c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"use 5.010;
 ;;
 my $s =
   'function convert(beg string, end string, read_date date, step char(6)) returns (date, date, char(1))';
 ;;
 my $rx_paren = qr{ ( [(] (?: [^()]*+ | (?-1))* [)] ) }xms;
 my $rx_identifier = qr{ \w+ }xms;
 ;;
 my $parsed_ok =
 my @ra = $s =~ m{
   \A \s*
   (private|public)? \s*
   (function|report) \s*
   ($rx_identifier)  \s*
   $rx_paren         \s*
   ((returns) \s* $rx_paren)? \s*
   \z
   }xms;
 ;;
 if ($parsed_ok) {
   dd @ra;
   }
 else {
   print 'parse failed';
   }
"
(
  undef,
  "function",
  "convert",
  "(beg string, end string, read_date date, step char(6))",
  "returns (date, date, char(1))",
  "returns",
  "(date, date, char(1))",
)