Answer by Ja͢ck for PHP Regex: match character set OR end of string

To match a string that starts with a slash, followed by six alphanumerics and is then followed by either the end-of-string or something that's not alphanumeric:

preg_match('~^/[a-z0-9]{6}([^0-9a-z]|$)~i', $str);

The original JavaScript probably used new RegExp(<expression>), but PCRE requires a proper enclosure of the expression; those are the ~ characters I've put in the above code. Btw, I've made the expression case insensitive by using the i modifier; feel free to remove it if not desired.

You were using | as the enclosure; as such, you should have escaped the pipe character inside the expression, but by doing so you would have changed the meaning. It's generally best to choose delimiters that do not have a special meaning in an expression; it also helps to choose delimiters that don't occur as such in the expression, e.g., my choice of ~ avoids having to escape any character.

Expressions in PCRE can be generalised as:

<start-delimiter> stuff <end-delimiter> modifiers

Typically, the starting delimiter is the same as the ending delimiter, except for cases such as [expression]i or {expression}i whereby the opening brace is matched with the closing brace :)