Not sure what you mean. There's no problem with adding <em>...</em> around the word "perl", it's just the \W+ that's inappropriate in the string to substitute...

Think of it this way: What concrete characters (and how many) should be substituted from the class of characters that \W stands for?

I would like to use \W in the following program to ignore special character(:) after word "perl" in above example. and want to highlight only word "perl". So how can i do it?

#!/usr/bin/perl -W
$str = <STDIN>;# suppose $str = "i am perl: a system";
chomp($str);
$query = <STDIN>;# suppose $query = 'perl';
chomp($query);
#$query = "perl";
$str =~ s/$query\W+/<em>$query<\/em> \W+/ig;

print " Value = " . $str;
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It'll ignore everything it doesn't match. You don't have to do anything.

If $query contains a literal text to match:

$str =~ s/\Q$query\E/<em>$query<\/em>/ig;
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If $query contains a regex pattern:

$str =~ s/($query)/<em>$1<\/em>/ig;
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...like I've already said: get rid of the \W+ in the substitution string:

$str =~ s/$query\W+/<em>$query<\/em> \W+/ig;
                                     ^^^
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The regex (i.e. s/$query\W+/) matches "perl: " which is then being substituted with the string "<em>$query</em> ".

Update: on second read it seems you want to keep the ":" (-> "ignore")...  in which case just don't match it in the first place, as ikegami said below.

What you may have had in mind could be written as

$str =~ s/$query(\W+)/<em>$query<\/em>$1/ig;
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that is, $1 would re-insert into the substitution string whatever the captured part in the regex (\W+) matched (i.e. ": ").  But as already said, that's not necessary if you don't include it in the match in the first place.

Sachin: Just out of curiosity: when you executed the code fragment in the OP with  -W 'no, really' warnings turned on, didn't you get a warning message?

>perl -WMstrict -le
"my $str = 'i am perl: a system';
 my $query = 'perl';
 $str =~ s/$query\W+/<em>$query<\/em> \W+/ig;
 print ' Value = ' . $str;
"
Unrecognized escape \W passed through at -e line 1.
 Value = i am <em>perl</em> W+a system


This should have been a clue.