Perl. Regexp work only once

Question

I need to process the string with regexp and change x->y if is number around.

String: 2x2x2 2x 2x2x 2x2x2x2x2

Regexp: s/([0-9])x([0-9])/$1y$2/g

my $string = "2x2x2 2x 2x2x 2x2x2x2x2";

$string =~ s/([0-9])x([0-9])/$1y$2/g;

print "$string\n";

I expect: 2y2y2 xx 2x 2y2x 2y2y2y2y2

But result: 2y2x2 2x 2y2x 2y2x2y2x2 (not all 2x2 changed)

What should I do?

Answer 1

Try the below regex:

s/(?<=\d)x(?=\d)/y/g

Answer 2

To be explicit: the reason "2x2x2" turns into "2y2x2" is that your expression first matches "2x2", replaces it with "2y2", then resumes searching after that match was found. The next bit is "x2", which doesn't match your pattern.

The reason @xdazz's solution works is that look-around assertions don't actually consume characters of the string. The portion matched in the string is just "x", whenever preceded and followed by numerals.

Incidentally, @xdazz's change from [0-9] to \d doesn't really change much, but it's slightly different: \d will also match other Unicode characters that are considered digits, whereas [0-9] only matches the exact 10 characters in the given range.