in reply to Lookahead assertion

Look-ahead is (?= ), not (?: ) !

13:46 >perl -wMstrict -MData::Dump -E "my $foo = '123456789'; my @w1 =
+ ($foo =~ /(?=(\d{3}))/g); dd \@w1;"
[123, 234, 345, 456, 567, 678, 789]

13:46 >

[download]

Update: The use of (?: ) instead of (?= ) appears to be a misprint in the Camel Book.

The following explanation from that same section (page 248) is worth noting (at least, it helped me to understand what is going on):

When the engine sees that it should try again because of the /g, it steps one character past where last it tried.

This explains why the final . in BrowserUk’s solution is not strictly necessary. But BrowserUk’s regex can be usefully generalised to step forward an arbitrary number of characters. For example, to capture 3 digits and then step forward 2 characters:

17:56 >perl -wMstrict -MData::Dump -E "my $foo = '123456789012345'; my
+ @w1 = ($foo =~ /(?=(\d{3})).{2}/g); dd \@w1;"
[123, 345, 567, 789, 901, 123, 345]

18:09 >

[download]

Hope that helps,

Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

Replies are listed 'Best First'.
Re^2: Lookahead assertion
by BrowserUk (Patriarch) on Oct 17, 2013 at 08:29 UTC
    When the engine sees that it should try again because of the /g, it steps one character past where last it tried.

    This explains why the final . in BrowserUk’s solution is not strictly necessary

    Indeed. The final '.' is not necessary with Perl's regex engine.

    But many other regex libraries -- including some that claim to be Perl-compatible -- do not have this pragmatic deviation from the classical regular expression operation; hence it has become a habit with me to do this explicitly.

    With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority".
    In the absence of evidence, opinion is indistinguishable from prejudice.
[reply]

      Thanks for your reply .. and I checked, the final dot makes no difference in the result .. 

        DB<6> @w3 = ( $foo =~ /(?=(\d{3}))/g );

  DB<7> x @w3
0  123
1  234
2  345
3  456
4  567
5  679
6  790
  DB<8> @w3 = ( $foo =~ /(?=(\d{3}))./g );

  DB<9> !7
x @w3
0  123
1  234
2  345
3  456
4  567
5  679
6  790

      [download]
      Good to know.

      Alex / talexb / Toronto

      Thanks PJ. We owe you so much. Groklaw -- RIP -- 2003 to 2013.

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Re^2: Lookahead assertion
by talexb (Chancellor) on Oct 17, 2013 at 11:35 UTC

    Thanks for your response -- I posted this problem last thing at night, then went to bed. When I woke up this morning, my mind returned to this problem, and I thought, "That's ridiculous -- could it be a mis-print?" 

      DB<6> @w3 = ( $foo =~ /(?=(\d{3}))/g );

  DB<7> x @w3
0  123
1  234
2  345
3  456
4  567
5  679
6  790

    [download]
    Yep. A ':' where it should have been a '='. Guess I better check to see what other errata there are.

    Alex / talexb / Toronto

    Thanks PJ. We owe you so much. Groklaw -- RIP -- 2003 to 2013.

[reply]
[d/l]

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