$var='Hobbs, F. 2005a. Examining American Household Composition: b1990 and 2000. U.S. Census Bureau, Census 2000 Special Reports, CENSR-24. I.S. Government Printing Office, Washington, DC.';

#$var=~s/(\w?) (\w?) ([0-9]{4}+[a-zA-Z]?) (\w?) (\w?)/&identify_year($1.$2.$3.$4.$5)/ge;

$var=~s/([0-9]{4})/&identify_year($1)/ge;

sub identify_year
{
  my ($input)=@_;

  print "$input\n";
  return ($input);
}