Hi a grep regex question I think

I have many cases of two similar terms in a file eg:

```
 'stop_codon,5UTR' & 'stop_codon'

```

The longer term is always the first term, a comma, then an additional term.
I want to use grep to find cases of one but not the other. So how can I tell grep to find only those terms that are not immediately followed by a ','.

Simple I am sure but I have little experience with regular expressions and I could'nt find a way to find an occurrence of one term unless it includes another term.

Thanks in advance,
Rubal