Regular expression matches last occurrence instead of first

ruby-talk
1

I've found an anomoly in the way Ruby handles non-greedy regular
expressions and wonder whether it's been discussed before. A search of
the documentation and a general Internet search didn't turn up
information on this issue.

When I want to match the first quoted string in a string such as:

  "aaaaa""bbb""ccc"

I match the last quoted string instead. The exact characters don't
matter.

Here's the sample code; note that (.*?) and ([^"]+) behave the same
way--and not the way I'd expect:

    str = '"aaaaa""bbb""ccc"'

    str.scan(/"(.*?)"/)
    puts $1
    # ccc

Andy Oram
    str.scan(/"([^"]+)"/)
    puts $1
    # ccc

    str.scan(/"(.*?)"(.*)/)
    puts $1
    # aaaaa

Adding an extra (.*) to the end produces the result I want, but I
don't believe it should make any difference.

Here is the equivalent Perl, which works as expected:

   $str = q{"aaaaa""bbb""ccc"};
   $str =~ /"(.*?)"/;
   print $1 , "\n";

   $str =~ /"([^"]+)"/;
   print $1 , "\n";
   # aaaaa

   $str =~ /"(.*?)"(.*)/;
   print $1 , "\n";
   # aaaaa

And the equivalent PHP:

    <?php

    $str = '"aaaaa""bbb""ccc"';
    preg_match('/"(.*?)"/', $str, $matches);
    echo $matches[1] , "\n";
    // aaaaa

    preg_match('/"([^"]+)"/', $str, $matches);
    echo $matches[1] , "\n";
    // aaaaa

    preg_match('/"(.*?)"(.*)/', $str, $matches);
    echo $matches[1] , "\n";
    // aaaaa

    ?>

2

andyo wrote:

Here's the sample code; note that (.*?) and ([^"]+) behave the same
way--and not the way I'd expect:

    str = '"aaaaa""bbb""ccc"'

    str.scan(/"(.*?)"/)
    puts $1
    # ccc

  Normal... #scan is not what you'r looking for:

------------------------------------------------------------ String#scan
     str.scan(pattern) => array
     str.scan(pattern) {|match, ...| block } => str

···

------------------------------------------------------------------------
     Both forms iterate through str, matching the pattern (which may be
     a Regexp or a String). For each match, a result is generated and
     either added to the result array or passed to the block. [...]

  scan find all successive matches for the pattern, and sets the
captured groups variables everytime it finds one. So, here, you simply
get the $1 for the last match, ie "ccc".

  What you're looking for is simply =~, as in Perl:

irb(main):001:0> str = '"aaaaa""bbb""ccc"'
=> "\"aaaaa\"\"bbb\"\"ccc\""
irb(main):002:0> str =~ /"(.*?)"/
=> 0
irb(main):003:0> $1
=> "aaaaa"

  Cheers,

  Vincent
--
Vincent Fourmond, PhD student (not for long anymore)
http://vincent.fourmond.neuf.fr/