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• JavaScript
• HTML
• CSS
• Result

var subject = "<h1> \
    one two<br /> \
    three<br /> \
    four \
    </h1>";
        
var content, 
    pattern = /<h1>([\S\s]*)<\/h1>/, 
    match = pattern.exec(subject);

if (match != null) {
    content= match[1];
    
    pattern = /<br\s*\/?>/g;
    final = content.replace(pattern, "&nbsp;$&&nbsp;"); // $& is a reference to the matched content
    
    alert(final);
    document.write('<div class="h1block"><h1>' + final + '</h1></div>');
}