VERY easy question:

$string=~s/41/4 1/g;

#it's not obvious what you might want if
#$string='a ab abcd  abcde abcdef abcdefg abcdefgh abcdefghi'
#some possibilities include:
($string2=$string)=~s/(\S{4})\B/$1 /g;
#or
$string2=join' ',($string=~/(....)/gs;

In fact, it was obvious...for me!
I just wanted to make group of 4 chars or digits, separated by one blank space.
The last answer is exactly what I was looking for.
you should answer now so I could grade you!!

In fact, it was obvious...for me!
I just wanted to make group of 4 chars or digits, separated by one blank space.
The last answer is exactly what I was looking for.
you should answer now so I could grade you!!

$string="1234123412341234"
$string =~ s/4/4 /g;

That puts a space at the end of $string which may not have been wanted.

What if $string contains non-digits, or groups of less than 4 digits separated by spaces?
Should those spaces be removed before regrouping them in fours?

Sorry hedj, but points are for ozo....
Ozo, no worrying about $string, they'll never have sapce or non digit char.
Thanks for all possibilities, that's what I'm looking for when I ask a question (especially as easy as this one) on experts-exchange..
You should answer!