Escaped hash character \# is somtimes parsed incorrectly

[p6rt]

Migrated from rt.perl.org#132666 (status was 'resolved')

Searchable as RT132666$

[p6rt]

From @smls

These regexes parse and run fine (they match a single `#` character)​:

  /\#/;
  /^\#/;

But this one doesn't​:

  /^ \#/;

The error thrown, is​:

  ===SORRY!===
  Regex not terminated.
  at -e​:1
  ------> /^ \#/⏏<EOL>
  Unable to parse regex; couldn't find final '/'
  at -e​:1
  ------> /^ \#/⏏<EOL>
  expecting any of​:
  infix stopper

It's as if the parser fails to recognize that the `#` character was
escaped (interpreting the backslash as unspace maybe?), and treats
everything until the end of the line as a comment.

This seems to be only happening when the escaped `#` character is
preceded by another token, *and* separated from it by whitespace.

Apparently this bug has always existed in Rakudo​:
https://gist.github.com/Whateverable/ca082a33d2ab689bf3ca4b6f7ae2d19f

[p6rt]

From @zoffixznet

On Fri, 29 Dec 2017 07​:05​:29 -0800, smls75@​gmail.com wrote​:

These regexes parse and run fine (they match a single `#` character)​:

/\\\#/;
/^\\\#/;

But this one doesn't​:

/^ \\\#/;

The error thrown, is​:

===SORRY\!===
Regex not terminated\.
at \-e​:1
\-\-\-\-\-\-> /^ \\\#/⏏\<EOL>
Unable to parse regex; couldn't find final '/'
at \-e&#8203;:1
\-\-\-\-\-\-> /^ \\\#/⏏\<EOL>
    expecting any of&#8203;:
        infix stopper

It's as if the parser fails to recognize that the `#` character was
escaped (interpreting the backslash as unspace maybe?), and treats
everything until the end of the line as a comment.

This seems to be only happening when the escaped `#` character is
preceded by another token, *and* separated from it by whitespace.

Apparently this bug has always existed in Rakudo​:
https://gist.github.com/Whateverable/ca082a33d2ab689bf3ca4b6f7ae2d19f

Thank you for the report. This is a duplicate of rakudo/rakudo#1324

The problematic cases are matched[^1] as unspaces. The `\#` thing was never meant[^2] to work, but it slipped through and now we probably have to deal with back-compat issues in order to fix it.

[1] rakudo/rakudo#1324 (comment)
[2] rakudo/rakudo#1324 (comment)

[p6rt]

The RT System itself - Status changed from 'new' to 'open'

[p6rt]

@zoffixznet - Status changed from 'open' to 'resolved'

[JJ]

I'm not sure about reopening this, but the error still seems to be there.