POSIX specifies two flavours of regular expressions: basic regular expressions (BRE) and extended regular expressions (ERE). There are subtle differences between the two and ERE supports more features than BRE. For example, what is written as a\(bc\)\{3\}d in BRE is written as a(bc){3}d in ERE. See https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1... for more details.

The regular expression engines available in most mainstream languages go well beyond what is specified in POSIX though. An interesting example is named capturing group in Python, e.g., (?P<token>f[o]+).