`{n}` iteration meta character after posix class in regex

Question

I am learning regular expressions. I have some doubts relating to that example :

I'd like to find all the files with conventional extensions in names. This works:

ls | grep '\.[[:lower:]][[:lower:]][[:lower:]]$'

but this does not:

ls | grep '\.[[:lower:]]{3}$'

As far as I understood the {n} iteration meta character results in matching with pattern that has exactly {n} occurrences of preceding character. Doesn't it work with POSIX classes? Or am I making some silly mistake here?

Answer 1

In POSIX basic (BRE), you need to escape the braces for this to work:

ls | grep '\.[[:lower:]]\{3\}$'

In POSIX extended (ERE), this requirement has been dropped, and ERE also finally adds alternation (|) to regular expressions (although some tools which use BRE do support alternation via \|).