I have string looks likes this

```
string="xxxxx.yyyyy[2].zzzzz"

```

I want to extract the number between the `[ ]`. I used the following `awk` command

```
echo $string | awk -F'[]' '{print $2}'

```

But this `awk` command returns error:

```
awk: bad regex '[]': Unmatched [ or [^

```

How to fix that?