use strict;
my $toFind="String";

while(<DATA>){
	my $line=$_;
	chomp($line);	
	if($line=~m/\b$toFind(?=[^\w\.]+|$)/i){
		print "$line\n";
	}
}

##</code><code>##

__DATA__
blah blah string blah
important stringstuff we wish to keep
Unimportant string
more stuff we wish to String.o
Some more unimportant String.c