Delimiters

When using the PCRE functions, it is required that
the pattern is enclosed by delimiters. A
delimiter can be any non-alphanumeric, non-backslash,
non-whitespace character.

Often used delimiters are forward slashes
(/), hash signs (#) and tildes (~). The
following are all examples of valid delimited patterns.

/foo bar/
#^[^0-9]$#
+php+
%[a-zA-Z0-9_-]%

It is also possible to use bracket style delimiters
where the opening and closing brackets are the starting and ending
delimiter, respectively. (), {}, [] and
<> are all valid bracket style delimiter pairs.

(this [is] a (pattern))
{this [is] a (pattern)}
[this [is] a (pattern)]
<this [is] a (pattern)>

Bracket style delimiters do not need to be escaped when they are
used as meta characters within the pattern, but as with other
delimiters they must be escaped when they are used as literal
characters.

If the delimiter needs to be matched inside the
pattern it must be escaped using a backslash. If the delimiter
appears often inside the pattern, it is a good idea to choose
another delimiter in order to increase readability.

/http:\/\//
#http://#

The preg_quote() function may be used to
escape a string for injection into a pattern and its optional
second parameter may be used to specify the delimiter to be
escaped.

You may add pattern
modifiers after the ending delimiter. The following is an
example of case-insensitive matching:

#[a-z]#i