Re: 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,

Comment on Re: Lookahead assertion Select or Download Code

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]
Re^3: Lookahead assertion by talexb (Chancellor) on Oct 17, 2013 at 11:39 UTC
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.	[reply] [d/l]
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]


Think about Loose Coupling
	PerlMonks