This got me farther in some perl5 regexp experiments, and more questions, now at http://blogs.perl.org/users/yary/2019/08/splitting-on-a-change-in-perl6.html
-y On Sun, Aug 25, 2019 at 12:55 AM William Michels <w...@caa.columbia.edu> wrote: > 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 <b2gi...@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 <eef...@gmail.com> wrote: > >> > >> On Thu, Aug 22, 2019 at 6:11 PM yary <not....@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. > >> >
