my $str = 'word1 <tag0> word2 <tag1>word3 word4</tag1> word5 </tag0> word6 <tag2>word7 word8</tag2> word9 <tag3>word10</tag3> word11';

## Examples:
# @tags = ('tag0', 'tag1'): 'word1 word2 word3 word4 word5 word6 word9 word11'
# @tags = ('tag3'): 'word1 word6 word9 word10 word11'
# @tags = ('tag1', 'tag2', 'tag3'): 'word1 word6 word7 word8 word9 word10 word11'