perl -wMstrict -e
"my @months = (qw(jan feb mar apr may jun jul aug sep oct nov dec));
 my $i = 1;  my %month_num = map { $_ => $i++ } @months;
 my $month_re = join '|', @months;
 print qq(o/p: \n);
 for (@ARGV) {
  print qq( $_: );
  s/($month_re)\w+/$month_num{$1} || $1/ei;
  print qq($_ \n)
  }"
jan JaN january jane
o/p:
 jan: jan
 JaN: JaN
 january: 1
 jane: 1