die "Unfit" unless defined
    my $fit =
      Test::CallFlow::ArgCheck::Regexp->new( qr/^..$/ )->check( 0, [ 'foo' ] );

Checks arguments against a regular expression. See base class Test::CallFlow::ArgCheck.

  $checker->check( 1, [ 'foo', 'bar' ] ) ? 'ok' : die;

Checks the argument at given position in referred array against a regular expression.