No number of regexes and/or if-clauses will reconcile a requirement statement that doesn't state what is required.

The first two requirements sound like the same thing: /([:alnum:]+[_]+/. But the third requirement is a subset of the first, consuming the alnum sequence with no need for underscore. Two regexes joined by || logical-or would do it. However, it just might be as simple as replacing the last + in the above regex with * to indicate "zero or more" underscores. The pattern now looks for one-or-more alnum characters followed by zero-or-more underscores.

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.

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Give a man a fish:  <%-{-{-{-<