In Perl, the following examples are functionally equivilant, though they get to that equivilancy in different ways.

if ( $string !~ m/pattern/ ) { ..... }
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And.....

if ( not $string =~ m/pattern/ ) { ...... }
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And.....

unless ( $string =~ m/pattern/ ) { ...... }
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Does your special language provide you with More Than One Way To Do It? ;)