note Enlil With a little [cpan://YAPE::Regex::Explain] magic: <code> $ perl -MYAPE::Regex::Explain -e 'print YAPE::Regex::Explain->new(qr/^(.+)_[^_]+$/)->explain' The regular expression: (?-imsx:^(.+)_[^_]+$) matches as follows: NODE EXPLANATION ---------------------------------------------------------------------- (?-imsx: group, but do not capture (case-sensitive) (with ^ and $ matching normally) (with . not matching \n) (matching whitespace and # normally): ---------------------------------------------------------------------- ^ the beginning of the string ---------------------------------------------------------------------- ( group and capture to \1: ---------------------------------------------------------------------- .+ any character except \n (1 or more times (matching the most amount possible)) ---------------------------------------------------------------------- ) end of \1 ---------------------------------------------------------------------- _ '_' ---------------------------------------------------------------------- [^_]+ any character except: '_' (1 or more times (matching the most amount possible)) ---------------------------------------------------------------------- $ before an optional \n, and the end of the string ---------------------------------------------------------------------- ) end of grouping ---------------------------------------------------------------------- </code> <br /> or a little more tersely. Match any string that contains at least one character before and one character after the last underscore that appears in the string. <p>-enlil 548901 548901