Shell Programming and Scripting

...
20120102 120352.5AM
...

...
2012 01 02 12 03 52.5 AM
...

sed 's/ //g' file | sed -e 's/.\{2\}/& /g' | sed 's/ //1; s/ \./\./;'

awk 'NF == 2 && $2 ~ /[aApP][mM]$/ && (length($2) == 9 || length($2) == 10) {
        l = length($2) == 10
        print substr($1, 1, 4), substr($1, 5, 2), substr($1, 7),
              substr($2, 1, l + 1), substr($2, l + 2, 2), substr($2, l + 4, 4),
              substr($2, l+8)
        #       YYYY            MM              DD
        #       H or HH         mm              ss.s
        #       AM/PM
        next
}
 {      print
}' file

#!/usr/bin/env perl

# @(#) p1	Demonstrate unpack, in this case characters.

use strict;
use warnings;

my (@a);

# Sample:
# input   20120102 120352.5AM
# output  2012 01 02 12 03 52.5 AM
while (<>) {

  # @a = unpack("a4a2a2xa2a2a4a2");
  @a = unpack("a4 a2 a2 x a2 a2 a4 a2");
  print "a = @a\n";
}

exit(0);

% ./p1 data1
a = 2012 01 02 12 03 52.5 AM