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#!/usr/bin/perl -ln

...
    }

__END__

echo pre_[aa,01:03,bb]_[x,y]_post | ./xstr.pl
pre_aa_x_post
...
pre_03_y_post
pre_bb_x_post
pre_bb_y_post

sub expand {
    doit split 
...
      (.*?)
      (?<!\\)\]/x, shift, -1;
}

sub expand {
    doit split /<SOME_REGEX>/, shift, -1;
}