The following regex will match words separated by spaces before a word starting with a @ character;
([^@\s]+ )+@[\w]{1,15}
For example, this will return two matches for the String:
User Name @username Text Text Text Text random @thing
- User Name @username
- Text Text Text Text random @thing
You can then select the second match to get the text you need. Alternatively, if you know what @thing will be before compiling the regex (e.g. @response, @config, etc.) you can replace the @[\w]{1,15} section of the regex with the specific @thing to only retrieve the text you want.