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interpolating the return from embedded code in a regexp

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Joseph Brenner
June 14, 2020 23:44
interpolating the return from embedded code in a regexp
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In part because of the recent discussion here, I decided to
play around with using Raku code embedded in a regexp.
I came up with a contrived example where I was going to
examine a product listing in a text block to see if the product
descriptions matched the product codes.  The valid associations
I have in a hash, so I'm (1) matching for product codes; (2)
using embedded code to look-up the associated description in the hash;
(3) using the returned description inside the regex.

my %products = ( 'P123' => "Green Labels That Say Magenta",
                 'P666' => 'Darkseid For President Bumpersticker',
                 'P912' => "Corn dogs",

my $text =  q:to/END/;
P123  Viridian Green Label Saying Magenta
P666  Yoda puppets
P912  Corn dogs

my @lines = $text.lines;
say @lines;

for @lines -> $line {
   say "checking line: $line";
   ## This line works, but it's not a complete solution:
   if $line ~~ / (^P\d+) \s+ <{ %products{$0}.subst(/\s+/, '\s', :g) }> / {
       say "Matched, line looks good";
   else {
       say "NO: bad line.";

I'd thought that a line like this would work:

    if $line ~~ / (^P\d+) \s+ <{ %products{$0} }> / {

The trouble though is I've got spaces inside the descriptions,
so if the returned string is treated as a regexp, I get these

   Potential difficulties:
       Space is not significant here; please use quotes or :s
(:sigspace) modifier (or, to suppress this warning, omit the space, or
otherwise change the spacing)

Reading a bit, I thought this should work

    if $line ~~ / (^P\d+) \s+ $( %products{$0} ) / {

That's supposed to use the return string as a literal match.
Instead I get a lot of strange messages like:

   Use of Nil in string context   in regex

Flailing around I considered lots of variations like this:

   if $line ~~ / (^P\d+) \s+ Q[<{ %products{$0}}>] / {

But I think that ends up treating everything inside the Q[]
literally, so you never do the hash lookup.

Another thing that might solve this problem is some sort of
regexp quote function I could use inside the code before
returning the string, but I don't know what that would be...

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