Brad: "Note that {} is there to update $/ so that $0 works the way you
would expect"I ran into that before & was trying to remember that detail... found it in https://docs.raku.org/language/regexes#Capture_numbers But the example is a bit on the obtuse side: These capture variables are only available outside the regex. # !!WRONG!! The $0 refers to a capture *inside* the second capture say "11" ~~ /(\d) ($0)/; # OUTPUT: «Nil» In order to make them available inside the regex, you need to insert a code block behind the match; this code block may be empty if there's nothing meaningful to do: # CORRECT: $0 is saved into a variable outside the second capture # before it is used inside say "11" ~~ /(\d) {} :my $c = $0; ($c)/; # OUTPUT: «｢11｣ 0 => ｢1｣ 1 => ｢1｣» say "Matched $c"; # OUTPUT: «Matched 1» As of Raku 2019.11 at least, the $0 construct works without the {} code block, in simple cases. > "11" ~~ /(\d) $0/ ｢11｣ 0 => ｢1｣ > '876554' ~~ /(\d) $0/ ｢55｣ 0 => ｢5｣ # Inside a new capture, $0 refers to inner capture > "11" ~~ /(\d) ($0)/ Nil > "11" ~~ /(\d) {} ($0)/ Nil > "11" ~~ /(\d) {} :my $c=$0; ($c)/ ｢11｣ 0 => ｢1｣ 1 => ｢1｣ # Let's use that inner capture > "1122" ~~ /(\d) {} :my $c=$0; ($c (\d) $0)/ ｢1122｣ 0 => ｢1｣ 1 => ｢122｣ 0 => ｢2｣ The Match docs can be clearer on when to use {} and when it isn't needed, opened an issue https://github.com/Raku/doc/issues/3478 -y On Mon, Jun 15, 2020 at 3:09 PM Brad Gilbert <b2gi...@gmail.com> wrote: > You don't want to use <{…}>, you want to use "" > > if $line ~~ / (^P\d+) \s+ {} "%products{$0}" / { > > Note that {} is there to update $/ so that $0 works the way you would > expect > > Although I would do something like this instead: > > my ($code,$desc) = $line.split( /\s+/, 2 ); > if %products{$code} eq $desc { > > On Sun, Jun 14, 2020 at 6:44 PM Joseph Brenner <doom...@gmail.com> wrote: > >> 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 >> END >> >> 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 >> warnings: >> >> 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... >> >
