Re: Regex question - capturing next char

multiple problems:

you say "word" but you mean string, right?
you don't anchor to the end of the string with $ but to the start with ^
you use an all quantifier * for the negated class, which also means 0-times, so r doesn't need to be rejected and the if-clause is entered.

Actually I don't know any more what your real goal is, so no recommendation other than

  DB<114> $in_str = 'aa<c><d>r';
 => "aa<c><d>r"

  DB<115> if ($in_str =~ /aa((<[^>]*>)*)$/) {print "$1"}
 => ""

  DB<116> $in_str = 'aa<c><d>';
 => "aa<c><d>"

  DB<117> if ($in_str =~ /aa((<[^>]*>)*)$/) {print "$1"}
 => 1
<c><d>
[download]

maybe try to show us better input and desired output.

Cheers Rolf

_{(addicted to the Perl Programming Language and ☆☆☆☆ :)}

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Re^2: Regex question - capturing next char by perluser4102 (Initiate) on Oct 10, 2014 at 06:11 UTC
Thanks for the replies. Let me explain my goal using a few examples: Case 1: $in_str = ‘somewordaa<metadata><moremetadata>r another wo<metadata>rd’; => aa should not be replaced by a, since aa is immediately followed by another alphabet 'r' once the metadata is ignored. Case 2: $in_str = ‘aword<metadata>aa<metadata> another wo<metadata>rd’; => aa should be replaced by a because it is at the end of a word. It is followed by space. Case 3: $in_str = 'wor<metadata>dsandmor<metadata>ewordsaa<metadata>;' => aa should be replaced by a because aa is at the end of the word (also end of string) once metadata is ignored. I now understand that the quantifier * on a negation of character class won't work because of the "0 times" match. I need to check if aa is at the end of a word. I plan to do this by checking if the following character (again, after ignoring metadata) is either a non-alphabet character or end of string. How would I go about doing this? `$in_str =~ s/aa((<[^>]>))([^a-zA-Z])/a$1$3/g;`	[reply] [d/l]
Re: Regex question - capturing next char by jonadab (Parson) on Oct 10, 2014 at 10:25 UTC
I think you can do this with a single substitution regex (s/pattern/replacement/;), ~~but it's going to need a lookahead assertion at the end~~. Also, this is only possible if you can write a regex segment that will consistently match the metadata. (Making it match metadata if there is any and not block matching if there isn't any is then trivial: precede the metadata-match portion in `(?:` and follow it up with `)`. Perhaps something along these lines... `s/(a(?:[<][^>]+[>]))a((?:[<][^>]+[>])*(?:\s+\|[;,.])+)/$1$2/;` That appears to work with the actual sample data you provided, but I don't know if it will work with your real data.	[reply] [d/l] [select]
Re^2: Regex question - capturing next char by QM (Parson) on Oct 10, 2014 at 13:43 UTC
And for the degenerative case: `[<][^>]+[>]` does not allow for an empty `<>` field. Perhaps `[<][^>]*[>]` is better? -QM -- Quantum Mechanics: The dreams stuff is made of	[reply] [d/l] [select]
Re^3: Regex question - capturing next char by AnomalousMonk (Archbishop) on Oct 10, 2014 at 15:03 UTC
I would approach the problem this way: c:\@Work\Perl>perl -wMstrict -le "use Test::More 'no_plan'; use Test::NoWarnings; ;; my $metadata = qr{ < [^>]* > }xms; ;; for my $ar_vector ( [ 'somewordaa<metadata><moremetadata>r another wo<metadata>rd', 'somewordaa<metadata><moremetadata>r another wo<metadata>rd', ], [ 'aword<metadata>aa<metadata> another wo<metadata>rd', 'aword<metadata>a<metadata> another wo<metadata>rd', ], [ 'wor<metadata>dsandmor<metadata>ewordsaa<metadata>', 'wor<metadata>dsandmor<metadata>ewordsa<metadata>', ], ) { my ($s, $expected) = @$ar_vector; $s =~ s{ aa (?! $metadata* r) }{a}xmsg; is $s, $expected, qq{'$expected'}; } " ok 1 - 'somewordaa<metadata><moremetadata>r another wo<metadata>rd' ok 2 - 'aword<metadata>a<metadata> another wo<metadata>rd' ok 3 - 'wor<metadata>dsandmor<metadata>ewordsa<metadata>' ok 4 - no warnings 1..4 [download] Note that making the character class match 'atomic' or possessive may speed things up a bit for large data (for some definition of 'large') by suppressing backtracking on subsequent match failure: `my $metadata = qr{ < (?> [^>]) > }xms;` and for Perl version 5.10 and above: `my $metadata = qr{ < [^>]+ > }xms;` (both variations tested). Your data will determine if possessiveness actually makes a difference. The regex compiler may be smart enough to make this optimization on its own (I haven't checked), but giving the compiler a hint never hurts. And of course, you will want to add many more test cases!	[reply] [d/l] [select]