The syntax for patterns used in these functions closely resembles Perl. The expression must be enclosed in the delimiters, a forward slash (/), for example. Any character can be used for delimiter as long as it's not alphanumeric or backslash (\). If the delimiter character has to be used in the expression itself, it needs to be escaped by backslash. Since PHP 4.0.4, you can also use Perl-style (), {}, [], and <> matching delimiters. See Pattern Syntax for detailed explanation.
The ending delimiter may be followed by various modifiers that affect the matching. See Pattern Modifiers.
PHP also supports regular expressions using a POSIX-extended syntax using the POSIX-extended regex functions.
Note: This extension maintains a global per-thread cache of compiled regular expressions (up to 4096).
You should be aware of some limitations of PCRE. Read » http://www.pcre.org/pcre.txt for more info.