Can you please use the code button (101010) to post any search queries and sample data? Looks like some special characters may have gotten lost!
^ and $ match start and end of the line. Since the string you want to extract is in the middle of the data, that doesn't work (assuming the sample you shared is the content of the pluginText field on which you apply the regex).
Probably this would work:
| rex field=pluginText "(?<fieldname>RES ONE Workspace Agent)"
Just out of curiosity: what is your purpose with extracting a literal string like that? Couldn't you just as well do: | eval field = "RES ONE Workspace Agent"?