I think what the OP is trying to do is to assemble the string once, in a single pass. In his added info, jeanluca, he said the assembling code is "very complex!".

I think eric256 in Re: Create string with a delay and Zaxo in Re: Create string with a delay are both on the right track, but eric256's "len($a . $b . $c . $d . $x)" represents a separate pass through the complexity. For Zaxo's solution, the second argument of the sprintf needs to be "length($str)+10", calculated just before $x is added.

Combining and extending the two approaches a little, I'd suggests 

$str = $a . $b . $c .$d;
$xpos_spot = length( $str );
$e = '0000000000';
$xpos_len = length($e);
$str .= $e;
$str .= $f;
...
$str .= $w;
substr $str, $xpos_spot, $xpos_len, 
        sprintf('%${xpos_len}d',length($str));
$str .= $x;
...

[download]
Note that this automatically adjusts for the length of the "fill-in" zeroes, if we change that part in a later revision, usually a good idea. Also the code still works if we insert a "$str .= $aplus", somewhere in the complexities of assembling the first part of the string.

There's a way to set up a kind of pointer into the part of the string you want to modify later, one that you can assign to. It uses a reference to "substr" with pre-defined parameters (known as currying). See BrowserUK's excellent Re: lhs substr(): refs vs. scalars writeup.

In reply to Re: Create string with a delay by rodion
in thread Create string with a delay by jeanluca

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