grep {CONSTANT} @list

creeble has asked for the wisdom of the Perl Monks concerning the following question:

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Re: grep {CONSTANT} @list by tobyink (Canon) on Nov 21, 2012 at 20:08 UTC
Nothing to do with `grep` in particular, or constants. `use constant REGEXP => qr(a); print grep { REGEXP } qw(a b c);` [download] ... is equivalent to: `my @tmp; foreach (qw(a b c)) { push @tmp, $_ if qr(a); } print @tmp;` [download] See the problem now? `if qr(a)` is always true. `if qr(anything)` is always true. Try: `print grep { $_ =~ REGEXP } @list;` [download] `perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'`	[reply] [d/l] [select]
Re: grep {CONSTANT} @list by LanX (Saint) on Nov 21, 2012 at 20:42 UTC
you are confusing a regex data-type with a regex operation. `grep {REGEXP}` uses a code-block which returns a regex which is always true if defined. `grep {/string/}` uses a `m//` (match operation) which defaults to test $_. Cheers Rolf	[reply] [d/l] [select]
Re: grep {CONSTANT} @list by afoken (Chancellor) on Nov 21, 2012 at 19:05 UTC
Documented in constant. Documented better (and fixed) in Readonly. Alexander -- Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)	[reply]
Re^2: grep {CONSTANT} @list by creeble (Sexton) on Nov 21, 2012 at 19:11 UTC
I don't see this documented in constant -- or at least, there's no mention of qr, grep, or regexp on the constant manpage.	[reply]
Re^3: grep {CONSTANT} @list by frozenwithjoy (Priest) on Nov 21, 2012 at 19:21 UTC
Perhaps this (constant#CAVEATS): You can get into trouble if you use constants in a context which automatically quotes barewords... And this (Readonly#COMPARISON_WITH_"use_constant"): You have to be very careful in places where barewords are allowed...	[reply]
Re^3: grep {CONSTANT} @list by Anonymous Monk on Nov 21, 2012 at 19:24 UTC
don't look for keywords, read docs, esp the notes and caveats	[reply]
Re: grep {CONSTANT} @list by frozenwithjoy (Priest) on Nov 21, 2012 at 19:52 UTC
The way I see it, you still need to put `//` around the pattern you are trying to match that is contained in `REGEX` (in this case it is `(?^:string)`). say grep { REGEX } @list; #doesn't work, because it isn't really doing what you think it is doin +g say grep { /REGEX/ } @list; #doesn't work because of the problems mentioned above when using const +ants in contexts that auto-quote barewords say grep { /(?^:string)/ } @list; #this works (and throws errors if // are removed) use Readonly; Readonly::Scalar my $regex_ro => qr/string/; say grep {/$regex_ro/} @list; #this works (and, as expected, gives same bad result you got if // is +removed) my $regex = qr/string/; say grep { /$regex/ } @list; #this works (and, as expected, gives same bad result you got if // is +removed) [download]	[reply] [d/l] [select]