You could just use /filter, and then /write -c to clear the file.

As an example: //filter -fk mymessages.txt myalias | write -c mymessages.txt

Or if you just want to test WITHOUT CLEARING THE FILE, try: /filter -fk mymessages.txt myalias
Code
alias myalias {
  ;$1 contains the whole line.
  echo -s Here's the whole line: $1

  ;This thing here splits the input line into tokens separated by character 32, which is a normal empty space.
  tokenize 32 $1
  echo -s There are $0 words! First word: $1 Second word: $2 Third word: $3 $+ ...
}


/help /filter
/help /write