Question

I’m implementing a keyword highlighter in Java. I’m using java.util.regex.Pattern to highlight (making bold) keyword within String content. The following piece of code is working fine for alphanumeric keywords, but it is not working for some special characters. For example, in String content, I would like to highlight the keyword c++ which has the special character + (plus), but it’s not getting highlighted properly. How do I escape + character so that c++ is highlighted?

public static void main(String[] args)
{
    String content = "java,c++,ejb,struts,j2ee,hibernate";
    System.out.println("CONTENT: " + content);
    String highlight = "C++";

    System.out.println("HIGHLIGHT KEYWORD: " + highlight);

    //highlight = highlight.replaceAll(Pattern.quote("+"), "\\\\+");
    java.util.regex.Pattern pattern = java.util.regex.Pattern.compile("\\b" + highlight + "\\b", java.util.regex.Pattern.CASE_INSENSITIVE);
    System.out.println("PATTERN: " + pattern.pattern());
    java.util.regex.Matcher matcher = pattern.matcher(content);

    while (matcher.find()) {
        System.out.println("Match found!!!");
        for (int i = 0; i <= matcher.groupCount(); i++) {
        System.out.println(matcher.group(i));
        content = matcher.replaceAll("<B>" + matcher.group(i) + "</B>");
        }
    }
    System.out.println("RESULT: " + content);
}

Output:
CONTENT: java,c++,ejb,struts,j2ee,hibernate
HIGHLIGHT KEYWORD: C++
PATTERN: \bC++\b
Match found!!!
c
RESULT: java,c++,ejb,struts,j2ee,hibernate

---

I even tried to escape ‘+’ before calling Pattern.compile like this,

highlight = highlight.replaceAll(Pattern.quote("+"), "\\\\+");

but still I’m not able to get the syntax right. Can somebody help me solve this?

Answer 1

This should do what you need:

Pattern pattern = Pattern.compile(
    "\\b" 
    + Pattern.quote(highlight)
    + "\\b",
    Pattern.CASE_INSENSITIVE);

Update: you are right, the above doesn’t work for C++ (\b matches word boundaries and doesn’t recognize ++ as a word). We need a more complicated solution:

Pattern pattern = Pattern.compile(
    "\\b" 
    + Pattern.quote(highlight)
    + "(?![^\\p{Punct}\\s])", // matches if the match is not followed by
                              // anything other than whitespace or punctuation
    Pattern.CASE_INSENSITIVE);

Update in response to comments: it seems that you need more logic in your pattern creation. Here’s a helper method to create the pattern for you:

private static final String WORD_BOUNDARY = "\\b";
// edit this to suit your neds:
private static final String ALLOWED = "[^,.!\\-\\s]";
private static final String LOOKAHEAD = "(?!" + ALLOWED + ")";
private static final String LOOKBEHIND = "(?<!" + ALLOWED + ")";

public static Pattern createHighlightPattern(final String highlight) {
    final Pattern pattern = Pattern.compile(
            (Character.isLetterOrDigit(highlight.charAt(0)) 
             ? WORD_BOUNDARY : LOOKBEHIND)
            + Pattern.quote(highlight)
            + (Character.isLetterOrDigit(highlight.charAt(highlight.length() - 1))
             ? WORD_BOUNDARY : LOOKAHEAD),
            Pattern.CASE_INSENSITIVE);
    return pattern;
}

And here is some test code to check that it works:

private static void testMatch(final String haystack, final String needle) {
    final Matcher matcher = createHighlightPattern(needle).matcher(haystack);
    if (!matcher.find())
        System.out.println("Failed to find pattern " + needle);
    while (matcher.find())
        System.out.println("Found additional match: " + matcher.group() +
                           " for pattern " + needle);
}

public static void main(final String[] args) {
    final String testString = "java,c++,hibernate,.net,asp.net,c#,spring";
    testMatch(testString, "java");
    testMatch(testString, "c++");
    testMatch(testString, ".net");
    testMatch(testString, "c#");
}

When I run this method, I don’t see any output (which is good :-))