Why does VIM have its own regex syntax?

Most vi (and therefore vim) features were derived from ed. vi and ed both predate perl by at least a decade or two. A better question might be "why doesn't Perl use the same regex syntax as vi?".

Of course, one could also argue that the kinds of regular expressions that one would wish to write inside a text editor to perform common tasks are probably rather different to those you might wish to write inside a programming language.

There is a plugin called eregex.vim which translates from PCRE to Vim's syntax. It takes over a thousand lines of vim to achieve that translation!

In addition to the mentioned reasons vim has some cards in the sleeve when it comes to regex, for example, to match positive look behind we do:

:%s/^foo \zsbar/moo/g

The above command will substitute "bar" for "moo" in the lines started with "foo".

the \zs makes easier to set a positive look-behind and \ze makes easier to make a positive look-ahead.

We also have a way to match only in the "visual area" -> \%V

:'<,'>s/\%Vthis/that/g

Although using the global flag "g" the substitution is restricted to the visual are due \%V

To read an awesome article about how amazing vim regexes can be, follow this link: https://bitbucket.org/snippets/sergio/7nzqxx

We can also use some "submatch" tricks on vim substitution https://dev.to/voyeg3r/vim-math-on-vim-substitution-4obo