import re
s='''
<div class="message">
----- BEGIN MESSAGE -----<br />
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<br />
----- END MESSAGE -----<br />
</div>'''
m = re.search(r'----- BEGIN MESSAGE -----(?:<br />\s*)*\s*(.*?)\s*(?:<br />\s*)*----- END MESSAGE -----', s)
print(m.group(1))
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