Well, you need fairly long file names for this to work so that eliminates DOS and the 14-character Unix file systems but also means that you'll have no problem using [-A-Za-z0-9._] in the file names. This gives us lots of room for "reversible fun".

I'd go with something like:

• [A-Za-z0-9.] remain unchanged
• [ ] becomes [_]
• Everything else becomes something like "-garbage-" where "garbage" is some reversible translation of the "everything else".

Now, with [A-Za-z0-9. ] already taken care of, we only need to encode the remaining 31 punctation characters, 33 ASCII control characters, and 128 eight-bit characters. And we can use [A-Za-z0-9._] (64 characters) for this encoding. That would make it easy to just go for a base-64 encoding, but that would make the filename longer than it needs to be for most titles.

Since I assume that control characters and 8-bit characters won't appear often in titles (international characters should be inserted using HTML escapes as 8-bit encodings vary), I wanted punctuation characters to be very compactly represented. After some playing around, I like converting "-" to "--", isolated punctuation characters to "-[a-z0-3]", and all other illegal sequences of characters to -([A-Z4-7]|[89_][\w.])+- where [A-Z4-7] represent a punctuation mark and [_89][\w.] represents a control or 8-bit character. Note that this leaves "-." available for future meanings.

Here is the code to do this packaged with a test program that encodes or decodes from STDIN to STDOUT. I tested it with:

    perl titles.pl <thispost.txt >thispost.enc
    perl titles.pl -d <thispost.enc >thispost.dec
    fc thispost.txt thispost.dec
    perl titles.pl -e <perl.exe >perl.enc
    perl titles.pl -e -d <perl.enc >perl.dec
    fc perl.exe perl.dec
[download]

The decoded files were identical to the originals. (-d means "decode" and "-e" means en/decode entire file rather than 1 line at a time.)

The encoding of this posting contained the follow lines as examples:

Well-l_you_need_fairly_long_file_names_for_this_to_work_so_that
eliminates_DOS_and_the_14--character_Unix_file_systems_but_also
means_that_you-gll_have_no_problem_using_-pcode-RU---A--Za--z0--9.-YWP
+M-code-r
in_the_file_names.__This_gives_us_lots_of_room_for_-breversible
fun-b.
-PM-p-RP-p-r
[...]
my_-deight1-q_pack_-bC-JBL-_0x80..0xBF-o
my_-deight2-q_pack_-bC-JBL-_0xC0..0xFF-o
my_-dctrl-q_pack_-bC-JBL-_0..31-l128-o
my_-dpunct-q_q---ABCDEFGHIJKLMNOPQRSTUVWXYZ4567----o
my_-dcode-q_join_-BBL-_-g0-g..-g9-GLG-a-g..-gz-GLG-A-g..-gZ-GLGYGLG-.-
+GO-
[...]
________-2_elsif-h__1_-QQ-_length-HD-1-i__-FF-__0_-PQ-_index-HD-punct-
+LD-1-i__-i_-0
[download]

Here is the code:

#!/usr/bin/perl -w
use strict;

# [A-Za-z0-9.] => unchanged
# " " => "_"
# "-" => "--"
# isolated punctation => "-[a-z0-3]"
# other => "-" . ( [A-Z4-7] | [_89][\w.] )+ . "-"
#                punctuation   _=control, 8,9=8-bit

my $eight1= pack "C*", 0x80..0xBF;
my $eight2= pack "C*", 0xC0..0xFF;
my $ctrl= pack "C*", 0..31,128;
my $punct= q-!"#$%&'()*+,/:;<=>?@[\\]^_`{|}~-;
my $code= join "", '0'..'9','a'..'z','A'..'Z','_','.';

sub encode {
    local( $_ )= shift;
    s{(-|[^-a-zA-Z0-9. ]+)}{
        if(  "-" eq $1  ) {
            "--";
        } elsif(  1 == length($1)  &&  0 <= index($punct,$1)  ) {
            my $p= $1;
            $p =~ tr.!"#$%&'()*+,/:;<=>?@[\\]^_`{|}~.a-z0-3.;
            "-$p";
        } else {
            my $g= $1;
            $g =~ s{([\Q$punct\E])|([$ctrl])|([$eight1])|([$eight2])}{
                my $c= $1 || $2 || $3 || $4;
                if(  $1  ) {
                    $c =~ tr.!"#$%&'()*+,/:;<=>?@[\\]^_`{|}~.A-Z4-7.;
                    $c;
                } elsif(  $2  ) {
                    "_" . substr( $code, index($ctrl,$c), 1 );
                } elsif(  $3  ) {
                    "8" . substr( $code, index($eight1,$c), 1 );
                } else {
                    "9" . substr( $code, index($eight2,$c), 1 );
                }
            }ge;
            "-$g-";
        }
    }ge;
    tr/ /_/;
    return $_;
}

sub decode {
    local( $_ )= shift;
    s{(_)|-(-)|-([a-z0-3])|-((?:[A-Z4-7]|[_89][\w.])+)-}{
        if(  $1  ) {
            " ";
        } elsif(  $2  ) {
            "-";
        } elsif(  ! $4  ) {
            my $g= $3;
            $g =~ tr.a-z0-3.!"#$%&'()*+,/:;<=>?@[\\]^_`{|}~.;
            $g;
        } else {
            my $g= $4;
            $g =~ s{([A-Z4-7])|([_89])([\w.])}{
                #{ local( $^W);
                #    warn "else: 1-($1) 2-($2) 3-($3)\n";
                #}
                my $g= $1 || $3;
                if(  $1  ) {
                    $g =~ tr.A-Z4-7.!"#$%&'()*+,/:;<=>?@[\\]^_`{|}~.;
                    $g;
                } elsif(  "_" eq $2  ) {
                    substr( $ctrl, index($code,$g), 1 );
                } elsif(  "8" eq $2  ) {
                    substr( $eight1, index($code,$g), 1 );
                } else {
                    substr( $eight2, index($code,$g), 1 );
                }
            }ge;
            $g;
        }
    }ge;
    return $_;
}

my $decode= grep /^-d/, @ARGV;
binmode( $decode ? \*STDOUT : \*STDIN );
if(  grep /^-e/, @ARGV  ) {
    my $data= do { local( $/ ); <STDIN> };
    print $decode ? decode($data) : encode($data);
} else {
    while(  <STDIN>  ) {
        s#\r?\n$##;
        print $decode ? decode($_) : encode($_), "\n";
    }
}
[download]

Enjoy!

        - tye (but my friends call me "-757-_-ZM-_-h--")

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