Perl 6: Backslashes in transliteration (tr///)

Question

I noticed while experimenting with tr///, that it doesn't seem to translate backslashes, even when escaped. For example,

say TR"\^/v"." given 'v^/\\';
say TR"\\^/v"." given 'v^/\\';
say TR"\ ^/v"." given 'v^/\\';

All of them output ...\ rather than what I expected, ....

There's some other weird behaviour too, like \ seemingly only escaping lowercase letters, but the docs page doesn't have much information... What exactly is the behaviour of backslashes (\) in transliteration (tr///)?

Yount

I don't think any of the user doc aspects of this gist helps but perhaps the links to roast tests and code at the end do – Stratosphere

It appears to me like a bug in the undocumented 'cc' role in QGrammar that gets used by the tr/// operator github.com/rakudo/rakudo/blob/master/src/Perl6/… – Inveracity

Answer 1

There is a bug caused by backslashes getting swallowed instead of correctly escaping things in the grammar for tr///.

say TR/\\// given '\\'
===SORRY!=== Error while compiling:
Malformed replacement part; couldn't find final /
at line 2
------> <BOL>⏏<EOL>

I have raised https://github.com/rakudo/rakudo/issues/2456 and submitted https://github.com/rakudo/rakudo/pull/2457 which fixes it.

The second part of the answer is that Perl 6 tries quite hard in some quoting constructs to only interpret \ as an escape for valid escape sequences, i.e. \n, \r, \s, \', etc. Otherwise it is left as a literal \.

Inveracity

Answer 2

I do not have an explanation for the observed problem. However, when you use the Perl 6 Str.trans method it looks like it's working as expected:

say 'v^/\\'.trans( "\\^/v" => "." );

Outputs:

....

Reference:

• https://perl6advent.wordpress.com/2010/12/21/day-21-transliteration-and-beyond/

Crossjack

I know about the trans routine (I could even use s:g/// if I wanted to be more regexey), but my question is specifically regarding the behaviour of tr///. trans simply takes Pairs, and the strings and regexes are evaluated on the usual way beforehand. – Yount