Just when we thought this couldn’t get any more confusing, we discover that POSIX also splits regular expression implementations into two kinds: basic regular expressions (BRE) and extended regular expressions (ERE). The features we have covered so far are supported by any application that is POSIX compliant and implements BRE. Our grep program is one such program.

What’s the difference between BRE and ERE? It’s a matter of metacharacters. With BRE, the following metacharacters are recognized: ^ $ . [ ] * All other characters are considered literals. With ERE, the following meta-characters (and their associated functions) are added: ( ) { } ? + |

However (and this is the fun part), the characters () {} are treated as metacharacters in BRE if they are escaped with a backslash, whereas with ERE, preceding any metacharacter with a backslash causes it to be treated as a literal.