#!perl -wl
use strict;

my $__item= 'abc_IoExtFscInjS_GetGlobalDatasetActvMethod';

warn $__item;
if ( ($__item =~ m/(?!Eth.+cfg)Eth/i) || ($__item =~ m/(?!abc_Eth.+cfg
+)abc_Eth/i) ) {
    print $`;
    print $&;
    print $';
    print "match";
} else {
    print "No match";
};
__END__
abc_IoExtFscInjS_GetGlobalDatasetActvMethod at tmp.pl line 6.
abc_IoExtFscInjS_GetGlobalDatasetActvM
eth
od
match
[download]

This had me stumped first, but when Perl claims it finds a match, it can also tell you, where :)

In mETHod there is one... matches due to /i

Thanks a lot!
I am getting code-blind..


Thank you for saving my day!
Tobias

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".

Although others have already helped out, thought I'd toss out another method. You can use DamianConway's Regexp::Debugger module to step through the matching process. For a video demo, you can watch his video presentation from YAPC::NA 2012 at http://youtu.be/zcSFIUiMgAs. (Note: At the time of the video recording, the module had not yet been released to CPAN.)