egrep regex: having [ and ] or dot in character class does not match

Question

I am trying to match an IPv6 address with square brackets but I am surprisingly stuck...

Works:

echo 'files ([2001:450:671d:200::121]:59464).' | egrep 'files \(.+\)\.$'

Does NOT match (but I think a dot in a character class is valid):

echo 'files ([2001:450:671d:200::121]:59464).' | egrep 'files \([.]+\)\.$'

Any other attempt such as

echo 'files ([2001:450:671d:200::121]:59464).' | egrep 'files \([][0-9:]+\)\.$'
echo 'files ([2001:450:671d:200::121]:59464).' | egrep 'files \([[]0-9:]+\)\.$'
echo 'files ([2001:450:671d:200::121]:59464).' | egrep 'files \([]0-9:[]+\)\.$'
echo 'files ([2001:450:671d:200::121]:59464).' | egrep 'files \([\[\]0-9:]+\)\.$'

Does not work either...

What the heck is wrong here?

Answer 1

That first one, \([.]+\), would match literal dots inside literal parenthesis, like (...) etc. The other ones don't match the hex digits above 9, i.e. abcdef. There's a d in 671d. Remember that IPv6 addresses are in hex.

You'd need to add those, so:

echo 'files ([2001:450:671d:200::121]:59464).' | egrep 'files \([][0-9a-f:]+\)\.$'

or more strictly:

echo 'files ([2001:450:671d:200::121]:59464).' | egrep 'files \(\[[0-9a-f:]+\]:[0-9]+\)\.$'

or use a-fA-F within the brackets if you need to deal with uppercase hex digits too.