What you are asking about are the delimiters. Read this article in PHP.net's manual: RegExp - 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.
And:
Often used delimiters are forward slashes (/), hash signs (#) and tildes (~). The following are all examples of valid delimited patterns.
