Tag: Regex

Sometimes we have the code with html tags that contain nothing but whitespace characters. Often those tags are nested. See a code below:

<div> 
 <div> 
  <div></div> 
 </div> 
</div>

What regex might be used to find and remove those tags?

Obvious solution is <div>\s*?<\/div> .

\s stands for “whitespace character”. It includes [ \t\n\x0B\f\r]. That is: \s matches a space( ) or a tab (\t) or a line(\n) break or a vertical tab (\x0B) sometimes referred as (\v) or a form feed (\f) or a carriage return (\r) .

General case

In general case, we use the following regex:
<(?<tag>[a-z]+?)( [^>]+?|)>\s*?<\/(\k<tag>)>

where <tag> is a named match group: [a-z]+?

JAVA code

When applying it recursively we might use the following code, JAVA:

public static String removeEmptyTags(String html) {
        boolean compareFound = true;
        Pattern pattern = Pattern.compile("<(?<tag>[a-z]+?)( [^>]+?|)>\\s*?</(\\k<tag>)>", Pattern.MULTILINE | Pattern.DOTALL);
        while (compareFound) {
            compareFound = false;
            Matcher matcher = pattern.matcher(html);
            if(matcher.find()) {
                compareFound = true;
                html = matcher.replaceAll("");
            }
        }
        return html;
    }

Recently I was asked to look at a brand- new online regex tester, regviz.org, developed as a collaboration of VISUS, University of Stuttgart and University of Trier. Though there are a lot of regex online testers on the market today, and many of them are quite good, let’s look at what is special about regviz.org and what it lacks.

Yes, I’m aware that using regex for HTML parsing is not the best idea. But still when I need to quickly extract some small portion of a web page I find myself applying regex more often than executing an XPath query, and its lookahead and lookbehind constructions may be quite helpful.

Now we want to review some email validation Regexes. We’ve chosen Regexes based on readability, complexity and RFC standarts relevance. For online Regex testing tools refer here.