how would I trim a mix of newline and space characters from the beginning and en

I would start by substituting all "new line" characters with a space; and then use trim() to remove all unnecessary spaces. So:

fn!trim(
  fn!substitute(
    ri!myString,
    char(
      10
    ),
    " "
  )
)

I would start by substituting all "new line" characters with a space; and then use trim() to remove all unnecessary spaces. So:

fn!trim(
  fn!substitute(
    ri!myString,
    char(
      10
    ),
    " "
  )
)

I just noticed the previous replies that reference use of char(13) alongside char(10)...so the principle remains the same, but you'd iterate over the string to substitute both with a space and then tidy it all up with trim():

fn!trim(
  fn!reduce(
    fn!substitute,
    ri!myString,
    {char(10),char(13)},
    " "
  )
)

Obviously you can extend the list of characters to be excluded as needed..

It's hard to know exaclty what the original poster's use case was from over 4 years ago, but it seems to me that they didn't want to remove all newlines, just extraneous ones (seemingly only at the beginning or end of a string).  I'm sure there's now a more graceful coding solution than the original one I posted so long ago, though it's always fun to see my name pop up in a newly-revived thread.