you could also try this

cat YourFile|perl -pe'$/="Title:";chomp;$_=/invalid url/?"Title:".$_:"
+"'
[download]

A quick explanation as this does look a bit line noise like. Perl is started with the options p and e. e gives the code to execute on the command line, p puts a while loop around the code with a continue block that just prints $_.

A very useful trick is to deparse a script to see better what is going on, lets do that now to make it more readable and so you can see the while and print added by the -p

pre> perl -MO=Deparse -pe'$/="Title:";chomp;$_=/invalid url/?"Title:".
+$_:""'
-e syntax OK
LINE: while (defined($_ = <ARGV>)) {
    $/ = 'Title:';
    chomp $_;
    $_ = /invalid url/ ? 'Title:' . $_ : ''
}
continue {
    print $_;
}
[download]

$/ is the input record seperator and by setting this to "Title:" I get perl to read one whole block at a time (up to and including the next occurance of Title: or EOF).

The chomp removes the record seperator (Title:) from the end of the record, trailing titles would be ugly.

The last part sets $_ depending on whether the block we are examining contains the string "invalid url". The Test?value1:value2 construct returns the first value if the test is true and the second if it fails. In this case we either fix up $_ for printing by prefixing "Title:" or we set it to an empty string so nothing is printed.

Cheers,
R.