The syntax for patterns used in these functions closely resembles Perl. The expression should 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.
The ending delimiter may be followed by various modifiers that affect the matching. See Pattern Modifiers.
Example 1. Examples of valid patterns
Example 2. Examples of invalid patterns
Note: The Perl-compatible regular expression functions are available in PHP 4 and in PHP 3.0.9 and up.