For egrep the limit on the number of allowed matches is expressed {lower,upper}

finds a line made only of one to six characters 'a' -s

Hi Ranj,
Your solution works for
egrep '^a(a)+$'

But can you tell me how to interpret this please?

How does this know it needs to select 2.. upto 6 a's ?

Thanks
Rohit

It doesn't limit the search to a maximum of 6 a's.
Example test with seven a's:
try it.

Hi Jim,
Thanks for your response but my requirement was really limiting the selection between 2 and 6 a's. (or for that matter a user specified number).
Any clues ?

Rohit