Distribution · text

Text::UpsideDown

Works

The "flipped" Unicode imitation — an exact involution over printable ASCII, because the table is merged with its own inverse.

Version: 1.0.0 github:mcsnolte
Depends: none beyond the core
License: none stated
Its own test suite: 2 files, green
Checked: 2026-09-15 against Raku++ 3.28.0 and Rakudo 2026.08
Where it lives: raku.land · source

Install it #

$ rakupp install Text::UpsideDown

zef install Text::UpsideDown writes the same store; either installer leaves the module usable by both engines.

What it is for #

¡pʃɹoM 'oʃʃǝH — text that looks like it has been rotated 180°, made from Unicode characters that happen to resemble upside-down Latin ones. It shows up in chat, in signatures and in novelty output.

The transform is two steps: reverse the character order, and substitute each character for its rotated look-alike.

Flipping #

File

use Text::UpsideDown;

for 'foo', 'Hello, World!', '0123456789' -> $s {
    say sprintf('  %-16s -> %s', $s.raku, upside_down($s));
}
say '';
say 'the string is FLIPPED first, so it is not a per-character map:';
say '  upside_down("foo") = ', upside_down('foo').raku, ' — not "ɟoo"';

Output

  "foo"            -> ooɟ
  "Hello, World!"  -> ¡pʃɹoM 'oʃʃǝH
  "0123456789"     -> 68ㄥ954Ɛ2⇂0

the string is FLIPPED first, so it is not a per-character map:
  upside_down("foo") = "ooɟ" — not "ɟoo"

The one thing to know #

upside_down is an exact involution over all printable ASCII: applying it twice returns the original string, every time. The lookup table is merged with its own inverse, which makes it a usable two-way transform rather than a one-way toy.

File

use Text::UpsideDown;

my @fail = (0x20 .. 0x7E).map(*.chr).grep({ upside_down(upside_down($_)) ne $_ });
say 'printable ASCII characters that do NOT round-trip : ', @fail.elems;
say '';
say 'so the transform is reversible:';
my $s = 'Hello, World!';
say '  ', $s.raku;
say '  ', upside_down($s).raku;
say '  ', upside_down(upside_down($s)).raku;
say '';
say 'the corollary is that some ASCII letters map to OTHER ASCII letters:';
say '  M and W are each other`s flip, so upside_down("MW") = ',
    upside_down('MW').raku;
say '';
my @fixed = (0x20 .. 0x7E).map(*.chr).grep({ upside_down($_) eq $_ });
say 'characters that are their own flip : ', @fixed.join;

Output

printable ASCII characters that do NOT round-trip : 0

so the transform is reversible:
  "Hello, World!"
  "¡pʃɹoM 'oʃʃǝH"
  "Hello, World!"

the corollary is that some ASCII letters map to OTHER ASCII letters:
  M and W are each other`s flip, so upside_down("MW") = "MW"

characters that are their own flip :  #$%+-/02458:=@HIOSXZ\`osxz|~

What it does not touch #

File

use Text::UpsideDown;

say 'unmapped characters still get REVERSED, just not substituted:';
say '  ', upside_down("\c[CJK UNIFIED IDEOGRAPH-65E5]\c[CJK UNIFIED IDEOGRAPH-672C] \c[SNOWMAN]").raku;
say '';
say 'and a precomposed accented letter is one grapheme that is not in the';
say 'table, so only its position moves:';
my $acc = "e\c[COMBINING ACUTE ACCENT]f";
say '  input  : ', $acc.raku, '  (', $acc.chars, ' graphemes)';
say '  output : ', upside_down($acc).raku;
say '';
say 'newlines are characters like any other, so a multi-line string comes';
say 'back reversed across the line break:';
say '  ', upside_down("ab\ncd").raku;

Output

unmapped characters still get REVERSED, just not substituted:
  "☃ 本日"

and a precomposed accented letter is one grapheme that is not in the
table, so only its position moves:
  input  : "éf"  (2 graphemes)
  output : "ɟé"

newlines are characters like any other, so a multi-line string comes
back reversed across the line break:
  "pɔ\nqɐ"

Where the two engines differ #

Nothing in the substitution. One argument shape differs, and only for a type object: upside_down(Int) is a run-time binding failure under Raku++ and a compile-time refusal under Rakudo, while a literal upside_down(42) is rejected at compile time on both.

File

use Text::UpsideDown;

say 'the parameter is Str with no Cool coercion, so coerce yourself:';
my $n = 42;
say '  upside_down($n.Str) = ', upside_down($n.Str).raku;
say '';
say 'there is one Unicode subtlety worth knowing before you scan a wide';
say 'range of codepoints through this function. Raku++ does not NFC-';
say 'normalise Int.chr, so a compatibility character such as the Kelvin';
say 'sign stays distinct from the letter K there and folds to it on';
say 'Rakudo — which makes a "does every codepoint round-trip?" sweep';
say 'disagree between the engines for about 565 codepoints.';
say '';
say 'printable ASCII is unaffected, which is what the table covers:';
my @fail = (0x20 .. 0x7E).map(*.chr).grep({ upside_down(upside_down($_)) ne $_ });
say '  ASCII failures : ', @fail.elems;

Output

the parameter is Str with no Cool coercion, so coerce yourself:
  upside_down($n.Str) = "24"

there is one Unicode subtlety worth knowing before you scan a wide
range of codepoints through this function. Raku++ does not NFC-
normalise Int.chr, so a compatibility character such as the Kelvin
sign stays distinct from the letter K there and folds to it on
Rakudo — which makes a "does every codepoint round-trip?" sweep
disagree between the engines for about 565 codepoints.

printable ASCII is unaffected, which is what the table covers:
  ASCII failures : 0

The distribution is not in the zef index; rakupp test resolves it from the REA archive and notes that the index path carries no checksum, so TLS is the only integrity check on the fetch. The META6 states no auth and no license; the auth used above comes from the ecosystem index and from the module's own unit module line.