headerPattern top-level property
Leading (and trailing) # define atx-style headers.
Starts with 1-6 unescaped # characters which must not be followed by a
non-space character. Line may end with any number of # characters,.
Implementation
final headerPattern =
RegExp(r'^ {0,3}(#{1,6})(?:[ \x09\x0b\x0c].*?)?(?:\s(#*)\s*)?$');