Post-grep-like operations

I am wanting to select lines containing 'fruit' or 'veg', and then carry out 
some operation on them, say substitution X for a.

Eg.

fruit banana
fruit fly
meat beef
veg carrot

to

fruit bXnXnX
fruit fly
veg cXrrot

I am trying to do this in a sed file:

/fruit/b
/veg/b
d
s/a/X/g

using the grep like command in the one-liners document.

The last expression doesn't get carried out.  I guess something is happening 
with the buffer, but do not understand what. I can't find '/b' in the 
documentation.

I am trying to avoid having to do multiple passes of the data, but maybe I 
can't.

Any help much appreciated,

Rob




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