str = <<'EOT'
Это <!--пример текста--> с
<a href="#">какой-то</a> <br>разметкой <br />
<b class="11">и</b> <amg src="1">
<area>непарными</area> <strong>
тегами</strong> и, <!--коментами-->,
<ul id="w"> <li id="id" class="class">1</li></ul>.
<u class="ну и даже так" />
EOT
puts str.gsub(/(<(?!\/?a\b)(?!\/?b\b)(?!\/?i\b)(?!\/?u\b)(?!\/?ul\b)(?!\/?li\b)(?!\/?ol\b)(?!\/?img\b)[^>]+\s*\/?>)/m,'')
.gsub(/(<a\b|b\b|i\b|u\b|ul\b|li\b|ol\b|img\b)(\s[^>]+?)(\s*\/?>)/m,'\\1\\3')
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