Which Regular Expression Characters Need Escaping in PHP?

In a PHP regular expression pattern, the following meta-characters (i.e. characters that have a special meaning in a regular expression pattern) must be escaped wherever in the pattern they appear (except within square brackets):

\ ^ $ . [ ] | ( ) ? * + { }

Within square brackets (i.e. []), only the following meta-characters have a special meaning, and they must be escaped:

\ ^ -

You can escape a regular expression meta-character using the general escape character (i.e. \). If you have the need to escape a run-time string that may contain regular expression meta-characters, then you can use the preg_quote() function.

You must remember to also escape the delimiter character if it is being used in the regular expression pattern.