use strict;

my $matchtext = shift || die &usage;

if ( ! is_valid_pattern( $matchtext ) ) {
    die "'$matchtext' is not a valid regular expression.";
}

my $page = "myfilename.html";
open  PAGE,  "< $page" or die "Cannot open $page for reading: $!";

while (<PAGE>) {
    print "There was an instance of $matchtext on line $.:\n\t$_\n" if /$matchtext/;
}

close PAGE;

sub usage {
    print "Usage: filematch.pl text_to_match";
}

sub is_valid_pattern {
    my $pattern = shift;
    return eval { "" =~ /$pattern/; 1 } || 0;
}