How could i use a RewriteRule accesing a variable?

If i have:

```
SetEnv MY_VAR (a|b|c)

RewriteRule ^%{ENV:MY_VAR}$ index.php?s=$1 [L,QSA]

RewriteRule ^%{ENV:MY_VAR}-some-one$ index.php?s=$1 [L,QSA]

```

I have these examples but doesn`t work.

**Later edit**

Ok Tim, thank you for the answer. Let say that i have:

```
RewriteRule ^(skoda|bmw|mercedes)-([0-9]+)-([0-9]+)-some-think$ index.php?a=$1 [L,QSA]

RewriteRule ^some-one-(skoda|bmw|mercedes)/pag-([0-9]+)$ index.php?a=$1 [L,QSA]

RewriteRule ^a-z-(skoda|bmw|mercedes)$ index.php?a=$1 [L,QSA]

```

(forget second part of RewriteRule) .. I don-t want to put everywhere (skoda|bmw|mercedes) this list. Is more quickly to make a variable then to use it in rule...