The question and example are very tricky! :) Basically, you can't. This is because it is valid to use spaces in Q2A in handles. That means from a given text such as:
@usera aaa bbb @userb ccc ddd eee @userc fff ggg
You could have 3 valid handles like these too: @usera aaa, @userb, @userc fff ggg
Now, assuming you're not allowing spaces in your handles you could use this regex: /@[^\s@\/\+]+/g
See this link: https://regex101.com/r/bN8kF0/1
This approach allows all characters to be part of handles but reject white spaces characters, ats, slashes and plus signs (as the core does). Additional filtering rules will depend on your configuration.