use YAPE::Regex::Explain; print YAPE::Regex::Explain->new(qr/^(?:(?:\d+|\d+-\d+|a)(,\s*|$))+$/)->explain; #### The regular expression: (?-imsx:^(?:(?:\d+|\d+-\d+|a)(,\s*|$))+$) 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, but do not capture (1 or more times (matching the most amount possible)): ---------------------------------------------------------------------- (?: group, but do not capture: ---------------------------------------------------------------------- \d+ digits (0-9) (1 or more times (matching the most amount possible)) ---------------------------------------------------------------------- | OR ---------------------------------------------------------------------- \d+ digits (0-9) (1 or more times (matching the most amount possible)) ---------------------------------------------------------------------- - '-' ---------------------------------------------------------------------- \d+ digits (0-9) (1 or more times (matching the most amount possible)) ---------------------------------------------------------------------- | OR ---------------------------------------------------------------------- a 'a' ---------------------------------------------------------------------- ) end of grouping ---------------------------------------------------------------------- ( group and capture to \1: ---------------------------------------------------------------------- , ',' ---------------------------------------------------------------------- \s* whitespace (\n, \r, \t, \f, and " ") (0 or more times (matching the most amount possible)) ---------------------------------------------------------------------- | OR ---------------------------------------------------------------------- $ before an optional \n, and the end of the string ---------------------------------------------------------------------- ) end of \1 ---------------------------------------------------------------------- )+ end of grouping ---------------------------------------------------------------------- $ before an optional \n, and the end of the string ---------------------------------------------------------------------- ) end of grouping ----------------------------------------------------------------------