Re: Regex matches string, i do not know why?!

[choroba]

Try adding grouping parentheses to see what is matching:

my $s = 'abc_IoExtFscInjS_GetGlobalDatasetActvMethod';
print "1 $1\n" if $s =~ /((?!Eth.+cfg)Eth)/i;
print "2 $1\n" if $s =~ /((?!abc_Eth.+cfg)abc_Eth)/i;

Output:

1 eth

You are using the /i modifier which makes the matching case insensitive. The string indeed contains "eth" which is not preceded by "eth.+cfg".