hrPattern top-level property
Three or more hyphens, asterisks or underscores by themselves. Note that
a line like ----
is valid as both HR and SETEXT. In case of a tie,
SETEXT should win.
Implementation
final hrPattern = RegExp(r'^ {0,3}([-*_])[ \t]*\1[ \t]*\1(?:\1|[ \t])*$');