create table foo (
   somevar1 sometype,
   somevar2 sometype null,
   somevar3 sometype not null,
   somevar4 sometype(20),
   somevar5 sometype(20) null,
   somevar6 sometype(10))

##</code><code>##

$str =~ s{
   (create\s+table\s+\w+\s*\()   # "create table foo (", save as $1
   (                             # Grab this pattern (each of the create table variables), save as $2
      (                          # Save as $3
         \s*                     # Any spaces
         \w+                     # Variable name
         \s+                     # At least one space
         \w+                     # Variable type
         (                       # Optional (nnn) for character string types, save as $4
            \(                   # Literal open paren
            \d+                  # Any number of digits
            \)                   # Literal close paren
         )?                      # The ? makes it optional
         \s*                     # Any spaces
      )                          # "varname vartype " (not the null/not null part)
      (                          # Save as $5
         [\w\s]*                 # Any words and/or spaces ("null" or "not null", specifically)
      ) 
      (                          # Save as $6
         ,?                      # Optional comma (optional as it may be the last element)
         \s*                     # Any spaces
      )
   )+                            # Match this whole section at least once
   (\s*\)\s*)                    # Closing paren for create table block with surrounding whitespace, save as $7
}
{
      print $1;
      print $3;
      if ($5 =~ /null/i) {
         print $5;
      } else {
         print " NOT NULL";
      }
      print $6;
      print $7;
}gex;

# Output: create table foo (somevar6 sometype(10) NOT NULL)