Implicit EVAL in regexes (trap?)

[AlexDaniel]

Let's say you have this code:

my $x = ‘hello’;
say ‘hello world’ ~~ /<{$x}>/

Result:

｢hello｣

So far everything looks alright. However, let's change the input data ($x) a little bit:

my $x = ‘#hello’;
say ‘hello world’ ~~ /<{$x}>/

Result:

===SORRY!===
Regex not terminated.
at /home/alex/EVAL_0:1
------> anon regex { #hello}⏏<EOL>
Regex not terminated.
at /home/alex/EVAL_0:1
------> anon regex { #hello}⏏<EOL>
Malformed regex
at /home/alex/EVAL_0:1
------> anon regex { #hello}⏏<EOL>
    expecting any of:
        infix stopper

So it turns out you cannot use variables with arbitrary data in <{ }>? To be honest, I'm not entirely sure what is happening here and if it should work like this. For example, if we're in EVAL, then why MONKEY-SEE-NO-EVAL plays no role here? Would be nice to have this explained somewhere.

[AlexDaniel]

Oh, actually, MONKEY-SEE-NO-EVAL does work here, but in a weird way:

Code:

my $x = ‘Hello{say ‘boo!’}’;
say ‘#hello world’ ~~ / <{$x}> /

Result:

===SORRY!=== Error while compiling /home/alex/EVAL_0
Prohibited regex interpolation (use the MONKEY-SEE-NO-EVAL pragma to override this error,
but only if you're VERY sure your data contains no injection attacks)
at /home/alex/EVAL_0:1
------> anon regex { Hello⏏{say ‘boo!’}}

[AlexDaniel]

And also, let's say I want to generate some string and match against it, is it possible to do it without creating a temp variable? For example, let's say I have $x but I want to use $x.lc inside a regex, or $x // ‘blah’ or whatever.

So far I came up with this:

my $x = ‘#Hello’;
say ‘#hello world’ ~~ / <{ my $foo = $x.lc; /$foo/ }> /

Maybe it works, I'm not entirely sure. But that's ridiculous anyway. What's the proper way to do it?

[jnthn]

/$($x.lc)/

[AlexDaniel]

Yup! This is it. Not sure if I've ever seen it, but definitely expected something like this. Cool, thanks.