What flavour of regular expression is grep?

Question

I\'m guessing it\'s not a Perl Compatible Regular Expression, since there\'s a special kind of grep which is specifically PCRE. What\'s grep most simil

Answer 1

There's a good write-up here. To quote the page, "grep is supposed to use BREs, except that grep -E uses EREs. (GNU grep fits some extensions in where POSIX leaves the behaviour unspecified)."

In other words, it's a long story. ;)