I like to take a very simple approach to such things, since the tricky methods usually confuse me when I come back to it 6 months later and have to modify it somehow. :-) 

my $s = 'ab';
my $pat = qr/^(a{1,3})(?=b)/;

while( $s =~ m/$pat/ )
{
    $s =~ s/$pat/$1a/g;
}

[download]

Or, if you prefer more of the 1-liner feel: 

my $s = 'ab';
my $pat = qr/^(a{1,3})(?=b)/;

do{ $s =~ s/$pat/$1a/g } while $s =~ m/$pat/;

[download]

I'm sure it is possible to do this in the regex itself (probably with /e). I'm sure other monks will enlighten both of us.

Update: I changed approaches mid-code and forgot to remove the superfluous m//. These solutions are not very perlish.

In reply to Re: Recursive substitution by bobf
in thread Recursive substitution by JadeNB

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