perl: REGEX help

I think this will do what you want.  I changed the geo portion slightly to do better checking based on the example - if other geo cases are valid, you can change it back - the only critical change to that portion was adding + after [a-z].  Most of your problem was due to anchoring the regex to the end of the string ($) but not giving full string matching in the regex (I prefer full validation so changed the match rather than remove the end-of-string anchor).

for my $metaKey (sort keys $metaHash){
  next if($metaKey =~ m/(exceptions|list/);
  if ($metaKey =~ m{^(?:[A-Z]{2,3}\.[a-z]+(?:[0-9]+|\.[a-z]+)|geo\.\d+\.[a-z]+)$}) {
      print "$metaKey good\n";
  }
  else{
     print "$metaKey bad\n";
  }
}

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print "$_ ".(/geo|exception|^[A-Z]{2,3}\.[a-z]+[.\d]+$/?"good":"bad")."\n" for sort keys %metaHash;

Hi,

pls try

^([A-Z]{2,3}\.[a-z]+\.?(\d+|[a-z]+)|geo\.\d.*[a-z])$

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REgards

maybe if exception

^([A-Z]{2,3}\.[a-z]+\.?(\d+|[a-z]+)|geo\.\d.*[a-z]|exception.*)$

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geo.lowercaseword  is already covered by "so unless geo or exception"
But if ABC.lowercaseword is valid, that could simplify it to /geo|exception|^[A-Z]{2,3}\.[a-z]+\.?(\d+|[a-z]+)$/

Is there anything containing "geo" or "exception" that is not valid?

in theory every thing starting with lower case   "geo" should also be valid as long as every thing after the first dot is either [a-z] or [0-9] or \.

That's a little different from how I originally interpreted the problem.
Is "every thing after the first dot is either [a-z] or [0-9] or \." meant to be the same condition as "DOT lowercaseword DOT or numbers"
or are there things that are valid following ^geo but not after ^[A-Z]{2,3}?
If the latter, then you might use
/exception|^(geo[^.]*(\.[a-z\d.]*)?|[A-Z]{2,3}\.[a-z]+\.?(\d+|[a-z]+))$/
Unless "every thing after the first dot is either [a-z] or [0-9] or \." is meant to imply that it is required that a first dot is present in which case
/exception|^(geo[^.]*\.[a-z\d.]*|[A-Z]{2,3}\.[a-z]+\.?(\d+|[a-z]+))$/,
could suffice

A few clarifications:

Is "geo" in the exceptions list?

Although you only stated restrictions on what follows the first dot, and did not state any restrictions on what can be between "geo" and the first dot, in your valid examples there is nothing between "geo" and the first dot. Should that be a requirement?

You say
geo.datum                =   Transvers mercator
is valid, but "                =   Transvers mercator" is after the first dot, and is not either [a-z] or [0-9] or \.
Should "                =   .*" be added to the list of every thing after the first dot?

When you have
AB.docid and ab.docid or ab.docID etc.
and you need to locate the second 2 and remove them, should the result be
AB.docid and  or  etc.
Or do you mean that whenever the beginning of a key looks valid except for some extra stuff following, everything after the valid first part should be removed?

Do you want to remove invalid keys and leave valid keys instead of just printing "good" after valid keys and "bad" after invalid keys?

If you change an invalid key into a valid key by removing invalid parts, what should happen if that key then collides with another valid key?  Should the data associated with both keys be combined somehow?

Based on the last few comments, this should get you closer:

$metaKey =~ m{^[A-Z]{2,3}(?:\.[a-z])+(?:\.?\d+)?$|^geo(?:\.\d+)?\.[a-z]+(?:$|\s)}

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You may want to remove the last (?:$|\s) - it was an attempt to validate geo keys but exclude the \s+=.*