Having searched the forums I thought I’d worked this out – didn’t, haven't – messed up.
Have title tag for example
“Happy Birthday, Sweet Sixteen - Neil Sedaka”
Want to remove “ - Neil Sedaka”
Created action
Field = Title
Regular Expression = (.)-(.)
Replace matches with = $trim($1)
Using this action title is renamed
“Happy Birthday”
Seems to work OK if there is no “, “ in title
i.e. “Happy Birthday Sweet Sixteen - Neil Sedaka”
Returns “Happy Birthday Sweet Sixteen”
Need to be shown the error of my ways – big please