@valid_entities= ('a','abbr','acronym','br');# remove tags
my %htmlenties = map { $_ =>1 } @valid_entities;
$line =~  s/
< # a tag open
( # begin capture group 1
\/? # an optional slash
( # begin capture group2
[^>]*? any number of characters that aren't closing tags
) # end capture 2
\/? # an optional slash (xhtml and all that)
) # end capture group 1
> # a closing tag
/exists $htmlenties{$2} ? "<$1>" : defined ($1) ? "&lt;$1&gt;" : "&lt;"/xeg;# different captures => different process