Perl regular expressions allow for look-ahead and look-behind assertions in both the positive and negative sense. However, look-behind assertions are a bit annoying, because they cannot be patterns of non-constant length. Meaning you can't say

print "whatever" if "foobar" =~ /(?<=o*)b/;
# print whatever if foobar has a b preceeded by any number of o's
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You'll have to work around it somehow.