Constant PRE_ENCAPSULATION_BOUNDARY 

pub(crate) const PRE_ENCAPSULATION_BOUNDARY: &[u8] = b"-----BEGIN ";

The pre-encapsulation boundary appears before the encapsulated text.

From RFC 7468 Section 2:

There are exactly five hyphen-minus (also known as dash) characters (“-”) on both ends of the encapsulation boundaries, no more, no less.