use strict;
my $str = 'As a 1st step, you can rewrite #21x*x - 1y = 1z + 1#';
my $regex = qr/(?![^#]*#[^#]*#)\b1([a-z])(?=[^#]*#)/p;
my $subst = '$1';
my $result = $str =~ s/$regex/$subst/rg;
print "The result of the substitution is' $result\n";
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for Perl, please visit: http://perldoc.perl.org/perlre.html