Regexp Magic in vim

Some characters in the pattern are taken literally. They match with the same
character in the text. When preceded with a backslash however, these
characters get a special meaning.

Other characters have a special meaning without a backslash. They need to be
preceded with a backslash to match literally.

If a character is taken literally or not depends on the 'magic' option and the
items mentioned next.

*/\m* */\M*
Use of "\m" makes the pattern after it be interpreted as if 'magic' is set,
ignoring the actual value of the 'magic' option.
Use of "\M" makes the pattern after it be interpreted as if 'nomagic' is used.

*/\v* */\V*
Use of "\v" means that in the pattern after it all ASCII characters except
'0'-'9', 'a'-'z', 'A'-'Z' and '_' have a special meaning. "very magic"

Use of "\V" means that in the pattern after it only the backslash has a
special meaning. "very nomagic"

Examples:

after: \v \m \M \V matches ~
$ $ $ \$ matches end-of-line
. . \. \. matches any character
* * \* \* any number of the previous atom
() \(\) \(\) \(\) grouping into an atom
| \| \| \| separating alternatives
\a \a \a \a alphabetic character
\\ \\ \\ \\ literal backslash
\. \. . . literal dot
\{ { { { literal '{'
a a a a literal 'a'

{only Vim supports \m, \M, \v and \V}

It is recommended to always keep the 'magic' option at the default setting,
which is 'magic'. This avoids portability problems. To make a pattern immune
to the 'magic' option being set or not, put "\m" or "\M" at the start of the
pattern.

Without \v:

:%s/^\%(foo\)\{1,3}\(.\+\)bar$/\1/

With \v:

:%s/\v^%(foo){1,3}(.+)bar$/\1/

See also :h /\v

Why not to use Magic Quotes

Portability

Assuming it to be on, or off, affects portability. Use get_magic_quotes_gpc() to check for this, and code accordingly.

Performance

Because not every piece of escaped data is inserted into a database, there is a performance loss for escaping all this data. Simply calling on the escaping functions (like addslashes()) at runtime is more efficient.

Although php.ini-dist enables these directives by default, php.ini-recommended disables it. This recommendation is mainly due to performance reasons.

Inconvenience

Because not all data needs escaping, it's often annoying to see escaped data where it shouldn't be. For example, emailing from a form, and seeing a bunch of \' within the email. To fix, this may require excessive use of stripslashes().