Regex teaser

Paul Makepeace paulm at
Wed Dec 4 07:55:16 GMT 2013

On Tue, Dec 3, 2013 at 5:03 PM, Mark Fowler <mark at> wrote:
> On Tue, Dec 3, 2013 at 6:54 PM, Paul Makepeace <paulm at> wrote:
>> $ perl -le '($a = "aabbb") =~ s/b*$/c/g; print $a'
> This is where tools like Regexp::Debugger shine.  Running
>  perl -le 'use Regexp::Debugger; ($a = "aabbb") =~ s/b*$/c/g; print $a'
> Shows exactly why it gives the output it does (if you hit "n" for next a lot)

Can't use an undefined value as an ARRAY reference at
/Library/Perl/5.16/Regexp/ line 499.

Glad we're not the only ones confused by it ;-) But yeah that's neat.
I don't agree it shows WHY as much as HOW.

The puzzle comes down to whether the $ is part of the first b*
capture. IMO it is (and python seems to agree). Why the engine
restarts having captured as much as it can to the very end strikes me
as counter intuitive. Almost, if not actually, bug-like.

So, Part II:

Construct an elegant* regex (in perl!) to ensure the end of the string
contains "string":

is(fqdn('foo'), '')
is(fqdn(''), '')


* I realise this is hilariously open to interpretation but you'll know
what I mean either way

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