(elbie): Regex not behaving as expected

in reply to Regex not behaving as expected

Your solution looks very close to a common trick I use, which is to assign a vairable to another variable, then do a regex search and replace on the new one:

my $file = 'file013.txt';
(my $sk = $file) =~ s/^.*?(\d+).*$/$1/;
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Which for the sake of this problem is a very similar solution to what others are proposing, the above method is much more useful when I want to keep most of the string rather than a portion of it.

Example, I want to emphasise the word keep above:

my $boring_string = '...when I want to keep most of the string...';
(my $exciting_string = $boring_string) =~ s/(keep)/<b>$1</b>/;
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elbieelbieelbie

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Re: (elbie): Regex not behaving as expected by ihb (Deacon) on Feb 01, 2003 at 16:30 UTC
If you use `.` as "everything else", then be sure to use the `/s` modifier. `s/^.?(\d+).$/$1/s` Beware of that if there is no digit in `$file`, then `$sk` won't change! This is bad. It would look like you need to change `\d+` to `\d`, but that would make it match "" at the beginning, and so `$1` would be empty, thus erasing the whole string. But this problem can be solved. By rewriting the pattern to a more natural (?) pattern we'll soon see the solution. First, your pattern can be rewritten to `s/\D(\d+)./$1/s` The anchors are removed, as they're unnecessary. They're unnecessary in your pattern too. Anyhow, now we can change `\d+` to `\d`, and `$sk` will be empty if no number was matched. So the result is `(my $sk = $file) =~ s/\D(\d)./$1/s;` But this still isn't fully analogous with `my ($sk) = $file =~ /(\d+)/;` since `$sk` will be the empty string in the former, and the undefined value in the latter. So in extraction situations I often stay away from that trick, and simply use the latter. `ihb`	[reply] [d/l] [select]

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Re: (elbie): Regex not behaving as expected
by ihb (Deacon) on Feb 01, 2003 at 16:30 UTC

If you use .* as "everything else", then be sure to use the /s modifier. s/^.*?(\d+).*$/$1/s Beware of that if there is no digit in $file, then $sk won't change! This is bad. It would look like you need to change \d+ to \d*, but that would make it match "" at the beginning, and so $1 would be empty, thus erasing the whole string.

But this problem can be solved. By rewriting the pattern to a more natural (?) pattern we'll soon see the solution. First, your pattern can be rewritten to s/\D*(\d+).*/$1/s The anchors are removed, as they're unnecessary. They're unnecessary in your pattern too. Anyhow, now we can change \d+ to \d*, and $sk will be empty if no number was matched. So the result is (my $sk = $file) =~ s/\D*(\d*).*/$1/s; But this still isn't fully analogous with my ($sk) = $file =~ /(\d+)/; since $sk will be the empty string in the former, and the undefined value in the latter. So in extraction situations I often stay away from that trick, and simply use the latter.

ihb

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