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;

[download]

Output: 

1 eth

[download]

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

لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ

In reply to Re: Regex matches string, i do not know why?! by choroba
in thread Regex matches string, i do not know why?! by tobias_hofer

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