s/// 
use warnings;
use strict;

while (<DATA>) {
    chomp;
    s/ ^ [0-9a-z]{3} $ /hi/xi;
    print "$_\n";
}

__DATA__
ABC
GHR
1DF
uy6
fore

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See also:

In reply to Re: Complex Regex by toolic
in thread Complex Regex by stallion

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