how to interpolate string containing capture-group parentheses as regex in Raku?

user_11064961 0
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I want to match against a programmatically-constructed regex, containing a number of (.*) capture groups. I have this regex as a string, say

my $rx = "(.*)a(.*)b(.*)"

I would like to interpolate that string as a regex and match for it. The docs tell me <$rx> should do the trick (i.e. interpolate that string as a regex), but it doesn't. Compare the output of a match (in the perl6 REPL):

> 'xaybz' ~~ rx/<$rx>/
｢xaybz｣

vs the expected/desired output, setting apart the capture groups:

> 'xaybz' ~~ rx/(.*)a(.*)b(.*)/
｢xaybz｣
 0 => ｢x｣
 1 => ｢y｣
 2 => ｢z｣

Comments

One unappealing way I can do this is to EVAL my regex match (also in the REPL):

> use MONKEY; EVAL "'xaybz' ~~ rx/$rx/";
｢xaybz｣
 0 => ｢x｣
 1 => ｢y｣
 2 => ｢z｣

So while this does give me a solution, I'm sure there's a string-interpolation trick I'm missing that would obviate the need to rely on EVAL..

regex-group raku regex
3 Answers

The result of doing the match is being matched when going outside the regex. This will work:

my $rx = '(.*)a(.*)b(.*)';
'xaybz' ~~ rx/$<result>=<$rx>/;
say $<result>;
# OUTPUT: «｢xaybz｣␤ 0 => ｢x｣␤ 1 => ｢y｣␤ 2 => ｢z｣␤»

Since, by assigning to a Match variable, you're accessing the raw Match, which you can then print. The problem is that <$rx> is, actually, a Match, not a String. So what you're doing is a Regex that matches a Match. Possibly the Match is stringified, and then matched. Which is the closest I can be to explaining the result

user_8914400

The problem is that things in <…> don't capture in general.

'xaybz' ~~ / <:Ll> <:Ll> <:Ll> /
# ｢xay｣

They do capture if the first thing after < is an alphabetic.

my regex foo { (.*)a(.*)b(.*) }

'xaybz' ~~ / <foo> /;
# ｢xaybza｣
#  foo => ｢xaybza｣
#   0 => ｢x｣
#   1 => ｢y｣
#   2 => ｢za｣

That also applies if you use <a=…>

'xaybz' ~~ / <rx=$rx> /;
# ｢xaybza｣
#  rx => ｢xaybza｣
#   0 => ｢x｣
#   1 => ｢y｣
#   2 => ｢za｣

Of course you can assign it on the outside as well.

'xaybz' ~~ / $<rx> = <$rx> /;
# ｢xaybza｣
#  rx => ｢xaybza｣
#   0 => ｢x｣
#   1 => ｢y｣
#   2 => ｢za｣

'xaybz' ~~ / $0 = <$rx> /;
# ｢xaybza｣
#  0 => ｢xaybza｣
#   0 => ｢x｣
#   1 => ｢y｣
#   2 => ｢za｣

Note that <…> is a sub-match, so the $0,$1,$2 from the $rx will never be on the top-level.

Brad_Gilbert_13370

You could do the following to expose the inner regex result to an outside variable:

my $rx = "(.*)a(.*)b(.*)";
my $result;

'xaybz' ~~ / $<result>=<$rx> {$result = $<result>}/;

say $result;

# OUTPUT:

# ｢xaybz｣
# 0 => ｢x｣
# 1 => ｢y｣
# 2 => ｢z｣
user_124682000
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