Interpolation of captures into error messages

[bpj]

I wish there was a way to interpolate captures into error messages.

As an example my DSL supports == <= => comparison but not any kind of not-equal operator because the comparison is always followed by an operator which indicates what to do if the comparison succeeds or fails — think ternary except that either the if-true part or the if-false part may be left out. I want to throw an error if the user tries to use a not-equal operator, but I also want to detect more than one (im)possible not-equal operator (!= <> ~=). So basically I want to do something like this:

not_equal:
  / ( [ BANG TILDE ] EQUAL | LANGLE RANGLE ) /
  `Not-equal operator ('$1') not supported.`

instead of this:

not_equal:
    / BANG EQUAL /
    `Not-equal operator ('!=') not supported.`
  | / TILDE EQUAL /
    `Not-equal operator ('~=') not supported.`
  | / LANGLE RANGLE /
    `Not-equal operator ('<>') not supported.`

As a compromise I do this:

not_equal:
  / (! [ BANG TILDE ] EQUAL | LANGLE RANGLE ) /
  `Not-equal operator not supported.`

but it is not really satisfactory because I want to show the offender in the error message.

So I'm thinking a simple syntax like $n or ${n} to interpolate a capture and $$ to get a single dollar:

$message =~ s{ \$ (\$) | \$\{ (?! 0 ) (\d+) \} | \$ (?! 0 ) (\d+) \b }{
    if ( defined $1 ) { $1 }
    elsif ( defined $captures[$+ - 1] ) { $captures[$+ - 1] }
    else { croak "No capture \$$+ for error message `$message`" }
}egx;