[za-pm] Introducing Darryl Trimming ... with a regex /x modifier problem

[James Wright]

  On 05/02/11 04:47, Anne Wainwright wrote:
>
>
>
> Hi Anne
>
> I've just started learning perl, and found your contact details here 
> (http://www.pm.org/groups/522.html) while looking for community help. 
> It seems a bit out of date, so please forgive me if I'm knocking on 
> the wrong door.
>
B::Deparse's -p option is helpful here:

> I was exploring the "/x" regexp modifier, and came upon some 
> unexpected results. Upon closer inspection, my problem lies with the 
> ternary operator. I can't see why the following results are 
> inconsistent ..
>
>           say '1:'.( 'hello' =~ /h e l l o/ )?'true':'false';        # 
> no '/x', no match
>
say((('1:' . ('hello' =~ /h e l l o/)) ? 'true' : 'false'));

it evaluates the string '1:' concated with the return of the regex 
evaluation, and '1:' is true.  This can be seen by replace '1:' with '0' 
which is false.
>
>           say '2:'.( 'hello' =~ /h e l l o/x )?'true':'false';       # 
> '/x' ignores spaces, produces '1'
>
say((('2:' . ('hello' =~ /h e l l o/x)) ? 'true' : 'false'));

While this is 'correct' as 'hello' does match /h e l l o/x, it isn't why 
it returns true, it is again a concatenation of '2:' with the result of 
the regex match, i.e. '2:1' which is also true.

>           say '3:'.( 'h e l l o' =~ /h e l l o/x )?'true':'false';   # 
> '/x' ignores spaces, no match
>
say((('3:' . ('h e l l o' =~ /h e l l o/x)) ? 'true' : 'false'));

The same as 1:
>
> say '1:'.( 'hello' =~ /h e l l o/ ).'.';        # no '/x', no match
>
> say '2:'.( 'hello' =~ /h e l l o/x ).'.';       # '/x' ignores spaces, 
> produces '1'
>
> say '3:'.( 'h e l l o' =~ /h e l l o/x ).'.';   # '/x' ignores spaces, 
> no match
>
>           say '1:'.( 'hello' =~ /h e l l o/ ) && 'true.';        # no 
> '/x', no match
>
>           say '2:'.( 'hello' =~ /h e l l o/x ) && 'true.';       # 
> '/x' ignores spaces, produces '1'
>
>           say '3:'.( 'h e l l o' =~ /h e l l o/x ) && 'true.';   # 
> '/x' ignores spaces, no match
>
More of the same:

> say '1:'.(( 'hello' =~ /h e l l o/ ) + 0) .'.';        # no '/x', no match
>
> say '2:'.(( 'hello' =~ /h e l l o/x ) + 0) .'.';       # '/x' ignores 
> spaces, produces '1'
>
> say '3:'.(( 'h e l l o' =~ /h e l l o/x ) + 0) .'.';   # '/x' ignores 
> spaces, no match
>
>           say '1:'.(( 'hello' =~ /h e l l o/ ) + 
> 0)?'true':'false';        # no '/x', no match
>
>           say '2:'.(( 'hello' =~ /h e l l o/x ) + 
> 0)?'true':'false';       # '/x' ignores spaces, produces '1'
>
>           say '3:'.(( 'h e l l o' =~ /h e l l o/x ) + 
> 0)?'true':'false';   # '/x' ignores spaces, no match
>
> .. where only the highlighted sections above yield the expected 
> results. All the others always return 'true'.
>
> If you can't help, then please point me in the right direction for 
> community support.
>
> Many thanks and kind regards
>
> DT
>
'.' is binding closer than the ternary operator, you want something like:
say '2:'.('hello' =~ /h e l l o/x ? 'true' : 'false');
to evaluate the regex match with the ternary operator, not the result of 
the concatenation.

>
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