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I want to write a regex that will match if and only if the pattern is not preceded by the characters "Etc/".

Strings that should match:

GMT+01:00
UTC+01:00
UTC+01
+01:00
Strings that should not match:
Etc/GMT+01:00
Etc/UTC+01:00
Etc/UTC+01
This is what I have so far:
(?<!Etc\/)((UTC|GMT)?(\+|\-){1}(\d{1,2})(:|\.)?(\d{1,2})?)
The right part of the above regular expression already matches the UTC and GMT offset and covers all the cases I need. But I don't manage to implement the exceptions mentioned above.
I expected the above regex to not match the string Etc/GMT+1:00. But in fact it matches the part +01:00 and only ignores Etc/GMT.
How can I achieve that the the following regular expression does not match if it is preceded with "Etc/"?
(UTC|GMT)?(\+|\-){1}(\d{1,2})(:|\.)?(\d{1,2})?
Here I have an example with most of the use cases I need.
You may add \S* after Etc/ to make sure Etc/ is checked even if there are any zero  or more non-whitespace chars between Etc/ and the expected match:
(?<!\bEtc/\S*)((UTC|GMT)?([+-])(\d{1,2})[:.]?(\d{1,2})?)
See the .NET regex demo
Details:
(?<!\bEtc/\S*) - a negative lookbehind that matches a location that is not immediately preceded with a whole word Etc/ and then zero or more non-whitespace chars
(UTC|GMT)? - an optional substring, UTC or GMT
([+-]) - + or -
(\d{1,2}) - one or two digits
[:.]? - an optional : or .
(\d{1,2})? - an optional sequence of one or two digits (equal to (\d{0,2})).
As you are already capturing all data in groups, another way could be getting all the matches of Etc/ out of the way, and use your pattern to capture what you want in the groups.
Note that you can change groupings of single chars like (:|\.) to a character class ([:.])
\bEtc/\S*|(UTC|GMT)?([+-])(\d{1,2})([:.])?(\d{1,2})?
\bEtc/\S* Match Etc/ and optional non whitespace chars
(UTC|GMT)?([+-])(\d{1,2})([:.])?(\d{1,2})? Your pattern with all the separate groups.
Regex demo
Or with just a single group:
 \bEtc/\S*|((?:GMT|UTC)?\+\d{2}(?:[:.]\d{2})?)
Regex demo
        
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