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Re: Lookbehind assertion weirdness
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From:
William Michels via perl6-users
Date:
August 24, 2019 16:55
Subject:
Re: Lookbehind assertion weirdness
Message ID:
CAA99HCzf+LCzeBd03Dq5NGf-+x+10DdkHbFvR1jdeSRLO258gQ@mail.gmail.com
Lookahead/lookbehind assertions: maybe the mnemonic "ABBA" will help?
In Markdown:
'Use *A*fter for a look-*B*ehind, use *B*efore for a look-*A*head',
or...
'For a look-*A*head' use *B*efore, for a look-*B*ehind" use *A*fter'.
As a trivial example of the first mnemonic in practice, below are
examples with html tags. Note the last two examples which differ in
the use of a 'greedy' (.*) pattern vs the use of a 'frugal' (.*?)
pattern, still give the same result:
> my $title = '<title>abcdefghijklmnopqrstuvwxyz</title>';
<title>abcdefghijklmnopqrstuvwxyz</title>
> $title ~~ /<?after '<title>' > .* <?before '</title>' >/;
｢abcdefghijklmnopqrstuvwxyz｣
> say ~$/ if $title ~~ /<?after '<title>' > .* <?before '</title>' >/;
abcdefghijklmnopqrstuvwxyz
> say ~$/ if $title ~~ /<?after '<title>' > .*? <?before '</title>' >/;
abcdefghijklmnopqrstuvwxyz
>
HTH, Bill.
https://docs.perl6.org/language/regexes
On Fri, Aug 23, 2019 at 2:28 PM Brad Gilbert <b2gills@gmail.com> wrote:
>
> `after` and `before` can be confusing, but I think it would be more confusing if it were the other way around.
>
> On Fri, Aug 23, 2019 at 2:15 PM Sean McAfee <eefacm@gmail.com> wrote:
>>
>> On Thu, Aug 22, 2019 at 6:11 PM yary <not.com@gmail.com> wrote:
>>>
>>> Perl 6 is doing the right thing. The dot matches any character. In
>>> this case, matching the final ':'. The next bit of the regex says the
>>> cursor has to be after 1:, and indeed, after matching the ':' the
>>> cursor is after '1:', so the substitution succeeds.
>>
>>
>> My real use case, that I tried to provide a simplified example of, was to process some pretty-printed JSON. Less simplified this time, I wanted to change all "foo": "whatever" strings to "foo": "*". In Perl 5 I would have done:
>>
>> s/(?<="foo": ")[^"]+/*/;
>>
>> Trying to express this in Perl 6, I thought "lookbehind" would naturally translate to a "before" assertion:
>>
>> s/<?before '"foo": "'><-["]>+/*/;
>>
>> ...but that didn't work. Various other attempts led to the simplified example I originally provided.
>>
>> Long story short, it seems that a Perl 5 (?<=...) lookbehind translates to a Perl 6 <?after ...> assertion, and likewise a Perl 5 (?=...) lookahead translates to a Perl 6 <?before ...> assertion. The terminology just confused me due to my prior Perl 5 experience.
>>
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