Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question: 

#!/usr/bin/perl
while($line = <DATA>){

@valid_entities= ('<a>','<abbr>','<acronym>','<br>');
my %htmlenties = map { $_ =>1 } @valid_entities;
#$line =~ s/(<\w*?>(?![^<\w*?>]*<\/\w*?>))/$1/g;
#$line =~ s/<\w*>(?![^<\/\w*>]*>)/&lt;/g;
$line =~  s/(<(\w*?)>)/exists $htmlenties{$1} ? $1 : defined ($2) ? "&
+lt;$2&gt;" : "&lt;"/eg;
print $line;
}

__DATA__
<helloe>How r u <a> www.google.com</a>
<hi>How r u </hi><et,-2><><br/>

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How to convert character '<' to '&lt;' and '>' to &gt; for all the characters which is not present in the array
and only if it doesnot have a closing end tag. in the above example, the output should look like 
&lt;helloe&gt;How r u <a> www.google.com</a>
<hi>How r u </hi>&lt;et,-2&gt;&lt;&gt;<br/>

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Replies are listed 'Best First'.
Re: convert characters
by Utilitarian (Vicar) on Aug 27, 2009 at 12:24 UTC
    Substitute the following 
    @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

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[reply]
[d/l]
      #!/usr/bin/perl
while($line = <DATA>){

@valid_entities= ('<a>','<abbr>','<acronym>');
my %htmlenties = map { $_ =>1 } @valid_entities;
@valid_entities= ('a','abbr','acronym','br');# remove tags
my %htmlenties = map { $_ =>1 } @valid_entities;
$line =~  s/<(\/?([^>]*?)\/?)>/exists $htmlenties{$2} ? "<$1>" : defin
+ed ($1) ? "&lt;$1&gt;" : "&lt;"/xeg;
print $line;

<helloe>How r u <a> www.google.com</a>
<hi>How r u </hi><et,-2><>

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      From the above code the output which i got is 
      &lt;helloe&gt;How r u <a> www.google.com</a>
&lt;hi&gt;How r u &lt;/hi&gt;&lt;et,-2&gt;&lt;&gt;

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      But the expected output is 
      &lt;helloe&gt;How r u <a> www.google.com</a>
<hi>How r u </hi>&lt;et,-2&gt;&lt;&gt;

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      As '<hi>' as the '</hi>' I shouldn't replace it.
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        Then add 'hi'to your valid entities array
        Did you write any of this code yourself?

        Update:My addition is not a complete solution by any means, for example it will fail to correctly interpret the following, (organising it so that it does could be a worthwhile exercise for you):

        <hi >
<a href="http://permonks.org">Link</a>
<input value="Next>" type="submit">

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[d/l]
[select]

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