What is the {L} Unicode category?

I don't see any explicit mention of it, but an example on this page indicates that \\p{L} means any letter:

Categories may be specified with the optional prefix Is: Both \p{L} and \p{IsL} denote the category of Unicode letters.

Taken from this link: http://www.regular-expressions.info/unicode.html

Check the Unicode Character Properties section.

\p{L} matches a single code point in the category "letter". If your input string is à encoded as U+0061 U+0300, it matches a without the accent. If the input is à encoded as U+00E0, it matches à with the accent. The reason is that both the code points U+0061 (a) and U+00E0 (à) are in the category "letter", while U+0300 is in the category "mark".