A simple way to accomplish this could be to use index in combination with substr:

use strict;
use warnings;

my $str = "ABCDEFGHIJKLMNOP";
my $seed = "FGHI";
my $length = 7;

my $slen = length $seed;
my $ipos = index $str,$seed;

my $lexpand = $slen < $length ? ($length - $slen)/2 : 0;
$lexpand = $ipos < $lexpand ? $ipos : $lexpand;
print +(substr $str,($ipos - $lexpand),$length),"\n";

[download]

This outputs DEFGHIJ as expected. The problem raises when the "expanded" substring reaches the beginning or the end of the reference string.

Update: Solved the limit cases:

"ABCDEFGHIJKLMNOP" "ABC" 7 => ABCDEFG
"ABCDEFGHIJKLMNOP" "BC" 7 => ABCDEFG
"ABCDEFGHIJKLMNOP" "MN" 7 => JKLMNOP

[download]

citromatik

In reply to Re: Can we "grow" a string? by citromatik
in thread Can we "grow" a string? by Anonymous Monk

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