine.charAt(i);

        if(c == '~')
            return true;
        else if(c == '#')
            return true;

        // etc...
    }
}

Is there a more elegant/efficient way of accomplishing this?

Answer 1

You can make use of Pattern and Matcher class here. You can put all the filtered character in a character class, and use Matcher#find() method to check whether your pattern is available in string or not.

You can do it like this: -

public boolean containsIllegals(String toExamine) {
    Pattern pattern = Pattern.compile("[~#@*+%{}<>\\[\\]|\"\\_^]");
    Matcher matcher = pattern.matcher(toExamine);
    return matcher.find();
}

find() method will return true, if the given pattern is found in the string, even once.

---

Another way that has not yet been pointed out is using String#split(regex). We can split the string on the given pattern, and check the length of the array. If length is 1, then the pattern was not in the string.

public boolean containsIllegals(String toExamine) {
    String[] arr = toExamine.split("[~#@*+%{}<>\\[\\]|\"\\_^]", 2);
    return arr.length > 1;
}

If arr.length > 1, that means the string contained one of the character in the pattern, that is why it was splitted. I have passed limit = 2 as second parameter to split, because we are ok with just single split.

Answer 2

I need the method to scan every character in the string

If you must do it character-by-character, regexp is probably not a good way to go. However, since all characters on your "blacklist" have codes less than 128, you can do it with a small boolean array:

static final boolean blacklist[] = new boolean[128];

static {
    // Unassigned elements of the array are set to false
    blacklist[(int)'~'] = true;
    blacklist[(int)'#'] = true;
    blacklist[(int)'@'] = true;
    blacklist[(int)'*'] = true;
    blacklist[(int)'+'] = true;
    ...
}

static isBad(char ch) {
    return (ch < 128) && blacklist[(int)ch];
}

Answer 3

Use a constant for avoids recompile the regex in every validation.

private static final Pattern INVALID_CHARS_PATTERN = 
                               Pattern.compile("^.*[~#@*+%{}<>\\[\\]|\"\\_].*$");

And change your code to:

public boolean containsIllegals(String toExamine) {
    return INVALID_CHARS_PATTERN.matcher(toExamine).matches();
}

This is the most efficient way with Regex.