Prompted by the responses in
Newbie Q:How do I compare items within a string?, in particular those of
Zaxo and
TedPride I had a go at expanding my attempt with some of their ideas. This lead to some confusion about the use of
pos() as shown
here. In order to explore this further I have written this script which attempts to find occurrences of one string within another and to annotate each occurrence with the offset (zero-based) within the string
use strict;
use warnings;
# 1 2
# 012345678901234567890123456789
my $string = q(The cat scattered caterpillars\n);
# ^ ^ ^
# 4 9 18
print "\n Match against string - $string\n";
print "Start matching\n\n";
my $match = 1;
while($string =~ /(cat)/g)
{
print "Match @{[$match ++]}\n";
print " found - $1\n";
print " Value of \$-[1] - $-[1]\n";
print " Value of \$+[1] - $+[1]\n";
print " Value of \$+[0] - $+[0]\n";
print " Value of \$#+ - $#+\n";
print "Value of pos(\$string) - @{[pos($string)]}\n";
}
print "\nStart annotation\n\n";
$string =~ s
{
(cat)(?{print "Value of pos(\$string) - @{[pos($string)]}\n"})
}
{
$1 . "[@{[pos($string)]}]"
}xeg;
print "\n Annotated string - $string\n";
which when run produces
Match against string - The cat scattered caterpillars
Start matching
Match 1
found - cat
Value of $-[1] - 4
Value of $+[1] - 7
Value of $+[0] - 7
Value of $#+ - 1
Value of pos($string) - 7
Match 2
found - cat
Value of $-[1] - 9
Value of $+[1] - 12
Value of $+[0] - 12
Value of $#+ - 1
Value of pos($string) - 12
Match 3
found - cat
Value of $-[1] - 18
Value of $+[1] - 21
Value of $+[0] - 21
Value of $#+ - 1
Value of pos($string) - 21
Start annotation
Value of pos($string) - 7
Value of pos($string) - 12
Value of pos($string) - 21
Annotated string - The cat[4] scat[9]tered cat[18]erpillars
As per the documentation, during the matching phase the value returned by pos() corresponds with the value of $+[1], pointing to just after the match. As Zaxo pointed out, $-[1] points to the start of the match.
In the annotation stage pos() again points to just after the match in the (?{ ... }) block, where the last / .../g match left off, I think the documentation says. However, when I use pos() as part of the substitution it seems to return values corresponding to $-[1] and not $+[1].
My question is, what could be causing this apparent change in behaviour?
Cheers,
JohnGG
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