Sub-pattern in regex can't be dereferenced?

Question

I have following Perl script to extract numbers from a log. It seems that the non-capturing group with ?: isn't working when I define the sub-pattern in a variable. It's only working when I leave out the grouping in either the regex-pattern or the sub-pattern in $number.

#!/usr/bin/perl
use strict;
use warnings;

my $number = '(:?-?(?:(?:\d+\.?\d*)|(?:\.\d+))(?:[Ee][+-]?\d+)?)';
#my $number = '-?(?:(?:\d+\.?\d*)|(?:\.\d+))(?:[Ee][+-]?\d+)?';

open(FILE,"file.dat") or die "Exiting with: $!\n";
while (my $line = <FILE>) {
        if ($line =~ m{x = ($number). y = ($number)}){
        print "\$1= $1\n";
        print "\$2= $2\n";
        print "\$3= $3\n";
        print "\$4= $4\n";
    };
}
close(FILE);

The output for this code looks like:

$1= 12.15
$2= 12.15
$3= 3e-5
$4= 3e-5

for an input of:

asdf x = 12.15. y = 3e-5 yadda

Those doubled outputs aren't desired.

Is this because of the m{} style in contrast to the regular m// patterns for regex? I only know the former style to get variables (sub-strings) in my regex expressions. I just noticed this for the backreferencing so possibly there are other differences for metacharacters?

`It seems that the non-capturing group with :? isn't working` That's not a non-capturing group, its a regular parenthesis. — TLP, Sep 03 '13 at 09:13
You should clarify your question. What is this subpattern you can't reference? Your code works like you say, is there a problem? — TLP, Sep 03 '13 at 09:25
Yes the code works like I said but I didn't want those doubled groups. Sorry for the unclear question. — EverythingRightPlace, Sep 03 '13 at 10:00

Ibrahim Najjar · Accepted Answer · 2013-09-03T09:41:32.800

2

The delimiters you use for the regular expression aren't causing any problems but the following is:

(:?-?(?:(?:\d+\.?\d*)|(?:\.\d+))(?:[Ee][+-]?\d+)?)
 ^^
Notice this isn't a capturing group, it is an optional colon :

Probably a typo mistake but it is causing the trouble.

Edit: It looks that it is not a typo mistake, i substituted the variables in the regex and I got this:

x = ((:?-?(?:(?:\d+\.?\d*)|(?:\.\d+))(?:[Ee][+-]?\d+)?)). y = ((:?-?(?:(?:\d+\.?\d*)|(?:\.\d+))(?:[Ee][+-]?\d+)?))
    ^^           first and second group               ^^      ^^    third and fourth grouop                     ^^

As you can see the first and second capturing group are capturing exactly the same thing, the same is happening for the third and fourth capturing group.

edited Sep 03 '13 at 09:41

answered Sep 03 '13 at 09:16

Ibrahim Najjar

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I was thinking typo too, but then it would be extremely odd to both mention it as `:?`, while using a ton of `?:` groups in the regex. – TLP Sep 03 '13 at 09:18
Awaiting clarification from the OP. I don't even really see what he is asking about. – TLP Sep 03 '13 at 09:23
Thx. Really stupid typo by me... I assumed there are differences through saving a sub-regex-string into a variable and using it in a regex. Seems this isn't the case. – EverythingRightPlace Sep 03 '13 at 10:02

score 0 · Answer 2 · answered Sep 03 '13 at 13:19

You're going to kick yourself...

Your regexp reads out as:

capture {
 maybe-colon
 maybe-minus
 cluster {     (?:(?:\d+\.?\d*)|(?:\.\d+))
  cluster {    (?:\d+\.?\d*)
   1+ digits
   maybe-dot
   0+ digits
  }
  -or-
  cluster {    (?:\.\d+)
   dot
   1+digits
  }
 }
 maybe cluster {
   E or e
   maybe + or -
   1+ digets
 }             (?:[Ee][+-]?\d+)?
}

... which is what you're looking for.

However, when you then do your actual regexp, you do:

$line =~ m{x = $number. y = $number})

(the curly braces are a distraction.... you may use any \W if the m or s has been specified)

What this is asking is to capture whatever the regexp defined in $number is.... which is, itself, a capture.... hence $1 and $2 being the same thing.

Simply remove the capture braces from either $number or the regexp line.

Like I wrote in the question with `It's only working when I leave out the grouping in either the regex-pattern or the sub-pattern in $number.` The problem is that I didn't understand this behaviour because I only wanted to match every entry once. And the only issue was the typo of `:?`, for which I am kicking myself indeed... — EverythingRightPlace, Sep 03 '13 at 13:22
Yes.... because `$number` contains a capture, then when you do '($number)' you do a **second** capture - meaning '$1' and '$2' - you then repeat the '($number)' later in the regexp, giving you '$3' & '$4'. — CodeGorilla, Sep 03 '13 at 13:30

Sub-pattern in regex can't be dereferenced?

2 Answers2