Your test results:

print-1

In "/G(\d+)\s*kg\s*/ig" you're making exactly the same mistake as you did in your OP and which I pointed out in my initial response: "('G' should be '\G')". There is no 'G' to match!

print-2

In "/(\d+)\s*kg\s*/ig", you've used the 'g' modifier. It's repeatedly matching each number followed by the units (one after another). As you say: "this one does the job".

print-3

Your explaination of "/(\d+)\s*kg\s*/i" is correct.

"Please add some insight on what is the importance of \G and what are some practice usage scenario of \G?"

'\G' is not an assertion I use that much: I can't really give you an "I often find it useful for ..." type answer.

There's more information in "perlre: Assertions"; as well as links to additional, related documentation.

Ohh my bad. I corrected G with \G but still no chnage
print "Weights are : $1 Kg\n" while $x=~/\G(\d+)\s*kg\s*/ig; #print -1

prints nothing. Still not sure what difference it was supposed to make. NO issues if you don't have much experience to share with this. I will try reading about it more.
Thanks for your help.

"print "Weights are : $1 Kg\n" while $x=~/\G(\d+)\s*kg\s*/ig; #print -1"

The condition for the first while iteration is FALSE: the while loop does not iterate.

Here's a blow-by-blow description of what's occuring. Bear in mind that character positions use a zero-based index: the first character in the string is at postion 0.

• We start at position 0 in $x (that's character "1").
• As no previous regex match with the 'g' modifier had occurred, the last match position is 0. The '\G' assertion is satisfied at the start of the string (i.e. position 0). That's a zero-width assertion, we stay at position 0.
• (\d+) matches "1". This is temporarily assigned to $1 (i.e. $1 eq '1').
• We move to position 1 in $x (that's character "" - a space).
• \s* matches "".
• We move to position 2 in $x (that's character "2").
• The literal sequence "kg" does not match "2".
• We move back to position 1 in $x and the regex engine backtracks to \s*.
• \s* means zero or more spaces greedily. Last time one space was matched. We can also satisfy this by matching zero spaces: position stays at 1.
• The literal sequence "kg" does not match "".
• The temporary value in $1 is removed. The regex engine backtracks to \G looking for another way to find a match from the current position 1.
• The last match position is 0; the current position is 1: the \G assertion is not satisfied.
• We now move to postion 2 in $x: again, the \G assertion is not satisfied.
• The regex engine moves along $x, one position at a time, attempting to find a match. Because none of these positions are 0, the \G assertion is never satisfied.
• Eventually, after 139 steps, the regex engine runs out of string (i.e. the end of $x is reached) and the match is FALSE.

I got all that information by running your code through Regexp::Debugger. I highly recommend this module: not only will you find bugs in your regex, you'll also learn a lot about them (in that respect, it's just as useful for regexes that work as those that don't).

You can fix your current problem by adding '.*?' after the '\G':

$ perle 'my $x = q{1 2 3kg 4 5 6 7 8 9 10Kg 11 12 13 kg 14 15}; say $1
+ while $x =~ /\G.*?(\d+)\s*kg\s*/gi'
3
10
13
[download]

" I will try reading about it more."

The link I provided before does lead to more links. One in particular, which you should definitely read, is "perlop: Regexp Quote-Like Operators". You'll need to scroll down a fair way: look for the "\G assertion" section.

— Ken

Ohh my bad. I corrected G with \G but still no chnage ... prints nothing. Still not sure what difference it was supposed to make.

c:\@Work\Perl\monks>perl -wMstrict -le
"my $x = '1 2 3kg 4 5 6 7 8 9 10Kg 11 12 13 kg 14 15';
 print qq{string \$x: '$x'};
 ;;
 printf qq{captured '$1'  } while $x =~ /\G(\d+)\s*kg\s*/ig;
 print '----------';
"
string $x: '1 2 3kg 4 5 6 7 8 9 10Kg 11 12 13 kg 14 15'
----------
[download]

The  \G anchor matches at the point (the exact character offset in the string) at which matching stopped in the last  /g global match iteration. But on the first  /g iteration, where is that point? On the first  /g iteration,  \G matches the same as  \A (the \Absolute-start-of-string anchor).

So what  /\G(\d+)\s*kg\s*/ig says is:

1. \G From the string offset at which the previous match stopped (or from the start of the string if it's the first match);
2. (\d+) Match and capture one or more decimal digits;
3. \s* Then match zero or more whitespace characters;
4. kg Then match the literal characters  'kg' case-insensitively (due to the  /i flag);
5. \s* Then match zero or more whitespace characters (this can't fail);
6. And this match iteration is finished.

But your  '1 2 3kg 4 5 6 7 8 9 10Kg 11 12 13 kg 14 15' string begins with some digits, some whitespace, and then some more digits, not the required  'kg' literals: the match immediately fails. There is a  '3kg' subsequence further on that could satisfy part of the overall match, but matching has already failed due to the  \G assertion.

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