"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
" 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.
