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. Leading whitespace before a valid delimiter is silently ignored. Often used delimiters are forward slashes (/), hash signs (#) and tildes (~). The following are all examples …
https://www.php.net/manual/en/regexp.reference.delimiters.php

