So  'a_________'  '%%a_____%%'  '123456789_' would be strings skooma would want to match? That wasn't my (limited) understanding after reading the OP.

c:\@Work\Perl\monks>perl -wMstrict -le
"for my $s (qw(a_________  %%a_____%%  123456789_)) {
   print qq{'$s' matches} if $s =~ /[[:alnum:]]+[_]+/;
   }
"
'a_________' matches
'%%a_____%%' matches
'123456789_' matches
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Update 1: And what about the  { } curlies?

Update 2: And please use code tags when you post. Please see Writeup Formatting Tips and Markup in the Monastery.