What's the difference between vim regex and normal regex?

Question

I noticed that vim's substitute regex is a bit different from other regexp. What's the difference between them?

Answer 1

"Regular expression" really defines algorithms, not a syntax. What that means is that different flavours of regular expressions will use different characters to mean the same thing; or they'll prefix some special characters with backslashes where others don't. They'll typically still work in the same way.

Once upon a time, POSIX defined the Basic Regular Expression syntax (BRE), which Vim largely follows. Very soon afterwards, an Extended Regular Expression (ERE) syntax proposal was also released. The main difference between the two is that BRE tends to treat more characters as literals - an "a" is an "a", but also a "(" is a "(", not a special character - and so involves more backslashes to give them "special" meaning.

The discussion of complex differences between Vim and Perl on a separate comment here is useful, but it's also worth mentioning a few of the simpler ways in which Vim regexes differ from the "accepted" norm (by which you probably mean Perl.) As mentioned above, they mostly differ in their use of a preceding backslash.

Here are some obvious examples:

Perl    Vim     Explanation
---------------------------
x?      x\=     Match 0 or 1 of x
x+      x\+     Match 1 or more of x
(xyz)   \(xyz\) Use brackets to group matches
x{n,m}  x\{n,m} Match n to m of x
x*?     x\{-}   Match 0 or 1 of x, non-greedy
x+?     x\{-1,} Match 1 or more of x, non-greedy
\b      \< \>   Word boundaries
$n      \n      Backreferences for previously grouped matches

That gives you a flavour of the most important differences. But if you're doing anything more complicated than the basics, I suggest you always assume that Vim-regex is going to be different from Perl-regex or Javascript-regex and consult something like the Vim Regex website.

Answer 2

If by "normal regex" you mean Perl-Compatible Regular Expressions (PCRE), then the Vim help provides a good summary of the differences between Vim's regexes and Perl's:

:help perl-patterns

Here's what it says as of Vim 7.2:

9. Compare with Perl patterns                           *perl-patterns*

Vim's regexes are most similar to Perl's, in terms of what you can do.  The
difference between them is mostly just notation;  here's a summary of where
they differ:

Capability                      in Vimspeak     in Perlspeak ~
----------------------------------------------------------------
force case insensitivity        \c              (?i)
force case sensitivity          \C              (?-i)
backref-less grouping           \%(atom\)       (?:atom)
conservative quantifiers        \{-n,m}         *?, +?, ??, {}?
0-width match                   atom\@=         (?=atom)
0-width non-match               atom\@!         (?!atom)
0-width preceding match         atom\@<=        (?<=atom)
0-width preceding non-match     atom\@<!        (?!atom)
match without retry             atom\@>         (?>atom)

Vim and Perl handle newline characters inside a string a bit differently:

In Perl, ^ and $ only match at the very beginning and end of the text,
by default, but you can set the 'm' flag, which lets them match at
embedded newlines as well.  You can also set the 's' flag, which causes
a . to match newlines as well.  (Both these flags can be changed inside
a pattern using the same syntax used for the i flag above, BTW.)

On the other hand, Vim's ^ and $ always match at embedded newlines, and
you get two separate atoms, \%^ and \%$, which only match at the very
start and end of the text, respectively.  Vim solves the second problem
by giving you the \_ "modifier":  put it in front of a . or a character
class, and they will match newlines as well.

Finally, these constructs are unique to Perl:
- execution of arbitrary code in the regex:  (?{perl code})
- conditional expressions:  (?(condition)true-expr|false-expr)

...and these are unique to Vim:
- changing the magic-ness of a pattern:  \v \V \m \M
   (very useful for avoiding backslashitis)
- sequence of optionally matching atoms:  \%[atoms]
- \& (which is to \| what "and" is to "or";  it forces several branches
   to match at one spot)
- matching lines/columns by number:  \%5l \%5c \%5v
- setting the start and end of the match:  \zs \ze

Answer 3

Try Vim's very magic regex mode. It behaves more like traditional regex, just prepend your pattern with \v. See :help /\v for more info. I love it.

Answer 4

There is a plugin called eregex.vim which translates from PCRE (Perl-compatible regular expressions) to Vim's syntax. It takes over a thousand lines of vim to achieve that translation! I guess it also serves as precise documentation of the differences.

Answer 5

Too broad question. Run vim and type :help pattern.