[Tarean]

Either hackernews or autocorrect ate the p, it was supposed to be \p{L} which is a unicode character class.

As the other comment mentioned pcre-compatible Regex are a standard, though the pcre spec isn't super readable. There are some projects that have more readable docs like mariadb and PHP, but it doesn't really make sense to repeat the spec in library docs https://www.php.net/manual/en/regexp.reference.unicode.php

There are libraries for pcre2 or gnu regex syntax with the same API if you prefer those