Hello Monks,
I would like some help in understanding the regular expression in the following code: 
sub TextFile 
{
    return 0 if (! -f $_[0]);
    return 0 if (! -r $_[0]);
    
    open FH, "<" . $_[0];
    my $block = " " x 4096;
    my $bytesread = sysread FH, $block, 4096;
    close FH;
    
    if (! defined $bytesread)
    {
        print "*** ERROR: TextFile: $_[0]: $!\n";
        return 0;
    }

    return $block =~ /^[\r\n\t -~]*$/s;
}

[download]
It attempts to guess whether a file is a text file based on what it sees in the first 4096 characters. I ran across it while seeking an alternative to -T to check for a text file. It seems to work in my tests, but I don't understand why because I don't understand what the regex is matching.

My understanding is that [] defines a character class. A ^ before [] means negation. * means zero or more. $ means end of line. So if I put that all together, it seems to mean that 
 
$block =~ /^[\r\n\t -~]*$/s

[download]
is true if $block does nor include any of \r\n\t -~ before an end of line. But that doesn't make sense. I'm also mystified by the inclusion of the characters -~ in the character class.

Can anyone unpack all this for me?

I am running strawberry perl 5.30 on windows 10.

Thanks!

In reply to Question regarding a regex by CrashBlossom

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