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Utility classes developed at Adrian Smith Software (A.S.S.)
There is a newer version: 21.0.1
package com.databasesandlife.util.wicket;

import org.apache.wicket.markup.ComponentTag;
import org.apache.wicket.markup.MarkupStream;
import org.apache.wicket.markup.html.basic.Label;
import org.apache.wicket.model.IModel;
import org.apache.wicket.util.string.Strings;

import java.io.Serializable;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

import static java.util.regex.Matcher.quoteReplacement;

/** 
 * Displays the text in the model over multiple HTML lines, and makes links clickable. 
 *
 * @author This source is copyright Adrian Smith and licensed under the LGPL 3.
 * @see Project on GitHub
 */
public class MultilineLabelWithClickableLinks extends Label {

    public MultilineLabelWithClickableLinks(final String id, Serializable label) {
        super(id, label);
        setEscapeModelStrings(false);
    }

    public MultilineLabelWithClickableLinks(final String id, IModel model) {
        super(id, model);
        setEscapeModelStrings(false);
    }

    /**
     * Matches "foo://foo.foo/foo".
     * 
     * Protocol is optional
     * 
Domain must contain at least one dot (TLD list is too long for whitelist)
     * 
Last part is optional and can contain anything apart from space and trailing punctuation
     *  (= part of the sentence in which the link is embedded)
     * 
     * Quotes are not allowed because we don't want <a href="foo"> to have foo containing quotes (XSS).
     */
    protected static final Pattern linkPattern = Pattern.compile(
        "(\\w{2,7}:/+)?([\\w-]+\\.)+[\\w-]+(/([^\\s'\"]*[\\w])?)?", Pattern.CASE_INSENSITIVE);

    /**
     * Takes some plain text and returns HTML with URLs marked up as clickable links.
     *    
     * 
Making links clickable is not as easy as it seems.
     * 

     * Conversion from plain text to HTML requires that entities 
     *   such as "&" get replaced by "&amp;".
     * 
Links such as "foo.com/a&b" need to get replaced by
     *   "<a href='foo.com/a&b'>foo.com/a&amp;b</a>".
     * 
     * 
     * Therefore,
     * 

     * One cannot firstly replace entities and then markup links, as the links should contain 
     *   unescaped "&" as opposed to "&amp;".
     * 
One cannot firstly encode links and then replace entities as the angle brackets in the link's
     *   "<a href.." would get replaced by "&lt;a href..." which the 
     *   browser would not understand.
     * 
     * 
     * Therefore, the replacement of HTML entities, and the replacement of links, must be done 
     *   in a single pass, not one after another.
     *   
     * @param plainText Plain text, no HTML.
     * @return Safe HTML with entities encoded and links marked up.
     */
    public static CharSequence encodeLinksToHtml(CharSequence plainText) {
        var m = linkPattern.matcher(plainText);
        var result = new StringBuffer();
        while (m.find()) {
            var replacement = "" + Strings.escapeMarkup(m.group()) + "";
            var backgroundStartInclIdx = result.length();
            m.appendReplacement(result, quoteReplacement(replacement));
            var backgroundEndExclIdx = result.length() - replacement.length();
            var background = result.substring(backgroundStartInclIdx, backgroundEndExclIdx);
            result.replace(backgroundStartInclIdx, backgroundEndExclIdx, Strings.escapeMarkup(background).toString());
        }
        var backgroundStartInclIdx = result.length();
        m.appendTail(result);
        var backgroundEndExclIdx = result.length();
        var background = result.substring(backgroundStartInclIdx, backgroundEndExclIdx);
        result.replace(backgroundStartInclIdx, backgroundEndExclIdx, Strings.escapeMarkup(background).toString());
        return result;
    }

    @Override public void onComponentTagBody(MarkupStream markupStream, ComponentTag openTag) {
        var plainText = getDefaultModelObjectAsString();
        var htmlWithLinksMarkedup = encodeLinksToHtml(plainText); 
        var htmlWIthLinksAndNewlines = htmlWithLinksMarkedup.toString().replace("\n", "
");
        replaceComponentTagBody(markupStream, openTag, htmlWIthLinksAndNewlines);
    }
}