regex.rb

text = %Q(%%BEGIN_GROUP_TEXT_7
    %BEGIN_TEASER
    The research on reversibly expandable structures focused ...
    %END_TEASER
    %BEGIN_EXPANDED
    ... to understand how to realize structures that can vary in shape and size, was translated into several US patents registered under the name of Charles Hoberman. These patents include precise drawings detailing the overall spatial formations as well as the form and dimension of individual joints and trusses. The exhibited patent 5,024,031 presents a summary of Hoberman’s approach, including spheres and iris domes.
    %END_EXPANDED
%%END_GROUP_TEXT
 
%%BEGIN_GROUP_TEXT_7
    %BEGIN_TEASER
    The research on reversibly expandable structures focused ...
    %END_TEASER
    %BEGIN_EXPANDED
    ... to understand how to realize structures that can vary in shape and size, was translated into several US patents registered under the name of Charles Hoberman. These patents include precise drawings detailing the overall spatial formations as well as the form and dimension of individual joints and trusses. The exhibited patent 5,024,031 presents a summary of Hoberman’s approach, including spheres and iris domes.
    %END_EXPANDED
%%END_GROUP_TEXT)

regexp = /%%BEGIN_GROUP_TEXT_(\d*).*?%BEGIN_TEASER(.*?)%END_TEASER.*?%BEGIN_EXPANDED(.*?)%END_EXPANDED.*?%%END_GROUP_TEXT/m

text.gsub!(regexp, 
  '<div class="div-\1">
    <div class="teaser">\2</div>
    <div class="expanded">\3</div>
  </div>')

puts text