Note: This would have served better as a reply to Re^2: /m pattern matching modifier, which first mentions the  "\nGTT\n" string.

The regex  /^.*$/mg matches the empty string (not 'nothing', i.e., no match) in the string  "\nGTT\n" because the /m regex modifier causes ^ to match at the start of a string (the default) and also immediately after an embedded newline, and causes $ to match its default and also just before an embedded newline.

A regex looks for the leftmost match. The leftmost position in the string above that matches the regex above is ^ (the absolute start of the string), .* (zero of any character except a newline), and $ (just before the first newline), and the string that exists at this position is the empty string.

Regexes are often counter-intuitive!

Updates:

1. And, as jethro said, even with the /g modifier, the regex matching in void or scalar context will still only return the leftmost of all possible matches on the first match attempt.
2. s/place/position/g, s/just before a newline/just before the first newline/ in the foregoing text.