I am trying to use the sed command to replace this line:

```
charmm.c36a4.20140107.newcali4.fixhcali.grange.b

```

with:

```
charmm.20140911.c36a4.3rd.ghost2.model3rd 

```

When I use:

```
sed -i '/s/firstline/secondline/g' 

```

It doesn't work. I think the periods are messing it up. How do I get around this?